The piston’s function is to act as a moveable plug in the cylinder, forming the bottom part of the combustion chamber.
There’s a gas-tight seal between the piston and the cylinder wall – so the only way for the hot combustion gases to expand is by forcing the piston down.
Over 60% of the friction inside the engine comes from the motion of the piston assembly, and so this is a major area of focus for improving the efficiency of engines.
The piston is still seeing ongoing development and research as we’ll see in more detail shortly.
Huge forces are generated during the change in direction of a piston on its up and down motion.
A lighter piston assembly has less momentum, thus exerting less force and allowing higher RPM engines.
This means that there’s a constant push to reduce the weight of the connecting rod and piston.
The piston is connected to the crankshaft through a connecting rod , often shortened to rod or conrod .
These parts together are known as the piston assembly .
Both ends of the connecting rod are free to pivot: The part of the connecting rod which connects to the piston is called the small end , and the end that attaches around the crankshaft is called the big end.
The big end will have bearing inserts which minimise friction and maintain an exact oil clearance with the rod journal on the crankshaft. The connecting rod is split into two – with a rod cap being used to clamp around the big end bearing and crankshaft.
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