The main source of lubricating oil contamination in a diesel engine's crankcase is combustion byproducts that are removed from the cylinder walls. During the combustion process, especially in diesel engines, various byproducts such as soot, unburned fuel, and water can be produced. These byproducts can accumulate and enter the crankcase oil.
As the engine runs, the pistons move up and down within the cylinders, and some of these combustion byproducts can adhere to the cylinder walls. When the pistons slide down, they can scrape these contaminates into the crankcase, mixing them with the lubricating oil. This contamination not only reduces the effectiveness of the oil in lubricating engine components, but it can also lead to the formation of sludge and cause other oil-related issues.
Understanding this source of contamination is crucial for maintaining the engine's performance, as well as ensuring the longevity of the oil and the engine itself. Monitoring oil quality and changing the oil regularly are practices that help mitigate the impact of such contamination.