Engine Construction & Installation Practice Test 2025 - Free Practice Questions and Study Guide

The efficiency of a piston ring in sealing cylinder pressure is most critical during the power stroke. During this stroke, the combustion of the air-fuel mixture occurs, generating the highest pressure within the combustion chamber. The piston travels down the cylinder under the force of expanding gases, which creates significant pressure that the piston rings must contain to prevent any leakage past them.

The sealing capability of the piston rings is essential during the power stroke because it directly impacts the engine's performance and efficiency. If the seals were to fail or not function optimally, pressure would escape, leading to a loss of power and reduced engine efficiency. Maintaining proper sealing during this phase ensures maximum energy transfer from the expanding gases to the piston, translating into effective work output from the engine.

In contrast, during the intake stroke, the pressure is lower as air is drawn into the cylinder, while in the compression stroke, the piston compresses the air-fuel mixture, and although sealing is vital, the pressure is lower than during the power stroke. The exhaust stroke, meanwhile, typically involves expelling spent gases and has significantly lower pressure compared to the power stroke. Therefore, the piston rings’ sealing ability is most crucial when the engine is producing its power.