Are diesels slipping back?

10 February 2017

By Tim Williamson

Cities around the world have proposed banning diesel powered vehicles and sales in several countries are falling. Does this risk the gains made on carbon or do we need something smarter, moving away from the one size fits all Euro Standards?

Diesels are bad, right? Cities all over the world are choking on air pollution and, in Europe especially, the finger of blame is pointing at the diesel engine, the fume belching, asthma inducing, test cheating pariah of the moment. Of course they should be banned, goes the cry.

But not so long ago, diesels were the future. By the late 1990s, the diesel had shrugged off its smoky, sluggish image and could compete on a par with petrol, at least for most uses. They were also more efficient, reducing emissions (of carbon) and providing a shining light in the fight against climate change. Previously the preserve of HGVs and busses, diesel engines were now popping up across the car fleet, even in small, nimble city cars.

Reflecting this, EU legislation setting fleet average targets for CO2 emissions encouraged manufacturers to push diesels further into their ranges. Tax regimes, in the UK and elsewhere, increased sales so that around half of all new cars sold were diesel. However, there are now signs that new registrations for diesel cars are tailing off in the UK, Germany and elsewhere, with the slack being taken up by electric and hybrid vehicles. There are also moves to find a viable replacement for heavy-duty vehicles (see ENDS, 501, November 2016). On the air quality side, there has been a long running disquiet about dieselisation and doubts cast about whether the carbon savings were real.

Back at the start of the auto-oil programme and the first “Euro” emission standards, both petrol and diesel engines were pretty gross polluters. It wasn’t unusual to see a smoky petrol engine and in that smoke were high levels of lead, carbon monoxide (CO) and hydrocarbons (HC). However, taking the lead and sulphur out of petrol not only reduced their emissions but allowed cars to be fitted with three way catalysts, with huge reductions in CO, NO2, HC and particles. While still not completely “clean”, petrol engines have generally responded well to successive tightening of the Euro standards, to the point where CO and lead are no longer considered priority pollutants and certainly aren’t the useful “markers” of vehicle pollution that they used to be.

The story is different for diesel. Three-way catalysts don’t work in a diesel exhaust – the chemistry is wrong – and so control systems have had to rely on “engine-out” rather than end-of-pipe solutions. This means using some fairly sophisticated engine management systems to ensure that conditions in the engine are set for low pollution. But we also expect our diesel cars to be fuel efficient and to compete in performance terms with petrol and these three aims are not compatible. So how did the manufacturers do it? Cynics have long since suspected some form of test-response has been built into the system although, until recently, there’s never been any proof.

Of course, there are end of pipe type systems for reducing polluting emissions from diesels. Particle filters have been shown to effectively eliminate particle emissions from diesels, certainly if you measure particles by mass which is why Euro 5b onwards includes a particle number limit. The addition of a urea based reagent, alongside a NOx catalyst, allows the removal of most of the NOx through Selective Catalytic Reduction (SCR), although the system needs to be at working temperature and the Ad-Blu reagent needs to be topped up, which is why a fully laden HGV can often have lower emissions than an empty one (AECC provide an excellent explanation of how exhaust catalysts work).

But fitting this miniature chemical factory under the floor of a small car is difficult and expensive and so the introduction of the full kit to the passenger market has been slow. As we’ve seen with the diesel-gate scandal, other methods have been used to demonstrate compliance with Euro standards. Reading through the recent report from Transport and Environment, not all methods are a subtle as those employed by Volkswagen.

So diesels are bad, right? Maybe not. Testing by Emissions Analytics and others have shown that there is still a carbon benefit from driving diesels. They have also shown that there are diesel engine cars, independently tested under real world conditions on real world roads, that, while not completely clean, are at least reaching equivalence with petrol engines. As clean as petrol and lower carbon? That doesn’t sound like something we should be banning, unless we ban petrol as well. The irony is that the leader in producing these new, “clean” diesels appears to be Volkswagen Group, based on R&D from before diesel-gate broke out. Unfortunately, it’s not easy to tell which diesels are clean and which are not; compliance with Euro 6 covers a wide range of emissions performance.

The Euro standard system has been successful in driving down vehicle exhaust emissions for most pollutants. The problems with diesels and NOx are by now well-known and much written about. What perhaps is less clear is how the Euro Standards got us into this position. If you look at the broad process for setting the standards, firstly the pollutants of concern are set and a technological solution to address those pollutants is identified. An emission standard is proposed which would effectively mandate that technology (even though the standards are “technology neutral”) and the industry lobby push back hard, usually on the basis of feasibility and cost. Finally, an introduction date is set to allow the industry time to adapt. Except that there are usually manufacturers who are ready with the technology at the very start of the process and may, in fact, already be fitting it to their models sold in other jurisdictions, such as the USA. The Euro timescale is set at the pace of the slowest and so encourages foot dragging by the industry – why should they invest when their competitors aren’t?

As the limits get tighter and harder to reach, so does the incentive to, I’ll say it, cheat. The current Euro test is carried out in a lab at 20oC and takes 20 minutes. T&E report shows that some manufacturers have set their emissions control equipment to switch off after 22 minutes, or when the temperature falls below 18oC which means that, for most of the time in the real world, it isn’t working at all. Therefore, real world emissions of modern diesels vary enormously, ranging from the vaguely honest to the how-do-they-get-away-with-it?

Emissions Analytics have developed the Equa index in an attempt to provide more precise information about the performance of different models and manufacturers. It’s a useful start and perhaps demonstrates that the EU should move away from standards that all cars must comply with to standards they should comply with, clearly identifying those that do not with a clear labelling system. Maybe a non-mandatory “Super 6” standard, with no margin of tolerance in the standard, tested in three different test centres and under real world conditions using genuine, road worthy models (no “golden vehicles” with their doors removed). If you could only drive cars made by X or Y in the centre of towns, because they’re the only ones compliant with the Super 6 standard, then surely there’s an incentive for all the other manufacturers to catch up, rather than the incentive for foot dragging that we have now.

So maybe not a ban but something smarter than low emissions zones based on Euro standards. What’s certainly clear is that we need more information, at the point of sale, about what’s really coming out of the exhausts of new vehicles, alongside real, workable alternatives to driving private cars in our urban centres.

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