Question 2 OSE02 - Assistant Engineer - OSV

The Correct Answer is C **Explanation for Option C (Correct):** In a diesel engine (which operates on compression ignition, unlike a gasoline engine which uses spark ignition), the ignition process is driven by the heat generated during the compression stroke. Air is drawn into the cylinder and then compressed significantly. This high compression ratio (typically 14:1 to 25:1) raises the air temperature within the cylinder far above the auto-ignition temperature of the diesel fuel. Fuel is then injected as a fine spray just before or very close to the point where the piston reaches Top Dead Center (TDC), which marks the **completion of the compression event**. Because the air is already intensely hot, the injected fuel spontaneously ignites almost immediately upon mixing with the superheated air. Therefore, ignition actually begins near the completion of the compression event. **Why Other Options Are Incorrect:** * **A) Near the completion of the expansion (power) event:** The expansion (power) event occurs *after* ignition and combustion have already taken place. Near its completion, the exhaust valve/port opens, and the pressure is dropping as the burnt gases are about to be expelled. Ignition cannot occur here. * **B) At the beginning of the compression event:** At the beginning of compression, the cylinder volume is at its maximum, and the pressure and temperature are low (close to ambient conditions). The air is not hot enough to cause spontaneous ignition of diesel fuel at this point. * **D) Near the completion of the scavenging event:** Scavenging refers to the process in some engines (especially two-stroke diesels) where exhaust gases are pushed out of the cylinder by incoming fresh air. This event occurs *before* the compression stroke begins (or overlaps slightly with the start of compression). The temperature is still relatively low, making ignition impossible.