Which function does an engine compression brake perform during the compression stroke?

The engine compression brake performs a crucial function during the compression stroke by opening the exhaust valve. This process allows the engine to utilize its own compression to slow down the vehicle, effectively transforming the engine into a brake. When the exhaust valve is opened during the compression stroke, it releases the compressed air rather than allowing it to be used for power generation in the next power stroke.

This braking mechanism is particularly useful for large vehicles, such as trucks and coaches, as it helps to manage speed on downhill grades, reducing reliance on the service brakes, which can heat up and become less effective over time. By allowing the compressed air to escape, it reduces power loss and creates a braking effect that enhances overall control and safety while driving.

In contrast, options that mention closing the intake or exhaust valves during the power stroke don't align with the function of an engine compression brake, as they pertain to standard valve operations rather than the specific actions involved in employing compression for braking.