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Even before the Euro-3 exhaust emission standard is finalised, diesel engineers are already turning their attentions to Euro-4. Meeting even lower NOx levels is proving a real cause for concern, but a new technology, called Selective Catalytic Reduction (SCR), could provide the answer—at a cost. Gibb Grace reports.
It will come as no surprise to anyone in the transport industry that the drive to even lower emissions in the next millennium will centre around NOx and particulates. NOx is the bete noire of the diesel engine as far as environmentalists and politicians are concerned, and it is often the stick they use to beat it with. It is true diesels produce more than their fair share of NOx, but the engineers have already achieved a lot. NOx has halved from 18.0 g/kWh at ECE Reg 49 to 9.0 g/kWh at Euro1. And is set to nearly halve again to 5.0 g/kWh at Euro-3.
But this is still too much in the eyes of some German politicians, who are pushing for a possible further halving at Euro-4 which could be introduced in 2005.
Even at Euro-3 the total amount of pollutants is already very small, and reducing the limits still further, particularly in terms of NOx, is proving a major challenge. Most engineers now agree that despite the widespread use of four valve heads, central injectors, high-pressure injection and so on, NOx can be improved further only by accepting a worsening in fuel consumption. The reason being that fuel, NOx and particulates are mutually interdependent, and simply reducing one, tends to produce an increase in the other two.
Given that the legislators are not about to relax the pressure on NOx and PM, it seems that the only way out is to settle for poorer fuel consumption. This might seem extraordinary given the history of diesel engine development, but that is the reality facing operators, unless a breakthrough can be found in the next three to four years.
Is SCR the answer?
It the combustion itself cannot be made inherently any cleaner, the alternative is to resort to cleaner fuels and/or some form of exhaust after-treatment. While some engineers are exploring the natural gas route, others are sticking with diesel. One relatively new, but promising area of research is an exhaust gas after-treatment called Sinox.
The name is a contraction of the name of the German company Siemens and NOx, and the process has been used successfully to control the large-scale atmospheric pollution emitted by power stations. A development group comprising Mercedes-Benz, MAN and Iveco is currently evaluating Sinox on the test bed and on test vehicles to see if it can be applied in an automotive context.
lveco has already run enough tests at its diesel engine research centre in Arbon, Switzerland to conclude that Sinox is effective. The centre's director Walter Knecht says: "Even using a relatively low-tech Euro2 engine we have demonstrated NOx levels in the order of 2g/kWh, so we know the technology works."
The heart of the Sinox system is a large stainless-steel tank which is referred to here as a catalyser as it forms part of the exhaust system and acts as an hydrolysis catalyst and an SCR catalyst. Prototype catalysers supplied by Siemens are as big as a good-sized fuel tank, and at this stage it is not clear how much smaller, if at all, they can be made and yet still remain effective.
Sinox works by injecting a urea/water mixture into the exhaust gas just before it enters the catalyser, so any system will have to include an electrically powered dosing unit as well as the catalyser. Urea is a complex substance containing nitrogen, hydrogen and carbon but is commercially available and widely used in a variety of industries such as agriculture, water treatment, medicine and pharmaceuticals.
For use in the Sinox catalyser, the solution containing 32% urea and 68% water comes made up and is carried in a dedicated tank on the vehicle. This solution is then injected into the exhaust pipe just ahead of the catalyser at the rate of 4 to 5% of the fuel used by the engine.
Practical considerations
Even at this early stage, it is clear Sinox can deliver the low NOx figures that the politicians are looking for, but, needless to say, there are many problems to be overcome before it could go into production.
Apart from the MD costs involved, provision will have to be made to make the essential urea solution easily and widely available. It has to be injected throughout the whole operating range of the engine in relation to the actual NOx flow rate and so, someone, either the urea supplier, or say, the petroleum companies, would have to invest in a suitable supply infrastructure consisting of storage tanks and dispensers.
From the vehicle point of view, packaging the urea tank, the catalyser and the dosing unit on a three-axle tractor unit will not be easy because of their bulk. Weight is not so much of a problem, though the urea tank would need to be around 60 litres to suit a maximum size fuel tank, and with its dosing pump could weigh up to 80kg. The 68% water content of the urea tank means the solution freezes at —11°C, and so the tank and the supply line would need to be heated, adding further cost and technicalities.
However, the Sinox process is not that sulphur sensitive and can produce acceptable levels of particulates using Ultra Low Sulphur Diesel with a 5Oppm sulphur content. Iveco's Dr Knecht says the need for a particulate trap will depend ultimately on the particulate limit set for Euro-4. Also on the plus side is the fact that Sinox makes the tra
ditional silencer redundant as the catalyser doubles as a silencer.
But the most attractive benefit remains its ability to produce low NOx, and that means the engine can be run efficiently, so the all-important fuel economy is unaffected.
On the face of it at least, it does look as though Sinox can provide an answer to the elusive goal of low NOx combined with good fuel consumption, and thus ensure the future of diesel well into the next Only time will tell if it becomes
Exhaust Gas the preferred technology.