Combustion Efficiency

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We learned in Thermal Energy 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 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 (1).

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.

Figure 1 Sport utilities had the highest rate of increased use in the last decade.
Figure 1 Sport utilities had the highest rate of increased use in the last decade.

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 1) (2). 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 (3). 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 (4).

Example: Which 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 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 1. Number of alternative fuel vehicles in use during 2004
Fuel Type Number Annual %
change since 1993
Reference: Davis, S. C., Diegel, S. W., US Oak ridge
National Laboratory, “Transportation Energy Data Book”,
Ed. 25, Table 6-1, ORNL-5198, 2006.

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.

Figure 2 Projected sales of advanced technology light-duty vehicles in 2020 by fuel type expressed in thousand vehicles sold.
Figure 2 Projected sales of advanced technology light-duty vehicles in 2020 by fuel type expressed in thousand vehicles sold.

As of 2003, over half a million alternative fuel vehicles were in operation in the United States (Table 1). 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 (5). The number of vehicles that use alternative fuels, as classified by the Department of Energy, is expected to increase and is shown in Figure 2.

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 or by liquid hydrogen stored in a pressurized tank placed in the trunk of the automobile (Figure 3). 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.

Figure 3 BMW 745hl, The first hydrogen-powered car using liquid hydrogen.
Figure 3 BMW 745hl, The first hydrogen-powered car using liquid hydrogen.

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 (6). 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.


(1) Bullis, K., “Better Than Hybrids,” Technology Review, April 2006.

(2) Davis, S. C., and Diegel S. W., “Transportation Energy Databook,” Ed., 24, US DoE, ORNL-6983, 2004.

(3) 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.

(4) Gibbs, W. W., “Transportation’s Perennial Problems,” Scientific American 277(4):54-57, October 1997.

(5) Alternatives to Traditional Transportation Fuels, Fuel Data Center, US DoE, Energy Information Administration, in Transportation Energy Data Book, Ed 23-2003.

(6) For additional information about the FreedomCAR project, visit the websites,, and

(7) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

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 (

US Department of Energy (

US Environmental Protection Agency (

National Energy Renewable Laboratory Hybrid Electric &Fuel Cell Vehicles (

FreedomCar (