dinsdag 1 april 2008

Nonelectric Hybrid Engines

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.

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