EcoMotors International, a startup based in Troy, Michigan, has a new approach to an old idea--the two-stroke engine--which it says is up to 50 percent more efficient than most vehicle engines and pollutes far less than a conventional two-stroke engines.

The company recently received a combined $23.5 million in investment from Bill Gates and Khosla Ventures. That money will go toward development of EcoMotors's opposed piston, opposed cylinder (OPOC) engine. The engine uses two piston movements per cycle, instead of four, and each cylinder contains two opposing pistons, instead of one. A single crankshaft sits in between pairs of cylinders. The design relies on precise computerized control of all the components.

A conventional car engine takes four piston movements, or strokes, to go through intake, compression, combustion, and exhaust. In a two-stroke cycle, these stages are completed with just two piston movements, delivering twice as many power strokes per revolution and requiring fewer parts. But two-stroke engines tend to spew out more unburned fuel in the exhaust, which is why the four-stroke design became more common.