Question 46 3AE01 - Third Assistant Engineer

The Correct Answer is C **Why option C ("diluted with fuel oil") is correct:** The flash point of an oil is the lowest temperature at which its vapors will ignite momentarily when an ignition source is applied. Diesel engine lube oil is a heavy lubricant with a high flash point (typically well above $200^\circ\text{C}$). Fuel oils, such as diesel or gasoline, are lighter, more volatile, and have significantly lower flash points (diesel flash point is typically $\sim 52^\circ\text{C}$ minimum). When lube oil is diluted with fuel oil (a common problem often caused by blow-by past piston rings or injector leakage), the mixture's overall volatility increases dramatically. The presence of these lighter, lower flash point hydrocarbons (the fuel) lowers the flash point of the entire lubricating oil sample. A decrease in flash point is the primary and most reliable indicator of fuel dilution in engine lubricating oil. **Why the other options are incorrect:** * **A) contaminated with sludge:** Sludge is a heavy, semi-solid contaminant composed of oxidized oil, water, and combustion byproducts. Sludge increases the viscosity of the oil but does not significantly increase its volatility, thus it does not typically cause a marked decrease in the flash point. * **B) contaminated with carbon:** Carbon (soot) is a solid combustion byproduct. Like sludge, soot increases viscosity and wear but is non-volatile. The addition of solid carbon particles does not decrease the temperature at which the liquid oil itself produces flammable vapors, so it does not lower the flash point. * **D) diluted with water:** Water dilution is detected by a "crackle" test or Karl Fischer analysis. While water can create steam/vapor which interferes with the flash point test process, pure water has no flash point and its presence does not introduce light, volatile hydrocarbons into the oil. Significant water contamination may slightly affect the reading, but a massive decrease in flash point is specifically indicative of fuel dilution, not water.