The key difference between diesel and petrol cars is their internal combustion engines. Unlike petrol models, which ignite the fuel with spark plugs, in diesel engines air is compressed to generate enough heat to light the fuel.
The energy generated by the explosion, be it compressed air or spark plugs, is converted into mechanical energy, driving pistons and rotaries to turn the wheels.
Compressed air works better than spark plug ignition, so it’s one up to diesels. But there are plenty of other factors to consider when comparing diesel and petrol cars.
New diesel cars tend to cost £1,000 to £2,500 more than petrol equivalents, according to the RAC. This is due to diesel engines being more expensive to produce, as they incorporate more technology to ensure better performance.
As a result, the re-sale price has traditionally been greater for diesels than petrol motors, which is unsurprisingly taken into account by retailers when setting the purchase price.
When it comes to which type of fuel will deliver a better, more efficient drive, diesel is, again, the winner over petrol.
Diesel engines are up to 40% more efficient than petrol motors thanks to its superior combustion system, which was developed to cater for the different fuel
Diesel is denser than petrol and releases around 15% more energy, meaning better road performance, according to the European Automobile Manufacturers Association
Although more powerful, diesel engines tend to be heavier than petrol engines, and then there is the cost of filling up the fuel tank
Petrol costs less than diesel, with the AA reckoning a difference of around 5p a litre. This is despite RAC Foundation research revealing the wholesale cost of both fuels is about the same.
The reason diesel is more expensive is twofold.
Two thirds of vehicles on UK roads are petrol. Most are privately owned, and it makes sense to appeal to the masses when selling a universal product, such as a car
Most of the other third are commercial diesel vehicles, which consume far more fuel than private cars. Also, HGV and haulage lorry drivers are unlikely to shop around for the cheapest service stations, especially as these are unlikely to be on their routes
A diesel engines’ greater performance capacity can mitigate the higher purchase cost and fuel price. For instance, Which? research shows the average miles per gallon for a diesel is 54mpg, while it is 45mpg for a petrol car.
Despite this cost difference, it could take years for a private car to make enough savings on fuel to balance the books, particularly if you don’t drive much, pooter round town, and rarely venture onto a motorway.
Which? worked out that buying a diesel VW Golf would take 16 years before it was cheaper than a petrol Golf.
If you do more than 20-25,000 miles plus a year, diesel may still be worth it
If you drive less than 20,000 miles stick to petrol – or consider a hybrid or EV
The superior performance of diesel cars over petrol comes into its own on motorways. The diesel engine has more power than a petrol engine, so it has the muscle to deliver slower-speed torque. This means it is better at towing, and more impressively, overtaking.
It’s worth noting that diesels have a diesel particulate filter (DPF) fitted to the exhaust pipe to capture harmful emissions. Captured soot builds up over time and can damage the car unless it’s cleared by driving enough miles to heat up the DPF and burn up the residue.
Additionally, diesels tend to require an AdBlue-reliant selective catalytic reduction system (SCR). This shoots a liquid-reductant agent through a special catalyst into the exhaust system to help control emissions without adversely affecting performance.
MOT test criteria were updated in 2018, with far greater emphasis on exhaust pipe emissions, engine management warning lights, and significantly for diesel cars, the diesel particulate filter (DPF).
If smoke of any colour can be seen coming from the exhaust pipe or there is any evidence that the DPF has been tampered with, the car will fail its MOT test.
Since 2018, there have also more rigorous emissions testing of petrol cars. Section 8 of the MOT Inspection Manual makes it clear that if a car emits blue or black smoke for a continuous period of five seconds at idle it will fail.
Diesel engines produce more noxious fumes than petrol ones, causing harm to the environment. Petrol was long seen as the greater polluter, but the ban of leaded fuel and introduction of catalytic convertors put pay to this perception.
Thanks to catalytic convertors, carbon dioxide emissions are now almost the same for diesel or petrol engines, removing one of the big advantages of diesel cars.
For decades petrol cars were seen as worse than diesel because they emit more CO2 than diesels. However, in recent years petrol engines have become more efficient, as well as being lead-free, and we now know more about the dangers caused by diesel emissions.
With a greater understanding of the dangers posed by airborne diesel particles it’s reckoned diesel fuel emissions cause 10,000 premature deaths each year in the UK, according to research from Oxford and Bath universities.
Dr Christian Brand, from the University of Oxford’s School of Geography and Environment, and Co-Director of the UK Energy Research Centre, said: “The valuation of health effects associated with diesel vehicles are at least five times greater than those associated with petrol vehicles, and around 20 times greater than battery electric vehicles.”
Diesel engine emissions can cause health and environmental problems because they contain the following:
Nitrogen oxides, which the National Atmospheric Emissions Inventory states lowers ‘people's resistance to pneumonia and bronchitis and other respiratory infections’. It maintains that most road transport emissions in 2017 came from diesel cars
Particulate matter, or PM2.5, which the Department for Environment, Food & Rural Affairs (DEFRA) describes as being ‘the mixture of solid particles and liquid droplets in the air. Particulate matter (including soot) is emitted during the combustion of solid and liquid fuels, such as in vehicle engines.’
Was Dieselgate the death-knell for diesel cars?
The 2015 VW dieselgate scandal did massive damage to the future of the diesel internal combustion engine as well as the reputation of Europe’s biggest car maker. VW’s emissions tickling speeded the end of diesel as a responsible, long-term fuel source for most cars.
The rigging was so serious that VW had to pull all diesel models out of the US for good. But the nuanced reality is that if diesel engines were abandoned much of the global supply chain would buckle.
Reports of diesel’s demise are exaggerated, especially if you’re talking truck traction. Diesel’s not going away here
Things are less clear for diesel-powered trains, as battery and hydrogen fuel cell technology improves
Ironically, dieselgate failed to kill VW and its share price (July 2020) is on the rise
Services, repairs and replacement parts for diesel cars are also more expensive than those designed for petrol cars, which is also taken into account when insurance premiums are set.
Despite the two fuels being taxed at the same rate, vehicle tax is different for diesel and petrol cars. New diesel cars may pay more than petrol cars due to the amount of emission they give off, which is dictated by the Real Driving Emissions 2 standards (RDE2).
New diesel cars not meeting RDE2 are taxed at the next rate up from their registered emissions in their first year. This means a new diesel in the 91-100 grams per km group would pay the sum set for the 101-110 bracket, which is £175 rather than £155.
After the first year, tax is paid at the same flat rate of £150 as applies to petrol cars.
Certain regions and roads in the UK impose restrictions on diesels and petrol cars, if they do not meet strict requirements on emissions. Although not strictly speaking a tax, drivers of these cars can face a charge for entering a restricted area.
The Ultra-Low Emission Zone (ULEZ) in London sees all motorists subject to a charge if their cars do not meet with minimum emission standards. These are Euro 4 standard for diesels and Euro 6 standard for petrol cars.
According to Transport for London, ULEZ standards are generally met by petrol cars first registered with DVLA after 2005, or after September 2015 for diesels. A daily £12.50 fee is payable if the standards are not met. This is in addition to the Congestion Charge.
London does not have a monopoly on driving restrictions. Bristol Council [recently approved plans to ban all privately-owned diesel cars from entering a clean air zone in the city centre between 7am and 3pm by March 2021.
Diesels and petrol cars that don’t meet Euro 4 and Euro 6 standards, respectively, will be banned from driving within the limits of Manchester’s A4540 Middleway Ring Road from 2021.
Other clean air schemes affecting car drivers are either being proposed or awaiting introduction in various cities, including:
Increased adoption of these schemes further highlights the government’s zeal to tackle vehicle emissions.
For decades diesel was seen as the smart option. Resale values on cars powered by the fuel were higher than for their petrol-driven equivalents. This was partly due to the fact that when diesel is ignited it lubricates the engine, meaning it lasts longer.
The knock-on effect of greater longevity is diesel cars have a higher resale value than like-for-like petrol models, despite the higher cost of replacement parts. However, the government has other ideas for the car sector.
The sale of new petrol and diesel vehicles will be banned from 2035, perhaps earlier, depending on the outcome of a climate change consultation. The ban would affect resale values. Both types of car are also likely to be affected by ever greater taxes and restrictions.
Currently, it will be legal to drive petrol or diesel cars on UK roads after the ban comes into force, as the it will only be the sale of new vehicles, not second-hand ones that will be affected.
However, considering the government has already changed key aspects of the ban, there is always the chance that further unwelcome revisions, if you drive a diesel, could appear.
Bringing forward the ban five years to 2035 is a case in point. The original 2040 deadline was rejected when it became clear that the 2050 target for the UK to achieve zero carbon emissions would be unrealistic. Likewise, hybrid cars were originally exempt from the ban.
After legislation banning vehicles powered by fossil fuels comes into force drivers will only be able to buy pure electric or hydrogen-powered cars. Hybrids will not be an option as they have a combustion engine as well as an electric motor, and will also be banned.
The ban will change the motoring landscape, and sooner than you might think. Yes, petrol cars dominate new sales, accounting for 65% in 2019, with diesels taking a 25% slice, according to the Society of Motor Manufacturers and Traders. But the tide is turning.
The ban on vehicles with an internal combustion engine may not come into force for over a decade, by which time most drivers will have bought at least one car, but as the deadline draws nearer the appeal of pure electric or hydrogen-driven cars will increase.
Electric cars are relatively cheap to recharge and are exempt from car tax. As further increases to vehicle tax for diesels, petrol and hybrid cars are introduced, and the purchase price of ‘green’ cars falls, sales can be expected to rise.
With this in mind, it seems likely that car manufacturers will gradually turn their backs on diesel and petrol cars in favour of ‘green’ models. Porsche has already stopped making diesels, and others will surely follow suit as the deadline approaches.