Kerosene and diesel are both hydrocarbon fuels refined from crude oil, occupying adjacent “cuts” in the fractional distillation process. This shared origin often leads to confusion, as they appear similar and can be used in some overlapping applications, such as home heating. Fundamental differences in their molecular structure define distinct physical properties, intended applications, and regulatory requirements. Understanding these specifics is necessary for the correct performance of equipment and compliance with fuel laws.
Fundamental Composition and Physical Properties
The core difference between kerosene and diesel lies in the length of their carbon chains, which results from their position in the refinery’s distillation column. Kerosene is a lighter petroleum fraction (C9–C16), collected at a lower boiling range (approximately 140°C to 250°C). Diesel is a heavier middle distillate (C12–C20), collected at higher temperatures (generally between 180°C and 370°C).
The shorter carbon chains make kerosene thinner and less viscous than diesel, impacting engine lubrication. Kerosene also exhibits a lower density, meaning a gallon contains slightly less energy (fewer BTUs) compared to the same volume of diesel fuel. A significant difference is volatility, measured by the flash point—the lowest temperature at which the fuel’s vapors will ignite when exposed to an ignition source. Kerosene typically has a flash point starting around 38°C (100°F), while diesel’s flash point is significantly higher, often above 52°C (126°F).
Kerosene’s lower flash point makes it easier to ignite and results in different classifications for storage and transport safety. Diesel’s lower volatility means it is less prone to producing ignitable vapors, contributing to its relative safety in high-volume transportation and storage. Kerosene’s lighter composition also provides better cold-weather performance, as it is less likely to experience “gelling” or wax crystallization, a problem that affects heavier diesel components in freezing conditions.
Primary Applications and Engine Compatibility
The differing physical properties dictate the specialized applications for each fuel, making them generally non-interchangeable. Diesel fuel is the standard choice for compression-ignition (CI) engines found in trucks, buses, trains, and heavy machinery. These engines rely on the fuel’s high cetane number—a measure of ignition quality—which allows the fuel to auto-ignite quickly and reliably under high pressure. Diesel typically meets a minimum cetane number of 40 to 51, ensuring efficient combustion and minimizing ignition delay.
Kerosene’s primary high-volume use is as jet fuel (Jet A or Jet A-1), where its low freezing point and stability are necessary for high-altitude flight. It is also widely used as heating oil (K-1) due to its clean-burning characteristics and ability to flow in cold storage tanks. When used in a diesel engine, kerosene presents problems, primarily its lower cetane number, which can lead to hard starting, rough running, and white smoke.
A serious concern is kerosene’s lower lubricity compared to diesel, which contains natural lubricating compounds that protect high-precision engine parts. Modern diesel engines rely on the fuel to lubricate the fuel pump and injectors. Using kerosene, which is too thin, can quickly lead to component wear and premature fuel system failure. Conversely, using diesel in a kerosene application, such as a heating unit, often results in excessive sooting and poor burner performance due to diesel’s heavier structure and higher viscosity.
Regulatory Distinction and Handling
Kerosene and diesel are separated by significant regulatory distinctions concerning taxation and identification. Fuel intended for use on public roads is subject to federal and state excise taxes to fund transportation infrastructure. Diesel fuel sold for motor vehicles is taxed, but diesel and kerosene destined for non-road uses (e.g., heating, agriculture, or construction) are exempt from this highway tax.
To enforce this tax distinction, untaxed fuels are legally required to be indelibly dyed. Fuel that is not taxed, including off-road diesel and heating kerosene, must be colored with a specific concentration of dye, most commonly Solvent Red 164, giving the fuel a noticeable red tint. Using this dyed, untaxed fuel in a highway vehicle is illegal and results in substantial fines and penalties.
The difference in flash point dictates separate storage and handling protocols. Kerosene’s lower flash point classifies it as a more hazardous combustible liquid than diesel, potentially requiring stricter ventilation and container labeling, especially in warmer climates. The safety margin for kerosene is narrower, increasing the risk of vapor ignition if proper storage procedures are not followed. Aviation kerosene is an exception to the dyeing requirement, as it is a non-taxable use requiring high purity, tracked through a separate registration system.