Advanced Automotive Technology: Visions of a Super-Efficient Family Car

The engine efficiency--the fraction of fuel energy that emerges as shaft horsepower--is about 22 percent

on the city part of the test and 27 percent on the highway, 24 percent composite. Strategies that increase

engine efficiency, by changing the engine type, improving its design and components, or helping it to

operate at its most efficient points attack the three quarters of fuel energy lost in the engines Raising

engine efficiency from 24 to 25 percent would reduce fuel consumption by 4 percent.

Of the energy that is converted by the engine to actual shaft horsepower:

* 16 percent (city), 2 percent (highway), 11 percent (composite) is lost because it cannot be used

when the vehicle is braking or idling. Systems that turn the engine off during braking and idle

(engine off or electric drivetrains), or store the energy produced (hybrid systems can do this), can

recover much of this 11 percent;

* 10 percent (city), 7 percent (highway), 9 percent (composite) is lost by transmission inefficiencies.

This is the target for improved transmissions or, for electric vehicles, avoiding the need for a

transmission;

* 11 percent (city), 7 percent (highway), 9 to 10 percent (composite) is used to power the

accessories. Aside from conventional strategies to improve accessory efficiency or to reduce

heating and cooling loads, electric vehicles have a different mix of accessories--some differences

help (no oil pump), and some hurt (may need a heat pump to generate cabin heat);

* 63 percent (city), 84 percent (highway), 71 percent (composite) is actually used to overcome the

tractive forces on the vehicle.

The three tractive forces play different roles at different speeds:

*

*

*

rolling resistance accounts for 28 percent of total tractive forces in the city, and 35 percent on the

highway, 31 percent composite. Both improvement to tires and weight reduction work to reduce

this large fiction of tractive forces;

aerodynamic drag accounts for 18 percent (city) and 50 percent (highway), 30 percent composite;

and

inertia (weight) force accounts for 54 percent (city) and 14 percent (highway), 40 percent

composite. Weight reduction directly attacks this force, or some of the energy used to overcome it

can be recovered by regenerative braking.

BASELINE

The analytical model used to forecast baseline fuel economy is the Fuel Economy Model
(FEM), used by the Department of Energy (DOE) Energy Information Administration as one of
the submodels in the National Energy Modeling System (NEMS). The fuel economy is forecast as
a function of input fuel prices, personal income and Corporate Average Fuel Economy (CAFE)
standards, and its methodology is summarized in appendix A. The FEM incorporates both