The compression stroke of a four-cycle diesel engine begins when ____________.

The compression stroke of a four-cycle diesel engine begins when the intake valve closes. This is a crucial phase in the engine cycle, as it marks the transition from the intake stroke to the compression stroke. During the intake stroke, the engine draws in a mixture of air (since diesel engines typically use air alone for combustion, we focus on air intake) and this occurs while the intake valve is open.

Once the piston reaches the bottom of the cylinder, the intake valve closes, sealing the chamber. At this point, as the piston begins to move back up, it compresses the air in the cylinder, raising both the temperature and pressure significantly. This compressed air will later ignite the diesel fuel that is injected into the cylinder, thus allowing for the power stroke to occur.

Understanding this sequence of events is essential, as it highlights the importance of valve timing and the role of the compression stroke in the overall efficiency and performance of a diesel engine.