Diesel fuel does burn, but the mechanism required for its ignition is vastly different from other fuels like gasoline due to its chemical structure and physical properties. Diesel is a heavier petroleum distillate than gasoline, composed of larger hydrocarbon molecules. These larger molecules give diesel distinct characteristics that influence how easily it vaporizes and ignites. This difference in molecular composition is the primary reason why diesel requires a specialized engine design to initiate combustion consistently.
Volatility and the Flash Point
The ease with which a liquid fuel changes into a flammable vapor is known as its volatility. Gasoline is highly volatile because it is composed of lighter, smaller hydrocarbon molecules that evaporate quickly, even at low temperatures. In contrast, diesel is considered a low-volatility fuel due to its heavier molecular structure.
The flash point is the lowest temperature at which a liquid produces enough vapor to briefly ignite when exposed to an external ignition source. Gasoline has an extremely low flash point, typically around -40°F (-40°C), meaning it is constantly producing ignitable vapor in almost any ambient environment. This high volatility is why gasoline spills pose an immediate fire hazard from the readily available vapor cloud above the liquid.
In contrast, the flash point for common No. 2 diesel fuel is higher, typically 125.6°F (52°C) or above. Since diesel must be heated well above typical ambient temperatures to produce sufficient ignitable vapor, dropping a lit match into a puddle of diesel at room temperature usually results in the match extinguishing itself. This difference is why diesel fuel is classified as a combustible liquid rather than a highly flammable one, reflecting the difficulty in starting combustion without heat application.
The Physics of Compression Ignition
The combustion of diesel fuel in an engine relies on a principle called compression ignition, which bypasses the need for a spark plug. This process leverages the fundamental physics of gases: rapidly compressing a gas increases its temperature.
Diesel engines are engineered with very high compression ratios, typically ranging from 14:1 to 25:1, which is much higher than in a gasoline engine. During the compression stroke, the piston rapidly squeezes the air inside the cylinder to a fraction of its original volume. This extreme compression raises the air temperature dramatically, often reaching between 1,300°F and 1,650°F (700°C and 900°C).
Once the air is heated to this intense temperature, the diesel fuel is finely misted directly into the combustion chamber via a high-pressure injector. The fuel immediately mixes with the superheated air, causing the fuel to spontaneously ignite without any external spark. This reliance on compression heat, rather than a separate ignition source, defines the diesel engine and allows it to burn its less volatile fuel.
Practical Differences from Gasoline
Because diesel fuel has a low vapor pressure and a high flash point, it presents a much lower risk of fire from accidental ignition during storage and transport. The reduced vapor formation means there is rarely an ignitable vapor-air mixture hovering above the liquid at normal temperatures.
This inherent safety feature makes diesel the preferred fuel for marine vessels, construction sites, and large transportation fleets. The engine itself is simplified because the high compression ratio eliminates the need for a separate electrical spark ignition system and its associated components. Instead, the engine’s design focuses on maximizing the pressure and temperature inside the cylinder to ensure consistent autoignition of the fuel spray. Ultimately, diesel is a potent combustible energy source, but its low volatility requires specific high-temperature, high-pressure conditions to unlock its power.