Why do diesel cars get better fuel economy than petrol cars?


You’ll often hear how diesel cars get really good gas mileage and often outperform similar petrol cars in the process. 

We could talk about some seriously complicated science stuff, or just tell you what you need to know. There’s a difference between fuel economy and fuel efficiency. 

Fuel economy is related to how you drive and what the car is used for. Fuel efficiency is, all things being equal, how efficient a source of fuel is. 

So how is a diesel engine more fuel efficient than a petrol?

Well the simple answer is that a diesel uses less fuel to run than a petrol. 

Because a diesel uses compression, not spark plugs, generally it will have a far higher compression ratio. 

A petrol car may have a compression ratio of 10:1, a diesel is more like 18:1. That means for every 1 part of fuel, you’d need 10 parts air for a petrol, a diesel needs 18 parts air.

So you’re using less diesel than petrol to achieve the power required.

How do you have an engine with no sparks?

A diesel injector sprays in an extremely fine mist of diesel that mixes with the pressurised air.

These two atomise and are squeezed by the piston. The compression then creates a lot of heat which leads the air/fuel mixture to explode. 

A petrol engine just uses the spark plug to create the explosion required.

This is why diesel engines will sometimes vibrate more than petrol engines, it’s why they sound different and also why they are used in different applications. 

So why are some cars petrol and some cars diesel?

A diesel engine will produce more torque than a comparable petrol engine, due to the compression nature of it’s engine. The strength of the explosion offers a lot of low down power and ‘grunt’.

You need to ‘rev out’ a petrol motor to get the same power figures. So trucks, vans, utes, SUVs and some sedan and hatchbacks use diesel. The power and efficiency. 

However, the fuel contains more oil than petrol and emits more particulates. This makes diesels worse for air quality via air pollution, and thus, world health.

Regardless of whether you have a diesel or petrol powered car, you need to take care of it. By using My Auto Shop you can easily get your diesel car serviced. Just as easily as getting your petrol car serviced.