Question 5 DDE04 - Designated Duty Engineer - 1000-4000 HP

The Correct Answer is C ### Explanation of Option C (Correct) Option C is correct because the simultaneous opening of both intake and exhaust valves—a condition known as **valve overlap**—occurs around Top Dead Center (TDC) as the engine transitions from the exhaust stroke to the intake stroke. This overlap is engineered into the camshaft timing of four-stroke engines, particularly diesel engines, for two main reasons: 1. **Scavenging:** It allows the outgoing inertia of the exhaust gases to help draw in the fresh incoming air charge, thoroughly clearing the cylinder of residual burned gases. 2. **Volumetric Efficiency:** By opening the intake valve slightly early, the engine maximizes the amount of air available for the subsequent compression and combustion phase, improving efficiency and power. ### Explanation of Incorrect Options **A) When the piston is at bottom dead center (BDC) transitioning from the power stroke to the exhaust stroke.** * At this point, the exhaust valve is either just beginning to open (or has opened significantly before BDC to release pressure), and the intake valve is still fully closed, having been closed since the compression stroke began. There is no valve overlap at this BDC position. **B) When the piston is at top dead center (TDC) transitioning from the compression stroke to the power stroke.** * Both the intake and exhaust valves must be tightly closed during the compression stroke and the beginning of the power stroke. If either valve were open here, the engine would lose compression, fail to ignite fuel effectively, and suffer severe power loss. **D) When the piston is at bottom dead center (BDC) transitioning from the intake stroke to the compression stroke.** * At this BDC position, the cylinder has just finished drawing in air. The intake valve is closing, and the exhaust valve is fully closed (and has been since the valve overlap point near the previous TDC). Both valves are definitely not open simultaneously.