Kerosene and diesel are hydrocarbon fuels derived from crude oil through fractional distillation. They are distinct petroleum products, separated by differences in their molecular structure and physical properties. This comparison involves looking at the fundamental chemical differences that influence their combustion characteristics, specifically how much energy they contain and how hot they burn.
Distinctions in Fuel Characteristics
The separation of crude oil into usable products occurs in a distillation column, where different compounds condense at varying temperatures. Kerosene is considered a lighter cut, typically condensing at a lower temperature range, approximately 140°C to 250°C. Diesel, being a heavier distillate, is collected at a higher temperature range, generally between 180°C and 370°C.
This difference in the distillation process results in distinct chemical structures for each fuel. Kerosene molecules are composed of shorter carbon chains, typically containing 12 to 15 carbon atoms per molecule. Conversely, diesel is made up of longer and more complex hydrocarbon chains, which generally range from 15 to 18 carbon atoms.
The molecular weight difference directly impacts the physical properties of the fuels. Kerosene is a thinner, less dense liquid with a lower viscosity compared to the heavier, thicker diesel. Kerosene also has a lower flash point, which is the minimum temperature at which the fuel produces enough vapor to ignite temporarily when exposed to a flame.
Comparing Heat Output and Flame Temperature
The question of which fuel “burns hotter” requires a distinction between the total heat energy released and the peak temperature of the flame. Heat output is quantified by energy density, measured in British Thermal Units (BTU) per unit of volume. Diesel fuel, due to its higher density and longer carbon chains, contains a greater mass of fuel per gallon.
This higher density means that a gallon of diesel (No. 2 fuel oil) typically holds more total energy, yielding approximately 137,381 to 138,500 BTU. Kerosene (No. 1 fuel oil) has a lower energy density, providing around 131,890 to 135,000 BTU per gallon. Therefore, diesel releases more total heat energy per unit of volume.
Flame temperature relates to the efficiency and completeness of the combustion process. In general, the peak temperature of a hydrocarbon flame burning in air is concentrated in a narrow range around 1,950°C. Kerosene’s lighter, more volatile nature allows it to vaporize and mix with air more readily, which can result in a cleaner and more complete burn.
This cleaner burn, characterized by less soot and uncombusted carbon, allows kerosene to sometimes achieve a slightly higher peak flame temperature under optimally efficient conditions. The slightly shorter carbon chains in kerosene combust more easily and uniformly than the more complex chains found in diesel. While diesel delivers more total heat per gallon, the cleaner combustion profile of kerosene can lead to a more intense, higher-temperature flame in specific burners.
Application of the Burning Differences
The differing properties of the two fuels determine their preferred applications in various systems. Kerosene is favored in applications where a clean burn and performance at low temperatures are prioritized. For example, it is the primary fuel used for jet aircraft because its lower freezing point is suitable for cold-weather operations at high altitudes.
The volatility and clean-burning nature of kerosene also make it suitable for non-pressurized heaters and lamps that rely on wicks for fuel delivery. Diesel, with its higher energy density per gallon, is the preferred fuel for heavy-duty engines, such as those in trucks and construction machinery. Its greater mass-per-volume translates directly to better fuel economy and range for these applications.
Diesel’s heavier composition also provides some natural lubricity, which is beneficial for the pumps and injectors in diesel engines. In contrast, kerosene offers very little lubricity, which can cause wear in standard diesel engines. In cold climates, kerosene is often blended with diesel to lower the overall cloud point and prevent the fuel from gelling.