Question 17 MODE01 - Chief MODU Engineer

The Correct Answer is C ### Why Option C (spring force) is Correct The intake and exhaust valves in an internal combustion engine (including a diesel engine) are opened mechanically, typically by a camshaft acting through lifters and rocker arms. However, to ensure the valve closes quickly, positively, and remains sealed against the valve seat when required, a **valve spring** is used. This spring is compressed when the valve is opened, and its stored mechanical energy provides the necessary force to push the valve back into firm contact with its seat, overcoming inertia and ensuring proper sealing against the high cylinder pressures. ### Why Other Options Are Incorrect **A) push rod pressure:** The push rod (or rocker arm) is responsible for *opening* the valve by applying downward force against the spring tension. It does not provide the force to return the valve to its closed position; in fact, the valve spring must overcome the mechanism's inertia as the cam lobe rotates away. **B) combustion pressure:** Combustion pressure occurs inside the cylinder and acts primarily on the face of the valve. While this pressure helps keep the valve seated *when it is closed*, it is not the mechanism that *returns* the valve to its seat. Relying solely on combustion pressure would result in the valve failing to close promptly during the exhaust or intake stroke when cylinder pressures are low or negative. **D) exhaust pressure:** Exhaust pressure is typically used to help scavenge the cylinder, but it is not the dedicated closing mechanism for the valve. Similar to combustion pressure, exhaust pressure varies greatly and cannot provide the reliable, constant closing force needed across all operating conditions (especially at low engine speeds or during the intake stroke).