A clean fight

Diesel cars offer good fuel economy and low greenhouse gas emissions. But they are facing increasing opposition from local politicians,

are blamed for pumping out too much NOx 

In the Thomas the Tank Engine books, the first truly evil locomotive character to be introduced to the idyllic railway world on the island of Sodor is the unnamed diesel loaned from The Other Railway and sent back when the Fat Controller discovers his Machiavellian machinations and his wicked ways with trucks. A second character, BoCo, also a diesel loco, is then regarded, unfairly, with deep suspicion and nicknamed the Diseasel by the other engines. 

The Society of Motor Manufacturers and Traders (SMMT) fears that diesel cars as a breed are being demonised in the same way these locomotives were, and that it’s equally unfair. But where the fictitious BoCo could redeem the reputation of his class through good deeds, diesel cars are not getting the same opportunity, says the SMMT.

Diesel vehicles are being blamed by local authorities for a rise in urban air pollution that could see cities such as London penalised by the European Commission. London Mayor Boris Johnson has mooted doubling the congestion charge for diesel vehicles in the centre of the capital, and Islington borough this month brings in a £96 surcharge for parking permits for all diesel cars. Other cities such as Oxford, and also Paris, are considering action too.

The SMMT says these policies are misguided and based on false information. “Today’s diesel engines are the cleanest ever, and the culmination of billions of pounds of investment by manufacturers to improve air quality,” says chief executive Mike Hawes. “Bans and parking taxes on diesel vehicles make no sense from an environmental point of view. We need to avoid penalising one vehicle technology over another, and instead encourage the uptake of the latest low-emission vehicles by consumers.”

Islington Council’s executive member for environment and transport, Claudia Webbe, is not convinced. “We don’t believe there are any clean diesels, not that have been created yet,” she told a north London newspaper. “We’re committed to tackling air quality and they’re proven to produce nitrogen oxide. Independent tests of the new Euro 6 vehicles show that they emit seven times more nitrogen oxide in the urban environment than the industry’s own tests suggest.”

Euro 6, the new EU emissions standard which comes into force in September, sets much stricter limits for emissions from all vehicles and continues the downward trend on carbon dioxide, particulates and nitrogen oxides (NOx) that has been in place for almost 20 years. Over this period, CO₂ has tended to make the headlines; the automotive industry trumpeted its achievement in reaching the 2015 CO₂ target two years ahead of schedule.

Some of the achievement on CO₂ is down to the rise in popularity of diesels over the past 15 years. In 2014, slightly more than half the cars sold in the UK were diesel; 10 years ago, it was barely a third, and before that even less. 

The reason numbers have been increasing is a bit of an anomaly, given the widespread view that diesels are environmentally unsound. It’s that CO₂ emissions, blamed for global warming, are directly linked to fuel economy, and significant gains in diesel engine efficiency have given a fuel advantage over petrol-driven vehicles. Their usage has been encouraged by the tax regime: Islington’s diesel car drivers pay lower vehicle excise duty than their petrol neighbours, but now have to pay more to park. 

The good side of diesel is all part of a broader set of political trade-offs, says Dave Greenwood, professor of advanced propulsion systems at the University of Warwick: “The political choice behind this has been whether you care more about global warming and energy security, in which case it’s about reducing CO₂ emissions and fuel consumption, or you care more about urban air quality and public health. Where does the balance lie?”

In Europe for the past 20 or so years, he says, policy on emissions has been driven in part by questions of energy security. “It’s about making better use of the crude stock you’ve got available, because when you break crude down you get a natural mixture of diesel and gasoline, and if you then burn only one of the two you’ve got to find uses for the other.” 

That requirement, coupled with greenhouse gas reduction, has stimulated European governments to back increasing the diesel share of the market: cautiously, in the UK’s case. In the US, where there are fewer concerns about energy security and urban pollution such as the infamous Los Angeles smog was seen as the most unacceptable result of emissions, the emphasis was different. “A diesel car in the US is more expensive than in Europe, because the level of aftertreatment needed to meet the very low NOx and particulates standards is higher and more expensive,” says Greenwood.

Drive to clean up: Particulate filters have cut NOx emissions as a spin-off

The trade-off in Europe does not mean that NOx and particulates have been ignored, and particulate filters are standard on heavy emitters such as trucks and buses. Rules such as low emission zones have had some effect, too. But now air quality and health have risen up the political agenda in Europe too, and targets for NOx in cities are being missed. 

Euro 6 aims to counter this, and sets stringent rules for NOx generation on individual vehicles. That it is happening just when local politicians are changing rules is an aspect of the ‘unfairness’ that grates particularly at the SMMT: blanket restrictions that apply to all diesel vehicles should not be used, it says, as modern diesels are significantly cleaner in all respects than old ones.  

But the anti-diesel politicos may have some points. The SMMT says that little over a quarter of NOx emissions in the UK come from road transport, and less than half of that from cars. But there is, says Greenwood, probably a specific problem with nitrogen dioxide, which pollutes locally in that it does not disperse rapidly from where it is emitted. So for vast swathes of the country, it’s barely a problem at all. But in cities, with stop-start motoring and heavy traffic and, in some places, hills and traffic lights, it is a problem in the immediate vicinity of roads. 

Up until now, a lot of the work on diesel emissions has been targeted at particulates, since soot particles ingested into the lungs have a specific carcinogenic role. Perversely, though, Greenwood believes that long-term work on particulate filters has been helpful for NOx reduction: “It might sound counter-intuitive, but when you optimise a diesel you’re trading off particulates against NOx emissions, so if you’ve got a particulate filter then you tune the engine for good NOx, because you know the filter will deal with the particulates.”

Euro 6, however, is much less forgiving about this kind of trade-off. Kathye Henderson, marketing manager at Eminox, a long-term leader in particulate traps, says there has been big growth in the market for systems that combine filters with selective catalytic reduction (SCR) to cut the NOx as well. 

Henderson quotes some impressive numbers for SCR systems. Particulate matter at the very small sizes now targeted can be reduced by 90%, NOx by 95%, and nitrogen dioxide – that seems to be the biggest problem – by 96%. But, she admits, the systems that work are for the trucks and other heavy-duty vehicles that are the biggest problem and are expensive: they’re not yet ready for mainstream cars.  

A further criticism is that the emission reductions that are promised by manufacturers, and that are the basis for the claims of substantial progress, are not borne out in the real world. No one is accusing the industry of falsifying results, but there has been increasing disquiet that the current vehicle test, the New European Driving Cycle, is not the way we drive nowadays. 

There may be a particular relevance here for NOx emissions, since the area of apparently greatest disparity seems to be urban driving, where the reality of stop-start is likely to be very different from the smooth, consistent speeds with few halts that were true in more relaxed times. It’s the stop-start nature of today’s driving that may produce the urban concentrations of nitrogen dioxide.

What’s now being developed is a new test cycle – the World Light-vehicles Test Cycle or WLTC. “It’s intended to be more representative, so there are more accelerations and decelerations and a bit of higher speed, too,” says Greenwood. WLTC may help make the figures more credible. Regulation in the heavier vehicles is based on tests that are even less grounded, since it’s almost impossible to devise a typical cycle. There’s more work needed here. 

More is needed elsewhere, too. One of the lessons to be derived from the Thomas the Tank Engine books is that you should be careful what you wish for. If one consequence of moves against diesel is to make petrol cars more attractive again, then there will be a price to pay in terms of fuel economy and CO₂ emissions.

Diesel engineers produced their gains on economy and CO₂ by raising temperatures and pressures; to bring petrol engines up to the same levels, similar engineering would be needed. That could then increase particulate and NOx formation from petrol engines too, as the trade-off. “We need to have a long-term balance on this,” says Greenwood. “When you get short-term shocks in policies, it may change buyers’ habits, and that’ll change manufacturers’ abilities to invest consistently.” And that would not be good for anyone.