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	In Sweden for an SAE gathering, JOHN DICKSON-SIMPSON observes that political pressure for ever tighter exhaust emission limits seems to be easing. Economic priorities are shifting as the soaring oil price frightens governments… A silver lining in the clouds of impending legislation could be sighted at a conference called by America's Society of Automotive Engineers in Gothenburg, Sweden, in late 2005 to discuss how to get bus and truck diesels to meet environmental ambitions for near-zero emissions. Byron Bunker from the US Environmental Protection Agency declared that there would be no further tightening of exhaust controls after 2010. The cost becomes too great compared with the benefits, he said, and the competitiveness of industry had to be protected. The EPA pronouncement was reinforced by a proposal beyond the conference - from the European Commission - that restrictions on nitrogen oxide emissions would be eased to make it more affordable to reduce particulates and improve fuel consumption. That theme was echoed at the conference by Dr Reinhard Schulter-Braucks from the European Commission. All this somewhat dampened European talk of 'beyond Euro 6'. Opinions at Gothenburg were that a pause for reconsideration was needed. The emphasis now had to be on getting international agreement on a world standard for exhaust emissions - most likely centring on the American pattern. And a prelude even to that would have to be to settle a world standard for the test procedure by which engine emissions are measured. There are three methods at present, and all are compromises attempting to reflect on an engine test bed what the emissions might actually be from a vehicle on the road. Really, a rolling-road cycle for measuring tailpipe emissions of vehicles would be the meaningful method, but one obstacle to that is the quite vast variety of commercial vehicles by comparison with cars. In the meantime, even Euro 5 and the American EPA regulations for 2007 will be costly. Plans are for 2010 emissions to be roughly a tenth of what they are now. Should the achievement of such cuts be by chemical treatment of the exhaust after the engine or by technical development inside the engine? Rivalry between the two philosophies was unabated at the conference.   resistance American resistance to after-treatment by spraying ammonia into the exhaust was softening, but operationally the method is still considered messy and dependent on yet more legislation - and more electronic sensing. So the in-engine champions have not given up. Researchers at Caterpillar, Hino and Scania are adamant that they can avoid elaborate after-treatment. Recirculation of cooled exhaust gas will not be sufficient, though. There will have to be two-stage turbo-charging and even higher injection pressures. They are also busy with a new combustion technique in which some fuel is injected on the compression stroke, vaporising for a pilot bang to boost purer - and rather cooler - combustion on the power stroke. Precision of control is the challenge. The fresh ray of hope for commercial-vehicle engineers lies in musing whether the Americans' shift to a pragmatic attitude might set a powerful example to Europe, where political ambitions for zero emissions still run wild. Preliminary proposals for Euro 6 level for 2012 are at best one gram per kilowatt-hour of nitrogen oxides and 0.015 g/kWh of particulates.   At worst they could be 0.5 g/kWh of NOx and 0.002 g/kWh of particulates if the Germans have their way - compared with 0.27/0.013 that is the United States' present aim for 2010. The 2006 European limits are 3.5 NOx, 0.02 particulates. For Euro 5 in 2009 NOx is supposed to be reduced to 2. This is all becoming extremely complicated and, indeed, uncertain as far as results go. Certainly there are now worries about diminishing returns. The costs are more and more out of proportion to the gains. Moreover, society's economic structure is changing, and so is political emphasis. Now coming to the fore are the cost of fuel and the desire to cut carbon dioxide. Already, so marked is the revision of perceptions in Germany that the call for natural gas as a cheaper fuel is returning. Inherently low emissions from burning gas are also being seen as a convenient way of by-passing the aggravation of meeting ever stricter diesel emission limits - and at a cost that is beginning to look competitive with cleaned up diesels.   German enthusiasm for gas has not yet reached the truck operators, however, because of the added weight of gas tanks and the dismaying cost of engines converted from diesel. That, however, could change by using spark-ignition power units - much cheaper and lighter than diesels. Maybe this could be the next big idea to occupy the emissions-reduction specialists.
The different global diesel emission standards that have been settled so far
Johnson Matthey's combined regenerating particulates filter and surrounding selective catalytic NOx reducer
Progressive percentage reduction of diesel emission limits in Europe since 1990
Pressure rise and energy output of homogeneous charge compression ignition - advance charge of fuel mist before main injection - plotted against crank-angle Caterpillar. Big reduction in NOx is obtained
Two-stage turbocharging at AVL has been found to improve response rate by 20 percent - useful in transient cycle emission tests

Such discussion about alternative fuels did not get much airing at the Gothenburg conference devoted to diesels, however. Worries about what legislators might actually want - and how to satisfy them - were more than enough to dominate engineers' minds. Nervousness is repeatedly aroused by lobbyists in the name of the environment. Whatever reduction in emissions are achieved, such pressure groups invariably want more, irrespective of scientific reality.

However, people in government departments do recognise these undercurrents. They are involved in the art of the possible rather than missions impossible.

There is too much impatience. Ever stricter limits run into futility. Presently planned levels need time to produce detectable benefits. As the European directorate's Schulter-Braucks said in Gothenburg: 'Without further regulations, in 10 years or so nitrogen oxides and particulates in the air will sink to targets set by the new Clean Air for Europe programme.'

He thought it would be more useful over the next few years to concentrate on fine tuning of regulations. 'Common order is needed. It would be sensible for Euro 6 just to match the US 2007 standards, to get an internationally agreed test cycle and to close the gaps exploited by manufacturers in cycle beating.' Further, he said: 'We're anxious not virtually to ban diesels. They're so good at reducing carbon dioxide emissions.'

His attitude was welcomed by Volvo's vice president Jarma Halonen: 'We can't afford to engineer for different standards and different monitoring across the world - and we want on-road and off-road standards to be the same. Besides which we need to rely on good fuel quality everywhere.'
When it came to discussions about how to achieve the regulatory ambitions, it did not take long for the rivalry to emerge between those who keep searching for answers in engine technology and those who want to avoid such expense by exhaust treatment outside the engine.
Pursuers of engine technology regard after-treatment as an act of despair. 'It gets manufacturers off the hook at the expense of operational inconvenience and regulationary burden'.

scorn

Professional scorn or not, the attractions of selective catalytic reduction - SCR - involving injection of diluted urea into a supplementary catalyst are tempting. It means that the basic engine performance can return almost to Euro 2 standard except for extra concentration on particulates reduction.
Fuel injection can be advanced again, vigorous air swirl restored and specific power increased again - all with the result of improving fuel consumption. The NOx conversion performance of SCR after-treatment is between 80 and 90 per cent, so the engine maker no longer has to compromise for fear of transgressing NOx levels.

One or two caveats linger. For SCR to work effectively, the exhaust temperature has to reach at least 250ºC. This can be a borderline condition in stop-start service, but it is of no practical concern to an engine manufacturer because the objective is just to conform to the official test bed cycle. Only in the US transient procedure does the approval cycle include a cold start. Helpfully, the upstream addition of an oxidation catalyst accelerates the SCR process.

The electronic control depends on the reliability and precision of NOx sensors. These seem to work within rather wide tolerances at present, although development engineers say that adequate improvements are under way.

The catalyser package is bulky. Its volume should be about double the engine's swept volume, says Corning. This tends to be awkward when it comes to the larger automotive diesels in the 12-16 litres band - implying a catalyser about 400mm diameter and more than a metre long.

Furthermore, a preceding filter to trap particulates might be necessary for Euro 4 and almost certainly for Euro 5. It is now usually allied with an oxidation catalyst to cut hydrocarbons and carbon monoxide - generating nitrogen dioxide to oxidise soot in the filter and relieve the demands on the SCR unit. This does not necessarily have to employ a precious metal, and Corning instances a vanadium-tungsten-titanium alloy.

However, Johnson Matthey has developed a more compact, if fat, compound catalyser that combines a particulates regenerating filter and a nitrogen oxides killer in a single unit. It does this by wrapping a particulate filter with an annular SCR stage. Exhaust gases do a double pass, firstly through the central filter and then back through the surrounding SCR before it emerges from the can.

Apart from cost, the most serious reason for operational hesitation on SCR is the need to have another tank for the urea solution, 1/20th the size of the fuel tank, and to be diligent about refilling it.
Commercial success will depend on whether the fuel economy hopes of SCR protagonists are fulfilled and how the savings actually turn out in service. Modest claims are 4 per cent, although Cummins, for one, talks of 7 per cent and points to subsidiary saving in doubling oil-drain intervals or else not needing expensive synthetic oil - though low-ash oils are desirable to inhibit filter blockages.

Real-life gain in fuel economy will be a critical issue. Preliminary estimates suggest that 4 per cent would not be good enough in the context of truck operating costs in Europe. It seems probable that recovering the initial cost of SCR over five years looks would take about 80,000 miles a year for an 18-tonner, 90,000 for a 26-tonner and more than 70,000 for a 40-tonner. Then there is the cost of the urea solution and a small amount of extra maintenance - but the savings on lubricating-oil expense should counterbalance the maintenance cost.

The, at this stage, rough estimates do indicate that operators' resistance or, at best, scepticism has some justification especially in the UK and the US. In Germany there is more willing acceptance because there is the incentive of reduced motorway tolls for vehicles conforming to Euro 5.
Alternatives to SCR that can at least satisfy Euro 5 requirements have been developed in Europe by MAN and Scania and for the US limits by all the engine manufacturers.

These involve assistance by regenerating particulate filters. This factor - coupled with the complications of exhaust gas recirculation, throttled turbo-charging and very high pressure multiple injections of fuel - adds cost approaching that for to SCR after-treatment. So the comparative cost disadvantage of SCR is whittled down, and this would leave vehicle buying decisions influenced more by the perceived inconvenience of handling the urea for SCR than by economics.

The more stringent US 2007 limits are difficult to attain, and some are saying that the savage cut in nitrogen oxides promised by the Americans for 2010 will be impossible for affordable engine technology. Hence the optimism about getting the Environmental Protection Agency to accept SCR. This got a sympathetic response from American researchers and law watchers at the Gothenburg conference - including Jeffrey Leet, principal engineer of South West Research Institute, and Rakesh Aneja of Detroit Diesel.

Nevertheless, at Gothenburg development engineers from Caterpillar, Hino and Scania remained perkily upbeat about prospects of avoiding SCR. They have a two-stage approach. Already they are bringing in more cooled exhaust gas recirculation with more precise electronic control and even higher rates of injection with two pilot and two post injections either side of the main injection that is retarded to top dead centre or even later. This technique is allied with less air swirl - following a philosophy of achieving a rather slower burn with lower peak temperature to subdue NOx.

Then there is a move to two-stage series turbo-charging, which gives more consistent air velocity relative to engine speed and better response in acceleration - therefore reducing emissions in the transient test cycle together with faster regeneration in the particulates catalyser.

Franz Moser from the research house AVL is a supporter of two-stage turbo-charging. He reported 20 per cent faster response. With variable geometry it could become even better over a wider range of speed and load, but the advance in performance was most significant with electronic simultaneous control of boost pressure in relation to exhaust gas recirculation.

Beyond these steps, most of which are in production, the future seems to lie in HCCI - homogeneous charge compression ignition. Stripped of jargon, this means a pilot charge of pre-mixed fuel and air. One method is to inject a bit of fuel into the inlet port. Or, simpler, to introduce a pilot injection on the induction or compression strokes.

The purpose is to restore thermal efficiency when the emissions have to be very small for the likes of Euro 6, when it comes, and for US 2010. But keeping the explosion-prone pre-mix charge under control is proving to be a tricky business. Timing is critical.

Such problems might be eased with variable valve timing, thinks Walter Knecht, advanced research director at Iveco. It is a way, he says, of varying the virtual compression ratio. Also, the pre-mix element might be variable according to load - an ideal being pursued by Bengt Johansson at Lund Institute. He thinks NOx can be brought down to future standards without resorting to after-treatment.
Much more work needs to be done, though, said Scania's Lars Dahlen at Gothenburg. Noise and cold start were 'not up to scratch yet'. Higher supercharge pressures seemed to help, he said. Multi-fuel capability is turning out to be an intriguing spin-off. For instance, when running on methanol, there was no need for exhaust gas recirculation.

At Caterpillar, said research manager Kevin Duffy, there was a preference with HCCI to inject a pilot charge in the compression stroke. More supercharge - 3:1 - was helpful to prevent the cylinder being starved of oxygen after the pilot burn and to compensate for late inlet-valve opening. This way, specific fuel consumption improved by 4 per cent, and power was well up - the 15-litre Caterpillar normally delivering 738 kW/550 bhp produces 823 kW/614 bhp in HCCI form.

Researchers are in no mood to discourage governments from ever stricter regulations. 'My staff will be looking for employment after 2010,' said an American delegate.

Many a true word is spoken in jest.