FR2907508A1 - Post-processing unit i.e. oxidation catalyst, internal temperature controlling method for internal combustion engine, involves adjusting gas composition to compensate difference between input and setting power by modifying potential power - Google Patents

Abstract

The method involves determining a value of thermal setting power (Wc) to obtain a setting temperature in a post-processing unit i.e. oxidation catalyst, from the estimation of rate (Qech) and temperature of exhaust gas before entering in the unit and based on the composition. Input power (We) supplied by the gas is determined based on losses (Wp), where the power (We) corresponds to a sum of heating power due to the temperature and potential power of exothermic reaction. The composition is adjusted to compensate difference between the power (We) and power (Wc) by modifying the potential power.

Description

1 Method for controlling the temperature of gases in a circuit

  The invention relates to a method for controlling the temperature of the gases in an exhaust system of an internal combustion engine and also covers a device for carrying out the method. It is known that it is necessary to reduce the pollution produced by internal combustion engines. To reduce pollutant emissions, increasingly complex gas after-treatment systems are located in the exhaust line of lean-burn engines. In particular, to reduce NOX nitrogen oxide emissions in a generally oxidizing mixture such as the exhaust gas of a lean-burn engine, there is usually available in the purification system a catalyst comprising a Nitrogen oxides accumulation known as "NOX Trap" which is integrated in the exhaust line and in which are trapped the nitrogen oxides emitted during normal operation of the engine in lean mixture. The operation of such a catalyst for the accumulation of nitrogen oxides is described in detail, for example, in EP-A-0580389. Such a catalyst must be regenerated periodically by operating the engine in a rich regime for a certain period of time. time to decompose the nitrates by releasing NOX which is then reduced to nitrogen by the reducing agents such as H 2, HC, and CO contained in the exhaust gas, as described in the document cited above. Similarly, to eliminate the soot particles present in the exhaust gas, the exhaust system normally comprises a particle filter, for example catalytic, whose inner wall is covered with a layer of impregnated material of precious metals, called "Wash Coat", providing an oxidation function intended to reduce the combustion temperature of the soot particles. To prevent clogging of such a particle filter by soot, it is necessary to periodically carry out a regeneration which consists of burning the soot by raising the temperature of the exhaust gas to around 600 C.

  This rise in temperature can be obtained by degrading the motor efficiency by appropriate means of regeneration aid. DB5052 EN DEM 0 2907508 2 To optimize the treatment of all pollutants, it is necessary to better manage the storage and regeneration phases of the post-treatment units by controlling, as much as possible, the thermal power developed within of these traps in order to optimize the soot combustion in the case of the particulate filter and the loading or reduction of the nitrogen oxides in the case of NOX trap or NOX Trap. Indeed, these combustion reactions, oxidation, adsorption or reduction are directly dependent on the temperature of the support of these traps and the gases that pass through them. The invention provides a solution to such problems through a method 10 for controlling the temperature of the gases in an exhaust circuit of an internal combustion engine and, in particular, to control the output thermal power for a first time. an oxidation catalyst or NOX Trap after-treatment system in a temperature window close to the maximum temperature of use, in particular avoiding possible temperature peaks.

In known manner, such a control can be performed from a measurement of the internal temperature of the catalyst by means of sensors placed inside thereof. However, the implantation of a sensor within the catalyst is difficult and presents risks for the integrity of the monolith constituting the catalyst. It is therefore preferable to place the temperature sensor downstream of the catalyst, the management of the thermal output of the first post-treatment system being ensured, in a control system called "by state feedback" from a measurement of the temperature of the gases at the outlet of this system. However, the response of such a system is slow and very delayed and, moreover, very sensitive to changes in the inertia of the monoliths caused by variations in the temperature of the gases at the inlet of the post-treatment system and / or variations in their flow. As a result, the temporal characteristics of the response of such a process to the control step limit the performance of a feedback control in terms of setpoint tracking and it may be impossible to avoid overruns. the maximum permissible temperature. Moreover, because of the sensitivity of the process to changes in operating conditions, it will be very difficult to achieve a fairly stable control over the entire operating field of the engine without describing its parameters for each of the operating conditions, which means that is inadmissible in terms of memory space dedicated to this function. In order to avoid these drawbacks, the subject of the invention is a new control method enabling regulation of the internal thermal of the catalyst without depending on the conditions of observation of the state of the system for the preparation of the adequate control.

In general terms, the invention therefore relates to the control of the internal temperature of a post-treatment unit placed in an exhaust circuit of an internal combustion engine, at least some pollutants contained in the exhaust gas. engine exhaust being stored, in normal operating phases, in at least one post-treatment unit which is alternately subjected to regeneration phases to remove stored pollutants, by raising and maintaining the temperature in the organ post-treatment in the vicinity of a set temperature. According to the invention, from a measurement or an estimate of the flow rate and the temperature of the exhaust gases before entering the post-treatment unit and taking into account their composition, it is determined on the one hand, the value of the reference power required to obtain the set temperature in the post-treatment unit, taking into account the losses and, on the other hand, the incoming heating power supplied by the gas in the post-treatment member, which is the sum of their direct heating power due to their temperature, and their potential exothermic reaction potency in the post-treatment member, and the composition of said exhaust gas, before entering the post-processing member, so as to compensate for the difference thus determined in advance between the incoming power and the target power by a corresponding modification of the potential power of reaction 25 exothermic. Preferably, the target power is determined from a measurement of the temperature of the gases at the outlet of the post-treatment unit, their flow rate and their heat capacity. Particularly advantageously, in order to compensate for the difference between the input power and the target power, the relative proportions of reducing agents and oxidants in the exhaust gas are adjusted in order to change the potential power of the exothermic reaction by a desired value. oxidation of the reductants in the post-treatment organ. According to another preferred feature, the proportion of reducing agents in the exhaust gas is adjusted by a post-injection of a controlled flow rate of fuel, either in at least one combustion chamber of the engine, or directly in the exhaust circuit, upstream of the post-processing unit.

For this purpose, a control unit determines the estimated values of the necessary setpoint power and the incoming thermal power supplied by the gases and a signal corresponding to the difference thus calculated is displayed on a control unit of a flow rate. post-fuel injection to adjust the potential power of exothermic reaction of the gases in the post-treatment organ. In a more advanced embodiment, to determine the fuel injection post-injection rate to compensate for the difference between the target power and the incoming thermal power, the control unit takes into account the variation of internal energy of the fuel. post-treatment unit due to the changeover to the set temperature. For this purpose, the instantaneous value of this variation of internal energy of the post-treatment unit is calculated taking into account its mass, its heat capacity and the temperature difference to be compensated. Other advantageous features of the invention will become apparent from the following description of certain particular embodiments given by way of example and shown in the accompanying drawings. Figure 1 is a general diagram of the intake and exhaust circuits of a combustion chamber of an internal combustion engine. Fig. 2 is a diagram of an exhaust circuit equipped with post-processing systems. FIG. 3 is a diagram showing an example of variation, over time, of the flow of the exhaust gases according to the engine speed and the corresponding variation of the incoming thermal power and the necessary reference power.

FIG. 4 is a diagram of a temperature control system for carrying out the method. Figure 5 is a diagram of a more advanced control system, proportional and integral control. FIG. 1 diagrammatically shows a conventional arrangement of the intake and exhaust circuits 2 connected to a combustion chamber 10 of an engine into which an injector 11 opens. In a conventional manner a part of the exhaust gas is returned to the intake circuit 1 by a recirculation circuit 12, the majority of the exhaust gas first passing through a variable geometry turbine 13 which drives a compressor 14 intake air. The diagram of FIG. 1 shows other well-known arrangements which can, moreover, be the subject of many variations and do not require a detailed description. At the exit of the turbine, the exhaust gases are returned to the atmosphere by an output circuit 20 which, to reduce pollution, comprises at least one post-processing member 21. This post-processing member, usually The oxidation catalyst 15 or NOX trap makes it possible to retain and store the nitrogen oxides contained in the exhaust gas and is usually associated with a particulate filter which retains, in the form of soot, the particles contained in in the gases. Thus, as shown schematically in FIG. 2, the exhaust circuit 2 of an internal combustion engine 11 usually comprises, downstream of the turbine 13, a first post-processing member 21 of the oxidation catalyst type and a second member 22 such as a particulate filter, the gases thus cleaned up being discharged into the atmosphere by an evacuation circuit 23. In a conventional manner, the pollutants accumulated, in normal operation, in the two post-treatment units 21, 22 are removed in regeneration phases, generally raising the temperature of the gases by adjusting the engine to a rich mixture. For this, the injector 11 is controlled, usually by a control unit receiving the information transmitted by various sensors and associated with a model that periodically controls the transition to the regeneration phase and maintaining the temperature at the desired desired level.

However, the temperature Te and the flow rate Qech of the exhaust gases measured, for example, by sensors 31 placed upstream or downstream of the turbine 13, vary at each instant as a function of the engine speed controlled by the driver. For example, in the diagram of FIG. 3, curve 3 shows the possible evolution, over time, of the flow rate of the exhaust gas, which varies according to the speed required of the engine. that is, the position of the acceleration pedal and gear engaged on the gearbox.

It is also known, by measurement, estimation or modeling, the composition of the exhaust gas and can therefore be deduced their heat capacity Cp. After having passed through the catalyst 21, the exhaust gases leaving via the pipe 20 'have the same flow Qech and their temperature Ts, which can be measured by a sensor 15, corresponds to the internal temperature of the catalyst 21. The problem of the invention is therefore to maintain this temperature Ts, throughout the duration of the regeneration phase, in the vicinity of a set temperature for which the operation of the catalyst is optimal but which must not be exceeded to avoid deterioration of the catalyst .

The outgoing thermal power, expressed in joule per second, is equal to: Ws = Ts * Qech * Cp (1) The principle of the thermal management is to provide in the catalyst 21 an average incoming thermal power substantially equal to this power thermal output, that is to say the power discharged to the desired set temperature. This incoming thermal power consists, on the one hand, of the thermal power supplied directly by the heat of the gases because of their temperature Te and of their flow Qech and, on the other hand, of the potential power indirectly supplied by the exothermic reaction capacity of the oxygen masses or reductants present in the exhaust gas and which react partially or totally in the catalyst. This exothermic reaction provides, in effect, a complement of energy W. which adds to the direct heating power of the gases to raise the temperature in the catalyst. The incoming power We, expressed in J / s, can therefore be written: We = Te * Qech * Cp + Wexo (2) DB5052 EN DEM 0 2907508 7 This incoming thermal power can therefore be calculated from a measurement, estimating or modeling the temperature Te and the flow rate Qech of the gases in line 20, at the inlet of the catalyst 21, as well as their composition, in particular oxygen or reducing emissions, which allow to determine their heat capacity Cp and the potential power of exothermic reaction Wexo. Similarly, it is possible to estimate or determine by modeling the possible thermal losses Wp in the catalyst 21.

In the diagram of FIG. 3, the curves 3 and 31 give, respectively, an example of evolution over time, as a function of the engine speed, the Qech gas flow and the incoming power We. Curve 32 indicates the corresponding change in the desired power Wo that the gases should bring to obtain the outgoing power Ws 15 which corresponds to the set temperature, taking into account the losses Wp according to the equation: Wc = Ws + Wp ( 3) 20 Given the engine speed, there is therefore, at each moment, a difference which must be compensated: A = WcuWe (4) between the input power supplied by the gases and the target power, this difference being able to vary at every moment. The idea of the invention therefore consists in piloting the potential energy WeXo that can be produced by the exothermic oxidation reaction of the reducing agents inside the catalyst 21, so as to supply the gases entering the catalyst with the complement 30 energy required to obtain the target temperature to be reached in the catalyst. For this, the composition of the exhaust gas, before entering the catalyst, is adjusted in order to modify the potential power of the exothermic reaction of the value necessary for the incoming power to be equal to the desired power to obtain the desired temperature. To achieve this adjustment of the composition of the reducing gases, the control unit can advantageously act on the fuel injection post-injection flow rate. For this purpose, as shown diagrammatically in FIG. 4, the control unit may comprise a calculation block 4 of the incoming power We which receives, on its inputs 41, 42, signals corresponding respectively to the temperature Te and Qech flow of gases measured, for example, by sensors 31 and, on its input 43, a signal corresponding to the potential power of exothermic reaction that can be estimated or calculated taking into account the composition of the gases. A calculation block 5 determines the desired power Wc necessary to obtain the desired setpoint temperature, based on the information displayed on its inputs 51, 52, 53 relating to the set temperature Ts, to the flow rate of the Qech gases and to the loss of energy Wp in the catalyst, which can be estimated or determined by modeling. The signals corresponding to the incoming power We and the setpoint power Wc, which are output at the outputs 44, 54 of the calculation blocks 4, 5 are displayed on a comparator 45 which outputs a signal corresponding to the difference AW, at the input 61 of a calculation block 6 which determines the correction to be made to the potential power of the exothermic reaction corresponding to the engine speed at the moment considered, in order to compensate for the calculated power difference A, so as to determine the potential exothermic power W'exo for obtaining, in the catalyst 21, the desired thermal power Wc according to the equation: Wc = Te * Qech * Cp + W'exo (5) The calculation block 6 thus emits at its output 62, a signal corresponding to this desired exothermic power. This signal is displayed on the control system 7 of the injector 11 which continuously adapts the post-injection flow to bring the missing power A, thus determined in advance, based on an estimate of the instantaneous thermal powers.

It is thus possible to maintain, at each instant, the temperature in the catalyst 21 at an optimum value while avoiding temperature peaks. Of course, the invention is not limited to the details of the embodiment which has just been described as a simple example but covers, on the contrary, all the variants remaining in the same protective frame. DB5052 EN DEM 0 2907508 9 For example, instead of adapting the gearbox flow in late post-injection into the cylinder, so as to increase the exothermic potential resulting from the initial adjustment of the operating point of the motor, it would be possible to place an injector in the exhaust circuit 20, upstream of the post-treatment unit 21, so that the release of heat on the catalyst, by oxidation of the additional flow of reducing agents, completes the energy flow brought by the On the other hand, it is also possible to add to the command which has just been described a second control term taking into account the internal temperature (s). s) of the monolith, these temperatures being measurable by means of sensors or determined using a thermal model. Such a system, shown diagrammatically in FIG. 5, thus also comprises calculation blocks 4 and 5 for respectively determining the incoming power We and the desired power Wc and deriving the missing power A ,,. However, in this system, there is added to the previous command a second control term developed by a calculation block 8 which receives on its inputs 81 the same information Ts, Te, Wexo, Qech, Wp to deduce the delta of the internal energy of the catalyst AE; nt as a function of the temperature, according to the equation: ## EQU1 ## where m is the mass or mass fraction of the monolith useful to the control , C (T) the heat capacity of the substrate which depends on the temperature and AT the difference between the set temperature and the temperature of the catalyst at the instant considered, which is given by the thermal model. The signal corresponding to this delta of the internal energy is displayed at the input 63 of the calculation block 6 which, as previously, receives on its input 61 the signal A corresponding to the difference between the power We and the power of 30 instructions. toilet. The calculation block 6 can thus calculate the instantaneous thermal power delta with respect to the difference between the target power and the incoming power, taking into account the delta of the internal energy resulting from the monolith temperature model, according to the equation Thus, the device 7 for controlling the injector 11 can adapt the post-injection flow rate so that the modified exothermic reaction power W'exo as well as 5 obtained allows, by compensating for the missing power A to obtain the necessary power required to reach, in the catalyst, the desired set temperature. In addition, the dynamic performance, particularly the rise time, could be improved by adding a thermal control mode using the knowledge of the internal energy of the monolith by means of a thermal model thereof. Similarly, a dynamic temperature setpoint would further improve the performance of the method according to the invention.

In the case just described of a NOX Trap type oxidation catalyst, the control of the temperature at the outlet of the catalyst also makes it possible to optimize the regeneration, by soot combustion, of the filter. 22. However, insofar as it makes it possible to constantly control the temperature in the post-treatment unit taking into account, at each moment, the characteristics of the exhaust gases resulting from the engine speed, the invention could also be adapted to other regeneration systems, such as the desulfation of a NOx Trap catalyst or a 4-way system placed downstream of the oxidation catalyst and would also optimize the adsorption efficiency of Nitrogen oxides in the catalyst or a 4-way system, as well as the efficiency of their treatment on a SCR type system. (7)

Claims (1)

  1. A method for controlling the internal temperature of a post-treatment member in an exhaust circuit (2) of an internal combustion engine (10) producing exhaust gases whose flow and temperature vary in according to the engine speed (10) at each moment, at least some pollutants contained in the exhaust gas being stored, in the normal engine operating phases, in at least one post-processing member (21) which is alternately subjected to regeneration phases to remove stored pollutants, by raising and maintaining the temperature in the post-treatment unit (21) in the vicinity of a set temperature Ts, characterized in that, starting from a measurement or estimate of the flow rate Qech and the temperature Te of the exhaust gas before entering the post-treatment unit (21) and taking into account their composition, it is determined, on the one hand, the value of the target power Wc necessary to obtain the set temperature in the post-processing unit (21), taking into account the losses Wp and, on the other hand, the incoming heat output We brought by the gases in the post organ -treatment (21), which corresponds to the sum of their direct heating power due to their temperature, and their potential exothermic Wexo reaction power in the post-treatment member (21) and that the composition is adjusted of said exhaust gas before entering the post-processing unit (21), so as to compensate for the difference thus determined in advance, between the incoming power We and the desired power Wc, by a corresponding modification the potential exothermic reaction power W exo 2 Control method according to claim 1, characterized in that the target power Wc is determined from the temperature Ts of the gases measured at the outlet of the post-treatment unit ( 21) their Qech flow rate and their heat capacity Cp. 3. Control method according to one of claims 1 and 2, characterized in that, to compensate for the difference between the incoming power We and the desired power Wc, the relative proportions of reducing agents and oxidants in the gases are adjusted. in order to change the desired power by the potential power of the exothermic oxidation reaction of the reducing agents in the post-treatment unit (21). 4. Control method according to claim 3, characterized in that the proportion of reducing agents in the exhaust gas is adjusted by a post-injection of a controlled flow rate of fuel, ie in at least one combustion chamber (10). ) of the engine, either directly in the exhaust circuit (2), upstream of the post-processing member (21). 5. Control method according to one of the preceding claims, characterized in that a control unit (4, 5) determines the estimated values of the necessary setpoint power Wc and the incoming thermal power We brought by the gases. and that a signal corresponding to the difference thus calculated is displayed on a control unit (6, 7) of a fuel injection post-injection rate for adjusting the potential exothermic power W. of the gases. 6. Control method according to claim 5, characterized in that, to determine the fuel injection post-injection rate for compensating for the difference between the desired power Wc and the incoming thermal power We, the control unit ( 6, 7) takes into account the variation of internal energy AEint of the posttreatment member (21) due to the change to the set temperature Ts. 7. Control method according to claim 6, characterized in that, in order to determine the exothermic reaction power to be provided by a fuel after-injection, the control unit (6, 7) adds to the estimated difference between the reference power Wc and the input thermal power We, the instantaneous value of the internal energy variation AEint of the post-treatment unit (21), taking into account its mass m, its heat capacity Cp and the difference 25 of temperature to compensate AT. DB5052 FR DEM 0