Land Vehicles
From Thermal-FluidsPedia
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 (a).
Table 1. Drag Coefficients Car CD Shape CD Trucks
Sedans
Porsche 924
GM Sunraycer0.70
0.55
0.34
0.13Flat Plate
Cylinder
Sphere
Teardrop1.17
1.10
0.41
0.04
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 ([[#References|1])). Newer trains using magnetic levitation have no contact with tracks and rolling friction is virtually eliminated (b).
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 1. Streamlining reduces the coefficient of drag and is particularly effective at high speeds where wind resistance 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,” (c) 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 (2).
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.
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References
(1) Wong, J. Y., Theory of Ground Vehicles, Second Edition, John Wiley and Sons, Inc., 1993.
(2) Katz, J. Race Car Aerodynamics, Robert Bentley, Inc., 1995.
(3) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Additional Comments
(a) 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.
(b) With 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.
(c) A type of vehicle construction in which the body is integral with the chassis.
Further Reading
Tillman, D., Fuels of Opportunity: Characteristics and Uses In Combustion Systems, Academic Press, 2004.
Sorensen, K., Hydrogen and Fuel Cells: Emerging Technologies and Applications, Academic Press, 2005.
Dhameia, S., Electric Vehicle Battery Systems, Academic Press, 2001.
Hussain, I., Electric and Hybrid Vehicles: Design Fundamentals, CRC Press, LLC. 2003.
Jefferson, C.M., and Barnard, R. H., Hybrid Vehicle Propulsion, WIT Press, 2002.
Spelberg, D. The Hydrogen Energy Transition: Moving Toward the Post Petroleum Age in Transportation, Academic Press, 2004.
Fuel, Direct Science Elsevier Publishing Company, Fuel focuses on primary research work in the science and technology of fuel and energy fuel science.
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.
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.
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.
External Links
US Department of Transportation (http://www.dot.gov).
US Department of Energy (http://www.doe.gov).
US Environmental Protection Agency (http://www.epa.gov).
National Energy Renewable Laboratory Hybrid Electric &Fuel Cell Vehicles (http://www.nrel.gov/vehiclesandfuels/hev).
FreedomCar (http://www.eere.energy.gov/vehiclesandfuels).