One of the biggest decisions behind car-buying is based on how it is powered. Whether you choose a petrol engine, a diesel engine, hybrid or even fully-electric, each have their own pros and cons that make them suitable for different drivers.

Diesel engines can be some of the most fuel efficient in terms of miles per gallon. But some diesel cars are excluded from city centres due to their emission levels. So can you get a clean diesel car?

How diesel engines work

Diesel engines have been around almost as long as cars have. Their comparatively simple design to petrol engines means they can be more fuel efficient. But how does that work?

Both petrol and diesel engines work through combustion. Fuel and air is injected into the engine cylinders where it is compressed by the pistons. This pressure makes the fuel ‘explosive’, i.e. volatile enough to get a reaction. To do this, an electric spark is used to ignite the compressed air. This creates the energy to power the car.

Diesel engines compress much more air than regular petrol engines. This creates a lot more pressure, making the energy output when this compressed air is ignited a lot stronger.

Does that make diesel more efficient?

This difference in how the engines work means that diesel cars are much more fuel efficient. They have to burn less fuel to get the energy release that petrol engine cars have to use for the same output. That means you get more miles to the gallon, less trips to the petrol station, more money in your pocket.

While diesel models tend to be a bit more expensive than petrol engines, with some estimates putting the average increase on the same make and model as £1,500 more for the petrol car. Diesel tends to be more expensive per litre too. But you will save more money as you don’t need to fill up your car as often as you would with a petrol car.

But are diesel engines clean?

Older diesel engines had a large amount of particulates in their emissions. This is the dirty excess from the diesel-burning process. This used to make diesel cars a lot more polluting than their petrol counterparts. In built up areas, this could cause air quality to drop dramatically.

Modern diesel engines are a different beast. This is because improvements in the quality of air filters. They now filter out up to 99% of those particulates, meaning that the emissions are far less toxic than they used to be.

These cleaner engines mean the cars are a much more attractive choice. In models like the BMW 320d (pictured), you can choose between the petrol and diesel and not have too much variance in the dirt of the emissions.

While they won’t match the hybrid or electric standards being set, a modern diesel car can help save you money without increasing your carbon footprint too much. Until the car-buying rules change in 2030, there’s no reason why you can’t see out the twenties in a diesel model.

Clean diesel engines – the numbers

There have been many studies over the years as car companies and emissions testers look to calculate whether diesel engines have become cleaner or not. One of the biggest studies was carried out by Emissions Analytics, an independent global testing firm who conduct scientific measurements of most makes and models.

The study, which took place back in 2018, examined diesel engines from a wide range of makes and models. The test looked at the emissions to see what level of CO2 was emitted, as well as how much Nitrous Oxide (NOx).

The results would to be very interesting to those who wanted to take advantage of a diesel car’s benefits without affecting their carbon footprint or air quality too much. When analysing models that fit the Euro 5, and Euro 6 category diesel cars with the best performing diesel engines emitted almost fifty per cent lest nitrous oxide than the top 10% of dirtiest petrol cars.

All the tests were conducted under the RDE standard, i.e. real world driving environment. That means the tests were carried out on the road and in real traffic, and not in laboratory or test conditions. Despite the findings of the test, some of the best-performing diesel cars did see an increase in the amount of vehicle excise duty they were charged. Why? Perhaps it is an image issue with diesel engines. While there are still older diesel models on the road with their more heavily polluting engines, cleaner diesels will still be seen as ‘dirty’.

What sort of cars are best for diesel engines?

You’ll tend to see a diesel engine in bigger types of cars. So in estates, SUVs or saloons. Mainly because the engines themselves tend to be larger, so they would take up more space in a smaller hatchback/supermini.

Can you get a hybrid diesel?

One way you might be looking to reduce your emissions while taking advantage of diesel’s affordability is through a diesel engine. While the vast majority of hybrids and PHEVs are in petrol format, there are some diesel options available. But there aren’t many. Some of the top rated diesel hybrids include the Audi A7 Sportback, Citroen DS5 and Hyundai Tucson.

Diesel engines and air quality tests

One of the biggest drivers against diesel engines is their affect on air quality. But a 2021 test by Green NCAP, a initiative set up to try and find and promote the development of cars that are clean, energy efficient and do not have such a negative impact on the environment.

Green NCAP carried out a test on the level of Nitrous Oxide being emitted by cars. Surprisingly, several diesel models appeared in the top ten, somewhat at contrast with their reputation.

The SEAT Leon, Skoda Octavia and Volvo XC60 4X4 not only scored well for clean air, but outperformed some hybrids. What that means is that, when a hybrid was not using its electric motor, the cars emission levels were more polluting than some diesels.

While the difference here is that exclusively diesel engines are always creating emissions, whereas hybrid engines aren’t, it’s interesting to see just how well these modern diesel engines perform should reassure those who think all diesel engines are filthy.