Environmental Engineering Reference
In-Depth Information
vehicle, is less than 50% for the average duty cycle. Nevertheless, for satisfactory performance
reserve power is required over the normal speed range of the vehicle.
Superimposed in Figure 8.6 are the relative efficiency curves of Figure 8.4 (dashed lines), for
fourth gear. The 100% peak is marked by
. It is readily apparent that, for a steady vehicle speed,
the engine relative efficiency is at best 50% for speeds less than half the cruise speed, rising to
about 80% at cruise speed. The normal practice of downshifting at low speeds leads to even lower
relative efficiencies. Taking into consideration the short periods of acceleration within a typical
driving cycle, the time-averaged relative engine efficiency is certain to be less than 80%, perhaps
closer to 60%. This performance could be improved a little by adding a fifth gear level, but that
would leave less power reserve for acceleration, necessitating downshifting before accelerating
at the higher speeds. An alternative is to employ a continuously variable transmission that can
reach higher relative engine efficiencies over a range of steady speeds, but will downshift when
acceleration is needed.
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8.5
VEHICLE FUEL EFFICIENCY
Traditionally, the efficiency of use of fuel by a vehicle is measured by the distance it moves in
a trip divided by the fuel consumed. 11 For road vehicles, this ratio is customarily reported in the
units of kilometers per liter (or, in the United States, miles per gallon). The vehicle's fuel efficiency
determines both the fuel cost of the trip and the accompanying carbon emissions to the atmosphere,
which are almost entirely in the form of carbon dioxide. The vehicle operator is concerned with
the fuel cost while national authorities are concerned with the effects of aggregate vehicular fuel
consumption on the problems of maintaining a reliable fuel supply and, most recently, on the
contribution of vehicles to the national budget of greenhouse gas emissions. In the United States,
where fuel retail prices are low compared with most other developed nations, fuel cost is a small
fraction of total operating costs of a passenger car, yet it is still a factor in consumer choice of vehicle.
Compared with European and Japanese owners, Americans on average drive larger, heavier, less
efficient, and more expensive automobiles, but pay less for fuel. National tax policies that greatly
affect the price of fuel play a large role in this difference.
The development of vehicle and engine technology that is more fuel efficient and less emitting
of pollutants is primarily a response by manufacturers to national policies of regulation and eco-
nomic tax disincentives that narrow the window of vehicle designs that appeal most to consumers.
In this section we consider the technological factors that directly effect vehicle fuel efficiency while
meeting the restrictions on vehicle emissions.
8.5.1
U.S. Vehicle Fuel Efficiency Regulations and Test Cycles
In the United States, fuel efficiency of new passenger vehicles and light trucks is regulated by the
National Highway Traffic Safety Administration of the U.S. Department of Transportation. This
regulation had its origin in the oil shortages of the 1970s caused by an embargo on oil exports
11 We use the technical term fuel efficiency interchangeably with the common term fuel economy, used in
government regulations. A fuel efficient vehicle would be economical to operate, having low fuel cost per
distance traveled.
 
 
 
 
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