How many times does any one piston change direction during one complete diesel cycle?

In a complete diesel cycle, each piston undergoes specific movements that correspond with the four main strokes: intake, compression, power, and exhaust. During this cycle, the piston moves downwards during the intake stroke and then moves back up during the compression stroke. After reaching the top of the compression stroke, the piston's direction changes again as it moves down during the power stroke. Finally, during the exhaust stroke, the piston moves back upwards to expel the exhaust gases.

In total, the piston changes direction four times throughout this entire process. This includes the initial downward movement during the intake, upward movement during compression, downward movement during power generation, and lastly, the upward movement during exhaust. This explains why the correct answer is four times, accurately reflecting the complete diesel cycle’s progression through its four strokes.