How Do Hybrid Cars Work?

Oct. 1, 2003 -- What makes a hybrid car so fuel-efficient? It's quite simple — they only burn gasoline when needed

That's because, as the term implies, hybrid cars have two different powerplants under the hood: a powerful electric motor and a conventional gasoline engine.

The electric motor, powered by a stack of rechargeable batteries, is the primary workhorse for moving the car during slow-speed driving — such as through a residential neighborhood or in stop-and-go urban traffic.

During coasting and slow-down phases when a driver lightly taps on the brakes, the car's wheels are automatically engaged to an electrical generator. The generator creates an extra "load" to assist the brakes in slowing the car down, but more importantly, it converts the car's mechanical energy back into electricity to recharge the car's batteries.

At higher speeds, such as steady highway cruising, computers automatically switch on the gas-burning engine, which then takes over as the primary driving force of the car. Typically, the small engine is designed with variable valve timing intelligence, or VVT-i, and other advances to ensure that the fuel is burned most efficiently and completely.

No Outlet, Just Gas and Go

But both the electrical motor and gas-powered engine can also operate in conjunction. For example, if more power is needed to merge onto a highway or pass a tractor-trailer truck, the on-board computers will automatically activate the electric motor to provided the needed acceleration boost.

And if the on-board batteries fall below a certain level of charge, the gas engine will also automatically turn on to charge the batteries and run the electrical motor if needed. That's why a hybrid car never needs to be plugged into an electrical outlet at night — a misconception that many hybrid-makers confess they still need to clarify among consumers and even some car dealers.

Hybrid proponents say the power combination offers consumers the best of both an electric car and a conventional gas-fueled vehicle. At complete stops — say, in rush-hour highway traffic where most car engines are spent idly wasting fuel — the car isn't using any gas at all. But like a pure-electric car, once a driver steps on the "gas" pedal, the electric engine comes to life and instantly propels the car forward.

And keeping a hybrid "ready to go" isn't any more difficult that fueling it with regular-grade gasoline. But with fuel efficiency ratings ranging from 30 to 60 miles per gallon, hybrid owners need to fill up their tanks far less frequently — typically, every 400 to 600 miles or so.