Only about 15% of the energy
in the fuel you put in your gas tank gets used to move your car
down the road or run useful accessories like air conditioning or
power steering. The rest of the energy is lost. Because of this
the potential to improve fuel economy with advanced technologies
is enormous.
Motor vehicles need energy
to accelerate (overcome inertia), to push the air out of their way
(aerodynamic drag), and to overcome the friction from tires, wheels
and axles (rolling resistance). Fuel provides the needed energy
in the form of chemicals that can be combusted (oxidized) to release
heat. Engines transform heat released in combustion into useful
work that ultimately turns the vehicle's wheels propelling it down
the road.
Even modern internal combustion
engines convert only one third of the energy in fuel into useful
work. The rest is lost to waste heat, the friction of moving engine
parts or to pumping air into and out of the engine. All of the steps
at which energy is wasted are opportunities for advanced technologies
to increase fuel economy.
The figure above illustrates the paths
of energy through a typical gasoline-powered vehicle in city driving.
Of the energy content in a gallon of gasoline, 62% is lost to engine
friction, engine pumping losses, and to waste heat. In urban driving,
another 17% is lost to idling at stop lights or in traffic. Accessories
necessary for the vehicle's operation (e.g., waterpump) or for passenger
comfort (e.g., air conditioning) take another 2%.
Just over 18% of the energy in gasoline
makes it to the transmission. Losses in the drive train to friction
and slippage claim more than 5%, leaving a bit less than 13% to
actually move the vehicle down the road. The laws of physics will
not permit all of these losses to be entirely eliminated. But improvements
are possible at every step.
The 12.6% of original fuel energy that
makes it to the wheels must provide acceleration (5.8 %) and overcome
aerodynamic drag (2.6%) and rolling resistance. In stop and go city
driving it is not surprising that acceleration is the biggest need,
rolling is next, followed by aerodynamic drag. On the highway the
order is reversed: aerodynamic drag, which increases at an increasing
rate with speed requires the most energy (about 10.9%). Each of
these final uses of energy also represents an opportunity to improve
fuel economy. Substitutions of high strength lightweight materials
can reduce vehicle mass and thus the energy required for acceleration.
Smoother vehicle shapes have already reduced drag significantly,
but further reductions of 20-30% are possible. Advanced tire designs
can cut rolling resistance.
|