How Does A Four-Stroke Internal Combustion Engine Work?

If combustion were much slower than it is, engines such as this single-cylinder example wouldn’t be able to operate at peak rpm. But thanks to turbulence, which spreads the flame rapidly in the combustion chamber, they can. There’s a lot of science to the shaping of ports and valves to allow maximum intake and exhaust flow to create this turbulence.

In this case, air flows down the intake pipe into the cylinder head. When the valve lifts that air flows around the valve and into the combustion chamber to follow the retreating piston. Intake air with fuel mixed into it by the carburetor comes out from under the valve at several hundred feet per second. This is essential for rapid combustion.

By itself, a mixture of evaporated gasoline and air burns quite slowly—a matter of a foot or so per second. But the fact that mixture comes out from under the intake valve at such great velocity puts a lot of energy into the air accumulating as the piston continues to complete its intake stroke.

When the piston comes back up and compresses that gas all the turbulent motion is confined to an ever-smaller space causing it to whirl faster. The spark plug produces a tiny flame kernel and the vigorous motion tears that kernel apart, shreds it, and distributes it around the combustion chamber, causing rapid combustion.

The velocity of the combustion process is of the order of 50 feet per second but it can be as high as 200 feet per second. Normally in an engine like this ignition spark occurs at about 36 degrees before top dead center. The combustion initiated by the spark continues for roughly 30 degrees after TDC.

The value of fast combustion is real because the longer you have flame in this chamber the more heat is being driven into the cylinder head and into the crown of the piston from which it is lost—it’s not there anymore to push the crankshaft around. So designers take care to make the combustion process pretty much as fast as they can.

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.”