The immediate answer to whether diesel floats on gasoline is no: diesel is denser than gasoline and will sink beneath it. Both fuels are complex mixtures of hydrocarbon molecules derived from crude oil, but they possess significantly different physical properties. Gasoline and diesel fuel are engineered for distinct engine types, and their separation is a direct result of their differing masses per unit of volume. When accidentally mixed, this density difference causes a layering effect, which has serious implications for a vehicle’s fuel system and engine performance.
Density: Why Diesel Sinks Below Gasoline
The primary factor determining the initial separation of the two fuels is density. Diesel fuel is consistently denser than gasoline. At a standard temperature of 59°F (15°C), diesel typically falls within a density range of 0.82 to 0.85 kilograms per liter, while gasoline is significantly lighter, ranging from 0.72 to 0.77 kilograms per liter.
This difference means that a given volume of diesel weighs more than the same volume of gasoline. If the two liquids are poured together into a static container, the heavier diesel will settle at the bottom, and the lighter gasoline will layer on top. In a vehicle’s fuel tank, this layering means the fuel pump’s inlet, which is typically near the bottom, will draw in the denser diesel first.
Key Chemical Distinctions
The reason for diesel’s higher density lies in the molecular structure of the hydrocarbons that make up each fuel. Both are refined from crude oil, but they represent different “cuts” from the fractional distillation process. Diesel molecules are significantly larger and heavier than gasoline molecules.
Gasoline consists mainly of short hydrocarbon chains, typically containing 4 to 12 carbon atoms per molecule. Diesel fuel is composed of longer chains, generally containing 12 to 20 carbon atoms. This greater number of atoms gives diesel a higher molecular weight and results in closer packing, which increases its mass per volume. Diesel is also less volatile and has a higher viscosity compared to gasoline.
Risks of Fuel Mixing and Contamination
Accidentally mixing diesel and gasoline in a vehicle’s tank leads to severe and distinct problems for both engine types. The specialized nature of modern fuel systems means that even a small percentage of contamination can be highly problematic.
Gasoline Engines
In a gasoline engine, the thicker, oilier diesel fuel can quickly clog the fine filters and injectors, which are designed for the lower viscosity of gasoline. This contamination can lead to poor combustion, misfiring, and eventually stalling. Unburned diesel can also damage the catalytic converter in the exhaust system.
Diesel Engines
Putting gasoline into a diesel engine is often more damaging due to the loss of lubrication. Diesel fuel is inherently oily and provides the necessary lubrication for the high-pressure fuel pump and injectors. Gasoline acts as a solvent, washing away this lubrication and causing increased friction and wear on these components. This lack of lubrication, coupled with the difference in ignition properties, can cause immediate and severe damage.