A new kind of hybrid vehicle could offer reduced fuel consumption to consumers concerned about gas prices. Mechanical engineers in the United Kingdom have developed a novel kind of combustion engine that is able to switch between being a two-stroke and a four-stroke engine. The system, they say, can reduce fuel consumption by 27 percent.

The improved fuel consumption essentially comes from downsizing the engine, says Neville Jackson, technology director of Ricardo UK, an engineering firm in Shoreham-on-Sea that developed the new engine. "A smaller engine has less internal friction and delivers better fuel consumption," he says.

But small car engines, which are usually based on a four-stroke design, don't offer a lot of power. They can be particularly problematic when operated at low speeds with a high load, such as when accelerating uphill. Such conditions can even make a small engine stall if the driver doesn't downshift.

"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.

"This is an interesting concept," says Martti Larmi, head of the Internal Combustion Engine Laboratory at Helsinki University of Technology, in Finland.

The main challenge in building such an engine is perfecting the scavenging process, he says, when the residual gases from the previous combustion cycle are replaced with fresh air and fuel.

"You need some kind of pressure on the intake side to push out the gases that have already burned," says Larmi.

In a traditional two-stroke engine, the force of the fuel and air intake drives out the exhaust. Unfortunately, this process causes some unburned fuel to be lost as exhaust, resulting in higher emissions. Four-stroke engines force the spent fumes out of the cylinder through a cam-controlled valve using an upward stroke of the piston. During the following downstroke, fresh air and fuel are injected into the cylinder while the exhaust valve is closed.