On which of the four strokes do the piston rings provide the maximum amount of seal against the cylinder wall?

The maximum seal provided by the piston rings against the cylinder wall occurs during the power stroke. This is the phase where the piston is forced down by the explosive force generated from the combustion of the air-fuel mixture.

During this stroke, the pressure inside the combustion chamber is at its highest, creating significant upward force on the piston. The design and function of piston rings become critical here as they need to ensure a tight fit against the cylinder walls to contain this high pressure and prevent any loss of combustion gases, which ensures efficiency in converting fuel energy into mechanical energy. A good seal also helps to maintain proper compression ratios and improves engine performance.

In contrast, during the intake, compression, and exhaust strokes, the pressure variations are not as high as during the power stroke, and there is less of a demand on the piston rings to maintain a perfect seal. This is why the power stroke is the moment when the piston rings achieve their greatest sealing efficiency.