Flashpoint Testing

Engine Oil Analysis using Flashpoint

Detecting diesel engine fuel through Flashpoint analysis?

All flammable liquids such as diesel fuel and petrol have a flashpoint. It is defined as the lowest temperature at which the liquid can form an ignitable mixture in air. The flammable liquid we are referring to in the case of oil analysis is usually diesel or petrol - fuel which has contaminated the oil.

All flammable liquids have a vapour pressure. The vapour pressure is related closely to the liquid's temperature. So as the temperature goes up, so does the vapour pressure. When the vapour pressure increases, the concentration of evaporated flammable liquid in the air increases.

It is therefore clear that the temperature determines the concentration of evaporated liquid at equilibrium. In essence, the flashpoint is the lowest temperature at which enough fuel vapour exists, that it ignites.

Flashpoint diagram

Engine oil contaminated by diesel fuel

The oil in your diesel engine can be badly affected by fuel contamination. Diesel fuel acts as a thinner to your engine oil and as a result the viscosity can drop dramatically. As we pointed out in the viscosity section, an oil's viscosity is one of the single most important defences against abnormal wear and/or equipment failure.

Should the flashpoint indicate the presence of diesel or petrol fuel, then it may suggest that fuel is entering the crankcase by way of the combustion chamber. This is called blow-by. Another cause of fuel dilution is by raw fuel entering the crankcase due to dripping faulty injectors.

The flashpoint test works hand-in-hand with the viscosity test and together they can help us tell the difference between an oil thinning due to oil transfer and an oil thinning due to the presence of diesel fuel/petrols.