Trapped by the
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Tighter diesel emission standards prompted truck and bus builders to look at regenerative particulate traps as a way to reduce soot. But are they really a serious option, or just a crude technological stop-gap?
en US and EC legislators first started talking about reducing diesel engine emissions in the early eighties many manufacturers looked at soot filters (or particulate traps, to give them their proper name) as one way to clean up exhaust fumes.
By the end of the decade prototype soot filters were being displayed on the show stands of environmentally conscious truck manufacturers but the demand for particulate traps, from hauliers and legislators alike, seems to have disappeared into thin air. What's gone wrong?
As particulate limits have tumbled from the 0.36g,/kWh level set in 1992 for Euro-1, to the current 0.15g/kWh for Euro-2, diesel engine makers have sought to solve the particulate problem within the combustion chamber.
So far they've been able to tackle it without resorting to exhaust after-treatment.
MAN's engineering boss Dr Klaus Schubert recalls the first discussions over soot filters: "The technology was created back in the early eighties when engines were smoking and nobody expected the improvements in combustion and electronics."
Inside the engine some fuel may not be fully burnt by the time combustion is over and the exhaust valve opens. "In the past 10-15 years we've learnt to control particulates by optimising combustion," says Schubert, "especially through reducing droplet size by higher injection pressures, using more but smaller diameter holes in the injectors and reducing swirl. We also now understand the trade-off between emissions and NOx and fuel consumption."
Schubert reckons that particulate trap's use is :" limited to special-purpose vehicles like municipal trucks or those running into covered areas".
Duplex
MAN developed a Duplex particulate trap 10 years ago using a pair of regenerative traps with exhaust gases deflected alternately through each trap by flaps. As one burner operates exhaust gases pass through the other trap.
This system has been fitted to around 150 innercity buses and a handful of trucks.
Although reliable it has proved rather expensive and there have been problems in dealing with the high exhaust gas temperatures created during the regenerative "burning off" process.
Cab heater and exhaust manufacturer Eberspacher has been working on a variety of particulate-trap systems for Mercedes and Iveco bus engines since the late eighties. But heater division marketing manager Herbert Barth says that in the absence of local or national Government interest, "We don't see any breakthrough in the market. We'll keep going with this but we won't be investing additional manpower."
Burner
Eberspacher sees the greatest potential for soot filters among city dwelling trucks and buses. It offers a variety of filters which combine its knowledge of exhaust systems and diesel-fuelled burner technology. They include both static and continuous regeneration systems. It also developed a double-trap system (as did heater rival Webasto) but has since concentrated on single burner-based traps for continuous regeneration.
According to Barth, double systems "involve twice the burn, twice the cost and twice the weight...the future lies in single stream". MAN agrees and has also worked on a single burner "full-flow" system.
While the principle of using heat to burn off trapped soot particles is straightforward, achieving it is anything but The choice of filter material used to collect the soot is critical, as is the temperature at which it is burnt off (regenerated). To prevent a filter becoming clogged and engine output being reduced by back pressure, regeneration must take place at regular intervals.
Under normal conditions exhaust gas temperatures from a truck engine vary from 120°C at idle to around 500°C on full load. For the collected soot to burn off temperatures inside the trap have to rise to 700°C but they musn't be allowed to build up too high: ceramic cores can melt down above 1,400°C while the alternative ceramic fibre filter material wrapped around the filter "candles" can become brittle if overheated.
An electronic control unit identifies the optimum time to burn off soot. Up to 50 grammes of soot are burnt off during the burn cycle; as gas temperatures are high it is clearly not desirable to start regeneration when the vehicle is stationary—especially if it has a kerb-level exhaust pipe! Eberspacher says that optimum regeneration time is when the vehicle's engine is on overrun.
A fully insulated ceramic wound filter is roughly 760mm long and 300nun in diameter and weighs 30kg without the burner. The projected operating life of a ceramic monolith filter is likely to be below 400,000km.
Eminox has produced a continously regenerating trap using Johnson Matthey's catalytic monolith. Instead of a burner it uses a platinum/ceramic oxidation catalyst to turn CO to CO, and NO to NO2 while oxydising hydrocarbons into carbon dioxide and water. Any carbon particulates are trapped, allowing the NO2 to oxidise the carbon into CO2 and NO.
For operators who want a particulate trap MAN has committed itself to offering three sizes, catering for engines up to 150hp; 250300hp; and 400hp-plus. "We've started with small ones first because there's greater potential in city delivery vehicles," says Schubert.
However, he remains sceptical about the widespread application of regenerative soot filters:"It may be the target for environmentalists for the operator to have a particulate trap. But the Euro-2 limit of 0.15g/kWh can be further reduced with an oxidation catalyst and sulphur-free fuel. When you get to the Euro-3 limit of 0.1g/kWh this is where we're at the limit of detection. If you're looking at kerbside particulates they're much higher."
Given the projected cost of a particulate trap of around DM10,000 (L4,200), Schubert insists: "We can spend a lot on engine technology for the same money" Given the sensitive operating environment for citybuses, and the current age of the UK bus parc, citybuses undoubtedly represent the greatest market for regenerative soot filters—especially if local authorities want to clean up the emissions from older buses through compulsory retrofitment.
However, it remains to be seen if EU countries are prepared to use soot filter retrofitment to cut down on diesel particulates for delivery trucks or citybuses. If they are, the particulate trap might catch some more customers.
Fl by Brian Weatherley