HERNDON, VA - Audi's A3 TDI diesel is one of the first cars on the road to feature an electric space heater, but it won't be the last.

A side effect to building more fuel-efficient vehicles is that they produce less excess heat to keep the passenger compartment warm. The heat engines produce is really wasted energy; it was turned into friction rather than being used to move the car.

Diesels have a higher level of thermal efficiency than gasoline engines. They produce less excess heat to warm the passenger compartment. That means it would take too long to warm the interior on a cold day, so the A3 TDI and Audi's other diesels use the space heater.

The heater turns on when the outside temperature is below 43 degrees Fahrenheit, the engine is running at 500 r.p.m. or more, and the driver turns the heat on. The auxiliary heater switches off when the engine's cooling fluid warms up enough to heat the car.