Under which of the following conditions would a piston ring seal more effectively?

A piston ring seals more effectively under high cylinder pressures due to the increased force applied against the cylinder wall. When cylinder pressure is high, the rings are pushed outward against the wall of the cylinder with greater force, enhancing the sealing capability and minimizing the likelihood of combustion gases escaping into the crankcase. This improved seal helps maintain effective combustion and engine efficiency.

At low cylinder pressures, however, the rings may not press against the cylinder wall with sufficient force, potentially leading to gas blow-by and reduced engine performance. High engine temperatures can also affect the materials of the piston rings and cylinder walls, sometimes leading to thermal expansion that doesn't favor optimal sealing. Conversely, low engine temperatures can cause the oil to thicken, impairing the flow and lubrication properties necessary for maintaining an effective seal. Therefore, the combination of high cylinder pressures is crucial for achieving optimal sealing performance in piston rings.