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Do diesels have a future?

The scandal engulfing the Volkswagen group - and very probably other major car manufacturers - is a serious one, but essentially about ethics and customer trust. To deliberately falsify the outcome of emissions tests is fraudulent and will have long-running consequences but it is not necessarily, as some commentators have suggested, the beginning of the end for diesel as a mainstream fuel.

Diesel engines have long been the motive power of choice for buses, commercial vehicles and (non-electric) trains. By compressing a mixture of air and low-volatility oil to a high degree, diesel engines create the conditions for high temperature ignition without the need for a spark. This means the fuel burns more efficiently. The engines are also rugged and create high levels of torque (pulling power) at low speed, all of which make them very suitable for their purpose.

The downside is that, despite the overall efficiency and fuel economy, combustion is incomplete and quantities of soot are produced on acceleration. This soot is comprised of tiny carbon particles which are small enough to be breathed deep into the lungs. Until recently, attention focussed on so-called PM10, particles with an maximum size of 10 micrometres, but it is now recognised that the most dangerous fraction is the even smaller PM2.5.

Early diesel engines were dirty, but used at a time when air pollution from domestic heating and industry was, by today’s standards, horrendous. As air quality has improved dramatically over recent decades, so more attention has been focussed on remaining sources of pollution. In urban areas, road transport is a major contributor, and a series of increasingly stringent EU regulations has been brought in to improve matters. The latest - Euro 6 - has recently been introduced and brings emissions of particulates to a very low level.

Diesel engines have been popular for many years across much of continental Europe, but today the ubiquity of turbochargers and improvements to fuel injection technology have reinforced their popularity and now also made them the preferred choice of many drivers in the UK. Recognition by the UK government of the lower CO2 emissions also led to diesels enjoying lower road taxes. The next market to be conquered by this new generation of diesels was to be the USA, and it was there that the discovery was made that VW were selling cars which performed much better under the stringent American emissions tests than in use on the road.

While fuel economy and CO2 and particulate emissions are all favourable, the problem comes with nitrogen oxides, which had previously not been widely talked about as a health hazard. There is now much greater recognition of their harmful effects on health, and it is these compounds which diesel engines produce in much larger quantities than petrol engines. In simple terms, the nitrogen oxides can be destroyed by reaction with urea injected into the exhaust stream, but this comes with some penalty to overall efficiency. The software controlling particular designs of diesel engine introduced over approximately the last five years can activate the urea feed when it senses the vehicle is being used on a rolling road test rig, but deactivates it for normal use.

Those are the facts as best as we can tell. The consequences for the Volkswagen Audi Group, now the world’s largest car manufacturer and owner of some of the most trusted global brands are at best severe, at worst catastrophic. One clear parallel is with BP and the Gulf oil spill, but this carries the added dimension of impacting two highly prestigious consumer brands; Volkswagen and Audi. The bigger they are, the harder they fall.

As for the rest of the car industry, it depends on whether VAG is an outlier or simply leading a trend. But there could also be a negative impact on the wider German economy, not just because VW is such a large manufacturer and exporter, but because the scandal could taint German engineering brands more generally.

The other question is what the overall impact on diesels will be. See for example, Will Volkswagen’s diesel scandal accelerate the shift to electric cars? This article quotes the reply from Dr Paul Nieuwenhaus of the Centre for Automotive Industry Research at Cardiff University when asked whether diesel could make a comeback: “It would be very difficult, particularly because there are these alternatives coming through. Diesel drivers would have benefitted from better fuel consumption and more lively performance but in the process they will have poisoned their neighbours.”

The alternatives are mainstream petrol-engined cars, battery, hydrogen or gas powered vehicles. LPG is widely used as a secondary fuel across Europe, but nowhere has it overtaken petrol, despite being somewhat cheaper and widely available at petrol stations. The problems of using hydrogen as a fuel except in very limited circumstances are far greater. It is extremely difficult to contain and so cannot be reliably stored for any length of time, needs to be highly compressed and or cooled to use in any volume, and is only as environmentally friendly as the energy used to generate it. Combine that with the lack of any significant breakthrough in fuel cell technology, and hydrogen seems destined to remain the fuel of the future for ever.

The latest generation of electric cars are admirable pieces of design and engineering. But even the highly-desirable Tesla models are more proof-of-concept than mass market. Until there is an affordable way to provide a country-wide fast charging or battery replacement system, their use will be limited to journeys which start and end in the same place or where there are known charging facilities and enough time to recharge. For cars driven only in urban areas, they could become an entirely sensible choice, but they are far from replacing the conventional car. If they were to, then a significant number of new power stations would be needed to provide the electricity for them. For electric cars, the future may not be as distant as for hydrogen, but they are unlikely to be the general vehicle of choice for another generation.

Which leaves us with a practical choice between petrol and diesel, increasingly in the form of hybrids. For mainly urban or short commute driving, hybrids make a lot of sense, and the extra cost and weight could be well justified. But the real choice is between the two alternative fuels for the main engine. Since the problem with dieselgate is simply betrayal of consumer trust rather than technical - since VW and Audi cars are well-liked for their overall quality and reliability and not bought primarily on environmental grounds - the real issue VAG has to address is how to regain that trust.

How this is addressed may make a difference to the brand people next purchase (although there are going to be some excellent deals to come, no doubt), but is it really going to have a major effect on the sale of diesels? An informal straw poll would suggest probably not, as long as the current problem is quickly and transparently solved. Many people buy diesel cars because of their demonstrably better fuel economy and their driving characteristics, and these will continue to be important. Beyond that, there will certainly be a revamp of the entire testing system, which everyone knows gives better fuel economy figures than real life driving for both petrol and diesel. Make the Euro 6 standard mean something and make sure it is enforced, and diesel is going to remain mainstream for many years to come.