How a Hybrid Car Works: Engine, Motor and Battery Explained

A hybrid car has two ways to move: an ordinary combustion engine and one or more electric motors. The two share the work, and a computer decides how much each one does at any moment. According to the US Department of Energy, the result is better fuel economy without giving up performance. This guide explains the parts, what happens as you drive, and what a hybrid cannot do.

The basic idea

The Department of Energy's Alternative Fuels Data Center describes a hybrid electric vehicle as one powered by an internal combustion engine and one or more electric motors that draw on energy stored in a battery. The pairing matters because a petrol engine is not equally efficient at every speed. The fuel economy site run by the US government says gasoline engines are least efficient at low speeds, and in some hybrids the electric motor alone moves the car at those speeds.

At other times the motor helps the engine instead of replacing it. Because the motor can add power when it is needed, the car can use a smaller, more efficient engine than it otherwise would. That is the heart of the design: the engine does not have to be big enough to deliver peak power on its own, because the motor covers the moments when extra power is needed.

The parts that matter

• Engine. A spark-ignited petrol engine. It can move the car, and it is one of the two ways the battery gets recharged.
• Electric traction motor. Drives the wheels, either on its own or together with the engine.
• Traction battery pack. The main battery. It stores the electricity that the motor uses.
• Electric generator. Makes electricity from motion, for example from the spinning wheels while the car brakes.
• Power electronics controller. The computer-controlled unit that manages the flow of electricity between the battery and the motor.
• Transmission. Passes power from the engine and the motor to the wheels.
• Everything else. A DC/DC converter and a separate auxiliary battery run the car's ordinary electrical equipment, and a cooling system keeps the engine, motor and electronics at a safe temperature.

The Department of Energy's list of components also includes the fuel tank, the fuel filler and the exhaust system, which do the same jobs as in any petrol car.

What happens as you drive

Exactly how a hybrid behaves depends on its design, but the main situations look like this.

• Pulling away and creeping along. In some hybrids the electric motor alone can move the car at low speed, with the engine off.
• Accelerating, overtaking or climbing a hill. The motor assists the engine, adding power when demand is highest.
• Steady driving. The engine can move the car and recharge the battery at the same time.
• Braking and slowing down. The wheels turn the motor, which acts as a generator and sends electricity back to the battery. This is called regenerative braking.
• Stopped at a light. The engine can shut off and restart when you press the accelerator, so no fuel is burned while idling.

Some hybrids can drive on the motor alone; others use the motor mainly to assist the engine. If you are comparing models, the manufacturer's own description will say which approach a particular car takes.

Why regenerative braking matters

In an ordinary car, braking turns the car's movement into heat in the brake pads and discs, and that energy is simply lost. In a hybrid, the Department of Energy explains, the electric generator produces electricity from the spinning wheels while the car brakes, and that energy goes back into the traction battery.

The fuel economy site describes the same thing from the motor's side: the wheels' forward motion turns the motor, which produces electricity and helps slow the car down. The stored energy is used later, for instance the next time the motor helps the engine accelerate. Energy that would have been wasted as heat becomes a small saving in fuel.

A hybrid does not need a charging cable

The Alternative Fuels Data Center is direct on this point: a hybrid electric vehicle cannot be plugged in to charge its battery. The battery is recharged by the engine and by capturing energy during braking, so you refuel at a petrol station exactly as you would with any petrol car. Plug-in hybrids are a separate type that can also be charged from an outlet; this article covers the conventional kind.

Why it saves fuel

The savings come from several small effects that add up. The motor lets the car use a smaller engine. In some hybrids the motor handles low-speed driving, where petrol engines are least efficient. Regenerative braking recovers energy that would be lost. Automatic stop and start reduces the energy wasted by idling, and the battery can also run accessories so the engine does not have to. The Department of Energy sums up the result as better fuel economy without sacrificing performance.

Sources

• How Do Hybrid Electric Cars Work? Alternative Fuels Data Center, US Department of Energy
• How Hybrids Work, fueleconomy.gov (US Department of Energy and EPA)