gas engines

Typically, in a full hybrid, the electric engine takes control when the car is cruising, at stop, or when slowly accelerating. When extra power is needed, the gas engine kicks in to give the acceleration expected from today's cars. By allowing the electric engine to take over, hybrids are able to get higher mpg than their sister cars with gas only engines. But since most of the energy is collected/saved when the car is stopped or in braking, hybrid cars tend to get better mileage in city driving. Which is opposite what gas only cars should expect, as gas engines are most efficient at high speeds (highway).

When comparing hybrid cars to plug-ins, hybrid engines have not only eliminated the need for plugging in, they have also increased the range that is possible.