Catalytic Converters
9.2 KHST-2010
Second mechanical durability test was the hot shake test (KHST-2010). This test includes three different load cases. Each period test time was 30 hours. New metallic catalytic converter was again in a very good condition after this test. We did not find any failure on the substrate nor the casing shell. As an example, figure 14. Shows gas inlet front face of a welded metallic converter 80 mm × 74.5 mm, 500 cpsi, tested over 90 hours according to the above proposed KHST-2010 test.
Figure 14: Gas inlet side of a close-coupled metallic converter after 90 h engine test bench running. Durability test parameter is KHST-2010.
Start of failure mechanism
10 Emissions, Back Pressure and Flow Distribution
Emission tests were carried out using as a test vehicle Fiat 600, may 2000. The engine dis- placement was 1,1 liter. Test converters were made as similar as possible. Total PGM- loading and wash coat formulation and cell densities were equal. Because of different structure the geometric surface area and volume of the converters differ slightly from each other. Physical parameters are shown in the table 5.
Table 5: Physical parameter
Parameter Standard metallic converter EcoXcell converter
Volume, dm³ 1,05 1,11
Cell density, cpsi 500 500(*
Total geometric surface area, m² 3,9 3,0
Total PGM loading g/cat 1,24 1,24
Pt:Pd:Rh 0:7:1 0:7:1
Aging of the converters was conducted by using in house aging cycle 20 h RAH. This ther- mally very severe cycle is described in figures 15 and 16.
Figure 15: Schematic test condition
Figure 16: Test cycle
MVEG-B emission test results are shown in the figure 17.
Figure 17: Test results
Results show that all emissions with the EcoXcell converter are better than with the standard converter despite of 23 % lower geometric surface area than in the conventional converter.
Emission test results of EcoXcell converter indicate improved mass and heat transfer from the bulk gas to the catalyst surface.
Back pressure measurements were conducted by engine test bench. Back pressure of the standard converter was slightly lower than in the EcoXcell converter where the corrugation angle was 20°. Results can be seen in the Figure 18.
Figure 18: Back pressure measurements
It was found that back pressure is quite sensitive to the angle in which corrugations are crossing each other. The smaller angle the lower pressure drop in the converter. In the Figure 19 are two similarly stacked EcoXcell converters with 10° and 20° angles is compared.
Figure 19: Back pressure measurements
0 50 100 150 200 250 300 350 400 450 500
2500 3000 3500 4000 4500 5000 5500
RPM
Pressure [mbar]
0 100 200 300 400 500 600 700 800
Temperature [°C]
EcoXcell20 ° Conventional T_average
By doubling the angle back pressure was more than doubled. So it is an important parameter when a converter design is optimised.
Gas flow distribution was measured with air in room temperature. With the same converters also gas flow distribution was measured. See the Figure 20.
Figure 20: Flow distribution
Flow distribution with EcoXcell converter was more even than in the standard converter.
Vmax/Vaverage was with EcoXcell converter 1.32 and with the standard converter the ratio was 1.70.
11 Summary and Conclusions
The objective of this study was to develop a more durable metallic substrate than any other of the existing substrates. There are today many close-coupled applications where all commer- cially available substrates have difficulties with mechanical durability. Driving by WOT under lambda control can increase temperature in the converter until 1100 °C. Starting point for this work was to analyse the weakest points of the existing products and to find improvements for those.
Our main focus was laying on the metallic substrates but also a ceramic close-coupled con- verter was tested as a reference. Three weakest points of metallic brazed substrates were found:
1. Fixing between the shell and the matrix. During fast acceleration temperature in the matrix rises faster than temperature of the shell. During deceleration the same happens opposite way. Different thermal expansion causes high tensile stress between the shell and the ma- trix.
2. Thermal shock and high vibration in radial direction causes higher tensile stress to the straight foil than to the corrugated foil so that the straight foil breaks normally first.
0 5 10 15 20
0 10 20 30 40 50 60 70 80 90 100 110 120
Distance [mm]
Velocity [m/s]
EcoXcell
Standard met. catalyst
3. Melting point of the brazing material is about 1160 °C. It is near the operation temperature.
In the new structure improvements for the above mentioned weak points are as follows:
1. Fixing between the matrix and the shell was made by welding. Ring weldings are locating near to each other to minimise tensile stress caused by different expansion of the shell and matrix.
2. By eliminating the straight foil completely the structure was made flexible in radial direc- tions. In axial direction the new structure is firm.
3. All the joints are made by welding. Melting point of the welding spots is about 1500°C.
Which is giving more margin to the operation temperature.
Mechanical durability test results were showing clearly superior behaviour of the new EcoXcell substrate compared to any of the commercial products. Duration time in the most demanding engine bench tests were more than ten times longer than with than the most com- mon metallic converters and more than double compared to the tested close coupled ceramic converters. According the conducted tests the main objective of the project could be achieved.
More tests will be needed.
All the other characteristics came as consequence of the new structure. Emission test results and gas flow distribution were improved due to the mixer structure, which enhanced the mass as well as heat transfer in the matrix.
There is plenty of room for further optimisation and improvements. Next step will be to study influence of different grossing angles to the emissions, back pressure and mass and heat trans- fer. Also tests with thinner foil materials will be included to the test programme in the near future.
12 References
[1] www.unifrax.com, unifrax Product Information Sheet
[2] Määttänen M. and Lylykangas R., Mechanical Strength of a Metallic [3] Catalytic Converter Made of Pre-coated Foil
[4] Määttänen M. and Avikainen T., Metallic Catalytic Converter Cross Axis Strength Con- siderations
[5] Luoma M., Lappi P. and Lylykangas R., Evaluation of High Cell Density Z-Flow Cata- lyst
[6] Luoma M., Härkönen M., Lylykangas R. and Sohlo J., Optimisation of the Metallic Three-Way Catalyst Behaviour
[7] T.R. Chandrapatala. A. D. Belegundu. Introduction To Finite Elements In Engineering.
Present Hall, Engelwood Cliffs, New Jersey 07632
[8] Dr. W. Elspass. Design of high precision sandwich structure using analytical and finite element models. MSC World User’s Conference Los Angeles, California March 26 – 30, 1990.
[9] Paul R. Woodmansee Master Student, Howard D. Gans, Ph.D Assistant Professor of Aerospace Engineering Air Force Institute of Technology. Finite Element Analysis of Porosity on Material Properties Using MSC/Nastran.
[10] Craig S. Collier, P.E and Kevin A. Spoth Lockheed Engineering and Sciences Co. Ther- momechanical Finite Element Analysis of Stiffened Unsymmetric Composite Panels With Two Dimenssional Models. NASA Langley research Center Hampton, VA.
[11] MSC/Nastran The Nastran Theoretical Manual Level 15.5. The Mac Neal-Schwendler Corporation
Material Aspects in Automotive Catalytic Converters, Hans Bode Copyright © 2002 Wiley-VCH Verlag GmbH &Co. K aA ISBN: 3-527-30491-6G
Claus-Dieter Vogt, Etsuji Ohara NGK Europe GmbH
Mikio Makino NGK Insulators Ltd
1 Abstract
The ceramic honeycomb substrate has been an integral part of automotive emission technology for the past 20 years. From the first ceramic monolithic substrates for catalytic converters to the new more advanced designs and shapes used in various applications and industries, the ceramic substrate has provided years of durability and performance. This paper will discuss the evolution of the ceramic honeycomb substrate from its origin through the latest advanced ap- plications.
2 Introduction
As concerns about increasing atmospheric pollution continue to rise around the world, special environmental interest and political groups continue to push for more demanding pollution regulations for industry to reduce further damage. The internal combustion engine used in automotive transportation has been cited as one of the major contributors to atmospheric pol- lution. By the year 2010 over 1 billion vehicles are projected to be on the road worldwide (1)
Technology continues to strive toward the maximum reduction of the main toxic and envi- ronmentally damaging components of exhaust emissions. An example of this technology is the three way catalytic converter which can convert more than 90 % of automobile exhaust pollut- ants into harmless gases. Since the beginning of emission regulations, the focus has been pri- marily directed at reducing hydrocarbons, carbon monoxide, and nitrogen oxides. Hydrocar- bons are organic compounds of hydrogen and carbon which when exposed to sunlight and nitrogen oxides, react to form oxidants which can irritate mucous membranes, and in some forms are considered carcinogenic. Carbon Monoxide is a colorless, tasteless, and odorless gas produced by the incomplete burning of carbon, which if inhaled with a volumetric concentra- tion of 0.3 % would result in death in 30 minutes. More than 90 % of the carbon monoxide emitted in cities comes from motor vehicles. Nitrogen monoxide is a colorless, tasteless gas produced by the internal combustion engine, but when exposed to air becomes NO2. Pure NOx
is a poisonous reddish-brown gas that can irritate mucous membranes. NO and NO2 are some- times referred together as nitrogen oxides (NOx) and are responsible for photochemical smog and acid rain (2). These represent the majority of toxic and environmentally detrimental gases in automotive engine exhaust.
Material Aspects in Automotive Catalytic Converters, Hans Bode Copyright © 2002 Wiley-VCH Verlag GmbH &Co. K aA ISBN: 3-527-30491-6G
3 History of Automotive Emission Control
The automotive industry has been an ongoing matter of interest concerning air pollution from the internal combustion engine since the early 1950’s, when the photochemical reactions that cause smog first became an issue in the Los Angeles area. Pollution attributed to the automo- bile slowly became evident through the 50’s and became a nationwide issue in the late 1960’s with the establishment of emission regulations by the state of California. This led to emission standard amendments to the Clean Air Act of 1965 passed by the Congress of the United States. In 1970, Congress passed the so-called Muskie amendments which required a 90 % reduction of automotive exhaust emissions between 1970 and the 1975–1976 model years.
Initially, exhaust emissions were reduced by engine calibrations such as leaner air/fuel ratio and spark retardation, but many auto manufacturers believed that the continued compromising of engine calibrations would lead to unacceptable fuel consumption and driveability. The solu- tion was an aftertreatment application called the catalytic converter, which reduced the harmful effects of automotive exhaust emissions without sacrificing engine performance.
The first catalytic converters, called oxidation catalysts, only controlled HC and CO, be- cause NOx regulations were not strict and could be met by using exhaust gas recirculation, which led to less nitrogen oxide formation from the engine. These catalyst systems were sup- ported by one-eighth inch diameter, thermally stable, transitional alumina coated pellets. Thus, these early converters were named pellet-type converters. They were used for the first few years until durability, available space under the car, and pressure drop issues became more important.
During the early 70’s another type of catalyst support, the ceramic monolith (honeycomb) substrate, was developed for the catalytic converter. Figure 1 shows various honeycomb sub- strates and a cut-away of a typical catalytic converter. The honeycomb substrate gets its name from the array of parallel channels or cells that create the look of a honeycomb. This new design possessed compatibility with catalysts and coatings, flexibility in shape and size, high geometric surface area, durability, low flow restriction, space efficiency, and fast warm-up.
Figure 1: Ceramic honeycomb substrate and automotive catalytic converter
In 1977 Congress amended the Clean Air Act and set revised standards to achieve a 90 % reduction in hydrocarbons by 1980, and 90 % carbon monoxide and 75 % nitrogen oxide re- duction by 1981. To meet the newer regulations, the requirements for the catalyst support
became more demanding, requiring higher thermal durability, better thermal shock resistance, better corrosion resistance, lower pressure drop, faster warm-up, and higher mechanical strength. This hastened the move toward the ceramic honeycomb substrate, which provided these characteristics, and by the mid 1980’s most automotive manufacturers had changed from the pellet-type support to the substrate support.
The original ceramic honeycomb substrates were primarily used with oxidation catalyst systems which required additional air (O2) for catalytic reaction. This air came from engine calibration adjustments (lean fuel ratio) and/or added pump configurations to inject air into the system. The oxidation systems, which by themselves could only control HC and CO, and relied on exhaust gas recirculation (EGR) to reduce NOx, were used until the three-way catalyst sys- tem emerged in the early 1980’s. The majority of the early substrates in the oxidation systems were designed to have 300 cells per square inch with a wall thickness of 12 mil (0.30 mm), known as a 12 mil/300 cpsi configuration. As emission standards tightened, the 6 mil/400 cpsi substrate appeared, which presented a larger geometric surface area, quicker warm-up, and lower pressure drop.
The three-way catalyst system controls all three main components using a loop fuel metering control system with an oxygen sensor and a catalytic converter. Most of these systems use the ceramic honeycomb substrate and rely on the loop fuel metering system to control the air/fuel ratio near the stoichiometric point, or the point where there is no excess fuel or air in the mix- ture. Near this stoichiometric point of air/fuel mixture, the following chemical reactions can optimally reduce the three main components of exhaust gas HC, CC, and NOx
2 2
2 6 2 2 2
2 2
2CO + O CO
2C H + 7O CO + 6H O 2NO + 2CO N + 2CO
®
®
®
Therefore a small window exists where the catalyst can reduce all three of these emission components as long as the air/fuel ratio is within this specified area, as shown in Figure 2.
Figure 2: Conversion efficiencies of a typical three-way catalyst
4 Substrate Requirements
Three-way catalyst technology is the preferred method of catalyst reaction, but tightening emission regulations and increasing performance needs continue to make the requirements for the catalyst support more stringent. The requirements and capabilities of the converter usually dictate the location in the exhaust flow, as shown in Figure 3. The thermal shock resistance, thermal durability, mechanical integrity, warm-up, and low pressure drop, of the catalyst sup- port continue to be improved, but at more critical levels and for a longer duration.
Figure 3: Converter location in passenger car
The catalytic support must be able to withstand the high and varying degrees of exhaust gas temperatures associated with the automotive engine. Catalytic converter support temperatures associated with normal stop and go driving can range from atmospheric temperature to 900 °C extreme exhaust temperatures, which usually occur due to engine ignition problems such as misfiring, can cause temperatures to exceed 1,000 °C . The ceramic honeycomb provides high temperature and thermal shock resistance due to a high melting point and low coefficient of thermal expansion (3,4).
The catalyst substrate requires high mechanical or structural strength due to the severe vi- bration that the substrate must endure by the tight mounting in the converter. The mechanical integrity of the substrate is directly proportional to the integrity of the cell structure of the honeycomb, and the shape of the honeycomb. The canning procedure inflicts a large amount of compressive pressure on the substrate during the clamping of the shell around the substrate.
The heat mass of the substrate is the main factor affecting light-off performance. The greater the heat mass, the longer it takes to transfer the needed heat across the substrate, and to reach full operating temperature. Light-off performance is important in the overall conversion effi- ciency of the catalyst.
The converter in the exhaust system produces pressure drop that has an immediate and counteractive effect on engine performance and fuel economy. This pressure change is related to the frictional loss across the honeycomb matrix, created by the cell wall thickness and cell pitch. Reducing the pressure drop is a function of increasing the open frontal area and creating larger cells, which the thinner wall configurations, such as the 6 mil/400 cpsi substrate, can accomplish.
5 Production Process of Ceramic Honeycomb
Since the ceramic honeycomb substrate’s inception in 1975, it has been made using Cordierite (2MgO + 2ALO3 + 5SiO2), which is mainly kaolin, talc, and alumina. This composition pro- vides low thermal expansion, high temperature stability, good porosity, and excellent oxidation resistance. Table 1 shows the typical properties of cordierite honeycomb.
Table 1: Typical material properties of cordierite honeycomb substrate
Item Properties
Crystal structure Cordierite
2MgO-2Al2O3-5SiO2 Thermal expansion
(x10– 6/°C) (40 °C–800°C) < 1.0 Specific heat (cal/g°C) 0.2 Softening temperature (°C) 1410 Thermal proper-
ties
Melting point (°C) 1455
Total pore volume (cm3/g) 0.2
Porosity (%) 35
Physical proper-
ties Mean pore diameter (µm) 4
A-axis > 85
B-axis > 11
Mechanical
properties Compressive strength (kg/cm2)
C-axis > 1
Thermal shock resistance
Electric furnace-room atmosphere
(°C Difference) > 650
The production process of ceramic honeycomb consists of four phases, the preparation of raw materials forming, firing, and inspection and testing of the finished product.
As for raw material, the mineral impurities and particle size of raw materials greatly affect the cordierite characteristic’s such as water absorption, coefficient of thermal expansion and thermal durability of the substrate. The raw material is mixed with organic binders and water, and kneaded to produce a uniform clay with desired hardness and temperature. These factor is important for forming process.
As for forming, the extrusion method performs two important tasks: providing the formation of the honeycomb in many sizes and shapes, and lowering the coefficient of thermal expansion of the ceramic material. These tasks are important in meeting the increasing requirements of catalyst supports for thermal shock resistance, and mechanical strength and durability.
Figure 4 illustrates the structure of a ceramic honeycomb extrusion die, The mechanism to form square cells begins as material flows into one of the back holes of the extrusion die (a), through the die (b), and out of the corresponding slit junction (c). As the material continues to flow out of the slit junction , it connects with the material flowing from other slit junctions to form a connecting point of the honeycomb wall (d).
There are possibility of square, hexagonal, triangular, or rectangular on the cell structure.
The square cell configuration has been widely selected and used in this field due to the balance
between mechanical strength and pressure drop. As the special application, the hexagonal cell also applied currently.
Figure 4: Ceramic honeycomb extrusion dies
The extrusion process also provides the necessary anisotropic arrangement of the kaolinite crystals for lower thermal expansion, as shown in Figure 5. These crystals are flat and the shearing force of the extrusion through the narrow channels of the die orients them. During the firing process the cordierite crystals are produced so that the negative thermal expansion c-axis is oriented vertically to the c-axis of the original kaolinite and in parallel with the honeycomb wall. This production of cordierite with favorable orientation (anisotropy) of the crystals is what allows the substrate to improve upon the coefficient of thermal expansion characteristics of the combined raw materials (3,4).
Figure 5: Orientation of cordierite crystal
The firing process affects important characteristics of the substrate such as coefficient of thermal expansion, water absorption, shrinkage percentage of the product, and thermal stabil- ity. The temperature is increased to approximately 1,400 °C and remains at this temperature to assure complete chemical reaction.
6 Advanced Ceramic Honeycomb
Automobile manufacturers and emission system suppliers continue to provide new technology for low temperature capability and high temperature durability. The targets of higher engine power (reduced pressure drop), higher fuel economy, and durability continue to be expected from the automotive industry as a whole. Various new technologies are presently available for these requirements and will be briefly discussed.
Thin Wall Substrate – Bulk density and cell structure have a direct effect on the warm-up capability of the substrate. By reducing the bulk density and increasing geometric surface area, the substrate not only warms up faster, but also reduces pressure drop. Existing technology has produced thin wall honeycombs with these attributes.
These substrates have 4 mil wall thickness and either 400 cpsi or 600 opal cell density for improved catalyst performance. The thin wall substrates 4 mil / 400 cpsi and 4 mil / 600 cpsi reduces bulk density 30 % and 15 % respectively compared to the 6 mil / 400 cpsi standard substrate. Geometric surface area increases by 5 % (4 mil / 400 cpsi), and 25 % (4 mil / 600 cpsi). The 4 mil / 400 cpsi reduces pressure drop 15 %, but the 4 mil / 600 cpsi increases pressure drop 26 % as shown in Figure 6.
Figure 6: Pressure drop and geometric surface area
Effect of Substrate Geometric Surface Area – Figure 7 shows two tested catalytic converter layouts. The catalyst have the identical diameter, and length, and catalyst coating consists of Pd, Pt and Rh with total precious metal loading of 150 g/ft3. The catalysts were aged at 850 °C for 50 hours with 60 seconds cruising and 5 seconds fuel cut cycle at engine dynamometer.