Abstract:
The exhaust system of an internal combustion engine is periodically dosed to recharge an SCR catalyst with ammonia. If dosing is scheduled and exhaust gas temperature is too low to ensure recharging, a forced increase in exhaust gas temperature is effected for the period of dosing. A diagnostic to confirm correct operation of a catalytic converter may advantageously run at the same time.

Description:
FIELD OF THE INVENTION 
     The invention relates to treatment of exhaust emissions of internal combustion engines, and particularly to engine exhaust systems incorporating selective catalytic reduction. Aspects of the invention relate to a method, to an engine and to a vehicle. 
     BACKGROUND 
     Legislation requires that noxious emissions of internal combustion engines be treated. Selective catalytic reduction (SCR) uses ammonia as a reducing agent to turn nitrogen oxides (NO x ) into nitrogen and water. Typically urea is injected periodically into the engine exhaust system upstream of an SCR chamber, where it mixes with hot exhaust gas to produce ammonia. This ammonia is absorbed onto the SCR catalyst in the chamber, and is effective at treating NO x . Over time the ammonia becomes depleted and a further injection of urea is required to recharge the catalyst. Depletion is estimated, usually in an engine management system, by monitoring engine activity over time. Systems for producing an injecting ammonia are also known. 
     An SCR catalyst is usually relatively remote from the engine; in a diesel engine it is for example downstream of the usual diesel oxidation catalyst (DOC) and diesel particle filter (DPF). As a consequence exhaust gas passing through the SCR chamber is often relatively cool, especially in city driving where vehicle speeds are low. 
     At low exhaust gas temperatures, thermal decomposition of urea to ammonia does not take place, and accordingly urea injection is inhibited by a control system. Such inhibition avoids formation of solid deposits in the chamber, and wastage of urea, but inevitably prolonged city driving may eventually result in a fully depleted catalyst. In the case of ammonia injection, deposits may also form at low exhaust gas temperatures. 
     It is against this background that the present invention has been conceived. Embodiments of the invention may provide an improved method or apparatus for treating exhaust emissions. Other aims and advantages of the invention will become apparent from the following description, claims and drawings. 
     SUMMARY  
     According to at least one embodiment of the invention there is provided a method of recharging an SCR catalyst of a vehicle exhaust system comprising the steps of determining that recharging of the SCR catalyst is scheduled, determining whether the temperature of the SCR catalyst is at or above a threshold; if yes, recharging the SCR catalyst from an external source, if no; raising the temperature of the exhaust gas to said threshold and recharging the SCR catalyst at said temperature from the external source. 
     In one embodiment of the invention there is provided a method of injecting urea into the exhaust gas stream of an internal combustion engine, comprising determining that an urea injection is scheduled, determining whether the temperature of an SCR catalyst is at or above a threshold; if yes, injecting a dose of urea; if no, raising the temperature of exhaust gas to said threshold and injecting a dose of urea at the raised temperature. 
     According to a further aspect of the invention there is provided a method of recharging an SCR catalyst of a vehicle exhaust system comprising the steps of determining that recharging of the SCR catalyst is scheduled, determining whether the temperature of the SCR catalyst is at or below a threshold, if no, recharging the SCR catalyst from an external source, if yes, raising the temperature of the exhaust gas to said threshold and recharging the SCR catalyst at said temperature from the external source. 
     In one embodiment of the invention there is provided a method of injecting urea into the exhaust gas stream of an internal combustion engine, comprising determining that a urea injection is scheduled, determining whether the temperature of an SCR catalyst is at or below a threshold; if no, injecting a dose of urea; if yes, raising the temperature of said catalyst to said threshold and injecting a dose of urea at the raised temperature. 
     The temperature of the SCR catalyst may be interpolated from exhaust gas temperature upstream thereof. 
     Embodiments of the invention therefore provide for the temperature of a cool exhaust gas stream to be raised sufficiently to ensure effective conversion of urea to ammonia, for the purpose of recharging the SCR catalyst. Continuous treatment of NO x  is thereby assured, even if the vehicle operating conditions tend towards low production of power and torque, such as in low speed city driving. 
     Furthermore, exhaust gas temperature may be raised by any suitable method. The engine operating map may be changed to cause the engine to work harder and thereby directly raise exhaust gas temperature, or electric heating may be provided in the exhaust tract to indirectly raise exhaust gas temperature. Alternatively a small amount of fuel is injected into the exhaust gas stream, and is burnt in the catalytic converter (e.g. the DOC); this raises exhaust gas temperature sufficiently to permit effective decomposition of urea. 
     One or more sensors may be provided to directly measure exhaust gas temperature at one or more locations of the exhaust tract. The outputs of such sensors may directly indicate the temperature of the SCR catalyst, or permit catalyst temperature to be interpolated or modelled from temperature measurement(s) made elsewhere. 
     After injection of urea, the vehicle engine may revert to normal operation, and the special measures to increase catalyst temperature are ceased. If the temperature of the SCR catalyst is raised by a different mode of driving, the special measures are also ceased. 
     In another embodiment ammonia may be directly injected into the exhaust gas stream. 
     A feature of legislation for treatment of engine exhaust emissions is that a periodic diagnostic is required to confirm that treatment is effective—in one example confirmation is required that a catalyst is working, and if not a malfunction signal is generated for the vehicle driver. If the exhaust gas stream is cool such a diagnostic may not be effective, with the result that a malfunction signal may be inappropriately generated. An advantage of the invention is that by raising exhaust gas temperature at the time of urea injection, the diagnostic can work effectively. Thus an embodiment of the invention includes the step of running such a diagnostic at the same time as gas temperature is raised for urea injection. 
     In an embodiment of the invention the diagnostic is for the DOC and comprises increasing the DOC temperature by post-combustion injection to cause an exothermic reaction therein, and confirming that the increase in DOC temperature meets or exceeds a pre-determined value. The advantage of this arrangement is that a single instance of raised exhaust gas temperature may be used both to ensure effective injection of urea for the recharging SCR, and for running the periodic diagnostic. 
     Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives, and in particular the individual features thereof, set out in the preceding paragraphs, in the claims and/or in the following description and drawings, may be taken independently or in any combination. For example features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic representation of an exhaust system of a diesel engine with urea dosing of the exhaust stream; and 
         FIG. 2  illustrates graphically the effect of an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a diesel internal combustion engine  11  has an exhaust tract, or system,  12  incorporating a diesel oxidation catalyst (DOC)  13 , a diesel particle filter (DPF)  14 , and a catalyst  15  for selective catalytic reduction. The exhaust tract terminates in an open tail pipe  16 . 
     An engine management system (EMS)  17  electronically controls operation of the engine, in particular fuelling thereof, according to programmed information, real time monitoring and driver demand. Such an engine management system is well known and need be not further described here. 
     A container  18  of liquid urea is provided for periodically dosing the exhaust tract, under the control of the EMS  17  as represented by arrow  19 ; as noted above urea mixes with exhaust gas to produce ammonia, with which to recharge the SCR catalyst. The mixing occurs in a region of the exhaust tract where the urea is provided, which is herein referred to as the recharging region. This region is upstream of the catalyst  15  but may encompass the catalyst. Systems for dosing urea into exhaust systems are well known, and need not be further described here. It should also be noted that urea is only an example of a thermally decomposable substance suitable for recharging the catalyst, and the use of other thermally decomposable substances is within the scope of the present invention. 
       FIG. 2  represents temperature (T) of the SCR over time (t). The temperature  20  may be sensed directly by a suitable sensor of the exhaust tract, or may be interpolated from sensors elsewhere in the exhaust tract, or may be calculated by the EMS  17  according to an algorithm, or other data relating to instant or cumulative engine performance. 
     In the invention the temperature of the SCR catalyst brick is determinative, but it will be appreciated that this temperature is very strongly influenced by exhaust gas temperature. 
     Exhaust gas temperature varies according to how hard the engine is worked. At time t 1 , gas temperature is high and consistent for example with highway driving. At time t 2  gas temperature is relatively low, and consistent with low speed city driving. A threshold temperature T a  indicates the minimum temperature necessary for thermal decomposition of urea to ammonia, and is around 170° C., or 180° C. Temperatures below the threshold temperature T a  therefore lie in a first range of temperatures where the urea cannot thermally decompose. Accordingly, temperatures above the threshold temperature T a  therefore lie in a second range of temperatures where the urea can thermally decompose. 
     At time t 3 , an injection of urea is scheduled due to depletion of ammonia on the SCR catalyst. Depletion, and hence injection, can be determined conventionally by any suitable method, but is typically calculated by the EMS  17  from continuous monitoring of engine operating condition. 
     Injection of urea is however inhibited at t 3  because gas temperature is below T a . Conventionally inhibition would continue until conditions of vehicle use changed to permit exhaust gas temperature to rise above T a . 
     According to one embodiment of the invention a forced increased of engine exhaust gas temperature is initiated to elevate the temperature above the threshold T a  and thus into the second range so that at t 4  injection of urea may occur with the certainty of thermal decomposition to ammonia. Hence the SCR catalyst may be recharged so as to effectively treat NO x . The dose of urea is governed by the size and nature of the SCR catalyst and the degree of depletion thereof; this dose can be determined conventionally. 
     As soon as the SCR catalyst has been sufficiently recharged, the engine reverts to normal operation, and for city driving the gas temperature may revert to less than T a , as indicated at t 5 . 
     Ammonia content of the SCR catalyst is schematically indicated by chain-dot line  21 , and show gradual depletion until t 4 , an increase as a result of injection of urea, and then gradual depletion. The characteristics of  FIG. 2  are somewhat exaggerated in order to illustrate the effect of the invention and the shape thereof may in practice not follow straight lines. 
     The alternative of injection of ammonia may be inhibited below a temperature of about 120° C. so as to avoid the formation of deposits within the SCR.