Diesel, also known as gas oil, is a petroleum-derived fuel used primarily in compression-ignition internal combustion engines, commonly called diesel engines.
Technically, diesel is a mixture of hydrocarbons, composed primarily of molecules with longer chains than those of gasoline. This gives it distinctive physical and chemical properties that make it suitable for applications requiring sustained power and thermal efficiency.
Next, we will explore in detail the physical and chemical properties of diesel, as well as their relationship to performance, safety and environmental impact.
Diesel composition
Diesel fuel is composed primarily of saturated hydrocarbons (paraffins) and aromatics with carbon chains typically ranging from 10 to 22 carbon atoms per molecule. The specific proportion of these compounds varies depending on the type of diesel and the refining process.
There are different grades of diesel, such as low-sulfur diesel and synthetic diesel, which are obtained from alternative sources such as natural gas or biomass.
In general, the hydrocarbons present in diesel give it a high energy density, which means that it can release a large amount of energy when burned.
However, this composition also influences other properties, such as viscosity, flash point and emissions generated during combustion.
Physical properties of diesel
Below I show you the main physical properties of diesel fuel:
Density
Diesel's density is higher than that of petrol and typically ranges between 0.82 and 0.85 g/cm³. This makes it heavier and denser, which has a positive impact on its energy density. At the same volume, diesel contains more energy than petrol, which contributes to the greater efficiency of diesel engines in terms of kilometres per litre. However, this higher density also means that diesel generates more CO₂ emissions per litre burned.
Goo
Diesel is more viscous than gasoline, meaning it is less fluid at room temperature. Its viscosity ranges from 2 to 4 centistokes at 40 °C, depending on the formulation and local regulations. This property is important because it affects the atomization of the fuel in the engine's injectors.
If the viscosity is too high, the fuel may form large droplets instead of a fine mist, reducing combustion efficiency.
To improve performance in cold climates, additives are added to reduce viscosity and prevent the formation of paraffin crystals that could clog fuel filters.
Flashpoint
The flash point of diesel is significantly higher than that of gasoline, ranging from 52°C to 96°C.
This property makes diesel fuel less volatile and therefore less flammable under normal storage and handling conditions.
This feature contributes to your safety, as diesel is less likely to accidentally ignite compared to gasoline.
Volatility
Diesel is less volatile than gasoline, meaning it evaporates more slowly at room temperature. This is due to its larger, heavier molecules. While its low volatility can make starting the engine difficult in cold weather, it also reduces evaporation losses during storage and transportation.
Color and smell
Diesel fuel has a colour that can vary from light yellow to brown, depending on its quality and sulphur content. Its characteristic odour, stronger and more penetrating than that of petrol, is due to the presence of aromatic hydrocarbons and sulphur compounds.
Chemical properties of diesel
Below I present the main chemical properties of this fuel:
Combustion
Diesel combustion occurs in an engine through a compression ignition process, where air is compressed to high pressures until it reaches temperatures high enough to ignite the fuel without the need for a spark. The general equation for the complete combustion of this fuel is:
C n H m + O₂ → CO v + H v O + energy
Under ideal conditions, diesel burns completely to form carbon dioxide (CO₂) and water (H₂O), releasing a large amount of energy. However, in practice, incomplete combustion can generate byproducts such as carbon monoxide (CO), unburned hydrocarbons, soot and nitrogen oxides (NOₓ).
Cetane number
The cetane number measures how easily diesel ignites under compression. A higher cetane number indicates that the fuel ignites more easily and produces a smoother burn, reducing engine knock and improving performance. Typical diesel fuels have a cetane number between 40 and 55.
This parameter is equivalent to the octane number in gasoline, although it measures a different property. Fuels with a lower cetane number can cause ignition problems and increase pollutant emissions.
Sulfur content
The sulfur content of diesel has been the subject of strict regulation due to its environmental impact. Sulfur, present in the form of sulfur compounds, generates sulfur dioxide (SO₂) when burned, a gas that contributes to acid rain and air quality problems. Modern diesels are typically low or ultra-low sulfur, with levels not exceeding 15 parts per million (ppm) in many countries.
4. Chemical stability
Diesel is relatively stable during storage, although it can oxidize and form gums and deposits if exposed to air for extended periods.
This oxidation can affect fuel performance and clog engine injectors. To mitigate this problem, antioxidants are added to diesel.
In addition, diesel fuel can be contaminated with water, which can encourage the growth of microorganisms in storage tanks. This problem is solved by the use of antimicrobial additives and the periodic removal of accumulated water.
Impact of additives
Additives in diesel improve both its performance and environmental sustainability. Some of the most common additives include:
- Cetane improvers: increase the cetane number and improve ignition.
- Antifreeze additives: prevent the formation of paraffin crystals in cold climates.
- Detergent additives: reduce the formation of deposits in injectors and valves.
- Stabilizers: improve fuel life by preventing oxidation.
Safety and handling of diesel fuel
Diesel is less flammable than gasoline, making it safer during transportation and storage.
However, handling them requires precautions, as prolonged exposure to the vapours or liquid can cause skin irritation and respiratory problems. Contact with soil or water sources can also have negative environmental effects due to their polluting nature.