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With ever-more-stringent worldwide fuel economy rules leading to a greater use of 3-cylinder engines for their smaller not-for-North America models, plus talk of even BMW cutting their vaunted 3-liter, 300+ hp twin turbo inline 6 in half to create a 1.5-liter, 150ish hp turbo 3-cylinder engine for base versions of their next-gen 1-Series and even 3-Series models, Ilmor Engineering's work on a five-stroke 3-cylinder engine isn't altogether surprising. An Autoblog article does a great job of summing things up:

Ilmor Engineering builds a five-stroke engine
by Jonathon Ramsey

English engineering firm Ilmor knows all about engine design, having produced powerplants for Formula One, IndyCar, and NASCAR. But its latest internal combustion creation, in contrast to those racing engines, is designed to burn fuel more frugally: a gas-powered five-stroke with diesel consumption.

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.

Ilmor Engineering builds a five-stroke engine — Autoblog

A Motor Authority article by James Martínez adds that "the engine runs an overall expansion ratio approaching that of a diesel engine – in the region of 14.5:1" and that "Eventually, the company hopes to achieve a power density figure of 150 horsepower per liter of displacement."

Ilmor Engineering Shows Off 5-Stroke Engine Concept - MotorAuthority

And, finally, a couple of pages on the Ilmor Engineering website add further, more detailed info:
Ilmor Engineering - 5-Stroke Concept Engine.
Ilmor Engineering - 5-Stroke Concept Engine - Operation
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