While diesel fuel is a combustible liquid, it does not ignite as easily as some other common fuels, such as gasoline. Diesel is a type of fuel derived from petroleum, primarily used in diesel engines, which operate on a compression-ignition principle. Unlike gasoline, which is highly volatile and readily forms ignitable vapors at typical ambient temperatures, diesel requires specific conditions to ignite. This difference stems from its distinct chemical and physical properties. Understanding these properties explains why diesel is considered safer to handle than gasoline, despite both being capable of combustion.
Diesel’s Unique Ignition Properties
The difficulty in igniting diesel stems from its higher flash point compared to gasoline. The flash point is the lowest temperature at which a liquid produces enough vapor to ignite briefly in air when an ignition source is present. For diesel fuel, this temperature ranges from 50°C to 100°C (122°F to 212°F). In contrast, gasoline has a significantly lower flash point, often around -43°C (-45°F). This means that at room temperature, gasoline constantly releases enough flammable vapors to create an ignitable mixture with air, making it far more prone to ignition from a small spark or flame.
Another important characteristic is the autoignition temperature, which is the lowest temperature at which a substance will spontaneously ignite without an external ignition source like a flame or spark. Diesel fuel has an autoignition temperature between 210°C and 329°C (410°F and 625°F). Gasoline’s autoignition temperature is typically higher, ranging from 247°C to 280°C (477°F to 536°F). This difference means that while diesel requires less heat to spontaneously ignite than gasoline, it also needs a much higher temperature to create ignitable vapors with an external source due to its high flash point.
How Diesel Can Ignite
Despite its higher flash point, diesel can ignite under specific conditions that facilitate the mixing of its vapors with air and provide sufficient heat. One method involves atomization, where liquid diesel is sprayed into a fine mist. This process increases the fuel’s surface area, allowing it to vaporize more rapidly and mix effectively with oxygen in the air. When atomized, diesel can ignite more readily, even at temperatures below its liquid flash point, if an ignition source is present.
Diesel engines utilize compression ignition. In these engines, air within the cylinder is compressed to high pressures, causing its temperature to rise, often exceeding 700°C (1292°F). When diesel fuel is injected into this superheated, compressed air as a fine spray, it spontaneously ignites without a spark plug. This demonstrates how intense heat and atomization lead to diesel combustion.
Beyond engine operation, sustained high heat sources can also ignite diesel. An open flame, a hot surface, or an electrical spark can raise the local temperature of diesel fuel above its flash point, or even its autoignition temperature, leading to ignition. For instance, a large pool of diesel might not ignite easily with a dropped match. However, prolonged exposure to a heat source, such as a welding torch or electrical short, can provide the energy needed to vaporize enough fuel and initiate a fire.
The Hazards of Diesel Fires
Once ignited, diesel fires present hazards, burning intensely and proving challenging to extinguish. While harder to start than gasoline fires, a diesel fire can generate substantial heat and spread quickly, particularly as a “pool fire” where spilled diesel ignites and burns across a surface. These fires can be difficult to control due to the sustained fuel supply and high burning temperatures.
A concern with diesel fires is the release of toxic fumes and thick, black smoke. The combustion of diesel fuel produces a complex mixture of gases and particulates, including carbon monoxide, nitrogen oxides, sulfur dioxide, and various hydrocarbons. The particulate matter, often visible as soot, can be inhaled deeply into the lungs and may contain carcinogenic compounds. The dense black smoke reduces visibility, complicating firefighting efforts and posing a respiratory hazard. Therefore, if a diesel fire occurs, safety measures prioritize containment and appropriate extinguishing agents to mitigate these dangers.