Question 3 3AE01 - Third Assistant Engineer

The Correct Answer is B **Explanation for B (piston movement):** In most two-stroke cycle diesel engines (which typically employ uniflow scavenging, such as those made by Detroit Diesel or opposed-piston designs), the intake ports (also known as scavenge ports) are located in the cylinder liner. These ports are opened and closed directly by the bottom edge of the **piston** as it moves up and down. As the piston moves past the ports, it exposes them, allowing pressurized air (scavenging air) to enter the cylinder; as the piston covers the ports, the intake process is terminated and compression begins. **Why the other options are incorrect:** * **A) camshaft:** While the camshaft is crucial in a uniflow scavenged two-stroke diesel engine (like a modern EMD or Detroit Diesel engine), its primary function is to operate the **exhaust valves** (and the fuel injectors), not the intake (scavenge) ports. The intake ports are fixed openings controlled solely by the piston position. * **C) exhaust valves:** The exhaust valves are located in the cylinder head and are controlled by the camshaft. They control the release of spent gases. They have no mechanical control over the separate intake ports located in the liner wall. * **D) vertical drive:** The vertical drive is a mechanical component (usually gearing or a shaft) used to transfer power from the crankshaft up to the camshaft and governor. It is a power transmission component, not a mechanism that directly opens or closes air ports.