Oil dilution

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Oil dilution is a term used in vehicle technology. Oil dilution describes the introduction of fuels into the lubricating oil of an internal combustion engine and the failure of the subsequent fuel evaporation in the oil, so that the oil is constantly further diluted and the lubricating effect of the oil continues to decline. This can result in major engine damage . Oil dilution is both gasoline - and diesel engines possible, with the latter particularly in versions with diesel particulate filter (DPF).

The main cause of oil dilution in gasoline engines is fuel, which after a cold start is deposited on the still cold cylinder walls and then passes the piston rings into the engine oil. This is aggravated by short journeys - especially in winter - or a defective thermostat in the cooling circuit. This effect is less pronounced in gasoline engines with electronic injection systems than in those with a carburettor , since the amount of fuel can be dosed more precisely during a cold start.

In diesel engines, the dilution of the oil after a cold start plays only a minor role, since the fuel hardly comes into contact with the combustion chamber walls even when the engine is cold. Already shortly after the start there is practically no wall wetting. The main cause of diesel engines can be found in the so-called post-injection, which is carried out to clean the diesel particulate filter - mainly when driving short distances. Unburned fuel gets into the crankcase through the so-called blowby .

In vehicles that are not only driven on short journeys and whose engines are otherwise well maintained, oil dilution can be recognized by a creeping increase in the oil level. The pulled oil dipstick can then smell of fuel. If this is observed, a check of the cooling system in the workshop is appropriate: the cooling thermostat is often defective, the engine is cooled too much and does not reach its normal working temperature. The cooling water temperature display is often too imprecise and insufficiently scaled for normal motorists to recognize this process.

With all engines, fuel that has been dragged into the oil in medium and long-distance operation can evaporate again, but never again completely. This applies to both gasoline and diesel fuel. The engine oil reaches maximum temperatures of around 120 to 130 ° Celsius, and this only at critical points in the engine; The operating temperatures of the oil are usually around 80 to 100 ° C. This means that only those fractions of the entrained fuel whose boiling points are at or below these temperatures are even separated from the engine oil; the rest remains in the oil. In the case of gasoline, up to about 80% of the fuel that has been introduced can be separated out again; with diesel fuel it is a maximum of about 40%.

The operation of diesel vehicles with biodiesel ( RME ) exacerbates the problem of oil dilution even more. Due to the higher boiling range, it is no longer possible for biodiesel to evaporate from the hot engine oil; Vehicles that are predominantly or exclusively operated with biodiesel should therefore strictly adhere to the prescribed oil change intervals . Finally, when using vegetable oils as fuel, the imported vegetable oils can polymerize with the additives contained in the motor oil . Over time, insoluble suspended matter and lumps form, which clog oil lines and lead to lubrication failure.

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