Question 52 3AE01 - Third Assistant Engineer

The Correct Answer is C **Why Option C is Correct:** The cetane number (CN) is the primary measure of the ignition quality of diesel fuel. Specifically, it measures the fuel's propensity to auto-ignite under the high temperature and pressure conditions found in a diesel engine cylinder. A high cetane number indicates that the fuel ignites very quickly after injection. The period between the start of injection and the start of combustion is known as the ignition delay or ignition lag. Therefore, fuels with a **high cetane number** have a **shorter (reduced) ignition lag**, leading to smoother engine operation and better cold starting performance. **Why the Other Options are Incorrect:** * **A) The cetane number affects the amount of injection lag.** This statement is conceptually confusing and uses imprecise terminology. Injection lag typically refers to the physical delay between the signal (or movement) and the actual fuel exiting the injector nozzle, which is a mechanical and hydraulic characteristic. The cetane number affects the **ignition lag** (the delay between injection and combustion), not the mechanical or hydraulic injection lag. * **B) The cetane number is an indication of the fuel's viscosity.** Viscosity is a measure of the fuel's resistance to flow and is critical for pumping and atomization. While viscosity is a physical property of the fuel, it is measured separately (e.g., in centistokes) and is not what the cetane number is designed to measure. The cetane number measures the chemical property of ignition quality. * **D) The cetane number is of little significance in the combustion process.** This is false. The cetane number is highly significant. It directly dictates the length of the ignition delay period, which is the most crucial factor determining how quickly, smoothly, and efficiently combustion begins. A poor (low) cetane number can cause excessively long ignition delays, leading to "diesel knock" and incomplete combustion.