Question 29 MODE01 - Chief MODU Engineer

The Correct Answer is D. **Explanation for why Option D is correct:** In a multi-cylinder diesel engine, starting is achieved by introducing high-pressure (HP) compressed air into some of the cylinders, forcing the pistons down to initiate rotation. When high-pressure air expands rapidly (adiabatically) inside a cylinder, it causes a significant drop in temperature. This chilling effect reduces the temperature of the air remaining in that cylinder (the compression temperature) and, more importantly, cools the cylinder liner and piston crown. Since auto-ignition in a diesel engine requires a high compression temperature (typically around 500-600°C), cooling these surfaces makes it harder to reach the necessary temperature for the injected fuel to ignite. The cylinders that **do not** receive starting air maintain their normal operating temperature (or are less chilled) and therefore achieve the required high compression temperature much more quickly during the initial rotations. Consequently, these cylinders are the first ones capable of igniting the fuel, providing the power needed to sustain combustion in the remaining cylinders and fully start the engine. **Explanation for why the other options are incorrect:** * **A) operation is under higher compression:** The compression ratio and resulting peak pressure are a design characteristic of the engine and are identical across all cylinders. A cylinder without a starting air valve does not inherently operate under higher compression than one with a starting air valve, assuming both are sealed and functioning correctly. * **B) fuel is admitted only to these cylinders during cranking:** During the starting sequence, fuel is typically injected into all cylinders that are programmed to receive it, regardless of whether they have a starting air valve. The issue is not the admission of fuel, but the ability of the air in the cylinder to ignite that fuel. * **C) compression is released during starting by opening the exhaust valve:** Compression release (decompression) is a technique used in some smaller engines to reduce the torque required for cranking, but it is not typically applied to large marine engines during the direct air start sequence. Furthermore, if the exhaust valve were opened to release compression, that cylinder definitely could not fire, as there would be no hot compressed air for ignition.