Environmental Engineering Reference
In-Depth Information
by OPEC nations. To ameliorate the nation's future dependence upon imported oil, Corporate
Average Fuel Economy (CAFE) standards were promulgated, beginning with the 1978 model year.
These standards required automobile manufacturers to design vehicles so that their sales-averaged
vehicle fuel economy
12
did not exceed the level designated for each of two vehicle classes: passenger
vehicles and light trucks (pickup trucks, vans, sport utility vehicles).
13
In 2000, the CAFE standards
for passenger cars and light trucks were 27.5 miles/gallon (11.7 km/L) and 20.7 miles/gallon (8.8
km/L), respectively.
The measurement of vehicle fuel economy is based upon dynamometer simulations of typical
driving cycles for urban and highway travel, originally devised by the U.S. EPA for evaluating
vehicle emissions (Urban Dynamometer Driving Schedule, UDDS). The test vehicle is operated
in a stationary position, while the drive wheels turn a dynamometer that is adjusted to provide
the acceleration/deceleration and air resistance loads as described in equations (8.10) and (8.13).
While the dynamometer does not precisely simulate these forces at each point in the test, it suf-
fices to provide a reasonable average for characterizing the emissions and fuel economy of the
test cycle.
Two test cycles are used for measurements of vehicle fuel efficiency, one for urban driving
and the other for highway travel. Figure 8.7 shows the vehicle speed versus time for each cycle.
The urban cycle has many starts and stops (25 in a 17.8-km ride lasting 23 minutes) and a low
average speed (while moving) of 31.4 km/h. For an average vehicle, about 34% of the energy
needed to propel the vehicle through this cycle is dissipated in braking. In contrast, the highway
driving cycle proceeds at a more uniform speed, averaging 78.5 km/h over a 16.5-km run, with
only one stop. The two cycles represent two common types of vehicle use: (a) stop-and-go driving
in congested urban streets and (b) uncongested freeway travel. For passenger cars, highway travel
is between 30% and 50% more fuel efficient than urban travel (see Table 8.2). New vehicle
purchasers are notified of the fuel economies for urban and highway travel determined from these
tests.
14
For the purpose of enforcing CAFE standards, a weighted average is used (55% urban and
45% highway).
15
12
While the average fuel economy is reported in units of miles per gallon, the averaging variable is the fuel
consumption, measured in units of gallons per mile. Thus the average fuel consumption per mile traveled is
regulated.
13
Passenger cars are called light-duty vehicles (LDV) to distinguish them from heavy-duty passenger vehicles
such as buses that carry more than 12 passengers. Light-duty trucks (LDT) are utility vehicles that can carry
freight or passengers, whose gross vehicle weight rating (GVWR) is less than 8500 pounds (3.856 t) and
frontal area is less than 45 square feet, or can operate off highways (e.g., sport utility vehicle). There are
four categories of LDT (LDT1, LDT2, LDT3, and LDT4) depending upon the vehicle GVWR (
<
6000 lb,
>
6000 lb ) and the loaded vehicle weight (LVW), which is the sum of the vehicle curb weight plus 300 pounds
(
5750 lb). The vehicle curb weight (VCW) is the manufacturer's estimated weight of a fueled
vehicle in operating condition.
14
The consumer guide to vehicle fuel economy discounts the highway and urban test fuel economies by 10 %
and 22 %, respectively, to account partially for disparities in the test procedure compared to average driving
conditions. Studies of fuel consumption of vehicles in use indicate higher consumption than the consumer
guide values.
15
The weighting factors are applied to the fuel consumption, which is the reciprocal of the vehicle fuel
economy.
<
3750 lb,
<
