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Common purpose
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01/03/2007
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Roger Bishop investigates the research and development directions being taken by two leaders in exhaust gas treatments and finds they have much in common
There are few more emotive subjects in the industry today than exhaust emissions and the systems being developed to deal with current and future legislative requirements.
Now two of the technology leaders in the field, Emitec and Purem, are proposing solutions that combine a diesel oxidation catalyst (DOC), particulate filter and selective catalytic reduction (SCR) within a single, space-saving silencer compartment.
In Emitec’s case, R&D engineers are talking about special metallic foils to enhance the mixing of the essential urea component in an SCR unit, solid urea systems to replace liquid injection, and homogeneous charge compression ignition (HCCI) developments.
Much of this current development work is focused on the truck and bus sector but it will also be of intense interest to car-makers who are looking at the types of systems that might be developed for them to meet Euro 6 requirements and beyond.
For thorough mixing of exhaust gas with injected urea Emitec has developed a twin-stage metal substrate that works using flat, perforated foils instead of metallic fleece. The initial section with MX structure of some 40cpsi ensures optimum gas mixing with a urea injection, which has to be largely evaporated. In the second stage, consisting of an LS/PE structure at 200/400cpsi, intensive amalgamation is continued and hydrolysis of urea to ammonia takes place. Both stages force the gases into ‘turbulent’ flows, so that they come into close contact with the catalytic coating and use the entire substrate cross-section due to radial flow into the adjacent channels.
In addition to the design of such a system, in which the individual components are arranged in sequence, a parallel arrangement of oxidation and hydrolysis catalytic converter has been put forward by truck-maker MAN whereby PM-Metalit and SCR-catalyst follow downstream. However, more benefits can be offered by a system that combines DOC, particle filter and SCR converter within one single space-saving silencer compartment.
The advantages of such a system have been demonstrated by recent research results. According to Emitec it offers a minimum of maintenance along with the lowest possible fuel consumption. Crucial to the long useful life of systems is the “proper coating” of all components which must be precisely matched one to the other.
The SCR process is not only suited to commercial vehicle diesel engines but also to passenger cars. For that reason diesel passenger cars with bigger displacements and higher weight class from Audi, BMW, Mercedes-Benz and Volkswagen are being fitted with SCR systems from 2008 in both the USA and Europe. In this way their NOx emissions will meet the stringent limits that become mandatory by 2010 in the USA and expected for Europe later. Only for engines in vehicles of lower weight and smaller displacements are NOx adsorbers being considered, says Emitec.
The design of SCR systems for diesel passenger cars is following the example of commercial vehicles. Their urea solution reservoir should be designed in a way that its content is sufficient for some 30,000km; in other words, it only has to be refilled in the course of periodic servicing. For passenger cars, too, Emitec has turbulent SCR-systems ready that it says are very compact but deliver high performance.
Even today, says Emitec, fitting DOC and particle filters close-coupled to the engine is causing problems for manufacturers as conventional wall flow particle filters must have about twice the volume of the engine’s displacement because of the need for ash storage capabilities. So a three-litre diesel needs a conventional particle filter of some six litres. Emitec’s ‘turbulent’ systems are said to require between 35 and 40% less volume for the same performance.
Solid urea systems
As the urea solution requires relatively large special tanks and only remains fluid below around 11°C, there is current R&D activity into the use of urea in solid form. This approach significantly reduces urea storage and heating problems and ensures the system remains functional even at very low temperatures. Active material pellets of about 2mm diameter are proposed. These would be warmed in a small heated reactor before being introduced into the exhaust gases.
A pre-condition for this approach would be a reliable feed system for the urea pellets and precise metering so that the required quantity of urea is dynamically injected into the exhaust gases. Among problems still to be solved before series production is coating the highly hydroscopic pellets with a hydrophobic layer.
One company at the forefront of urea gas dosing technology is the UK’s IMI Vision, which has prototyped an SCR system called Sigma (European Automotive Design, September 2005).
Over the last 20 years of Emitec’s improvements to gasoline three-way converters, conversion rates of toxic hydrocarbons, carbon monoxide and nitrogen oxides have been achieved by improvements to catalytic coating as a result of lowering the mass of substrates while increasing their geometric surface area. At the same time engine developers have reduced engine emissions significantly. Now, though, the high cost pressure on diesels with regard to specific engine components and expensive particulate filters and deNOx systems (compared with gasoline engines) is making homogeneous charge compression ignition (HCCI) a realistic prospect.
HCCI and synthetic fuels
First published results point to the potential of HCCI to significantly reduce NOx and PM emissions. Currently this state can only be achieved over a relatively small sector of the engine map but there are indications that the technology can improve fuel consumption by around 5%. Over the entire engine map this might increase to as much as 15% compared with today’s diesel engines. Much R&D remains to be done along with the development of new components such as very high pressure injection.
Future systems will also take into account the future increasing availability of synthetic diesel fuels (BTL, GTL, CTL) which contain no sulphur and no high boiling point aromatic substances. They burn even in today’s engines cleaner than conventional diesel fuel and might be another enabler towards achieving homogeneous combustion. However, even HCCI combined with synthetic fuels will require efficient DOC, SCR and PM filter aftertreatment.
Systems call for sound design
The exhaust gas aftertreatment methods, deNOx-ing and particle filtering, each achieve efficiency rates of up to 90% or more. The de-NOx route is currently the most widespread in series production across Europe, while particle filter technology is more common in the USA.
From 2010 permissible NOx and PM emissions levels will again be drastically reduced when EPA10, which has already been passed for the NAFTA region, comes into force. Correspondingly low limit values are currently in the resolution phase in Europe (Euro 6) and Japan (PNLTE).
Purem of Germany says these low emission rates can no longer be achieved by exhaust gas after-treatment methods alone proposes a combination of technologies – filters to reduce the particle emissions, and deNOx to cut down on nitric oxides.
The combination of a particle filter with an upstream oxidation-type catalytic converter generating NO2 together with an SCR catalytic converter with upstream injection of a reducing agent is known by Purem as SCRT. Indeed, as early as the end of the 1990s, the use of these systems as a retrofit unit in city buses was investigated.
To make them acceptable in OEM on-road sector, high performance must be ensured in all operating states. This requires the use of additional components which will then be put together in series development, as illustrated in the diagrams from Purem. These show:
HC doser (1) with oxidation-type catalytic converter (2)
Particle filter (3), possibly with a catalytic layer for oxidation purposes
Oxidation-type catalytic converter (4) for converting NO into NO2
Dosing device (5) for aqueous urea solution
SCR catalytic converter (6)
Oxidation-type catalytic converter (7) for eliminating any excess ammonia
Acoustic modules (8)
Additionally, sensors for determining pressures, temperatures and concentrations of nitric oxide
It would be all-too-easy to forget the acoustic requirement of a combination system which are subject to separate regulations. At present, Noise Emission Directive 92/97 EEC is applicable in Europe. Europe is currently voting to decide the new restrictions planned for 2009 which will, of course, entail more stringent stipulations. The reduction of noise in exhaust gas cleaning systems must be incorporated as an essential element of the overall system as early as the concept phase, says Purem. This is the only way to ensure that appropriate constructive measures concerning acoustics, such as the resonance volume or absorption chambers, can be addressed if the need arises.
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Author
Roger Bishop
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