/ Motoring

Would you still buy a diesel?

Amid headlines of manufacturers cheating emission tests and the harmful effects of NOx on our health, is there any room left for a defence of diesel?

Diesels are distinctly out of fashion. With consumer confidence in them shot after various emissions scandals, higher UK road taxes, new MOT rules and some cities around the world banning them outright, driving a diesel seems to be becoming taboo.

Add to this growing public awareness of the effects of NOx emissions and the subsequent plummeting sales, and it seems it may be curtains for diesels.

Decline and fall

Since VW’s ‘Dieselgate’ rocked the world in 2015, sales of diesel cars have plummeted in the UK, falling from 50.1% of overall new car sales in 2014 to just 31.9% as of July, according to data published by the SMMT (Society of Motoring Manufacturers and Traders).

And it’s not without reason. Public awareness about NOx (oxides of nitrogen) emissions from diesels – linked to tens of thousands of premature deaths around the world – has grown massively.

Air pollution from vehicle emissions is a very real problem. In 2017, for example, it took just five days for Brixton Road in London to break its NO2 (nitrogen dioxide) limit for the entire year.

And the results from our new research paints a fairly dirty picture for diesel: on average, the 61 diesel cars we tested produced 0.27g/km of NOx – nearly three and a half times the official Euro 6 limit (0.08g/km).

Diesel defence

But does it have to be this way? Diesels generally produce less carbon dioxide (CO2) and carbon monoxide (CO) than equivalent petrol cars.

And they don’t inherently have to be NOx-spewing monsters. Some manufacturers are starting to produce some squeaky clean diesel cars.

Currently they are the exception rather than the rule, but we’ve found two Mercedes and one BMW diesel car that produce less or the same NOx emissions as your average petrol.

AdBlue & SCR make a difference

More and more diesel cars are using Selective Catalytic Reduction (SCR). It’s an active emissions system that injects urea, most commonly AdBlue, into the exhaust system to cancel out harmful emissions.

And it makes difference. We found that diesel cars with an SCR system, on average, produced about a third of the NOx than cars without one.

So fitting these systems on all diesels could make a huge difference to NOx emissions.

But it’s really a question of whether manufacturers think it’s worth the cost to add these sophisticated systems to their entire range of diesel cars – something that will no doubt be influenced by the tough new on-road emission tests that will be in force for all new cars by September 2019, known as RDE step 1, and will require better real-life emission control than before.

Room for reform?

Some manufacturers clearly do think there’s public appetite for cleaner diesels, a Mercedes spokesperson telling us previously:

‘Our new engines are both highly efficient and produce low levels of NOx. It’s a fact that it’s worth improving the modern diesel instead of banning it.’

That’s one opinion, but it is true that relatively ‘clean’ diesel cars are possible – and available, if you’re willing to pay for them.

The problem is these ‘clean’ examples really are the exception and, it seems, time may already have been called on diesel generally.

But what do you think? Do you agree that diesel has a future if it can be improved? Is the public snubbing of diesel going too far?

What about if someone presented you with a clean diesel car – it produces less NOx, CO2 and CO, and costs the same as an equivalent petrol car – would you buy it or not?


Transport for London says, in connection with the proposed ULEZs, that retro fitting emissions reduction technology is unlikely to become available for cars and small vans, because it would too expensive to develop. Is this pessimistic view generally accepted? Will Which be monitoring how the situation might evolve?
Robert T

They have a site about filters, but this seems only to include commercial vehicles:
Petrol cars since 2005 are OK, but diesels sold before Sept 2015 may not be Euro 6. I guess converting them will be uneconomic so best to sell before the rush and buy a compliant car. however, seems to be some moves in Germany to reduce NOx emiisions in older cars http://europe.autonews.com/article/20170802/ANE/170809901/automakers-offer-to-upgrade-5-million-diesel-cars-in-germany
This site shows a retrofit filter designed for bigger vehicles http://www.wandhgroup.co.uk/index.php/ulez

From what I have read, there simply is not room for these modifications in most modern cars, where the engine is crammed into such a small space that even a routine maintenance job such as changing the timing belt is a difficult and expensive job. On commercial vehicles, more space is available to fit diesel particulate filters and reservoirs for urea solution, both of which are bulky.

Air Quality is the biggest health emergency bar none (wars, obesity) worldwide and where I live in Southampton. I cannot see any discussion on particulates which are known to be hazardous, especially the very fine PM2.5’s, known to cross membranes even into the brain and affect babies in the womb. Bad, we just don’t know how bad. Diesels I believe produce very much more particulate as well in general as NOx than Petrol. Electric cars produce the same tyre but less brake dust (due to regenerative braking) but the health cost are very much less at only £13 a year. Averages for diesel cars is £258 per year, petrot £37 a year. We love our electric car (a Nissan Leaf) which we bought second hand (for £8000) and does nearly everything we need – but for that which it, train, e-bicycle or bicycle do not, we very occasionally hire a car for short periods. It works out so much cheaper. Hybrids are an unsatisfactory halfway house for us – more complex and expensive, and all that polluting fossil fuel gubbins has be dragged around with you all the time. The main hurdle is whether you can charge (cheaply) at home. Houses without their own parking place and flats need investment & regulation from Government via councils.

Those health costs look impressive, but where do they come from and what else should be in the equation? Until the government can sort out a transport policy that makes the train a more viable option, then electric cars only work for very short journeys and commutes, so car hire remains the alternative. It is still much less convenient to collect a car from some place miles away than to walk to one outside your house. At present convenience wins over the cost argument!

Nigel Reese says:
1 January 2019

Modern diesels are hard to beat, the hysteria whipped up around the VAG cheating software and the obvious problems with old diesels has skewed the debate. The high levels of NOX exist as a by product of highly efficient high temperature combustion, they occur in less efficient lean burn petrol engine emissions too. We are in danger of throwing out the baby with the bath water. Battery power will have significant impact on our environment, not least our ability to supply the power to recharge them:10,000 miles driven is approx equivalent to the total power used in a 4 bed house per year. We need to invest in a variety of solutions, and efficient clean diesels are part of the solution.

Elizabeth MacCallum says:
25 January 2020

There seems to be very little awareness of the reduction in emissions that ad-blue makes. I have been persuaded to buy a small SUV fitted with this because apparently I can even drive through London without being penalised now. Recent radio and TV discussions about climate emergency & the need for more electric cars overlook the fact that such cars at present are totally impractical till all chargers are standardised, thousands more charging points are fitted and a massive increase in the production of electricity occurs. There is also the problem of the extremely limited availability if lithium for batteries – a new battery for storage needs to be developed that is lithium free.

Which? News that appeared today says “Petrol cars produce a lot of carbon monoxide (CO). In our tests, which are tougher than the official tests, one in five models produced CO in quantities that exceed all emission limits from the past 27 years……..

Euro car-emissions laws limit the amount of CO cars can produce (in official test conditions). This started with CO being limited to 2.72g/km in 1993, when Euro 1 was introduced. Years later, when Euro 2 replaced Euro 1, the limit was dropped to 2.2g/km. We are currently on Euro 6, and the limit is 1g/km of CO. Our tough tests are more challenging than the official tests, and include a unique motorway cycle. We believe our tests to be more realistic, and therefore more indicative of what your car actually produces. “….

Read more: https://www.which.co.uk/reviews/new-and-used-cars/article/air-pollution-and-car-emissions/cars-that-produce-the-most-co – Which?

This sort of article is fine if it simply shows the output from different vehicles (assuming the tests are conducted under identical conditions). However what is misleading is to link the results to the EU limits. As with all emissions, the limits given by the EU are only valid when measured against the accompanying standard test regime. They cannot be cited when a different test regime is used. This, in my view, is a failing Which? have shown in several criticisms of cars’ mpg and emissions. It might make better headlines but I want accuracy, not misleading reporting.

If our government is so concerned about diesel car pollution then why did they make it more attractive to buy a diesel car from a road tax perspective. Since 2001 it has always been cheaper to tax a diesel car when comparing it to a similar sized engine petrol car. After owning my last car, Mazda 6 2.0 diesel I vowed never again to buy another diesel caused by the horrendous problems that this car suffered with dpf issues. It wasn’t the cars fault, just a poor design of the dpf regeneration system. However, 40mpg around town, 55-60mpg on a run, cheaper to tax when comparing to the 2 litre. The petrol variant of that year wouldn’t come anywhere close. I just changed my car for another diesel. Not Mazda as you may have guessed. Again it’s a 2 litre diesel, 150BHP, does 38mpg around town and I am a little heavy footed. Not sure about long runs yet and it’s £120 a year to tax. Ford offer a 150BHP 1.5 petrol turbo in the same car but that didn’t interest me at all. Small engine in a car that weighs what the Kuga does, no way, and from reports I see on the forums there are loads of complaints about economy. The diesel car is not dead yet and offerings by the manufacturers of small petrol turbo engines tuned to get the power up is just shortening the life of the engine. I didn’t buy a QASHQAI for that reason. My next car will be a diesel also and perhaps more pressure should be put on the oil companies to find a solution that produces a cleaner fuel which produces less emissions and toxic gases.

I wonder if being heavy-footed might be a factor in DPF and EGR problems. In the days before they were fitted, rapid acceleration produced clouds of black smoke, but now the soot is retained in the engine.

I thought the billowing black smoke was because someone had cut the governor cable to get a few extra BHP at the expense of loads of unburnt fuel?

I had a Montego estate many years ago with a Perkins diesel engine. Leaving my drive one day, late for work, I put my foot down to get away from a car approaching up the road. It promptly disappeared in my cloud of black smoke and emerged with its lights flashing in a rather angry mood. Not something I repeated.

Penny Price says:
8 June 2019

I would not buy a diesel car again as I have gone electric, I would not buy petrol either. I have a BMW i3 with a range extender. This car really means you can have your cake and eat it, all the benefits of an electric car, quiet, whizzy acceleration, low carbon footprint, with the range anxiety taken away. The range extender means that if you have to travel out of range of your battery, a small petrol driven motor kicks in to generate power for the battery. In reality after two years I perhaps have to resort to the range extender 10% of the time to complete a journey just beyond range, that I do every month. The car costs peanuts to run, I have solar panels at home, so plug it in during the day, as I work from home. My home electric bill for house and car is £15 a month. The only downside is because it’s not classed as a full electric I pay £140 road tax, but sooner or later all electric cars will have to or there will be no money in the treasurys kitty.

Like all other vehicles there is pollution from tyre wear and tear, but much less from brake pads- you don’t use brakes much in an electric car, because power is generated from deceleration instead. The battery ethics is a lot more worrying, and that is my main concern as the cobalt used may well come from conflict zones like Congo. This is something that Tessa are starting to change.
Penny Price

It seems that the ‘range extender’ version has been dropped: https://www.which.co.uk/reviews/new-and-used-cars/bmw-i3/review

I agree with you about the ethical concerns, but hopefully batteries from electric vehicles will be effectively recycled, which does not alway happen with batteries small rechargeable products.

The government will want to make up for lost revenue from vehicle and fuel duty but maybe for the time being there is an incentive to go electric.

The Range Extender i3 will still be available in North America and Japan, where demand remains higher than Europe and the rest of the world.

The larger capacity battery is said to give the i3 sufficient range without the supplementary 650cc engine. Personally I’d prefer the security of alternative power until we have an adequate network of fast charging points for those longer distance drives.

Recycling is one aspect, but re-use is better. Batteries in comparatively small products when they reach their “half life” are pretty useless for most things so get recycled if you’re lucky or just go into the land fill. However, electric vehicle batteries have such a high power density that they could be useful for many other things (emergency lighting, energy storage, storage heaters… plenty of others) once they are not good enough for an EV but still have plenty of uses. This needs a change of culture of course and collaboration – but would mean recycling is delayed keeping carbon footprints lower for longer.

That’s how we have to think, Roger. Perhaps the secondhand value of EV batteries that have lost some of their original capacity will be what is needed to encourage reuse.

I don’t expect the scale of personal transport we currently have to be sustainable. At some point we will see towns and cities with out-of-town interchanges and public transport serving the built area. I wonder whether anyone has the organisational ability to set that in motion? We should ban polluting vehicles in those environs sooner not later and provide decent, regular and wide-ranging electric bus and taxi services.

I travelled down a crowded M5 yesterday. The M4 was queuing and Bristol at 7pm was very busy too. Millions of people using their cars to go about doing things. How do we change such a life style and the desire to move at will at anytime of our choosing to live as we wish? How do we change the public attitude to the car? True, better alternatives will help, but this current way of life has been with us for decades and is inbuilt into the way we live. Also true, it is getting progressively more difficult to get about and, left to our own devices, we might probably solve the problem by gridlocking the road system and be forced into change. Already, I choose my journey times carefully and avoid likely jams and hot spots when this is possible. The whole area of population movement needs to be looked at as complete survey -why people move, what they do in their cars and what consequences might there be for any change. Once this is understood, plans can be developed to compensate for any restriction and disruption to life style and essential travel – children to activities -visiting relatives- shopping and so on. The business world also needs understanding and looking at with care for what it does and how it does it. This is joined up thinking, removed from knee jerk politics of bans and instant remedies that suddenly change things that have been going on for ever. It doesn’t have to be protracted either. Getting on with it is essential so we can change together.

As a flippant corollary to the above, it would seem that we have to suffer to save the planet. The common perception is that the more we suffer the more the planet is saved. I’m looking in the cupboard for my ash birch and sack clothing so that I can prepare for what’s in store.! My yurt is on order and as is the bow and arrow so I can hunt the local oxen. This will have to be cooked in a solar oven to avoid burning fossil fuels and wood. Hope it doesn’t rain.

I think it is time we abandoned “landfill” as a way of disposing of our non-degradable rubbish. We should have national centres for recovering materials to which all local authorities send their segregated “waste” materials, and local centres to segregate all materials. Cost? We must bear that as a means of preserving our future. Degradable rubbish? Presumably much of it can be composted and put to use.

A national, legally-enforced, system is needed. Together with legally-enforced reduction in unnecessary and non-recyclable packaging.

However, this comment arose from the related question of what to do with spent EV batteries. Maybe we could put them to other uses if those uses are genuine and worthwhile, but we should not create uses for the sake of it.

Our current methods of landfill management are actually preserving degradable waste by hermetically sealing it in and preventing its decomposition. There was a TV programme a few months ago on our waste management problems that opened a landfill site and found forty-year old newspapers still intact and readable, textiles still useable, plastic carrier bags still functional, and so on. There are good reasons for entombing the waste in clay-lined pits to prevent harmful leachate from polluting watercourses but topping them off with more layers of clay prevents any organic decomposition of the material inside. The alternatives to landfill are not always satisfactory either, so the real answer is to eliminate waste.

John Neeves says:
21 August 2019

In the current discussion of the relative merits of petrol and diesel cars I have never seen any comments on safety. Diesel fuel is far less flammable than petrol as is well known to a woman friend of mine. Her husband’s petrol car was hit by a lorry at a motorway exit and the car went up in flames. I do not know whether he would have survived the crash if his car had been a diesel car but he would certainly not have burned to death.

That’s a very sad story, John. When I bought my first diesel car the lower fire risk was one of the attractive features, though I have never looked at statistics. Diesel engines have replaced petrol inboard engines in boats because explosive petrol fumes can accumulate in the engine bay if there is a leak.

Diesel fuel is still a fire risk and there is a current recall of half a million Volvo vehicles with Diesel engines.

Where does Which? stand on the subject of climate change? Pollution is something which needs to be tackled but CO2 does not contribute to climate change.

Perhaps, MartinK, you could expand on your brief comment?

What is causing climate change?
Geological records stretching back millions of years indicate a number of large variations in Earth’s past climate. These have been caused by many natural factors, including changes in the sun, volcanoes, Earth’s orbit and CO2 levels.

However, comprehensive assessment by scientists shows that it is extremely likely that human activity has been the dominant cause of warming since the mid-20th Century.

Greenhouse gas emissions
Evidence that CO2 emissions are the cause of global warming is very robust. Scientists have known since the early 1800s that greenhouse gases in the atmosphere trap heat.

Global CO2 emissions from human activity have increased by over 400% since 1950. As a result, the concentration of CO2 in the air has reached more than 400 parts per million by volume (ppm), compared to about 280ppm in 1750 (around the start of the Industrial Revolution).”


You state CO2 does not contribute to climate change.

But could you share any evidence you have for that? We know that most heat from the Sun is absorbed by so-called ‘greenhouses gases’ and reflected around the atmosphere, warming the planet.

We also know there are four main greenhouse gases:

Nitrous Oxide
Water Vapour
Carbon Dioxide

Of those we know, with certainty, that humans have increased atmospheric CO2 (Carbon Dioxide) concentration by more than a third since the Industrial Revolution began

Methane is a hydrocarbon gas produced both through natural sources and human activities, including the decomposition of wastes in landfills, agriculture, and especially rice cultivation, as well as ruminant digestion and manure management associated with domestic livestock. On a molecule-for-molecule basis, methane is a far more active greenhouse gas than carbon dioxide, but also one which is much less abundant in the atmosphere.

MartinK – I suspect that climate change is beyond the remit of Which?, though some of our Conversations have discussed it: https://conversation.which.co.uk/page/3/?s=%22climate+change%22&cat=0#038;cat=0

The extent of human impact on carbon dioxide production is rather contentions but there is absolutely no doubt that we are responsible for poor air quality in cities, polluting our waterways with chemicals, and plastic waste in the environment.

I have never understood why vehicle taxation is related to carbon dioxide production rather than toxic materials emitted.