die zukunft der heterogenen katalyse im automobil ... · die zukunft der heterogenen katalyse im...

Die Zukunft der heterogenen Katalyse im Automobil;„Turbolente“ Katalysatorenfür Otto- und Dieselanwendungen

The Future of Heterogeneous Catalysisin Automotive Applications;„Turbulent“ Catalystsfor Spark- and Compression Ignition Engins

Dipl.-Ing. W. Maus, Emitec GmbHDipl.-Ing. R. Brück, Emitec GmbH

26. Internationales Wiener Motorensymposium 28. - 29. April 2005

26th International Vienna Engine Symposium 2005

Dipl.-Ing. Wolfgang Maus Dipl.-Ing. Rolf Brück

EMITEC Gesellschaft für Emissionstechnologie mbH, Lohmar

Die Zukunft der heterogenen Katalyse im Automobil; „Turbulente“ Katalysatoren für Otto- und Dieselanwendungen

The Future of Heterogeneous Catalysis in Automotive Applications; “Turbulent” Catalysts for Spark and Compression Ignition Engines

Kurzfassung Die Entwicklung der Katalysatortechnologie für den automobilen Bereich begann in den 60er Jahren. Aus der chemischen Industrie wurden zunächst die als hocheffektiv bekannten Schüttgutkatalysatoren mit sehr gutem, einer turbulenten Strömung vergleichbarem, Stofftransport übernommen. Mechanische Schwingungen und Gaspulsationen führten zu Abrasion. Daher wurde dieser Weg aus Gründen der Dauerhaltbarkeit verlassen und zunächst metallische Wabenkörper, die in Raffinerieprozessen eingesetzt wurden, verwendet. Diese Folien-Substrate waren ebenfalls nicht dauerhaltbar, da die Verbindungsverfahren zur Herstellung monolithischer Strukturen noch nicht zur Verfügung standen. Keramische, monolithische Wabenkörper wurden später entwickelt, die ebenfalls mit katalytisch aktiver Beschichtung versehen waren. Der Nachteil monolithischer Wabenkörper besteht allerdings in der laminaren Kanalströmung, die den Stofftransport und damit die volumenspezifische katalytische Wirksamkeit begrenzt. Neuartige Katalysatorträger-Entwicklungen für Otto- und Dieselmotoren nutzen den Effekt der „Turbulenz“ gezielt aus. Die Konvertierung der limitierten Abgaskomponenten basiert auf Mechanismen des turbulenten Stofftransports. Im Folgenden werden die zugrunde liegenden Gesetzmäßigkeiten für den Einsatz im automobilen Katalysator hergeleitet und mit Testergebnissen untermauert.

Abstract The development of catalyst technology for automotive applications started in the early sixties with the adoption of highly efficient pellet catalysts that were used in chemical engineering processes. This catalyst type produced very high mass transfer rates, which were comparable to those found in turbulent flow. However, catalyst beds made of this type of pellet failed because mechanical vibrations and gas pulsations caused abrasion. In order to increase durability, metallic honeycombs as those used in oil refining processes were seen as a possible solution. Since the technology for joining foils into a monolithic structure was not available at the time the durability of the substrates was also limited. Ceramic honeycomb structures with straight channels coated with the catalytic material were developed later. Their smooth channels meant that the chemical reaction was limited by a poor mass transfer rate to the channel wall. New catalyst developments for spark and compression ignition engines make specific use of “turbulent” effects. The conversion of regulated pollutants is based on the mechanisms of turbulent mass transfer. The principles behind their application in automotive catalysts are described, and substantiated by test results, in the following chapters.


1. Introduction In the sixties and seventies the number of vehicles on the road grew rapidly as a result of increasing wealth and the desire for greater mobility that accompanied it. Initially in the United States and later also in Europe the negative effects of this trend were felt in the form of smog. Public and political pressure to build more environmentally friendly vehicles was set against an increasing demand for mobility and new vehicles. Pellet fixed-bed reactors as those used in the chemical industry were seen as an obvious solution (figure 1). This technology had been used successfully in the chemical industry for a long time. Compared to channels with laminar flow profiles this technology achieves high Nusselt or Sherwood numbers similar to those observed in turbulent flow profiles, which helped to achieve a high level of overall catalytic efficiency [1, 2]. Initial tests with these catalyst systems, which were still open-loop systems at the time, produced an effective reduction in gaseous emissions.

Abbildung 1: Pellet-Katalysator für den automobilen Einsatz

Figure 1: Pellet catalyst for automotive applications In contrast to the static operating conditions of the chemical industry, pellet catalysts in vehicles were exposed to much higher stresses. Vehicle reactors were subject to gas pulsation, mechanical vibration and thermo-mechanical stresses under dynamic operating conditions. Thermal variation in stress causes differential expansion between the mantle and the ceramic pellets. The vibrations of the vehicle set off relative motion of the pellets against each other and the mantle leading to catalyst abrasion. The ceramic pellets simply turned to dust [3]. The catalytic advantage of this type of catalyst was therefore accompanied by a serious disadvantage with regard to mechanical, and hence also chemical, durability. The obvious solution was a metallic honeycomb of the type used in some applications of the chemical industry, which was made up of smooth and corrugated steel foils. Initially, these systems also failed because of the stresses inherent in vehicles and because the technology to join the separate foils into a monolithic structure was not available at that time. Extruded ceramic honeycombs were developed as an alternative. The advantage of these catalyst substrates – initially with 300 cells per square inch (cpsi) and a wall thickness of 0.2mm – was that the thermo-mechanical stresses were borne by a stable matrix and that the differential


expansion against the mantle was absorbed by flexible “embeddings” [4]. The first of these consisted of wire mesh; later so-called swelling ceramic fibre mats were used to compensate for the differential expansion between the metallic mantle and the ceramic honeycomb. Reports on the various fault hazards of ceramic systems are being published to this day. The technical reliability of metallic METALIT® substrate catalysts has been proven since their introduction in 1986. However, straight smooth channels with laminar flow vectors and profiles, i.e. those running parallel to the catalyst axis, have a negative catalytic effect. As described below the pollutants are transported from the centre of the channel to the catalytically active wall solely by diffusion. The crucial factor, however, was achieving mechanical durability. Furthermore, the efficiency of the initially open-loop catalysts was more dependent on fuel mixture generation than on mass transfer. The continuing growth in vehicle traffic meant that the open-loop catalyst systems whose efficiency was inherently limited to between 50% and 70% were unable to prevent a rise in environmental pollution. The United States again led the rest of the world in adopting gradually tighter emission limits, which today demand over 99% efficiency. The requirements of exhaust gas after-treatments increased accordingly. The three-way catalyst was introduced in 1976 [5] as a first step in this direction. The application of a so-called lambda sensor enabled catalysts to operate in the stoichiometric range and at the same time ensured an optimum conversion of hydrocarbons (HC), carbon monoxide (CO) via oxidation and the reduction of nitrogen oxides (NOx). The demand for higher conversion rates required a lot of work on both the engines and the catalysts. New engine generations with reduced raw emissions and optimised, faster lambda control became the standard as did close-coupled, very thin-walled catalyst substrates (30 m) with a high cell density and high-temperature-resistant coatings, which Emitec and Toyota introduced for the first time in 1997. With stricter legal requirements with regard to efficiency and durability the technical complexity and hence cost pressures increased. Catalyst substrates and configurations (close-coupled, thermal cascade) helped to reduce relative costs through improved catalytic coatings. The design of the monoliths with their straight, smooth channels was not changed at first. Tests that aimed to recreate the positive properties of pellet systems and introduce “turbulent” catalysts failed because of the inadequate production engineering available at the time and because of unsuitable coating technology [6, 7]. Catalyst substrates with 900 cpsi were used where even higher volume-specific efficiency was required. Substrates with as many as 1,200 to 1,600 cpsi were developed to production stage in order to improve mass transfer [8, 9, 10, 11]. Another advantage of higher cell densities is the increase of the volume-specific geometric surface, which has a direct effect on catalysis. Ever since its foundation Emitec had been stressing the importance of the catalytic surface per unit volume to achieving good conversion rates. The related, lowest possible, cross-section-related mass that had to be heated was in turn a factor in catalyst light-off. Progressively thinner and at the same time more corrosion-resistant metal foils it have made it possible to increase cell density from 400 cpsi (s.a.) by over a factor of 3 without increasing the proportion of cold-start emissions. A smaller hydraulic


diameter improves heat transfer (improved “light-off”) and increases the conversion rate by reducing diffusion paths. The manufacturers of ceramic substrates followed this technological trend after many years of hesitation but the potential of “turbulent” reactors was never achieved (figure 2).

Abbildung 2: Stofftransportkoeffizient Beta als Funktion der Strömungsgeschwindigkeit bei unterschiedlichen Zelldichten und Strömungsformen (T = 600 °C)

Figure 2: Mass transfer coefficient beta as a function of flow rate at various cell densities and laminar/turbulent flow (T = 600 °C)

Turbulent flow can be generated by an increase in the flow rate and hence the Reynolds numbers in the catalyst (reduction of the flow cross-section). Conventional catalysts are unable to do this for pressure loss reasons. This is only possible in engines with an exhaust gas turbocharger where the catalyst can be positioned in front of the turbine since the turbocharger itself causes a very high loss of pressure whereas the effect of an upstream catalyst is much smaller by contrast. This type of pre-turbocharger catalyst acts as a pulsation damper and flow straightener for the flow entering the turbine and has demonstrated its much higher turbulent catalytic efficiency. It took the growing pressure of the car industry for cheaper catalysts with smaller volumes and an even higher level of efficiency to give new impetus to the development of “turbulent” catalysts. But it was not just the production of these catalyst substrates but also appropriate catalytic coatings and coating processes that had to be put into practice. In addition, pressure loss was to be reduced for higher engine performance and lower CO2 emissions.


2. Heterogeneous catalysis The rate of heterogeneous catalytic conversion processes that take place in the vehicle catalyst depends especially on the mass transfer of the substances involved as well as the temperature and the generation of an optimum fuel mixture. The term mass transfer refers to the transport of the gaseous emissions from the gas phase core to the catalytically active wall of the catalyst channel. Because of the small size of the channel (small hydraulic diameter), laminar flow profiles with relatively poor transport conditions develop only a few millimetres behind the gas inlet so that mass transfer takes place in the form of diffusion. According to Fick’s laws this means that the transferred mass depends on the mass transfer coefficient on the one hand and directly on the concentration gradient between the gas phase core and channel wall on the other. From this it follows that in a catalyst at operating temperature approx. 90% of the emissions have already been converted after approx. 10% of its length (figure 3).

Abbildung 3: Abnahme der Emissionen über Katalysatorlänge (exemplarische Berechnung

auf Basis einer stofftransportlimitierten Umsetzung)

Figure 3: Emission reduction vs. catalyst length (calculated on the basis of mass transfer limitation)

Figure 3 shows that a large part of the catalyst and hence the precious metals convert only a relatively small proportion of the emissions because the transport from the gas phase core (cell channel in the laminar flow regime) to the wall limits the rate of the overall process. While it is possible to remedy this problem by increasing cell densities again and thus reducing channel size and diffusion paths this would lead to a disproportionate rise in pressure loss. Even a reduction in catalyst length made possible by this, which has a linear effect on pressure loss, cannot compensate for this disadvantage (cf. figure 4).


Abbildung 4: Druckverlust als Funktion der Zelldichte und der Katalysatorlänge (Berechnung,

Kat-Ø 105 mm, 400 bzw. 800 cpsi, 50 m, 400 kg/h, T = 100 °C, unbeschichtet)

Figure 4: Pressure loss as a function of cell density and substrate length (calculation, catalyst Ø 105mm, 400/800 cpsi, 50 m, 400kg/h, T = 100°C, uncoated)

There is another trend that requires new methods in catalyst technology: Modern engines typically produce increasingly lower raw emissions. As mentioned above, lower concentrations of pollutants also reduce diffusion-based mass transfer. So measures to increase mass transfer have to be found. The following offer a potential solution:

A. Radial flow near the wall in the channels (TS design) B. A reduction of diffusion paths and of the hydraulic diameter and a repeat of the

entrance flow (LS design) C. Radially open, perforated structures (PE design)

3. Structures with radial flow components (TS design) In 1990 radial flow components were successfully realised using the so-called transversal structure (TS design) [12, 13]. Tests on a flow model, which used a change of colour caused by a chemical reaction with the wall to qualitatively or semi-quantitatively visualise the amount of a gaseous substance (ammonia) reaching the substrate wall, demonstrated the advantage of the TS structure [14], see figure 5.


Abbildung 5: Genutzte Kanalwandung (Abwicklung über Zellumfang) in einem glatten Kanal (links)

und einem Kanal mit transversal strukturierter Folie (TS, rechts), dargestellt als Schwärzungsgrad in Abhängigkeit von der Position auf der glatten bzw. gewellten Lage

Figure 5: Intensity of contact between gas and channel walls (flat and corrugated layer), shown by adsorption of ammonia; smooth channel (left) compared to a TS-structured channel (right)

The increase of mass transfer coefficient beta caused by the TS indentations is shown qualitatively in figure 6.

Abbildung 6: Aufbau der TS-Struktur und Vergleich des Stofftransportkoeffizienten Beta eines

glatten Kanals mit einem Kanal mit transversaler Struktur (berechnet, T = 700 °C, w = 50 m/s, Zelldichte = 200 cpsi)

Figure 6: Design of TS structure, comparison of mass transfer coefficient beta between a smooth channel and a TS-structured channel (calculated, T = 700°C, w = 50m/s, cell density = 200 cpsi)

Improved utilisation of the catalyst wall, as shown in figures 5 and 6, was used to reduce cell density and thus backpressure while maintaining the same level of efficiency and also to reduce the dimensions of the catalyst. Since it was possible to coat this substrate by standard processes it was widely used in mass-produced catalysts with low cell densities from 200 to 500 cpsi. The products achieved significant market penetration.


4. Structures to reduce diffusion paths (LS design) Papers on developments dealing with unfavourable mass transfer conditions and measures to increase mass transfer were published as early as the seventies. The fact that, for instance, the turbulent and hence more effective entrance flow could be repeated if the honeycomb was divided into suitable, short circular discs with axial clearances (cf. figure 7a) was described as a very effective solution. Results published at a later date highlighted the possibility of reducing the catalyst volume by increasing efficiency through this disc catalyst design [15]. These positive findings did not find their way into mass production for cost and technical reasons. However, tests clearly showed the potential of “turbulent” flow control. Substrate structures, whose conventional smooth channel walls had been replaced by walls that had regular gaps and were positioned along the flow, were introduced by Behr as early as 1989 and a short time later by Emitec. The so-called Behr-SQ and Emitec-LS technologies are described in [16] and [17] (cf. figures 7b and 7c). The effect of the substrate designs was to repeat the entrance flow and reduce the diffusion paths (s.a.).

Abbildung 7: a) Scheiben-Kat-Technologie (links) b) SQ-Technologie (rechts) c) LS-Technologie (Mitte)

Figure 7: a) Disc catalyst technology (left) b) SQ technology (right) c) LS technology (middle)

The first hurdle was to find suitable production methods for the metal substrate since apart from corrugating, the foil profiles also required the structure to be embossed and cut. Since, as mentioned above, an adequate coating process was not available even reproducible research projects were doomed to fail. The early theories can now be put into practice because new production technologies, process technologies and materials have become available. New coating processes – adapted to structured metal substrate technologies – are now being mass-produced. A common feature of the SQ and the LS designs is that the pollutant particles do not have to rely solely on diffusion to travel from the channel core flow to the wall and that a new wall projecting into the channel is positioned directly in the middle of the flow core. An LS blade is, in fact, pushed into the channel before laminar flow and a resulting concentration profile are able to develop (figure 8).

LS - Design SQ - Design


Abbildung 8: Radiale Strömungen und Konzentrationsprofile in einem LS-Kanal

Figure 8: Radial flow profiles and concentration profiles in an LS channel

This effect is further explained by calculations. The alternating arrangement of a normal flow channel and an LS blade is effectively comparable to a packed column of Raschig rings (figure 8) where flow passes through hollow catalyst cylinders as the bulk material. This effect of a multiple entrance flow is also similar to the multiple alternating arrangements of short catalyst discs and corresponding clearances as shown in figure 7a. The LS design therefore represents the first “turbulent” catalyst system for automotive applications based on systems found in the chemical industry. Figure 9 shows the effect on the mass transfer along an LS channel compared to a standard channel. The designation of the LS-structure requires a new nomenclature. The base structure (main corrugation), which defines the amount of material used, forms the first part of the designation. The effective part of the structure (main corrugation cell density plus LS fins), including the LS fins that are visible in when looking through the catalyst, forms the second part of the designation. The effectiveness of the LS structure is dependent on an optimum layout of the LS fins.


Abbildung 9: Stofftransportkoeff. Beta in einem Standard- und einem LS-Kanal (berechnet, T =700 °C, w = 50 m/s, Zelldichte = 200/400 cpsi, Schaufellänge = 7 mm, Abstand = 17 mm)

Figure 9: Mass transfer coefficient beta in a standard channel and an LS channel (calculated, T = 700°C, w = 50m/s, cell density = 200/400 cpsi, blade length = 7mm, distance between blades = 17mm)

Figures 10a, 10b and 10c show the effects of different blade lengths, blade numbers or blade depths for an LS cell density of 200/400 cpsi

Abbildung 10a: Einfluss Einprägungsanzahl (Schaufellänge 2mm, Einprestiefe 50 %): Effizienzen /

Druckverluste div. LS-Geometrien (Messung, Eff.: T = 600 °C, w = 10 m/s, Propen 7 mM/m

3, Lambda = 1, Vkat = 3,2 cm

3, PM = 30g/ft, Pd:Rh=5:1; Gegendruck:

m = 10 kg/h, T = 100 °C)

Figure 10a: Effect of blade number (blade length 2mm, blade depth 50%): efficiency/pressure loss of various LS designs (measurement, eff.: T = 600°C, w = 10m/s, propene 7mM/m

3,

lambda = 1, Vkat = 3.2cm3, PM = 30g/ft, Pd:Rh=5:1; pressure loss: m = 10kg/h,

T = 100°C)


Abbildung 10b: Einfluss Einprägungslänge (Schaufelanzahl 2 + 1, Einpresstiefe 50 %, 200/400 LS):

Effizienzen und Druckverluste div. LS-Geometrien (Messung, Eff.: T = 600 °C, w = 10 m/s, Propen 7 mM/m


3, PM = 30g/ft, Pd:Rh=5:1;

Gegendruck: m = 10 kg/h, T = 100 °C)

Figure 10b: Effect of blade length (blade number 2 + 1, blade depth 50%): efficiency and pressure loss of various LS designs (measurement, eff.: T = 600°C, w = 10m/s, propene 7mM/m

3, lambda = 1, Vkat = 3.2cm

3, PM = 30g/ft, Pd:Rh=5:1; pressure

loss: m = 10kg/h, T = 100°C)

Abbildung 10c: Einfluss Einprägungstiefe ET (Schaufelanzahl 2 + 1, Schaufellänge 7 mm):

Effizienzen und Druckverluste div. LS-Geometrien (Messung, Eff.: T = 600 °C, w = 10 m/s, Propen 7 mM/m


3, PM = 30g/ft, Pd:Rh=5:1;

Gegendruck: m = 10 kg/h, T = 100 °C)

Figure 10c: Effect of blade depth ET (blade number 2 + 1, blade length 7mm): efficiency and pressure loss of various LS designs (measurement, eff.: T = 600°C, w = 10m/s, propene 7mM/m

3, lambda = 1, Vkat = 3.2cm

3, PM = 30g/ft, Pd:Rh=5:1; pressure

loss: m = 10kg/h, T = 100°C)


The blade length of the substrate designs shown above was varied irrespective of the number of blades (LS structures); the channel length was identical in each case. Figure 10a shows that an excessive number of blades causes disproportionately high pressure loss. Therefore the length and the depth of the blades was changed in the next steps (Figures 10b and 10c). The resulting design, marked with arrows, was chosen for series production (7mm length at 70% ET). The pressure loss of this design, which is used with a cell density of 200/400 cpsi LS, is equivalent to that of a conventional substrate with 400 cpsi. Generally the effects of the blade design can also be applied to higher cell densities. Due to the usually poor flow distribution in close-coupled locations with notional cylinder exhaust entry points, radial equalisation between the individual exhaust entry points of the catalyst face is desirable. On the one hand this improves the utilisation factor of the catalyst while on the other the lambda values for each cylinder, which differ slightly during actual operating conditions, are combined in a stoichiometric mixture that is ideal for catalytic efficiency. The unstructured flat layer in the LS structure limits the extent of radial flow equalisation in the catalyst in individual layers of the LS structure.

5. Radially open structures (PE design) It is possible to achieve effective radial flow equalisation inside a METALIT® catalyst substrate through the appropriate perforation of the foils. The improvement of mass transfer and flow equalisation to compensate for a reduced geometric surface area is a design criterion for the size of the cut and the density of the holes. The catalytic coating is of particular interest in this case since loss of surface area at a constant washcoat quantity per volume results in a thicker washcoat, which in turn impairs the pore diffusion properties of the reactants inside the washcoat and causes higher pressure loss. The coating manufacturers have successfully worked on a solution and developed new washcoats with a lower washcoat amount but with the same oxygen storage capacity. Perforated foils have a dual effect: The first relates to the holes of one layer “permitting” the flow to change from one channel to another because of pressure loss differences, which facilitates flow equalisation (figure 11). The second effect is based on the geometric structure of the winding form with perforated foils. The perforations are arranged so that they are positioned on successive cross-sections along the direction of the axis. The winding method produces cavities in an offset successive arrangement along the axis (caves, see figure 12). This generates a “zigzag” flow from the channels into the caves and back into the channels at the end of the caves. The direction of the flow in the caves depends on the pressure difference and the pressure loss of adjacent channels. The changeover from cave to channel generates the same “turbulent” effect as the entrance flow of a catalyst.


Abbildung 11: Schematisch dargerstellter Strömungsausgleich und Strömungsverteilungsergebnisse

Figure 11: Diversion of channel flow (schematic) and results of flow distribution measurements

The fact that diffusion equalisation takes place as a positive effect in addition to flow equalisation has to be taken into consideration. Figure 13 shows the course of the mass transfer coefficient along the channel length of a PE structure compared to a conventional substrate and an LS substrate. A significant increase in the mass transfer coefficient can be observed in the LS structure.

Abbildung 12: PE-Struktur mit Kavernenbildung

Figure 12: PE structure with “caves”

This is due to the constant repetition of the entrance flow and the reduced hydraulic diameter in the area of the indentation. The pressure loss of the PE structure is dramatically reduced in the caves because of a significantly greater hydraulic diameter and the resulting reduction of side friction so that the repeated entry loss in the formation of the flow from the caves into the cell channels is overcompensated for. Figure 14 shows pressure loss as a function of hole size on which this function depends. In series production, the hole diameter was specified as 8mm and porosity as ~40%.


Abbildung 13: Stofftransportkoeffizient Beta über Kanallänge eines Standardträgers / LS-Trägers /

PE-Trägers (berechnet, T= 700 °C, w = 50 m/s, Zelldichte = 200/400 cpsi, Einprägungslänge = 7 mm, Abstand = 17 mm, Lochdurchmesser = 8 mm, Lochabstand = 3 mm)

Figure 13: Mass transfer coefficient beta along channel length of a standard substrate / LS substrate / PE substrate (calculated, T = 700°C, w = 50m/s, cell density = 200/400 cpsi, blade length = 7mm, distance between blades = 17mm, hole diameter = 8mm, distance between holes = 3mm)

Abbildung 14: Druckverluste von Substraten mit diversen PE-Lochgrößen (Messung, T = 100 °C,

m = 300 kg/h, Träger Ø 118 x 74,5 mm/ 400 cpsi / 50 m, unbesch., Porosität = 37 %)

Figure 14: Pressure loss of substrates with various PE hole sizes (measurement, T = 100°C, m = 300kg/h, substrate Ø 118 x 74.5mm/ 400 cpsi / 50 m, uncoated, porosity = 37%)

As mentioned above, apart from flow distribution, the equalisation of the cylinder-dependent gas concentration plays is particularly important. Depending on the position of the lambda sensors, different measurement results for the pre-catalyst


and post-catalyst lambda sensor may be obtained, especially in close-coupled catalyst systems that consist of only one substrate. In extreme cases the pre-catalyst sensor (e.g. integrated in the manifold) detects an ideal mixture in all cylinders while the post-catalyst sensor is primarily exposed to the exhaust gas of only one cylinder. Clogging of the fuel injectors can result in incorrect fuel mixture generation or even the activation of the OBD lamp. Figure 15 shows the effects of the PE structure on the equalisation of the lambda signals in front of and behind the catalyst with a cylinder-selective leaning of the mixture by 20% even at relatively short substrate lengths of 50.8mm.

Abbildung 15: Vergleichmäßigung der Lambdasignale vor und hinter Kat durch die PE-Struktur bei schrittweiser Abmagerung eines Zylinders (Messung am Motorprüfstand, Katalysator Ø 110 x 50,8 mm/600 cpsi/40 m), delta Lambda = Lambda nach Kat – Lambda vor Kat

Figure 15: Equalisation of lambda signals in front of and behind the catalyst by using a PE-structured substrate with gradual leaning of the mixture of one cylinder (measurement on roller test bench, catalyst Ø 110 x 50.8mm/600 cpsi/40 m), delta lambda = lambda behind catalyst – lambda in front of catalyst

These research results fully validated earlier considerations [18] to insert a perforated metal substrate structure at the front of the substrate to improve light-off behaviour (by reducing the mass that had to be heated). The fluidic and mass transfer-related advantages could not be determined theoretically, let alone practically, at that time. The results presented here clearly demonstrate that only an axially completely structured matrix is capable of releasing the full fluidic, thermodynamic and chemical potential [19]. Combination of PE and LS structures In this design an LS structure is added to the corrugated layer of the metallite, the flat layer is perforated according to the specifications of the PE design (figure 16).


Abbildung 16: Kombination der LS- und PE-Strukturen in einem Träger als LS/PE-Struktur

Figure 16: Combination of LS and PE structures in one substrate to form an LS/PE structure The potential applications of “turbulent” catalysts are exemplified in the following chapter.

6. Application examples

The structured foils described above can be used to generate completely new application-dependent advantages:

I. Reduction of pressure loss II. Equalisation function of the concentrations III. Flow distribution IV. Improved mass transfer

The PE structure is already being used in sports application with high requirements regarding pressure loss. In the course of the development of catalysts for EU4, ULEV and SULEV spark ignition engines, substrates with high cell densities (s.a.) have become the standard because of their efficiency, size and low system costs. High-performance sports cars were accordingly equipped with substrates with up to 600 cpsi and corresponding pressure loss disadvantages. The PE design now makes it possible, for example, to replace a standard 600-cpsi substrate with a 600-cpsi PE substrate of the same volume and the same level of efficiency but with a pressure loss of a 400-cpsi substrate. Last year, the Audi RS6 was the first vehicle with this substrate to go into series production [20]. Other vehicles, such as the Maserati V8, followed. Ever since catalysts have first been used in car racing, metal substrates have been the preferred choice because of their pressure loss advantage. In 2004, Volvo tested the PE structure on the racing circuit in an extremely successful S60 vehicle. In 2005, most racing cars are being converted to PE. This METALIT®

type is therefore rebuilding the reputation of metal substrates for sports cars.


6.1. Applications for spark injection engines The exhaust gas after-treatment of spark injection engines has been incorrectly described as having reached the end of its development since existing technologies had already made it possible to build cars that cleaned the environment. Because modern standard catalysts are already able to achieve the required level of efficiency the aim of “turbulent” substrates is to reduce catalyst and system costs. This must include above all the functions relating to lambda control and catalyst diagnostics. A modern standard catalyst system usually consists of two substrates that are arranged behind each other inside a mantle. In order to improve cold starts the first substrate has a smaller diameter than the second; this is known as a so-called cascade system [21]. The gap between the two substrates is generally used to accommodate the second lambda sensor. The gap also causes flow mixing and flow equalisation. In these very effective systems, which are in common use today, both the LS design and the PE design produce significant functional and cost advantages. Both structural types have a lower thermal mass, which makes it possible to dispense with the first, smaller substrate in the cascade. By adding another innovation to this structure, that is, the so-called lambda sensor catalyst [22], the gap between the substrate becomes superfluous, because this type of substrate allows one or more lambda sensors to be positioned directly inside the substrate. The radial “permeability” in the substrate now also allows diffusion equalisation of the pollutant concentration, which has a positive effect on the control accuracy of fuel mixture generation. The production method of metallic catalyst substrates makes it possible to punch a hole for the lambda sensor in almost any axial position during production through a simple die cutting process. The holes punched into the metal foils combine to form a lambda sensor hole when the substrate is assembled. In combination with the PE structure a single substrate system with the functions of a double substrate system is created. Another advantage of the LS/PE structure is the reduced thermal mass, which improves cold start behaviour and renders the cascading of the substrates obsolete. Figure 17 contains a comparison between a traditional cascade and a single brick system with an integrated lambda sensor and a PE structure. This shows that system costs can be significantly reduced by combining the LS/PE structure with an integrated lambda sensor.


Abbildung 17: Vergleich von Kaskadensystem und Einzelträgersystem mit integrierter Sonde

und PE-Design

Figure 17: Comparison of a catalyst cascade design and a single brick catalyst with integrated lambda sensor and PE design

Figure 18 shows the gaseous emission of a catalyst system using LS technology compared to a standard substrate in a spark ignition engine. A 600-cpsi system can therefore evidently be replaced by a 300/600-cpsi LS substrate.

Abbildung 18: Emissionen eines Katalysators Ø105x135mm (70 g/ft

3; 1:7:1) im FTP-75 Test

gemessen an einem1,8l-Turbo-Motor

Figure 18: Emissions of a substrate Ø105x135mm (70g/ft3; 1:7:1) in an FTP-75 test cycle using

a turbocharged 1.8-litre engine


6.2. Applications for compression ignition engines 6.2.1. Oxidation catalysts; application of LS structure: A benchmark test was carried out on a medium-sized vehicle with a 2.2-litre turbodiesel engine. A 200/400-cpsi LS substrate and a 400/800-cpsi substrate with a smaller volume were compared to a 400-cpsi standard substrate. Each catalyst was subjected to an NEFZ cycle.

Abbildung 19: CO-Emissionsergebnisse bei einem Trägervergleich an einem 2,2 l Dieselmotor

(Messung am Rollenprüfstand, Träger Ø 118mm, Länge 150 mm (V=1,64 l) bei 400 cpsi und 200/400 LS bzw. Länge 110 mm (V=1,20l) bei 400/800LS, alle 50 m, PM = 90 g/ft

3 Pt:Pd:Rh = 1:0:0) ECE, EUDC und Gesamttest

Figure 19: CO emission results for a 2.2-litre diesel engine (roller test bench, substrate Ø 118mm, length 150mm (V=1.64l) at 400 cpsi and 200/400 LS or 110mm (V=1.20l) at 400/800 LS respectively, all 50 m, PM = 90g/ft

3 Pt:Pd:Rh = 1:0:0) ECE, EUDC and

overall test result

In this special application, the advantage of the 200/400 LS structure in the ECE part of the EU test cycle (cf. figure 19) is due to lower thermal capacity and hence faster heating behaviour (figure 20), while the catalyst volume is significant at low temperatures (diesel) because of the limited reaction rate compared to the 400/800 LS substrate. The mass transfer advantage of LS structures, which is able to overcompensate for the cold start disadvantage, becomes clear at operating temperature (EUDC part) (figure 19).


Abbildung 20: Strukturtemperaturen der einzelnen Katalysatoren 50 mm hinter der

Gaseintrittsseite

Figure 20: Structure temperatures of individual catalysts 50mm downstream from the gas inlet side

The 400/800 LS catalyst has the same level of efficiency as the 200/400 LS substrate despite its 27% smaller volume (1.2 instead of 1.64l). Overall the efficiency of the LS structures in the EUDC part of the cycle is a good 30% above that of the normal structure. The overall test result reflects the individual parts of the test. In this case the cold start advantage of the 200/400 LS substrate had the greatest impact. The HC test results of the individual designs correspond to the CO results. The LS structure either opens up the possibility of saving material by using a 200/400 LS instead of a 400-cpsi standard substrate with identical volume, which reduces costs and weight, and/or the possibility of reducing the catalyst volume at the same cell density, which saves on precious metals. In case of poor flow distribution caused by problematic locations a combination of LS and PE can produce an optimum result (s.a.). In September 2004 the LS/PE structure had its world premiere as an oxidation catalyst used in front of a particle filter.


6.2.2. Reduction catalysts; application of LS/PE structure: Other areas of application for the LS/PE structure include substrates for so-called SCR catalysts, where, apart from mass transfer, it is above all the distribution of urea or ammonia inside the substrate that is an important factor to ensure optimum metering over the entire substrate without breakthrough. Figure 21 shows the NOx reduction rates of different substrate types (full extrudate, standard metal substrate with the same cell density, LS/PE substrate) at different temperatures. The space velocity was 100,000h-1 for each catalyst. Ammonia feed was controlled by ammonia slip set at 5ppm. The rising maximum conversion, which is explained by an optimum ammonia distribution and improved mass transfer, is clearly evident.

Abbildung 21: NOx-Reduktion mit SCR-Katalysatoren als Funktion der Temperatur auf Basis

verschiedener Strukturen (Raumgeschwindigkeit 100 000 1/h)

Figure 21: NOx reduction with SCR catalysts as a function of temperature based on various substrate structures (GHSV 100,000 1/h)

7. Summary The new “turbulent” catalyst substrates represent the logical further development of the advantages of “turbulent” catalyst systems that were previously utilised in static applications. Ever since catalyst technology was first used in vehicles over three decades ago it had not been possible to meet the requirements of mobile applications with these substrate systems. As a result honeycombs with laminar flow regimes of the type commonly used today became established at the time. Because of their mass transfer, flow distribution and pressure loss advantages they can be used in a wide range of applications for spark and compression ignition engines. The main development objective, which was to produce more compact and cheaper catalyst systems for spark ignition engines, has therefore been achieved.


For diesel catalysts still under development the “turbulent” METALIT® types offer functionally specialised solutions for oxidation, hydrolysis, mixer and reduction catalysts. New control strategies, which ensure improved catalytic efficiency and more stable long-term behaviour, can be developed in combination with an integrated lambda sensor or integrated gas sensors, for example, in NOx adsorbers.

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[17] R. Brück, J. Diringer, U. Martin, W. Maus; Emitec GmbH; ”Flow Improved Efficiency by New Cell Structures in Metallic Substrates”; SAE 950788

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[20] M. Ganz, S. Hackmayer; Quattro GmbH; C. Kruse, A. Reck; Emitec GmbH; “Advanced Catalyst System for the Audi RS6, 8 Zyl, 4,2 ltr, 331 hp with LEV Certification”; Wiener Motoren-Symposium 30.04.2004

[21] United States Patent 54 03 559, 1995

[22] Mats Laurell, Jan Dahlgren, Mats Karlflo; Volvo Car Corp.; Kait Althöfer, Rolf Brück; Emitec GmbH; “A Metal Substrate with Integrated Oxygen Sensor; Functionality and Influence on Air/Fuel Ratio Control”; SAE 2003-01-0818

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