Which of the following is NOT one of the strokes in a 4-cycle engine?

In a 4-cycle engine, the process consists of four distinct strokes: the intake stroke, the compression stroke, the power stroke, and the exhaust stroke. Each of these strokes plays a crucial role in the functioning of the engine.

The intake stroke involves drawing in the air-fuel mixture into the cylinder, which is then compressed in the compression stroke. After this, the power stroke occurs, during which the ignition of the fuel-air mixture produces a force that drives the piston down. Finally, the exhaust stroke expels the burnt gases from the cylinder.

The term "vacuum" does not refer to any stroke in the 4-cycle engine cycle. While the term may relate to the vacuum created in the cylinder during certain processes, it is not a recognized stroke like the others mentioned. Since "vacuum" does not accurately describe part of the operational cycle of a 4-cycle engine, it is not one of the strokes involved in this type of engine.