Variable Stroke Motorcycle Engine Feasible?


What goes up, they say, must come down. And for a piston in a combustion engine, that's certainly true. But does it actually need to come down the same distance as it originally rose?

It's another sticking point with the basic layout of a combustion engine. During the intake stroke, the amount of fuel and air drawn in must be at a particular ratio - 14.7:1 - and it must be compressed the right amount to make it burn efficiently (anything up to around 14:1 compression ratio, but normally nearer 10:1).

The problem is that the fuel-air mixture when it burns, can actually expand much more, creating valuable power. Conventional engines simply can't use this extra expansion; once the piston reaches the bottom of its power stroke, any further expansion of the gases inside must be got ride of through the exhaust valve.
But, with a clever crankshaft and con-rod arrangement, it's actually possible to create an engine that has a power stroke that's longer than its intake stroke.

Honda is just one of the companies working on this new engine arrangement, using a secondary shaft, turning at half engine speed, connected to the lower part of a two-piece con-rod, its movement means that during the intake and compression strokes, the piston travels only a relatively small distance, but then moves much further during the power and exhaust strokes. Effectively, it creates a combustion engine that that has a small capacity when it's drawing fuel in, but a large capacity when it's pumping power out.

Honda's prototype engine – an air-cooled single cylinder – has an intake capacity of 135cc but an expansion capacity of 203cc, a 50 percent increase. On a motorcycle, that would equate to a four-cylinder engine that's 540cc during intake, but 812cc during the power stroke.

Pumping losses are reduced, as is friction, because the piston doesn't travel as far as it would in a conventional engine. The prototype engine has shown Honda that in the near future they can improve fuel efficiency well past 20 percent without sacrificing raw engine power performance.
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