The Flash point is defined as the minimum temperature at which a liquid develops enough vapors to ignite (flash) when it is exposed to some igniting source.
Most gasoline oils possess a flash point of about 380 °F while in the case of diesel oil samples, the flash point temperature goes up to 410 °F.
For all engine oils, this temperature is required to be higher in order to resist prior combustion. The inclusion of contamination causes the flash point of the oil to get lower over the passage of time.
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Why flash point Change?
One factor is the dilution of oil.
This occurs due to prolonged usage which causes it to get mixed up with fuel.
Besides mixing with fuel, other foreign factors like the mixing of water vapors, dust, dirt, or any other contamination also result in dilution.
If the flashpoint temperature of motor oil falls below a certain level which is normally 150 °C, the oil will be burnt completely beforehand and therefore must be replaced immediately.
Some other factors that cause variation in oil’s flash point are as follows:
| Reasons | Decrease in Flash point | Increment in Flash point |
| Variation in the chemistry of oil | Thermal Cracking-Micro dieseling -Radiation by gamma rays | Polymerization |
| Removal from oil | Thermal Evaporation-Vacuum Dehydration | |
| Inclusions to the oil | Solvents-Natural gas-Water-Diesel Fuel |
Decrease in Flashpoint:
- Thermal Cracking of Oil
Sometimes an extremely high temperature of the oil causes the evolution of hot gases which act as one of the crucial reasons for a lower flash point.
Other uninviting factors like improper functioning of the heater tank also act as a source of heat generation.
Regardless of the source of heat generation, if the temperature of the oil rises above 550 °C the cracking of the oil occurs.
This cracking of the oil further leads to the formation of coke and low-weight molecules that readily decreases the flash point temperature.
Furthermore, the exposure of oil to gamma rays also causes gas to evolve thus dwindling its flash point temperature.
- Contaminants within the oil
The presence of contaminants also causes the flashpoint of oil to drop significantly.
Some major constituents that cause a lower flash point include gasoline, diesel, natural gas, and solvents.
Solvent contamination occurs due to various reasons like when using naphtha or kerosine chemical for cleaning the gearbox.
Water contamination also gives false flash point temperatures during testing.
For mitigating this problem, calcium carbonate is added prior to identifying the flash point of the oil.
Apart from that, glycol and coal dust are also found to be effective in removing these water contaminants.
Increase in Flashpoint
- Polymerization
The flashpoint of the oil can be incremented by the polymerization process. It is the process in which monomers (hydrocarbon compounds) react together to develop long-chain compounds (polymers).
The polymers not only assist in increasing the oil’s flashpoint temperature but also helps to decrease its pour point temperature and modify its viscosity.
- Thermal Evaporation
It is the process of removing solid contaminants from the engine oil in the form of a thin layer.
For this purpose, the oil is exposed to a resistive burning source in a vacuum chamber to generate vapor pressure.
The contaminants are evaporated as a result, in the form of a fine layer in the chamber which can then be removed easily.
- Vacuum Dehydration
In this process, contamination in the form of water or vapors are removed from the oil using a dehydrator in a vacuum environment.
The removal of these contaminants increases the flashpoint of the oil.
How to determine the Flashpoint?
Flashpoint is one of the most important constants when determining fuel characteristics.
Every engine oil has a particular flash point so a separate determining test is required to be taken for each oil sample.
This test is extremely important for consumers to find out if the oil is suitable for their system or not.
For example, if the oil has a flash point of 180 °C it cannot be used for a system where a peak temperature reaches above 180 °C therefore the foregoing oil will not be useful.
According to Cleveland, the oils having flash points greater than 79 °C are determined by ISO 2592 standard in an open crucible. On the contrary, the oils which possess flash points lower than 79 °C are measured in closed crucibles as per Abel-Pensky (DIN 51755) or Pensky-Martens (DIN 51758).
Flash points of Engine oils
The flash point of all fluids depends upon their specific applications. For instance, paraffin-based fluids have a density between 860 and 890 kg m-3 their flash point generally falls between 200 and 280 °C, and for naphthalene-based fluids, their maximum flash point is 235 °C.
When it comes to engine oils since they are exposed to high temperatures, they are required to possess higher flash points.
The minimum flash point temperature possessed by most of the oils is 200 °C with some oils even exhibit higher flash points up to 280 °C.
Some average flash points of different viscosity grade oils are as follows:
| SAE Oil Grades | Lowest Temp °F (°C) | Highest Temp °F (°C) | Average °F (°C) |
| 20W-50 | 390 (198) | 507 (263) | 448 (231) |
| 20W-40 | 415 (212) | 500 (260) | 452 (233) |
| 15W-50 | 415 (212) | 503 (261) | 447 (230) |
| 5W-50 | 437 (225) | 457 (236) | 450 (232) |
| 5W-40 | 450 (232) | 450 (232) | 450 (232) |
| 15W-40 | 399 (203) | 495 (257) | 432 (222) |
| 10W-30 | 390 (198) | 520 (271) | 424 (217) |
| 5W-30 | 354 (178) | 480 (248) | 420 (215) |
Flash point in 2-stroke engines
A lot of 2-stroke oils are ignited by mixed lubrication within the engine along with the fuel.
Thus, these oils exhibit lower flash point temperatures as compared to other normal motor oils.
Their flash point on average does not exceed 130 °C.
However, some 2-stroke oils possess higher flash point temperatures, specifically racing motorbikes where the flashpoint acts as a quality feature as they require the oil to burn slowly, and the addition of different supplements further helps to enhance engine life.
Conclusion
Flashpoint is one of the most important factors in determining fuel economy.
For the oil possessing a low flash point, its fuel consumption will be greater, therefore, its fuel economy will be lower and vice versa.
Different factors like oil cracking, contamination (addition of solvents, gasoline), and interaction with gamma rays all cause a decline in the oil’s flashpoint.
The flash point testing of the oil can be done either in an open crucible or a closed crucible depending on its flash limit.
Normally, the flash point of 2-stroke oils is lower than normal 4-stroke oils except for racing motorbikes that exhibit higher flashpoints.
All in all, flash point is one of the key factors before buying any type of motor lubricant.