Question 34 3AE01 - Third Assistant Engineer

The Correct Answer is D **Explanation of Option D (heating the engine coolant) - Correct:** Heating the engine coolant (preheating the engine) is the most effective method for easing the start of a large, low-speed diesel engine in cold weather. When the engine structure and cylinders are cold, the compression temperature is significantly lowered due to heat loss to the cold cylinder walls. This low temperature prevents the atomized diesel fuel from reaching the auto-ignition point (the point where the fuel spontaneously combusts). By heating the coolant, the temperature of the cylinder liners, pistons, and cylinder heads is raised, which in turn: 1. **Increases Compression Temperature:** Reduces heat loss during the compression stroke, allowing the air to reach a temperature necessary for reliable ignition. 2. **Reduces Viscosity:** Slightly reduces the viscosity of the lube oil (though not the primary goal, it helps), easing rotation by the starting air. 3. **Ensures Reliable Ignition:** Guarantees successful and smooth combustion immediately upon injection, which is the primary challenge during a cold start. **Explanation of Incorrect Options:** **A) increasing the quantity of starting air:** While adequate starting air pressure is essential to turn the engine, merely increasing the *quantity* (volume/reservoir capacity) does not directly address the fundamental problem of poor ignition caused by low compression temperatures. If the air pressure is sufficient to turn the engine, increasing the total stored volume will not make the cold start *easier* or guarantee ignition. **B) increasing the lube oil pressure:** Lube oil pressure must be established *before* starting to ensure bearing protection, but increasing the pressure beyond the required operating specification offers no benefit for cold starting. High viscosity in cold weather makes it difficult to maintain normal pressure, but changing the pressure setting itself does not improve the ignition conditions in the cylinder. **C) heating the engine fuel supply:** Heating the diesel fuel slightly improves atomization and lowers viscosity, which is marginally beneficial. However, in a large, low-speed engine, the overwhelming factor determining a successful start is the temperature of the air *inside* the cylinder at the point of injection (compression temperature). Heating the fuel supply is a secondary measure; it cannot compensate for excessively low cylinder wall temperatures that quench the combustion process. Engine preheating via the coolant (D) addresses the root cause directly.