Question 36 3AE01 - Third Assistant Engineer

The Correct Answer is D 1. **Explanation for why option D ("individual cam for each pilot valve") is correct:** In large, slow-speed, two-stroke diesel engines, the injection of starting air into the cylinders must be precisely timed relative to the piston position and direction of rotation (Ahead or Astern). The air pilot starting system uses pilot valves to control the much larger main starting air valves. These pilot valves are operated by timing mechanisms. The two main types of mechanisms used to time and actuate the pilot valves are: * A single **ported distributor disc** (driven by the camshaft or crankshaft), which uses compressed air directed through ports to signal the pilot valves. * **Individual cams for each pilot valve**, mounted on a dedicated control shaft or the main camshaft. These cams physically lift or depress tappets/rollers to open and close the corresponding pilot valves directly, ensuring exact timing for both Ahead and Astern operation. 2. **Explanation for why each of the other options is incorrect:** * **A) regulator valve:** A regulator valve controls or reduces the pressure of the starting air supply, but it is not a mechanism used for the timing or actuation of the pilot valves themselves. * **B) quick opening main air valve:** The quick opening main air valve (or master starting valve) controls the flow of high-pressure air to the engine manifold. It is controlled by the starting system, but it does not actuate or time the smaller pilot valves. * **C) pilot air start check valve:** A check valve is a non-return valve, ensuring air flows only in one direction (usually from the pilot valve to the main air valve actuator). It is a component within the system but does not serve as the alternative timing and operating mechanism to the ported distributor disc.