Diesel power has seen a huge upsurge in popularity with new car buyers over the last few years, driven not least by diesel cars – claimed – highly impressive fuel economy coupled with the perception that they are greener than their petrol-powered brethren. The slightly hysterical publicity given to the – admittedly high – levels of atmospheric pollution affecting most of the south and east of England in the last few weeks has, however, highlighted to the general public a few issues which may just gain wider publicity and eventually cause diesel power to be seen as far from an environmentally acceptable transport solution. Let me put my cards on the table here. I’m no sandal-wearing promoter of renewable energy and all that good stuff – rather I’m an engineer with a lifetime spent in vehicle development, admittedly mostly of trucks and buses, but for the purposes of this article the principles are the same whether we are talking about a bus or a Bentley. As such, many years of my life have been spent grappling with ever-tightening levels of diesel emissions legislation on new vehicles under the auspices of European-wide legislation – Euro 1 through to the current level, Euro 6. These Euro standards have admittedly raised the quality of vehicle emissions dramatically, and every credit is due to the European powers-that-be for that.
And yet the Euro emissions standards have also produced some unintended consequences, which in some respects have made emissions issues worse rather than better. To explain further… I need, with apologies, to try to delve into the technicalities of emissions in a little more detail. Vehicle emissions broadly break down into four components: hydrocarbons, which are broadly the unburned elements of the fuel used, Oxides of Nitrogen, or NOx, Carbon Dioxide or CO2, and Particulates, PM or “soot”. The relative levels of each pollutant are different for petrol and diesel power, with diesels being high emitters of particulate matter whilst petrol engines generate higher CO2 levels.
Now, Hydrocarbons are relatively benign in terms of their damaging effects, and have therefore not been a major source of EU vehicle emissions legislation attention. Neither, oddly, has CO2, which has instead attracted UK and other governments focus due to the Kyoto objectives to reduce greenhouse gas, or CO2 levels. Particulate matter (PM) was historically the real focus of the Euro standards, since it was, quite rightly, seen as both a visible pollutant and a very real health hazard. Despite many raised voices from the automotive industry proclaiming loudly that the legislated levels could not be achieved, Euro levels 1 through 3 did in fact drive noteworthy levels of PM reduction.
This, however, is where the wheels started to come off the legislative standards. Achieving the legislated reduction in particulates was achieved initially by vehicle and engine manufacturers in a couple of alternative ways. Firstly, by making the engine run lean, thus effectively reducing the quantity of particulates emitted: this however had the side-effect of increasing the levels of NOx emitted. Alternatively, and perhaps paradoxically, manufacturers forced their engines to run rich, hence emitting more particulates but keeping Nox levels lower, and then using a filter to remove the majority of this particulate matter.
Unfortunately, the test regime dictated by Europe for these tests fell somewhat short of true operational conditions, resulting in a number of problems in service. Firstly, where vehicles were fitted with exhaust filters, known typically as diesel particulate filters, or DPF’s, these tended to clog up, causing poor running particularly in stop/start traffic conditions, as the filters rely almost entirely for successful operation on the entering exhaust gas being very hot. Indeed, there were numerous recalls and campaigns on heavy vehicles to introduce improvements to these systems. The alternative “lean burn” engines were no less troublesome, although in a much more insidious way. Their high level of NOx production were not immediately obvious, until air quality monitoring stations began to pick up significant increases in NOx, the cause of which was eventually traced to these supposedly low-emission engines. Unfortunately, NOx is known to be a particular factor in exacerbating some respiratory conditions, thus this increase was, and is, extremely unwelcome. The later Euro standards, and particularly the current Euro 6, therefore focussed on reducing NOx levels, which is being achieved by vehicle manufacturers by producing engines with increasingly complex emissions control equipment, most particularly for diesel engines.
In the case of most Euro 6 diesel engines, both types of emissions controls discussed above need to be fitted in order to meet the required emissions levels, leading to highly complicated engine packages. Indeed, one truck manufacturer was at one time strongly encouraging their customers to buy the simpler late Euro 5 equipped vehicles rather than waiting for Euro 6.
This high level of complexity of modern diesel engines has for some time been causing the more technical of automotive writers to advocate a return to petrol engines, at least for motorists doing average annual mileages. Honest John, the respected Daily Telegraph motoring writer, holds particularly strong views on this subject. Yet even the warnings of Honest John and the like overlook the real concern about diesel emissions – namely that whilst the amount of particulates has reduced dramatically with successive Euro levels, the damaging effects of the remaining particles has not.
To explain, and again necessarily delving into the technical detail a little: unlike NOx and other emissions, particulate matter, although PM can to a degree be minimised by either chemical means or engine combustion settings, the main way of reducing PM emissions remains as noted above by adding a filter to the exhaust system. Now any filter, almost by definition, tends to be better at stopping large particles than small ones, thus DPF’s and other exhaust filters do exactly that, stopping the larger particles and unfortunately allowing the smaller ones an easier passage. Putting some numbers around this, whilst such filters are pretty effective at trapping the larger hazardous particles between 2.5 and 10 micron in size (known as PM 10), they are much less effective at dealing with particles smaller than 2.5 microns, known as PM 2.5. To put this into perspective, a human hair is anywhere from 40 to 120 microns thick, so PM 2.5 particles are tiny indeed. European legislation has, perhaps unwittingly, not helped here in the slightest, for the mandated levels of particulate emissions are measured by weight (or, more technically, mass). Big particles clearly weigh more than small ones so a test pass can be obtained by trapping a comparatively small number of large, i.e. PM10 or above, particles whilst leaving these smaller, ultra fine, particles free to enter the atmosphere. Yet medical research has for some years increasingly been showing that of particulate particles, the really damaging ones are the “ultrafines” – those of size PM 2.5 or below, which can be drawn deep into the respiratory system and lungs. This knowledge has been something of an elephant in the room for some time, as finding a suitable truly solution continues to be elusive, yet it is only in recent weeks that it is starting to become more common, public, knowledge.
Taking this barely publicised health issue with ultrafine particulates together with the high levels of Nox being pumped out by many diesels of the earlier pre-Euro 4 levels, the diesel vehicles produced over the last few years, and indeed those still being produced today, start to look much less the green icons we have been led to believe, and something perhaps more malevolent altogether. Where will all this lead? It may yet be the case that the combined knowledge of the automotive manufacturing world will overcome the problems of trapping ultrafine particles, driven by either public or government pressure, or more likely further Euro emissions standards, although at this time nothing is forthcoming from the EU legislators. Indeed, one of the most challenging issues facing the legislators is how to measure these ultrafine PM2.5 particles, since the traditional measurement techniques are too crude. Or possibly the veil will again be drawn over the issue, and governments continue to promote diesel power over petrol as the best option to help meet their CO2 targets, quietly ignoring the damaging health effects of doing so.
More likely, at least in my view, is that petrol-powered cars will become more popular at the expense of their diesel brethren, as the perceived reduced operating cost benefits of diesels disappear – not least if present Government CO2 – based financial inducements to buy diesel cars such as lower road fund licences are steadily unwound. Indeed, this process has probably already started. The latest crop of petrol engines is substantially more economical than even their very recent predecessors, with in some cases approaching diesel levels of both fuel economy and CO2 emissions. More significantly, almost clandestine legislative action may already be starting to discriminate against diesel power. Tellingly, Transport for London last year quietly amended the Congestion Charge exemption conditions to rule out all the low CO2 emissions diesel cars which were hitherto exempt.
It is anyone’s guess how this will pan out, In conclusion, I can only say that my own car-ownership policy has now certainly changed, from an unquestioning diesel-only approach to once again becoming a true petrol-head! Interesting times!