Question 16 3AE01 - Third Assistant Engineer

The Correct Answer is B. ### Why Option B is Correct The camshaft in a diesel engine (or any four-stroke engine) is a crucial mechanical component designed to precisely actuate the intake and exhaust valves, and often the fuel injection pump plungers (especially in older or large medium-speed diesels). 1. **Valve Timing:** The angular position (or phase) of the cam lobe relative to the crankshaft determines *when* the valve opens and *when* it closes during the piston stroke—this is valve timing (e.g., ignition timing, duration). 2. **Valve Lift:** The physical shape and size (specifically, the distance between the base circle and the nose of the lobe) determine *how far* the valve opens—this is valve lift. 3. **Firing Order:** The specific sequential arrangement of the cams (e.g., injection pump cams or exhaust/intake cams) along the length of the shaft dictates the order in which cylinders receive their charge or fuel and thus the sequence in which they fire. The camshaft is responsible for keeping the valve events coordinated according to the engine's designed firing order (e.g., 1-5-3-6-2-4). The arrangement and shape of the cams are the direct mechanical control mechanism for these three specific characteristics. ### Why Other Options Are Incorrect **A) Speed, torque, and horsepower production:** These are performance outputs (results). While camshaft design drastically *influences* these factors, they are not *directly controlled* by the fixed shape of the cams alone. Speed is controlled by fuel rack position and load; torque and horsepower are derived metrics of the engine's performance across its operating range. **C) Fuel consumption, efficiency, and cylinder pressure:** These are outcomes related to the combustion process and thermal performance. While optimal valve timing (controlled by cams) is necessary for high efficiency and pressure, fuel consumption and efficiency are also heavily influenced by injector design, fuel quality, turbocharging, and engine load. The cam shape only *sets the stage* for the mechanical process, not the thermodynamic results. **D) Scavenge pressure, compression ratio, and exhaust pressure:** * **Compression Ratio:** This is a fixed geometric ratio determined by the piston design, connecting rod length, and cylinder head volume. It is not influenced by the cam shape. * **Scavenge/Exhaust Pressure:** These pressures are outputs largely controlled by the turbocharger (or blower) system, engine load, and exhaust manifold restrictions, not the fixed profile of the cam lobes.