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How pistons work (part 1)


by Henri Lebarbé ,

As one of the main engine components, the piston must operate properly. First, it sucks the air-fuel mixture; then, it helps compress said mixture and, finally, it needs to withstand the sudden rise in temperature caused by the ignition. Of course, when all this is done, the piston has to expel the exhaust gases.

Conventional engines rely on "exponential combustion": once the air-fuel mixture is ignited by the spark plug, a flame wave spreads through the combustion chamber at extreme speeds. The radical temperature increase must be controlled by an external cooling system (using oil or air). The latter dissipates heat while the engine is running.

Obviously, the piston rings play different roles. For one, they transfer 60 percent of the heat from the piston to the cylinder wall. (Read more on piston rings later in this article.)

Pistons are made of aluminum and various nickel-based alloys, ensuring hardness and the strength to fulfill their mission. Depending on the application, engineers will put them to the test in a pressure chamber.

When I began my career in the motorcycle industry, at Peugeot Cycles and Motorcycles, I remember performing breaking tests on small-size pistons. It took 7 metric tons before a crack started to appear, which immediately stopped the pressure machine. I was amazed to see how much load pistons can withstand. Just think about what goes on in an engine when a piston breaks; there's an awful lot of pressure involved, especially in the case of a turbocharged engine.

In my opinion, the piston is the most active part of a motorcycle engine. It must withstand heat, friction and inertia, not to mention all those vibrations caused by the crankshaft and rods. The engine needs to be perfectly balanced to make sure no unwanted vibrations overload the pistons.

When pistons are manufactured, their weight is calculated in grams and specialists randomly select a few units out of the whole production to perform quality check-ups. At the very beginning of the manufacturing process, right after the casting, pistons are closely monitored and the slightest flaw means they will be re-cast.

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