Which statement describes the timing of the power stroke in a two-stroke diesel engine?

• A. The power stroke starts after the piston passes near TDC
• B. The power stroke occurs before TDC
• C. The power stroke occurs after BDC
• D. The power stroke occurs at the same time as exhaust

Answer: (A)

In a two-stroke diesel, the air is compressed as the piston moves up toward top dead center, and fuel is injected toward the end of that compression, causing combustion to heat the trapped air. The resulting high pressure pushes the piston downward during the expansion stroke, so the power stroke begins just after the piston passes or is near top dead center. If it tried to start before TDC, there wouldn’t be any combustion pressure acting to push the piston down yet. It also isn’t tied to when the piston is at bottom dead center or during exhaust timing—the expansion (power) stroke starts right after ignition near TDC and continues as the piston moves downward.