Question 39 AEL01 - Assistant Engineer - Limited

The Correct Answer is A. ### Explanation for Option A (Correct) A supercharged diesel engine uses a mechanical compressor (the supercharger) driven directly by the engine's crankshaft to force a higher mass of air into the cylinders than could be achieved by natural aspiration alone. This process is called "boosting." Consequently, the air pressure within the intake manifold and, subsequently, the cylinder air charge pressure at the start of the compression stroke is significantly **higher** than in a naturally aspirated engine. This higher pressure and density of air allows more fuel to be efficiently burned, leading to greater power output. ### Explanation for Other Options (Incorrect) **B) reduced cylinder mean effective pressure:** This is incorrect. Mean Effective Pressure (MEP) is a theoretical pressure that, if applied uniformly throughout the power stroke, would produce the same net work output as the actual cycle. Since the supercharger forces more air into the cylinder, allowing for the combustion of more fuel and the production of significantly more power, the **Mean Effective Pressure (MEP) is actually increased**, not reduced. **C) less valve overlap:** This is generally incorrect and often the reverse is true. Valve overlap (the period where both intake and exhaust valves are open simultaneously) is often **increased** in high-performance or boosted engines. In turbocharged and supercharged engines, designers may increase overlap to use the pressurized intake air to help "scavenge" (push out) the remaining exhaust gases more effectively before the compression stroke begins. The fundamental reason for supercharging (forcing air in) is independent of the mechanical requirement for less valve overlap. **D) reduced blow-by:** This is incorrect. "Blow-by" refers to the leakage of combustion gases past the piston rings into the crankcase. Blow-by is primarily dependent on factors like cylinder pressure (which is higher in a supercharged engine), engine condition, piston ring design, and wear. Since a supercharged engine operates with significantly higher peak combustion pressures, the tendency for blow-by is generally **increased**, not reduced, unless special ring packages are employed to counteract this effect.