Most of today’s automobiles run on engines based on fossil fuels. Many types of engines have been developed, the two most common types of engines are Petrol engines and Diesel engines.


Petrol engines are mostly used in cars and smaller vehicles. Petrol engines are internal combustion engines which have spark ignition.  They run on relatively explosive fuels such as petrol. In petrol engines, air and fuel are mixed before compression. Once the air and fuel are compressed, the fuel is ignited by using an electric spark. Petrol engines work on the Otto cycle which explains how in petrol engines chemical energy is converted into thermal energy and the into motion.

       How Petrol Engines Work

How petrol engine works

Step 1: Fuel (petrol) and air are added in the cylinder and mixed.

Step 2: The mixture of petrol and air is compressed by the crankshaft.

Step 3: Sparks are used in the mixture to ignite it and the mixture forces the piston down.

Step 4: Gasses go out from the exhaust valve.


Generally, trucks, bakkies and other heavy vehicles run on diesel engines. The diesel engine is also an internal combustion engine. In diesel engines, the fuel (diesel) is injected into a combustion chamber and is then ignited by the high temperature of the air in the chamber. The engines only compress the air and not the fuel.  When injected into the combustion chamber, the fuel undergoes spontaneous ignition. Diesel engines work on the Diesel cycle which consists of a constant pressure process, a constant volume process and two isentropic processes.

       How Diesel Engines Work

How a diesel engine works

Step 1: Induction Stroke – Piston of the engine goes down and air comes into the cylinder.

Step 2: Compression Stroke – Piston goes up and compresses the air, now the air is getting hot.

Step 3: Power Stroke – The fuel is added in the cylinder.

Step 4: Exhaust Stroke – Fuel starts burning due to ignition temperature provided by the hot air gasses/smoke go out from the exhaust valve.


Petrol Engine Diesel Engine
Type of fuel use – Petrol Type of fuel use – Diesel
Presence of spark plugs Presence of fuel injectors
Works on the Otto cycle Works on the Diesel cycle
The fuel is mixed in a carburettor The fuel is mixed inside the cylinder
The fuel and air mixture are compressed together The fuel and air are not compressed together


Ignition is induced by spark Ignition is induced by heat generated by compression of air
Fuel consumption is higher Fuel consumption is lower
Engine has lower efficiency than diesel engine Engine has higher efficiency than petrol engine



What happens when you put petrol in a diesel engine?

As well as powering the car, diesel acts as a lubrication oil that keeps the fuel pump and other components running smoothly. Petrol, acting as a solvent when mixed with diesel, has the opposite effect.

Adding petrol to a diesel engine increases friction between parts, causing damage to the fuel lines and pump. Diesel is meant to self-light and therefore do not have spark plugs like petrol engines. Therefore, if you put the wrong type in and start the engine, it can cause damage to the fuel injector and other parts of the engine. If you drive away or even start the car with petrol in the mix, you could cause expensive damage to the engine.

What happens when you put diesel in a petrol engine?

Luckily, putting diesel in a petrol engine isn’t as bad as the other way around. Petrol is made of a mixture that is designed to be stable and not set fire to itself – instead, the engine has a spark plug that sets fire to the petrol in order to start the car. Because diesel needs to be compressed before it’ll ignite, chances are you won’t even be able to start the engine.

You put in the wrong fuel, what do you do now?

Whether you’ve added diesel instead of petrol, or vice versa, to your car, there’s still time to avoid a huge repair bill. The best thing is to switch the vehicle off and have it towed to a workshop. You should avoid driving the vehicle. The longer the vehicle is driven, the higher the level of damage. At a workshop they should be able to drain, flush and refuel your vehicle.

Which vehicle components are harmed by driving with the wrong type of fuel for your vehicle?

Depending on the contamination ratio, you can receive different levels of damage.

Typically, if you have a diesel contamination in a petrol vehicle:
• No permanent damage to major components.
• You will need to drain and flush the whole fuel system.
• Replace fuel filter.
• In extreme cases, drain engine oil and replace oil and oil filter.

Typically, if you have petrol contamination in a diesel vehicle:

Here at VAG Spec Centre we do replacement of damaged engine parts and complete engine overhauls. Trust your Audi and Volkswagen Specialists to help and to determine the best and fastest route to get you back on the road!