Question 17 MODE02 - Assistant MODU Engineer

The Correct Answer is D. ### Why Option D ("designed engine speeds are decreased") is correct: The ignition quality of diesel fuel, typically measured by the **cetane number**, is a measure of how quickly and effectively the fuel ignites under compression. When the designed engine speed is decreased (i.e., the engine runs slower), the piston takes a longer duration to travel from the point of fuel injection to Top Dead Center (TDC). This increased time allows the injected fuel droplets more time to heat up, vaporize, and mix adequately with the air before ignition occurs. This longer "ignition delay period" is more forgiving to fuels with lower cetane numbers (poorer ignition quality). Therefore, as engine speeds decrease, the requirement for exceptionally high ignition quality in the fuel becomes less critical. Slow-speed diesel engines (like large marine diesels) can typically tolerate much lower cetane number fuels compared to high-speed automotive engines. ### Why the other options are incorrect: **A) lube oil additives are increased:** Lube oil additives primarily affect engine wear, corrosion, and lubrication performance. They have no direct or significant impact on the chemical combustion properties (ignition quality) of the separate diesel fuel itself. **B) designed piston speeds are increased:** Increasing the designed piston speeds (or engine RPM) **decreases** the time available for ignition delay. A shorter delay time demands that the fuel ignites almost instantly upon injection. This requires a fuel with **higher** ignition quality (higher cetane number), making the ignition quality **more** critical, not less. **C) designed injection pressures are decreased:** Decreasing the injection pressure generally leads to poorer atomization (larger fuel droplets) and reduced penetration of the fuel spray. Poor atomization hinders vaporization and mixing, which in turn *increases* the ignition delay period. To compensate for this longer delay and ensure stable combustion, the fuel would need *better* ignition quality. Therefore, decreasing injection pressure makes ignition quality **more** critical, not less.