Why Diesel Mixes With Oil in Diesel Generators?

Diesel and engine oil in diesel generators serve completely different functions and must never be mixed. If fuel and oil do mix, it indicates a seal failure in the unit, a problem that occasionally occurs during operation.

So, how does diesel enter the engine and mix with oil? Generally, there are two main pathways for diesel to contaminate engine oil in generator sets:

First, through the gap between the piston and cylinder wall into the combustion chamber.

During normal operation, diesel is injected into the combustion chamber by the injector mounted on the cylinder head at a precise timing, quantity, and pressure. It then ignites and burns, generating high-temperature, high-pressure gas that pushes the piston downward to do work. Ideally, all injected diesel should be completely burned into exhaust gas.

The diesel injector is directly installed on the diesel engine's cylinder head, with the injector nozzle tip in contact with the high-temperature combustion chamber. Its working conditions are extremely harsh, making it a critical component affecting the diesel engine's design specifications and performance.

What causes the injector to over-inject fuel? The primary reason is a stuck injector needle valve, which allows diesel to flow directly into the combustion chamber. In this case, diesel is not injected only at the end of the compression stroke but continuously feeds into the combustion chamber, resulting in a large amount of unburned diesel flowing into the oil pan. Additionally, unburned diesel in the combustion chamber may condense and drip into the oil pan.

Second, diesel leaks from the high-pressure fuel pump into the oil pan.

The high-pressure fuel pump requires oil lubrication during operation, so a dedicated oil passage leads to its lower part, with the pump's plunger located above. If the plunger is cracked, stuck, or scratched, diesel will leak through the gap between the plunger and its barrel, mix with the oil below, and flow back into the oil pan.

The first pathway is the more common cause. The injector needle valve is an extremely precise mating component, ground in pairs during manufacturing with a clearance of only 0.003~0.005 mm—so tight that even air barely passes through, let alone diesel. However, it has extremely high requirements for diesel cleanliness; even minor impurities in the diesel can cause it to stick. Therefore, the diesel filter is a vital component and must be regularly replaced and maintained.

Among the three key precision components in a diesel engine's fuel system, the injector needle valve assembly has the lowest reliability and shortest average service life. Since the injector needle valve assembly is so crucial, what causes its damage and how can it be prevented? Detailed analyses of these questions are available on the Dingbo Power website for further reading.

Dingbo Power has advanced testing equipment, modern production technology, professional manufacturing technology, perfect quality management system, and strong technical research and development capabilities. It can provide 3KW～2500KW various specifications of ordinary, automatic, four protection, automatic switching, low noise and mobile generator sets, high quality and low energy consumption to meet the diverse power needs of customers, and can also meet users with different voltages and different frequencies. It is required to create a parallel power supply system for multiple units. If you are interested in our product or any question on generator, welcome to send email to [email protected].