Petrol or Diesel cars?

The inexorable rise of the diesel took a hammer blow with the VW emissions scandal in 2015 but nonetheless diesel powered cars still make-up almost 50% of the UK’s new car sales. Let’s cut through the emotion which can muddy the water and examine the pros and cons of the diesel engine in comparison with its petrol cousin…

car exhaust fumesOver the last twenty years’, millions of drivers have swapped their allegiance from petrol to diesel. Undoubtedly the wallet has been the main driver but what of other considerations? To help you come to a rational decision we compare diesel and petrol engines from three angles:

Environmental Considerations

You would be forgiven for being confused about the environmental credentials of the diesel engine because even the experts can’t agree. For many years, the diesel engine was lauded as the more environmentally friendly engine. It was argued that it was more efficient in producing energy and discharged much less carbon into the atmosphere than the petrol engine. Indeed, in the belief that they were helping the environment most European countries promoted diesel cars by providing fiscal incentives for lower CO2 emissions and taxing diesel at a lower rate than petrol.

All well and good, until the scientists changed their minds and said actually, although diesel produces less CO2 than petrol it discharges other noxious gases (nitrates and particulates) which are even worse for the environment. But the merry-go-round doesn’t stop there; manufacturers of diesel engines are now claiming that use of particulate filters and SCR technology to inject a combination of water and urea into the flow of exhaust gases, break down the mono-nitrogen oxides in diesel exhaust which makes it a cleaner alternative than petrol.

So where does that leave us? Trying to make sense of the fast moving and sometimes confusing science is difficult for the layman but it’s probably a reasonable assumption to say that that the jury is still out on the comparative environmental impact of diesel and petrol cars. In simple terms they both spew out harmful gases into the atmosphere, we know that but which combination is better or worse is less clear.

If you believe the manufacturers, diesel engines made in Europe today are pretty clean and certainly no worse than their equivalent petrol engines when it comes to pollution. The problem, we are told, lies with older diesels (of which there are millions on the road) which are running without the necessary filters and SCR technology which neutralise the particulates, nitrogen-oxides and hydro-carbons.

Whatever your take on this debate it’s fair to say the manufacturers are making massive strides to reduce the impact of emissions, both from diesel engines and petrol engines. There is hybrid technology, greater fuel efficiency, stop-start functionality, cleaner fuels and improved filters all of which will help make the motor car less polluting than it was.

So if the environment if an important consideration for you I am afraid the difference between petrol and diesel is not at all obvious. You may wish to consider hybrid technology which combines fossil fuels with electricity.


Both diesel engines and petrol engines convert fuel into energy through a series of small explosions. The technical difference between diesel and petrol is the way these explosions happen. In a petrol engine, fuel is mixed with air, compressed by pistons and ignited by spark plugs. In a diesel engine, the air is compressed first, and then the fuel is injected. The fuel ignites as the compressed air heats up.

What does this actually mean for the motorist? For driving enthusiasts, the petrol engine is still the preferred option with its sportier feel and without the unappealing ‘clatter’ which characterises the diesel engine at idle and low speed. It’s hard to imagine Jeremy Clarkson or Richard Hammond getting hyperbolic about a diesel engine. They are pretty quick these days and have undoubtedly come on a heap but they don’t have that je ne sais quoi; that certain throaty roar that only comes from a petrol engine being put through its paces. For some drivers a diesel will never do.

But on the other hand diesel engines generally run colder and at lower revs than petrol engines and therefore tend to last longer and require less maintenance. This of course has a knock on effect on residual values which are usually higher on diesel cars. Diesel cars too, are generally torquier than their petrol equivalent which makes them great workhorses and ideal for pulling heavy loads and driving over mountains. The majority of estate cars are diesel on the assumption, presumably that estates are ‘working’ cars. Diesel engines suit turbo chargers and mid-range acceleration can be exceptional. So, on balance not as exciting as the petrol but the modern diesel is no slouch and certainly long gone are the days when they described as ‘tractors in disguise’.


Diesel cars are more expensive to manufacture than petrol cars and hence are more expensive to buy new. Historically however, they have retained their value well and resale prices on the second-hand market are generally higher for diesels than for their petrol equivalents. Although, with the bad press received over the last year or two, this may change and we would certainly expect older diesels to suffer higher depreciation.

The raison d’etre for the existence of the diesel engine car is fuel efficiency. People buy diesels, in the main because they wish to reduce their fuel bill. The diesel engine is more efficient than the petrol engine and therefore will achieve higher MPG figures.

So, if you drive a lot of miles a diesel engine will save you money but if you pootle around town and cover only a few thousand miles a year it will make little difference. There is generally not a great deal of difference in the cost of servicing and maintenance either, although diesels will generally last longer as they are under less stress.

In summary, the point at which diesels become the sensible ‘financial’ choice is somewhere around the 10,000 miles per year figure. Much more than that, fuel will become an important cost consideration and a fuel efficient diesel may offer you considerable savings. If you drive much less than this, you may be advised to go for a petrol which will be less expensive to buy.