Petrol in a diesel car? The difference between petrol and diesel

Petrol in a diesel car? The difference between petrol and diesel


Whoops, you managed to put petrol in a diesel car. So what? So the two fuels are actually very different, and have quite different functions, and getting it wrong means you’ll need to drain your fuel system and start again with the correct fuel.  

On the bright side, once you know why it’s a bad idea to get your fuels all mixed up and put petrol in a diesel car, it means you’re less likely to do it again. Knowledge really is power. Here’s the information you need to help you avoid fuel mix-up issues in future.

What is petrol?

According to Wikipedia, petrol is a:

“colourless petroleum-derived flammable liquid that is used primarily as a fuel in spark-ignited internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives.”

The chemicals usually added to petrol improve its stability and performance, also helping to keep engine corrosion at bay and keep the fuel system clean. It can also include chemicals that contain oxygen, stuff like ethanol and MTBE, which help it combust better.

What is diesel fuel?

The most common type of diesel fuel is a distillate of petroleum fuel oil, but many modern environmentally friendlier alternatives have nothing to do with petroleum, fluids like biodiesel, BTL (biomass to liquid) and GTL (gas to liquid) fuels. To help people tell them apart, they’re often called petrodiesels. You really don’t want to put any of these kinds of fuel into your petrol car, either.

What does petrol do inside an engine?

In a combustion engine the sparks from the spark plugs ignite a mix of fuel and air, which causes a tiny explosion. The fuel burns instantly, generating hot gas that forces the pistons down, the energy it releases powers the crankshaft, and off you go. Put petrol in a diesel car and the process just doesn’t work.

What does diesel do inside an engine?

Diesel engines have far better thermodynamic efficiency than petrol ones, which also means they use fuel more efficiently. Diesel engines don’t have spark plugs. The diesel internal combustion engine uses highly compressed hot air to ignite the fuel instead, harnessing compression ignition rather than spark ignition.

Avoid petrol in a diesel car – 5 top tips to tell diesel and petrol apart

  1. Petrol for ordinary road vehicles is almost colourless in its natural state, as is diesel, so that doesn’t help much. But they do feel different. Rub a tin amount between your fingers. Diesel feels oily, while petrol feels cold because it evaporates on your skin, and feels more lot like water.
  2. If you happen to have a container of what you think is petrol, it helps to check the container, as long as it’s in the original container the fuel was bought in. In the UK a red can is for 4 star or leaded petrol, a green is for unleaded and black is for diesel fuel.  
  3. They smell completely different. Most of us know what petrol smells like. It’s worth sniffing them both, one after the other, so you can learn the difference
  4. Diesel feels denser and thicker than petrol
  5. We don’t recommend you burn either fuel to see which is which. But when burning, diesel produces dark smoke and petrol only gives off light, see-through smoke  

Now you know the differences between the engines and why they use the fuels they use. What if you still manage to put put petrol in a diesel car? Don’t tro to fix it yourself. Call the experts in and we’ll get you back on the road within an hour and a half or so, including the time it takes us to get to you.

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