Question 4 3AE01 - Third Assistant Engineer

The Correct Answer is D **Explanation for Option D (compression stroke):** The timing of fuel injection is critical for efficient combustion in a four-stroke cycle diesel engine. Fuel oil injection must occur when the air inside the cylinder is at its highest temperature and pressure, ensuring auto-ignition (combustion) begins immediately upon injection. This condition is reached near the end of the **compression stroke**, just before the piston reaches Top Dead Center (TDC). Injecting the fuel during this phase allows the combustion process to start right at or shortly after TDC, maximizing the force exerted on the piston during the subsequent power stroke. **Why the other options are incorrect:** * **A) intake stroke:** During the intake stroke, the cylinder is filling with fresh air (or air/fuel mixture in gasoline engines). The pressure and temperature are low, making auto-ignition impossible. * **B) exhaust stroke:** During the exhaust stroke, the piston is pushing burned gases out of the cylinder. Injecting fuel at this point would waste fuel as it would be immediately expelled through the exhaust valve. * **C) power stroke:** The power stroke immediately follows injection and combustion. If injection started during the power stroke, the engine would have already completed most of its expansion, resulting in very low efficiency and power output.