It starts with a three-cylinder, 700cc, turbocharged engine with 130 hp and 122 lb-ft. There are two overhead camshafts: a high pressure shaft turning at half the crank speed working on the two high pressure cylinders, and a low pressure shaft running at crank speed for the third cylinder. The two outside, high-pressure cylinders work like a normal four-stroke, but alternate their exhaust flow into the third, central low-pressure chamber. That cylinder's expansion and compression strokes aren't fixed, so it can be selectively tuned for the best expansion ratio.
The principle is that extra work gets done for the same amount of gas, and Ilmor claims increased fuel efficiency over conventional gas engines. The five-stroke is made with current, in-use technology so there'd be no extra costs for exotic materials or processes, and it's said to be "relatively compact." Ilmor says the next step is to build another working prototype with targets of 10% better fuel efficiency and 20% less engine weight compared to an equivalent four-stroke engine.
[Source: Ilmor via Motor Authority]