Petroleum-based fuels like gasoline and diesel are the primary energy sources for global transportation, but they possess distinct physical and chemical characteristics. Understanding the difference between these two common fuels requires looking beyond the pump to the intrinsic property that determines their relative weight: density. Gasoline is significantly less dense than its counterpart. This difference in density dictates their refining process, energy content, and practical application in vehicles.
Comparing Physical Properties
The perception of a liquid’s “lightness” or “heaviness” is scientifically defined by its density, which is the mass of a substance contained within a specific volume. Density is measured in units like kilograms per liter (kg/L) and is directly influenced by temperature, so industry measurements are standardized at 15°C (59°F). Under these conditions, the density of finished gasoline typically falls within a range of approximately 0.71 to 0.77 kg/L.
Diesel fuel, by comparison, is substantially denser, with a standard range between 0.82 and 0.86 kg/L. This means that one liter of diesel weighs about 10% to 15% more than one liter of gasoline. Refineries and distributors often use a standardized measure called API Gravity, developed by the American Petroleum Institute.
API Gravity works inversely to density; a higher API number signifies a lighter, less dense product. Gasoline generally possesses a higher API Gravity than diesel, confirming its lighter nature. This standardized measurement system allows the petroleum industry to classify and manage crude oil and its refined products.
Molecular Basis for Density Differences
The fundamental reason for the density difference lies in the molecular structure of the two fuels, both of which are mixtures of various hydrocarbons. Gasoline is composed of shorter hydrocarbon chains, typically ranging from 4 to 12 carbon atoms per molecule. These molecules are smaller and more compact, leading to less mass packed into the same volume, which results in lower density.
Diesel molecules, conversely, have longer carbon chains, generally ranging from 8 to 21 carbon atoms. These larger, heavier molecules naturally occupy more space and contribute more mass, making the liquid denser overall. The separation of these molecules occurs during the fractional distillation of crude oil in a refinery.
Crude oil is heated, and different components vaporize and condense at different temperatures, known as boiling ranges. Gasoline vaporizes at a lower temperature and is collected higher up in the distillation column.
Diesel is a “middle distillate,” meaning it has a higher boiling range and condenses lower down the column, retaining the heavier, longer-chain hydrocarbons. This difference in the refining fraction determines the final molecular makeup and density of the product.
Real-World Effects of Varying Density
The higher density of diesel fuel has significant practical consequences, particularly concerning energy content. Because diesel is denser, a specific volume, such as a gallon or liter, contains a greater mass of fuel. This increased mass translates directly into a higher volumetric energy content.
On average, a gallon of diesel contains approximately 137,381 British Thermal Units (BTU) of energy, while a gallon of gasoline contains about 120,476 BTU. This difference explains why diesel engines often achieve better fuel economy than gasoline engines, as more energy is delivered with each volume of fuel consumed. The heavier nature of the fuel also impacts the engine’s fuel injection system.
Diesel engine calibration must account for the fuel’s density because the mass of fuel injected per stroke varies with it, affecting engine power and emissions. Furthermore, the density difference has implications for handling and spills; if diesel and gasoline were to mix, the less dense gasoline would float on top of the denser diesel. Both fuels possess an API Gravity greater than 10, ensuring they float on water in the event of a spill.