The Physical Properties of Diesel Fuel

old diesel tank image by Charles Taylor from

Diesel was once thought to a be dirty, low-quality fuel source, but increased standards and regulations caused the quality of diesel to improve dramatically.

The Engine Manufacturers Association, with approval from the United States Department of Energy, put out a recommended guideline designed to define the limitations of key physical properties for diesel fuel in order to maintain its newly improved quality level.

Flash Point

When provided with an ignition source, fuel ignites or flashes at its flash point. A higher flash point means a fuel only becomes volatile at a high temperature. Diesel fuel has a flash point of 55 degrees Celsius, or 55 degrees Celsius.


The viscosity of a liquid refers to its resistance against being deformed by shear forces. Fuels under viscosity requirements often provide insufficient lubrication, while fuels above those requirements cause poor combustion. Viscosity for diesel fuel at 40 degrees Celsius, or 40 degrees Celsius, should be between 2 to 4.4 * 10-5 ft^2/s.


The relative density of fuel, sometimes called the API specific gravity, refers to the amount of fuel per unit. A greater, more compact density leads to increased fuel performance. The relative density of diesel should be between 31 and 40.

Vapour Pressure

The Reid vapour pressure measures the volatility of fuels. The absolute vapour pressure of a liquid occurs at 37.8 degrees Celsius or 37.8 degrees Celsius. The vapour pressure of diesel should be 0 kPa.

Pour Point

The pour point of a liquid refers to the minimum temperature that liquid will pour or flow. The pour point for diesel fuel rests at 10 degrees Fahrenheit or -12 degrees Celsius.


Lubricity refers to a fluid's ability to minimise friction between two surfaces in motion with each other. The lubricity of diesel fuel under ideal conditions is 3100.


Distillation measures the temperature range at which a fuel either becomes volatile or becomes a vapour. Number one diesel usually has greater volatility than number two diesel, and therefore has a lower distillation. The 95 per cent distillation end point sits at 288 degrees Celsius for diesel one and 355 degrees Celsius for diesel two.

Accelerated Stability

Accelerated stability tests for the stability of diesel fuel under normal storage conditions, and unstable fuels darken and create gum residues. Diesel fuel reaches its accelerated stability at 15 mg/L.

Water and Sediment

Under ordinary circumstances, diesel fuel appears clear and remains free of water and other sediments. Water and sediments shortens the life of fuel filters and negatively affects corrosion and microbial growth. Diesel fuel can have a maximum 200 ppm of water and 10 ppm of sediments and still be of good quality.

Ash Content

Ash content measures the amount of metal within diesel fuel. A high ash content leads to plugging and injection system wear. The ash content of diesel fuel should be no more than 0.01 per cent.