IMI Magazine

IMI Magazine

Is diesel losing its allure?

So, PSA Peugeot-Citroen claims that the increasingly tougher emissions regime is making diesel cars more expensive. Didn’t we all know that? Costs have been on the rise since the imposition of the first emissions limits in the US in the 1960s.

For petrol-engined cars, the really big jump came with the arrival of the catalytic converter. But the three-way cat converter solution doesn’t work for the diesel, which operates ‘lean’ except at maximum power. Simple oxidising converters reduce diesel hydrocarbon and carbon monoxide emissions, but these are not the real problem, which is created by oxides of nitrogen and, above all, by particulate emissions. For these, the answer is either the theoretically simple but practically complex and expensive particulate filter, or the alternative, less familiar selective catalytic reduction (SCR) which relies on the injection of a reducing agent into the exhaust gas stream.

Of the two, the particulate filter has proved by far the most popular for cars (although SCR has done better in heavy truck fleets). It might well be argued that the particulate filter is to the diesel what the three-way catalytic converter was to the petrol engine. The question is, why is Peugeot-Citroen making its point now, especially since the group pioneered the mass application of particulate filters to passenger cars? According to Gilles Michel, the company’s executive vice president of technology and purchasing, there are two possible reasons for Michel’s scaremongering. One is that the present European regulations are delicately poised. The recently introduced Euro 4 emission regulations set diesel limits which can be met by small, modern engines without needing a particulate filter. The break-point falls at about 2-litre capacity, above which a filter becomes essential. If the limits could be “frozen” at Euro 4, then diesel cars in the B and C segments, with engines up to 1.6-litre capacity, would be spared the extra expense of a filter and the associated control system. Small diesel cars would therefore be cheaper (or rather, no more expensive); and it is in these smaller car classes that the greatest potential for increased diesel penetration remains. European diesel sales in the larger D-segment already exceed 50%.

However, the proposed Euro 5 limits have recently been published (see table) and there seems no chance of keeping them on the shelf, and this brings us to the second possible reason for the PSA argument. Undoubtedly the most troublesome aspect of the proposal, for the car manufacturers, is that the particulate limit for diesels has been lowered by four-fifths. From 2008, if the legislation goes ahead as planned, this will effectively make highly efficient particulate filters mandatory for all new diesel cars (and the same limit will apply to cars using “lean-burn” direct petrol injection technology).

The worry is that the requirement will make diesel cars, and especially small cars, considerably more expensive to buy. The result, as Gilles Michel warns, could be a reversal of the strong European trend away from petrol and towards diesel. In the view of some, this would be no bad thing: they would argue that city-centre air quality standards demand it. Yet there are strong counter-arguments, and these are what Peugeot-Citroen has most in mind. The shift to diesel has been helping European car manufacturers edge towards their commitment to achieve average CO2 emissions of less than 140g/km by 2008 - though it seems likely the target will be missed, except by a handful of companies. Also, potentially, the move will hurt the pro-diesel side of the technical fuel-economy argument between diesels and hybrid petrol-electric cars (although the “best of both worlds” solution of a diesel-electric hybrid is often, and apparently deliberately, overlooked). Finally, if new diesel cars are made more expensive, owners will be more tempted to keep older, less fuel efficient and higher-polluting cars.

Thus run the arguments, and they should not be lightly dismissed. Whether it is helpful for an interested party like PSA Peugeot-Citroen to make them is another matter.  Not that it really makes much difference: there is no evidence that such special pleading has never really influenced the legislators, who look in the first instance to those who elected them. The vision of ever-tightening exhaust emission regulations (the European authorities are already talking about Euro 6 standards) leading to ever-cleaner air in our cities is an easy one to sell to the average citizen, though he may protest when he discovers, every two or three years, how much his new car is going to cost. It is much too easy for the ecologists, pressing on towards their ultimate goal of zero emissions, to dismiss the counter-arguments as special pleading by vested interests. That is no reason to allow the regulatory noose to tighten without protest.

The counter-arguments are valid, and in a sense we now have different ecological interests arguing against each other. Those who want zero emissions of noxious exhaust gases are at odds with those who want minimum “greenhouse” CO2 emissions.

Regenerative particulate filters are cleaned, at appropriate intervals, by the injection of “post-combustion” fuel pulses which raise the filter temperature and burn off the accumulated soot. The burning both of the extra fuel and of the soot creates additional CO2, and imposes a fuel consumption penalty. Exhaust emissions from Euro 4 compliant cars are already extremely low, to the point where they actually “scrub” the air in highly polluted locations, leaving it cleaner than when it entered the engine. An excellent case can be made for switching the emphasis to encouraging the replacement of elderly vehicles, including the “gross polluters”; yet this has never been politically popular, perhaps because it requires original thinking. To the zero-emissions zealots, it represents an unwelcome diversion.

Whatever the vehicle manufacturers say, however logical their arguments may be, it is difficult to see what will prevent the legislative steamroller lumbering towards its ultimate goal.

By 2008, certainly by 2010, all new diesel cars will be equipped with particulate filters. Consumers will be paying the price, dealer workshops will be equipped for checking their correct operation, and our diesel cars will be creating slightly more CO2 than would otherwise have been the case. Whether the result will indeed be a skewing of the car market away from diesel remains to be seen, but Europeans would do well to remember that diesels are much less popular in other parts of the developed world.