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Transportation “The Stone Age did not end because people ran out of stones- it ended because they found something better.” ~ Zaki Yamani, Saudi Arabia’s Oil Minister CHAPTER 14 The world’s transportation systems are expanding every day, and with expansion comes the rapid depletion of natural resources, ever increasing environmental degradation, more traffic congestion, and a higher frequency of accidents. The United States, with four million miles of highways, railroads, and waterways, 130 million passenger cars, 88 million buses and trucks, 230,000 aircrafts, 1.4 million miles of oil and gas pipelines, 40,000 ships, numerous railroad cars and boats, and 20,000 airports, has the largest transportation system in the world.1 Every day about 13 million barrels of petroleum, more than 2/3 of total US petroleum consumption and 27% of its entire energy use, are required to operate this huge system (Table 14-1).2 Because most of these fuels are imported, the US must seek ways to reduce its reliance on foreign oil either by increasing fuel efficiency and switching to alternative fuels or by reducing total miles traveled by investing in mass transit systems and carpooling, bike riding, and walking. Although most transportation issues are global in nature, because of the sheer size of the US transportation system, its role as a major technological powerhouse, and due to the fact that it is the biggest contributor to environmental pollution, this chapter focuses mainly on US data. Table 14-1. Transportation Energy Use by Mode, 2005 Modes Highway Automobiles Light Trucksa Motorcycles Buses Heavy Trucks Non-Highway Air Water Rail Pipeline Total Trillion BTU 22,043 9,140 8,108 27 191 5,342 5,721 2,348 1,300 657 842 27,065 a Percent of total 81.8 35.0 26.5 0.1 0.7 19.5 18.2 8.4 3.9 2.4 3.5 100 Including SUVs Reference: Davis, S. C., Diegel, S. W., Table 2.6. “Transportation Energy Data Book,” ORNL-6978, Edition 26, 2006.. Power Requirements We discussed the concept of energy and power in some detail in Chapter 2. Briefly, energy is what is needed to carry out a given task without concern for the time it takes to accomplish it. Power is the rate at which the energy is used and therefore determines the speed at which the task is performed. Without sufficient energy, we cannot go very far without having to refuel. If everything else remains the same, a larger fuel tank (more energy) makes it possible to drive longer distances, whereas a larger engine (more power) allows a vehicle to reach its destination faster, carry a greater load, or climb steeper roads. A major concern in any design of land, air, or marine vehicle is the power required for operation under various conditions. 1 2 Transportation Statistics, Annual Report 2003, US Department of Transportation, Bureau of Transportation Statistics, Washington, DC 20590, 2003. Davis, S. C., and Diegel, S. W., Transportation Energy Data Book, Oak Ridge National Laboratory, Edition 24, 2004. Did You Know That ...? • • • • • • • • • • US Transportation: The Facts There are over 220 million vehicles in the United States; this equals roughly 760 cars for every 1000 people. There are enough roads in the United States to circle the globe 157 times. Europeans and Japanese pay about three times as much for a gallon of gasoline as Americans. US carbon emissions alone are more than those of China, India, Germany, and Japan combined. According to the 2000 US Census, 88% of all people used personal vehicles to commute to work, of which 76.3% drove alone. Only 5.2% of people used public transportation, nearly all of whom lived in low-income neighborhoods. Road accidents cost about $90 billion and kill 40,000 Americans annually. The social costs associated with the loss of productivity, health, and road repairs approach one trillion dollars a year. As a result of congestion in US metropolitan areas each American wastes an average of 50 hours every year in traffic. Transportation accounts for 2/3 of all US petroleum consumption. In 2001, Americans used 100 million gallons of fuel to travel a distance of 2.8 trillion vehicle-miles, an average of 9,800 miles per capita. Annually, American cars burn enough gasoline to equal their own weight. Land Vehicles The most popular method used to power a vehicle is to burn fuel (mainly fossil fuels) in an onboard engine. Alternatively, energy can be stored in some other form (batteries, fuel cells, flywheels, or compressed air) and used as is needed. No matter which approach is used, to propel land vehicles, power is needed for four purposes.3 1. Overcoming the rolling friction of the tires - A heavier vehicle, a rougher road, and faster movement all result in a greater rolling resistance. Other parameters that affect rolling friction are the size and inflation pressure of the tires and the type, material, and age of the treads. At low speeds a vehicle’s rolling resistance is large relative to other resistances, and most power is dissipated at the wheels. For tracked vehicles such as tanks, heavy construction equipment, and rail systems, rolling friction is much greater and dominates all other types of resistance.4 Newer trains using magnetic levitation have no contact with tracks and rolling friction is virtually eliminated.5 2. Overcoming aerodynamic (wind) resistance – Aerodynamic resistance is the result of the interaction between a moving vehicle and the fluid through which it travels. The magnitude of the force increases with the density, the square of the vehicle’s velocity relative to oncoming wind, the projected area of the vehicle in the direction of travel (called the frontal area), and the shape of the vehicle. The effect of body shape on drag is usually expressed in terms of the coefficient of the drag. The drag coefficients for some common vehicles and body shapes are given in Table 14-2. Streamlining reduces the coefficient of drag and is particularly effective at high speeds where wind resistance Table 14-2. Drag Coefficients Car Trucks Sedans Porsche 924 GM Sunraycer CD 0.70 0.55 0.34 0.13 Shape Flat Plate Cylinder Sphere Teardrop CD 1.17 1.10 0.41 0.04 Here we concern ourselves mainly with passenger cars. Similar analysis can be carried out for other types of land vehicles such as buses, trucks, tanks, or railcars. Wong, J. Y., Theory of Ground Vehicles, Second Edition, John Wiley and Sons, Inc., 1993. 5 W ith a top speed of 430 km/h (267 mph), China’s maglev train is considered to be the world’s fastest commercial train. An experimental Japanese mag-lev train set the current speed record of 581 km/h (361 mph) on a test track near Tokyo in 2003. 3 4 334 Chapter 14 - Transportation is dominant. For most passenger cars, drag coefficients vary from 0.3 to 0.7 and aerodynamic resistance becomes significant at speeds exceeding 30-50 km/hr (20-30 mph). 3. Climbing (overcoming gravitational resistance) – As a vehicle climbs a slope, it must overcome gravity. The resistance increases with the grade of the road and the weight of the vehicle. When the vehicle is going downhill, this force is negative; therefore the total power required to propel the vehicle downhill is actually less than what is needed on a flat surface. 4. Accelerating – The acceleration force is equal to the vehicle’s mass times the acceleration. When the vehicle is decelerating (braking) this force is negative, which helps to reduce the overall power (energy) requirements. We will see later that electric and hybrid vehicles can regain a large fraction of the energy that would otherwise be lost during deceleration by regenerative braking. In addition, engines must provide enough power to operate a number of accessories such as heater, air conditioner, lights, wipers, horn, power steering, and a variety of microprocessors. Of the four types of resistance, only the first two contribute significantly to power requirements during cruising. Climbing and acceleration are mainly important during city driving, in stop-and-go traffic, and when passing other vehicles on highways. During idling, engine provides no useful work, and all energy goes to overcome engine friction. Whether a vehicle is cruising at a constant speed on a flat plain or accelerating over an incline, to reduce power it is desirable to reduce various resistances. This can be done: a. By making vehicles lighter. Heavier cars waste energy by flexing and heating up the tires. Both rolling and acceleration forces increase with vehicle mass and are therefore reduced proportionally to weight in lighter vehicles. There are new fiber composite materials (carbon, glass, and Kevlar foam) that are many times stronger than steel and weigh only one third to one half as much. Composites also make it possible to build frameless “monocoques,”6 making manufacturing easier with considerable savings in both material and energy. Reducing the frame weight by a certain amount makes the vehicle lighter by more than that amount; the synergistic effect resulting from lighter frames makes it possible to have a lighter suspension to carry the weight, a smaller engine, and less fuel to move it. b. By making vehicles more aerodynamic. Sleeker, sportier shaped bodies and smoother underbodies reduce air friction, allowing cars to move faster and consume less fuel. Convertible cars, cars with rolled down windows, and cars with large frontal areas are considerably less aerodynamic.7 6 7 A t ype of vehicle construction in which the body is integral with the chassis. K atz, J. R ace Car Aerodynamics, Robert Bentley, Inc., 1995. 335 c. By proper maintenance. Fuel economy can be considerably improved by keeping tires inflated, air filters clean, and the engine tuned. Driving at the cruising speeds, using accessories less frequently, and avoiding fast braking and acceleration can also help. d. By using hybrid technology. Hybrid vehicles have two modes of propulsion, usually an internal combustion engine and an electric motor. The vehicle operates as an electric car during city driving and in stop-and-go traffic, but is essentially a conventional gasoline or diesel vehicle during cruising and highway driving. Since the power required during cruising is low, a much smaller engine is needed. The electric motor delivers additional torque when accelerating or climbing steep grades. Thus, the fuel economy of these vehicles is significantly improved. d. By using alternative fuels. Contemporary automobiles are highly inefficient; almost 80% of the fuel energy is lost in the exhaust and dissipated to the environment as waste heat. Certain fuels have been shown to have marginally better efficiencies than gasoline and diesel oil, but are of interest mainly due to their reduced emissions. Marine Vehicles As ships and other marine vehicles move through water, they experience both water and air resistances. The resistance through water is the greater of the two and depends on many factors including ship speed, hull form, and water temperature. The total hull resistance consists of several components: a. Skin friction drag due to friction between the hull and water b. Form drag due to pressure forces acting on the hull c. Wave drag due to energy lost in creating and maintaining the ship’s characteristic bow and stern waves e. Wind resistances Frictional losses are the function of the hull’s wetted surface area, surface roughness, and viscosity. At low speeds, viscous resistance is dominant and can account for up to 50-80% of total resistance. As speed increases, the wave-making resistance increases rapidly and eventually dominates all other resistances. Air drag can also play a role and may contribute between 4 to 10% to total ship resistance, depending on speed, the shape of the ship above the waterline, and the area of the ship exposed to the air. Wind and current resistances can be significant in rough waters and when a ship runs into strong headwinds. The resistance is not proportional to velocity, but increases as the square of velocity at low speeds and more rapidly as velocity to the fifth power at higher speeds (Figure 14-1). The power required to propel a ship through water is the product of total hull resistance and ship speed (P = F.V). Therefore, the power required can be proportional up to ship speed raised Total Resistance Total Resistance Air Resistance Wave Making Resistance Viscous Resistance Ship Speed (knots) Figure 14-1 Components of hull resistance 336 Chapter 14 - Transportation to the 6th power! The fuel consumption rate also increases accordingly; it takes much more fuel to travel a given distance at a faster speed than traveling the same distance at a slower speed. Aircrafts The resistance forces acting on aircrafts are similar to those discussed for marine vehicles and consist mainly of pressure and viscous forces. Pressure forces act normal to the surface and arise from differences in air pressure along the body. Viscous or shear forces are surface resistances close to the airplane body due to the fact that it moves in a viscous fluid. The magnitudes of these resistances vary widely depending on the airplane’s shape, size, speed, and altitude.8 Normally, the resultant aerodynamic forces are resolved into two components: that perpendicular to the flight path (lift) and that parallel to the flight path (drag). During cruising, the engine must be able to overcome drag forces. Maximum power is needed during takeoff and increases with the lift and the rate of the climb. Example 14-1: To ferry a space shuttle from Edwards Air Force Base in California to its launching site at NASA’s Kennedy Space Center in Florida, it is bolted on the back of a modified Boeing 747-400 jumbo jet. The volume of fuel used for this journey is huge, as it will take one gallon of fuel to travel only one length of the plane (231 feet). Because of many refueling stops along the way, the trip takes two days to complete. Calculate the amount of fuel it takes to complete the 2,600 mile journey. Solution: The fuel efficiency of the Boeing 747-400 is around 0.2 mpg. Because of the heavy payload and loss of much of the aerodynamic advantages, with the space shuttle piggy-backed, the fuel efficiency drops to only 231 feet per gallon (0.04 mpg). For the 2,600 mile trip, 2,600 mi/(0.04 mi/gal) = 65,000 gallons of fuel is needed. The Boeing 747-400 can carry 32,750 gallons (215,000 liters) of fuel, which is about half the required fuel. For safety reasons, planes carry at least twice the amount of fuel required to complete a trip. This means the plane has to make at least 3 or 4 stops for refueling. Ex 14-1 Internal Combustion Engines Internal combustion engines have found major applications in many industries; particularly in transportation, home appliances, and stationary power generation systems. Depending on their size, they can deliver power in the range of a few to several thousand kilowatts. The principle of operation of internal combustion engines is simple -- a mixture of fuel and air is burned inside a combustion chamber and the product of the combustion is expanded, turning the energy of fuel into useful (shaft) work. Examples of internal combustion engines are gasoline engines, diesels, and gas turbines. 8 Schaufele, R. D., The Elements of Aircraft Preliminary Design, Aries Publications, Santa Ana, Ca, 2000. 337 There is no single person who can be named as the inventor of the internal combustion engine; the process was an evolutionary one that started with Christian Huygens in 1680 who designed (but never built) an internal combustion engine working with gun powder. Over the next two centuries many inventors perfected combustion engines, among them Nicholas Otto (1876), who proposed a four-step “Otto” cycle, Gottlieb Daimler (1885), who invented the prototype gas turbine, and Karl Benz (1889), who built the first practical four-stroke, two cylinder engine and the first four-wheeled automobile. Wilhelm Maybach (1890) then used Benz’s design to build the first four-cylinder, four-stroke engine.9 The first automobile manufacturing company in the United States was established by Henry Ford in 1903. The company named its automobiles according to the letters of the alphabet. In 1908 it introduced the Model-T (Figure 14-2), the first mass-produced vehicle, and offered it at prices affordable to a great number of people. With increased reliability and cheap, widely available petroleum, the number of privately owned cars increased rapidly. Figure 14-2 1908 Ford Model T To meet their transportation needs, Europeans and Americans followed different paths. Europeans invested mainly in trains, trolleys, and subways. American policy makers, under pressure from the booming automotive industry, promoted private car ownership. Some American car companies went as far as buying and dismantling streetcars and replacing them with diesel buses, which in many instances were poorly designed, effectively forcing the public to purchase private cars for transportation. Following WWII, US interstate highways expanded rapidly, literally paving the way for automobiles as the principal source of transportation.10 Of the 700 million cars in the world, over one-third belong to Americans. Today, Europeans have the same ratio of cars to people as the US had a quarter century ago. Similarly, present per capita car ownership in the former Soviet Union is comparable to that of the US in 1923 (See Figure 14-3). In 2003, Americans traveled more than 4.1 trillion miles, over 87% of which was by automobiles, SUVs, and other light trucks. As the data presented in Table 14-3 indicate, the miles traveled by commuters using rail systems totaled only about 0.6% of that traveled by passenger cars. Bus and train occupancy rates were very low; as a result, fuel efficiency measured as BTU/passenger-miles were roughly the same for passenger cars and other modes of transportation. Public transportation in Europe is, however, significantly more efficient as trains and buses run with much heavier occupancy rate; four to five- fold increases in efficiency (730 BTU/ passenger-miles for trains and 1050 BTU/passenger-miles for buses) are common in most European countries. 900 800 700 Vehicles per 1000 People 600 500 400 300 200 Eastern Europe 1994 100 0 Middle East 2005 China 1994 1900 1910 1920 1930 1940 1950 1960 1970 Central & South America 2005 U.S. Canada 1994 Western Europe 2005 Canada 2005 Paci c 2005 Paci c 1994 Western Europe 1994 1980 1990 2000 Figure 14-3 Vehicles per thousand people for various regions in the world. Source: US Department of Transportation, Federal Highway Administration, Highway Statistics 2007, Washington D.C. (http://www. fhwa.dot.gov) 9 10 Brittanica Online Encyclopedia (http://www.britannica.com). See for example Jane Holtz Kay, A sphalt Nation: How the Automobile Took over America and How We Can Take It Back , New York: Crown Publishers, 1997, p. 171. 338 Chapter 14 - Transportation Whether the European or American model of transportation is superior is a matter of taste and public policy. In the United States, cars are a necessity and are well-entrenched in the American lifestyle. Distances between homes and offices and between different cities are much longer, and public commuter systems are vastly inadequate. As a result, the use of public transportation is largely limited to the low-income and the disadvantaged. In Europe, on the other hand, distances are shorter, the infrastructure is more developed, and a vast network of railways and other public transportation systems for commuting and long distance travel already exist. Affordable and easily accessible public transportation has removed the need for personal vehicles for most people, making cars a luxury - nice to have for special occasions, leisurely evening activities, and weekend excursions. Spark Ignition (Gasoline) Engines Most passenger cars operating today are spark ignition; their engines work by allowing a mixture of fuel and air prepared in a carburetor to burn and expand inside the cylinders. The downward motion of a piston is converted to rotational motion of the crankshaft by means of connecting rods and eventually, by way of a transmission, to the wheels. The rotary (Wankel) engine is a special type of spark ignition engine in which the piston-cylinder assembly is substituted by a three-edged rotor mounted off-center of a specially designed housing. Unlike conventional engines where the four strokes are accomplished by linear up and down motions of the pistons, rotary engines work by varying the volume as the rotor rotates in the housing. The main advantages of rotary engines are their lower mass and volume as compared to reciprocating engines of comparable power rating and their simpler construction. Furthermore, because rotary engines deliver power directly in rotational form, their overall efficiency is higher. The main disadvantage is that rotors are more difficult to seal, which makes hydrocarbon emission higher. Basic operation of reciprocating and rotary engines is described in Figure 14-4. Table 14-3. US Passenger Travel and Energy Use, 2004 Mode Number of Vehicles 136 million 81 million 5.4 million 695,000 219,000 18,600 Passengermiles (billions) 2,669 1,479 11 21 549 31 % of total travel 56 31 0.2 0.3 12 0.6 Load Factor (persons/ vehicles) 1.6 1.7 1.1 8.7c 90 24 BTU/ passengermiles 3,496 4,329 2,272 4,318c 3,959 2,978 Automobiles Light trucks Motorcycles Busesa Planesb Rails a Includes transit, intercity, and school; b General aviation; c Data available for transit buses only Ref: Davis, S. C., Diegel, S. W., Table 2.12. “Transportation Energy Data Book,” ORNL6978, Edition 26, 2006. 339 Charged Ignition (Diesel) Engines Instead of compressing the air/fuel mixture to high temperatures and pressures as is done in gasoline engines, diesel engines operate by compressing air alone. Because there is no fuel present, air can be heated to pressures and temperatures well over the ignition temperature of fuel without concern for engine knock. Unlike gasoline engines, diesels require no carburetors or spark plugs. Instead, tiny droplets of diesel fuel are injected directly into the cylinder, where they mix and react with already heated air and burn (Figure 14-4). Since injection times are relatively long, the piston travels an appreciable distance before fuel is cut off, keeping the pressure in the cylinder nearly constant. intake valve open all valves closed exhaust valve open cylinder piston connection rod (1) (2) (3) (4) (1) (2) (3) (4) Four-stroke Otto/Rotary Engines 1. Intake stroke: the intake valve (port) opens and the air-fuel mixture is sucked into and fills the cylinder. 2. Compression stroke: The intake valve closes, and the crankshaft helps the piston to move up (or the rotor to rotate), compressing the mixture to a high temperature and pressure. 3. Power stroke: A spark plug ignites the mixture. The pressure suddenly rises, forcing the piston to move up (or the rotor to rotate), expanding gases and providing shaft work. At the end of the power stroke, the exhaust valve opens and the pressure suddenly drops to near atmospheric conditions (blow-down). 4. Exhaust stroke: The piston moves upwards (or the rotor continues its rotation and clears the exhaust port), exhausting the burned gases. Four-stroke Diesel Engine 1. Intake stroke: As the piston slides down, the intake valve opens and pure air is sucked into and fills the cylinder. 2. Compression stroke: The intake valve closes, and the crankshaft helps the piston to move up, compressing the air to a high temperature and pressure. The temperature is now above what is necessary for self-ignition of the fuel. 3. Power stroke: At TDC, an injection pump gradually injects fine fuel droplets into the combustion chamber. As fuel droplets meet particles of hot air they burn, increasing the pressure and forcing the piston to move down, expanding gases and proving shaft work. At the end of the power stroke, the exhaust valve opens and pressure suddenly drops to near atmospheric conditions (blow-down). 4. Exhaust stroke: As the piston makes the second round of upward motion, burned gases are exhausted. The exhaust valve then closes in preparation for the cycle to begin again. Figure 14-4 Four-stroke internal combustion engines for reciprocating (top) and rotating (bottom) engines. 340 Chapter 14 - Transportation Generally speaking, diesels are less efficient than gasoline engines with similar compression ratios. However, because diesels are designed to operate at higher compression ratios than spark-ignition engines (20-22 compared to 8-9 for spark ignition engines), they have higher efficiencies (~40% compared to ~30% for the petrol engines). Their main drawbacks are that they are bulkier, emit more, have a higher capital cost, and accelerate more slowly. These characteristics make diesels particularly attractive in stationary applications and for buses and large trucks. Because of the stricter air pollution standards, diesels are less common in the United States than Europe. New technological inventions are underway that makes diesels cleaner, and thus sales of diesels are likely to increase in the US in the near future. Gas Turbines The basic operation of a gas turbine is relatively simple. Air is drawn through a nozzle and compressed in a compressor to a very high pressure and temperature before entering a combustion chamber. Fuel is injected through a fuel injector into the combustion chamber where it mixes with the air and burns. Depending on the application, kerosene, natural gas, or jet fuel can be used. The product of combustion is expanded and runs a turbine that provides power. The shaft work can be used to propel a helicopter, provide thrust to a jet engine, drive a tank, or turn a generator and produce electricity (Figure 14-5). The main advantages of gas turbines over gasoline and diesel engines are their relatively low emissions, very high power density (kW/m3) and specific power (kW/kg), multi-fuel capability, and smooth, vibrationfree power delivery. New ceramic materials allow gas turbine operation at higher temperatures with efficiencies exceeding those of conventional engines by as much as 50%. Transportation Fuels The most common fuels used for automobiles are still gasoline and diesel oil. They are liquid, are relatively inexpensive, and have good combustion characteristics. Furthermore, the necessary infrastructure for production, Turbine cross section view compressor turbine cold air air warms up energy released combustion chamber Figure 14-5 Gas turbine operation follows the Brayton cycle, consisting of four processes: 341 storage, and distribution of these fuels is fully in place. These fuels, however, are becoming increasingly more scarce and have been associated with most of our air pollution problems. In the search for better fuel economy and reduced emissions, much research has been done on the use of alternative fuels. However, these fuels have not found wide acceptance and their use has been limited to fleet and government vehicles. Suitable alternative fuels can only find wide-scale acceptance if they meet many of the same requirements as gasoline and diesel fuels. To get a better understanding of the issues in using alternative fuels, various transportation fuels as well as their advantages and drawbacks are summarized below. Gasoline (Petrol) is the most common fuel used in vehicles. It is a colorless and volatile liquid made up of many hydrocarbons, but is conveniently represented as a single compound with a molecular structure approximated as C8H17. Like most other liquid fuels, gasoline is derived primarily from crude oil. To aid combustion and reduce hydrocarbon emissions, it is advantageous to add some oxygen to gasoline. The product, called reformulated gasoline, has been found to reduce smog in cities with a large concentration of cars. Additionally, adding some detergents to gasoline will help to prevent the build-up of engine deposits, keeping engines working smoothly and burning fuel cleanly. Diesel oil, like gasoline, is a mixture of light distillate hydrocarbons. Its boiling point is somewhat higher than that of gasoline, allowing for engine operation at an increased compression ratio with less concern over the engine knock, common in gasoline engines. Kerosene is a mixture of heavier hydrocarbons; it is mostly used as heating oil. Kerosene is lighter than gasoline and has a greater heating value. It is used as jet fuel in pure distillate ( JP5) or is blended with gasoline ( JP4). Kerosene is often represented as a single hydrocarbon with the formula C12H26. Biodiesel is derived from vegetable oils, seeds, animal fats, and algae. Biodiesels are, however, more viscous than fossil fuels; this makes coldtemperature starts more difficult. Because biodiesel is a biomass, it is biodegradable, less toxic, has fewer emissions, and does not contribute to global warming. Biodiesel is marketed as pure biodiesel (B-100), or as a blend of 20% biodiesel and 80% petroleum diesel (B-20). Natural gas is about 90% methane and 10% heavier hydrocarbons. Its high octane number (ON = 130) allows engines to operate at compression ratios as high as 12:1, compared to 8:1 for gasoline engines. The main problems with methane are its toxicity and lack of odor, which makes leaks undetectable. Furthermore, methane-powered cars have inferior 342 Chapter 14 - Transportation power and energy densities, resulting in roughly 20% less fuel efficiency than gasoline and diesel engines and a relatively low cruising range. Because of its low energy density, natural gas must be compressed or liquefied before it can be used for vehicles. To produce compressed natural gas (CNG), methane must be compressed to 15-25 MPa (2,400-3,600 psi) and stored in special containers. At ordinary room temperatures, it is impossible to liquefy methane by compression alone and must be cooled to cryogenic temperatures. CNG is generally considered a relatively clean fuel, as the emission of carbon monoxide, hydrocarbons, and particulate matter is substantially less than that of gasoline or diesel fuel. Reduced emissions are offset, however, by an increase in the emission of unburned methane, a potent greenhouse gas which traps heat about twenty times more effectively than carbon dioxide. At this time, methane emission accounts for 13% of all greenhouse gases, but it would become a major concern if natural gas were to become a dominant fuel for transportation. Liquefied Petroleum Gas (LPG) is natural gas leftover after methane is removed. It is mostly propane, but some butane and higher hydrocarbons are also present. Unlike methane, propane can be liquefied at room temperature and relatively low pressure; this makes it suitable for storage in light fuel tanks and gives cars a driving range comparable to that of gasoline. Methanol (also called methyl or wood alcohol) is the simplest alcohol. Many consider methanol to be the fuel of choice. It is liquid, like gasoline, but has a higher octane rating (ON = 100), and can be produced by utilizing a variety of methods from gasoline, natural gas, and other fossil fuels in large quantities and at a low cost; it may also be produced from the distillation of wood chips, garbage, and animal manure. Since methanol has a high H/C ratio, it is a relatively clean fuel and, except for aldehydes, methanol emissions are considerably lower than those of gasoline. Unfortunately, methanol has several drawbacks which limit its use as a viable transportation fuel. Methanol has an invisible flame. To enhance flame luminosity, methanol is usually mixed with a small percentage of gasoline and is often used as M85 (85% methanol, 15% gasoline) instead of pure methanol (M100). Methanol is so toxic that even ingesting a small amount can cause blindness or death. Another problem is that methanol is corrosive to aluminum and other materials commonly used in seals and pipe fittings for transporting gasoline. Therefore, methanol cannot be transported through existing pipelines and must be trucked. Methanol has only half the energy density of gasoline, which means for a given driving range about twice the amount of fuel will be needed. Because of these limitations, the number of methanol vehicles has declined steadily in recent years. Because methanol can be converted to hydrogen at relatively 343 low temperatures, it remains an attractive fuel for producing the hydrogen needed to operate fuel cell vehicles. Ethanol (also called ethyl or grain alcohol) is made through the biochemical conversion (fermentation) of sugar and perennial grasses (such as miscanthus), or by the hydration of ethylene from petroleum. Ethanol can be used directly in fuel cells, used in the pure form or mixed with gasoline to power internal combustion engines. Since ethanol is a biomass, there is no net carbon dioxide emission and it does not contribute to global warming. Like gasoline, ethanol is a liquid, making it possible to use in existing fuel tanks, fuel pumps, and fueling stations. Engine performance is also comparable to that of engines using gasoline. NOx emissions are lower and formaldehyde higher, but other emissions are about the same as gasoline and slightly greater than methanol. Ethanol has a lower heating value than gasoline and burns at a lower A/F (9:1 as compared with 15:1 for gasoline). So, for a given engine more fuel can be introduced per cycle, compensating for its lower heating value, and an overall higher power output is possible. Ethanol has a very high octane rating (ON = 113) and is therefore most suitable for use in highcompression engines. Ethanol, like methanol, burns with an invisible flame. To increase visibility, ethanol is often blended with gasoline to produce gasohol (a mixture of 10% ethanol and 90% gasoline), E85 (85% ethanol, 15% gasoline), and E95 (95% ethanol, 5% gasoline). E85 is usually used for light-duty applications, whereas E95 is best for heavyduty vehicles. Gasohol is not common in the United States. Hydrogen is considered by some to be the fuel of the future; it is the simplest and most abundant element in the universe. Pound for pound, hydrogen has the highest energy content of all fuels, and when burned in oxygen the only by-product is water. Hydrogen is also an ideal fuel to produce electricity – through fuel cells – directly eliminating carbon dioxide and other pollutants that are associated with burning of fossil fuels. There are, however, a number of factors that prevent hydrogen from being utilized as the most promising transportation fuel, and some even have dismissed hydrogen as hype. Unlike petroleum and methane, hydrogen cannot be mined but must be produced in a number of ways, ranging from hydrocarbon reformation, to coal gasification and pyrolysis, to the electrolysis of water. Although hydrogen is a clean fuel, converting conventional fuels to hydrogen does not necessarily improve air quality. Cleanliness depends on how the hydrogen is produced. If hydrogen is I f hydrogen is burned in an internal combustion engine then, besides water vapor, some NOx will also be produced. The high temperatures of hydrogen flames cause air to be ionized i nto atomic nitrogen and oxygen, which subsequently react to produce nitric monoxide. When hydrogen is used in fuel cells, the reaction is at low temperatures, and the only product is w ater vapor. 11 344 Chapter 14 - Transportation produced by reforming ethanol, then the large amount of energy required for the distillation of ethanol will take away most of the hydrogen advantage; if hydrogen is produced by reforming fossil fuels, then most of its environmental benefits disappear as well. Renewable sources such as solar, wind, and hydropower are ideal because they can electrolyze water in large quantities without polluting the atmosphere. Nuclear sources are another option. Currently, most commercial hydrogen is produced by steam reformation of natural gas. Natural gas is a particularly good candidate for producing hydrogen at the wellhead. Instead of piping crude or natural gas, large reformers can be installed to strip off the hydrogen and ship it through the pipelines. The byproduct of the reformation process, mainly carbon dioxide, can then be sequestered by injecting it back into the gas field, raising the pressure and improving extraction efficiency (See Chapter 7, Enhanced Recovery). Hydrogen safety and storage are two other concerns most often raised in regard to hydrogen. Hydrogen is highly explosive and burns easily with an invisible flame. When compared to most other gaseous fuels, hydrogen’s flammability range is unusually broad, from 4 to 75 percent. Natural gas, in contrast, burns between 5 and 15 percent and requires much less energy to ignite. Table 14-4 compares the ignition characteristics of hydrogen with those of methane and gasoline. Table 14-4. Ignition Characteristics of various gaseous and liquid fuels Hydrogen Ignition Temperature Flammability Limits Explosion Limits Energy Density Ignition Energy 585 oC 4-75% by vol. 20-65% by vol. 120 MJ/kg 8.5 MJ/L 0.02 mJ Methane 540 oC 5-15% by vol. 6-14% by vol. 50 MJ/kg 40 MJ/L 0.2 mJ Gasoline 230-480 oC 1.4-7.6% by vol. 1-7.6% by vol. 44 MJ/kg 21 MJ/L 0.2 mJ Source: Flynn, T., Cryogenic Engineering, Second Ed., CRC Press, 2005. Hydrogen storage is also a problem, as hydrogen must be compressed to very high pressures, cooled to very low temperatures, or stored as chemical or metal hydrides, adding considerable weight to the storage container. Hydrogen has a low energy density; one kilogram of hydrogen contains about the same amount of energy as a gallon of gasoline and about one third that of natural gas. Another concern is hydrogen embrittlement. Due to the small size of its molecules, hydrogen is far more prone to leak out, causing pipes to deteriorate faster. Existing distribution channels used for transport and delivery of gasoline are not suitable for hydrogen and the cost of developing new systems is very high. Aside from these considerations, some concerns have recently been 345 raised in regards to the effect of potential gas leaks to the environment. Normally, hydrogen is stored at high pressures and about 10% of all hydrogen manufactured is leaked through the atmosphere. Being so light, hydrogen rapidly rises through the atmosphere, eventually reacting with oxygen to form water vapor. The additional water makes the stratosphere wetter, cooling the lower atmosphere, particularly in the polar regions where most hydrogen is converted to water vapor. This would disrupt the ozone layer, causing 7-8% more depletion over the poles.12 Emissions from Internal Combustion Engines When automobiles were marketed in the early twentieth century, they were seen by many as a clean form of transportation, doing away with the nuisance of horse feces in large urban cities. In the decades that followed, as the numbers of vehicles increased, the impact of this technology on air quality and health became more pronounced and cars became the main source of environmental pollution. Today, motor vehicles are responsible for roughly four-fifths of all carbon monoxide emissions and nearly half of all hydrocarbons and nitrogen oxides released into the atmosphere globally (Table 14-5). In addition, motor vehicles produce carbon dioxide, a potent greenhouse gas and a major contributor to the production of ozone in the lower atmosphere. Compared to reciprocating engines, gas turbines produce lower emissions. This is because gas turbines operate under ultra-lean combustion. Furthermore, gas turbines do not respond well to transient operations and must operate in a continuous mode. Table 14-5. US Total emission of criteria pollutants in 2002* Source CO NOx HC (VOC) PM10 PM2.5 SO2 NH3 Total Emission 108 million tons 20 million tons 16 million tons 21 million tons 7 million tons 14 million tons 7 million tons Mobile 82% 55% 42% 2.2% 6.1% 4.4% 6.3% Stationary 3.8% 38% 6.6% 6.2% 17.9% 86.3% 1.4% Industrial 2.3% 3.9% 41.5% 3.1% 7.1% 8.9% 3.4% Others** 12.6% 2.2% 10.2% 88.5% 69.0% 0.3% 88.8% * To the closest percent ** Waste disposal, recycling, etc. Source: US EPA, National Emission Inventory Air Pollutant Emission Trends website http://www.epa.gov/oar/oapqs The main sources of engine emissions are unburned hydrocarbon from the crankcase, carburetor, and fuel tank, as well as tailpipe emissions. Most emissions come from the exhaust pipes and include carbon monoxide, nitrogen oxides, soot, and various volatile organic compounds. Exhaust pipe emissions are largely a result of incomplete combustion processes and are affected by many factors. Three parameters have profound effects on how combustion proceeds and how much pollution is formed. These 12 NaTrompme, et al., “Potential Environmental Impact of a Hydrogen Economy on the Stratosphere,” Science , 300, 1740-1742, 2003. 346 Chapter 14 - Transportation parameters are referred to as the three Ts of combustion - temperature, time, and turbulence. Temperature determines the rate of reaction and heat release. If everything else remains the same, higher temperatures increase production of nitric oxides and limit the production of hydrocarbons, carbon monoxide, and soot. Temperature is highest near the stoichiometric conditions and for fuels with high hydrogen to carbon (H/C) ratios. Stoichiometric conditions occur when an air/fuel mixture burns to completion and when, under equilibrium conditions, the only products of combustion are carbon dioxide and water. The time that reactants have in a flame reaction zone determines the extent to which reactions go to completion. Greater residence times generally favor more stable molecules (carbon dioxide, nitrogen, water vapor) and lower overall emissions. Turbulence affects the rate of mixing of fuel and air molecules. Better mixing prevents localized pockets of very rich or very lean mixtures, thus allowing the reaction to complete and produce fewer contaminants. A brief discussion of major pollutants from automobiles and other internal combustion engines and factors affecting their formation is given below. Nitric oxide results from a reaction between oxygen and nitrogen heated to combustion temperatures. As we may expect, maximum NOx occurs near stoichiometric conditions where temperature is at its peak and sufficient oxygen is available. Too much air dilutes the mixture and reduces the flame temperature; with too much fuel, little oxygen is left over for reacting with nitrogen. The best way to reduce NOx emission is by lowering the combustion temperature and limiting the availability of air. Recirculating a portion of exhaust gases back to the intake manifold is shown to be effective in reducing NOx emission by lowering the air to fuel ratio and cooling the flame. Unfortunately, exhaust gas recirculation (EGR) valves must operate with a rich mixture, which promotes production of hydrocarbons and carbon monoxides. Another method to reduce NOx emissions is to add some hydrogen to the gasoline/air mixture. The drawback is backfiring, a result of hydrogen’s low ignition temperature and lower power rating. Carbon monoxide results from incomplete reactions. It is formed when there is insufficient oxygen, temperature, or time to oxidize all carbon to carbon dioxide. The obvious way to reduce carbon monoxide emissions is by increasing the compression ratio (increasing temperatures) or by maintaining a lean fuel mixture. Advancing the spark raises peak temperatures and increases the time available for reaction, both of which inhibit carbon monoxide formation. Hydrocarbon emission can be divided into two categories, unburned and partially burned. Roughly half of all hydrocarbon emissions come from unburned vapor during fill ups, evaporation from the carburetor, and from other hot surfaces when the engine is shut off. Other sources 347 of hydrocarbon emission are crankcase blow-by and emission from leaky valves, piston rings, and gaskets. The hydrocarbon release during fueling is reduced by installing vapor recovery nozzles on gas pumps. The crankcase emission is practically eliminated by closing off the vent to the atmosphere and installing a positive ventilation valve (PCV) to recycle blow-by back into the engine’s intake. Particulates– Besides gaseous pollutants, most internal combustion engines produce some solid carbon particles called soot. Soot particles are large clusters of carbon atoms generated in the combustion chambers and aggregated to sizes ranging from a few nanometers to hundreds of microns. Soot can also be formed from the lubrication of oil next to cold surfaces and from the rapid expansion cooling during the power stroke. Although some soot is generated in gasoline engines under heavy loads, diesels are by far the main source of soot. This is due to the relatively poor mixing of the fuel and air in the combustion chamber. Diesel particulate problem has largely been solved in Europe with superior fuel injection systems and specially manufactured catalytic filters. In the US, improved fuel injection systems have helped remove odor and visible smoke. However, because of the high sulfur content of the US diesel fuel, catalytic filters have yet to be deployed. Lead was a major source of particulate emission prior to the 1980s, during which time it was added to gasoline as an anti-knocking agent. Knock occurs as a result of premature ignition in engines with high compression ratios. A small amount of lead additive had the effect of increasing the octane number (and ignition temperature), which allowed building engines with higher compression ratios and better efficiencies. Lead, however, is a poisonous metal with detrimental health effects. It also contaminates catalytic converters that are designed to reduce exhaust emissions. Because of this, use of lead additives was gradually phased out and, except for very old cars, all cars operate with lead-free gasoline now. In addition to carbon particles, many combustion systems such as industrial boilers and kilns produce sulfate. Because sulfur in gasoline and diesel fuels is removed before they are sold, few sulfate particles are formed in automobiles. Catalytic Converters Vehicular exhaust emissions not only contain carbon dioxide and water vapor, but also a significant amount of carbon monoxide, nitrogen oxides, and hydrocarbons. Generally speaking, nitric oxides are produced during cruising and acceleration (driving mode) where the mixture burns at its maximum temperature near stoichiometric conditions. On the contrary, during braking and when the engine idles (stopping mode), some exhaust enters the intake manifold, causing carbon monoxide, hydrocarbon, and particulate emissions to be high. 348 Chapter 14 - Transportation For the best performance - maximum efficiency and minimal environmental pollution - it is ideally desirable to convert all carbon into carbon dioxide, all hydrocarbons into water vapor, and all nitrogen into molecular nitrogen. To convert carbon monoxide and hydrocarbon we need a high temperature and an oxidizing atmosphere. The problem is that these same conditions provide a favorable environment for the oxidation of nitrogen to nitric oxides. Three-way catalytic converters can achieve successful modifications of CO and HC while at the same time reducing NOx to molecular nitrogen. This is achieved in two steps: Convert all nitric oxides into nitrogen in a reducing atmosphere (fuel rich). NO Rh N2 + O2 (14-1a) Convert all hydrocarbons and carbon monoxide into carbon dioxide and water in an oxidizing atmosphere (air rich). CO + HC Pt, Pd CO2 + H2O (14-1b) 100 Catalytic Convenrter Efficiency (%) Generally, catalytic converters are made of a stainless steel canister mounted along the exhaust pipe. Inside the container is either a porous ceramic honeycomb or is filled with ceramic pellets embedded with small particles of alumina with catalytic materials deposited on their surface. Rhodium is the catalyst of choice for reducing, and platinum and palladium work best for oxidizing reactors. Three-way converters employ redox (for reduction/oxidation) catalysts, which are essentially made of two reactors, a reducing reactor followed by an oxidizing one. CO + HC + NO reducing reactor CO + HC + N2 80 60 HC NOx 40 20 0 CO Lean Rich 0.96 0.97 0.98 0.99 1.00 1.01 Equivalence Ratio, 1.02 1.03 1.04 1.05 oxidizing reactor CO2 + H2O (14-1c) Figure 14-6 Conversion efficiency of the catalytic reactors as function of the fuel/air ratio. NOx removal efficiency is very low under lean conditions where efficiency is highest for CO and HC. In a narrow region near the stoichiometric ratio the conversion efficiency is high for all three contaminants. Figure 14-6 shows the conversion efficiencies of the catalytic converter for CO, HC, and NOx at different fuel/air ratios. As can be seen, for redox reactors to work, they must operate within a narrow range of air fuel ratios (around stoichiometric). When in good working conditions, these reactors reduce in excess of 98% of CO and 95% of HC and NOx emissions. Emission Standards Prior to 1970, there were no standards regulating automobile emissions. In 1970, the Environmental Protection Agency (EPA) was established and, as its first act, it regulated the emissions of carbon monoxides to less than half their previous level. Complying with this standard was easy; all 349 Table 14-6. US Federal emission standards for gasoline and diesel passenger cars HC g/mile Uncontrolled 1970 1972 1975 1981 2004 11.0 2.2 2.9 1.5 0.41 0.41 CO g/mile 80 23 28 15 3.4 3.4 NOx g/mile 4.0 --3.1 1.0 0.4 Source: 40 CFR 86.000.8 Office of Air and Radiation, US EPA (http://www.epa.gov). that was needed was to adjust the carburetors in order to provide a slightly leaner mixture. The result, however, was that the concentration of nitric oxides in the atmosphere climbed to unreasonable levels, forcing the EPA to adopt new emission standards for the control of both nitric oxide and carbon monoxide. As the number of cars grew and air quality deteriorated, the EPA became more stringent. The Clean Air Act Amendments (CAAA) of 1990 defined two sets of standards, Tier-1 and Tier-2, for light-duty vehicles (vehicles under 6,000 pounds such as passenger cars, sport utility vehicles, minivans, and pickup trucks).13 Tier-1 regulations have already been implemented. Tier-2 standards began being phased-in in 2004 and are to be completed by 2007. Because emission standards are expressed in grams of pollutants per mile, light trucks, SUVs, and larger engines will have to utilize more advanced emission control technologies in order to meet the standards. Table 14-6 shows the minimum EPA standards for vehicles in the United States. California has devised its own standards which have always been more stringent than the Federal standards. Europeans and Japanese impose their own standards, which are generally comparable to the US standards. There is currently no particulate emission standard for passenger cars and light trucks. For big diesel trucks and buses, starting with model year 1994, the EPA has required a reduction in particulate emissions by 90%. Combustion Efficiency We learned in Chapter 5 that for better thermal efficiencies, combustion must be carried out at a high temperature. This is accomplished by using better fuels, and by further compressing the air/fuel mixture. Diesel engines can operate at significantly higher compression ratios than gasoline engines. In gasoline engines, however, there is a limit on the amount that a mixture can be compressed before temperature exceeds the ignition point. If this happens, the entire charge is exploded at once in what we commonly call engine knock. In addition to the uncomfortable noise it creates, knocking reduces the life of the engine. Most modern gasoline cars have compression ratios of 8-10 and thermal efficiencies of around 20-25 percent; that means that roughly three quarters of the fuel energy is lost through the exhaust, releasing a substantial amount of air pollutants into the atmosphere. One way to reduce knock in gasoline engine is to use fuels with higher octane numbers. This has been done by switching to alternative fuels (like methanol), or by mixing gasoline with fuels of high octane numbers (such as gasohol). A team of investigators at MIT have recently reported a factor of two improvements of fuel efficiencies by reducing engine size but operating with a turbocharger. Turbochargers are devices used to pre-compress air, allowing more air to be ingested into the cylinders. The fuel burned will be proportionally higher and more power is obtained. The knocking problem was solved by injecting a precisely controlled 13 US Environmental Protection Agency website (http://www.epa.gov). 350 Chapter 14 - Transportation amount of ethanol into the combustion chamber at exact instance when mixture reaches its maximum pressure. The injected methanol cools the mixture, effectively raising the fuel octane number to 130 – comparable to that of natural gas vehicles and high performance racing cars.14 Following the oil shock of 1973, the US government has introduced the Corporate Average Fuel Economy (CAFÉ) standard, which has gradually doubled the 14 miles per gallon (mpg) average of 1975 to 27.5 mpg in 1990. CAFÉ standards have been frozen at this level ever since. Even though cars are becoming consistently more efficient, the savings in fuel has, unfortunately, not translated into reduced consumption. Instead, better fuel efficiency along with the cheap price of petroleum has triggered an upsurge in the use of sport utility vehicles, pickups, and minivans (cumulatively called light trucks). For example, the US Department of Transportation reported that between 1990 and 2000, vehicle miles traveled by passenger cars have increased by around 15%; during the same period, the total vehicle miles traveled by light trucks have increased by 60% (Figure 14-7).15 Even those who own the more compact economy cars have not helped reduce overall energy use, as the continuous drop in gasoline cost has encouraged more and more use of private vehicles.16 It is not clear that further increases in efficiency will ultimately lead to a lower rate of gasoline consumption. Making cars more fuel efficient does not necessarily save us energy. What is needed most are better designed neighborhoods that reduce our need for commuting. Unfortunately, Americans have opted for decentralization - moving away from population centers to suburbs. In the last three decades, 86% of the nation’s growth was suburban. This resulted in heavier reliance on personal transportation and increased the number of miles traveled. European cities, on the other hand, have become more centralized. The larger population density was accommodated by better architectural planning, reducing the need for cars and transit. In Europe, 40-50% of trips are taken walking and biking and about 10% are by transit. In contrast, in the United States, 87% of trips are by private cars and only 3% are by transit.17 Example 14-2: W hich one is a more efficient (economical) mode of transportation, a car or an airplane? Solution: Jet aircrafts use huge amounts of fuel but also carry a lot of passengers at a very fast speed. Cars, on the other hand, are relatively slow and carry only a few passengers. However, they are not big gas-guzzlers like jet aircraft. For example, a Boeing-747 needs 12,000 liters of kerosene per hour to lift its 300-ton body. Assuming Bullis, K., “Better Than Hybrids,” Technology Review, April 2006. Davis, S. C., and Diegel S. W., “Transportation Energy Databook,” Ed., 24, US DoE, ORNL-6983, 2004. Paul McCready points out that in 1986 dollars, fuel cost for driving 25 miles has dropped from $4 in 1929 to $3 in 1949, $2 in 1969, and $1 in 1989. 17 Gibbs, W. W., “Transportation’s Perennial Problems,” Scientific American 277(4):54-57, October 1997. 14 15 16 Figure 14-7 Sport utilities had the highest rate of increased use in the last decade. Source: Transportation Energy Data Book, Ed. 25,2005. 351 a density of 0.8 kilogram per liter of kerosene, the Boeing-747 uses about 3% of its weight in fuel every hour it flies. The aircraft has the capacity to carry 400 passengers at a speed of 560 mph (900 km/hr). This is about 12,000/(400x900) = 0.033 liters per passenger-km. A typical passenger car, on the other hand, gets about 30 mpg (9 km/l). Even if there are four passengers riding together in the car (which is very rare in the United States), we consume 1/(4x9) = 0.03 liters per passenger-km. This is roughly the same as the airplane, though it travels at a much slower speed. Therefore, flying is a more efficient way to travel. Haven’t birds known that for a very long time? Alternative Fueled Vehicles Table 14-7. Number of alternative fuel vehicles in use during 2004 Fuel Type Number Annual % change since 1993 1.3 12 20 -14 66 13 9.3 LPG CNG LNG M85 E85 Electricity Total 194,000 144,000 3,000 4,600 146,000 56,000 548,000 To increase efficiency, improve air quality, and reduce dependence on foreign oil, the US, Japan, and many European governments have passed laws and enacted taxes and other incentives to promote the use of alternative fuel vehicles. Two US laws, the 1990 Clean Air Act Amendments and 1992 Energy Policy Act, require certain fleets (government agencies, buses, taxis, etc.) to acquire (purchase, lease, convert) vehicles that operate on fuels other than petroleum as a portion of their fleet. The US Department of Energy has established programs to work with automotive manufacturers and various state agencies to achieve these objectives. The task of administering these programs is delegated to the EPA. As of 2003, over half a million alternative fuel vehicles were in operation in the United States (Table 14-7). This is in addition to about 20 million vehicles that have been using various blends of biodiesel fuels. The most common fuel by far was propane (LPG), followed by natural gas (CNG), and ethanol (E85). In addition, about 46,000 electric vehicles were on the road, mainly in California and Arizona.18 The number of vehicles that use alternative fuels, as classified by the Department of Energy, is expected to increase and is shown in Figure 14-8. The best possible fuel is, of course, pure hydrogen. Since no carbon is present, with the exception of trace amounts of hydrocarbon and carbon monoxide (the result of lubricating oil being swept into the combustion chamber), no such emissions are produced. The only products are water and a small amount of nitric oxide. Hydrogen, however, costs considerably more than petroleum fuel – about five times more for the same amount of energy. In addition to cost and inherent problems with production and handling, no commercial hydrogen-powered passenger cars are yet available. Daimler-Chrysler has two prototypes; NECAR-4 is running on liquefied hydrogen, and NECAR-4a is using compressed hydrogen. GM has developed the HydroGen, which uses liquefied hydrogen. BMW’s H2R hydrogen-powered concept car uses a modified 6-liter, 12-cylinder internal combustion engine for its propulsion. In addition, BMW has introduced a modified V-12 engine that can be fueled either by gasoline Reference: Davis, S. C., Diegel, S. W., US Oak ridge National Laboratory, “Transportation Energy Data Book”, Ed. 25, Table 6-1, ORNL5198, 2006. 1,000 --- No. of Vehicles 750 500 ----- 250 --- 0 Figure 14-8 Projected sales of advanced technology lightduty vehicles in 2020 by fuel type expressed in thousand vehicles sold. Source: National Renewable Energy Laboratory Website (www.nrel.gov/). 18 A lternatives to Traditional Transportation Fuels, Fuel Data Center, US DoE, Energy Information Administration, in Transportation Energy Data Book, Ed 23-2003. 352 Chapter 14 - Transportation or by liquid hydrogen stored in a pressurized tank placed in the trunk of the automobile (Figure 14-9). The car can deliver 232 horsepower (210 kW) and has a range of 300 kilometers using hydrogen alone. The engine will automatically switch to gasoline to extend the range for an additional 650 km. In addition to the engine, an onboard 5-kW fuel cell is used to operate all the electrical functions as well as auxiliary systems like the air conditioner. In addition to these vehicles, the US government has partnered with the three big automakers (Ford, General Motors, and Daimler-Chrysler) to initiate the Freedom Cooperative Automotive Research (FreedomCAR) program.19 Its aim is the advancement of high-efficiency vehicle technology, focused on fuel cells and hydrogen produced from renewable sources. The long-term goal is to develop a hydrogen-based economy to clean the environment and reduce the US dependence on foreign oil. Figure 14-9 BMW 745hl, The first hydrogen-powered car using liquid hydrogen. Electric Propulsion At the second half of the nineteenth and early twentieth centuries were the advents of innovations, as horse-drawn carriages were replaced with steam locomotives, petrol engines, and electric trolleys. In 1900, of the 4,200 automotives sold in the US, 40% were steam, 38% electric, and 22% were powered by gasoline.20 By 1912, electric cars were dominating the automobile market, surpassing all other vehicles equipped with internal combustion engines. The demise of electric cars came with the invention of the starter motor by Charles F. Kittering in 1911. The Kittering starter was a very small electric motor that produced enough power to crank an engine for a very short time, eliminating the need for much more tedious hand cranks. The popularity of gasoline engines continued to increase as they become more reliable, eventually making the steam engine obsolete and electric vehicles (EV) inferior. Mass production of Ford’s Model-T, with affordable prices and a range twice that of the best electric cars available, helped make internal combustion engines even more popular. As electric vehicles became less and less attractive and the number of internal combustion engines grew, so did the concern for automotive pollution. The sudden increase in the price of gasoline in early 1970, following the Arab oil embargo, revived the interest in electric vehicles. The main problems that have precluded a wide acceptance of electric vehicles are their higher cost, limited range, and the inconvenience these vehicles present in comparison to internal combustion engines. Most electric vehicles use lead-acid batteries, which are heavy and of relatively For additional information about the FreedomCAR project, visit the websites http://www.eere.energy.gov/hydrogenfuel, http://www.eere.energy.gov/hydrogenfuelcells, a nd http://www. eere.energy.gov/vehiclesandfuels. 20 S hacket, S., The Complete Book of Electric Vehicles, Domus Books, 1981. 19 353 low power density. This means that, for the same weight, EVs have a much shorter range than gasoline and diesel engines. Furthermore, charging the batteries takes a long time and, unlike gasoline filling stations that are widely available throughout the world, battery charging stations are few and far between. The obvious challenge to electric vehicles’ popularity is to develop compact batteries with similar energy density to gasoline at a reasonable price. Battery-operated Electric Vehicles (BEV) Lead-acid batteries are the primary power source for many of today’s plug-in electric vehicles. More advanced nickel batteries (such as nickeliron, nickel-cadmium, and nickel-metal hydride) offer better ranges, shorter recharge times, and longer lifetimes, but certain safety concerns and higher costs have precluded their widespread use. Cadmium is very toxic, while nickel-iron batteries tend to produce a buildup of hydrogen during charging. The battery with the greatest potential is the nickelmetal hydride (Ni-MH), which is nontoxic, has about four times the range of lead-acid batteries, and can be recharged in a few minutes. These batteries are currently used in the Honda EV Plus and Toyota RAV4-EV. A close competitor to the Ni-MH battery is the lithium-metal hydrite (LiMH) battery. These batteries have a very high specific energy but must be operated at temperature of about 70o C. Li-MH batteries are currently being tested in the Nissan Altra. Currently, the relatively low price of gasoline probably remains the most important impediment to wide acceptance of electric cars in the United States. As the price of gasoline rises, it is expected that electric cars will claim a larger fraction of the total vehicle market. Out of 230 million cars operating in the United States, about 40% are used as a second car, and 87% of automobile trips are less than 50 kilometers. Electric vehicles can satisfy some of these needs as around-town cars. Furthermore, EVs may have utility as fleet vehicles in inner city short route buses, delivery vehicles, airports, and mail delivery services. Advantages and Disadvantages of Electric Vehicles Torque ICE ICE Horsepower The major advantages of electric over conventional vehicles are: 1. Better torque characteristics. Electric motors are simpler and have torque characteristics that match those demanded by vehicles more closely (high torques at low speeds and acceleration, relatively low torques during cruising). Internal combustion engines, on the other hand, deliver maximum torque at an optimum cruising speed (Figure 14-10). Because little or no torque is delivered during start ups and at lower speeds, internal combustion engines need starter motors. Electric motors produce the highest torque at zero speed when it is most needed. An electric motor can therefore be attached directly to Figure 14-10. Performance characteristics of electric motors versus internal combustion engines 354 Chapter 14 - Transportation the drive wheels and can accelerate the vehicle from rest to the desired speed without the need for a transmission or torque converter. EVs’ power trains are simpler and don’t usually need more than one or two gear ratios. Reverse gears are also absent because their function can be achieved simply by reversing the polarity of the electrical input. 2. Regenerative Braking. Electric motors can run as a generator by running in reverse. Electric vehicles take advantage of this feature by employing regenerative braking, where up to 50% of the kinetic energy of the vehicle can be reclaimed during urban stop-and-go traffic to recharge the battery. The experimental data shows that, depending on design and the driving cycle, regenerative braking extends driving range between 5 to 15 percent. 3. Lower noise and emission. Electric vehicles are much quieter during operation and do not consume any power or produce any emission when stopped. This is not true with gasoline cars, which continue to consume a substantial amount of fuel and produce pollution even when they idle. Some emissions, such as hydrocarbons, are actually higher during idling than when cruising at optimal speeds. 4. Superior efficiencies. Electric vehicles have efficiencies in the range of 40-45% compared to efficiencies of 18-25% common for most conventional vehicles.21 However, when losses associated with the generation of electricity and transport are considered, depending on how the electricity is generated, EV efficiency advantages are only 1030%. The major disadvantages of electric vehicles over conventional vehicles are: 1. Batteries have a relatively small capacity to store energy. Compared to gasoline, which has an energy density of 12,000 watt-hours per kilogram, lead-acid batteries have only 40 watt-hours per kilogram. Lower battery storage capability results in limitations on the distance electric vehicles can travel before they must be recharged. More batteries add to vehicle weight which indirectly limits performance. With current technology, EVs that are equipped with lead-acid batteries offer a range of 60-80 kilometers and take six to eight hours to charge. Ni-MH batteries offer a range of 400 kilometers and can be recharged faster. The EVs available in the market are, on average, 300 to 1000 kilograms heavier than similar conventional vehicles. 2. Infrastructure does not exist. There is no network of recharging stations. Charging must be accomplished in much faster times that are currently possible and in all types of weather conditions. 3. Electric vehicles cost more. Cost of the ownership (initial purchase price plus costs associated with maintenance and repair) are higher for electric vehicles. 21 It should be noted that the efficiencies quoted here are first law efficiencies, the fraction of the input energy (electricity for EV and petrol for ICE) that is used to drive the vehicle. 355 Fuel Cell Vehicles (FCV) Fuel cells produce electricity through a chemical reaction between hydrogen and oxygen. The fuel cell was invented in 1839 by the British physicist Sir William Grove but, because of inherent problems associated with its operation, was soon forgotten. It was only in the 1960s that the first fuel cell was first employed aboard Gemini and Apollo capsules to replace heavy batteries used in previous space missions. Ever since, fuel cells have been used in a number of applications such as in airplanes, stationary power generators, and now automobiles. Figure 14-11 In NECar-5 by Daimler-Benz methanol is reformed to produce hydrogen to run a PEM fuel cell. In 2002, the Honda FCX and Toyota FCHV-4 became the first fuel cell vehicles to enter the market. Daimler-Chrysler has also introduced a limited number of their newest fuel cell model in 2006 (Figure 14-11). Principle of Operation Fuel cell operation can be accurately described as reverse electrolysis. In electrolysis, an electric current is passed across two electrodes immersed in an electrolyte solution, splitting water into its constituents – hydrogen and oxygen. The fuel cell works in the opposite direction; hydrogen and oxygen (usually from air) recombine in an electrochemical reaction to form water and an electric current is produced. The reaction is different from combustion, as it usually occurs at lower temperatures and electricity (instead of heat) is the output. Since fuel cells are not heat engines, their efficiencies can be quite high and are not constrained by Carnot principles. Efficiencies of around 50% are typical. The most common fuel cell for passenger cars and light-duty trucks is Proton Exchange Membrane (PEM). It consists of a positive and a negative electrode separated by a membrane or electrolyte (Figure 14-12). Hydrogen gas is introduced at the negative electrode (anode) where a catalyst strips electrons from the hydrogen atom. Hydrogen ions migrate through the membrane toward the positive polymer electrode (cathode) and react with oxygen to form water. Without the membrane, hydrogen and oxygen mix and react directly in a chemical reaction, generating heat instead of electricity. The membrane works by allowing the hydrogen ions (protons) to pass through while blocking electrons. Electrons are then diverted through an external wire before arriving on the other side where they are recombined with protons and oxygen to complete the reaction. As long as the flow of hydrogen gas is not interrupted, the electrical current is maintained, which can turn a light on or be used to power an electric motor. The PEM operates best at temperatures of around 80o C. To summarize, the reaction at each electrode and the overall reaction can be written as: Figure 14-12 Principle of operation of a PEM Fuel Cell. Source: NASA Website (http://www.nasa. gov/). 356 Chapter 14 - Transportation Anode: Cathode: 2 H2 4 H+ + 4 e2 H2O 2 H2O O2 + 4 e- + 4 H+ (14-2) Overall Reaction: 2 H2 + O2 Besides PEM fuel cells, three other technologies show promise for various transportation applications. The Phosphoric Acid Fuel Cell (PAFC) is most appropriate for locomotives, heavy-duty trucks, and urban transit buses. These fuel cells are already used for stationary power generation. Two other technologies, Solid Oxide and Direct Methanol fuel cells, are mainly of interest for on-board fuel processing and reformer technologies. The major problem with all these fuel cells is that they operate at much higher temperatures and at very high costs. Reformers “Water will one day be employed as fuel, that hydrogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light...” ~ Jules Verne (1828-1905) Fuel cells suitable for a car must be small, have a high power density, refuel in a short time, store sufficient hydrogen for a range of 500 kilometers or better, and be available at a cost comparable to conventional internal combustion engines. When fuel cells use pure hydrogen as fuel, there are no emissions except for the pollutants produced in generating the hydrogen. Producing hydrogen in large quantities and storing it in convenient containers is not always practical, however. Hydrogen has a low volumetric energy density, and storing it requires very high pressures or cryogenic temperatures. Hydrogen can also be stored as metal or chemical hydrides. Compressed gas occupies the largest volume, while hydrides are the heaviest of all options. Storing hydrogen in liquid form is impractical and not an option at this time. To travel 500 kilometers, a fuelcell car requires three kilograms of hydrogen. If stored at 20 MPa (3000 psia) pressure tanks, it occupies a volume of 190 liters. Furthermore, the current lack of convenient refueling stations is a big impediment in marketing fuel cell vehicles using pure hydrogen. R ather than relying on storing hydrogen and using it directly, a number of car companies are concentrating on developing on-board reformers. Reformers use a variety of fuels to produce hydrogen by reformation, where it is subsequently used in the fuel cell. Gasoline, methanol, or any other fuel can be used. Reforming natural gas does not seem to be the best option as natural gas is most suitable to be used in combined cycle turbines to generate electricity relatively cleanly and at a high efficiency. Methanol is considered to be a favorite because it is liquid at room temperature and can be produced from biomass. Although methanol has lower energy content than gasoline, the higher efficiency of methanol reformation can compensate for lower energy density and vehicle range remains relatively the same. 357 Advantages and Disadvantages of Fuel Cell Cars The advantages of fuel cells are many. They are efficient, operate virtually vibration-free and silently, have a high energy density, have no moving parts, and produce little or no emissions. Furthermore, fuel cells have no exhaust and thus have a low thermal signature. They can be customized to any shape and for any power demand, which makes these devices ideal for numerous applications. They will eventually replace a major portion of internal combustion engines and power future cell phones, computers, camcorders, and other cordless devices. The major barriers to commercial development of fuel cell cars are the public’s concern over safety and the cost. If hydrogen is used directly as the fuel instead of being produced by an on-board reformer, then other issues such as hydrogen distribution, delivery, and storage would also become a problem. The issues of hydrogen storage were briefly discussed above, namely, the low density of hydrogen even at very high pressures or at liquid temperatures. At present, hydrogen can be stored at 1-2% by weight, at best. To make fuel cell cars practical, the Department of Energy has set a goal of 62 kg hydrogen per cubic meter for storage space. Metal hydride and carbon nanofibers are two promising technologies. Concern about hydrogen safety is partially justified, but mostly overexaggerated. Hydrogen gas is highly explosive, and its flame is invisible. Images of the catastrophic explosion of the hydrogen-propelled “Hindenburg Zeppelin” in 1937 are still alive in many people’s minds,22 even though some recent studies have shown that the cause of explosion was not the hydrogen, but the electrostatic charge accumulated by an oily substance used to treat the cotton skin of the airship. The lack of flame visibility can be easily resolved with certain additives. New advances in composites allow the design of hydrogen tanks that can withstand major crashes and falls from buildings several stories tall. In addition, hydrogen sensors are available that can detect and shut down hydrogen flow as the smallest leaks are detected. Finally, since hydrogen is the lightest of all gases, hydrogen flames are highly buoyant and spread in the form of jets diffusing upward and away from the body of the car and its occupants, as demonstrated by several crash and fire simulations (See Figure 14-13).23 Lack of a hydrogen distribution and delivery system is another important factor delaying the introduction of fuel cell equipped passenger cars. To make fuel cells viable, a sizable investment in hydrogen generation and distribution infrastructure is required. This is an uphill battle for hydrogen enthusiasts, who have to compete with already established gasoline infrastructure. The United States now has more than a dozen hydrogen stations, mostly in California, used only for demonstration and research Time = 0 min, 0 sec Time = 0 min, 3 sec Time = 1 min, 0 sec Figure 14-13. Fuel leak simulation. Hydrogen-powered vehicle is on the left, and gasoline-powered vehicle is on the right. Notice that after 1 minute, hydrogen flame is subsiding, whereas gasoline flame is engulfing the entire vehicle. (Ref. 22) H indenburg Zeppelin was a giant hydrogen-filled airship (at 245 meters, it was the largest ever to fly) that operated regularly in 1930s taking transatlantic passengers between Germany and both North and South America before it was destroyed by a fire in 1937. 23 Swain, M. R., “Fuel Leak Simulation”, Proceedings of the 2001 DoE Hydrogen Program Review, NREL/CP-570-30535. 22 358 Chapter 14 - Transportation purposes. Fuel cells produce power at considerably higher costs than internal combustion engines-- at around $4 a watt compared to $0.05 a watt. To make fuel cells a viable alternative to batteries and internal combustion engines, their cost must be brought down, they must work virtually maintenance free, and their safety must be improved. This cannot happen unless fuel cell-operated devices can be produced at a fraction of what it costs today. The prices cannot fall unless fuel cells can be massproduced, which happens only when prices are already low and sufficient infrastructure (such as hydrogen filling stations) is in place - a classic chicken and egg problem!24 Emissions from Electric and Fuel Cell Cars Electric and fuel cell vehicles don’t have tailpipe emissions. This does not, however, mean that these cars are truly zero emission vehicles (ZEV); there are still some emissions from the tailpipe to the smoke stacks where electricity or hydrogen is generated. For fuel cell vehicles, if hydrogen is produced by on-line reformers, additional emissions are introduced. Nevertheless, electric and fuel cell vehicles are far cleaner and pose fewer health risks than internal combustion engines. Among the reasons are: 1. There are options other than fossil fuels for generating electricity or hydrogen. In fact, if nuclear energy or renewable sources such as wind, solar, or hydropower are used, no pollution is produced and these vehicles would truly be zero emission vehicles. 2. Most power plants are located in rural areas and areas away from city centers and other population centers. The health impact is expected to be lessened as pollutants are diluted and scattered over larger distances. Furthermore, it is easier to control emissions from relatively few power plants and stationary sources than from millions of automobile sources on the road. 3. The majority of trips are during city driving when vehicles are not fully warmed up and a significant portion of travel is in stop and go traffic. Conventional vehicles produce a large amount of HC and CO during cold starts, acceleration, and braking. All in all, substituting EVs and FCVs for conventional vehicles would significantly reduce urban emissions. According to one study,25 if all conventional vehicles were replaced with electric vehicles, overall hydrocarbon and carbon monoxide emissions would drop by 95-99% and nitric oxides by up to 90%. Ozone and the volatile organic compounds that contribute to urban smog will be eliminated altogether. Sulfur dioxide and particulate emission, however, may drop or rise depending on how much of the electricity comes from coal and oil power plants. 24 25 “ The Ultimate Incinerators,” Scientific American , September 1995, p. 180. E lectric Power Research Institute Website, http://www.epri.com. 359 Hybrid Vehicles The much greater range offered by gasoline engines and the much cleaner operation of battery-electric and fuel cell vehicles are the major impetus for the development of hybrid vehicles. A hybrid vehicle is defined as a vehicle with at least two modes of propulsion. Today, most hybrid vehicles combine an internal combustion engine (either gasoline or diesel) and an electric motor powered by batteries. The choice is not, however, limited to these and other primary sources of power such as gas turbines and fuel cells; other types of storage devices, such as flywheels and ultracapacitors, are occasionally being used. The idea of hybrid vehicles is not new. The French Kriéger Company, originally established to produce electric cars, introduced the first alcoholelectric hybrid car in 1902, a gasoline version in 1904, and a turbine-electric hybrid shortly before it became bankrupt in 1909. With the advent of the starter motor in 1911, much of the lure of electric and hybrid vehicles vanished and, until the oil crisis of 1973, no major research was carried out. To secure a foothold in the future automotive market, Japanese companies initiated a major investment in producing hybrid vehicles; in 1999, Honda released its two-door Insight, the first hybrid car to hit the American markets. The Insight received an EPA mileage rating of 61 mpg city and 70 mpg highway. Until recently, the Honda Insight, Honda Civic, and Toyota Prius were the only hybrid vehicles available to the public in the United States. The performance of the three major Japanese hybrids are compared and given in Table 14-8. As of 2005, almost all major automotive manufacturing companies have been introducing passenger, SUV, and pickup hybrids. Efficiency Hybrid vehicles offer superior efficiencies over both petrol and electric vehicles because they are operated at their most efficient modes and they can recover some energy through regenerative braking. For example, to double the fuel efficiency (2X hybrid-electric), we can use an engine only half the size and supplement the power through recovering much of the losses during braking. Let’s consider two cars, one conventional and another hybrid as depicted in Figure 14-14. The conventional car (1X) consumes 100 units of fuel, losing 73 units through the exhaust, radiator, friction, and other accessories and another 11 units during standby, thereby delivering 16 units of energy to propel the car. The hybrid car (2X or two times efficiency) deploys an internal combustion engine half the size using 50 units of fuel and losing 38 units to the ambient. The standby losses are eliminated because the vehicle operates as an electric vehicle when the car stops and no motor is left running. An additional 4 units of energy can be recovered through regenerative braking as the vehicle decelerates. Like the gas-powered car, the net useful energy is 16 units. 1-X GASOLINE 16 100 11 Available for propulsion 73 Stand by losses Thermal & frictional losses 2-X HYBRID 12 38 16 Available for propulsion 50 Regeneration 4 Thermal & frictional losses Figure 14-14 Comparison between a pure internal combustion engine and a 2x hybrid vehicle. 360 Chapter 14 - Transportation Table 14-8. Specifications of most popular hybrid vehicles available in the US in 2005 Honda Insight Configuration Engine/Electric Motor Power Engine compression ratio Fuel economy (city/highway) Emission rating Battery Passenger capacity Weight Range Series 65 h.p./9.7 kW 10.8:1 57/56 mpg SULEV* NiMH 2 1975 lb 528 miles Honda Civic Series 85 h.p./9.7 kW 10.8:1 48/47 mpg ULEV NiMH 5 2736 lb 554 miles Toyota Prius Parallel 76 h.p./50 kW 13:1 60/51 mpg SULEV* NiMH 5 2890 lb 589 miles * Super Ultra Low Emission Vehicle Source: Manufacturers published data. In 1993, the US government, partnered with national laboratories and three major US automakers, Daimler-Chrysler, Ford, and General Motors, ventured the Partnership for New Generation of Vehicles (PNGV), whose long term goal is to come up with a 3X car that can triple fuel economy to 80 mpg. A similar program called FreedomCAR is also being undertaken to develop new and alternative technologies that improve fuel efficiency and reduce the US dependence on imported foreign oil. Configurations Almost all hybrid vehicles operating today utilize an auxiliary power unit (APU), such as a small spark ignition engine, a diesel, or a gas turbine to complement the batteries in providing power directly, or by acting as a generator to charge the batteries. The APU can be fueled by gasoline, diesel fuel, or any number of alternative fuels and is operated at relatively steady-state optimum condition to produce very low emissions and high efficiency (Figure 14-15). Hybrid vehicles are commonly classified as either series or parallel. The main distinctions between the two configurations are in the way the APU transfers power to the wheel and whether batteries become fully discharged (charge-depleting) or retain the charge by continuous charging (charge-sustaining). In a series configuration, all motive power comes from the electric motor powered by batteries. The gasoline engine drives a generator to produce electricity, which either supplies power to an electric motor or charges the battery. With series hybrids, there is no mechanical connection between the engine and the wheels; power is transferred electrically to an electric motor that drives the wheels. Because electric motors generate torque that matches requirements at different speeds, series hybrids have a simpler transmissions or no transmission at all (Figure 14-16a). As a result, modern hybrid-electric drive systems are much lighter, weighing about one third to one half the weight of batteries required for a purely battery-operated electric car. 361 Figure 14-15 Plug-in hybrid vehicle and its components. Image Courtesy of Argonne National Renewable Energy. Engine Generator Battery Pack Motor Wheels a. Series Hybrid Generator Clutch Transmission Wheels Battery Pack Motor/ Generator Clutch Figure 14-16 Series and parallel hybrids. b. Parallel Hybrid In the simplest configuration, the vehicle operates as a pure electric vehicle until batteries are depleted to their preset thresholds, at which time the internal combustion engine begins to recharge them. Since the engine does not have to meet changing power demands directly, it can be set to operate in a narrow speed range where it has the highest efficiency, the lowest emissions, or a combination of both. Alternatively, control strategies can be devised where a small IC engine operates continuously at its optimal point so as to keep the battery at or near its full charge. One major advantage of a series hybrid is that it can attain a long range with an engine only a fraction of the size of the conventional engine and with a battery pack weighing far less than those of pure electric vehicles. In a parallel configuration, the engine, the electric motor, or both supply power to the wheels (Figure 14-16b). Parallel hybrids are primarily used in electric-only mode for short trips and city driving, whereas long trips and highway cruising are carried out by engine-only operation. The electric motor can be used to help overcome hill climbs, to accelerate quickly, and in instances when the engine cannot single handedly meet the power demand. Because the engine, electric motor, or both must deliver the power, a clutch/transmission assembly is necessary in parallel hybrids. Air Hybrid Vehicles Instead of storing the energy as electric energy in a battery in an electric car, air hybrid vehicles work by storing the energy needed to propel the engine in an onboard tank of compressed air. Like a conventional engine, a piston compresses air in a cylinder. As the piston reaches the top, a small amount of compressed air is released into the expansion chamber to create a low pressure, low temperature pressure wave that drives the piston to power the engine. When vehicle brakes, the engine is used as an air compressor to absorb the braking energy and store it into the air tank. The engine is essentially shut off when it stops behind a traffic light. As the car accelerates, more and more air is allowed into the cylinder until the compressed air is depleted. The tank can be refilled in a refill station or by 362 Chapter 14 - Transportation a compressor operated by a conventional internal combustion engine. Advantages and Disadvantages of Hybrid Vehicles Hybrid vehicles have advantages over both electric and conventional vehicles by combining features and enhancing performance to a degree not possible using either propulsive system, and without their limitations. For example, ICE performance is a strong function of engine speed, designed to operate optimally at cruising speeds around 90-100 km/hr (roughly 50-60 miles per hour); efficiency drops rapidly at both very high and low speeds. On the other hand, electric motors have their highest efficiency at low speeds, generate no on-board emissions, have favorable torque characteristics, and can utilize regenerative braking. These characteristics make conventional engines ideal for freeway driving and constant speed operation and electric vehicles best suited for city driving and transient conditions. Another advantage of the hybrid system is that individual components can be sized to fit different driving conditions. Since the engine is not directly coupled to the wheels, engine size can be selected to run near its optimal conditions at all times. In most designs, the engine is sized to meet only the cruising demands, whereas power for acceleration and hill climbing is offered by batteries and other storage devices. The major disadvantages of hybrid vehicles are additional complexity of dealing with two power systems and potentially higher capital cost. Social Costs of Transportation Many of the costs associated with driving have little to do with fuel use. Traffic congestion, lost productivity, air pollution, stress, damage to roads and other properties, accidents, and health costs are among many indirect costs attributed to driving.26 In 2002 alone, Americans lost over 3.5 billion hours and wasted 5.7 billion gallons of gasoline , because of traffic delays. Furthermore, during the same year there were over six million accidents, resulting in about 43,000 deaths. The loss of productivity due to traffic delay alone is estimated at around 62 billion dollars. If costs associated with stress and health effects were included, the cost of owning a car would be much higher.27 These costs are borne by everyone. The solution suggested by car manufacturers, politicians, and policy makers has been to promote heavier and bigger cars, pack cars with more air pollution control devices, add lanes, build more roads and parking structures, and to move many industries and large corporations away from metropolitan areas and large population centers. The solution may actually be the exact opposite. For example, it may be wiser to discourage car ownership by providing a more 26 27 Hawkens, Paul, Lovins, A, and Lovins, L. H., “Natural Capitalism: Creating the Next Industrial Revolution,” Rocky Mountain Institute, p. 41. 1999. Schrank, D., and Lomax, T., “The 2004 Urban Mobility Report,” Texas Transportation Institute, Texas A&M University, September 2004. 363 efficient and cheaper public transportation system and, at the same time, making personal car ownership costlier and less convenient. This may require raising automobile sales taxes, insurance fees, and maintenance costs. The revenues could then fund development of comprehensive public transportation structures. Restricting driving is also an effective way to reduce congestion and improve air quality. Some countries have enacted laws to make certain parts of large metropolitan areas off limits to passenger cars. People are to leave their cars outside the restricted zones and take public transportation to downtowns and populated city centers. Police cars, ambulances, and other emergency vehicles are exempt. In another scenario, drivers have access to certain regions during certain hours or days, and only on an as-needed basis. For example, cars with license plates that end with an odd number can travel in a city only on Sundays, Tuesdays, and Thursdays, or they have to pay an extra toll charge during rush-hours. Office managers and business owners can encourage their employees to carpool or to use public transportation by providing free parking spaces to carpools or by paying for their bus or train fares. They may also help in paying part of the home mortgages of employees who live within a short distance of the place of employment. Some companies and organizations have successfully implemented flex-hours, allowing some employees to shift their work hours or even work part of their time at home. The local city government can participate in the effort by providing bikes to the general public that can be used free of charge in central districts where most government offices are. Architects and city planners can be especially effective in this effort by designing convenient shopping centers and large office buildings to satisfy multiple needs, thus reducing the need for frequent visits. Summary To cope with ever-increasing problems associated with transportation, various approaches have been proposed. Among options considered are: 1) reduce vehicle use, 2) increase the efficiency and reduce the emissions of conventional gasoline-powered vehicles, 3) switch to less noxious fuels, and 4) use cleaner alternative systems. The first choice calls for developing a large-scale mass transit system, carpooling, reducing travel by working from home, and removing the need for excessive driving by implementing more effective community designs. This will also reduce much of the social costs such as traffic congestion and delays, pollution, accidents, repair costs, and other social ills such as fatigue and stress. Although this option helps with many social and environmental problems, by itself it seems to have limited success. The second and third approaches are to make existing transportation systems better by designing more efficient cars or vehicles that use 364 Chapter 14 - Transportation better and less-polluting fuels. Internal combustion engines have been around for a long time and their efficiencies have been greatly improved. It seems that improving efficiencies beyond current levels only comes in small steps and no major breakthrough is eminent in the near future. Alternative fuels like methanol and natural gases have been tried by a number of investigators with marginal improvements in emissions and fuel efficiencies. Hydrogen seems to be the obvious alternative fuel. Problems associated with hydrogen storage, safety concerns, and the lack of a hydrogen distribution infrastructure have been major obstacles to commercial development of hydrogen-propelled internal combustion engines, although a number of automotive manufacturers have developed prototype units. The fourth option is to do away with current technologies in favor of newer and cleaner alternatives. In order for electric, hybrid, or any other types of cars to find mass market acceptability, they must meet certain requirements. They must have a full-size trunk, be able to travel large distances, recharge in a short time, have all the comfort and luxury of the conventional cars, and be offered at costs comparable to conventional internal combustion engines. Unless there are major breakthroughs in advanced battery technologies, it does not appear that battery-powered electric vehicles, in their current form, can make headway in achieving a major market share.28 Their uses would probably be limited to small service vehicles, fleet operation, and in areas where pollution, not driving range, is of primary concern. W hen and if fuel cell cars become popular and infrastructure is in place, fuel cells will not only be cost effective, but may also be a source of income for their owners. As US data shows, Americans drive their cars only 4% of the time. The remaining 96% of the time cars sit idly in parking garages either at work or at home. During these times, cars can be used as tiny power plants, producing electricity which can be put on grids and sold to utility companies. In this way, a large fraction of the cost of owning and maintaining the cars may be recovered, and our reliance on foreign oil and other nonrenewable technologies reduced. For now, hybrid vehicles consisting of a petrol engine and a batterypowered electric motor seem to be the preferred option. In the long term, fuel cells using pure hydrogen offer the best hope. In the meantime, petroleum-based reformers may be the solution as we transition to onboard hydrogen storage in the future. A technological revolution is underway which makes future transportation systems even more efficient. New advanced polymer and carbon-fiber I n hope of acquiring a great market share and to meet the California mandate of replacing 2% of the automobile sales by electric vehicles, GM introduced Impact (to be later called E V-1) to the market. The car, although well-designed technically, did not meet public acceptance, and GM had to stop its production. California also had to back down from its stated goals. Ford’s Think! was somewhat more widely accepted, as it was marketed as a second (neighborhood) car to be used for city driving and only for short trips with a range of about 50 m iles. Ford also had to close down its plants eventually. 28 365 composites are developed that are lighter and safer to use. Composites can be made that design frame-less “monocoques” that cost less, are many times stronger than conventional frames, are lighter than steel by two to three times, and absorb five times more energy. This helps not only in designing safer cars, but also in reducing rolling resistance and power requirements for climbing and accelerating. The reduction in required power results in smaller engines, transmissions, and other components, which in turn makes it possible to make the vehicle even lighter. Additional Information Books 1. Tillman, D., Fuels of Opportunity: Characteristics and Uses In Combustion Systems, Academic Press, 2004. 2. Sorensen, K., Hydrogen and Fuel Cells: Emerging Technologies and Applications, Academic Press, 2005. 3. Dhameia, S., Electric Vehicle Battery Systems, Academic Press, 2001. 4. Hussain, I., Electric and Hybrid Vehicles: Design Fundamentals, CRC Press, LLC. 2003. 5. Jefferson, C.M., and Barnard, R. H., Hybrid Vehicle Propulsion, WIT Press, 2002. 6. Spelberg, D. The Hydrogen Energy Transition: Moving Toward the Post Petroleum Age in Transportation, Academic Press, 2004. Periodicals 1. Fuel, Direct Science Elsevier Publishing Company, Fuel focuses on primary research work in the science and technology of fuel and energy fuel science. 2. Transportation Research Part C: Emerging Technologies, Direct Science Elsevier Publishing Company; this journal focuses on scholarly research on development, application, and implications in the fields of transportation, control systems, and telecommunications, among others. 3. Fuel Cells Bulletin, Direct Science Elsevier Publishing Company, Fuel Cells Bulletin is the leading source of technical and business news for the fuel cells sector. 4. International Journal of Hydrogen Energy, Direct Science Elsevier Publishing Company, Quarterly journal covering various aspects of hydrogen energy, including production, storage, transmission, and utilization, as well as economical and environmental aspects. Government Agencies and Websites 1. US Department of Transportation (http://www.dot.gov). 2. US Department of Energy (http://www.doe.gov). 366 Chapter 14 - Transportation 3. US Environmental Protection Agency (http://www.epa.gov). 4. National Energy Renewable Laboratory Hybrid Electric &Fuel Cell Vehicles (http://www.nrel.gov/vehiclesandfuels/hev). 5. FreedomCar (http://www.eere.energy.gov/vehiclesandfuels). 367 Exercises I. Essay Questions 1. What are the major sources of resistance in a car? When are they dominant? How can they be reduced? 2. What are the major sources of resistance in a ship? When are they dominant? How can they be reduced? 3. What are the major sources of resistance in an airplane? When are they dominant? How can they be reduced? 4. How do spark ignition internal combustion engines work? What are their advantages and disadvantages over other types of engines? 5. How do diesel engines work? What are their advantages and disadvantages over spark ignition gasoline engines? 6. How do gas turbines work? What are their advantages and disadvantages over gasoline and diesel engines? 7. What are the major emissions from an internal engine? What are the conditions that these pollutants produce? What can we do to reduce these emissions? 8. Describe the operation of a catalytic reactor. How does it simultaneously reduce NOx, CO, and HC emissions? 9. Why are diesels more popular in Europe? What needs to be done to make diesels appropriate for the US market? 10. Name five alternatives to gasoline and diesel fuels. W hat are the advantages and disadvantages of each? 11. Which of the transportation modes (automobiles, trains, barges, and ships) is more energy efficient for carrying passengers? How does it differ in 368 Europe and the United States? 12. What are problems associated with hydrogen storage, delivery, and safety? What are the sources of emissions from hydrogen combustion? 13. What are the advantages of hydrogen as a transportation fuel? 14. How is hydrogen stored in a vehicle and in bulk quantities? 15. What are the current problems with hydrogen as a transportation fuel? 16. How did hydrogen contribute to the Hindenburg explosion? 17. How does a fuel cell work? Why is it advantageous compared to batteries and other modes of propulsions? 18. What are the impediments to large-scale production of fuel-cell vehicles? 19. What is a hybrid vehicle? When are hybrid vehicles advantageous over conventional engines? 20. What is the difference between a series and a parallel hybrid? 21. What are the major manufacturers of hybrid vehicles, and what fraction of the market do they own? II. Multiple Choice Questions 1. The main strokes of a four-stroke internal combustion engine are a. Intake, compression, ignition, and exhaust b. Compression, ignition, expansion, and exhaust c. Intake, compression, power, and exhaust d. Intake, compression, ignition, and expansion e. Intake, ignition, expansion, and exhaust 2. To increase the efficiency of a spark ignition engine we must Chapter 14 - Transportation a. b. c. d. e. Increase the compression ratio Use fuels with higher flame temperatures Reduce exhaust temperature Prevent heat losses All of the above a. Are technologically fully developed with the possibility of marginal improvements in their qualities b. Are able to produce maximum torque only over a small speed range c. Do not produce any torque at start d. Are the biggest source of pollutant emission e. All of the above 9. The main advantage of diesels over spark-ignition engines is that a. They are heavier and therefore more durable b. Diesel fuel is cheaper than gasoline c. They can be constructed at higher compression ratios and therefore their efficiency is higher d. They do not require any spark for ignition e. All of the above 10. The main advantage of gas turbines over gasoline and diesel engines is a. Their relatively low emission b. Their high power density c. Their multi-fuel capability d. Their smooth, vibration-free power delivery e. All of the above 11. Nitric oxides are produced mainly as a result of a. Incomplete combustion b. Quenching by the cold surfaces c. The reaction of nitrogen and oxygen in the air at high temperatures d. Burning a very rich mixture e. All of the above 12. Carbon monoxides are produced mainly as a result of a. Incomplete combustion b. Quenching by the cold surfaces c. Poor mixing of fuel and air d. Burning a very rich mixture e. All of the above 13. The main function of catalytic converter in an automobile is a. To increase efficiency b. To increase power c. To reduce noise 369 3. In the past ten years, the average miles per gallon of gasoline used by vehicles in the US has a. Increased b. Decreased c. Remained about the same 4. The power required to propel a vehicle over a given distance is proportional to a. Speed divided by distance b. Power times distance c. Speed times distance d. Force times distance e. Force times speed 5. The main barrier to wide-scale use of electric vehicles is a. Finding or developing light electric motors and generators b. Developing faster charging systems c. Developing lighter and stronger chassis d. Reducing cost e. Developing batteries with higher energy and power densities 6. Most power required to propel a passenger car is for a. Acceleration and climbing b. Cruising c. Braking d. Idling e. Accessories 7. Aerodynamic resistance dominates all other resistance in vehicles a. Cruising at speeds below 40 km/hr (25 mph) b. Only during stop-and-go traffic c. During acceleration d. At speeds exceeding 40 km/hr (25 mph) e. Never; it is always less than rolling resistance 8. Internal combustion engines d. To reduce pollution e. To reduce vibration 14. Which of the following statements are not correct? a. The major ingredients contributing to both gasoline and diesel emissions are carbon monoxide, nitric oxides, and unburned hydrocarbon. b. Sulfur dioxide emissions are not generally high, because sulfur is removed from the gasoline and diesel fuel before they are sold. c. Catalytic converters are effective for removal of carbon monoxide, nitric oxides, and hydrocarbons. d. Catalytic filters can be used to remove much of the fine particulates from diesel exhausts. e. The most efficient method to reduce nitric oxide emission is by operating the engine near its stoichiometric condition. 15. Which of the following emissions cannot be controlled by catalytic converters? a. Carbon monoxide b. Carbon dioxide c. Nitric oxide d. Hydrocarbons e. None of the above 16. LPG is mainly a. Natural gas with some propane and butane b. A blend of methanol and ethanol c. Distilled gasoline d. Leftover natural gas after methane is removed e. Used as a jet fuel 17. Which of the following cannot be used to power an alternate-fuel vehicle? a. Methanol b. Ethanol c. LPG d. Coal e. Electricity 18. The major disadvantage(s) of methanol as an alternative fuel is/are that a. It has an invisible flame 370 b. c. d. e. It is toxic It is corrosive It has low energy density All of the above 19. What is the main advantage of hydrogen over other fuels? a. Hydrogen can be stored at very high pressures so we can store a lot in a small volume. b. Hydrogen is easily produced from a variety of sources. c. Hydrogen flame is invisible so it is relatively safe. d. When produced from renewable sources, hydrogen is the cleanest of all fuels. e. All of the above. 20. Which of the following statements is not correct? a. Hydrogen can be substituted for domestic energy sources. b. Hydrogen offers better utilization of resources. c. Hydrogen can be used as a convenient medium for storing solar, wind, and nuclear energies. d. Hydrogen can be substituted for most fuels now in use. e. None of the statements are incorrect. 21. Hydrogen is a clean source of energy. When burned in air the only product(s) it produces is (are) a. Water vapor b. Water vapor and carbon dioxide c. Water vapor and carbon monoxide d. Helium e. Deuterium and tritium 22. Hydrogen is often cited as the fuel of the future. This honor is granted mainly because a. Hydrogen is clean and available in an unlimited supply b. Hydrogen is the main source of nuclear energy c. Hydrogen is light and easily combustible d. Hydrogen is the largest component of the earth’s atmosphere e. All of the above 23. The main disadvantage of battery-operated electric Chapter 14 - Transportation vehicles is a. That they produce the highest torque during start ups and climbing where speed is low b. That their overall efficiency is less than that of internal combustion engines c. That we still need fossil fuel to produce electricity and charge the battery d. That they can travel considerably shorter distances e. All of the above 24. Most electric vehicles operating today use a. Lead-acid batteries b. Iron-nickel batteries c. Nickel-cadmium batteries d. Nickel-metal hydride batteries e. Zinc-air batteries 25. Which of the following can be said about fuel cells? a. Fuel cell is a well-developed technology with most infrastructure already in place. b. Fuel cell vehicles can go thousands of miles between each charging. c. Fuel cells are relatively cheap to manufacture. d. Their efficiency can be higher than those dictated by the Carnot efficiency. e. All of the above. 26. Natural gas vehicles (NGV) a. Are considered truly zero emission vehicles b. Are considered highly unstable and dangerous c. Produce no nitric oxide because there is no nitrogen in the fuel d. Are considerably cleaner than gasoline-fueled vehicles e. All of the above 27. Which of the following is true about hybrid vehicles? a. They are available for purchase today. b. They give better mileage than regular vehicles. c. They use regular gasoline. d. They use two modes of propulsion. e. All of the above. 28. Fuel cells are regarded as promising technology because a. They were used in the space program b. They use fossil fuel c. Their only by-product is water d. They are cheap to produce e. We have huge reserves of hydrogen 29. Which of the following is true about electric vehicles? a. The main disadvantage of electric vehicles is the cost of batteries. b. The number of battery-powered electric vehicles in California will increase to 20% by 2010. c. Electric vehicles have limited range. d. Most electric vehicles are powered by fuel cells. e. Electric vehicles are superior during highway driving and at cruising speeds. 30. Which vehicle whatsoever? a. LEV b. ZEV c. ULEV d. SULEV e. ICE type emits no pollution 31. Reformulated gasoline a. Is lower in carbon monoxide emission because of additional oxygen dissolved into the gasoline b. Is lower in particulate emission because of additional hydrogen dissolved into the gasoline c. Has a higher fuel efficiency d. Is the same as regular gasoline e. All of the above 32. Which of the following is true of methanol as a fuel? a. It is not corrosive, so it can be transported through existing pipeline. b. It has a higher energy density than gasoline. c. Because it has a higher H/C ratio than gasoline, it is relatively clean. d. Unlike gasoline it does not produce any 371 formaldehyde. e. It is relatively nontoxic. 33. You are trading your car for a hybrid car that gives you 20 more miles per gallon. If the distance you travel each year is 20,000 miles and the average price of gasoline is $2.40 a gallon, how much do you save annually in fuel cost? a. $ 416.67 b. $ 1,000 c. $ 2,400 d. It depends on how much you paid or received in the trade. e. Cannot tell from the data given. 34. You have a sport utility car which gives you 20 miles per gallon. You are trading your car for a hybrid car that gives you 50 miles per gallon. If the distance you travel each year is 20,000 miles and the average price of gasoline is $2.40 a gallon, how much do you save annually in fuel cost? a. $ 960 b. $ 1,440 c. $ 2,400 d. $ 2,500 e. Cannot tell from the data given. 35. An air hybrid vehicles a. Is another name for existing hybrid cars b. Is a type of internal combustion engine that operate without any fuel c. Is similar to a conventional engine except that compressed air from a tank provides the propulsive power d. Violates the first law of thermodynamics because gets energy without burning any fuels e. Violates the second law of thermodynamics because it compresses air without doing any work on the pistons III. True or False? 1. Cylinders offer considerably less drag compared to spheres of the same diameter. 2. Rolling up the windows and replacing outside mirrors with inside mirrors can reduce aerodynamic drag and improve fuel efficiency. 372 3. To minimize aerodynamic drag, airfoils should be shaped as close as possible to tear drops. 4. To reduce rolling drag, it is best to keep tires underinflated. 5. During cruising operation, the engine produces enough power to overcome aerodynamic resistance. 6. To reduce drag in trucks, it is best to make trucks boxier in shape. 7. For ships and boats moving at low speeds, viscous drag accounts for most of the resistance. 8. A rotary engine works similarly to a reciprocating engine, except the piston is substituted with a triangular rotor. 9. A four-stroke cycle engine goes through one revolution for each power stroke. 10. Rotary engines employ rotors instead of pistons to achieve compression. 11. Two out of every three gallons of petroleum consumed in the United States are for transportation. 12. Gas turbines have relatively lower emissions than gasoline and diesel engines. 13. The higher the C/H ratio, the cleaner the fuel. 14. Electric vehicles were more popular than petrol engines during late nineteenth and early twentieth centuries. 15. Electric motors produce maximum torque at cruising speeds. 16. Gasoline engines are most efficient at cruising speeds. 17. Electric vehicles are much quieter during operation and do not consume any power or produce any Chapter 14 - Transportation emission when stopped. 18. Conventional vehicles produce a large amount of HC and CO during cold starts, acceleration, and braking. 19. In a series hybrid configuration, all motive power comes from the electric motor powered by batteries. 20. EV performance is a strong function of engine speed, designed to operate optimally at cruising speeds but drop rapidly at both very high and low speeds. IV. Fill-in the Blanks 1. ____________ is the dominant drag force for automobiles at slow speeds. 2. ____________ is the result of energy lost in creating and maintaining the ship’s characteristic bow and stern waves. 3. Instead of a piston, rotary engines use triangularly shaped ___________ to achieve compression. 4. Electric motors produce the highest torque at _________ speed, when it is most needed. 5. In a _________ hybrid configuration, the engine, the electric motor, or both supply power to the wheels. V. PROJECT I - Energy Use and Pollution from Passenger Cars In this assignment you are asked to estimate the total petroleum consumption by passenger vehicles in the United States and their contribution to the air pollution problem. a. What kind of car do you drive? What year and model? b. What is the power rating for your car? c. How many miles per gallon do you get for your car in city driving? In highway driving? d. What is the average price of gasoline in your area? e. Besides yourself, how many other people usually ride in your car? Now answer the following questions: 1. How many gallons of fuel do you use annually? W hat is the mass of fuel? 2. What is the annual cost of petroleum? Compare this with the cost of bottled water or soda you consume every year. 3. How many kilograms of air do you pollute for every mile traveled? Gasoline is a mixture of many hydrocarbons, but the chemical formula can be adequately represented as C8H16. Assume complete combustion. 4. How much carbon dioxide does your car produce? Give your results in grams/mile. What is the yearly emission of carbon dioxide in kilograms? 5. What is the total annual emission of carbon monoxide and nitric oxides for your car? The CO, NOx, and particulate emissions Federal Standards set by the EPA are 3.4, 1.0 and 0.2 grams per mile respectively, for cars less than 5 years old, and 4.2, 0.6, and 0.10 for cars older than 5 years (See Table 14-5). 6. What are the emission rates per person per year? 7. Extend your calculations to estimate the total amount of gasoline consumed by Americans every year. How does your estimates compare to published data by the Department of Transportation? How much pollution is from cars? Assuming that there are 120 million vehicles in the United States, what is the average amount of petroleum used per vehicle? Is it higher or lower than your rate of consumption? 8. Assume that you are converting your car to run on methanol (CH3OH), a synthetic liquid produced from wood. How are the emissions affected? 9. Repeat the calculation, assuming a switch from gasoline to natural gas. Assume natural gas is primarily methane gas with chemical formula CH4. 10. What is the impact of natural gas vehicles on global warming if there is a leakage of 1% of methane gas into the atmosphere? As a greenhouse gas, methane is about 40 times more potent than carbon dioxide. 373 Data and Conversion factors: - The LHV for gasoline is 44 MJ/kg - 1 US gallon of fuel has a mass of 2.85 kg. Hint: Assume stoichiometric reaction: C8H16+12 (O2 + 3.76 N2) 1 kg HC+14.7 kg air 8 CO2 + 8H2O + 45.1 N2 3.1 kg CO2+1.3 kg H2O +11.3 N2 Project II - Cars versus Bikes In this project you will compare the amount of energy used to travel from your home to school and back by either driving your car or by riding your bike. 1. What is the round-trip travel distance between your house and the school or work? 2. How long does it take you to get to work or school if you ride a bike? Assume average speed of 20 km/h. 3. How much muscle energy do you use to make the round trip travel by bike? 4. How much food energy do you need for taking this trip? 5. How long would it take you to travel the same distance by a car? 6. What is the fuel efficiency (mpg) of your car? How many liters of gasoline do you consume? W hat is the total energy content of gasoline required for the trip? 7. How much money do you save annually in gasoline and other car expenses, if you bike to work or school? 8. What are the advantages and disadvantages of driving over bicycling? Discuss in terms of convenience, energy efficiency, health benefits, etc. 374