You don’t! A Hybrid-Electric Vehicle (HEV) uses both a gasoline engine and an electric DC motor. There are two main facts which make the combination of these two very efficient:
– The DC motor is very powerful, giving alot of torque at low RPM’s, right where a gasoline motor is weakest.
– A gasoline engine uses the most fuel while accelerating, so the DC motor assists at these times.
Basically, the electric motor kicks in to bring the vehicle from a stop. The gas engine starts up at around 40km/h and you run off the gas engine while cruising.
If you need to accelerate, the electric motor will help the gas engine, giving you the feel of a more powerful engine. When you hit the brakes, energy from your momentum is captured and used to recharge the battery.
If this were a perfect system, the motor would accelerate you, breaking would charge the battery an equal amount, and the gas engine would be only used for maintaining speed. Of course, when you add in friction, wind resistance, and a less than perfect battery system, we’re not quite there, but the idea is to get close to that.