Question 14 MODE02 - Assistant MODU Engineer

The Correct Answer is D 1. **Explanation for why option D ("individual cam for each pilot valve") is correct:** The purpose of the pilot valves in an air pilot starting system is to accurately time the admission of high-pressure starting air into the main engine cylinders. This timing must be synchronized with the crankshaft position. There are two primary mechanical mechanisms used to achieve this synchronization: * **Ported Distributor Disc:** A rotating disc (driven by the engine camshaft or crankshaft) with ports that sequentially align with the openings leading to the pilot valves. * **Individual Cams:** A camshaft (or a separate distribution shaft) dedicated to operating the pilot valves. Each pilot valve has a dedicated cam profile that physically pushes the valve open at the precise moment starting air is required for that cylinder. This method offers very precise timing control, often preferred on larger or more modern engines. 2. **Brief explanation for why each of the other options is incorrect:** * **A) regulator valve:** A regulator valve controls or reduces air pressure to a usable level (e.g., controlling pressure for control air or auxiliary systems). It does not function as a timing device to operate the pilot valves. * **B) quick opening main air valve:** This valve (often called the Master Air Starting Valve) is the main isolation valve that admits high-pressure starting air into the starting manifold, enabling the system. It is operated by the control air signal, but it does not determine the *timing* of the individual cylinder pilot valves. * **C) pilot air start check valve:** A check valve allows flow in only one direction. The check valve is typically located downstream of the pilot valve (or integrated into the main cylinder air start valve) to prevent combustion gases from entering the starting air manifold. It does not operate the pilot valves.