How Does A Motorcycle Combustion Engine Work?

Cycle World Technical Editor Kevin Cameron answers your motorcycle engineering and mechanical questions

Until the electric-vehicle revolution has advanced quite a bit from where it is at the moment, motorcycles remain largely powered by combustion engines. And that means pistons going up and down in cylinders connected to rotating crankshafts by connecting rods.

In the case of a four-stroke engine, it takes two revolutions of the crankshaft to complete the cycle of intake, compression, power, and exhaust. But in two-stroke engines, those same processes all happen within one crankshaft revolution.

Central to both conditions is a piston moving up and down in a cylinder. When the piston rises in the cylinder to compress the mixture of fuel and air above it, a spark plug screwed into the head ignites that mixture. The result is not an explosion. It is simply deflagration.

Combustion begins in the gap of the spark plug. A little flame kernel is then torn to pieces by vigorous air motion in the mixture and that causes a rapid spread of flame. When the flame spreads right out to the edge, the fuel and air are consumed.

Energy is transformed into heat, which causes the pressure of the gas in the combustion chamber to rise. That chunk of energy drives the piston downward on the power stroke. Sometimes that lovely, normal, gradual combustion process goes wrong.

Pre-ignition is exactly what it sounds like: ignition of the fuel-air mixture before the spark. Detonation, which occurs after normal ignition but near the end of combustion, is another type of failure. Both of these are worth avoiding.

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for Cycle World. Kevin’s unparalleled experience and knowledge of the sport were—and continue to be—prompted by a lifetime of curiosity. His willingness to share that information with anyone who is willing to listen is likewise unique.

Kevin’s greatest strength lies in his ability to present complex subjects in simple terms with clarity and, often, humor. In this video series, shot in his home shop, Kevin draws upon his vast historical references to address modern-day questions. As Kevin has written, “Emotions bring us to engineering, but engineering then becomes a special way of confronting reality.”

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