Ricardo's engine, called 2/4SIGHT, uses valves like a four-stroke engine, but in two-stroke mode, the engine keeps both the intake and exhaust valves open at the same time so that the fuel and air in the cylinder are replenished each cycle, rather than every other cycle. Ricardo's prototype, an adapted 2.1-liter V6 engine, has been tested by researchers at the University of Brighton and has been found to be able to produce the kind of performance one would normally expect from a three-to-four-liter engine. Based on the New European Driving Cycle, which is a standard performance test designed to gauge engine efficiency and emissions under typical car usage, the prototype has demonstrated fuel savings of 27 percent, and it reduces emissions by a similar amount. The next phase is to try to incorporate a prototype engine into a working vehicle, says Jackson.
"Four strokes are most efficient at full throttle; with two strokes, it's the opposite," says Robert Kee, a mechanical engineer who specializes in combustion engines at Queen's University, in Belfast, Northern Ireland.
The difference between two- and four-stroke engines is that the latter carry out the four stages of air intake, compression, combustion, and exhaust in four strokes of a piston. A two-stroke engine, in contrast, does this in just two piston strokes.
Two-stroke engines are intrinsically simpler by design and have higher power-to-weight ratios at high loads and low speeds because they get twice as many power strokes per revolution. But traditional two-stroke engines require oil to be mixed in with the fuel, and therefore produce higher emissions. Because of this, they aren't typically used in cars. Instead, they're used for lightweight applications such as chainsaws, lawnmowers, and some motorbikes.
But now, researchers at Ricardo have developed a piston head that operates in both two- and four-stroke mode, and it can switch automatically between the two modes, depending on the needs of the engine. This allows a smaller engine to handle the low-speed, high-load conditions without stalling.