How Do Hybrid Autos Work and Improving Fuel Efficiency
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.
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It's no accident that the two highest fuel economy vehicles for 2002 are hybrid vehicles. Hybrid electric vehicles combine the best features of internal combustion engines (1) and electric motors (3). |
In the Honda Insight and Toyota Prius both the engine (1) and the electric motor (3) are connected to the wheels by the same transmission (2). With the assistance of the electric motor the engine can be smaller.
Intelligent power electronics (4) decide when to use the motor and engine and when to store electricity in advanced batteries (6) for future use. The electric motor is used primarily for low speed cruising or to provide extra power for acceleration or hill climbing.
When braking or coasting to a stop, the hybrid uses its electric motor (3) as a generator to produce electricity, which is then stored in its battery pack (6).
Unlike all-electric vehicles, hybrid vehicles do not need to be plugged into an external source of electricity. Gasoline stored in a conventional fuel tank (5) provides all the energy the hybrid vehicle needs.
Three hybrid vehicles are currently available:
For fuel economy information on these vehicles, please visit the Find and Compare Cars Section.