Question 42 AEL01 - Assistant Engineer - Limited

The Correct Answer is C ### 1. Why Option C (10° to 20°F) is Correct A temperature rise ($\Delta T$) between the inlet and outlet jacket cooling water (JCW) of a medium or high-speed diesel engine operating at normal load is a critical indicator of the engine's thermal stability and the cooling system's efficiency. * **Optimal Engine Design:** Modern diesel engine cooling systems are designed to maintain the engine components (liners, cylinder heads) within a narrow, stable temperature range to prevent thermal stress and maximize efficiency. A typical design aims for a modest temperature differential across the engine block. * **Engineering Standard:** The standard operating range for the $\Delta T$ of jacket water cooling systems in marine and industrial medium/high-speed diesel engines is generally between **10°F to 20°F (approximately 5.5°C to 11°C)**. This range ensures adequate heat removal without excessive flow rates (which would cause high pumping energy consumption and cavitation risk) or inadequate flow rates (which would lead to overheating and high thermal gradients). * **Conversion:** $10^\circ\text{F}$ is equivalent to $\approx 5.5^\circ\text{C}$, and $20^\circ\text{F}$ is equivalent to $\approx 11^\circ\text{C}$. This range is narrow enough to maintain consistent metal temperatures, minimizing thermal fatigue and ensuring optimal running clearances. ### 2. Why Other Options Are Incorrect **A) 1° to 5°F (Too Low)** A temperature rise of only 1°F to 5°F would indicate that the cooling water flow rate is excessively high, meaning the pumps are consuming too much energy, or that the engine is operating significantly below normal load. While a very low $\Delta T$ ensures maximum cooling, it is inefficient and not representative of normal operation under full load. **B) 5° to 10°F (Suboptimal/Slightly Low)** While close to the acceptable lower boundary, this range is typically considered slightly too low for optimal thermal efficiency and is generally less representative of the designed $\Delta T$ for normal load operation. Many engine manufacturers target the 10°F minimum to optimize heat recovery and combustion efficiency. **D) 50° to 100°F (Dangerously High)** A temperature rise of 50°F to 100°F would indicate a severe restriction in cooling water flow or a gross failure of the cooling system (e.g., failed pump, severely fouled heat exchanger). This level of $\Delta T$ would result in extreme thermal gradients across the cylinder head and liner, leading rapidly to localized boiling, thermal stress, warping, cracking of components, and potential engine seizure or shutdown.