FR2832184A1 - Emission control system e.g. for motor vehicle diesel internal combustion engine, comprises a butterfly on-off valve and an exhaust gas recirculating system - Google Patents

Abstract

The system of emission control comprises a butterfly on-off valve to control the airflow, control of the valve by sensors, an exhaust valve for recirculation of exhaust gases to the inlet, a nitrogen oxides (NOx) storage-reduction catalyst in the exhaust line, and a supply line for reducing agent to the catalyst (fuel supply). The system is also controlled by a programmed control unit. The emission control system comprises: (1) a sensor (11) for detection of quantity of air admitted; (2) a stop-valve (13) to control the flow rate of admitted air in the induction manifold (8); (3) a passage for recirculation of part of the exhaust gases to the inlet manifold, together with a control valve (26) to adjust the flow rate, linked to the control unit (30); (4) a storage-reduction NOx-type catalyst situated in the exhaust line of the engine; (5) a supply line for reducing agent (60) to deliver the agent to the catalyst; and (6) a control system (30) to determine the need for delivery of the reducing agent dependent on the functioning status of the engine. The system also comprises a pressure sensor in the inlet manifold and a second control system for the on-off valve which operates when the quantity of admission is less than a predetermined level. An Independent claim is also included for an emission control process involving the stages of: (1) detection of a quantity of air admission; (2) control of admission by operation of a butterfly stop-valve; (3) effecting recirculation of part of the exhaust gases via a recirculation route (25); (4) adjustment of the flow rate of recirculating exhaust gas by means of a valve (26) on the basis of the functional state of the engine; (5) determination of the need to deliver a reducing agent on the basis of the functional state of the engine; and (6) delivery of a reducing agent to the NOx catalyst.

Description

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procédé de commande'd'émission pour un moteur à combustion interne comprenant un système EGR, etc. SYSTEM AND METHOD FOR CONTROLLING THE TRANSMISSION OF AN ENGINE
INTERNAL COMBUSTION
The present invention relates to a system and a method for controlling the emission of an internal combustion engine and, more particularly, to a system and to a

emission control method for an internal combustion engine comprising an EGR system, etc.

 In general, lean internal combustion engines, of which diesel engines are examples, incorporate a wide variety of measures to reduce the emission of nitrogen oxides (NOx) and smoke. As one such measure, a transmission control system as disclosed in Japanese patent publication No. 2,845,056 has been proposed.

 An emission control system disclosed in its publication includes an intake butterfly valve disposed in an intake passage and an NOx catalyst of the known storage-reduction type and a reducer supply device, both disposed in a exhaust gas passage.

 This emission control system is designed to reduce the amount of opening of the intake throttle valve or the supply of a reducing agent in the exhaust gas by using the reducer supply device during the increase in the temperature on the NOx catalyst side of the storage reduction type, or during the improvement of the removal of nitrogen oxides (NOx) adsorbed in the NOx catalyst of the storage-reduction type.

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 The storage-reduction type NOx catalyst (hereinafter called "NOx catalyst" when appropriate) works to purify the exhaust gas in the following manner. Namely, when the oxygen concentration in the exhaust gas is high, that is to say when the air-fuel ratio of the exhaust gas is poor, the NOx catalyst adsorbs nitrogen oxides (NOx ) contained in the exhaust gas. Conversely, when the oxygen concentration in the exhaust gas is low, i.e. when the air-fuel ratio of the exhaust gas is rich, the NOx catalyst reduces the adsorbed nitrogen oxides (NOx) by nitrogen dioxide (NO2) or nitrogen monoxide (NO) and then release them into the exhaust gas. At this time, the nitrogen dioxide (M02) and the nitrogen monoxide (NO) released are oxidized with the unburned fuel components (CO, HC) contained in the exhaust gas and are thus converted into vapor d water (H20) and non-harmful carbon dioxides (C02).

 Furthermore, the reducer supply device is adapted to forcibly reduce the oxygen concentration in the exhaust gas by supplying engine fuel (light oil) serving as a reducing agent in the exhaust gas. exhaust flowing into the NOx catalyst. When the reducing agent (engine fuel) is supplied to the reducer feeder, the previously described exhaust gas purification effect is improved and the temperature of the NOx catalyst increases due to heat of reaction generated during the reaction between the reducing agent and the catalytic substances.

 At the same time, when the intake butterfly valve is substantially closed, the following advantageous effects are obtained:

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 (1) Since the amount of air (oxygen) to be used in the combustion of the engine is reduced, the amount of supply of the reducing agent is reduced accordingly.

(2) When the engine is running under a low load, the cooling (heat radiation) of the NOx catalyst caused by an excessive amount of exhaust gas flowing through it is prevented.

(3) Sufficient time is allowed for the reaction between the reducing agent and the catalytic substances, thus the exhaust gas purification efficiency is improved.

 Furthermore, in recent years, internal combustion engines capable of operating with a high EGR rate have greatly attracted the attention of people. An internal combustion engine of this type has an EGR system (an exhaust gas recirculation system) in addition to an intake butterfly valve, and is adapted to prevent the generation of soot (smoke) substantially from completely by implementing an EGR rate of approximately 65% or more. Namely, an engine operation with such a high EGR rate is effected by opening the EGR valve substantially completely while closing the intake butterfly valve substantially completely, so as to fundamentally prevent the generation of soot and suppress the production of smoke .

 In addition, when the engine is operated with a high EGR rate, the air-fuel ratio of the exhaust gas becomes rich (close to or equal to the stoichiometric air-fuel ratio). This provides another advantage in that the temperature of the exhaust gas purification catalyst can be immediately increased. This is equivalent to saying that if the engine control described above is arranged to reach the temperature of the exhaust gas purification catalyst which is higher than

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 at its activation temperature, the engine control can be called catalyst heating control.

 However, when the intake throttle valve is substantially closed, the amount of air (intake air) to be used in the combustion of the engine is significantly reduced while the amount of EGR gas increases in turn. In such a case, in contrast to normal combustion, various problems can occur such as a misfire due to a lack of oxygen and an increase in EGR gas as inert gas, a misfire due to a decrease in pressure compression resulting from an excessive reduction in the amount of opening of the intake throttle valve and a decrease in atmospheric pressure, an increase in the amount of unburned fuel components (HC), and so on.

 Thus, when the intake throttle valve is substantially closed, the engine is forced to operate under severe conditions, as a result, the combustion of the engine becomes easily unstable.

 In the conventional emission control system, therefore, as a solution to the above problems, the amount of opening of the EGR valve is corrected while monitoring the output of an air flow measuring device, so to introduce an appropriate amount of EGR gas continuously and thereby ensure stable combustion of the engine. More specifically, feedback control of the EGR valve is performed based on the output of the air flow measuring device to operate the engine with a desired EGR rate.

 However, research which has been conducted in a dedicated manner by the present inventors and others has discovered various problems with such EGR rate control.

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 First, we will focus on the feedback control performed during the EGR rate control. An air flow is measured using an air flow measuring device as a feedback control parameter. To this extent, sufficient accuracy may not be ensured unless the air flow is higher than a certain value. In a conventional system, however, the intake throttle valve is closed in the previously described engine operating condition, therefore, the air flow rate in the intake passage may not be maintained high enough. In such a case, errors in the output of the area flow measuring device can be induced, which makes it difficult to perform the feedback control. Likewise, in such a feedback control based on the output of the air flow measuring device, drastic changes in atmospheric pressure are not reflected towards the control. In fact, the feedback control performed in the operating state of the engine described above needs to be much more precise than that performed when the engine is running with a large amount of intake air. In the conventional system, however, the accuracy of the feedback control and changes in air pressure are not reflected back to the feedback control and, therefore, it is assumed that the feedback control does not work as well. effectively than expected.

 Likewise, in view of the increase in the temperature of the exhaust gas purification catalyst, when the amount of EGR gas is reduced in the feedback control, the amount of air increases as a consequence (the air ratio -fuel becomes lean), thereby reducing the heating efficiency of the exhaust gas. It is therefore preferable to obtain a desired EGR rate by correcting the opening amount of

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 the intake butterfly valve. In the conventional system, however, the EGR rate is controlled by correcting the amount of EGR gas, therefore, the temperature of the exhaust gas may decrease in some cases.

 An EGR rate is a relative rate represented as "amount of EGR gas / (amount of EGR gas + amount of air)". Thus, in a case where the EGR rate is modified by controlling only the EGR gas, the total quantity of gas to be used in an engine combustion (an absolute quantity: quantity of EGR gas + quantity of air) decreases significantly when the quantity of admission is made small (when the required EGR rate is high). In conventional EGR rate control, therefore, the total amount of gas may be insufficient, thereby causing a decrease in compression pressure.

In such a case, a misfire and a decrease in the exhaust gas temperature are possibly caused.

 Likewise, research by the present inventors and others has revealed that the stability of engine combustion performed with the throttle valve open depends greatly on humidity.

 More specifically, when the humidity is high, a large amount of moisture (water content) is mixed with the air to be used in the combustion of the engine. In such a case, the ignition delay (a factor of engine combustion) increases due to the humidity. Consequently, when the ignition delay as a factor of engine combustion increases in the already severe engine operating condition as described above, the stability of engine combustion is certain to be significantly impaired.

 In view of the above situation, the invention has been made to provide a transmission control system

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 for an internal combustion engine capable of ensuring stable engine combustion even when the amount of air to be used in the combustion of the engine is significantly reduced due to change in the operating state of the engine.

 To solve the problems mentioned above, the transmission control system of the invention is constituted as described in the following.

 The emission control system of the invention includes: an intake amount detecting means for detecting the intake amount; an intake butterfly valve for regulating the flow of intake air flowing from the intake passage; first intake throttle valve control means for opening or closing the intake throttle valve in accordance with the intake quantity detected by the intake quantity detection means; an exhaust gas recirculation passage for recirculating a portion of the exhaust gases ejected from an internal combustion engine to its intake system; an exhaust gas recirculation valve for adjusting the flow rate of the exhaust gases flowing in the exhaust gas recirculation passage; exhaust gas recirculation valve control means for opening or closing the exhaust gas recirculation valve based on the operating state of the internal combustion engine; a storage-reduction type NOx catalyst disposed in an exhaust gas passage of the internal combustion engine;

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 reducer supply means for supplying a reducing agent to the storage-reduction type NOx catalyst; determination means for determining whether or not to deliver the reducing agent based on the operating state of the internal combustion engine.

 The emission control system is arranged to deliver the reducing agent from the reducing feed means when the determining means determines to deliver the reducing agent, and is characterized by further including: means intake pressure detection for detecting the pressure in the intake passage; second intake throttle valve control means for opening the intake throttle valve in accordance with the pressure in the intake passage detected by the intake pressure detection means when the intake quantity detected by the intake amount detecting means is less than a predetermined intake amount.

 In accordance with a further aspect of the invention, the emission control method of the invention comprises the steps of: detecting the quantity of admission; control the intake quantity by opening and closing the intake throttle valve in accordance with the detected intake quantity; recirculating part of the exhaust gases ejected from the internal combustion engine to its intake system via the exhaust gas recirculation passage; adjusting the flow rate of the exhaust gas flowing in the exhaust gas recirculation passage by means of the exhaust gas recirculation valve;

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 open or close the exhaust gas recirculation valve based on the operating condition of the internal combustion engine; determining whether or not to deliver the reducing agent based on the operating condition of the internal combustion engine; and delivering the reducing agent from the reducing means supplying the storage-reduction type NOx catalyst when it is determined to deliver the reducing agent.

 The emission control method of the invention is characterized by further comprising the steps of: detecting the pressure in the intake passage detecting the intake pressure and controlling the air flow intake by opening or closing the intake throttle valve in accordance with the pressure detected in the intake passage when the detected intake amount is less than a predetermined intake amount; and controlling the intake air flow by opening or closing the intake throttle valve in accordance with the intake amount when the detected intake amount is equal to or greater than the predetermined intake amount.

 In the previously described emission control system and method of the invention, the intake amount is detected using the intake amount detecting means. Likewise, the intake throttle valve is disposed in the intake system, and its opening amount is controlled in accordance with the intake amount detected by the intake amount detection means. Besides this, the exhaust gas recirculation passage and the exhaust gas recirculation valve are arranged in the exhaust system.

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 The exhaust gas recirculation valve is opened or closed by the exhaust gas recirculation valve control means based on the operating state of the internal combustion engine. In addition, in the exhaust system, the storage-reduction type NOx catalyst and the reducing agent supply means are arranged. Likewise, the determination means is adapted to determine whether or not to deliver the reducing agent on the basis of the operating state of the engine. When the determining means determines to deliver the reducing agent, the reducing agent is delivered from the means of supplying the reducing agent to the NOx catalyst of the storage-reduction type to promote the removal of nitrogen oxides (NOx) adsorbed by the NOx catalyst.

 In addition, the intake passage detection means for detecting the passage in the intake passage is provided. When the intake amount detected by the intake amount detection means is less than the predetermined intake amount, the intake pressure detected by the intake pressure detection means is used to control the opening and closing the intake throttle valve. In the invention, More specifically, the first means for controlling the intake throttle valve and the second means for controlling the intake throttle valve are provided.

The first intake throttle valve control means is adapted to open or close the intake throttle valve in accordance with the intake quantity, while the second intake throttle valve control means is adapted to open or close the intake throttle valve in accordance with the intake pressure. When the detected intake quantity is equal to or greater than the predetermined intake quantity, the intake air flow is controlled by

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 opening and closing the intake throttle valve in accordance with the intake quantity. On the contrary, when the intake quantity decreases as the opening quantity of the intake throttle valve decreases, it is difficult to adjust the flow rate by controlling the intake throttle valve using the first means of controlling the intake throttle valve. In this case, therefore, the control is switched to use the second intake throttle valve switching means to continue controlling the intake air flow. In this case, the amount of opening of the intake throttle valve is adjusted by opening or closing the intake throttle valve in accordance with the intake pressure.

More specifically, the intake pressure in the intake passage is detected, and the intake throttle valve is opened or closed in accordance with the detected intake pressure.

 In accordance with a further aspect of the invention, the emission control system of the invention preferably also includes the humidity detection means and the injection synchronization correction means. The humidity detecting means is intended to detect the humidity in the intake passage, and the injection timing correction means is intended to effect advance correction of the engine fuel injection timing internal combustion. The fuel injection synchronization advance correction is performed when the intake amount detected by the intake amount detecting means is less than the predetermined intake amount and the humidity detected by the intake means humidity detection is equal to or greater than the predetermined humidity.

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 Likewise, in accordance with an additional aspect of the invention, it is preferable to detect the humidity in the intake passage and to carry out a correction in advance of the timing of fuel injection of the internal combustion engine. when the intake amount detected by the intake amount detection means is less than the predetermined intake amount and the humidity detected by the humidity detection means is equal to or greater than the predetermined humidity.

dans le passage de re-circulation de gaz d'échappement ; un moyen de commande de soupape de re-circulation de gaz d'échappement destiné à ouvrir ou fermer la soupape de re-circulation de gaz d'échappement sur la base de l'état de fonctionnement du moteur à combustion interne ; In accordance with a further aspect of the invention, the emission control system of an internal combustion engine comprises: intake amount detecting means for detecting the intake amount; an intake butterfly valve for controlling the flow of intake air flowing in the intake passage; first intake throttle valve control means for opening or closing the intake throttle valve in accordance with the intake quantity detected by the intake quantity detection means; an exhaust gas recirculation passage intended to recirculate a part of the exhaust gases ejected from the internal combustion engine to its intake system; an exhaust gas recirculation valve for adjusting the flow rate of the circulating exhaust gases

in the exhaust gas recirculation passage; exhaust gas recirculation valve control means for opening or closing the exhaust gas recirculation valve based on the operating state of the internal combustion engine;

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 a storage-reduction type NOx catalyst disposed in the exhaust gas passage of the internal combustion engine; reducer supply means for supplying the reducing agent to the storage-reduction type NOx catalyst; determining means for determining whether or not to deliver the reducing agent based on the operating state of the internal combustion engine.

 The emission control system is arranged to deliver the reducing agent from the reducing detection means when the determining means determines to deliver the reducing agent, and is characterized by further including: means humidity detection for detecting humidity in the intake passage; injection timing correction means for performing a fuel injection timing timing correction of the internal combustion engine when the intake amount detected by the intake amount detection means is less than the predetermined intake amount and the humidity detected by the humidity detecting means is equal to or greater than the predetermined humidity.

 In accordance with a further aspect of the invention, the emission control method of an internal combustion engine comprises the steps of: detecting the amount of intake; control the intake quantity by opening or closing an intake throttle valve in accordance with the detected intake quantity; recirculating part of the exhaust gas ejected from the internal combustion engine to the emission control system of the internal combustion engine via the exhaust gas recirculation passage;

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 adjusting the flow rate of the exhaust gas flowing in the exhaust gas recirculation passage by means of the exhaust gas recirculation valve; open or close the exhaust gas recirculation valve based on the operating condition of the internal combustion engine; determining whether or not to deliver the reducing agent based on the operating condition of the internal combustion engine; and delivering the reducing agent from the reducing means to the storage-reduction type NOx catalyst when it is determined to deliver the reducing agent.

 The emission control method of the invention is characterized by further including the steps of: detecting humidity in the intake passage; and performing advance correction of the fuel injection timing of the internal combustion engine when the detected intake amount is less than the predetermined intake amount and the detected humidity is equal to or greater than the humidity predetermined.

 In the previously described emission control system and method of an internal combustion engine, the humidity detection means for detecting humidity in the intake passage, and the injection timing correction means intended to correct the synchronization of fuel injection of the internal combustion engine are provided or used in addition to the intake quantity detection means. The fuel injection synchronization correction means is adapted to carry out a correction of advance of the fuel injection synchronization of the internal combustion engine when the intake quantity detected.

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 by the intake amount detecting means is less than the predetermined intake amount and that the humidity detected by the humidity detecting means is equal to or greater than the predetermined humidity.

 This is equivalent to saying that, when the intake quantity is low and the humidity is high, an advance correction of the fuel injection timing is performed to encourage the gasification of the fuel so that the second is restricted . Thus, a misfire due to the endothermic effect of humidities (water content) or a slow increase in compression pressure is prevented when the engine is forced to run under severe conditions. At the same time, the advance correction of the fuel injection timing can be carried out in various ways as required.

 For example, the injection timing can be advanced by a predetermined amount, or the advance timing of the injection timing can be changed gradually based on the humidity.

 In addition, it is best to detect humidity when the wiper is in operation. In this case, humidity detection can be easily performed using devices or devices that are already installed on the vehicle.

 The exemplary embodiment previously mentioned and the other exemplary embodiments, aims, characteristics, advantages, meaning, techniques and industry of this invention will be better understood by reading the following detailed description of the exemplary embodiments of the invention, when considered in relation to the accompanying drawings, in which:

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 Fig. 1 is a view showing, in a simplified manner, a construction of an internal combustion engine in each embodiment of the invention; Fig. 2 is a view showing a section through a particle filter arranged in an exhaust gas passage of the internal combustion engine of FIG. 1; Fig. 3 is a flow chart used to explain an EGR rate control in a first embodiment of the invention; Fig. 4 is a flowchart used to explain a command intended to correct the synchronization of the fuel injection in a second embodiment of the invention.

 In the following description and in the accompanying drawings, the invention will be described in more detail in terms of exemplary embodiments.

 It should be understood that the constructions of internal combustion engines, which will be described later, are simply engine constructions employed in the first and second embodiments, respectively. Consequently, the details of the constructions can be modified without departing from the scope of the invention as defined in the claims described later.

(First embodiment)
An internal combustion engine 1 in a first embodiment is a diesel engine used in a vehicle. As shown in Fig. 1, the internal combustion engine 1 comprises, as its main components, four cylinders 2 forming combustion chambers, a fuel supply system, an intake system, an exhaust system, a control system, and and so on.

 The fuel supply system comprises fuel injection valves 3, a common rail (an accumulator) 4, a fuel supply pipe 5,

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 a fuel pump 6, and other components, and is operative to deliver fuel to each cylinder 2. Each fuel injection valve 3 is an electromagnetically driven open-closed valve and is operative to inject fuel into the a correspondent of the cylinders 2. Each fuel injection valve is connected to the common rail 4 serving as a fuel distribution pipe. The common rail 4 is connected to the fuel pump 6 via the fuel supply pipe 5. The fuel pump 6 is driven to rotate by the rotation of a crankshaft serving it as the output shaft of the internal combustion engine 1 .

 In the fuel supply system constructed as described above, the fuel pump 6 delivers fuel into the fuel tank (not shown) to the common rail 4 via the fuel supply pipe 5. The fuel delivered to the common rail 4 is pressurized to a predetermined pressure therein and is distributed to each fuel injection valve 3. When the drive voltage is applied to each fuel injection valve 3, it opens to inject fuel into cylinder 2.

 Furthermore, the intake system comprises an intake pipe 9, an intake butterfly valve 13, an intake manifold 8, an air filter box 10, an intercooler 16 and other components, and forms an intake passage through which air is supplied to each cylinder 2.

 The intake pipe 9 forms a passage which guides the air (intake air), which has been introduced via the air filter box 10, towards the intake manifold 8. The intake manifold 8 forms passages each of which guides the air distributed via the intake pipe 9 into a corresponding one of the cylinders 2. Likewise, an air flow meter

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 11 and an intake temperature sensor 12 are arranged in the vicinity of a location to which the intake pipe 9 and the air filter box 10 are connected to each other. The air flow meter 11 is used to detect the flow of air flowing in the intake pipe 9, while the intake temperature sensor 12 is used to detect its temperature.

 In addition, the intake butterfly valve 13 is disposed just upstream of the intake manifold 8 and is adapted to increase or decrease the amount of air circulating in the cylinders (the combustion chambers) 2. The amount of opening of the intake throttle valve 13 is modified by an actuator 14 comprising a stepping motor and the like. Likewise, an intake temperature sensor 24 and an intake pressure sensor 23 are arranged just downstream of the intake butterfly valve 13. The intake temperature sensor 24 is used to measure the temperature in the intake manifold 8 while the intake pressure sensor 23 is used to measure the pressure in the intake manifold 8. Next to this, a compressor housing 15a of a turbo compressor 15 and an intercooler 16 are arranged in a part of the intake passage between the air filter box 10 and the intake butterfly valve 13. The housing 15a of the compressor is operational for compressing the intake air, while the intercooler 16 is operational for cooling the air which has been compressed in the housing 15a of the compressor.

 In the intake system constructed as described above, the air supplied to the respective cylinder 2 is first supplied as intake air to the air filter box 10 by a negative pressure generated in the operation of the engine. The air supplied to the air filter box 10 is then supplied to the housing 15a of the

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 compressor of the turbo compressor 15 through the intake pipe 9 after the dust and the like contained therein have been removed in the air filter box 10. The air supplied to the housing 15a of the compressor is compressed by a wheel from the compressor (not shown) and is subsequently cooled in the intercooler 16. Thereafter, air is supplied to the intake manifold 8 after its quantity is approximately adjusted by the intake butterfly valve 13. The air delivered to the intake manifold 8 is distributed through each branched pipe to each cylinder 2 and is burned with the fuel injected from the fuel injection valve 3. The outputs of the air flow meter 11 and the intake pressure sensor 23 are entered into an electronic control unit 30, which will be described later, and are used in feedback control of the EGR rate, and so on.

 The control system will be described below in the following.

 The control system is provided with advance correction which is called an ECU (electronic control unit) 30 comprising a MEM (read-only memory) 32, a MEV (random access memory) 33, a CPU (central unit) 34, an input terminal 35, and an output terminal 36.

 As with the outputs of the previously indicated sensors, the outputs of other sensors such as a load sensor 41, a crankshaft angle sensor 42, and a vehicle speed sensor 43 are input to the input terminal. 35 via corresponding A / D converters 37 or directly. The load sensor 41 is adapted to detect the amount of depression of the accelerator pedal 40, the crankshaft angle sensor 42 is adapted to detect the speed of rotation of the crankshaft la, and the vehicle speed sensor 43 is suitable for detecting vehicle speed supply. In addition, the

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 fuel injection valve 3, a reducer supply valve 61, the actuator 14 operational for driving the intake butterfly valve, the EGR valve 26 and other components are respectively connected to the output terminal 36 via corresponding driver circuits 38.

 MEM 32 stores programs for controlling the aforementioned devices and apparatus, control look-up tables to which reference is made when each control program is executed, and so on. MEV 33 stores an operating history of an internal combustion engine, in which the output signals input to the input terminal 35 from the respective sensors and the control signals input to the output terminal 36 are recorded. The CPU 34 is arranged for centralized control of the respective devices and apparatus. More specifically, the CPU 34, in operation, executes each control program, in which the sensor output signals recorded in the MEV 33 and the control look-up tables stored in the MEM 32 are compared with each other, to output a control signal, determined as a result of the comparison process, to a corresponding device or apparatus via the output terminal 36.

 The exhaust system comprises the exhaust manifold 18 and an exhaust pipe 19, and forms an exhaust gas passage through which the exhaust gas from each cylinder 2 is ejected out of the engine body. The exhaust system further includes an EGR system 20, a catalytic converter 52, a reducer supply device 60, and so on, and also functions as an emission control system for reducing the amount nitrogen oxides (NOx) and materials

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 particulate matter (for example soot) contained in the exhaust gas.

 The exhaust manifold 18 is connected to the exhaust terminals 18a provided at the respective cylinders 2 and forms a passage through which the exhaust gases ejected from the exhaust ports 18a converge and are then guided into the housing of the turbine 15b of the turbo compressor 15. Likewise, the exhaust pipe 19 forms a passage for the housing 15b of the turbine towards a silencer (not shown).

 The EGR system 20, like one of the emission control systems, has an EGR passage 25, an EGR valve 26, an EGR cooler 27, and so on, and is used to reduce the amount of nitrogen oxides (NOx) generated in the combustion of engine air and fuel. Besides this, the EGR passage 25 is a passage which communicates the exhaust manifold 18 and the intake manifold 8 bypassing the engine body. The EGR valve 26 is an electrically driven open-closed valve and is disposed in the EGR passage 25. The EGR valve 26 is adapted to adjust the amount of exhaust gas (EGR gas) flowing in the EGR passage 25 in accordance with the EGR rate control by UCE 30 and others. The EGR cooler 27 is used to cool the exhaust gases flowing in the EGR passage 25 using an engine coolant as a means of heat transfer.

 Thus, the EGR passage 25 forms a part of the exhaust gas recirculation passage, likewise, the EGR valve 26 serves as an exhaust gas recirculation valve. This is equivalent to saying that the ECU 30, an EGR rate control program incorporated therein, and so on, provides means for controlling the exhaust gas recirculation valve. at the same time, in the following description, the exhaust

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 circulating in the EGR passage 25 will be called "EGR gas" where appropriate.

 In the system, EGR 20 constructed as described above, part of the exhaust gas flowing in the exhaust manifold 18 is delivered in the EGR passage 25 at a flow rate corresponding to the amount of opening of the EGR valve 26. The EGR gas (the exhaust gas) delivered to the EGR passage 25 then enters the EGR cooler 27 disposed in an intermediate position of the EGR passage 25. After the EGR gas is cooled as it passes through the EGR cooler 27 , it is then delivered to the intake manifold 8. The EGR gas delivered to the intake manifold 8 circulates in each cylinder 2 while being mixed with the intake air delivered from a location upstream of the manifold intake 8 and is burned with fuel injected from each fuel injection valve 3.

 EGR gas (exhaust gas) contains an inert gas such as water vapor (H20) and carbon dioxides (COz). When the exhaust gas containing such inert gases is supplied to each cylinder 2 at the same time as the intake air, the combustion temperature decreases due to the inert gases, thereby suppressing the generation of oxides of nitrogen (NOx).

 Next, the catalytic converter 52 will be described. The catalytic converter 52 consists of a box 53 and catalysts 52a, 52b for purifying exhaust gases disposed towards the box 53. The catalytic converter 52 is suitable for purifying the exhaust gas ejected from the engine body by reducing the amount of harmful substances contained therein.

 More specifically, the catalytic converter 52 is disposed in the vicinity of the outlet orifice of the housing 15b of the turbine and comprises a catalyst 52a of NOx from the

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 storage-reduction type and a particle filter 52b housed in the box 53. The storage-reduction type NOx catalyst 52a is placed in the box 53 so as to be closer to the upstream side of the catalytic converter 52 than to the particle filter 52b. at the same time, in the following description, we will simply call the NOx catalyst 52a of the storage reduction type "the NOx catalyst 52a" where appropriate.

 The NOx catalyst 52a, as one of the exhaust gas purification catalysts, is able to reduce the amount of nitrogen oxides (NOx) mainly in the exhaust gas. The purification of exhaust gases by the NOx catalyst 52a will be more specifically described in the following. When the oxygen concentration in the exhaust gas is high, i.e. when the air-fuel ratio of the exhaust gas is poor, the NOx catalyst 52a absorbs the nitrogen oxides (NOx) contained in the exhaust gas. exhaust. Conversely, when the oxygen concentration in the exhaust gas is low, i.e. when the air-fuel ratio of the exhaust gas is rich, the NOx catalyst reduces the absorbed nitrogen oxides (NOx) by nitrogen dioxide (NO2) or nitrogen monoxide (NO) and release them into the exhaust gas. At this time, the nitrogen dioxide (N02) and the nitrogen monoxide (NO) released are oxidized with unburned fuel components (CO, HC) contained in the exhaust gas and are thus converted into vapor d non-harmful water (H20) and nitrogen dioxide (C02).

 The NOx catalyst 52a comprises a support formed of, for example, alumina (AIzOs), at least one element carried on the surface of the support is chosen from alkali metals such as potassium (K), sodium (Na), lithium (Li ) and cesium (Cs) an alkaline earth metal such as barium (Ba) and

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 calcium (Ca) and a rare earth metal such as lanthan (La) and yttrium (Y), and a noble metal such as platinum (Pt), also carried on the support.

 Regarding the purification of exhaust gases, in general, the combustion of a diesel engine is carried out in an oxygen-rich atmosphere. As a result, the oxygen concentration in the exhaust gas emitted during the combustion of the engine hardly decreases and the amount of unburned combustible components (CO, HC) is extremely low until the previously described reduction and release of the substances take place.

 In the embodiment, thus, the engine fuel (HC) serving as the reducing agent is injected into the exhaust gas to decrease the oxygen concentration and add hydrocarbons and the like as unburnt fuel components, to encourage purifying the exhaust gas, at the same time, the reducing agent is injected using the reducing gear supply device 60, as described later in detail.

 Furthermore, the particle filter 52b is able to oxidize (burn) the particulate matter such as soot, contained in the exhaust gas. more specifically, the particulate filter 52b includes a filter 58 carrying an active oxygen emitting agent so that the particulate matter trapped on the filter 58 is burned (oxidized) by the active oxygen and thus removed.

 The filter 58 is formed from a porous material such as cordierite and has a honeycomb pattern, as shown in FIG. 2, a plurality of passages (inlet and outlet exhaust gas passage) 55, 56 extending parallel to each other are formed in the filter 58. The lower end of each gas passage

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 incoming exhaust 55 is closed by a stop 55a while the upper end of each outlet exhaust gas passage 56 is closed by a stop 56a. The passages 55 and 56 are arranged in vertical and lateral directions of the filter 58, fine partitions 57 being interposed therebetween.

 A support layer formed of alumina (Al 2 O 3) or the like is provided on the surfaces of the partitions 57 and in pores of the partitions 57. In addition, an active oxygen-emitting agent is carried on the support as well as the catalyst. noble metal formed from platinum (Pt) or the like. The active oxygen emitting agent has a property of adsorption of oxygen when an excessive quantity of oxygen is present around it and of release of the oxygen adsorbed after its conversion into active oxygen when the concentration of oxygen decreases.

 The active oxygen emitting agent consists of at least one element chosen from an alkali metal such as potassium (K), sodium (Na), lithium (Li) cesium (Cs) and rubidium (Rb), a metal alkaline earth such as barium (Ba), calcium (Ca) and strontium (Sr), a rare earth metal such as lanthan (La) and yttrium (Y) and a transition metal such as cerium (Ce) and tin ( Sn).

 At the same time, it is preferable to use, as active oxygen emitting agent, an alkali metal or an alkaline earth metal such as potassium (K), lithium (Li), cesium (Cs), rubidium (Rb) , barium (Ba) and strontium (Sr), which has a higher iconicity than calcium (Ca).

 In the particulate filter 52b constructed as previously described, the exhaust gas passes through the incoming exhaust gas passage 55, the partitions 57 and the outgoing exhaust gas passage 56 as shown by the arrows a at the Fig. 2. When the exhaust gas passes through the partitions 57, the

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 particulate matter in the exhaust gas, such as soot, is trapped on the surface and inside each separation. The particulate matter trapped in the separations 57 is oxidized (burned) and thus removed from the filter 58 without emitting light flames due to the presence of active oxygen, the amount of which has been increased as the concentration of oxygen in the exhaust gas. circulating in the partitions 57 has been changed multiple times.

 In the embodiment, as described above, the amount of particulate matter contained in the exhaust gas, such as soot, is reduced by providing the storage-reduction type NOx catalyst 52a and the particle filter 52b in the exhaust passage. In the embodiment, the NOx catalyst 52a and the particle filter 52b are arranged in series as described above. This arrangement is mainly intended to allow the temperature of the particle filter 52b to be increased by using the heat of reaction generated in the oxidation and reduction reactions in the NOx catalyst 52a and to use the active oxygen, which has been emitted as a result of oxidation and reduction reactions in the NOx catalyst 52a, for the purification of exhaust gases by the particle filter 52b. at the same time, as is apparent from the foregoing descriptions, the NOx catalyst 52a carries a substance or substances which are the same or similar to those used as an active oxygen emitting agent. Thus, the NOx catalyst 52a can be judged to be capable of functioning as an active oxygen emitting agent. Also in the embodiment, an oxidation catalytic converter 59 which oxidizes and thereby removes unburnt fuel components contained in the gas

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 exhaust is arranged downstream of the catalytic converter 52.

 The reducer supply device 60 includes the reducer supply valve 61, a reducer supply passage 62, a fuel pressure control valve 64, a fuel pressure sensor 63, a shutoff valve d 66, etc., and is adopted to deliver an appropriate amount of the reducing agent (engine fuel) to a portion of the exhaust gas passage upstream of the catalytic converter 52, thereby serving as a means of supplying reducer.

 The reducer supply valve 61 is an electrically driven open-close valve and is driven to open when a predetermined voltage is applied. The reducer supply valve 61 is arranged in a part of the exhaust manifold 18 at which the exhaust gases ejected from the respective cylinders converge. The reducer supply passage 62 guides part of the fuel supplied by the fuel pump 6 to the reducer supply valve 61. The fuel pressure control valve 64 is arranged in an intermediate part of the supply passage reducer 62 and is adapted to maintain the fuel pressure in the reducer supply passage 62 at a predetermined pressure. The fuel pressure sensor 63 is adapted to detect the fuel pressure in the reducer supply passage 62. The emergency cut-off valve 66 can be implemented to stop the supply of fuel to the passage of reducer supply 62 when an abnormality occurs in the pressure of the reducer supply passage 62.

 In the reducer supply device 60 constructed as described above, the fuel ejected at

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 from the fuel pump 6 is first adjusted to a predetermined pressure by the fuel pressure control valve 64 and is thereafter supplied to the reducer supply valve 61 via the supply passage reducer 62. When a predetermined voltage is applied to the reducer supply valve 61 to open, the fuel in the reducer supply passage 62 is injected into the exhaust manifold 18 via the reducer supply 61. The fuel (reducing agent) supplied to the exhaust manifold 18 is then agitated within the turbo compressor 15 and is subsequently supplied to the catalytic converter 52 via the exhaust pipe 19.

Thus, the exhaust gas having a low concentration of oxygen and comprising hydrocarbons (HC) as unburned combustible components is delivered into the catalytic converter 52 thereby promoting the purification of the exhaust gases described above.

 At the same time, one of the conditions for delivering the reducing agent requires increasing the bed temperature of the catalytic converter 52 to its activation temperature. This amounts to saying that, in a case where the reducing agent is delivered when the bed temperature is lower than the activation temperature, it is feared that the reducing agent will not work effectively but, moreover, will cool the catalytic converter 52.

 When the engine is running effectively, however, a sufficient temperature of the catalytic converter 52 cannot be ensured in many cases. For example, in a case such as when the vehicle is traveling at low speed in a cold area, when the engine is idling for a long time, immediately after the engine starts, and when the vehicle is traveling at low speed on a long descent, the temperature of the exhaust gas circulating in the

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 catalytic converter 52 becomes low, by this effect, the temperature of the catalytic converter 52 decreases.

This amounts to saying that, when the temperature of the air circulating in the combustion chambers is low and when the quantity of fuel consumed in the combustion of the engine is low, the temperature of the exhaust gas decreases, thus the temperature of the bed of the catalytic converter 52 moves away from the activation temperature.

 To promote purification of the exhaust gas by oxidizing (burning) particulate matter such as soot, it is necessary to maintain the temperature of the catalytic converter 52 at approximately 500 C or higher. In general, however, the exhaust gas temperature of a diesel engine is much lower than this target bed temperature. Thus, in a diesel engine, the temperature of the catalytic converter 52 cannot be maintained at 500 C or more by using the heat energy of the exhaust gas in a normal operating state.

 In the internal combustion engine in the embodiment, "a catalyst preheating control" is performed when the bed temperature of the catalytic converter 52 has not yet reached its activation temperature and, in addition, "a low temperature combustion control "is carried out when it is necessary to increase the temperature of the bed of the catalytic converter 52 to 5000 C or more. In the low temperature combustion advance correction control, the supply of the reducing agent and the preheating of the catalyst control are implemented in a combined manner, in order to allow the purification of exhaust gases even in the severe engine condition described above.

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 The contours of the catalyst preheating control and the low temperature combustion control will be described below. In the preheating control of the catalyst, the quantity of air to be supplied to the combustion chambers is greatly reduced when the quantity of EGR gas to be introduced into the combustion chamber is increased, thereby allowing the engine to operate with a high EGR rate.

In this control, moreover, a secondary fuel injection is carried out in a subsequent stage of combustion to greatly increase the temperature of the exhaust gas and thereby increase the temperature of the catalytic converter 52 immediately. In summary, this command is an engine command which reduces the opening quantity of the intake butterfly valve 13 while increasing that of the EGR valve 26 and performs the secondary fuel injection in a later stage of the combustion in order to greatly increase the temperature of the exhaust gas.

 With regard to the combustion of the engine with a high EGR rate, when the engine is operated with a high EGR rate, the air-fuel ratio of the exhaust gas becomes rich and the temperature of the exhaust gas increases accordingly. By using the exhaust gas whose temperature has thus been increased, the temperature of the exhaust gas purification catalyst can be immediately increased.

 As mentioned in the previous description concerning the conventional system, when the system operates with a high EGR rate, a misfire or a decrease in the compression pressure occurs due to an insufficient quantity of air (oxygen) and an amount of insufficient total gas, the combustion of the engine becomes, consequently, unstable. As a result, various feedback commands are made to

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 achieve and maintain a desired EGR rate and thereby ensure stable engine combustion. EGR rate feedback commands will be described in detail later.

 The low temperature combustion control is also an engine control which reduces the opening quantity of the intake butterfly valve 13 disposed in the intake pipe 9 while increasing that of the EGR valve 26 in order to ensure engine operation with a high EGR rate. In this command, moreover, the reducing agent is delivered into the exhaust gas using the reducing device 60 to cause the generation of heat from the catalytic converter 52 and thereby increase the temperature of the bed. from the catalytic converter 52 immediately.

 In accordance with the embodiment, to obtain engine operation with a high EGR rate, the EGR rate is greatly modified to preheat the catalytic converter 52 as described above.

 Fig. 3 is a flowchart showing a control program which is carried out when an indicator indicative of a requirement to recover the catalytic converter 52 (the particle filter) is ACTIVE namely when it is required to remove particulate matter such as soot. Hereinafter, an EGR rate control in the embodiment will be described based on the preheating control of the catalyst and the low temperature combustion control with reference to FIG. 3.

 First of all, the ECU 30 determines in step S101 whether or not to carry out the preliminary heating control of the catalyst on the basis of the outputs of the respective sensors when the recovery indicator has been activated. In this step, more specifically, the ECU 30 determines whether the conditions for carrying out the heating control

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 The catalyst prerequisite was satisfied based on the engine rotation, the amount of throttle opening, the exhaust gas temperature, and so on, detected by the corresponding sensors.

 The conditions for carrying out the preheating of the catalyst control require: (1) the speed of rotation of the engine is equal to or less than a predetermined speed of rotation and the amount of depression of the accelerator pedal 40 is equal to or less than a predetermined amount (i.e. the engine load is low) and (2) the temperature of the exhaust gas flowing out of the catalytic converter 52 is equal to or less than a first set temperature (i.e., the catalytic converter 52 n 'has not been activated). When conditions (1) and (2) are both satisfied, the preheating control of the catalyst is performed to increase the temperature of the catalytic converter 52 (step S102).

 Here, the previous requirement intended to ensure the low engine load (condition 1) is carried out for the need to close the intake butterfly valve 13 of the intake pipe 9 during the prior heating control of the catalyst. This amounts to saying that if the intake throttle valve 13 is closed when the engine is operating under a high load, it can induce a significant reduction in the engine output, leading to a torque shock. Besides this, the requirement of ensuring that the temperature of the exhaust gas flowing out of the catalytic converter 52 is equal to or lower than the first established temperature (condition 2) is fulfilled for the following reason. Namely, in a case where the temperature of the exhaust gas flowing out of the catalytic converter 52 is sufficiently high, it is not necessary to carry out the prior heating control of the

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 catalyst because the catalytic converter 52 has been activated. In the embodiment, the first established temperature is set at approximately 150 C. thus, in the embodiment, when the temperature of the exhaust gas is lower than 150 C, the ECU 30 determines whether it is necessary to '' perform the preheating of the catalyst.

 Next, in step S102, the opening amount of the EGR valve 26 is adjusted to a predetermined amount and the opening amount of the intake butterfly valve 13 is reduced while monitoring the output of the pressure sensor intake 23 so as to control the EGR rate and thus obtain engine operation with a high EGR rate. This amounts to saying that a feedback control of the intake throttle valve 13 is carried out to obtain a desired EGR rate by using the pressure in the intake pipe 9 as a parameter.

 As previously described, the EGR rate is controlled using the intake throttle valve 13, rather than using the EGR valve 26. This is due to the following reasons: (1) the EGR rate can be greatly changed without changing the quantity of total gas (amount of EGR gas + amount of air) more than necessary (2) the amount of EGR gas introduced can be relatively increased, etc.

Likewise, the pressure in the intake pipe 9 is used as a parameter due to the following reason: (3) sufficient reliability of the recirculation control can be ensured.

 The EGR rate is defined as "amount of EGR gas / (amount of EGR gas + amount of air)", therefore, it is possible to obtain a desired EGR rate by changing the relative ratio between the amount of EGR gas and the amount of air. With a small amount of air, i.e. with a high EGR rate, even if the amount of gas

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 EGR is modified in the order, it influences difference the EGR rate to change.

 On the contrary, when the amount of air is controlled, the EGR rate changes greatly in synchronization with changes in the amount of air. This amounts to saying that the EGR rate can be greatly modified by slightly changing the amount of air and that it is therefore possible to obtain a desired EGR rate without reducing the total amount of gas to be used by the combustion of the motor (corresponding to the reason (1) described above). Thus, sufficient compression pressure can be ensured in compression processes so that problems resulting from reduced compression pressure, such as misfire and reduced engine output, can be avoided.

 From a difference point of view from the description above, when the EGR rate is modified by controlling the EGR valve 26, its quantity of control needs to be large. Consequently, it is inevitable in some cases to significantly reduce the amount of EGR gas, which can hinder a sufficient increase in the temperature of the exhaust gases. Furthermore, when the EGR rate is modified using the intake throttle valve 13, a desired EGR rate can be obtained without reducing the quantity of EGR gas introduced (quantity of EGR gas). When the EGR rate is controlled using the intake throttle valve 13, therefore, the amount of EGR gas (the amount of EGR gas introduced) can be relatively increased compared to when the EGR rate is controlled using the EGR valve 26 (corresponding to the reason (2) described above), it is thus possible to further increase the temperature of the exhaust gas.

 With regard to the reliability of the feedback control, when the prior heating control of the

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 catalyst is carried out, the amount of air circulating in the intake pipe 9 decreases greatly as the amount of opening of the intake butterfly valve 13 reduces. At this instant, the air flow decreases below the measurement limit of the air flow measurement device 11. In this case, the precision of the estimation of the amount of air by using the device for measuring the air flow 11 decreases, it therefore becomes difficult to precisely control the EGR rate. In the embodiment, however, the amount of air required in controlling the EGR rate is estimated based on the pressure change in the intake pipe 9, rather than depending on the air flow, the EGR rate feedback control is therefore precisely performed even when the catalyst is heated beforehand when the amount of air is low (corresponding to reason 3 described above).

In the embodiment, therefore, a predetermined intake amount, so called in the invention, is set to an air intake amount obtained as the opening amount of the inlet throttle valve 13 is reduced.

 In the embodiment, at the same time, the intake pressure sensor 23 is adapted to measure the pressure in the intake pipe 9 using the pressure in a vacuum state (an absolute pressure) as its pressure. reference, therefore, sufficient measurement accuracy can be obtained even if the air flow is very low. In the embodiment, moreover, changes in ambient pressure can be reflected in the estimation of the amount of air, therefore, significantly high accuracy can be obtained compared to when the amount of air is estimated. based on the output of the air flow meter 11. Thus, the combustion of the engine can be kept stable

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 even during engine combustion with a high EGR rate, when the engine is forced to operate under severe conditions as previously described.

 As previously described, in step S102, by performing the feedback control of the intake throttle valve 13 based on the pressure of the intake pipe 9, a desired EGR rate is obtained without reducing the amount total gas to be used in engine combustion and the amount of EGR gas introduced. As a result, even when the amount of air to be used in the combustion of the engine is significantly reduced, stable engine combustion can be ensured and a decrease in the exhaust gas temperature can be suppressed.

 Subsequently, when it is determined in step S103 that the temperature of the catalytic converter 52 has reached a second established temperature, the ECU 30 advances to step S104 to perform a low temperature combustion command. When it is determined in step S103 that the temperature of the catalytic converter 52 has not yet reached the second established temperature, conversely, the ECU 30 continues to carry out the preheating control of the catalyst by repeating the 'step S102. Surely, the second set temperature needs to be set higher than the first temperature. In the embodiment, the second set temperature is set at about 180 C, which is presumably high enough to ensure that the temperature of the catalytic converter 52 has reached its activation temperature.

 In step S104, the feedback control of the intake throttle valve 13 is performed based on the pressure of the intake pipe 9 in step S102 described above. The advantage obtained by performing the feedback control of the intake butterfly valve 13 is as mentioned in the description.

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 previous concerning the control of the preheating of the catalyst and, therefore, will not be described again.

 The ECU 30 then determines in step S105 whether the temperature of the exhaust gas flowing out of the catalytic converter 52 has reached a third established temperature, a temperature at which particulates such as soot can be oxidized and thus removed. If the exhaust gas temperature has reached the third set temperature, an appropriate amount of the reducing agent for reducing the particulate matter is supplied to the exhaust gas to oxidize (burn) the particulate matter (step S106).

 In the first control, as described above, the pre-heating control of the catalyst and the low-temperature combustion control are carried out to oxidize (burn) and thus remove the particulate matter trapped on the catalytic converter 52 by increasing the temperature of the converter catalytic 52 immediately.

 After step S106, an SOx poisoning recovery command is performed in step S107 and the program is then finished. "SOx poisoning" is a phenomenon where sulfur oxides (SOx) contained in the exhaust gases are converted to hydrosulphates (BaS04) and are accumulated in the catalytic converter 52 thus reducing its exhaust gas purification capacity.

 The SOx poisoning recovery command consists in recovering the exhaust gas purification capacity of the catalytic converter 52 by releasing the hydrosulfates (BaSO 4) which are accumulated there. To release hydrosulfates (BaS04). the reducing agent is delivered into the exhaust gas while maintaining the temperature

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 from the catalytic converter 52 to 500 C or more. In SOx poisoning control, more specifically, the pyrolysis of hydrosulphates (BaSO4) is accelerated and the pyrolyzed hydrosulphates (BaSO4) are released by being gasified via reaction with a reducing agent such as hydrocarbons (HC) and carbon monoxide (CO). In the control program, i.e., the SOx poisoning recovery control is performed using the temperature of the catalytic converter 52 made equal to or greater than 500 C by the low temperature combustion control. At the same time, step S105 does not always need to be carried out and, therefore, the need for this method can be determined when appropriate, therefore, the need for this method can be determined when appropriate.

 The previously described steps are implemented if a positive determination is made in step S101.

If a negative determination is made at this stage, conversely, the ECU 80 implements methods as follows.

 First, when a negative determination is made in step S101, the ECU 30 determines in step S108 whether or not to perform the low temperature combustion control.

 In step S108, the low combustion command is determined to be executed when the following execution conditions are satisfied: (1) the engine speed is equal to or less than a predetermined speed and the amount depressing the accelerator pedal 40 is equal to or less than a predetermined amount (i.e., the engine load is low) and (2) the temperature of the exhaust gas ejected from the catalytic converter 52 has reached the second established temperature (i.e. the catalytic converter

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effectuées comme décrit précédemment. 52 has been activated). When these conditions (1) and (2) are both satisfied, the ECU 30 advances to step S104 to effect the combustion control at low temperature and deliver the reducing agent to oxidize and thereby remove particulate matter. This amounts to saying that the methods of steps S104 to S106 are

performed as described above.

Conversely, when a negative determination is made in step S108, this means that the ECU 30 judges that either the preheating control of the catalyst or the low combustion control is not necessary or is difficult to perform. In this case, the ECU 30 implements the EGR command corresponding to the normal operating state (step S109) without carrying out either the command for heating the catalyst beforehand or the command for combustion at low temperature and, thereafter, ends the program.

 The preceding "EGR command corresponding to the normal operating state" is a command intended to obtain an appropriate EGR rate in order to suppress the generation of nitrogen oxides (NOx).

 The target EGR rate in this EGR control is low compared to that obtained during the preheating control of the catalyst and the combustion control at low temperature.

 In normal operating condition, the intake butterfly valve 13 is substantially completely open to ensure that a sufficient amount of intake air is supplied to the respective combustion chambers, and the amount of valve opening EGR 26 is corrected while monitoring the output of the air flow meter 11 so as to obtain a desired EGR rate. This means that a feedback control of the EGR valve 26 is carried out using the air flow

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 circulating in the intake pipe 9 as a parameter to obtain the desired EGR rate.

 In normal operating condition, the EGR rate is controlled using the EGR valve 26 for the following reasons: (1) the EGR rate can be greatly changed without reducing the amount of air to be used in the combustion of the engine, ( 2) the amount of air to be used in the combustion of the engine can be relatively increased, etc.

Likewise, the pressure in the intake pipe 9 is used as a parameter for the following reason: (3) the response of the feedback control can be improved.

 As described above, the EGR rate is defined as "quantity of EGR gas / (quantity of EGR gas + quantity of air)". In a case where a sufficient amount of air is supplied (when the target EGR rate is low), a large amount of change in the EGR rate can be obtained by controlling the amount of EGR gas compared to that obtained by controlling the amount of air (corresponding to the reason (1) described above).

 From a point of view different from the preceding description, when the EGR rate is modified by controlling the intake throttle valve 13, its quantity of control needs to be large. Thus, in some cases, the air flow rate to be used in the combustion of the engine can be greatly reduced and it can cause the generation of smoke or the like. On the other hand, when the EGR rate is changed using the EGR valve 26, a desired EGR rate can be obtained without reducing the amount of air. thus, when the EGR rate is controlled using the EGR valve 26, the amount of air is relatively increased compared to when the EGR rate is controlled using the intake butterfly valve 13 (corresponding to the reason (2) described previously), and it is therefore possible to reduce the generation of smoke or the like.

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 Hereinafter, the response characteristic of the feedback control will be described. In general, the feedback control based on the output of the air flow meter 11 provides a good control response, since the amount of air is directly calculated from the air flow. In the feedback control based on the output of the intake pressure sensor 23, on the other hand, the amount of air is indirectly calculated by correcting the pressure in the intake pipe 9. Its response characteristic becomes, by consequence, somewhat worse than that of the feedback control based on the output of the air flow meter 11 when the air flow is high.

Thus, in the normal operating state of the engine when the air flow is high and the engine rotation speed changes greatly and frequently, the feedback control based on the output of the air flow meter 11 provides a additional precision (corresponding to reason 2 described above).

 In step S109, therefore, the feedback control of the EGR valve 26 is performed based on the flow of air flowing in the intake pipe 9 to obtain a desired EGR rate without reducing the amount to be used in combustion of the engine and, thereby, further optimization of the EGR rate control can be obtained.

 It should be understood that the previously described program is only an embodiment of the invention, therefore, various changes can be made to the details of the program when appropriate.

In the program described above, for example, the EGR rate control is performed on the basis of the feedback control by the use of the intake butterfly valve 13 when starting either the prior heating control of the catalyst or control of combustion at low temperature. However,

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 since the EGR rate gradually changes over time after the start of control of the intake butterfly valve 13 and of the EGR valve 26, the timing at the level of switching between the feedback control using the butterfly valve d The intake 13 and the intake using the EGR valve 26 can be corrected in accordance with changes in the EGR rate which occur over time.

 The program can also be changed as follows.

The amount of air to be used in an actual combustion of the engine is estimated on the basis of the air flow circulating in the intake passage, the amount introduced of EGR gas, the target EGR rate, the time elapsed after switching the combustion mode, and so on. With such an arrangement, the ECU 30 is arranged to start the control of the EGR rate on the basis of the feedback control using the intake butterfly valve 13 when the estimated quantity of air has reached a predetermined value. This is equivalent to saying that details of the previously described control program can be changed as long as the EGR rate control can be performed based on the feedback control using the intake throttle valve 13 when the amount of air delivered to the combustion chambers reaches or becomes less than the predetermined amount.

 While the invention is applied to a diesel engine in the embodiment as an example, it should be understood that the invention is not confined to use in diesel engines but can be used in other engines internal combustion systems which include an EGR system and an intake butterfly valve and which need to be operated with a greatly reduced amount of air available for combustion of the engine.

Likewise, the intake butterfly valve 13 is used as an example means to reduce the amount

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 of admission into the embodiment. However, if a variable displacement turbo compressor is provided, the intake quantity can be controlled using the turbo compressor in addition to, or instead of, the intake throttle valve 13.

(Second embodiment)
Hereinafter, a second embodiment of the invention will be described below. In the embodiment, the correction of the synchronizations to inject the fuel is carried out in addition to the control of the EGR rate, in order to further stabilize the combustion of the engine. Here, whether or not to make the correction and determine on the basis of the air humidity applied to the respective combustion chambers.

 Fig. 4 is a flowchart showing a control program intended to correct the synchronization of the fuel injection. The correction of the timing of the fuel injection will be described below with reference to this flowchart. At the same time, while the correction of the timing of the fuel injection is carried out as an independent control in the second embodiment, it can be carried out as a part of the catalyst pre-heating control program and the low temperature combustion control.

 In this program, the ECU 30 firstly determines in step S201 whether at least one of the commands for the prior heating of the catalyst and the command for combustion at low temperature has been started. If a positive determination is made at this step, the ECU 30 advances to step S202 to estimate whether the humidity of the air to be supplied to the combustion chambers is equal to or less than a predetermined humidity. Conversely, if a negative determination is made at let S201, namely if the ECU 30 determines whether neither the prior heating command of the

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 catalyst or the low temperature combustion control must not be performed in accordance with this circumstance, the control program is complete.

 Regarding the estimation of humidity in step S202, an increase in humidity is determined when a wiper switch (not shown) is operated. More specifically, when the wiper switch is operated, the ECU 30 determines that the vehicle is moving in the rain and performs the process in the next step to correct the timing of the fuel injection.

 When the ECU 30 determines in step S202 a decrease in humidity based on the operation of the wiper switch, the estimation of the humidity can be performed by another arrangement of devices and d 'devices. By way of example, it is possible to estimate the humidity on the basis of the output of a humidity sensor and also to determine the humidity from the humidity on the road surfaces captured by an on-board road surface surveillance camera, or meteorological information transmitted via VICS (vehicle information communication system). This amounts to saying that the humidity detection means is only necessary to be able to detect a decrease in humidity. however, it is preferable to use the humidity detection means consisting of an on-board device or apparatus such as a wiper switch, a road surface surveillance camera, and the VICS.

 Next, the ECU 30 performs advance correction of the timing of the fuel injection so as to optimize the timing of the fuel injection in step S203, and then ends the program. The means for correcting the synchronization of the fuel injection in the second embodiment of the invention is

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 arranged as described above. At the same time, the humidity predetermined in the embodiment is equivalent to a temperature with which the combustion of the engine becomes easily unstable during the preheating control of the catalyst or the combustion control at low temperature.

 Likewise, if necessary, the advance correction of the timing of the fuel injection based on the humidity can be changed so as, for example, to advance the timing by a predetermined basic amount or to increase its amount of advance in accordance with humidity.

 In this command, as described above, the humidity advance correction is carried out when the amount of air circulating in the intake passage is low and the humidity is equal to or less than the predetermined amount such as during the pre-heating catalyst control or low temperature combustion control. Thus, a misfire resulting from the endothermic effect of humidities (water content) or a slow increase in compression pressure can be prevented and stable engine combustion can be ensured even when the engine is forced to operate under severe conditions such as during the preheating control of the catalyst or the low temperature combustion control.

 In the invention, therefore, stable combustion of the engine can be ensured even when the amount of air to be used in the combustion of the engine is significantly reduced due to change in the operating state of the engine. Likewise, it is possible to prevent the temperature of the exhaust gas from decreasing while ensuring a sufficiently stable combustion of the engine in an operating state of the engine where the temperature of the exhaust gas needs to be increased.

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 An emission control system for an internal combustion engine is provided, which includes an intake butterfly valve (13) and an EGR valve (26) and is adapted to obtain an EGR rate suitable for an operating condition required by performing feedback controls on the intake throttle valve (13) and the EGR valve (26). When the opening quantity of the intake throttle valve (13) is large and the intake quantity is consequently large, the intake throttle valve (13) is controlled in accordance with the outlet d '' an air flow meter (11). When the opening quantity of the intake throttle valve (13) is small and the intake quantity is consequently low, the control is switched to open or close the intake throttle valve (13 ) based on the output of an intake pressure sensor (23). Likewise, when the amount of air to be used in the combustion of the first is low and the humidity is high, an advance correction of the timing of the fuel injection is carried out.

Claims (8)

 CLAIMS 1. An emission control system for an internal combustion engine, comprising: intake amount detecting means (11) for detecting an intake amount; an intake butterfly valve (13) for controlling a flow of intake air flowing in an intake passage (8); first intake throttle valve control means (11,26,30, S109) for opening and closing the intake throttle valve (13) in accordance with the intake quantity detected by the intake detection means quantity of admission; an exhaust gas recirculation passage (25) for recirculating part of an exhaust gas ejected from an internal combustion engine (1) to an intake system of the internal combustion engine (1); an exhaust gas recirculation valve (26) for adjusting a flow rate of an exhaust gas flowing in the exhaust gas recirculation passage (25); exhaust gas recirculation valve control means (30) for opening and closing the exhaust gas recirculation valve based on the operating condition of the internal combustion engine (1); a storage-reduction type NOx catalyst (52a) disposed in an exhaust gas passage (19) of the internal combustion engine (1); <Desc / Clms Page number 48>  reducer supply means (60) for supplying a reducing agent to the NOx catalyst of the storage reduction type (52a); and determining means (30, S105) for determining whether or not to deliver a reducing agent based on the operating state of the internal combustion engine (1), the emission control system being arranged to delivering a reducing agent from the reducing agent supply means (60) when the determining means (30) determines to deliver the reducing agent, characterized in that it further comprises: pressure sensing means intake (23) adapted to detect pressure in the intake passage (8); and a second intake throttle valve control means (23,26, 30, S102, S104) for opening and closing the intake throttle valve (13) in accordance with a pressure in the intake passage ( 8) detected by the intake pressure detection means (23) when the intake quantity detected by the intake quantity detection means is less than a predetermined intake quantity. 2. Emission control system according to claim 1, characterized in that it further comprises: humidity detection means for detecting humidity in the intake passage (8); injection timing correction means (30, S203) for performing advance correction of a fuel injection timing of the internal combustion engine (1) when an intake amount <Desc / Clms Page number 49>  detected by the intake amount detecting means (11) is less than a predetermined intake amount and that the humidity detected by the humidity detecting means is equal to or greater than a predetermined humidity. 3. An emission control system for an internal combustion engine (1), comprising: an intake amount detecting means (11) for detecting an intake amount; an intake butterfly valve (13) for controlling a flow rate of an amount of air flowing in an intake passage (8); first intake throttle valve control means (11,26,30, S109) for opening and closing the intake throttle valve (13) in accordance with the intake quantity detected by the intake detection means quantity of admission; an exhaust gas recirculation passage (25) for recirculating a portion of the exhaust gases ejected from the internal combustion engine (1) to an intake system of the internal combustion engine (1); an exhaust gas recirculation valve (26) for adjusting a flow rate of an exhaust gas flowing in the exhaust gas recirculation passage (25); exhaust gas recirculation valve control means (30) for opening and closing the exhaust gas recirculation valve (26) based on an operating state of the internal combustion engine (1 ); <Desc / Clms Page number 50>  a storage-reduction type NOx catalyst (52a) disposed in an exhaust gas passage (19) of the internal combustion engine (1) a reducing agent supply means (60) for supplying a reducing agent to the NOx of the reduction storage type (52a); and determining means (30, S105) for determining whether or not to deliver a reducing agent based on the operating state of the internal combustion engine (1), the emission control system being arranged to delivering a reducing agent from the reducing supply means (60) when the determining means (30) determines to deliver the reducing agent, characterized in that it further comprises: a humidity detecting means for detecting humidity in the intake passage (8); injection timing correction means (30, S203) for effecting a timing correction of a fuel injection timing of the internal combustion engine (1) when the intake quantity detected by the detection means the intake amount (11) is less than a predetermined intake amount and that the humidity detected by the humidity detecting means is equal to or greater than a predetermined humidity. 4. Emission control system according to claim 2 or 3, characterized in that: the humidity detection means is adapted to detect humidity when a wiper is operating. <Desc / Clms Page number 51> 5. Emission control method for an engine

 internal combustion (1), comprising the steps of: detecting an intake amount (11); controlling the intake quantity by opening and closing an intake throttle valve (13) in accordance with the detected intake quantity; recirculating part of an exhaust gas ejected from the internal combustion engine (1) to an intake system of the internal combustion engine (1) via an exhaust gas recirculation passage (25 ); adjust the flow rate of an exhaust gas circulating in the

 exhaust gas recirculation passage (25) by means of an exhaust gas recirculation valve (26); opening and closing the exhaust gas recirculation valve (26) based on an operating condition of the internal combustion engine (1); determining whether or not to deliver a reducing agent based on the operating condition of the internal combustion engine (1); and delivering a reducing agent from the reducing means (60) to a storage-reduction type NOx catalyst (52a) when it is determined to deliver the reducing agent, the emission control method being characterized in that it further comprises the steps of: detecting a pressure in the intake passage (8); <Desc / Clms Page number 52>  detecting an intake pressure and controlling a flow of the intake air by opening and closing the intake butterfly valve (13) in accordance with the pressure detected in the intake passage (8) when the quantity d the detected admission is less than a predetermined admission amount; and controlling the intake air flow by opening and closing the intake throttle valve (13) in accordance with the intake amount when the detected intake amount is equal to or greater than the intake amount predetermined. 6. Emission control method according to claim 5, characterized in that it further comprises the steps consisting in: detecting a humidity in the intake passage (8); and carrying out a correction of advance of the injection timing of the fuel of the internal combustion engine (1) when the detected intake quantity is less than a predetermined intake quantity and the detected quantity is equal to or greater than one predetermined humidity. 7. An emission control method for an internal combustion engine (1), comprising the steps of detecting an intake amount (11); controlling the intake quantity by opening and closing the intake throttle valve (13) in accordance with the detected intake quantity; recirculating part of an exhaust gas ejected from the internal combustion engine (1) to a system <Desc / Clms Page number 53>  exhaust gas recirculation passage (25) by means of an exhaust gas recirculation valve (26); opening and closing the exhaust gas recirculation valve (26) based on an operating condition of the internal combustion engine (1); determining whether or not to deliver a reducing agent based on the operating condition of the internal combustion engine (1); and delivering a reducing agent from a reducing feed means (60) to a storage-reduction type NOx catalyst (52a) when it is determined to deliver the reducing agent, the control method of emission being characterized in that it further comprises the steps of: detecting humidity in the intake passage (8); carry out an advance correction of the fuel injection synchronization of the internal combustion engine (1) when the detected intake quantity is less than a predetermined intake quantity and the detected humidity is equal to or greater than one predetermined humidity.

 intake of the internal combustion engine (1) via an exhaust gas recirculation passage (25); adjust the flow rate of an exhaust gas circulating in the 8. A method of emission control according to claim 6 or 7, characterized in that: the humidity is detected when a wiper is operating.