NO SOONER have you turned the ignition key than a shock wave rips through your car’s engine. It might sound like a recipe for disaster, but engines that rely on shock waves to compress air and fuel could make hybrid cars much more efficient.

So says Norbert Müller at Michigan State University in East Lansing. He has developed a prototype of such an engine, which was presented last week at a meeting organised by the US Department of Energy’s Advanced Research Projects Agency – Energy. At its heart is a rotor containing a number of radial channels (see diagram).

As the rotor spins, the channels allow an air-fuel mixture to enter via central inlet ports. The mixture would escape through the outlet ports in the walls of the surrounding chamber, but by now the rotor has turned to a position where the channels are not pointing at the outlets.

The resulting sudden build-up of pressure in the chamber generates a shock wave that travels inwards, compressing the air-fuel mixture as it does so. Just before the wave reaches the central inlet ports, these too are shut off by the turning of the rotor.


The compressed mixture is then ignited. By this time the rotor’s channels are pointing towards the outlet ports again, releasing the hot exhaust. As the gas escapes at high speed, it pushes against the blade-like ridges inside the rotor, keeping it spinning and generating electricity.

The design does away with many of the components of a conventional engine, including pistons, camshaft and valves. This makes it much smaller and lighter than a conventional engine. A car fitted with the new engine could be up to 20 per cent lighter overall, Müller claims. By eliminating losses associated with mechanical components, it will also make cars more fuel-efficient, he says.

Müller says the engine can be adapted to run on a variety of fuels, including hydrogen. Having built a small prototype, he hopes to have a 25-kilowatt version ready by the end of this year.

Rui Chen, who studies combustion systems at Loughborough University in the UK, says the design could significantly reduce the weight of a car’s power train. “In terms of fuel, the design is much more flexible than a conventional piston engine,” he adds.