Abstract
A critical review of recent work on fuel lubricant interactions is undertaken. The work focusses on liquid fuels used in diesel and gasoline vehicles. The amount of fuel that contaminates the lubricant depends on driving conditions, engine design, fuel type, and lubricant type. When fuel contaminates a lubricant, the viscosity of the lubricant will change (it will usually decrease), the sump oil level may increase, there may be a tendency for more sludge formation, there may be an impact on friction and wear, and low speed pre-ignition could occur. The increased use of biofuels (particularly biodiesel) may require a reduction in oil drain intervals, and fuel borne additives could contaminate the lubricant. The move towards the active regeneration of particulate filters by delayed fuel post-injection and the move towards hybrid electric vehicles and vehicles equipped with stop-start systems will lead to increased fuel dilution. This will be of more concern in diesel engines, since significant fuel dilution could persist at sump oil temperatures in the range of 100–150 °C (whereas in gasoline engines the more volatile gasoline fuel will have substantially evaporated at these temperatures). It is anticipated that more research into fuel lubricant interactions, particularly for diesel engines, will be needed in the near future.
Highlights
If fuel viscosity is assumed to be 1 mPa.s, the effect of fuel dilution, with the assumption that fuel dilution is only 1%, would be to reduce the viscosity to only 2.97 mPa.s. Because of their higher boiling point range, less evaporation will occur compared to gasoline, and it is possible that higher levels of diesel fuel dilution could persist at oil temperatures in the range of 100–150 ◦ C
For diesel engines, significant amounts of diesel fuel could remain in the sump oil at elevated temperatures and so there is concern that diesel fuel dilution could lead to lower viscosities that could lead to thinner oil films and wear issues at higher temperatures
Fuel dilution of the lubricant can affect lubricant viscosity, which can impact friction and wear, and can lead to increases in the sump level
Summary
Fuel/lubricant/engine interactions are not as important as fuel/engine or lubricant/engine interactions, and so are not as high on the list of priorities for either fuel scientists or lubricant scientists, unless field issues emerge. It is well known that unburnt fuel, and fuel additives, can accumulate in lubricants, and in sufficient concentration can cause issues, such as (1) altering (usually lowering) the viscosity of the lubricant, (2) helping sludge to form in engines, (3) altering the oxidation properties of the lubricant (leading to lower oil drain intervals), and (4) potentially affecting the frictional properties of the lubricant. The recent wider spread use of biofuels (mainly ethanol or methanol in gasoline fuel, or biodiesel in diesel fuel) has occasionally led to field issues ( in diesel engines). Recent interest in fuel/lubricant interactions has surged due to (1) many more engines having stop-start systems, and (2) many engines having active aftertreatment systems in which extra fuel is injected (and burnt) to help with tailpipe emissions. This paper attempts to critically review work over the last 30 years, to give a fuller picture of what is currently known and what gaps remain
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