Question 2 3AE01 - Third Assistant Engineer

The Correct Answer is C ### Why Option C is Correct Modern marine diesel engines, even those equipped with traditional mechanical fuel injection (as opposed to common rail or electronically controlled systems), operate on the **Diesel Cycle**. 1. **Compression Stroke:** Air is compressed (approaching adiabatic compression). 2. **Combustion Phase (Heat Addition):** This is the crucial phase for classification. In a theoretical *pure* Diesel cycle, heat addition occurs entirely at **constant pressure**. However, the theoretical **pure** Diesel cycle is less efficient for high-speed or high-power engines. 3. **The Actual/Modern Cycle (Mixed/Sabathe Cycle):** Modern high-speed and medium-speed diesel engines (including marine applications) use a **mixed cycle** (also known as the Sabathe or Dual Cycle) to achieve higher efficiency and smoother operation. * **Phase 1 (Rapid Ignition):** Fuel injection often begins slightly before or right at top dead center (TDC), leading to a rapid, initial pressure spike. This initial part of the combustion occurs very quickly, approximating **constant volume** heat addition. * **Phase 2 (Controlled Burning):** The remainder of the fuel is injected and burned while the piston is moving down. The injection rate and timing are controlled to maintain combustion at a relatively high but controlled pressure, approximating **constant pressure** heat addition. Therefore, the actual combustion process in a modern marine diesel engine is a **combination of constant volume and constant pressure** heat addition. ### Why Other Options Are Incorrect **A) entirely constant pressure:** This describes the theoretical (pure) Diesel cycle. While a fundamental concept, actual modern engines deviate to include a constant volume phase for improved efficiency and power output (the Sabathe Cycle). **B) entirely constant volume:** This describes the theoretical **Otto Cycle** (used in spark-ignited gasoline engines). In the Otto cycle, combustion occurs theoretically instantaneously at TDC (constant volume). Diesel engines, being compression-ignition engines, must control the burn rate through continuous fuel injection, meaning the combustion process extends over a period of piston movement, thus preventing it from being purely constant volume. **D) a combination of constant temperature and constant pressure:** Constant temperature (isothermal) processes are extremely difficult to achieve in an internal combustion engine and are not representative of the heat addition phase. The heat addition phase in the diesel cycle involves a substantial increase in temperature and pressure.