What is the purpose of the deep cutouts in the piston crowns in a modern sportbike engine? How close the piston comes to the cylinder head at top dead center has an important impact on combustion-chamber turbulence, as explained in this video by Cycle World Technical Editor Kevin Cameron.
I want to talk about the real heart of the matter, the combustion chamber, the place where the chemical energy of the fuel is turned into heat to generate the pressure that pushes the pistons downward to spin the crankshaft.
Here is a piston and a connecting rod. You’ll see the crown of the piston has a complicated shape. It’s made this way to provide clearance for the four valves as the piston comes close to the cylinder head at top dead center. The piston cannot touch the cylinder head or it would pound itself to pieces. But exactly where it stops has an important influence on the outcome.
When the piston performs its intake stroke, the intake valves open, the piston moves away from the combustion chamber, and atmospheric pressure pushes fresh fuel-air mixture into the chamber. At high power, when the engine is really spinning over, intake air velocity is hundreds of feet per second. So there is really quite a lot of turbulence.
That turbulence turns out to be quite important to fast combustion. Why do we care how fast the combustion is? Because the longer combustion takes, the longer the cylinder head and piston crown are exposed to high temperature, the more heat is lost from the combustion gas, the more its pressure drops, and the less power the engine makes.
We want quick combustion quickly completed so the high-pressure gas that is generated can push the piston promptly on its way. The clearance spaces for the valves exist because, in order to get the valves open fully when we need them to be open, the intakes have to already be opening before top dead center and the suction stroke. That is why the deep cutouts in the piston crown are necessary.
As the piston is compressing the fuel-air mixture into a smaller and smaller space, the original hundreds of feet per second of intake velocity dies away from friction so the motion of the fresh charge slows down. To speed up that motion one last instant before the spark, we have what are called squish areas.
The flat surfaces in the piston crown come quite close to the four surfaces in the combustion chamber at top dead center, and the mixture trapped between them is squished out of those spaces and shoots in toward the center of the chamber, giving it a last-moment squirt of extra turbulence.
In a race engine, squish-area clearances would be carefully controlled down to a very small number, so small that if it were made 0.005 closer, you would see bright areas indicating some contact. We don’t want any contact. But in a production engine, we can’t bring things so close together because we can’t guarantee that precise an assembly.
How close the piston comes to the head at top dead center depends on the length of the connecting rod, the length of the crankshaft stroke, the height of the cylinder, and the height of the piston above the wrist pin. We don’t want tolerance stack-up, where an unfortuitous series of errors adds up to piston-to-cylinder-head contact.
So a production engine has all the features of a race engine but it isn’t pushed to the nth degree. A central spark plug and these valve-clearance pockets means this is a very compact, fast-burning combustion chamber, which is just what a sportbike engine needs.
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