When does the exhaust valve open during engine operation?

The exhaust valve opens during the exhaust stroke of the four-stroke engine cycle. This is the phase where the combustion gases that were created from the previous power stroke are expelled from the combustion chamber.

During the exhaust stroke, after the power stroke has taken place and the piston moves up in the cylinder, the exhaust valve opens to allow these gases to exit the engine, making room for a fresh air-fuel mixture to enter during the subsequent intake stroke. Timing is crucial in this process; the opening of the exhaust valve must be coordinated with the movement of the piston to efficiently purge the combustion chamber of exhaust gases.

Understanding this sequence is key to recognizing how internal combustion engines operate, as the timing of valve operations significantly affects engine performance and efficiency.