Question 215Motor Plants - Assistant Engineer

The correct answer is D) combustion by-products removed from the cylinder walls. In a normally operating diesel engine, the main source of lubricating oil contamination in the crankcase is the combustion by-products that are removed from the cylinder walls during the engine's operation. These by-products, such as soot, partially burned fuel, and other combustion residues, find their way into the crankcase and contaminate the lubricating oil. The other options are incorrect because: A) metal particles loosened by wear are not the main source of contamination, B) the use of air cleaners is not relevant to the source of contamination, and C) while condensation of water vapors can contribute to oil contamination, it is not the primary source in a normally operating diesel engine.