Patent Publication Number: US-2009217647-A1

Title: Method and device for regenerating the particle filter of an internal combustion engine during the transient operating phases thereof

Description:
The present invention relates to a method and a device for regenerating the particle filter positioned in the exhaust line of an internal combustion engine of the diesel engine type. 
     Because of increasingly strict emissions standards, particle filters are being fitted to motor vehicles as standard. These particle filters are able to filter the exhaust gases of internal combustion engines and trap the harmful particulates. These particle filters have to be regenerated at regular intervals so that they do not become blocked and prevent the engine from operating correctly. 
     One known regeneration solution described in particular in US 2004/204818 A1 has been to position a diesel-fuel injector directly on the exhaust line. During regeneration, this injector injects diesel fuel into the exhaust line. The diesel fuel reacts in an oxidation catalytic converter and produces heat. This heat heats the particle filter to a temperature of the order of 650° C., allowing the particulates caught in the filter to be burnt off. 
     In the device described in that document, the oxidation catalytic converter is positioned upstream of the particle filter and the diesel-fuel injector is positioned upstream of the oxidation catalytic converter. 
     The process of regenerating the particle filter, that is to say of injecting diesel fuel into the exhaust line, is triggered when the pressure drop caused by the presence of the particulates in the filter reaches a certain threshold. This pressure drop is measured using pressure sensors positioned upstream and downstream of the particle filter. 
     In the embodiment described in patent application FR0453187 dated 23 Dec. 2004, the depollution device comprises two oxidation catalytic converters, the first situated close to the internal combustion engine and the second close to the particle filter. 
     However, it has been found that the volume of the second oxidation catalytic converter was not large enough for the injector into the exhaust to be used over the entire engine operating range. Above and beyond certain engine speed and load limits, the catalytic converter was no longer capable of treating the diesel fuel injected into the exhaust by the injector. This leads to three undesirable phenomena:
     1. It is no longer possible to maintain a temperature of 650° C. in the catalytic converter. If the temperature drops below 580° C., it is no longer possible to sustain regeneration.   2. The amount of unburnt hydrocarbons entering the filter considerably increases. If the particle filter is at a temperature of above 620° C. then this substantial addition of hydrocarbons may cause soot-regeneration “runaway” and possibly even destroy the particle filter as a result of excessively high thermomechanical stresses.   3. If the filter is relatively cold, then the unburnt hydrocarbons cannot be treated by the filter and pass directly through to the exhaust, generating unpleasant smells, white smoke and pollution.   

     In order to remedy these problems, the Applicant Company has, in a patent application filed at the same time as the present application, described a method in which, during regeneration:
         the injection of fuel into the exhaust tube is interrupted when the engine operation enters a critical zone defined by speed and load upper limits,   injection of fuel is resumed when engine operation returns to a non-critical zone.       

     In addition, according to this method, the reference temperature at the inlet of the oxidation catalytic converter is set to 250-550° C. when engine operation is not in the critical zone and this temperature is set to 650° C. when the engine is in the critical zone. 
     This method guarantees that the particle filter can be regenerated over the entire engine operating range. However, when the vehicle is used very transiently, spikes of unburnt hydrocarbons that greatly exceed the expected level for steady-state operation are found at the outlet side of the catalytic converter and these may cause particle filter regeneration runaway and possibly lead to the particle filter being destroyed. These spikes are furthermore undesirable when attempting to comply with emissions standards. 
     It is an object of the invention to overcome these disadvantages. 
     According to the invention, this object of the invention is achieved by means of a method for regenerating a particle filter positioned in the exhaust line of an internal combustion engine, in which method, while the particle filter is being regenerated, fuel is injected into the exhaust tube upstream of an oxidation catalytic converter positioned upstream of the particle filter. This method is characterized in that, during regeneration:
         the injection of fuel into the exhaust tube is interrupted when the engine operation enters a critical zone defined by speed and load upper limits,   injection of fuel is resumed when engine operation returns to a non-critical zone,   during transient phases of operation between the two zones the volume flow rate of exhaust gases is measured and the fuel injection flow rate is reduced or the injection of fuel is deferred on the basis of this measurement.       

     This method makes it possible to avoid spikes of unburnt hydrocarbons during the aforementioned transient phases. 
     In a first embodiment of the method, fuel injection is resumed once the volume flow rate of exhaust gases has dropped back below a predetermined threshold. 
     In a second embodiment of the method, a correction is applied to the maximum flow rate at which fuel is injected, on the basis of the volume flow rate of exhaust gases. 
     According to another aspect, the invention relates to a device for implementing the method according to the invention. This device is characterized in that it comprises:
         means for interrupting the injection of fuel into the exhaust tube when the operation of the engine enters said critical zone,   means for resuming fuel injection when engine operation has returned to a non-critical zone,   means for measuring or estimating the volume flow rate of exhaust gases during the transient phases of operation,   means for controlling the injection of fuel when the volume flow rate of exhaust gases has dropped back below a predetermined threshold,   or means for correcting the maximum flow rate at which fuel is injected on the basis of the volume flow rate of the exhaust gases.       

     As a preference, the device comprises a flow meter positioned on the exhaust tube or equipment that estimates the volume flow rate of the exhaust gases. 
     In a first version of the device, the means of measuring the volume flow rate of the exhaust gases interface with a hysteresis module for controlling the injection of fuel. 
     In a second version, the means for correcting the maximum injection flow rate comprise a map which interfaces with the means of measuring the volume flow rate of the exhaust gases. 
     Other particulars and advantages of the invention will become further apparent throughout the description which follows. 
    
    
     
       In the attached drawings, given by way of nonlimiting examples: 
         FIG. 1  is an overall diagram of a known device for regenerating a particle filter, 
         FIG. 2  is an overall diagram of the intake and exhaust system of a turbocharged diesel engine, 
         FIG. 3  is a curve showing the change in engine load C as a function of engine speed R and illustrating two engine operating zones, 
         FIG. 4  is a diagram illustrating a first embodiment of the device according to the invention, 
         FIG. 5  is a diagram illustrating a second embodiment of the device according to the invention. 
     
    
    
       FIG. 1  schematically illustrates a device for cleaning the exhaust gases of a combustion engine of the diesel type, as described in French Patent Application FR0453187 dated 23 Dec. 2004. 
     This device comprises a particle filter  1  positioned on the exhaust line  2  of the combustion engine, in which exhaust line there are a first oxidation catalytic converter  3  located near the engine and a second oxidation catalytic converter  4  located just upstream of the particle filter  1 . 
     Positioned upstream of the second oxidation catalytic converter  4  is a diesel-fuel injector  5  connected to a pump  6  so that diesel fuel can be injected into the exhaust line  2 , the burning of which fuel raises the temperature of the oxidation catalytic converter  4  during phases in which the particle filter  1  is being regenerated. 
     Upstream of the catalytic converter  4  and between it and the filter  1  there are temperature sensors  7 ,  8 . 
     The reference  9  denotes a sensor capable of measuring the differential pressure between the inlet to the catalytic converter  4  and the outlet from the filter  1 . 
     Regeneration of the filter  1  is triggered when the differential pressure measured by the sensor  9  reaches a certain threshold. In this instance, diesel fuel is injected by the injector  5  into the exhaust line. The burning of the diesel fuel raises the temperature of the catalytic converter to about 650° C. so that the particles of soot present in the filter  1  can be burnt off. 
       FIG. 2  depicts a diesel engine  10  comprising a turbocharger  11  the compressor  12  of which compresses air into the intake duct  13  which is connected to the intake manifold  13   a  of the engine  10 . 
     Downstream of the turbocharger  11 , the exhaust line  2  comprises the device depicted in  FIG. 1 , namely, in succession, a first oxidation catalytic converter  3 , a diesel-fuel injector  5 , a second oxidation catalytic converter  4  and the particle filter  1 . A temperature sensor  7  is positioned close to the inlet to the oxidation catalytic converter  4 . 
     However, the Applicant Company has found that regeneration cannot be performed in all the internal combustion engine operating zones. 
     Thus, the curve in  FIG. 3  shows two zones S 1  and S 2 . The operating range S 1  is bounded by speed and load maximum values, that is to say by a collection of speed/load pairs which, for a given load (or torque), defines a maximum torque (or load, respectively) value. In the zone S 1 , engine operation lies below a load C and speed R defined by the curve. In the zone S 2 , engine operation on the other hand lies above the load C and speed values defined by the curve. 
     According to the method described in a patent application filed by the Applicant Company on the same day as the present application, during regeneration:
         the injection of fuel into the exhaust tube is interrupted when the engine operation enters a critical zone S 2 , and   injection of fuel is resumed when engine operation returns to a non-critical zone S 1 .       

     When the engine is in zone S 1 , the reference temperature at the inlet to the oxidation catalytic converter is set to a value of between 250 and 550° C. 
     When the engine enters the critical zone S 2 , because the injection of fuel has been interrupted, the above reference temperature is set to 650° C. 
     This method is able to guarantee that the particle filter can be regenerated over the entire engine operating range. However, when the vehicle is used very transiently, spikes of unburnt hydrocarbons that greatly exceed the expected level for steady-state operation are found at the outlet side of the oxidation catalytic converter and these may cause particle filter regeneration runaway and possibly lead to the particle filter being destroyed. These spikes are furthermore undesirable when attempting to comply with emissions standards. 
     The objective of the method described in the present invention is to reduce these hydrocarbon emissions at the outlet of the oxidation catalytic converter during transient phases. 
     In transient phases of use of the vehicle, a distinction can be made between two particular operating scenarios: 
     1. The engine is in the zone S 1  in which the injector into the exhaust has been in use for a number of minutes. The exhaust line upstream of the oxidation catalytic converter is at a temperature of between 250° C. and 550° C. and the driver decides to accelerate the engine. For that, the engine load C increases instantly and the speed R increases as the vehicle accelerates. If the demand for acceleration is very strong, the engine leaves the zone S 1  in which the injector is used. The exhaust line upstream of the oxidation catalytic converter must therefore reach 650° C. However, the temperature on the inlet side of the catalytic converter increases only very gradually during the time it takes for the exhaust line upstream of the catalytic converter to heat up and reach the temperature of 650° C. 
     2. The engine has, for several minutes, been in the zone S 2  in which the injector into the exhaust is not used, for example because the vehicle is on a steep slope (heavy demand for load placed on the engine). The exhaust line upstream of the oxidation catalytic converter is at 650° C. If the slope becomes less steep, then the engine load needed to maintain the vehicle speed becomes lower and returns to zone S 1  in which the injector into the exhaust is used. Given the thermal inertia of the exhaust line, the temperature at the inlet to the catalytic converter does not drop immediately from 650° C. to the target value of between 250° C. and 550° C. 
     The ability of the oxidation catalytic converter to treat the hydrocarbons is connected in particular with the volume flow rate of the exhaust gases passing through the catalytic converter. The greater the volume flow rate, the more hydrocarbons will leave the catalytic converter. 
     At the moment of entering zone S in which the injector into the exhaust is used, the volume flow rate is higher than it would be if the engine were operating in the steady state. It is at this time that the ability of the catalytic converter to treat the hydrocarbons is exceeded. During the time that it takes for the temperature at the inlet to the catalytic converter to reach its reference temperature and therefore that it takes for the volume flow rate to return to the value it would have in the steady state, hydrocarbons pass through the catalytic converter. 
     It may therefore be seen that it is the difference between the volume flow rate in transient conditions and the volume flow rate in the steady state that causes the hydrocarbon spikes. To remedy this problem, it is therefore necessary to alter the volume flow rate or, at the very least, to take this into consideration. 
     Thus, according to the method of the invention:
         during transient phases of operation between the two zones S 1  and S 2  the volume flow rate of exhaust gases is measured or estimated and the fuel injection flow rate is reduced or the injection of fuel is deferred on the basis of this measurement.       

     In practical terms, fuel injection is not resumed until the volume flow rate of the exhaust gases has dropped back below a predetermined threshold. 
     A correction may also be applied to the maximum flow rate at which fuel is injected, on the basis of the volume flow rate of the exhaust gases. 
     The device for implementing the above method comprises:
         means for interrupting the injection of fuel into the exhaust tube when the operation of the engine enters the critical zone S 2 ,   means for resuming fuel injection when engine operation has returned to a non-critical zone S 1 ,   means such as a flow meter for measuring the volume flow rate of exhaust gases during the transient phases of operation,   means for controlling the injection of fuel when the volume flow rate of exhaust gases has dropped back below a predetermined threshold,   or means for correcting the maximum flow rate at which fuel is injected on the basis of the volume flow rate of the exhaust gases.       

     The diagram in  FIG. 4  shows that the flow meter  14  that measures the volume flow rate of the exhaust gases interfaces with a hysteresis module  15  to control the injection of diesel fuel by the injector  5 . 
     When the volume flow rate of the exhaust gases is above the second threshold of the hysteresis module  15 , the output from this hysteresis module becomes equal to zero and the selector  16  switches to zero, dictating a zero value on the command  17  that controls the flow rate of the injector  5 . 
     When the volume flow rate of exhaust gas drops back below the first threshold of the hysteresis module  15 , the output from this hysteresis module becomes equal to one and the selector  16  switches to the position that allows the command  17  to trigger injection of fuel by the injector  5 . 
     The two thresholds of the hysteresis module may possibly be programmed into a map as a function of engine speed and load. 
     The use of a hysteresis module allows the injection of fuel into the exhaust tube to be activated and cut off cleanly. 
     The diagram in  FIG. 5  illustrates means for correcting the maximum flow rate at which fuel is injected. These means comprise a map  18  which interfaces with the flow meter  14  which measures the volume flow rate of exhaust gases and with a MIN unit  19  capable of saturating the flow rate of the injector on the basis of the volume flow rate of the exhaust gases.