Rajan Wilcox
Diesel is a combustible fuel that has a higher flash point than gasoline, making it less flammable. The flash point of diesel is generally accepted to be 140°F, while gasoline has a flash point of -45°F. This means that gasoline will ignite immediately when exposed to a flame or spark, whereas diesel will not. To burn diesel, it must be heated above its flash point, at which point it will vaporize and ignite when mixed with air. This is why diesel engines use compression ignition and glow plugs to heat the combustion chamber, as diesel fuel will not ignite with a spark. When burning diesel, it is important to note that it is a polluting and hazardous substance, and that it should not be mixed with gasoline or other fuels, as this can be risky.
| Characteristics | Values |
|---|---|
| Flash point | 140°F |
| Ignition method | Compression or heat |
| Fuel type | Combustible |
| Fuel properties | Thick density, evaporates slowly |
| Fuel safety | Less toxic and flammable than gasoline or kerosene |
| Fuel mix | 3:1 diesel to gasoline for a "hot mix" |
| Fuel ignition | Burns fumes, not liquid |
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What You'll Learn
Diesel fuel flash points
Diesel fuel has a higher flash point than gasoline, typically ranging between 52 and 96 °C (126 and 205 °F), although some sources state the range as 52 to 93 °C (126 to 200 °F). Diesel fuel flash points can vary depending on their type, with red diesel, for example, having a flash point of 125 °F. The generally accepted value for the flash point of diesel is 140 °F, with diesel fuel above this temperature considered combustible rather than flammable.
The flash point is the lowest temperature at which a liquid fuel releases vapours capable of forming an ignitable mixture with air. This distinction between flammable and combustible fuels is important for fire safety, as it determines the appropriate firefighting methods and equipment required. Diesel is a combustible fuel and requires a higher temperature to ignite compared to flammable fuels.
Due to its higher flash point, diesel fuel is less volatile than gasoline, and therefore safer to transport and store. Diesel engines lack spark plugs because a spark alone cannot ignite diesel fuel. Instead, diesel engines use compression ignition and glow plugs to warm the combustion chamber. Once the combustion chamber reaches operating temperature, the diesel fuel begins to vaporise and ignite.
It is important to note that the flash point of diesel fuel can be affected by contamination, particularly the accidental introduction of gasoline. This can significantly reduce the flash point, leading to safety hazards during handling and transportation.
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How to ignite diesel
Diesel engines ignite due to the auto-ignition of diesel fuel when it is injected into the engine. Unlike gasoline engines, diesel engines do not require a spark plug to ignite the fuel. Instead, they rely on the heat generated by the compression of air to spontaneously ignite the fuel. This compression results in increased fuel efficiency compared to other engine types.
To ignite diesel fuel, the following conditions are necessary:
- High compression ratio: Diesel engines have a higher compression ratio than gasoline engines, typically ranging from 14:1 to 25:1. This higher compression ratio increases the pressure and temperature in the cylinder.
- Direct fuel injection: Diesel fuel is injected directly into the cylinder, where it mixes with the compressed air.
- Auto-ignition temperature: The high temperature and pressure in the cylinder cause the diesel fuel to rapidly evaporate into vapour. When the temperature exceeds the auto-ignition temperature, the vapour ignites spontaneously.
In cold conditions, diesel engines may require additional assistance to ignite the fuel. This is typically achieved through the use of a glow plug, which is an electrically heated wire that facilitates fuel ignition when the engine is cold.
It is important to note that diesel fuel does not ignite easily under all conditions. For example, if you drop a match into a cup of diesel fuel, the match will go out. This is because diesel fuel requires extreme pressure or high temperatures to evaporate into vapour and ignite.
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Diesel fuel vapors
Diesel fuel, also known as diesel oil, heavy oil, or simply diesel, is a liquid fuel designed for diesel engines. Rudolf Diesel's original prototype was designed for liquid fuels, and he tested various types of fuel, including crude oil, kerosene, lamp oil, petrol, ligroin, coal tar creosote, paraffin oil, and fuel oil. Diesel fuel is a distillate of petroleum fuel oil, but biodiesel, biomass-to-liquid (BTL), and gas-to-liquid (GTL) diesel are also available as alternatives.
Diesel engines do not use spark plugs because diesel fuel does not ignite with a spark. Instead, diesel engines use compression ignition and glow plugs to warm the combustion chamber. Once the combustion chamber reaches operating temperature, diesel fuel begins to vaporize. The vapors mix with the air and ignite, burning the fuel. The flash point of diesel, or the temperature at which it vaporizes and ignites, is between 100 and 180°F, with 140°F being the generally accepted value. Diesel with a flashpoint below 140°F is flammable, while diesel above this temperature is combustible.
Due to its higher flash point, diesel is less flammable than gasoline. Gasoline has a flash point of -45°F and will ignite immediately when exposed to a flame or spark. In contrast, diesel takes longer to ignite and is more challenging to evaporate. However, diesel fires tend to last longer, and diesel is often used for bonfires as it burns hot and long.
To burn diesel fuel, it must be heated sufficiently to turn into vapors, which are flammable. These vapors then need to mix with air for the diesel to burn. While a lighter may not be sufficient to ignite diesel, spreading the fuel over a large surface area can help it evaporate, and holding a lighter close by for a short period can ignite the vapors. Alternatively, a blow torch or a drip torch with a "hot mix" of diesel and gasoline can be used to ignite diesel fuel.
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Diesel fuel mixtures
Diesel fuel is a complex mixture derived from crude oil through fractional distillation. The most common type of diesel fuel is a specific fractional distillate of petroleum fuel oil, known as petroleum diesel. However, there are also alternative diesel fuels that are not derived from petroleum, such as biodiesel, biomass-to-liquid (BTL), and gas-to-liquid (GTL) diesel.
The process of fractional distillation involves heating crude oil between 200 and 350 °C (392 and 662 °F) at atmospheric pressure. This results in a mixture of carbon chains containing between 9 and 25 carbon atoms per molecule. The fraction is then treated with hydrodesulfurization to remove sulfur, which can cause issues with diesel engines.
Diesel fuel typically contains a range of aliphatic and aromatic hydrocarbons with boiling points between approximately 163 and 357 °C. The specific composition of diesel fuel can vary, but it often includes branched-chain alkanes (paraffins), cycloalkanes (naphthenes), aromatics, and mixed aromatic cycloalkanes. Kerosene, a type of distillate with a similar boiling range to diesel, is sometimes added to diesel fuel to modify its viscosity.
In some cases, diesel engines have been fuelled with mixtures of diesel and other fuels, such as petrol, kerosene, rapeseed oil, or lubricating oil. These mixtures were often used in the past due to their lower cost, as they were not taxed. However, modern diesel engines typically require higher-quality fuels with proper ignition characteristics. Mixing diesel with other fuels can cause issues with performance and engine damage. Therefore, it is generally not recommended to mix diesel with other fuels unless specifically designed for that purpose.
When burning diesel fuel, it is important to note that it has different properties compared to petrol. Diesel does not ignite with a spark, and a higher temperature is required for ignition. The diesel fuel needs to be heated sufficiently to vaporize and form flammable vapors, which then mix with air and ignite. This is why diesel engines use compression ignition and glow plugs to warm the combustion chamber before fuel injection.
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Diesel fuel safety
Diesel fuel burns hotter and longer than other ignition sources. It is a very good fuel source for burning wood piles made up of tree limbs, lumber, and other organic debris. Diesel fuel is also useful for burning through wet leaves and green branches.
However, there are important safety considerations to keep in mind when burning diesel fuel. Firstly, diesel fuel must be stored safely and in compliance with relevant regulations. In Australia, for example, the storage and handling of flammable and combustible liquids are governed by specific standards. It is crucial to determine the flash point of your diesel fuel, as this will indicate whether it is classified as a flammable or combustible liquid. Flammable and combustible liquids can burn and explode, so sufficient controls must be implemented when handling and storing diesel fuel. Always store diesel fuel in a safe location, away from heat sources and open flames.
When burning diesel fuel, it is important to follow local regulations and obtain any necessary permits. Create a safe burning area by choosing an open space away from structures, decks, and flammable materials such as propane tanks. Be mindful of overhead wires and underground utility lines, as heat and flames can cause damage. Keep the wood pile at a manageable size, no larger than 6 feet wide and 4 feet tall.
To ignite the diesel fuel, it is recommended to use a tool like a drip torch to pour metered fuel onto the burn material. Do not attempt to ignite diesel fuel by directly lighting it with a match, as it requires intense pressure or sustained flame to ignite. Instead, use a paper towel or sawdust as an ignition source. Saturate a paper towel with diesel fuel, or insert handfuls of sawdust saturated with diesel fuel into the wood pile, and ignite the paper towel or each pile of sawdust. Always monitor the fire until it burns out, and have a source of water readily available for extinguishing any unwanted flames.
It is important to note that burning diesel fuel can be polluting and hazardous. Mixing diesel with other fuels, such as gasoline, can be risky and may produce a large gout of flames that quickly die out. Always burn dry materials to reduce the production of carbon monoxide (CO).
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Frequently asked questions
The flashpoint of diesel is between 100°F and 180°F, with 140°F being the generally accepted value. Diesel fuel with a flashpoint below 140°F is flammable, while diesel fuel with a flashpoint above 140°F is combustible.
Diesel fuel cannot be ignited by a spark or by directly lighting it with a match. It must be heated above its flashpoint to vaporize and ignite. The vapors, not the liquid, are what burn. Diesel engines use compression ignition and glow plugs to heat the combustion chamber and ignite the fuel.
A 3:1 ratio of diesel to gasoline is recommended for a "hot mix" in a drip torch. For a longer burn time and a less intense flame, a 4:1 or 5:1 ratio can be used, but this is more difficult to ignite.
