How does diesel fuel differ from petrol?

Diesel fuel and petrol differ in their chemical composition, energy content, and the way they ignite in an engine.

Diesel fuel and petrol, also known as gasoline, are both derived from crude oil but they are processed differently, resulting in distinct chemical compositions. Petrol is a mixture of hydrocarbons, primarily containing alkanes, cycloalkanes and alkenes. Diesel fuel, on the other hand, is composed of larger and heavier hydrocarbon chains, including paraffins, olefins, aromatics, and naphthenes. This difference in composition affects their physical properties such as viscosity and volatility. Diesel is denser and less volatile than petrol, which means it evaporates slower and has a higher energy content.

The energy content of a fuel is measured in British Thermal Units (BTUs). Diesel fuel has a higher energy content than petrol. This means that diesel engines, which are designed to utilise this high energy content, are generally more fuel-efficient than petrol engines. They can extract more energy from a given volume of fuel, leading to better fuel economy.

The ignition process in diesel and petrol engines also differs. Petrol engines use spark ignition, where a spark plug ignites the air-fuel mixture. Diesel engines, however, use compression ignition. In this process, air is compressed until it heats up to a temperature high enough to ignite the diesel fuel. This difference in ignition process is due to the different properties of the fuels. Diesel fuel has a higher flash point (the lowest temperature at which it can vaporise to form an ignitable mixture in air) than petrol, making it suitable for compression ignition.

In summary, diesel fuel and petrol differ in their chemical composition, energy content, and the way they ignite in an engine. These differences have implications for the design and operation of engines, as well as their fuel efficiency and emissions.