The four-stroke engine has dominated internal combustion for more than a century, but a British engineering firm is ready to topple that technology with what it calls a 150-horsepower "five-stroke" engine said to offer the fuel economy of a diesel without the particulate emissions.

Ilmor Engineering, a firm that is co-owned by Roger Penske and supplies Honda engines to the Indy Racing League, spent almost 20 years developing the three-cylinder engine. Two cylinders operate on the conventional four-stroke cycle and empty their exhaust into a third low-pressure expansion cylinder, which allows the expansion and compression processes to operate independently. The prototype engine was first displayed at the 2009 Stuttgart Engine Expo, and its being readied for real-world, under-the-hood testing.

The Otto cycle four-stroke engine has pretty much been the standard for 132 years, but the five-stroke engine might be a great stopgap between current technology and the Next Big Thing.

According to Ilmor, the 0.7-liter engine is capable of 150 brake horsepower and weighs 20 percent less than engines with a similar output. Brake specific fuel consumption of the prototype is 226 g/kWh, which is a 10 percent improvement over current four-stroke technology. Even better, Ilmor says the technology is "100% conventional" and doesn't require any new manufacturing techniques.

Ilmor says that the prototype five-stroke engine, based on a design by Gerhard Schmitz, has an overall expansion ratio "approaching that of a diesel engine – in the region of 14.5:1." Along with its light weight and relatively high output, the extra work done in the low pressure (LP) cylinder provides for better fuel economy.

"Running of the concept engine has produced impressive fuel consumption readings over a very wide operating range," the company said in a statement. "This is because at the onset of knock a greater percentage of work can be extracted in the LP cylinder, giving a degree of self compensation."

Ilmor says the prototype is ready for road testing and could be used as a conventional engine or as part of a hybrid drivetrain. The company says it is "keen" to find a partner to help develop the engine and it is talking to automakers and top suppliers about the technology.

Photo of the "five-stroke" engine: Ilmor Engineering.