Micro-engine. Coloured scanning electron micro- graph (SEM) of combustion chambers in the world's smallest combustion engine. This is a rotary Wankel engine, which uses a rotating rotor instead of pistons and cylinders. Each chamber contains a fuel injection inlet and an exhaust outlet, and the cog at the centre allows the transmission of power. The entire engine is smaller than most batteries, and it is hoped that it could power a variety of electronic devices. It runs on liquid fuels such as butane or propane. It will lead the way to even smaller engines etched on a silicon wafer. The chip was built at the University of California at Berkeley, USA, in 2001.