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Posted on: 27/04/2017

Diesel & Petrol - what's the difference? CPS Fuels Information Sheet

Diesel & Petrol differences 

Diesel and unleaded petrol are both hydrocarbon mineral fuels derived from crude oil extracted from the earth by drilling and pumping. Varoius liquid fuels (Diesel, Gas Oil, Heating Oil & Petrol), gasses, plus other useful hydrocardon derivatives, are all refined from crude oil by a process of boiling and condensation known as fractional distillation. At the refinery distillation columns are used to separate the crude oil into different molecules and substances, at different temperatures.

Humber Refinery

Diesel is refined at a higher temperature than petrol and has a certain viscosity according to the different surrounding temperatures (unlike petrol). Diesel also has denser and less `explosive’ properties, making it more suitable for heating applications, as well as for fuelling the internal combustion engines of vehicles. The fuels in the Diesel range are known as middle distillates, whereas Petrol, is known as coming from the Petroleum spirit range. 

Combustion differences

Each fuel requires a different combustion process in order for the fuel to burn and the engine to operate. They are similar but there are key differences to note. The most obvious difference is that for the pistons to be powered in an engine, they depend upon different ignition systems.

The petrol engine needs a spark to ignite the vapour and air mixture produced by the carburettor when it is compressed in the combustion chamber. A series of four piston strokes completes the cycle. Initially, the vaporised fuel and air is drawn into the combustion chamber via an open intake valve; the piston then compresses the mixture as it moves up the chamber and at the correct moment a plug produces a high voltage spark that explodes the gas and forces the piston back down. As the piston moves back up the chamber the intake valve is shut and an exhaust valve opened by way of a connecting cam-shaft operating the valves timing and allowing the spent gasses to be expelled. Finally, the exhaust valve shuts and the intake valve opens again allowing the fuel mixture back into the chamber as the piston moves downward, completing the cycle.

Diesel Petrol PumpsThere are normally four pistons fitted on a crankshaft so that they complement each other in performing and continuing each of the four functions. The vertical motion of the pistons translate mechanically through a connecting crank and drive shaft to the wheels, propelling the vehicle forwards or backwards depending on gear selection.


The diesel engine generally has a similar four stroke/four piston configuration as the petrol engine, except it does not have a sparking ignition system in order to motivate the pistons, but instead relies initially on an electrically heated glow-plug to ignite the fuel injected into the combustion chamber when the engine is cold and first being started. Further ignition is generated simply through the act of the piston compressing the air, which heats up through friction to such a degree that it ignites any diesel fuel on contact. Similarly to the petrol engine, valves connected to a cam-shaft operate in synchronisation with the pistons’ four stroke timing.

Diesel engines are usually considered more efficient on fuel economy than petrol engines, and more robust. They are more difficult to stall as they operate at lower rpm (the revolutions of the shaft per minute) and have higher torque making them good at towing and suitable as work vehicles, such as tractors. Torque is a little like the mechanical advantage you exert through leverage, however, this doesn’t ordinarily translate into speed. Petrol cars tend to have more power and higher rpm, equating to faster speed, more wear and tear on the engine, and less fuel economy.

CPS Fuels can supply bulk diesel locally to your own fuel station, or nationally using the extensive Keyfuels fuel card bunker network to your vehicles on the road.

Call our friendly Commercial Team today!