Question 45 QMED01 - Junior Engineer

The Correct Answer is A **Explanation for Option A (misfiring):** A leaking (or 'burnt') exhaust valve in a diesel engine compromises the integrity of the combustion chamber seal, particularly during the compression stroke. When the valve leaks, high-pressure hot combustion gases escape past the valve seat into the exhaust manifold (a phenomenon known as 'blow-by'). Critically, during the compression stroke, the required compression pressure and temperature are not achieved because air leaks out. Since the temperature must be high enough to reliably ignite the injected fuel (the principle of compression ignition), insufficient compression leads to the failure of ignition, which is defined as **misfiring**. **Why the other options are incorrect:** **B) pre-ignition:** Pre-ignition is the ignition of the fuel/air mixture *before* the intended spark timing (in a spark-ignition engine) or *before* the fuel is injected (in a diesel engine). It is typically caused by hot spots (like glowing carbon deposits) igniting the charge prematurely. While a very hot, leaking valve could potentially contribute to hot spots, the primary and immediate effect of the leak is the *loss of compression*, leading to *failure* of ignition (misfiring), not premature ignition. Pre-ignition is fundamentally a problem of timing in the combustion cycle, whereas valve leakage is a problem of compression integrity. **C) interrupted scavenging:** Scavenging is the process of pushing burnt exhaust gases out of the cylinder using incoming fresh air (common in two-stroke engines or during valve overlap in four-stroke engines). If an exhaust valve leaks, the timing of the gas flow is maintained, but the pressure and volume of the gas being pushed out might be slightly affected. However, the fundamental *process* is not interrupted; rather, the volumetric efficiency might be reduced. This term is imprecise and not the most direct or serious consequence. **D) reduced scavenging:** Reduced scavenging means less efficient removal of exhaust gases. While a leaking exhaust valve *could* potentially lead to reduced scavenging efficiency (as the pressure might be slightly off during the overlap period or the backflow might interfere), this effect is secondary. Furthermore, reduced scavenging typically manifests as reduced power and higher exhaust temperatures, but it is not the immediate, primary mechanical symptom of a leaking valve, which is the immediate loss of compression leading directly to **misfiring** during the power stroke.