Question 26 MODE02 - Assistant MODU Engineer

The Correct Answer is C ### Explanation for Option C (Correct Answer) An increase in the air inlet manifold pressure (often achieved by turbocharging or supercharging) means more air mass is packed into the cylinders during the intake stroke. This results in: 1. **Higher Air-Fuel Ratio (A/F) at a given load:** More oxygen is available to burn the fuel completely. 2. **Improved Combustion Efficiency:** The increased density leads to better mixing and more complete oxidation of the fuel, reducing unburnt hydrocarbons. 3. **Higher Thermal Efficiency:** Because the combustion is more complete and efficient, a greater percentage of the chemical energy in the fuel is converted into useful work. 4. **Lower Specific Fuel Consumption (SFC):** Specific fuel consumption is the measure of fuel used per unit of power output (e.g., horsepower-hour). Since the engine is producing more power or the same power using the fuel more efficiently, the amount of fuel required per horsepower-hour (or brake horsepower-hour, BSFC) **decreases**. --- ### Explanations for Incorrect Options **A) decrease in maximum cylinder pressure** **Incorrect.** Increasing the mass of air in the cylinder leads to a higher effective compression ratio (as the compression pressure is higher) and provides more oxygen for complete combustion. This typically results in a **higher** (not lower) maximum firing pressure ($\text{P}_{\text{max}}$) for a given fuel injection setting, especially when operating under load. **B) increase in ignition lag** **Incorrect.** Ignition lag (or delay) is the time between the start of injection and the start of combustion. It is primarily influenced by temperature and pressure at the end of the compression stroke, and the physical/chemical properties of the fuel. Increasing the inlet manifold pressure increases the density and, critically, the **temperature** and **pressure** at the start of combustion. Higher temperature and pressure tend to **decrease** (shorten) the ignition lag, promoting quicker and more stable ignition. **D) decrease in exhaust manifold pressure** **Incorrect.** In a turbocharged engine (the most common source of increased inlet manifold pressure), the turbocharger turbine is driven by the exhaust gas energy. To drive the compressor and increase the inlet pressure, the turbine creates a restriction, which necessarily leads to a **higher** (not lower) pressure upstream in the exhaust manifold (back pressure). Even in a naturally aspirated engine operating at higher load due to denser air, the increased mass flow rate of exhaust gases tends to increase the exhaust pressure.