Question 13 MODE02 - Assistant MODU Engineer

The Correct Answer is B **Explanation of Correct Option (B):** Starting air check valves (or non-return valves) are critical components in the starting air system of internal combustion engines, particularly large marine diesel engines. Their primary function is to allow high-pressure starting air into the cylinder during the starting sequence and, critically, to prevent the hot, high-pressure combustion gases within the cylinder from flowing back into the starting air line and manifold. To ensure the valve closes immediately and securely after the starting air pressure drops and before combustion occurs, it must be held firmly on its seat. This force is typically provided by a robust **spring force** acting directly on the valve stem or differential piston, holding the valve closed whenever the starting air pressure is equal to or less than the pressure within the cylinder (or zero when the engine is stopped). **Explanation of Incorrect Options:** * **A) cam rollers on the camshaft:** Cam rollers are used for operating timed valves (like inlet, exhaust, and fuel injection valves) which must open and close precisely relative to the piston position. Starting air check valves are typically automatic (pressure-actuated) non-return valves, not mechanically timed by the camshaft. * **C) air pressure on top of the valve differential piston:** The starting air pressure must act *underneath* the differential piston/valve head to lift the valve off its seat, allowing air *into* the cylinder. If air pressure acted on top, it would actively help hold the valve closed, preventing the engine from starting. While external control air sometimes assists in closing pilot valves, the primary holding force for the main check valve seat is the spring force. * **D) air pressure on the bottom of the valve differential piston:** Air pressure on the bottom of the differential piston is what *opens* the valve against the spring force during the starting process. When the starting air stops, this pressure disappears, allowing the spring force (and the high cylinder compression/combustion pressure) to immediately snap the valve shut. If this pressure were present continuously, the valve would remain open, which would be extremely dangerous.