How hybrid-electric cars and trucks work

For many years, it seemed that the internal combustion engine was superior. No serious contender had won out, with electric and steam fading early in the automobile's history, and Chrysler's "burn anything" turbine efforts finally ended by the Federal government (despite promising results). Slowly, though, the technology for merging electric and gasoline vehicles started to arise, with on-board computers, new materials, and new ideas. The combination is ideal in many ways: electric motors have very high torque, to get a car rolling almost immediately; gas motors are more efficient when running at a constant speed (e.g. to produce electricity); and, if you use electric power, you can generate it while braking, recapturing energy otherwise lost as heat. Now, nearly every automaker is working on hybrid systems, with Chrysler focusing on a "through the road" system (where one set of wheels is gas powered, another set electrically powered) and General Motors very excited about a new development, hub motors, where small electric motors can be fitted into the wheel hubs for strong, quick acceleration, without the lag time of gas motors.

But how do they actually work?

We present this rather good description provided by Toyota, makers of the world's first production hybrid-electric car, the Toyota Prius. We have made many edits, so any errors are probably ours.

A hybrid powertrain uses an engine that's burning a fossil fuel (e.g. gasoline), combined with an electrical system made up of a motor, generator and battery. Depending upon the system, the gasoline engine may be able to drive the vehicle by itself, or it may drive the electrical system only (which in turn will actually drive the vehicle). Or the electrical system might be able to drive the vehicle by itself, or both systems may be able to work together.

A typical four-door sedan may have an engine rated at, say, 200 horsepower. It requires the full 200 horsepower very little of the time, normally only for quick passing maneuvers or while climbing steep hills. The vast majority of the time the engine is operating at a small fraction of its full output. Once the sedan is at freeway speed, as little as 20 or 30 horsepower may be needed to keep it moving. In fact, many drivers may seldom, if ever, call upon the full power output of the engines under their cars' hoods. What people really need is 200 horsepower every once in a while, maybe 100 horsepower from time to time, and about 30 or 40 horsepower most of the time.

Could an electric car do that? The pure electric vehicle is quiet and smooth and generates none of the emissions currently regulated for vehicles with gasoline engines, but after over a century of research the electric vehicle still lacks a suitable battery and there is not a likely prospect of finding one on the horizon. The pure electric car has the same handicap it had 100 years ago -- limited range. Also, while a car with a gasoline engine can be completely refueled in a few minutes, hours are required to charge up an electric car. The further an electric car goes, the more its performance drops.

In simple terms, the electric car doesn't have enough when it's needed; the conventional gasoline car has too much when it's not needed.

The hybrid solves both those issues. One of the most elemental forms of the hybrid is the familiar diesel-electric railroad locomotive. These have huge diesel engines, which drive generators, which supply the electrical power for electric motors, which in turn drive the wheels. The diesel engine operates within its most efficient speed range, and varying the speed of the train is done through the electric motors. This makes for a very fuel efficient, and reliable, powertrain. But of course, once trains are up and running they tend to run at fairly constant speeds anyway.

The road vehicle, because it has to deal with the widely varying speeds and conditions of traffic, has a more difficult duty cycle. Starts, stops, short trips, family vacations, stuck in traffic jams -- all these create fuel consumption and emissions problems. To deal with this, the typical automotive hybrid system is comprised of a relatively small gasoline engine, which drives either the wheels directly, or a generator, or both. There's also an electric motor, which drives the wheels, sometimes alone, or sometimes in concert with the engine. A battery pack supplies the electric motor, and a generator makes the electrical power to recharge the battery. Sophisticated electronic controls watch over all these parts. As software is to computers, it's the controls that make the whole package work in harmony.

The most sophisticated production hybrid is currently the 2004 Toyota Prius. The Prius has a 1.5-liter, four-cylinder gasoline engine of 78 horsepower. It's linked to drive the wheels directly via a transmission and, whenever it's running, it also drives a generator that keeps the battery charged. The generator supplies electrical power to the electric motor or the battery, as needed.

Whenever the Prius is stopped, the gasoline engine is shut down. This means no unnecessary idling or fuel waste while stuck in traffic or at stop signs. When accelerating from rest at a normal pace, and up to mid-range speeds, the Prius is powered by the electric motor, which is fed by the battery. As the battery charge is depleted, the gasoline engine responds by powering the electric generator, which recharges the battery. Once up to speed and driving under normal conditions, the engine runs with its power split: part of this power goes to the generator, which in turn supplies the electric motor, and part drives the wheels. The distribution of these two power streams from the engine is continuously controlled to maintain the most efficient equilibrium. If the need arises for sudden acceleration, such as a highway passing maneuver or a quicker start from rest, both the gasoline engine and the electric motor drive the wheels.

And during braking and other types of deceleration, the kinetic energy of the moving vehicle is converted into electrical energy, which is then stored in the battery. At all times the state of charge of the battery is constantly monitored, and whenever needed the generator is powered by the gasoline engine to provide the necessary charge.

The result is a vehicle powered by a gasoline engine, in that it's the engine that drives the wheels or drives the generator that supplies (either directly or through the battery) the electric motor. But the engine is only as big as it needs to be. It isn't even running all the time, and if sudden acceleration is called for, both the gasoline engine and electric motor share the load. The engine in hybrid vehicles like the Prius run exclusively on gasoline, while the electrical portion of the power system never needs to be plugged in for a charge. There's no cord and no waiting. You can fill up at any normal gas station anywhere.

But the real benefit, to both the owner and driver of a hybrid like the Prius, and the environment, is in the numbers. The Prius is roomy enough inside to meet the EPA's Midsize category, just like the Camry. It will accelerate from 0 to 60 mph in about 10 seconds (roughly equal to a four-cylinder Toyota Camry), and will deliver fuel economy in the mid-50-miles-per-gallon range, making it the most fuel efficient of any midsize vehicle sold in America, and delivering twice the combined mileage rating of its closest competitor. In addition, Prius will be certified as SULEV, or "Super Ultra Low Emission Vehicle.