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"Are there any acceptable levels of fuel dilution in diesel engines? What is the best test for fuel dilution?"
As fuel is burned during the combustion cycle, it can enter the crankcase and be introduced to the lubricating oil. The process of the raw or unburned fuel mixing with the oil is known as fuel dilution. There are many causes of fuel dilution, with most related to some sort of mechanical issue.
Perhaps the most common reason fuel dilution occurs is due to a process known as blow-by. If the piston rings are damaged or dirty, they may not seal properly. These rings are designed to keep the oil and the exhaust separated and are critical in ensuring proper compression in the cylinder. During combustion, any unburned fuel may leak past the rings and get into the oil. Another way this can take place is through defective fuel injection. If the fuel isn’t properly atomized, it may not burn completely and will have a better chance of entering the oil.
Once fuel is in the oil, it begins to affect both the oil and the engine. The most notable change in the lubricating oil will be a decrease in viscosity. Fuel thins the oil to the point where it cannot support the load in the engine or build a sufficient film to keep the engine parts separated. The fuel will also lead to cylinder liner washdown in which the fuel removes the oil and additive films from the cylinder wall. This results in accelerated wear during boundary lubrication conditions and premature engine failure.
When performing oil analysis, you can detect fuel dilution through several different tests, such as viscosity. A reduction in viscosity can often be linked to fuel dilution, but you may not always see this because of an offset in soot loading. Also, shearing of the viscosity index improvers can cause a reduction in viscosity, so another test is frequently needed to confirm fuel contamination.
Fourier-transform infrared (FTIR) spectroscopy is a readily available test that most oil analysis laboratories use. It can detect fuel, but you must have a new oil reference and a fuel reference to know where to look on the spectrum to determine if fuel is in the oil sample.
Perhaps the most effective technology for fuel dilution is gas chromatography. In this test, hydrocarbons in the oil sample are separated and measured. Since fuel has significantly smaller hydrocarbons than the lubricant base oil, it can be monitored more accurately. The results are typically given in percentages, with a recommended cautionary limit of 1.5 percent and a critical limit of less than 5 percent. When fuel levels are much higher than 5 percent, there is a risk of engine fire.
One additional test worth mentioning is flash point. When the flash point of an engine oil drops by 20 degrees C, there likely is a problem with fuel dilution. This test is qualitative but not quantitative for fuel.
Keep in mind that as an oil is in service, the fuel levels will continue to rise. Extended drain intervals put your equipment at higher risk for problems with contaminants building to unsafe levels, so routinely sample and analyze your engine oil to mitigate this risk.