Question 38 AEL01 - Assistant Engineer - Limited

The Correct Answer is C ### Explanation of Correct Option (C) **Option C: decrease in fuel consumption per horsepower-hour** An increase in the air inlet manifold pressure of a diesel engine (often achieved via turbocharging or supercharging) is known as **boost pressure**. This practice increases the density and mass of the air charged into the cylinders (a process called scavenging or boosting). 1. **Improved Combustion:** More air allows for a higher air-fuel ratio, resulting in more complete and efficient combustion of the injected fuel. 2. **Higher Power Density:** Because the engine can burn more fuel efficiently, it produces more power for the same size/displacement. 3. **Increased Thermal Efficiency:** By increasing the compression pressure and allowing for a higher expansion ratio (in relation to the amount of fuel burned), the overall thermodynamic cycle efficiency improves. 4. **Specific Fuel Consumption (SFC):** SFC is the measure of fuel consumed per unit of power produced (e.g., grams per kilowatt-hour, or pounds per horsepower-hour). Since the engine is producing more power more efficiently, the fuel consumed per unit of power output decreases. Therefore, there is a decrease in fuel consumption per horsepower-hour. ### Explanation of Incorrect Options **Option A: decrease in maximum cylinder pressure** This is incorrect. Increasing the air manifold pressure leads to a higher initial pressure at the start of the compression stroke. Consequently, the pressure at the end of the compression stroke and the **maximum pressure during combustion** (peak cylinder pressure) both significantly **increase**. **Option B: increase in ignition lag** This is incorrect. Ignition lag (or delay) is the time between the start of fuel injection and the start of combustion. It is primarily influenced by the pressure and temperature inside the cylinder at the time of injection. Increasing the air manifold pressure (boosting) substantially **increases** the compression pressure and temperature. Higher temperature and pressure promote faster fuel evaporation and chemical reaction, thus **decreasing** the ignition lag (or ignition delay). **Option D: decrease in exhaust manifold pressure** This is generally incorrect. For the increased power produced due to boosting, more mass (air and fuel products) is flowing through the engine. While the turbocharger (which is driven by exhaust gases) extracts energy from the flow, the overall mass flow rate is higher, and the resulting gas volumes are expanded. Therefore, the exhaust manifold pressure typically **increases** (or remains high), as the exhaust system must handle a greater flow of high-energy gases corresponding to the increased power output.