Question 29 QMED01 - Junior Engineer

The Correct Answer is A. ### Why Option A is Correct A naturally aspirated diesel engine relies solely on the pressure difference created by the downward movement of the piston during the intake stroke to draw air into the cylinders. This intake pressure is typically slightly below atmospheric pressure. A **supercharged** diesel engine (or a turbocharged engine, often grouped similarly in this context) uses a mechanical compressor (supercharger) driven by the engine, or an exhaust-gas-driven turbine (turbocharger), to actively compress the incoming air *before* it enters the cylinder. This process, known as boosting, forces a greater mass of air into the cylinder at a pressure significantly **higher than atmospheric pressure**. Therefore, compared to a naturally aspirated engine, a supercharged diesel engine has **a cylinder air charge of higher pressure**. This higher pressure (and resulting higher density) allows more fuel to be burned, leading to higher power output. ### Why Other Options Are Incorrect **B) reduced cylinder mean effective pressure:** The mean effective pressure (MEP) is the theoretical constant pressure that would generate the same net work per cycle as the actual varying cylinder pressure. Because supercharging forces more air into the cylinder, allowing more fuel to be burned, it dramatically increases the power output and thus the average pressure exerted on the piston during the power stroke. Therefore, a supercharged engine has a significantly **increased**, not reduced, cylinder mean effective pressure. **C) less valve overlap:** Valve overlap is the duration (measured in crankshaft degrees) during which both the intake and exhaust valves are simultaneously open. In high-performance or boosted engines, valve timing is often specifically optimized to take advantage of the forced induction. For a supercharged engine, engineers often design the system with **increased** valve overlap to improve scavenging (using the incoming high-pressure charge to help push remaining exhaust gases out). While timing varies, supercharging itself does not inherently cause *less* valve overlap; in fact, the need for good scavenging often leads to specialized or even greater overlap. **D) reduced blow-by:** Blow-by is the leakage of combustion gases from the cylinder, past the piston rings, into the crankcase. Blow-by is primarily influenced by the peak cylinder combustion pressure, the quality and fit of the piston rings, and the engine wear. Since supercharging significantly **increases** the peak combustion pressure within the cylinder (because more air and fuel are burned), it tends to **increase** (or at least not reduce) the forces driving blow-by. Therefore, supercharging does not inherently reduce blow-by.