The Euro exhaust emissions regulations

Alan Bunting

Sunday 26 November 2006 23:47

Emissions regulations seem to dominate the development of vehicle technology, and it is instructive to look at the history of regulations governing commercial vehicle emissions.

In 1971 British Standard BS AU 141 laid down procedures for measuring a commercial vehicle’s performance, in particular distinguishing net and gross horsepower figures. It was part of the then new type-approval/homologation process. At the same time it enabled different manufacturers’ powertrains to be fairly compared by prospective buyers. The standard also introduced for the first time a rudimentary exhaust emission requirement based on exhaust gas opacity. Even though the so-called ‘smoke limit’ was defined in universally recognised Hartridge units, there was controversy over possible variations in the meter reading with changes in ambient temperature and humidity.

It was a controversy that continues to this day in roadside smoke checks which the authorities concede are rough and ready. Visible smoke can however be correlated with more accurately measurable levels of particulate matter (PM) in the exhaust, though no one used the term in the context of emissions legislation until Euro 2 requirements came into force in 1995/6.

Euro 1

Euro 1 regulations in 1992, enacted according to EU Directive 88/77/EEC, relating to all vehicles of more than 3.5 tonnes gross vehicle weight, were concerned only with gaseous emissions. For diesel vehicles, oxides of nitrogen (NOx) were the prime concern. But carbon monoxide (CO) and gaseous hydrocarbon (HC) limits were included as well so as to align the regulations with those introduced around the same time for petrol-engined vehicles, even though actual diesel CO and HC levels were, and remain, way below the set limits.

Euro 2 – Particulate emissions

Euro 2 saw the introduction of the first PM limit. There was overwhelming medical evidence that excessive airborne particles could get into the lungs and cause or aggravate respiratory problems. At 0.15 grams/kilowatt-hour (g/kWh) it allowed over seven times the mass (weight) of particles – principally soot – than today’s Euro 4 legislation permits. There is ongoing debate about the health effects of particle size and number, as opposed to their total mass (weight), as used until now to decide compliance or otherwise. The likelihood of a PM size/number requirement being built into Euro 6 legislation in six or seven years time has wavered, partly in line with views on accurate measurement feasibility.

Euro 3

Directive 1999/96/EC published seven years ago, finalised – and at such short notice that manufacturers were up in arms – the previously unconfirmed Euro 3 limits for implementation in 2000/2001. (NB: From Euro 2 onwards, each new round of emission limit reductions has embodied two implementation dates, one year apart, for example, in the case of Euro 4 requirements, October 1 2005 and October 1 2006. The first relates to ‘new vehicle types’ introduced to the market from that date; the second is the date affecting all newly-registered vehicles.)

Euro 4

With the advent of Euro 4 legislation, which came fully into force on October 1 2006, a new certification test procedure (Directive 1999/96/EC) was introduced for diesels. Engines had to be subjected to the European Transient test Cycle (ETC) as well as the 13-mode steady-state cycle (ESC) adopted five years earlier for Euro 3. It was originally intended that the ETC would supplant the ESC altogether. But the ‘cycle beating’ scandal in the US – where every manufacturer was found guilty of exploiting a loophole in the EPA transient cycle, allowing NOx levels to exceed the legal limit under some steady-state cruise conditions, to the benefit of fuel consumption – persuaded EU legislators to make the ETC provisions additional to the ESC.

Euro 5

On the short notice issue, the European Commission redeemed itself somewhat in 1999 by simultaneously announcing proposals for future Euro 4 and 5 limits up to a decade into the future. However, the limits for Euro 5 have, even now, not been set in stone, despite their already being used as the basis for ‘early compliance’ incentive schemes like the German Maut autobahn-toll-reduction programme. Since last year the German government has been urging the EC to tighten the PM limits for Euro 5 yet further, but that now looks increasingly unlikely to happen.

A World Harmonised Test Cycle

There is currently a lot of pressure from manufacturers, especially DaimlerChrysler and Volvo, whose interests cover the US and European as well as the Japanese market, for a common certification procedure to be adopted globally in what is being termed a world harmonised test cycle (WHTC). The EPA emissions limits are set using a quite different test cycle from the European version.

Critics say there would have to be three ‘versions’, in order to simulate the very different patterns of truck and bus use in the three regions, because of differences in terrain, traffic congestion and hence average speeds and engine loads. But harmonisation of actual emission limits would become more attainable. And the huge costs involved today in two- or three-fold R&D and subsequent homologation testing for essentially the same engine installation could be brought down. A DaimlerChrysler engineer recently expressed optimism that Euro 6 requirements would correlate with those for EPA 2010.

Euro 6 and beyond

Firm proposals on Euro 6 limits and certification are due from the EU during 2007. Officials in Brussels are understood to be considering six NOx/PM limit pairings, from 2.0/0.03 (unchanged from Euro 5) to a very severe 0.2/0.02g/kWh. Whatever limits are applied, it looks certain that DPFs (diesel particulate filters) will be needed, regardless of the system used to reduce NOx. But customers, for operating cost reasons, as well as environmental legislators, increasingly concerned as they were about CO2 emissions, would expect Euro 6 vehicles to be more fuel efficient than their Euro 5 counterparts.

The EEV standard – Enhanced Environmental Vehicles

As long ago as 1999, when Euro 4 and 5 limits were set, the European Commission also issued a recommended (ie non mandatory) ‘super clean’ standard for use by member states or city authorities as the basis for environmentally-driven incentive schemes. This EEV (enhanced environmental vehicle) standard is based on the Euro 5 NOx limit (of 2g/kWh) in combination with a) a PM limit one-third lower than Euro 5, at 0.02g/kWh, determined using the ETC and b) a 70% lower smoke opacity limit. This latter requirement is thought to have been included under political pressure designed to force the need for a DPF – something which Brussels perceives as a tangible step towards emissions reduction in the eyes of the public.

Manufacturers continue to lobby the EU, questioning the cost-benefit ratio of ever more stringent limits on those pollutants, viz PM and NOx, which threaten local air quality. Huge R&D investment is implied in driving forward the technology – in fuel systems, air management (ie turbocharging refinement), machining precision (to cut lube oil consumption), aftertreatment catalyst chemistry and filter development – if those limits continue to be tightened. There is some evidence to suggest that the European Commission has, in the last year or so, adopted a more pragmatic approach, particularly in the light of the EU’s commitment a) to stimulate the price competitiveness of European products (obviously including trucks and buses) and b) to minimise the trickle-down financial burden to EU citizens (through higher transport costs).

The case for postponing indefinitely any further tightening of PM and NOx limits beyond Euro 5 levels has been reinforced by the groundswell of concern about climate change. The technology focus of vehicle and engine manufacturers attention should, it is argued, be switched to that of CO2 reduction, through improvements in fuel efficiency.

As it happens many of those engine refinements directed at boosting fuel economy will also help cut particulates. Higher injection pressures can be harnessed in two ways to increase the percentage of injected fuel that is ‘productively’ burned. They allow the fuel to be injected through smaller spray holes, which increases fuel atomisation, to the benefit of fuel-air mixing, so that more of the fuel injected is burned and turned into motive energy, while less finds its way through to the exhaust in the form of soot particles. Increased pressure alternatively allows the fuel to be injected more rapidly. Injection can, in crank angle terms, start later (closer to top-dead-centre), which cuts the amount of undesirable ‘negative work’, resisting the still-rising piston. Combustion can also finish earlier (sooner after TDC), which reduces the likelihood of incomplete combustion through the falling-away of pressure as the piston descends.

With metaphorical sticks and carrots at their disposal, legislators will certainly continue to drive emissions-related technology. If, as many now predict for the early part of the next decade, a breakthrough comes in HCCI (homogeneous charge compression ignition) technology, allowing the air and fuel to be thoroughly mixed before combustion starts, then both NOx and PM emissions generated in the cylinder will be spectacularly reduced, accompanied by dramatic fuel savings with corresponding cuts in CO2. That will lead the environmental law-makers to require all manufacturers to emulate the technology leaders.

NOx – nitrogen oxides

Concern about NOx from vehicle exhausts originated in California and the greater Los Angeles area especially, where its reaction with HC in hot sunny weather was found to contribute significantly to the choking photochemical smog which regularly blights the city. NOx was also deemed to be a contributor to so-called acid rain.

Excessive diesel NOx results from unduly high peak combustion temperatures, which are an unwelcome invariable accompaniment to increased engine efficiency. The problem was tackled initially by retardations in injection timing. That had the effect of staggering the peaks of the two pressure rises which occur in the cylinder: one caused by the rising piston, the other through combustion of the fuel. The two thus became less cumulative, so that peak pressure, and with it peak temperature, were reined back. But retarding injection had a serious downside – combustion became less efficient and fuel consumption suffered.

As the allowable NOx level came down, notably from 5g/kWh at Euro 3 to just 3.5g/kWh at Euro 4, further injection retardation became impractical. It was necessary to address the NOx problem more fundamentally. Five years ago MAN led the way with the inclusion of cooled exhaust gas recirculation (EGR) on many of its Euro 3 engines. All diesels ‘breathe in’ much more air than they need, but by displacing some of the oxygen-rich intake air with exhaust gas, the fuel-air mixture is made to burn ‘less fiercely’, so that peak combustion temperatures – and NOx formation – are held down.

EU publications and programmes

The Auto-Oil II Programme is an EU project to look at future trends in emissions and air quality.

An official report from the Netherlands, The Impact of Euro 5: facts and figures, points out that light commercial vehicles and passenger cars will not need NOx reducing equipment to meet the Euro 5 regulations in 2010, but suggests that a lower NOx limit would be beneficial as a cost-effective way of improving air quality.