Application Guideline to Define a Catalyst Layout for ...

1 2001-01-0929 Application Guideline to Define a Catalyst Layout for MaximumCatalytic EfficiencyPer Marsh,Dr. Filip AckeVolvo Konieczny,Rolf Br ck,Peter HirthEmitec GmbHCopyright 2001 Society of Automotive Engineers, influence of physical parameters of the Catalyst ssubstrate such as thermal mass, hydraulic diameter andgeometric surface area on Catalyst s efficiency is wellknown as published in numerous works. This paper willshow interactions of these parameters and will provide aguideline on how to design the optimum system for aspecific Application , taking into account system s backpressure and system costs. Based on engine test benchresults that show the influence of the physicalparameters, the results for the optimized design regardingemission tests and maximum conversion rate at higherloads will be INTRODUCTIONThe tightening of emission legislation and the compulsionto guarantee of maintaining to these limits even after80,000 miles, for example, led to an ongoing reduction ofenvironmental pollution caused by motor vehicles inconjunction with On Board Diagnosis demands on reduced engine-out emissions and thequality of exhaust gas aftertreatment systems grewaccordingly.

2 Figure 1 shows the required Hydrocarbon(HC) conversion rates depending on the level of untreatedengine-out emissions and the respective HC-limits ofCalifornian exhaust gas conversion rates are based on HC engine-outemission levels of g/mile resp. g/mile in theFederal Test Procedure (FTP). It is striking that even inthe case of reduced engine-out emissions, catalystefficiencies increases continuously starting from"Transient Low Emission Vehicles" (TLEV) through to the"Super Ultra Low Emission Vehicles" (SULEV).Figure 1: Required HC conversion rates dependend onCalifornian emission legislation and on different levels ofHC engine out emissionsIt has been shown in various studies [1, 2, 3], that coldstart emissions in particular can be significantly reducedby moving the Catalyst in a position closer to the enginebecause of the more rapid heating of the Catalyst addition, an increase in cell density led to an increaseTLEVLEVULEVSULEVNZEVHC-Conversio n Rate [%] g/mile HC Engine Out Emissions2 g/mile HC Engine Out Emissions99,89896,2593,7599,510098969492 90in efficiency at operating temperature and a reduction inthe Catalyst volume [4, 5, 6].

3 Due to existing conditionssuch as lambda control or system pressure-lossrequirements it is, however, necessary to makecompromises which are influencing both catalystefficiency as well as the costs of the exhaust gasaftertreatment , the objective for the future has to be thedevelopment - without compromise - of the most costeffective exhaust gas aftertreatment system with aconversion rate as close as possible to 100% for allpollutants, so that motor vehicles do not place anyadditional burden on the environment - even in areas withlow background emissions.

4 This new generation of carswill help to clean up even these areas with regard to thelimited Catalyst EFFICIENCYO verall Catalyst efficiency depends on cold startbehaviour and the efficiency at operating in the past cold start emissions representedapproximately 80% of the total emissions, developmentwas focussed mainly on improving light-off behaviour. Insummary it can be said that primarily due to a reductionin thermal mass [7, 8, 9] and in Catalyst diameter [10]assisted by higher exhaust gas temperatures in the frontof the Catalyst it has been possible to significantlyimprove cold start efficiency.

5 Today the development ofclose-coupled Catalyst systems is almost the norm andmore and more vehicles have appropriate passive UltraLow Emission Vehicle (ULEV) aftertreatment systems asstandard. The parallel development of enginemanagement systems as well as an increase in celldensity from 400 cpsi to 800 or up to 1200 cpsi made itpossible to further reduce emissions at operatingtemperature as EXHAUST GAS TEST ANALYSISA nalysing the HC emissions of an ULEV vehicle with aclose couples Catalyst system (Figure 2), it can benoticed that even a larger proportion of total emissions isproduced during the cold start phase, as compared tocatalyst systems designed for the TLEV problem that at the beginning of the test before light-off the Catalyst has an effect of a heat sink, has becomeeven greater regarding the ever increasing exhaust the other hand, a greater significance is placed on theemissions at normal operating temperature due to theweighting of "Bag 2" in the FTP-cycle.

6 Due to theweighting (Equation 1), emissions in FTP bag 2 haveapproximately 2,5 times greater an effect on the overallresult than the cold start 2: Analysis of accumulated HC emissions in theFTP-cycle for a TLEV- and a ULEV-vehicleEgew = (EBag1 + E Bag 2)/(DBag1 + DBag 2) x + (EBag 2 + EBag 3)/(DBag2 + DBag 3) x (1)with EBag 1 -Emissions in Bag 1,and accordingly in Bag 2 and 3 DBag 1 -Distance driven in Bag 1,and accordingly in Bag 2 and 3 Therefore its obvious that almost 100 percent conversionof emissions are necessary at operating temperaturesince the emission standard has to be adhered to even inaged condition, when significant increases in cold startemissions the next generation of exhaust gas emissionstandards such as SULEV, "Near Zero Emission Vehicle"(NZEV) or the European standard "EnhancedEnvironmentally Friendly Vehicles" (EEV), however,catalytic efficiency has to be increased again by factor of4 to 10.

7 Expressed in other words, the decisive numberis not the increase in the absolute conversion rate from98 to (ULEV => SULEV), but the reduction oftailpipe emissions from to g/m. Due to theweighting of the individual exhaust gas bag results in theFTP-cycle, now the Catalyst efficiency at operatingcondition (Bag 2) gains increased importance, too. In thefollowing section primarily the optimum Catalyst layoutfor normal operating temperature will be the limiting factors for Catalyst efficiency in aheterogenious Catalyst (Figure 3), we find there thelambda control to be of great importance as well as themass-transfer of the pollutants from the channel center tothe HC-Emissions [g/mile]Operating Temperature (Phase III)Hot Start(Phase IV)TLEV-StandardULEV-StandardCold-Start( Phase I + II )Bag 1 Bag 2 Bag 3 Figure 3.

8 Limiting factors for Catalyst efficiency in theheterogenious catalysis [11, 12] MASS TRANSFERU nder normal operating conditions the reaction is limitedby mass-transfer. Caused by the chemical reaction, theconcentration of pollutants is decreased dramaticallywithin the channel in flow direction towards the rear end ofthe Catalyst . This results in decreasing difference ofpollutant s concentration between center and wall andtherefore in a decrease of the driving force for from that, due to the laminar flow, themass transfer rate numbers are comparably poor. Mass-transfer usually is described using the mass transfercoefficient Beta.

9 DDhSh =12 (2)withSh = a Rem Scn-Sherwood numberD12 -Binary diffusion coefficientRe-Reynolds numberSc-Schmidt numberdh-Hydraulic diameter of channelEquation (2) exhibits the dependency of the mass-transferrate on both the velocity of flow in the channels(described with the Reynolds number), as well as on thechannel s characteristic size described with the hydraulicdiameter. Discussing now the influence of the catalystdiameter, its obvious to receive a significant gas velocitylevel, when the Catalyst s diameter is reduced. Accordingto equation (2) that results in inceased numbers of themass transfer coefficients, as plotted in figure smaller Catalyst diameters for a givencatalytic reaction, which is limited by mass transfer reveal higher efficiency.

10 Consequently, in figure 5 thedependence of the Catalyst s HC-conversion efficiency oncatalyst s diameter and on the exhaust gas mass flow 4: Mass transfer coefficients as a function ofcatalyst diameter and of exhaust gas mass flow (celldensity 600 cpsi)In these experiments, the Catalyst length was keptconstant. Accordingly, Catalyst volume increases withincreased diameter. The results plotted in figure 5 show,that for each mass flow level there exists an optimumcatalyst diameter range, giving an maximum conversionrate result. It can furthermore be observed, that, at thelow mass flow level of 50 kg/h the Catalyst with 70 mmdiameter reveals a higher conversion rate compared tothe Catalyst with 127 mm diameter, even though theformer has a volume deficit of 70%.