Exhaust gas purification system for lean burn engine

Abstract: An exhaust gas purification system for a lean burn engine comprising a thermal mass unit (32) and a NO x conversion catalyst unit (34) downstream of the thermal mass unit (32). The NO x conversion catalyst unit includes at least one catalyst section (36). Each catalyst section includes a catalytic layer (38) for converting NO x coupled to a heat exchanger (40). The heat exchanger portion of the catalyst section acts to maintain the catalytic layer (38) substantially at a desired temperature and cools the exhaust gas flowing from the catalytic layer into the next catalytic section in the series. In a further aspect of the invention, the exhaust gas purification system includes a dual length exhaust pipe (20) upstream of the NO x conversion catalyst unit (34). The dual length exhaust pipe (20) includes a second heat exchanger (24) which functions to maintain the temperature of the exhaust gas flowing into the thermal mass (32) downstream near a desired average temperature.

Description: The present invention relates generally to exhaust systems for engines and, more particularly, to an exhaust after-treatment system for a low emission, lean-burn internal combustion engine. Catalysts are generally used as a means for removing pollutants such as HC, CO, and NO x from the exhaust gas of internal combustion engines. The ability of a catalyst to remove NO x in the exhaust gas falls rapidly, however, when the air-fuel ratio of the exhaust gas becomes leaner. Therefore, in engines operating in a lean air-fuel ratio environment, such as stratified charge lean-burn engine, it is difficult to remove NO x from the exhaust gas using a conventional catalyst. Similarly, in the case of diesel engines, a reductant is required to reduce NO x since the exhaust gas has a relatively high concentration of oxygen. Accordingly, to achieve NO x control, exhaust after-treatment systems have included a NO x conversion device. Presently, however, the performance of NO x conversion technology is limited in several respects. NO x conversion efficiency is affected by the operating temperature of the device, and requires a relatively narrow temperature operating window. In particular, the HC reductant used in diesel engine systems results in a highly exothermic reaction that heats the catalyst. As the catalyst and exhaust gas temperature increases, the HC reductant tends to react more readily with the oxygen than the NO x , thereby reducing the NO x conversion efficiency. Accordingly, there is a need to maintain the temperature of the exhaust gas within a narrow range such that the HC reductant is more selective towards NO x reduction. In addition, because the efficiency of catalysts and NO x conversion devices are temperature dependent, a significant portion of harmful emissions can be generated during the cold start portion of the engine cycle, i.e., the first 100-200 seconds after cold start. Thus, there exists a need for an exhaust after-treatment system which provides an effective means of reducing cold start NO x emissions as well. United States Patent 5,474,745 is concerned with a catalytic converter for exhaust gas No x purification comprising a plurality of purification units serially arranged in a housing, each of which comprises a catalytic layer for reducing No x and a separate cooling core without catalyst for cooling exhaust gas admitted through said catalyst layer, through which said exhaust gas flows sequentially, wherein said cooling core transfers heat from said exhaust gas to outside said converter. EP 0886039A1 relates to an assembly for eliminating nitrogen oxides present in the exhaust gas of an internal combustion engine, said assembly being part of the exhaust line coming from the engine and comprising: a nitrogen oxides conversion means and a heat exchanger placed in a bypass line upstream from the conversion means intended to cool said exhaust gas. According to the present invention there is provided an exhaust gas purification system for a lean burn engine comprising: a thermal mass unit; and a NO x conversion catalyst unit downstream of said thermal mass unit, said NO x conversion catalyst unit including at least one catalyst section, and wherein said at least one catalyst section comprises a catalytic layer for converting NO x coupled to a heat exchanger for maintaining said catalytic layer substantially at a desired temperature and for cooling exhaust gas flowing from said catalytic layer. The heat exchanger portion of the catalyst section acts to maintain the catalytic layer substantially at a desired temperature and transfer exothermic heat from the reacting HC reductant of the exhaust gas flowing through the catalytic layer. Each of the several serially arranged catalyst sections are spaced apart from each other within the NO x conversion catalyst unit. This acts to increase the gas turbulence entering each catalyst section thereby increasing NO x conversion efficiency. Further at least two NO x conversion catalyst units are serially arranged downstream of the thermal mass unit. These additional units can be added as long as there are significant hydrocarbons passed through to the following unit. In such a case, the first NO x conversion catalyst unit obtains a higher NO x conversion efficiency than can be attained with a single NO x conversion catalyst unit. In a further aspect of the invention, the exhaust gas purification system includes a dual length exhaust pipe including a first path and a second path for cooling the exhaust gas, and an exhaust valve positioned within the exhaust pipe. The second path can include another heat exchange unit for transferring heat from the exhaust gas flowing through the second path. The exhaust valve controls the mixing of the exhaust flow through the first and second paths to maintain the temperature entering the thermal mass downstream near a desired average temperature. A heater is included upstream of the NO x conversion catalyst unit to improve the light-off time of the NO x conversion catalyst unit during engine cold-start. One advantage of the embodiment of the present invention is that it improves NO x conversion efficiency under transient or steady-state engine operation due to the controlled exhaust gas inlet temperature to the catalysts. Another advantage is that, as compared to conventional catalyst systems, the higher NO x conversion efficiency level results in less HC reductant being used for a given NO x emission reduction level. A further advantage is improved reductions in HC and NO x generated during engine cold start. This results from both the heater and the fact that the catalyst can be located closer to the engine because of the dual length exhaust pipe and heat exchanger. The present invention will now be described further, by way of example, with reference to the accompanying drawings, in which: FIGURE 1 is a schematic view of an exhaust system according to one embodiment of the present invention illustrating its operational relationship with an internal combustion engine; FIGURES 2A and 2B are a cross-sectional view of one embodiment of the catalyst section unit of Figure 1 along line 2-2; FIGURE 3 is an isometric view of another embodiment of the catalyst section unit of Figure 1; and FIGURE 4 is a schematic view of another embodiment of a NO x conversion catalyst unit according to the present invention. Turning now to Figure 1, an exhaust gas purification system 10 is illustrated in operational relationship with a lean burn internal combustion engine 12 such as a direct injected diesel engine for an automotive vehicle. The engine 12 has an exhaust manifold 14 to direct the exhaust gases from the engine 12 to the exhaust system 10. The engine 12 includes sensors, indicated generally at 16, for providing information about engine performance to the engine control unit 18. Such information includes the crankshaft position, angular velocity, throttle position, air temperature, engine coolant temperature, etc. The information from the sensors 16 is used by the engine control unit 18 to control operation of the engine 12. The exhaust gas purification system 10 is coupled to the exhaust manifold 14 by way of an exhaust flange 15 and includes a dual length exhaust pipe 20 which provides a first path 20A and a second path 20B for exhaust gas flow. The dual length exhaust pipe 20 further includes an exhaust valve 22 which is controlled by the engine control unit 18 via a downstream exhaust gas temperature sensor 23 to regulate the flow of exhaust gas through the first and second paths, 20A and 20B. The second exhaust path 20B is preferably longer than the first exhaust path 20A to provide additional cooling to the exhaust gas flowing through the second exhaust path 20B. In addition, the second exhaust path 20B also preferably includes a heat exchanger 24. The heat exchanger 24 further lowers the bypassed exhaust gas temperature. In low temperature environments, however, the heat exchanger 24 may also aid to increase the exhaust gas temperature. The exhaust valve 22 controls the mixing of the bypassed exhaust flow through the second path 20B with the engine-out exhaust flow through the first path 20A to maintain the temperature entering the catalyst system near a desired overall average. One advantage of the dual length exhaust pipe 20 is that it allows the catalyst system to be more closely coupled to the engine 12, thereby providing faster light-off of the catalyst. The dual length exhaust pipe 20 transports exhaust gas produced from combustion of an air/fuel mixture in the engine 12 to the catalyst system 26. The catalyst system 26 is contained within one or more housings 28. The catalyst system 26 preferably includes a heater 30 to increase the exhaust gas temperature entering the catalyst system during engine-cold start. This is done to further improve the light-off time within the catalyst and may also be designed to burn some or all of the hydrocarbons in the exhaust gas flow during the light-off phase. It can be used to raise the exhaust gas temperature under light load conditions and/or under cold environment conditions. The heater 30 may be made of a catalyst-coated, electrically heated ceramic substrate, such that the catalyst is formulated to light-off HC above the normal exhaust gas temperature operating range. A thermal mass unit 32 acts to dampen the temperature fluctuations of the exhaust gas entering the catalyst system during transient speed and load engine conditions. The thermal mass unit 32 can take many forms such as a heat exchanger, however, it is important that its thermal inertia be such that the exiting exhaust gas flow under relatively large changes in speed and load have a substantially steady-state temperature. The thermal mass unit 32 can be constructed of a ceramic or metal honeycomb structure, or constructed similar to an automotive radiator. For efficiency, the heater 30 and thermal mass unit 32 would, preferably be combined together in a single unit. Exhaust gas purification system 10 of Figure 1 includes two NO x conversion catalyst units 34. Each NO x conversion catalyst unit 34 includes at least one catalyst section 36 which includes a catalytic layer 38 coupled to a heat exchanger 40. The catalyst sections 36 are arranged serially in the housing 28 in a spaced apart relationship. This space increases the gas turbulence entering the catalyst section 36 thereby increasing the NO x conversion efficiency of the catalyst layer 38. Flow vanes (not shown) may also be used in this area to improve the temperature uniformity of the exhaust gas flowing into the catalyst section 36. The catalytic layer 38 is composed of the catalytic component such as platinum, palladium, rhodium and transition metals combined therewith carried on a substrate having a washcoat to increase its surface area. The substrate is preferably formed into a honeycomb structure made of a good heat conducting material such as metal. This aids in the exothermic heat transfer to the heat exchanger plate 40. In the NO x purification process of the present invention, a reductant such as diesel fuel or HC is reacted with the NO x to be purified in the catalytic layer 38. The reductant may be from a late post injection in the cylinder, and contained in the exhaust gas flowing through the system, or added to the exhaust gas such as shown in Figure 1 through reductant injectors 42. The reductant injectors 42 are located in relation to the catalyst inlet so as to distribute the reductant across the front of the catalyst substrate. Preferably, the catalytic layer 38 loading matched to the amount of hydrocarbons burning within it and corresponding substrate heat - exchanger heat transfer capacity, thereby keeping the exhaust gas temperature flowing across the active catalyst sites within the desired temperature range to improve HC-NO x selectivity. In other words, in lean-burn engines such as the diesel engine of Figure 1, there exist excess amounts of O 2 in the exhaust gas, and the O 2 concentration is substantially higher than the NO x concentration. Increasing the catalyst-exhaust gas temperature tends to facilitate oxidation between O 2 and the reductant rather than the reductant and NO x . This is a result of the increase in exothermic heat resulting from the oxidation process which tends to increase the catalytic temperature and decrease the reductant NO x selectivity. By reducing the length of the catalytic layer 38, it reduces the amount of reductant burning within it, thereby inhibiting the generation of exothermic heat. In addition, since the catalytic layer 38 does not burn all the reductant passing through it, some of the reductant is available to reduce the NO x in the following catalyst section 36. Accordingly, the number of catalyst sections 36 within each NO x conversion catalyst unit 34 may be more than the two shown, and may be repeated as long as there is a significant amount of reductant passed through to the following unit. It has been found that higher NO x conversion efficiencies are obtained with multiple catalyst sections than those obtained with a single catalyst section. Similarly, even higher NO x conversion rates may be obtained by the use of multiple NO x conversion catalyst units depending upon the level of NO x conversion desired. The heat exchanger 40 can comprise an air-to-air, air-to-coolant, cross-flow, or heat-pipe type heat exchanger. The heat exchanger preferably includes fluid-flow conduits 35 to aid in the heat transfer. The structure of the heat exchanger 40 will be described in further detail below with reference to Figures 2A and 2B. Alternatively, the heat exchanger can be formed in a finned design with controlled external forced air cooling or fluid cooling. In such a case, the heat exchanger would include a heat-pipe transfer to air as liquid-cooled fins. Preferably, the thermal inertia of the heat exchanger 40 is optimised to reduce temperature variations of the exhaust gas entering the subsequent catalytic layer during transient engine operating conditions. In addition, the heat exchanger 40 operates to maintain the bed temperature of the catalytic layer to which it is coupled within its optimised temperature range for NO x conversion efficiency. To further control the temperature of each catalyst section 36, it is desirable to use a washcoat for the catalytic layer having reduced insulation properties (i.e., a higher heat transfer coefficient) to increase the heat transfer from the catalyst component to the substrate to further reduce localised high temperature regions within the catalytic layer 38. An additional oxidation catalyst or three-way catalyst 44 may also be included downstream of the NO x conversion catalyst units 34 to further purify HC, CO and the like in the exhaust gas. In operation, a flow of exhaust gas from the engine 12 is regulated by the engine control unit 18 via the exhaust valve 22 through the first exhaust path 20A and second exhaust path 20B to regulate the temperature of the exhaust gas flowing into the catalyst system 26. Control logic within the engine control unit 18 monitors the temperature of the exhaust gas flowing through the first path 20A, second path 20B, as well as the heat exchanger 24 and controls the exhaust valve 22 by known methods to maintain the exhaust gas temperature entering the catalyst system within a desired overall average range. One example of such a control system is disclosed in U.S. Patent No. 5,855,113 assigned to Ford Global Technologies, Inc.. If the engine is operating in a cold start mode, the heater 30 is activated to burn HC on its catalyst-cooled surface to increase the temperature of the exhaust gas flowing into the NO x conversion catalyst unit 34 to improve the light-off time within the catalyst. As soon as HC light-off occurs within the catalyst, electricity to the heater 30 is cut off, and HC burning occurs only in the catalyst 38. Reductant injection, such as HC or diesel fuel, can also occur upstream of the NO x conversion catalyst unit 34 to rapidly light-off the catalyst. If the heater 30 is located downstream of the reductant nozzle 42, it may also be used to burn off some or all of the HC during the light-off phase. The exhaust gas then flows through the thermal mass unit 32 which aids in maintaining the temperature of the exhaust gas flow and damping fluctuations in the exhaust gas temperature passed to the NO x conversion catalyst units 34. The exhaust then flows into the catalyst section 36 and, in particular, through the catalytic layer 38 wherein the NO x conversion takes place. At the same time, the heat exchanger 40 maintains the catalytic layer 38 within a desired temperature range for optimum NO x conversion efficiency. Any remaining HC, CO or NO x is converted by the catalytic converter 44 downstream of the NO x conversion catalyst units 34. The exhaust gas then flows out of the catalyst system 26 to the tailpipe (not shown), or additional exhaust gas purification components such as a particulate filter. Figure 2A shows a cross-sectional view of part of a NO x conversion catalyst unit 34 of Figure 1 along lines 2-2. Figure 2B shows a detail of a portion of Figure 2A. Referring now to Figures 2A and 2B, a NO x conversion catalyst unit 34 is shown including a catalyst section 36 including a catalytic layer 38 as well as a heat exchanger 40. As shown in Figure 2B, a substrate wall 50 creates a plurality of channels 52 through which the exhaust gas flows. The substrate wall 50 is formed of a good heat conducting material such as metal and is washcoated with a high surface area Zeolite material. Substrate wall 50 carries the catalytic layer 38 which carries out the NO x conversion. A plurality of heat sinks or fins 54 are included adjacent portions of the substrate wall 50 to transfer heat generated during the NO x conversion process to the heat exchanger 40. The heat exchanger 40 preferably includes a plurality of fluid flow conduits designated alternatively as vertical conduit 56 or horizontal conduits 58. Air or liquid flowing through the conduits 56, 58 maintain the temperature of the substrate 50 and hence the exhaust gas flowing through the NO x conversion catalyst unit 34 within a narrow temperature range optimum for NO x conversion efficiency. For example, in one test, a temperature of 200°c within a range of 190-210°c provided near optimum NO x conversion with the catalyst under consideration. Of course, the more narrow the window about the optimum temperature, the better. Fluctuations, however, will occur due to tradeoffs between the temperature regulating devices and engine operating conditions. Specifically, the size and complexity of the heat exchangers may outweigh the additional benefits of more closely regulating the exhaust gas temperature. In addition, the optimum temperature will depend on the catalyst composition. Figure 3 shows an isometric view of an alternative embodiment of a catalyst section of a NO x conversion catalyst unit of Figure 1. The catalyst section 60 includes an upstream catalyst layer 62 and a downstream heat exchanger 64. The heat exchanger 64 further includes a fluid conduit 66 having an inlet 68 and outlet 70 for maintaining the temperature of the catalyst layer 62 within its optimal range for NO x conversion efficiency. A fluid such as glycol under pressure having a heating point of 197°c at 1.0 atm, can be circulated through the conduit 66. Catalyst section 60 can be conveniently manufactured by applying the catalyst and washcoat to only one-half of the structure forming the catalyst section 60. No catalyst would be present on the rear section of each catalyst section 60. As a result, no exothermic reductant reaction would be created in the heat exchange portion 64 of the catalyst section. In addition, the thermal mass would aid to dampen exhaust gas temperature fluctuations. The addition of heat sink fins (not shown) would further dampen exhaust gas temperature fluctuations, and aid to maintain the catalyst layer 62 within a desired temperature range. The cross-flow heat exchanger 64 of the rear portion of the catalyst section 60 could also consist, for example, of a rectangular flow section similar to those found in automotive radiators with a fluid coil (liquid or gas) to aid in the heat transfer. Figure 4 is a schematic view of another embodiment of a NO x conversion catalyst unit 434 according to the present invention. Referring to Figure 4, the heater and thermal mass unit have been combined as a single unit 430. The NO x conversion catalyst unit 434 includes a front heat exchanger portion 440 and a rear catalyst section 438. The combined structure is formed similar to that described above with reference to Figure 2B. Accordingly, the catalyst section 438 includes a washcoat and catalyst carried on a substrate which is coupled to a heat exchanger 443. In addition, because there is only one NO x conversion catalyst unit 434 in the exhaust gas purification system of Figure 4, one or more additional heat exchangers 441, 442 are included upstream of the catalyst section 438 having corresponding fluid conduits 450. The fluid conduits preferably channel cooling air or fluid in alternating directions as it flows from inlet to outlet. The first heat exchanger 441 includes a fluid conduit (not shown) carrying a cooling fluid which acts to cool the exhaust gas flowing through the substrate channel 452. The operating temperature of the first heat exchanger 441 depends upon the transient or steady state engine operation and load conditions. The second heat exchanger 442 acts to further cool or maintain the temperature for NO x conversion within the catalyst. The third best exchange 443 likewise maintains the bed temperature of the catalyst near the optimum temperature for NO x conversion and cools the exhaust gas flowing through the gas channel 452. The advantage of the single unit embodiment of Figure 4 is simplified construction and fewer components as compared to the multiple catalyst system of Figure 1. From the foregoing it will be seen that there has been brought to the art a new and improved exhaust gas purification system having increased NO x conversion efficiency.

Claims: An exhaust gas purification system connected to the exhaust manifold (14) of a lean burn engine comprising: a NO x conversion catalyst unit (34) including at least one catalyst section (36) and wherein said at least one catalyst section (36) comprises a catalytic layer (38) for converting NO x coupled to a heat exchanger (40) for maintaining said catalytic layer (38) substantially at a desired temperature and for cooling exhaust gas flowing from said catalytic layer, characterised by a thermal mass unit (32) located upstream of the at least one catalyst section (36). An exhaust gas purification system as claimed in claim 1 including at least two NO x conversion catalyst units (34) serially arranged in the exhaust gas flow. An exhaust gas purification system as claimed in claim 1 or claim 2, wherein the catalyst unit (34) includes a plurality of serially arranged catalyst sections (36) spaced apart from each other. An exhaust gas purification system as claimed in any one of the preceding claims, wherein said heat exchanger (40) is a fluid-cooled heat exchanger. An exhaust gas purification system as claimed in any one of the preceding claims further comprising a heater (30) upstream of said thermal mass unit (32) and said NO x conversion catalyst unit (34). An exhaust gas purification system as claimed in any one of the preceding claims further comprising an oxidation catalyst (44) downstream of said NO x conversion catalyst unit (34). An exhaust gas purification system as claimed in any one of the preceding claims, wherein said catalytic layer (38) comprises a catalyst carried on a substrate wherein said catalyst comprises one or more materials selected from the group consisting of platinum, palladium, rhodium, and transition metals combined therewith. An exhaust gas purification system as claimed in any one of the preceding claims, wherein the NO x conversion catalyst unit (34) includes at least one upstream heat exchanger coupled to the catalyst section. An exhaust gas purification system as claimed in any one of the preceding claims combined with a dual length exhaust pipe located immediately downstream of the exhaust manifold (14), said dual length exhaust pipe including a first path (20a), a second path (20b) which is longer than the first path (20a), an exhaust valve (22) positioned in said exhaust pipe for regulating the flow of exhaust gas between said first path (20a) and said second path (20b), and a control logic operative to control said exhaust valve (22) so as to regulate the temperature of the exhaust gas. An exhaust gas purification system as claimed in claim 9 in which the second path (20b) includes a heat exchanger (24).

Inventors: HAINES LELAND MILBURN, HAINES, LELAND MILBURN
Applicants: FORD GLOBAL TECH LLC, FORD GLOBAL TECHNOLOGIES, LLC
Application year: 2000
Main classification: F01N3
IPC codes: F01N7, F01N3, B01D53