(Credit: iStock)
If the diesel engine were a celebrity, its story would make for a good movie. Once akin to a Cinderella, fit only for the dirty service on tractors and heavy trucks, the technology rose in popularity among fuel-consumption and carbon emissions-conscious drivers in the late 1990s. But the stardom didn’t last long – two decades later, diesel engines became a target of the health-conscious public’s disdain for their excessive contribution to harmful air pollution.
In the UK, with about 11.2 million diesel cars on the roads, owners of older diesel vehicles could soon be paid up to £2,000 to scrap them and replace them with a cleaner car – as part of a new government strategy to tackle air pollution. In Europe as a whole, in 2016 diesel car sales dropped below 50% for the first time in a decade. Other parts of the world report similar trends: in India, for instance, sales fell from 52% in 2012 to only 26% last year – after the Indian Supreme Court imposed an additional 1% sales tax on all diesel cars in an attempt to discourage sales.
The recent revelations of Volkswagen cheating in emissions tests have contributed to diesel’s fall from grace, as have numerous studies quantifying the impact of diesel-generated pollution on human health. The public debate has gained prominence, inspiring authorities in many European cities to pledge to ban diesel vehicles completely in less than a decade.
But is this necessarily the right move? Yes, diesel cars emit nitrogen oxides (NOx), said to cause lung conditions such as asthma, and older models are believed to be especially polluting. But is current diesel really so inferior to its combustion sibling petrol that it needs to be totally shunned if we want to breathe better air?
NOx dilemma
University of Bath researchers measure vehicle emissions, taking into account different driver behaviours (Credit: University of Bath)
Not everyone thinks so. According to Chris Brace, an engineer at the new Centre for Low Emission Vehicle Research (CLEVeR) at the University of Bath, one has to look at the current diesel emissions problem in the context of the past 25 years of technology development. “If you looked at combustion engine exhaust – both diesel and petrol – prior to 1992, you would find carbon monoxide, which is a serious poison,” he says. You’d also find lead in petrol, and unburned hydrocarbons and nitrogen oxides in quite large quantities, he adds – and the diesel engines back then were producing a lot of soot.
Brace first joined the Bath engineering team in 1992, the year when the first European legislation limiting combustion engine emissions – the Euro 1 standard – came into force. And even though older diesel cars are indeed rather “dirty,” he says that modern ones paint a different picture. “Today, you’ll find that all these pollutants have been reduced by the order of 10 and in some cases 100,” he says. “We have now reached the point when we have mechanisms to deal with all of the serious pollutants in both gasoline and diesel, and that includes the nitrogen oxides and particulate emissions.”
Nitrogen oxides emissions were at the heart of the Volkswagen scandal, first indicated in research by the International Council on Clean Transportation (ICCT) and West Virginia University in 2013 and 2014. And while NOx produced by petrol engines can be eliminated by the same unit – the three-way catalytic converter – that also eliminates carbon monoxide and hydrocarbon residues, the procedure is much more complicated in diesel engines.
Chris Brace's team uses a robot that replicates real-world driving situations (Credit: University of Bath)
“Petrol engines run at what we call stoichiometric air-fuel ratio, which means that all the oxygen is consumed with the fuel and there is no oxygen left in the exhaust,” explains Brace. “With a diesel engine, you always have oxygen in the exhaust – they always run lean. And if you have a lot of oxygen in the exhaust, it’s very difficult to perform reduction chemistry.”
The gas needs to be broken down by an aftertreatment system that most frequently relies on the urea-based selective catalytic reduction. It was this technology that the Volkswagen defeat devices were designed to selectively turn on in laboratory emissions tests. In normal operations, the nitrogen oxides emissions scrubbing system wouldn’t turn on at all, leading to emission levels far exceeding legal limits.
Clean diesel?
However, does diesel technology as such deserve a bad name based on one manufacturer’s transgression? And is the nitrogen oxides problem unsolvable? Arvind Thiruvengadam, a mechanical engineer at West Virginia University’s Center for Alternative Fuels, Engines and Emissions, who was part of the team that uncovered what was going on at Volkswagen, believes that’s not the case.
“Current technology diesel engines have the potential to be near-zero emissions,” says Thiruvengadam. “Clean diesel is not a myth. The same study that found the problem in VW vehicles also showed that a BMW X5 with the same technology as VW has emissions below the standard during most operating conditions.” And, he adds, technology for near-zero emissions in diesel engines does exist – and there is ongoing research to make it cost-effective and more durable.
Peter Mock, managing director at ICCT Europe, agrees. “The latest tests by German autoclub ADAC found that, while some vehicles perform very well in laboratory as well as in road tests, others still show massive discrepancies between the acceptable laboratory performance and the quite unacceptable real driving emissions.”
European legislators have been forcing automotive engineers to push the boundaries of the combustion technology's efficiency and cleanliness (Credit: iStock)
Europe will introduce mandatory road testing for new models later this year. The problem is that the pace of technology development is much faster than the speed at which people buy new cars. In Europe, systems removing NOx have only been made mandatory since 2015, when the Euro 6 standard came into force. Particulate matter filters removing all particles down to the size of 20 nanometres have been required since the 2009 Euro 5 norm. Buyers who opted for diesel before that will be obliged to pay £12.50 on top of the existing congestion charge for entering central London from 2019.
Carbon footprint
As the technology has been promoted for its superb fuel consumption and better carbon footprint, the proportion of diesel passenger cars has increased in some parts of the world from nearly zero in the early 1990s to half of the fleet today. From the air pollution perspective, these vehicles were apparently not quite ready. If we take Brace, Thiruvengadam and Mock’s word for it, the technology is now there to make diesel engines ‘clean’ – the question is whether the public would be willing to give it another chance.
Diesel engines have “higher engine efficiencies than gasoline engines,” says Thiruvengadam – so “they offer better miles per gallon compared to gasoline”. And it’s exactly the same feature that makes diesel more efficient, and in turn leads to it producing less CO2, which makes it challenging to remove NOx from the exhaust.
The VW emissions scandal tarnished the reputation of diesel technology (Credit: Getty Images)
“Global warming is perhaps the most serious and difficult challenge that we have to face as a society, so any technology that allows us to be more efficient and reduce our CO2 emissions is really important,” says Brace, who together with his colleagues have spent years fine-tuning diesel engines. Their cooperation with Ford in the 1990s resulted in a 15% reduction in diesel engine fuel consumption. Ford used the results in its award-winning Duratorq and EcoBoost diesel engines, which reportedly produce a combined 150,000 tonnes of CO2 per year less than earlier technologies, equivalent to the annual CO2 emissions of 32,000 passenger cars.
“If you look at road transport, particularly the larger, heavier vehicles, there is no viable alternative to diesel engines if you want efficient road haulage and efficient engines,” says Brace. Modern diesel engines can achieve up to 50% thermal efficiency, and, he thinks, potentially more.
“The gasoline engine can just about approach that in extreme conditions, say in Formula One – but these systems operate at very high load. In real life, when you are driving around the city, you would see that the diesel engine would be on average 25% more thermally efficient than a gasoline engine vehicle.”
Brace has no doubt that diesel has a place in future cities. While electric vehicles would help clean up the city air faster, they are still far too expensive, and their real environmental impact depends on the cleanliness of the electricity used to charge them. In China, which relies largely on coal-fired power plants, electric vehicles actually ‘produce’ up to five times more particulate matter than combustion engines, according to a series of studies by Tsinghua University. And hybrids, while promising, are still too costly and struggle with battery limitations, says Brace.
So he and his team are not giving up on diesel any time soon. The university’s planned £60 million Institute for Advanced Automotive Propulsion Systems will look into some innovative concepts that could further clean up combustion engine’s act, such as cold or split cycle combustion.
“It could be possible to get up to 60% thermal efficiency with the right technologies, and that would be a very significant change over what we can achieve today,” says Brace. “The other thing to bear in mind is that the waste from that system can also be used, so, although you might be converting half of the chemical energy into the work directly, that still leaves you with maybe another half of the energy that can be harnessed to do useful things outside of the engine.”
It will take years for such new technologies to reach the market, of course – but, with the continuing research efforts, diesel may not be about to say its last word just yet. n