Question 10 MODE01 - Chief MODU Engineer

The Correct Answer is B **Explanation of Option B (Early fuel injection):** If the fuel injection timing is too early (advanced), combustion begins while the piston is still moving significantly upwards toward Top Dead Center (TDC). Injecting and igniting the fuel earlier means that the heat release occurs in a smaller volume than intended by the engine designer. Since the engine cylinder pressure is directly related to the temperature and the inverse of the volume (according to the ideal gas law, $P \propto T/V$), reducing the volume during the critical combustion phase leads to a significant increase in the maximum pressure reached—the firing pressure ($P_{max}$). Crucially, the problem states that the **compression pressure is normal**. Compression pressure is measured just before ignition and is solely dependent on the mechanical components (piston rings, valves, liner condition, valve timing) and the resulting compression ratio. If the compression pressure is normal, the mechanical condition of the cylinder (piston rings, valves) and the compression ratio are fine, isolating the problem to the combustion event itself, which is controlled by the fuel injection timing. **Explanation of why other options are incorrect:** * **A) Worn piston rings:** Worn piston rings would allow combustion gas to leak past the piston (blowby), leading to a *decrease* in both compression pressure and maximum firing pressure. Since the compression pressure is stated to be normal, worn rings are unlikely. * **C) Late fuel injection:** Late (retarded) fuel injection means combustion occurs primarily after the piston has already started moving downward. This increases the volume during the combustion phase, leading to a *lower* maximum firing pressure and often higher exhaust temperatures and poor efficiency, opposite to the observation. * **D) High engine load:** While increased load generally results in slightly higher average firing pressures across all cylinders due to more fuel being injected, it should affect *all* cylinders equally (assuming the engine is balanced). If only **one** cylinder has a significantly higher firing pressure than the average, the cause must be specific to that cylinder's injection mechanism (like a fault in its specific fuel pump or injector control), not a generalized load increase.