Question 38 QMED01 - Junior Engineer

The Correct Answer is C ### Explanation for Option C (Power Stroke) The starting air is admitted to the cylinder at the beginning of the **power stroke** because this is the point in the cycle where the piston is positioned at Top Dead Center (TDC) and is scheduled to move downward. 1. **Timing and Function:** The purpose of starting air is to mechanically rotate the crankshaft by pushing the piston down, mimicking the action of combustion. 2. **Position:** The power stroke begins immediately after the compression stroke (when the piston is at TDC). By admitting high-pressure air at this exact moment, the air expands, forcefully drives the piston down, and generates the necessary rotational torque to turn the engine over. ### Why the Other Options Are Incorrect * **A) intake stroke:** The intake stroke is the piston's first downward movement, drawing in fresh air. Admitting starting air here would be inefficient, as the piston is just starting a long cycle and the pressure push would not optimally align with the natural timing needed for the engine to fire subsequently. * **B) compression stroke:** The compression stroke is an upward movement of the piston (moving from Bottom Dead Center to Top Dead Center). Admitting starting air would fight the piston's direction of travel, preventing the engine from rotating. * **D) exhaust stroke:** The exhaust stroke is an upward movement of the piston, expelling burnt gases. Admitting starting air would fight the piston's upward movement and potentially damage components or exit prematurely through the open exhaust valve.