BMW is reviving steam engine principles in an auxiliary drive that it told Automotive News Europe can both boost power and fuel efficiency in ordinary piston-driven cars.

The automaker is testing a two-stage device called the Turbosteamer on a stationary conventional 1.8-litre BMW engine at BMW headquarters in Munich. The device works entirely on the waste heat from the engine. It converts more than 80% of the heat energy in the exhaust into usable power, said Raymond Freymann, head of advanced research and development for BMW.

BMW could start building production vehicles with the system by the first half of the next decade, Burkhard Gšschel, BMW board member for r&d and purchasing, told the paper. It can be used on any combustion engine.

In trials on a test rig, the system when attached to a regular BMW four-cylinder 1.8-liter Valvetronic engine reduced fuel consumption figures by 15% - and also generated an extra 13hp and 20Nm of torque.

The concept is hardly new - the first patent for a steam auxiliary drive harnessing exhaust was issued in 1914 to Wilhelm Schmidt of Germany.

The biggest problem so far has always been the sheer size of a heat-recovery system. But the BMW Turbosteamer fits in a regular 3 series body. The entire system weighs 100kg.

"All we lose is a bit of ground clearance," said Freymann.