What does the opening of the exhaust valve during the fourth piston stroke achieve?

The opening of the exhaust valve during the fourth piston stroke is crucial for removing the byproducts of combustion from the combustion chamber. As the piston moves upwards during this stroke, the exhaust valve opens to release the burned gases created during the previous combustion cycle. This process is essential for maintaining engine efficiency, as it ensures that fresh air-fuel mixture can be drawn into the cylinder for the next cycle, without the interference of residual exhaust gases that could hinder performance.

Effective exhaust gas removal is key in maintaining optimal engine operation and preventing issues such as incomplete combustion or reduced power output. Thus, the correct answer highlights the importance of exhaust valve operation in the overall functioning of an engine.