power steering. And, unless the engine is turned off, during idle and braking the engine energy is
largely wasted because it is not being used to provide motive force.To produce usable energy, the vehicle must take fuel energy and translate it to shaft power
through the engine; most of this power is then directed through the remainder of the vehicle’s
drivetrain to drive the wheels. Generally, this is a relatively inefficient process. Energy is lost
because moving parts in the engine create friction; because air and fuel must be pumped through
the engine, causing aerodynamic and fluid drag losses; because much of the heat generated by
combustion cannot be used for work and is wasted; and because slippage in the transmission
causes losses. As discussed later, a conventional vehicle drivetrain generally will be able to
transform about 14 (city) to 23 (highway) percent of the fuel energy into usable power at the
wheels.^5
In an attempt to reduce vehicle fuel consumption, vehicle designers can work to reduce all of
the forces acting on the vehicle (the tractive forces), as well as the losses in turning fuel into
motive power. Tractive forces may be reduced by smoothing out body surfaces to reduce
aerodynamic drag, by redesigning tires to reduce their rolling resistance, or by making the vehicle
lighter, through use of lighter materials and redesign of the vehicle structure and interior, to
reduce inertia forces as well as to further reduce rolling resistance. Accessory losses may be
reduced by improving the design of air conditioners, water and oil pumps, power steering, and
other power equipment, or by reducing the work these accessories must do (for example, heating
and cooling loads can be reduced by providing insulation and coating window surfaces with
coatings that reflect unwanted solar radiation). Drivetrain losses may be reduced through various
strategies--ranging from redesign of conventional engines and transmissions to shifting to
alternative types of drivetrains that may offer increased efficiency.
Fuel consumption may also be reduced by sacrificing consumer amenities--reducing the size of
the passenger compartment (and, consequently, the size and weight of the vehicle), using a less
poweful engine that cannot provide the same acceleration (and that may cause greater noise and
vibration), designing transmission shifts that achieve higher efficiency at the cost of more
harshness, reducing the number of accessories such as air conditioning or power locks and
windows, and so forth. Most modem attempts to reduce fuel consumption do not contemplate
sacrificing these amenities,^6 but some types of vehicle redesigns may achieve higher efficiency
only at the cost of such a sacrifice.^7 As discussed later, comparisons of vehicle fuel economy
achievements should carefully consider of any differences in vehicle performance or amenities.
To obtain an idea of target areas for saving fuel the following are a few quantitative indicators
for a typical mid-size car that gets 27.7 mpg on the EPA test cycle (22.7 mpg city; 38.0 mpg,
highway):(^5) Counting the energy not used for power during the time the vehicle is idling and braking.
(^6) For example, the Partnership for a New Generation of Vehicles has as a key goal the development of an 80 mpg vehicle thatessentially matches
the
7
performance of the current class of intermediate autos.
With vehicles that rely on batteries or chemical fuels with low energy densities for energy storage, designers may have to sacrifice range to
maintain efficiency.