ENERGY FOR A SUSTAINABLE WORLD - World Resources Institute

the next several decades was 37 mpg (6.4 lhk)
because of its estimate that the total cost of
owning and operating a car would rise sharply
at higher fuel economy levels. 37
Soon after the study was published, however,
the more efficient Volkswagen Rabbit
diesel and the Honda City Car were introduced.
In the last couple years, the number of
fuel-efficient models available has grown considerably.
(See Table 10.) The most energyefficient
model available in the United States at
this writing is the four-passenger, gasolinefueled
Sprint, which has an estimated on-theroad
fuel economy of 57 mpg (4.1 lhk), some
50 percent higher than the NAS estimate of the
"technological limit." 38
Even these impressive new cars exploit only
a fraction of the presently available technology
for improving fuel economy. Most of the improvements
to date are from reducing weight
through use of lightweight materials, reducing
rolling resistance through radial tires, reducing
aerodynamic drag, and using pre-chamber
diesel engine and lean-burn gasoline engines.
Options for making further gains with present
technology include shifting to direct-injection
diesel engines (the kind used in trucks) or
spark-ignited, direct-injection diesel engines
(which have multifuel capability), introducing
the continuously variable transmission (CVT),
introducing the feature of engine-off during
idle and coast, using lightweight materials more
extensively, and further reducing aerodynamic
drag. Several prototype cars indicate what can
be achieved using some of these technologies.
(See Table 10.) The most recently introduced
prototype, the Toyota AXV, a four- to five-passenger
lightweight car with a direct-injection
diesel engine, CVT, and a drag coefficient of
0.26, gets 98 mpg (2.4 lhk). (See Figure 4.)
But doesn't good fuel economy imply a sluggish
vehicle that will make highway entry or
passing difficult or dangerous? Not necessarily.
The Volvo Light Component Project 2000 vehicle
gets 65 mpg (3.6 lhk) but requires only 11
seconds to accelerate from 0 to 60 mph (96 km/
hour), compared to 17 seconds for the popular
automatic Chevrolet Cavalier, which has a fuel
economy of only 28 mpg (8.4 lhk). 39 (See Table
10.)
Doesn't high fuel economy mean a tiny,
cramped vehicle? Again, not necessarily. Most
of the new highly efficient cars and prototypes
listed carry four or five passengers comfortably.
Aren't fuel-efficient cars unsafe? Here too,
good engineering design can improve the
structural strength and safety of even very
small cars. For example, the lightweight (707
kg) Volvo LCP 2000 can withstand 35-mph
(56-km/hour) front and side impacts and
30-mph (48-km/hour) rear impacts—meeting
stricter safety standards than do cars currently
sold in the United States. 40 Moreover, advanced
technology still under development will
make it possible to build "heavy" fuel-efficient
cars, such as the Cummins/NASA Lewis car
design. (See Table 10.) This 80-mpg (2.9-lhk)
car, equipped with a CVT and an advanced
multifuel-capable, direct-injection adiabatic
diesel engine and turbocompounding, would
weigh 1,360 kg, approximately the average
weight of new cars sold in the United States
today.
One problem posed by diesel engines is that
they emit on average about 100 times the
weight of particulates produced by gasoline
engines of comparable performance. However,
Mercedes Benz and Volkswagen have developed
emission-control devices that permit
diesel cars to meet the strict 1986 California
particulate limit of 0.2 grams per mile. 41 Moreover,
the use of diesel fuel is not necessary to
realize high fuel economy. Several highly efficient
cars have gasoline engines, and sparkassisted
diesel engines can have the fuel
economy of diesels operating on gasoline or
alternative fuels (e.g., ethanol or methanol).
(See Table 10.)
Often the technologies added to cars to improve
fuel economy cost more than the tech-
63