Insight: Is it time to give up on the diesel engine?

Recent figures from the SMMT, the UK’s car manufacturers’ club, indicate that CO2 emissions have been reduced for the past 19 straight years and are now well over 30% lower (at an average for a new car of around 120g/km) than in 2000.

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Second, the automotive industry is understandably reluctant to criticise the cars it has already put on the road, on the grounds that they complied with the legislation of the time. UK car users have bought roughly a million diesels per year and there are an estimated 12m diesel cars and vans already on our roads. Penalising them would create havoc. Completely changing the car parc, if you started now, could take 20 years.

Third, Europe’s motor industry needs to preserve its markets, viability and infrastructure to fund new, electrified cars planned along its ‘glidepath’ towards the 95g/km manufacturer fleet average that's required by 2020 – and onward towards a hoped-for zero-emissions future in 2050. (In the UK, Jaguar Land Rover has just opened a new diesel plant in Wolverhampton and Ford builds most of its world requirement for diesels in Dunton).

Fourth, its bosses are extremely reluctant to wade into a complex, illogical debate that has conflated Volkswagen’s highly publicised diesel emissions scandal in the US with a 15-year-old progression of EU emissions standards – currently at EU6 – whose fuel consumption results bear so little resemblance to owners’ experience that they are presumed to be dishonest. ‘They’re all at it’ is the common accusation.

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The testing regime (called NEDC or New European Driving Cycle) attached to that 15-year progression is more at fault than the actual emissions standards. The impending WLTP will change all that and is welcome. The WLTP standards will be accompanied by new real-world driving tests, called RDE, and it is the combination of the two that will deliver new, believable fuel consumption and emissions figures.

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The Government, meanwhile, is hamstrung (and embarrassed) by its promotion over past decades of a legislative framework that has appeared to encourage diesel adoption as a way of lowering CO2 – evidently at the expense of cutting NOx. Knowledge of the harmful effects of NOx (about a quarter of which emanates from transport across the UK, but about half in London) has increased greatly in the past five years, although government agencies have lately been made unpleasantly aware of university studies that started issuing NOx health warnings 20 years ago.

Officialdom is very reluctant to offend car users in large numbers, such as by outlawing or cost-loading diesel cars they bought in good faith. Lobbyists say offending the motorist has in the past proven to be about the most vote-sapping action a government can take. Even London’s mayor, Sadiq Khan, who has loudly expressed a determination to improve London’s air quality by charging diesels £10 to enter the London congestion zone from October, is applying the charge to only pre-EU4 diesels - those made before 2005.

“When EU4 was introduced for 2005, the industry’s attention turned to diesel particulates,” says Mike Hawes, CEO of the SMMT. “By the time EU5 arrived in 2010, they were the big issue. NOx wasn’t seen as damaging the way it is now, but I’m definitely not saying the industry disputes the effects of NOx on human health: we’re no experts. Our concern shows in the speed we’ve brought anti-NOx after-treatments to market.”

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What about diesel particulates? Consisting mainly of the ash left over from combustion, these are acknowledged as an important problem and damaging to human health, but the automotive industry believes it has already moved to cut them dramatically by fitting cars with particulate traps (DPFs) from 2010 when EU5 was introduced.

Why isn’t there such concern over the pollution contribution of petrol cars? Because experts say that can be more easily controlled by increasingly effective catalytic converters, first seen in the early 1990s. Levels of petrol exhaust pollutants rise less rapidly than diesels with extra speed, extra load, difficult gradients and hard driving (which is why diesels have attracted all of those headlines about reputable cars producing 10 times their advertised output of pollutants in real-world tests).

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The lower compression ratios of petrol engines have traditionally resulted in their production of considerably less NOx than diesels from combustion. Neither in the past have they had the diesel’s exhaust ash problem, requiring a particulate filter. But the situation could be about to change. The rise of more frugal, harder-working, smaller-capacity turbocharged petrol engines – with higher resultant compression ratios – is likely to mean they will also make more NOx. There are also strong indications that they may produce finer particulates that may be harmful and may need filters of their own. Research continues…

The most urgent problem, identified by London mayor Khan, appears to be the profusion of old-school diesels – notably, well-worn and decades-old taxis, trucks and delivery vans as well as passenger cars – on our roads. The mayor has already hit the headlines, and been rebuffed by the government, for proposing a £500m scrappage scheme that would pay diesel owners up to £3500 for ditching old diesel cars.

Before he left office, London’s previous mayor, Boris Johnson, also proposed a 2020 clean air directive that would penalise pre-EU6 (2014) diesels and pre-EU4 (2005) petrol cars by charging their drivers £12 on top of the regular daily congestion charge to enter London’s congestion zone. Latest indications are that Khan’s administration will keep the idea but bring it forward by a year. The writing seems to be on the wall for old diesels. Those who need older cars for inner London will do better owning petrol models.

What should the concerned private car owner do about diesel ownership? First, say the experts, consider the kind of driving you do. If you have a healthy early diesel and never drive in urban areas, it’s no crime to keep it. You won’t be contributing to urban pollution, the urgent problem.

Steve Gill, director of powertrain engineering at Ford of Europe, says: “Our concern is that customers will move away from diesels as a result of unbalanced coverage of the emissions situation. We’re not trying to force diesels on customers – they should buy what they want – but we’ve had to reposition some of our products and we’re having discussions now about where to put our money in the future.”

In Autocar’s opinion, diesel car ownership must not be off the agenda, because a bedrock of diesel sales is necessary to keep CO2 levels falling, the latest cars are clean and the industry needs the sales to fund the electrification that is coming. There is no doubt that the cleanest cars, with both their particulate and NOx outputs ‘trapped’, are EU6 models, produced from 2014. We’d choose one of those.

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Were we about to buy a new diesel car, we’d consider delaying the purchase until models tested under the new, real-world WLTP process enter the market at the end of this year. Then, despite all the fuss, we’d feel we’d chosen a car that was clean, efficient, legal and great to drive.

David Greenwood, head of advanced propulsion at Warwick University, says: “A 2016 EU6 car emits less than 1% of the particulates a similar car did in 1992. For NOx, the output is well below 10%. If you read some media, you’ll see headlines that say air quality is getting worse in cities. It’s not. The data says it’s getting better. But it’s not getting better at the rate we’d like it to.”

The bottom line is this: the cleanest diesel you can buy is probably the one you’ll be able to buy late this year, which not only meets current EU6 standards but has also survived the latest WLTP ‘real-world’ test procedures. If you need an older car and are worried about urban pollution, go for a post-2014 EU6 diesel or a post-EU3 petrol model, 2005 or later.

Diesel engine DOS

1. Do think about the kind of driving you do. Diesels, even older models, are fine if you don’t drive in high-pollution areas.

2. Do consider an electric or plug-in hybrid if you have a two-car family fleet. Your second car can be a diesel. You may find your EV’s smoothness makes it your first choice.

3. Do spread the word that better times are imminent for diesels, brought by stringent standards and a better testing process.

4. Do study the facts about diesel vehicles and air pollution, and pass them on. Disinformation has the potential needlessly to threaten jobs and cripple an important industry.

Diesel engine DON'TS

1. Don't get the idea that criticism of diesel cars is all wrong. There are at least seven levels of car diesel ‘cleanliness’. Only recent versions can truthfully be termed clean.