Question 20 MODE02 - Assistant MODU Engineer

The Correct Answer is A **Explanation for Option A (Correct):** When switching from Heavy Fuel Oil (HFO) back to Diesel Oil (DO) in a main propulsion diesel engine, the temperature of the fuel supplied to the fuel pumps must be managed carefully. HFO requires high temperatures (often around 120-140°C) for proper viscosity, atomization, and combustion. DO, however, has a much lower required viscosity and flash point, and supplying it at HFO temperatures can cause issues like fuel pump cavitation, premature vaporization (vapor lock), and injector tip overheating. Therefore, after the three-way valve is switched to allow the flow of DO, the temperature supplied by the preheater must be **gradually reduced**. This gradual reduction prevents thermal shock to the engine components (especially the fuel pumps and injectors) and allows the system to smoothly transition the viscosity to the optimal level for the less viscous DO. **Explanation for Other Options (Incorrect):** * **B) The heating steam must be secured before the diesel oil passes through the three-way valve:** This is incorrect. If the heating steam is secured too early (before or immediately upon switching the valve), the residual HFO in the lines downstream of the preheater will rapidly cool and increase in viscosity, potentially leading to fuel starvation, poor atomization, or stalling of the engine. The temperature reduction must be gradual, as described in Option A, not immediate isolation. * **C) The diesel oil must never be allowed to mix with the heavy fuel:** This is technically impossible in a practical marine fuel system during a switchover. Some mixing will inevitably occur at the interface in the piping, pumps, and engine components. Furthermore, DO is often used to "flush" the system and cool down the components. While excessive or uncontrolled mixing in storage tanks is avoided, the residual mixing during a proper transition is managed and is not the primary operational precaution. * **D) The heating steam to the preheater should be increased as soon as the diesel fuel passes through the three-way valve:** This is incorrect and highly dangerous. Increasing the heat when switching to DO would exacerbate the problems of vapor lock and cavitation due to the much lower flash point and viscosity of DO compared to HFO. The goal is always to **reduce** the temperature to the appropriate range for DO (typically 20-40°C).