Diesel vs Petrol: What’s The Difference?


Diesel vs Petrol: What’s The Difference?


This blog covers:

  • The Science
  • The History
  • How do Diesel and Petrol Engines Work?
  • Efficiency?
  • Emissions?
  • Biodiesel


The Science

Diesel is made from crude oil. Crude oil liquid contains hydrocarbons and the carbon atoms in crude oil link together in chains. These hydrocarbon molecule chains have different lengths, properties and behaviours.

The crude oil hydrocarbon chain begins with methane (CH4), then ethane (C2H6), propane (C3H8), and then butane (C4H10). Chains from CH4 up to C4H10 are gases at room temperature. Chains from C4H10 to C18H32 are liquids, and chains from C18H32 to C19 onwards are solids.

Petrol, or ‘Gasoline’, is also made from crude oil. The chains from C7H16 to C11H12 are combined to make petrol. This fuel compound’s boiling point is below the boiling point of water and it is highly volatile/ evaporates quickly.

Diesel, or ‘diesel oil’ is known for its heavier/ thicker consistency due to it containing more carbon atoms in longer chains. A typical chain for diesel is C14H30. Diesel’s boiling point is higher than the boiling point of water and it evaporates much slower than petrol.


The History

In 1876, German engineer Nikolaus August Otto developed the compressed charge internal combustion engine that ran on petroleum. However, with these engines, only approximately 10% of fuel created useful energy and the rest was waste or useless energy.

In 1892, however, Rudolf Diesel, a German inventor, believed that engines could be made to run much more efficiently. Diesel developed a diesel engine that was just over 25% efficient, which was more than half as efficient as the current steam-powered engines. This four-stroke internal combustion engine involved the processes of intake, compression, ignition and exhaust. These processes work to convert chemical energy from fuel into mechanical energy by moving pistons in a linear motion to connect to a crankshaft.

Petrol engines have spark plugs that ignite the gas-air mixture causing combustion. On the other hand, diesel engines comprise of a hot air-diesel mixture that combusts when highly compressed. The engine intakes air, compresses it and then injects the fuel directly into the combustion chamber (direct injection). With direct injection, diesel fuel is sprayed directly into the cylinder.

Some petrol engines use port fuel injection or carburettors to help inject the fuel just prior to the intake stroke outside of the cylinder so it can mix with air.




How do Diesel and Petrol Engines Work?

Both diesel engines and petrol engines can use a four-stroke combustion cycle.

The four-stroke cycle for diesel engines is:

  1. Intake stroke: Intake valve opens to let air in. This moves the piston down.
  2. Compression stroke: The piston moves back up and compresses the air.
  3. Combustion stroke: When the piston reaches the top, fuel is injected and ignited, which causes the piston to move back down.
  4. Exhaust stroke: The piston moves back to the top to eject the exhaust out of the exhaust valve created from the combustion.

The difference between the diesel engine combustion cycle and the petrol engine combustion cycle is that the diesel engine does not have a spark plug. Instead, the diesel engine works on self-ignition temperature and compression. The higher the compression, the higher the efficiency and power.

The four-stroke cycle for petrol engines is:

  1. Intake stroke: The piston descends to pull air into the cylinder through open air intake valves as fuel is injected.
  2. Compression stroke: The valves close and the piston moves back up the cylinder to compress the air and fuel mixture.
  3. Combustion stroke: When the piston reaches the top, the spark plug emits a spark to ignite the petrol fuel. This forces the piston back down.
  4. Exhaust stroke: Exhaust is pushed out of the cylinder by the piston through the exhaust valves.



Diesel has a much higher compression rate, thus equalling to higher efficiency and power. Diesel contains a high energy content, with 1 gallon of diesel fuel containing approximately 139,000 BTUs (British Thermal Units, which is the amount of heat required to raise the temperature).

On the other hand, 1 gallon of petrol fuel contains approximately 124,000 BTUs.



Due to the hydrocarbons it contains, diesel fuel emits smaller amounts of carbon dioxide (CO2) and carbon monoxide (CO). However, products such as nitrogen (N) and particulate matter (soot) can sometimes be difficult to remove. Consequently, thanks to improving technology and advancements in the diesel industry, greener alternatives, such as ‘biodiesel’ are also being developed.

Petrol fuel emits much more carbon monoxide (CO) and high hydrocarbons (HC) than diesel fuel, however emit slightly fewer nitrogen oxide (NOx) and particulate matter (soot) emissions.



Biodiesel is a fuel produced from biologically raw materials, such as vegetable oils or animal fats.

Often, these alternative sources are too thick/ viscous to be used immediately as a replacement for diesel fuel. Instead, the oils can be converted into fatty esters through the process of transesterification. Transesterification is the reaction of a fat or oil triglyceride with an alcohol and catalyst to form esters and glycerol to reduce the viscosity thus then to be used as fuel.

Despite biodiesel being further developed over recent years, this isn’t the first time the idea of ‘biodiesel’ has been generated. In fact, Rudolf Diesel, the inventor of the diesel engine, designed his engine to use a variety of fuels, from coal dust to vegetable oils. In 1900, at the Paris World Fair, he demonstrated a model based on peanut oil!


28 February 2020