The present invention relates to a- method and apparatus for reducing emission of particles and NOx.

As is well known to those expert in the art, reduction of the level of mixed oxides of nitrogen, or NOx, in the exhaust gases of internal combustion engines can be achieved by a process generally known as exhaust after-treatment. One particular type of NOx control device relies on the addition of a reductant which is combined with the exhaust gases. The exhaust gases and the reductant together are passed over a catalyst which selectively promotes the reaction of the reductant and NOx to produce nitrogen and other compounds such as water and/or carbon dioxide. This type of system is commonly referred to as a Selective Catalytic Reduction (SCR) system. Exhaust after-treatment can equally be used as a term to include the process whereby particulate matter is removed from the exhaust gases of an internal combustion engine, in a device known to those expert in the art as a particle filter. Particle filters have traditionally been associated with application to diesel engine exhaust systems, but it is well established to those expert in the art that a type of spark ignition engine in which the fuel is directly injected into the combustion chamber produces levels of particulate emissions which may require the use of a particle filter. The combination of systems to control NOx and particulate emissions can therefore be held to apply to exhaust gas streams from all internal combustion engines fuelled by hydrocarbon fuel.

Particle filters and NOx reduction systems have been under development for a number of years, and patents have been issued in respect of various types of exhaust after- treatment comprising a particle filter and separately a NOx control device. Combinations of particle filter and NOx control device have also been marketed to reduce harmful pollutants from diesel engines.

An example of an open loop type of SCR system available from KleenAir Systems Inc. of California, USA can be found in US Patents 5,224,346, 5,609,026 and 5,992,141. The principle on which this type of system works lies in predicting the level of NOχ, and injecting a proportionate amount of ammonia (NH3) as the reductant into the exhaust gases upstream of a catalytic device. Reduction of NOx principally to nitrogen occurs through reaction of N0χ with NH3 both in the exhaust gas stream and in the catalytic device. The principle of the NOx control system is described in the US Patents listed above, although details of the control method are not given. In the US Patents listed above, the method of predicting N0χ at any point of engine operation is described as fundamentally linked to exhaust temperature, which in turn is linked to the load placed on the engine. In one example given in one of the Patents^ where fuel is injected into the engine by means of electronic devices, electrical signals from the injection devices are used to control the flow of ammonia, on the assumption that NOx emissions will be proportionate to the fuel admitted to the engine. This type of control system is applicable to a spark ignition engine: in such engines electronic control of fuel injectors is very common.

For compression ignition engines, commonly described as diesel engines, which frequently have mechanical control of the fuel injection process, this approach is not appropriate. No detail of the control strategy adopted is given in the US Patents listed above. However, the general principle declared is that NOx emissions are strongly dependent on exhaust gas temperature, and that the control strategy uses this basis to control tailpipe NOx emissions, through the medium of controlling ammonia addition upstream of the selective reduction catalyst. However, in such a system, if the ammonia reductant is not inhibited at temperatures too low for the required reactions to take place, the reductant will pass through the SCR catalyst unreacted and will be emitted from the tailpipe as an additional pollutant. Conversely if the ammonia reductant is not inhibited at temperatures outside the upper limit for the satisfactory reduction of NOx in the selective reduction catalyst, NOx emissions will increase through the oxidation of nitrogen in the catalyst. It is therefore very important in a control system using exhaust gas temperatures to predict NOx levels, and thus to control ammonia flow to the selective reduction catalyst, that the control system accurately relates exhaust temperatures to NOx levels in the exhaust gases. The accurate prediction of ammonia flow, to control and reduce NOx in an exhaust system in which the SCR is a singular item of exhaust after-treatment, has apparently been satisfactorily achieved and is covered by the above listed US Patents, although the precise details of how this is achieved are not disclosed. However, we have discovered that the insertion of a particle filter between the engine and the SCR system, surprisingly affects the control of ammonia flow to the SCR catalyst, with the result that NOx reduction can be impaired and pollutant emissions can actually be increased.

The particulate filter is placed upstream of the SCR system because it requires the heat from the exhaust for regeneration of the filter.

By way of background, reference will now be made to Figures 1 to 4 of the accompanying drawings in which: Figure 1 is a schematic block diagram of a previously proposed exhaust system including a predictive NOx control system; Figure 2 is a schematic block diagram of another previously proposed exhaust system including a predictive NOx control system; and Figures 3 and 4 show test results of operating the system of Figure 2.

Practical examples of the predictive NOx control system applied to diesel engine exhaust gases have been proposed, hi Figure 1 an engine 2 which in this example is a diesel engine has an exhaust system 4 including a catalytic device 8. Thermocouples are used as a means to provide information with which to control the flow of ammonia to the injector. Two thermocouples are used in the exhaust system 4 as shown in Figure 1. A first thermocouple designated Tl is mounted in a location upstream of the catalytic device 8, and also upstream of an ammonia injector 14 which is upstream of the catalytic device 8. A second thermocouple T2 is fitted to the body of the catalytic device 8. Electrical voltage signals from the thermocouples are input to an electronic control unit (ECU) 6 which computes the required flow of ammonia from the two signals and controls an ammonia control valve 12 which controls supply of ammonia to the injector 14. The ECU 6 is available from KleenAir B2005/003154

Systems Inc. of California, USA. The precise details of the control method are not known and have not been published or disclosed. However, it is believed that if the temperature of the exhaust gases is too low to permit reaction between NH3 and NOχ, the control valve remains shut to prevent the flow of ammonia. Similarly if the exhaust gas temperature is sufficiently high to allow NH3 to decompose in such a way as to increase NOx emissions from the tailpipe, the control valve shuts off the flow of ammonia. At temperatures in between these upper and lower limit values, it is believed that ammonia flow is controlled in proportion to the exhaust gas temperature. The proportionality is not known or defined and has not been disclosed, but it is believed that as exhaust gas temperature increases, NOx output from the engine also increases, and ammonia flow increases to provide enhanced NOx reduction capability.

Figure 2 shows an exhaust system similar to that of Figure 1. It differs in that a diesel particulate filter (DPF) 16 is provided in the exhaust system 4 upstream of the first thermocouple Tl and upstream of the NOx emission control device 8. Particle filters accumulate carbonaceous material, commonly referred to as "soot", and as is well known to those expert in the art, the accumulation of soot increases back pressure or resistance to the flow of exhaust gases through the filter. To overcome this problem it is necessary from time to time to dispose of the accumulated soot, a process known as regenerating the particulate filter. The simplest way to regenerate the filter is to burn the soot. As is well known to those expert in the art, various techniques and methods have been developed to achieve filter regeneration, including but not by way of limitation, catalytic coatings on the surface of the filter medium, the use of a catalytic oxidation device which encourages the production of NO2 in the exhaust gas stream which subsequently reacts with accumulated soot in the filter, this being a device well known as the CRT ™, the use of burners or electrical heating devices and the use of fuel additives, also known as fuel borne catalysts, to assist the process of regeneration. A feature common to most if not all the techniques and methods used to secure filter regeneration is that exhaust heat plays a role in the process of filter regeneration. For this reason, conventional experience and logic suggests placing the particle filter upstream of the device 8 because it is helpful to use this location to assist filter regeneration. Placing the particle filter close to the engine provides the greatest heat input to assist filter regeneration. Tests were carried out with the DPF 16 placed upstream of the NOx control system on the diesel engine 2 as shown in Figure 2. The test results are shown in Figures 3 and 4. The arrangement shown in Figure 2 has been used previously for certain types of diesel particulate filter, typically Cordierite, and is well known to those expert in the art. The choice of DPF material used in our tests was made on the basis of resistance to thermal damage, e.g. melting or cracking, since it is known to those skilled in the art that some previously used ceramic DPF materials are prone to thermal damage, such as may occur when burning out soot in the DPF. The use of ceramic material resistant to damage during the burning of soot in the DPF, represents a considerable improvement in reliability and durability in service. Emissions measurements showed that the DPF 16 eliminated a very significant proportion of the solid particulate material present in the exhaust gases mainly as soot. However, it has been found as a result of these tests, that the combination of the DPF 16 placed upstream of the NOx control system, adversely affects the NOx prediction algorithms employed in the ECU.

The present invention thus seeks to provide reliable control of NOx and particle emissions when a particulate filter is present in an exhaust system in addition to the NOx emission control system.

According to an aspect of the invention, there is provided a system for controlling the emission of particles and NOx from a hydrocarbon fuelled engine, the system comprising an exhaust system including a series arrangement of an NOx emission control device having a controller which controls the operation of the device in dependence on exhaust gas temperature, and a particulate filter, and a sensor upstream of the particulate filter arranged to indicate to the controller the temperature of the exhaust gas upstream of the particulate filter.

According to one aspect of the invention there is provided a system for controlling the emission of particles and NOx from a hydrocarbon fuelled engine, the system comprising an exhaust system including the series arrangement of an NOx emission control device, the NOx emission control device having a controller which controls the operation of the device in dependence on exhaust gas temperature and a particulate filter; and an exhaust gas temperature sensor upstream of both the particulate filter and the emission control device, the sensor being arranged to indicate, to the controller, the temperature of exhaust gas upstream of both the particulate filter and the emission control device.

By placing the exhaust gas temperature sensor upstream of the particulate filter, for example upstream of both the particulate filter and the NOx emission control device, the temperature of the exhaust gas upstream is correctly indicated to the controller substantially independently of any effect on the system of the particulate filter.

hi an embodiment of said one aspect, the NOx emission control device is upstream of the particulate filter.

hi an embodiment the NOx emission control device comprises a catalytic device wherein the NOx emission control device comprises a catalytic device which is operable to reduce NOx by reacting NOx with reductant. By placing the NOx emission control device upstream of the particulate filter it heats up more quickly improving performance.

It has been found that the particulate filter, when placed downstream of the emission control device, still operates satisfactorily.

A further aspect of the invention provides, in a system for controlling the emission of particles and NOx from a hydrocarbon fuelled engine, the system comprising an exhaust system including the series arrangement of an NOx emission control device, the NOx emission control device having a controller which controls the operation of the device in dependence on exhaust gas temperature and a particulate filter a method of sensing exhaust gas temperature upstream of both the particulate filter and the emission control device, and controlling the NOx emission control device in dependence upon the sensed temperature. In an embodiment the NOx emission control device is upstream of the particulate filter.

According to another aspect of the present invention there is provided a system for controlling the emission of particles and NOx from a hydrocarbon fuelled engine, the system comprising an exhaust system including: an NOx emission control device, the NOx emission control device having a controller which controls the operation of the device in dependence on exhaust gas temperature: a particulate filter upstream of the NOx emission control device; and an exhaust gas temperature sensor upstream of the particulate filter, the sensor being arranged to indicate, to the controller, the temperature of exhaust gas upstream of the particulate filter.

Yet another aspect of the invention provides, in a system for controlling the emission of particles and NOx from a hydrocarbon fuelled engine, the system comprising an exhaust system including: an NOx emission control device, the NOx emission control device having a controller which controls the operation of the device in dependence on exhaust gas temperature: and a particulate filter upstream of the NOx emission control device, a method comprising the steps of sensing the temperature of exhaust gas upstream of the particulate filter and controlling the NOx emission control device in dependence upon the said sensed temperature.

In experiments on a system having a DPF and a NOx emission control device having a catalytic device, we found that, depending on the vehicle operation pattern, heat stored in the DPF can either decrease or increase the exhaust gas temperature sensed by a thermocouple fitted downstream of the DPF, causing the predictive NOx control system to alter the flow of reductant, in this case ammonia, resulting in unexpected and undesirable effects. As discussed above in our tests we used, in the DPF, ceramic materials resistant to thermal damage. In the system in which the DPF is upstream, of the catalytic device, the DPF itself may include a catalyst which converts NO to NO2. That improves the overall conversion of NOx to N2.

Ceramic materials used for the particulate filter, hereinafter referred to by way of example as a DPF principally, though not by way of limitation, are of two types, namely Cordierite, and silicon carbide (SiC). Cordierite is the proprietary name given to a product produced by Corning Inc. Properties of these two materials are shown in Table 1, from which it will be seen that the specific heat of SiC is 750J/kg/°C compared to 600 J/kg/°C for Cordierite. However, Cordierite is much less dense than SiC, having a bulk density of lg/cm3 compared to a figure of 1.6g/cm3 for SiC When this factor is taken into account, the thermal capacity of a SiC DPF is twice that of a Cordierite DPF of the same volume. The typical volume of a DPF for a light duty application, for example but not by way of limitation, would be 3.3 litres (144mm diameter by 204mm length). Using the figures for bulk density given in Table 1, thermal capacity of this size of DPF would be 3.98 kJ/°C for SiC, compared to 1.98 kJ/°C for Cordierite. The thermal capacity of a SiC DPF is regarded as high in the context of the proposed combination of DPF placed upstream of a NOx control catalyst in the exhaust system of a diesel engine. Similarly the thermal capacity of a Cordierite DPF is regarded as low in the context of the existing prior art where a DPF made of this material has been placed upstream of a NOx control catalyst in the exhaust system of a diesel engine.

Table 1 of Annex 1 also shows that the melting point of SiC is 2300 0C compared with circa 1200 0C for Cordierite, and further that the critical maximum DPF temperatures are circa 1600 0C and 1000 0C respectively during soot combustion or regeneration. Overall, the thermal properties of SiC in comparison with those of Cordierite, are judged by many expert in the art, to be particularly suitable for DPF application. This makes the combination of a SiC DPF with a NOx control catalyst potentially attractive, where it is desired to control both particulate and NOx emissions from an internal combustion engine. Other suitable materials for DPF construction are metal products, typically sintered metal products such as those made by HJS in Germany, though not by way of limitation. Sintered metal products typically employ stainless steel alloys to resist corrosion, and are very suitable for DPF manufacture. Like SiC DPFs, such materials also have a high thermal capacity.

Where an installation of the type depicted in Figure 2 employs a DPF of high thermal capacity, and the vehicle is operated from cold, the thermal storage effect of the high heat capacity DPF greatly reduces exhaust gas temperatures downstream of the DPF. This has the effect of slowing down the rate of temperature rise at the point of measurement indicated by Tl. In cold start emissions testing, the thermal storage effect of the high heat capacity DPF has the effect of delaying for a considerable time, the instant at which ammonia begins to flow into the SCR catalyst employed to reduce NOx emissions. As a result, NOx reduction in a standard European emissions test of the type known to those expert in the art as the ECE plus EUDC test cycle is much lower than would be recorded by a system shown in Figure I5 where no DPF is used.

Similarly, the use of a high thermal capacity material to trap particulates stores heat during times when the vehicle is driven at high speed and load. When the vehicle is subsequently stopped and operated at low load or idle conditions, the thermocouple at Tl records exhaust gas temperatures which are interpreted by the ECU as representing high speed and load conditions. Operation at low speed and load under the same circumstances, with a system depicted in Figure 1, would not result in ammonia flow with a NOx control system in isolation, because the temperature information would be consistent with the vehicle operational mode. However, for an installation of the type shown in Figure 2, and where the thermal capacity of the DPF is such that it materially affects exhaust gas temperatures, the control system is disturbed, with the result that the flow of ammonia is mismatched to the actual operating conditions of the vehicle.

The difficulties of inserting a high heat capacity DPF upstream of the SCR catalyst and thermocouples in an arrangement as shown in Figure 2 were demonstrated by emissions testing. In a normal emissions test using the standard ECE plus EUDC test cycle method, which requires the test vehicle to be started from cold, and also requires that the test vehicle should be maintained at a controlled ambient temperature between 20 0C and 30 0C for a minimum of 6 hours prior to test, NOx reduction demonstrated with the combined system as depicted in Figure 2 was significantly reduced compared with results known to have been obtained with the same SCR system depicted in the arrangement shown in Figure 1. These tests are described in more detail in Annex 2.

Similarly, when the test vehicle was operated for about 30 minutes under conditions of moderate to high speed and load immediately prior to conducting an emissions test using the ECE plus EUDC test cycle without the cold start, the thermal storage of the high heat capacity DPF resulted in high initial gas temperatures at the point of initial measurement Tl at the start of test. The differences between cold and hot start tests can be seen on temperature traces from the tests as shown in Figure 3 for the cold start test and Figure 4 for the hot start test. As a result of the high initial exhaust gas temperature at a time when the test vehicle speed and load was low, excessive ammonia flow to the SCR catalyst occurred, resulting in the production of polluting gases. This is clearly shown in Figure 4, and is clearly absent in Figure 3.

Under such circumstances, the predictive algorithm used to control ammonia flow to the NOx control catalyst breaks down. Excessive ammonia flow at low vehicle speed and load leads to the emission of unreacted ammonia from the tail pipe. This phenomenon is known as ammonia slip to those skilled in the art, and it represents a potential health hazard, as ammonia is a noted toxic gas. Furthermore, excessive ammonia injection is interpreted as NOx by gas analysers employed to measure emissions in standard exhaust emissions test equipment. Where vehicle operation leads to thermal storage in a high thermal capacity DPF combined with a NOx control system relying on predictive ammonia flow techniques, indicated NOx levels in emissions tests are likely to be increased compared to those where no NOx control system is employed. Even though the apparent NOx increase may not accurately reflect reality, the intent to reduce recorded NOx emissions will be thwarted under these circumstances. Perhaps of more significance for actual vehicle operation in cities and other congested areas, the scenario of a vehicle operating for some period of perhaps ten to twenty minutes so that it has become thoroughly warmed up, followed by a halt and subsequent idling or sustained slow speed operation because of traffic congestion, is a very real one. With the combination of DPF and predictive NOx control system shown in Figure 2 and tested, with the results depicted in Figures 3 and 4, there is a very real possibility of ammonia slip releasing toxic gases into city streets, whereas the after-treatment system was designed to reduce emissions.

The results observed from the testing of the system of Figure 2 were surprising given that systems combining a DPF upstream of an SCR catalyst and employing an arrangement as depicted in Figure 2 are known in the prior art. However, these known systems have used different DPF materials which prove to be of very much lower thermal storage capacity than the DPF material used in our tests. It is our assertion and belief that the difference in thermal storage capacity is responsible for the effects observed in our testing. The choice of DPF material used in our tests was made on the basis of resistance to thermal damage, e.g. melting or cracking, since it is known to those skilled in the art that some previously used ceramic DPF materials are prone to thermal damage, such as may occur when burning out soot in the DPF. The use of ceramic material resistant to damage during the burning of soot in the DPF represents a considerable improvement in reliability and durability in service. However, the use of this material for the DPF when combined with an SCR system in an arrangement shown in Figure 2, unexpectedly and surprisingly had the effects detected when emissions tests were conducted.

In accordance with the embodiments of the invention in which the diesel particulate filter is upstream of the catalytic device, by placing the exhaust gas temperature sensor upstream of the DPF, the effect of the thermal capacity of the DPF on the measurement of exhaust gas temperature is at least much reduced. Thus if a DPF of high thermal capacity is used, the adverse affect of the DPF on the NOx control algorithm is much reduced and may be eliminated. It will be appreciated that the invention is not limited to use of a DPF of high thermal capacity; the invention may be used with the DPF of low thermal capacity as previously used in the art. By placing the exhaust gas temperature sensor up stream of the DPF, manufacturers of emission control systems can freely choose between high and low thermal capacity DPFs without concern for the operation of the control system. A preferred embodiment of the present invention is concerned with the combination of a high thermal capacity particle filter and a NOx control device of the SCR type with open loop control of the addition of reductant. In particular this preferred embodiment is concerned with the benefits that ensue from the utilisation of the thermal capacity of the particle filter to increase the degree of conversion of N0χ and to minimise the possibility of excess reductant being introduced into the exhaust system.

Placing the exhaust gas temperature sensor upstream of the DPF, has the benefit of allowing the use of a high heat capacity DPF in combination with an open loop SCR system and preventing the disruption of the control algorithm which results from the arrangement shown in Figure 2. An additional benefit is that the system can use a high heat capacity DPF made of materials which are more robust than a low heat capacity DPF as used in the known systems.

Providing a DPF of high thermal capacity and placing the exhaust gas temperature sensor upstream thereof has the following beneficial effects.

1. In a normal cold start emissions test carried out to the formal ECE plus EUDC protocol, exhaust gas temperatures sufficient to allow ammonia to flow are sensed much earlier in the test cycle, allowing NOx reduction over a greater portion of the test cycle, which results in a greater overall reduction in NOx emissions.

2. In a test cycle such as that used in the ECE plus EUDC protocol, in the ECE phase representing city driving, the vehicle is accelerated and decelerated and subject to significant periods of idling. The effect of including a DPF of high thermal capacity into the exhaust line is to help to maintain exhaust gas temperatures by conserving heat. The thermal capacity of this type of particle filter stores heat and later releases it into the exhaust gas stream. The present invention allows reductant to be added and reacted over a wider range of the vehicle's operating cycle, therefore increasing the degree of N0χ reduction over the test cycle.

3. In operation in actual driving conditions as opposed to use in emissions testing, where short periods of inactivity may follow periods of fully warmed-up operation, the invention will prevent heat stored in the high heat capacity DPF from influencing the control system and releasing excessive reductant into the SCR catalyst. This action will eliminate the possible passage of unreacted reductant into the atmosphere and will also prevent excessive reductant from potentially increasing emissions of NOx. This benefit will be of greatest significance where the vehicle fitted with the combined DPF and SCR after-treatment system is used for a significant number of hours during the day in urban or city areas and is also subject to stops where the engine is shut down for periods during which the DPF does not fully cool. Examples but not by way of limitation include buses, taxis, ambulances and other municipal vehicles.

In further experiments in which we placed the catalytic device of the NOx emission control device upstream of the DPF and placed the exhaust temperature sensor upstream of both we found generally similar results.

A yet further aspect of the invention provides a system for controlling the emission of particles and NOx from a hydrocarbon fuelled engine, the system comprising an exhaust system including a series arrangement of an NOx emission control device, the NOx emission control device having a controller which controls the operation of the device in dependence on exhaust gas temperature, and a particulate filter downstream of the NOx emission control device; and a temperature sensor in the NOx emission control device upstream of the particulate filter, the said temperature sensor being arranged to indicate, to the controller, the temperature of exhaust gas upstream of the particulate filter and the operating temperature of the NOx emission control device. Thus a temperature sensor in the NOx emission control device upstream of the particulate filter has the dual function of sensing the exhaust gas temperature upstream of the particulate filter and the operating temperature of the NOx emission control device, thus sensing a temperature approximately to the exhaust gas temperature immediately upstream of both the particulate filter and the NOx emission control device.

In an embodiment the NOx emission control device comprises a catalytic device which is operable to reduce NOx by reacting NOx with reductant.

In an embodiment the said temperature sensor is arranged to sense the temperature of the catalyst and indicate the sensed temperature to the said controller.

An embodiment of the system comprises a reductant injection arrangement controlled by the said controller and arranged to inject reductant into the exhaust system upstream of the NOx emission control device.

The invention also provides in a system for controlling the emission of particles and NOx from a hydrocarbon fuelled engine, the system comprising an exhaust system including a series arrangement of an NOx emission control device, the NOx emission control device having a controller which controls the operation of the device in dependence on exhaust gas temperature, and a particulate filter, a method comprising the steps of sensing exhaust gas temperature in, and the operating temperature of, the emission control device upstream of the particulate filter and controlling the NOx emission control device in dependence upon the sensed temperature.

For a better understanding of the present invention, reference will now be made by way of example to the accompanying drawings in which: Figure 5 which is a schematic block diagram of an example of an engine and exhaust system including a particulate filter and NOx control system in accordance with the present invention, Figure 6 is a schematic block diagram of another example of an engine and exhaust system including a particulate filter and NOx control system in accordance with the present invention; Figure 7 shows test results of operating the system of Figure 5 under hot start conditions; Figure 8 shows test results of operating the system of Figure 5 under cold start conditions; Figure 9 shows test results of operating the system of Figure 6 under hot start conditions; Figure 10 shows test results of operating the system of Figure 6 under cold start conditions; and Figure 11 is a schematic block diagram of yet another example of an engine and exhaust system including a particulate filter and NOx control system in accordance with the present invention.

Referring now to Figure 5, engine 2 is a diesel engine having an exhaust system 4. A diesel particulate filter DPF 16 is arranged in the exhaust system upstream of a catalytic device 8.

The DPF 16 removes particles from the exhaust emitted by the engine 2. As known in the art, the particles (soot) collected in the filter are burnt out to regenerate the filter. For that purpose it is known to provide additives in the fuel of the engine. As is known in the art, a range of additives or fuel borne catalysts may be used to assist regeneration. These include but not by way of limitation formulations containing the elements iron, cerium, platinum, strontium, sodium, calcium, manganese, lead, copper and combinations thereof. The additives reduce the combustion temperature of the soot: see for example co-pending International Patent Application WO 02/097256. In this example the DPF includes material chosen on the basis of their resistance to thermal damage, since it is known to those skilled in the art that some previously used ceramic DPF materials are prone to thermal damage, such as may occur when burning out soot in the DPF. The use of ceramic material, for example SIC, resistant to damage during the burning of soot in the DPF, represents a considerable improvement in reliability and durability in service.

A DPF of SiC has a high thermal capacity. By high thermal capacity we mean a thermal capacity which is such that it significantly influences the temperature of the exhaust gas downstream of the DPF. For a filter volume of 3.31itres for example, thermal capacity may be in the range 3kJ/°C to 6kJ/ 0C , preferably 3.5kJ/°C to 5kJ/ 0C. A preferred example is about 4k J/ 0C .

As discussed in more detail in Annex 2, the DPF 16 of Figure 5 may include Platinum or other catalyst which produces a beneficial overall reduction in NOx. The catalyst may be a coating on the ceramic of the DPF. However, providing such a catalyst significantly increases costs. Such a catalyst is not essential.

The catalytic device 8 is part of an NOx emission control system also comprising an electronic control unit (ECU) 6 known in the art, temperature sensors Tl and T2, which in this example are thermocouples Tl and T2, a source 10 of reductant which in this example is ammonia, a reductant injector 14 and a reductant flow control valve 12 which controls flow of ammonia from the source 10 to the injector 14. The ECU 6 is available from KleenAir Systems Inc. of California, USA. The source 10 comprises a bottle or other container for storing reductant. The reductant reacts with the NOx to convert it to less polluting nitrogen and water. In this example, the ECU 6 provides open loop control of the addition of ammonia. The injector 14 is upstream of the catalytic device 8 and downstream of the DPF 16 and injects ammonia into the exhaust 4. NOx production is believed to be strongly related to exhaust gas temperature. The amount of reductant required thus depends at least partly on exhaust gas temperature. The electronic control unit 6 controls the flow of reductant in dependence on the temperatures sensed by the thermocouples Tl and T2. Thermocouple T2 senses the temperature of the catalytic device or of the catalyst therein. In accordance with the example of the present invention shown in Figure 5, the thermocouple Tl is upstream of the DPF 16 and senses the temperature of the exhaust gas from the engine. The thermocouple Tl is placed sufficiently far from the DPF 16 that the temperature it senses is relatively uninfluenced by the DPF 16.

The results of testing the system of Figure 5 are shown in Figure 7 and 9 and discussed in Annex 2.

Figure 6 shows a modification of the example of Figure 5 in which the catalytic device 8 is upstream of the particulate filter 16 and the thermocouple Tl is upstream of both the catalytic device 8 and the particulate filter 16. The catalytic device 8 of Figure 6 is as described above with reference to Figure 5. The particulate filter 16 of Figure 6 is as described above with reference to Figure 5.

The example of Figure 6 operates in the same way of the example of Figure 5. The example of Figure 6 has been found to produce better results than the example of Figure 5 because the reducing catalyst heats up more quickly.

The results of testing the system of Figure 6 are shown in Figures 9 and 10 and discussed in Annex 2.

In a modification of the example of Figure 6, the DPF 16 comprises a filter cartridge having a heater, for example a diesel engine glow plug, for regenerating the filter as is described in corresponding UK patent application (00504771.7). However experiments have operated satisfactorily without such a heater

Figure 11 is a schematic view of an alternative system comprising an engine 2 which is for example a diesel engine having an exhaust system 4. A catalytic device 8 which is part of an NOx emission control system is upstream of DPF 16. The NOx emission control system comprises the device 8, a source of reductant 10, in this example ammonia, a reductant flow control valve 12, a reductant injector 14, upstream of the catalytic device 14 and an ECU 6'. As so far described the system functions in the same way as the system of Figure 6. The ECU 6' controls the valve 12 to supply reductant into the exhaust gas.

In this alternative system a temperature sensor, for example, a thermocouple T12 is in the emission control device and senses both the exhaust gas temperature and the operating temperatures of the catalytic device 8 upstream of the DPF 16.

The invention is not limited to diesel engines: it is applicable to any internal combustion engine fuelled by hydrocarbon fuel. For example, the engine 2 may be a spark ignition engine fuelled by petrol (gasoline).

The invention is not limited to a particular filter of high thermal capacity. It may be used with a particulate filter of low thermal capacity.

The invention is not limited to the use of ammonia as the reductant. Other reductants may be used, for example urea or hydrocarbons such as propane amongst other hydrocarbons.

The invention is not limited to a particulate filter of ceramic material. Other designs, but not by way of limitation, include sintered metal or metal wire particulate filters. Annex 1

Table 1: Properties of SiC and Cordierite (from Stobbe et al SAE 932495,1993)

This is taken from "SiC as a substrate for Diesel Particulate Filters" by P Stobbe,

J.W.Hoj, H. Pederson, and S. C. Sorensen, SAE Transactions, paper 932495 published

by SAE International, The Engineering Society for the Advancing Mobility Land Sea

Air and Space, Warrendale. PA, USA. Annex 2 Tests carried out with Taxi and combined DPF and SCR system

To establish the effects of fitting combined diesel particulate filter (DPF) and selective catalytic reduction (SCR) catalyst to a light duty diesel passenger vehicle, a London 'black cab1 taxi powered by a 2.71itre diesel engine was subjected to emissions tests. The tests were carried out on a specially developed and equipped chassis dynamometer. This equipment is fitted with pairs of rollers on which the driving wheels of the taxi were mounted such that any attempt to drive the vehicle rotates the rollers. The rollers of the chassis dynamometer drive resistance equipment with electrical and electronic control so that the effect of driving the taxi on the rollers is similar to driving on the road. Using this equipment it is possible to simulate a wide variety of road driving conditions in a controlled and repeatable manner. For the purposes of emissions testing, a standardised test cycle simulating city driving in dense traffic, followed by higher speed operation simulating driving outside the city, was used.

The chassis dynamometer was mounted in a large chamber employed specifically for the purpose of measuring exhaust emissions. To this end, the chamber was fitted with temperature control devices to maintain ambient temperature between 20 0C and 30 0C and with humidity between 5.5 and 12.2g H2O/kg dry air, as required by the European Union (EU) Directive 70/220/EEC, as currently amended. The chamber was also equipped with appropriate piping, exhaust gas dilution equipment, sample handling equipment and specialised exhaust analysis equipment, such that the total entity constituted a dedicated exhaust emissions test facility. The exhaust emissions facility, and the method of conducting tests and measuring exhaust emissions, had previously been assessed and audited by the United Kingdom Vehicle Certification Agency (VCA) with the result that the measurement of exhaust emissions in the test facility was accredited by the VCA. After the vehicle had been prepared for testing, emissions measurements were carried out to the standard required by EU directive 70/220/EEC as currently amended (the Directive). This method calls for pre-conditioning of the test vehicle (normally this constitutes driving three sets of Extra Urban Drive Cycles (EUDC) as specified in the Directive on the chassis dynamometer), after which the vehicle was retained in the test chamber under conditions of ambient temperature and humidity control for at least 6 hours before the actual emissions test was carried out. The emissions test itself is carried out from a cold start and constitutes both the city centre and extra urban drive cycles, known as ECE + EUDC.

Tests were carried out using the cold start procedure both with no exhaust after- treatment and with the combined DPF and SCR system in place. The arrangement of the DPF and SCR units and all other parts of the after-treatment system was as depicted in Figure 2. Results obtained have been compared and are shown in Figure 3.

Tests were also carried out using a hot start test method. Although this does not comply with the EU Directive 70/220/EEC, a similar test cycle is used, except that the engine is fully warmed up prior to starting the test. The warm up procedure adopted was the same as the pre-conditioning method used for the cold start test, namely driving three sets of extra urban drive cycles on the chassis dynamometer immediately before starting the combined ECE plus EUDC emissions test cycle. The principle difference between a cold emissions test as specified by EU Directive 70/220/EEC as currently amended, and the hot test procedure adopted, was the omission of the 6 hour minimum interval between vehicle pre-conditioning and the execution of the emissions test.

The hot start test method described was adopted with no exhaust after-treatment in place, and with the combined DPF and SCR system in place. The arrangement of the DPF and SCR units and all other parts of the after-treatment system was as depicted in Figure 2. Results obtained have been compared and are shown in Figure 4. Figure 4 shows that with a hot start, and the temperature sensor Tl positioned as shown in Figure 2 there is a "spike" of pollutants for the first 200 seconds; thereafter NOx is generally less than with no after-treatment.

Figure 7 shows a test result for the arrangement of Figure 5 with a hot start. Comparing Figure 7 with Figure 4, there is no spike at the start. Figure 8 shows the test result for a cold start. Figures 7 and 8 are test results from the arrangement of Figure 5 using a DPF 16 having a Platinum coating. NOx from the engine comprises a major proportion of NO and a minor proportion of NO2. The Platinum coating increases the NO2 content in the exhaust gas by oxidising the NO. That leads to better conversion of NOx to nitrogen in the SCR catalyzer 8 downstream of the DPF 16, because NO2 is more reactive than NO.

Catalysts other than Pt can be used. Examples include Pt, Pd, Rh, W, V, Fe, Ti, Ce, Cu, Cr and Co. The choice depends in part on the sulphur content of the diesel fuel. The catalyst may be combined with Zr, Y, Mn, La, Sr, Ru, Sn, Ni, Al, Ba or Nd (amongst other possibilities) which may affect the selectivity, activity and/or tolerance to poisoning of the catalyst.

Figures 9 and 10 show test results, for hot and cold starts respectively, of the arrangement shown in Figure 6, where the DPFl 6 in Figure 6 does not have a Platinum coating and is downstream of the SCR catalyser 8.

Figure 9 has no "spike" as in Figure 4. Both Figures 9 and 10 show a reduction in NOx compared with no after-treatment.

Figures 9 and 10 show that good performance can be obtained in the arrangement of Figure 6 without using a Platinum coating on the DPF.