Exhaust gas purification system for internal combustion engine

Abstract

When the temperature of the exhaust gas flowing into an NSR catalyst exceeds a specific threshold temperature that is determined on the basis of the cetane number of fuel in such a way that the specific threshold temperature is set lower when the cetane number of the fuel is low than when it is high, fuel is supplied to the exhaust gas by fuel supply device to perform an NO.sub.X reduction process for the NSR catalyst. If the quantity of heat generated in the NSR catalyst per unit time is smaller than a specific value while the NO.sub.X reduction process is being performed, the NO.sub.X reduction process in progress is suspended, and the NO.sub.X reduction process is performed later on when the temperature of the exhaust gas flowing into the NSR catalyst exceeds an updated threshold temperature higher than the specific threshold temperature.

Claims

1. An exhaust gas purification system for an internal combustion engine comprising: an NO.sub.X storage reduction catalyst provided in an exhaust passage of the internal combustion engine, which stores NO.sub.X in exhaust gas when the exhaust gas has a lean air-fuel ratio higher than a theoretical air-fuel ratio and desorbs NO.sub.X stored in it to let the desorbed NO.sub.X be reduced by unburned fuel in the exhaust gas when the exhaust gas has a rich air-fuel ratio lower than the theoretical air-fuel ratio; a fuel supply device for supplying fuel to the exhaust gas upstream of the NO.sub.X storage reduction catalyst; and a controller comprising at least one processor, wherein the controller configured to: determine, after fuel is supplied to a fuel tank of the internal combustion engine, a cetane number of the fuel used in the internal combustion engine after the supply of fuel; and perform an NO.sub.X reduction process by supplying fuel to the exhaust gas through the fuel supply device when an amount of NO.sub.X stored in the NO.sub.X storage reduction catalyst is larger than or equal to a specific NO.sub.X storage amount and a temperature of the exhaust gas flowing into the NO.sub.X storage reduction catalyst exceeds a specific threshold temperature that is determined on the basis of the cetane number of the fuel, the threshold temperature being set lower when the cetane number of the fuel is low than when the cetane number of the fuel is high, wherein when a quantity of heat generated in the NO.sub.X storage reduction catalyst per unit time is smaller than a specific value while the NO.sub.X reduction process is being performed, the controller suspends the NO.sub.X reduction process in progress, and performs the NO.sub.X reduction process later on when the temperature of the exhaust gas flowing into the NO.sub.X storage reduction catalyst exceeds an updated threshold temperature higher than the specific threshold temperature.

2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the controller is further configured to perform, when the air-fuel ratio of the exhaust gas to which fuel has been supplied by the fuel supply device is leaner than a specific air-fuel ratio suited to the NO.sub.X reduction process while the NO.sub.X reduction process is being performed, a fuel-increasing process of increasing a quantity of fuel supplied through the fuel supply device in the NO.sub.X reduction process so as to make the air-fuel ratio of the exhaust gas equal to the specific air-fuel ratio, wherein when the quantity of heat generated in the NO.sub.X storage reduction catalyst per unit time reaches or exceeds the specific value while the fuel-increasing process is being performed during the NO.sub.X reduction process, the controller continues the NO.sub.X reduction process with the fuel-increasing process being performed.

3. The exhaust gas purification system for an internal combustion engine according to claim 2, wherein when the quantity of heat generated in the NO.sub.X storage reduction catalyst per unit time is smaller than the specific value while the fuel-increasing process is being performed during the NO.sub.X reduction process, the controller suspends the NO.sub.X reduction process and the fuel-increasing process in progress and performs the NO.sub.X reduction process later on when the temperature of the exhaust gas flowing into the NO.sub.X storage reduction catalyst exceeds the updated threshold temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram showing the general configuration of an exhaust gas purification system for an internal combustion engine according to the present disclosure.

(2) FIG. 2 is a graph showing relationship between the cetane number of fuels used in an internal combustion engine and a threshold Tf of the exhaust gas temperature for starting an NO.sub.X reduction process that is determined on the basis of the cetane number.

(3) FIG. 3 is a flow chart of a cetane number setting process executed in the exhaust gas purification system for an internal combustion engine shown in FIG. 1.

(4) FIG. 4 is a flow chart of an NO.sub.X reduction control executed in the exhaust gas purification system for an internal combustion engine shown in FIG. 1 according to a first embodiment of the present disclosure.

(5) FIG. 5 is a flow chart of a process for setting the threshold of the exhaust gas temperature for starting the NO.sub.X reduction process.

(6) FIG. 6 is the first flow chart of the NO.sub.X reduction process executed in the exhaust gas purification system for an internal combustion engine shown in FIG. 1 according to a second embodiment of the present disclosure.

(7) FIG. 7 is the second flow chart of the NO.sub.X reduction process executed in the exhaust gas purification system for an internal combustion engine shown in FIG. 1 according to the second embodiment of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

(8) In the following, specific embodiments of the present disclosure will be described with reference to the drawings. The dimensions, materials, shapes, relative arrangements, and other features of the components that will be described in connection with the embodiments are not intended to limit the technical scope of the present disclosure only to them, unless particularly stated.

First Embodiment

(9) FIG. 1 shows the general configuration of an exhaust gas purification system for an internal combustion engine 1 according to the present disclosure. The internal combustion engine 1 shown in FIG. 1 is a diesel engine for driving a vehicle. The internal combustion engine operates by burning an air-fuel mixture having a lean air-fuel ratio higher than the theoretical air-fuel ratio. The internal combustion engine 1 is connected with an exhaust passage 2. The exhaust passage 2 is provided with an NSR catalyst 3, which stores NO.sub.X contained in the exhaust gas when the exhaust gas has a lean air-fuel ratio and desorbs NO.sub.X stored therein to let the desorbed NO.sub.X be reduced by unburned fuel in the exhaust gas when the exhaust gas has a rich air fuel-ratio. The exhaust passage 2 is also provided with a particulate filter 4 (which will be simply referred to as the filter hereinafter) that traps PM in the exhaust gas, which is arranged downstream of the NSR catalyst 3.

(10) The exhaust passage 2 is also provided with a fuel supply valve 5 arranged upstream of the NSR catalyst 3. The fuel supply valve 5 supplies fuel (unburned fuel) to the exhaust gas flowing into the NSR catalyst 3. The exhaust passage 2 is provided with a first temperature sensor 6 and a second temperature sensor 7. The first temperature sensor 6 is arranged upstream of the NSR catalyst 3 to measure the temperature of the exhaust gas flowing into the NSR catalyst 3, and the second temperature sensor 7 is arranged downstream of the NSR catalyst 3 to measure the temperature of the exhaust gas flowing out of the NSR catalyst 3. The exhaust passage 2 is provided with an air-fuel ratio sensor 8, which is arranged downstream of the NSR catalyst 3 to measure the air-fuel ratio of the exhaust gas flowing out of the NSR catalyst 3. The exhaust passage 2 is also provided with a differential pressure sensor 13, which are arranged near the filter 4. The differential pressure sensor measures the differential pressure of the exhaust gas across the filter 4, namely the difference between the exhaust gas pressure in the exhaust passage 2 upstream of the filter 4 and the exhaust gas pressure in the exhaust passage 2 downstream of the filter 4.

(11) The intake passage 15 of the internal combustion engine 1 is provided with an air flow meter 10 capable of measuring the flow rate of the intake air flowing in the intake passage 15. An electronic control unit (ECU) 20 is provided for the internal combustion engine 1. The ECU 20 controls the operation state of the internal combustion engine 1. The ECU 20 is electrically connected with the fuel supply valve 5, the first temperature sensor 6, the second temperature sensor 7, the air-fuel ratio sensor 8, the differential pressure sensor 13, the air flow meter 10, a crank position sensor 11, and an accelerator opening degree sensor 12. The fuel supply valve 5 controls supply of fuel to the exhaust gas according to instructions from the ECU 20. Measurement values of the sensors are given to the ECU 20. For example, the crank position sensor 11 measures the crank angle of the internal combustion engine 1 and sends it to the ECU 20. The accelerator opening degree sensor 12 measures the accelerator opening degree of the vehicle provided with the internal combustion engine 1 and sends it to the ECU 20. The ECU 20 calculates the engine speed of the internal combustion engine 1 from the measurement value of the crank position sensor 11 and calculates the engine load of the internal combustion engine 1 from the measurement value of the accelerator opening degree sensor 12. Moreover, the ECU 20 can determine the temperature of the exhaust gas flowing into the NSR catalysts 3 on the basis of the measurement value of the first temperature sensor 6 and estimate the temperature of the NSR catalyst 3 on the basis of the measurement value of the second temperature sensor 7. The ECU 20 can determine the state of deposition of PM in the filter 4 on the basis of the measurement value of the differential pressure sensor 13.

(12) The internal combustion engine 1 has a plurality of cylinders, each of which is provided with a fuel injection valve 1a. The fuel injection valve 1a has a pressure sensor (not shown) built in it, and the fuel injection pressure measured by the pressure sensor is sent to the ECU 20. The ECU 20 sends a drive command to an ejection drive unit (EDU) 21 on the basis of the engine load and the engine speed of the internal combustion engine 1 and the fuel injection pressure measured by the pressure sensor, and the fuel injection valve 1a is controlled by a drive signal sent from the EDU 21.

(13) Fuel for the internal combustion engine 1 is stored in a fuel rank 30 and supplied to the fuel injection valve 1a and the fuel supply valve 5 through fuel supply lines. The fuel tank 30 is provided with an opening and closing sensor 31 that senses opening and closing of a tank cap that is opened for refueling. The opening and closing sensor 31 informs the ECU 20 of the opening and closing status of the tank cap.

(14) In the exhaust gas purification system for the internal combustion engine 1 configured as above, NO.sub.X contained in the exhaust gas is generally stored into the NSR catalyst 3 to reduce emissions to the environment. Another catalyst for exhaust gas purification that is not shown in the drawings may be additionally provided. If the NO.sub.X storage amount of the NSR catalyst 3 (i.e. the amount of NO.sub.X stored in the NSR catalyst 3) increases to reach the saturation storage amount of the NSR catalyst 3 (i.e. the NO.sub.X storage amount that saturates the NO.sub.X storage capacity of the NSR catalyst 3), there arises a possibility that NO.sub.X that the NSR catalyst 3 does not afford to store is emitted to the external environment. To avoid such circumstances, when the NO.sub.X storage amount in the NSR catalyst 3 reaches or exceeds a predetermined NO.sub.X storage amount, an NO.sub.X reduction process is performed to desorb and reduce NO.sub.X stored in the NSR catalyst 3. In the NO.sub.X reduction process, specifically, a predetermined quantity of fuel is supplied to the exhaust gas by the fuel supply valve 5 to make the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 3 rich, whereby NO.sub.X stored in the NSR catalyst 3 is desorbed and reduced. The predetermined NO.sub.X storage amount is set to a value equal to the saturation storage amount minus a certain margin.

(15) When fuel is supplied to the exhaust gas by the fuel supply valve 5 for the NO.sub.X reduction process, it is necessary that NO.sub.X be satisfactorily reduced in the NSR catalyst 3 by the supplied fuel. If the reduction reaction does not progress satisfactorily, there can arise troubles such as a decrease in the activity of the NSR catalyst 3 due to adhesion of supplied fuel to the NSR catalyst 3. Volatility of fuel is important in order for the reduction of NO.sub.X by fuel to progress satisfactorily in the NSR catalyst 3. More specifically, the higher the volatility of fuel is, the more the reduction reaction is likely to progress satisfactorily in the NSR catalyst 3. If the volatility of fuel is high, fuel can be supplied to the exhaust gas without causing the aforementioned trouble even if the temperature of the NSR catalyst 3 is relatively low, namely even if the temperature of the exhaust gas flowing into the NSR catalyst 3 is relatively low. Then, the chance of performing the NO.sub.X reduction process is increased.

(16) Given the above-described relationship between the volatility of fuel and the reduction reaction in the NSR catalyst 3, the exhaust gas purification system for the internal combustion engine 1 according to the present disclosure is configured to adjust a threshold temperature Tf according to the cetane number of fuel, which correlates with the volatility of fuel to some extent. The threshold temperature Tf is a threshold of the exhaust gas temperature (namely, the temperature of the exhaust gas flowing into the NSR catalyst 3) for performing the NO.sub.X reduction process. Generally speaking, the smaller the cetane number is, the higher the volatility of fuel tends to be. Therefore, in the exhaust gas purification system for the internal combustion engine 1 according to the present disclosure, the smaller the cetane number of fuel is, the smaller the value of the threshold temperature Tf is set, as shown in FIG. 2. By setting this relationship between the threshold temperature Tf for the NO.sub.X reduction process and the cetane number, the value of the threshold temperature Tf is set small when a fuel having high volatility (i.e. having a small cetane number) is supplied to the internal combustion engine 1, because the reduction of NO.sub.X by fuel in the NO.sub.X catalyst 3 tends to progress more excellently with such a fuel. Thus, it is possible to increase the chance of performing the NO.sub.X reduction process.

(17) Now, setting of the cetane number of fuel needed to carry out the NO.sub.X reduction process according to the cetane number of fuel will be described with reference to FIG. 3. FIG. 3 is a flow chart of the process of setting the cetane number of fuel used in the internal combustion engine 1. This cetane number setting process is a control process that is performed repeatedly by execution of a certain control program in the ECU 20. Firstly in step S101, it is determined whether or not the determination of the cetane number by the cetane number determination process executed in step S106 (described later) has been undone since the supply of fuel to the fuel tank 30 (or refueling). In detecting the supply of fuel to the fuel tank 30, it may be determined that the supply of fuel to the fuel tank 30 is performed when the opening and closing sensor 31 detects the opening of the tank cap. Alternatively, the supply of fuel may be detected by measuring the amount of fuel contained in the fuel tank 30 by another sensor. When the supply of fuel to the fuel tank 30 is performed, there is a possibility that the cetane number of the fuel in the fuel tank 30 may be different from the cetane number that has been known before, namely there is a possibility that the cetane number of the fuel becomes unknown. In the case where the supply of fuel to the fuel tank 30 was performed and the determination of the cetane number by the cetane number determination process in step S106 has been undone after that, an affirmative determination is made in step S101, and the process proceeds to step S102. In the case where the determination of cetane number has been done, a negative determination is made in step S101, and this cetane number setting process is terminated.

(18) In step S102, since the cetane number of fuel after the supply of fuel is unknown, a standard cetane number is set as a provisional cetane number of fuel. The standard cetane number may be set to a value, in the range of cetane numbers that common fuels available in the market can have (e.g. between 45 and 60), that makes the threshold temperature Tf as high as possible in order to avoid adhesion of fuel to the NSR catalyst 3, namely the highest cetane number in that range. Alternatively, the standard cetane number may be set to the cetane number used before the supply of fuel is performed this time, namely the cetane number determined by the cetane number determination process performed after the previous time fuel supply. After the completion of the processing of step S102, the process proceeds to step S103.

(19) In step S103, it is determined whether or not the fuel in the fuel supply line (including the common rail) between the fuel tank 30 and the fuel injection valve 1a has been consumed completely. It is probable that the fuel of the kind used before the supply of fuel to the fuel tank 30 is remaining in the fuel supply line. As will be described later, in the cetane number determination process in step S106, the cetane number of the new fuel is determined on the basis of the rotational fluctuation that occurs when a small quantity of fuel is injected through the fuel injection valve 1a. Therefore, it is undesirable that the old (or previous kind of) fuel is remaining in the fuel supply line when the cetane number determining process is performed. In step S103, if the integrated amount of fuel consumed by operation of the internal combustion engine 1 after the supply of fuel to the fuel tank 30 exceeds the amount equal to the capacity of the fuel supply line, it may be determined that the fuel in the fuel supply line has been consumed completely. If an affirmative determination is made in step S103, the process proceeds to step S104. If a negative determination is made, the cetane number setting process is terminated.

(20) In step S104, it is determined whether or not fuel cut process is being performed in the internal combustion engine 1. The fuel cut process is the process of suspending the fuel injection by the fuel injection valve 1a, for example, during deceleration of the internal combustion engine 1. If an affirmative determination is made in step S104, the process proceeds to step S105. If a negative determination is made, the cetane number setting process is terminated.

(21) In step S105, it is determined whether or not conditions for determining the cetane number of fuel are met. Specifically, the conditions for determining the cetane number are met when the following two conditions are both met. The first condition is that the high load operation or the low load operation was not performed in the internal combustion engine 1 immediately before the fuel cut process is performed. In the case where the high load operation was performed immediately before the fuel cut process, the temperature of the inner wall of the cylinder of the internal combustion engine 1 during the fuel cut process is relatively high. Then, an environment in which fuel is more apt to be vaporized due to external factors is established. This is not an environment that allows accurate determination of the cetane number, which relates to the volatility of fuel. In the case where the low load operation was performed immediately before the fuel cut process, the temperature of the inner wall of the cylinder of the internal combustion engine 1 during the fuel cut process is relatively low. Then, an environment in which fuel is less apt to be vaporized due to external factors is established. This is not an environment that allows accurate determination of the cetane number, which relates to the volatility of fuel. Hence, in the above cases, it is determined that the condition for determining the cetane number is not met, because it is difficult to determine the intrinsic volatility of the fuel accurately in these cases.

(22) The second condition is that the engine speed of the internal combustion engine 1 is lower than a standard engine speed. As will be described later, in the cetane number determination process in step S106, the cetane number is determined on the basis of the rotational fluctuation that occurs when a small quantity of fuel is injected through the fuel injection valve 1a. Therefore, when the engine speed is excessively high at the time of determination, it may be difficult to measure the rotational fluctuation. For this reason, the second condition about the engine speed is set. If it is determined in step S105 that the conditions for determining the cetane number including the first and second conditions are met, the process proceeds to step S106. If it is determined in step S105 that the conditions for determining the cetane number including the first and second conditions are not met, the cetane number setting process is terminated.

(23) In step S106, the cetane number determination process is performed. Specifically, a very small quantity of fuel is injected by the fuel injection valve 1a in one or some of the plurality of cylinders in the internal combustion engine 1 while the fuel cut process is being performed, and the cetane number is calculated on the basis of the fluctuation of the engine speed (or rotational fluctuation) with the combustion of the fuel thus injected. Since the quantity of fuel injected to determine the cetane number is very small, the quantity is controlled using the fuel pressure measured by the pressure sensor provided in the fuel injection valve 1a as described above. When measuring the rotational fluctuation, the fuel injection timing is adjusted in such a way that the ignition of fuel occurs at a specific time, and the cetane number is calculated larger when the rotational fluctuation is large than when it is small. After the completion of the processing of step S106, the process proceeds to step S107.

(24) In step S107, it is determined whether or not the determination of the cetane number of the fuel has been done by the above-described cetane number determination process. If an affirmative determination is made in step S107, the process proceeds to step S108. If a negative determination is made, the cetane number setting process is terminated. In step S108, the cetane number of the fuel used in the internal combustion engine 1 after the supply of fuel is set to the cetane number determined by the above-described cetane number determination process, and then the cetane number setting process is terminated.

(25) As above, in the cetane number setting process shown in FIG. 3, after the supply of fuel to the fuel tank 30 is performed in the internal combustion engine 1, the standard cetane number is set as a provisional cetane number of the fuel until the completion of the cetane number determination process. After the completion of the cetane number determination process, the cetane number determined by the cetane number determination process is set as the proper cetane number of the fuel. The cetane number thus set is used in calculation of the threshold temperature Tf for the NO.sub.X reduction process based on the relationship shown in FIG. 2.

(26) As described above, the volatility of the fuel is generally correlated with the cetane number of the fuel, but there are fuels of which the actual volatility of the fuel does not agree with the volatility estimated from the cetane number of the fuel. This is because the cetane number is originally an indicator of the self-ignitability of fuels. Therefore, when the threshold temperature Tf is calculated on the basis of the relationship shown in FIG. 2, there may be cases where the threshold temperature Tf is not appropriate for the volatility of the presently used fuel. In such cases, there is a possibility that the fuel supplied through the fuel supply valve 5 for the NO.sub.X reduction process is not used sufficiently in reduction of NO.sub.X in the NSR catalyst 3 but adheres to the NSR catalyst 3. If the fuel supplied by the fuel supply valve 5 adheres to the NSR catalyst 3, there is a possibility that the activity of the NSR catalyst 3 may be deteriorated. The exhaust gas purification system for the internal combustion engine 1 according to the present disclosure is configured to perform the NO.sub.X reduction control shown in FIG. 4 in order to avoid the adhesion of fuel to the NSR catalyst 3 and to enable fuel to be appropriately used in reduction of NO.sub.X in the NSR catalyst 3, even in cases where the actual volatility of the fuel and the volatility estimated from the cetane number of the fuel do not agree with each other as described above. This NO.sub.X reduction control is a control process that is performed repeatedly by execution of a certain control program in the ECU 20.

(27) Firstly in step S201, it is determined whether or not the amount of NO.sub.X stored in the NSR catalyst 3 (the NO.sub.X storage amount) reaches or exceeds the predetermined NO.sub.X storage amount to require the execution of the NO.sub.X reduction process. In other words, it is determined whether or not a NO.sub.X reduction request is made. The amount of NO.sub.X stored in the NSR catalyst 3 is determined by another process by integrating the quantity of NO.sub.X stored into the NSR catalyst 3 per unit time since the end of the last time execution of the NO.sub.X reduction process. The quantity of NO.sub.X stored into the NSR catalyst 3 per unit time is calculated on the basis of the operation state of the internal combustion engine 1. Alternatively, the quantity of NO.sub.X stored into the NSR catalyst 3 per unit time can be calculated by integrating the difference between the quantity of NO.sub.X flowing into the NSR catalyst 3 per unit time and the quantity of NO.sub.X flowing out of the NSR catalyst 3 per unit time. The quantity of NO.sub.X flowing into the NSR catalyst 3 per unit time may be estimated on the basis of the operation state of the internal combustion engine 1 or calculated as the product of the measurement value of an NO.sub.X sensor provided upstream of the NSR catalyst 3 and the exhaust gas flow rate. The quantity of NO.sub.X flowing out of the NSR catalyst 3 per unit time may be calculated as the product of the measurement value of an NO.sub.X sensor provided downstream of the NSR catalyst 3 and the exhaust gas flow rate. If the NO.sub.X storage amount determined as above is larger than or equal to the predetermined NO.sub.X storage amount, it is determined that a request for the NO.sub.X reduction process is made. If an affirmative determination is made in step S201, the process proceeds to step S202. If a negative determination is made in step S201, the NO.sub.X reduction control is terminated.

(28) In step S202, the aforementioned threshold temperature Tf as the threshold of the exhaust gas temperature for starting the NO.sub.X reduction process is read from the memory of the ECU 20. The threshold temperature Tf is set by a threshold temperature setting process shown in FIG. 5, which is executed separately form the NO.sub.X reduction control, and memorized in the memory of the ECU 20. As described above, the threshold temperature Tf is a threshold of the exhaust gas temperature that is set in order to allow the supply of fuel to be performed at as low an exhaust gas temperature as possible taking account of the reaction of fuel and NO.sub.X in the NSR catalyst 3. The threshold temperature setting process will be specifically described later. After the completion of the processing of step S202, the process proceeds to step S203.

(29) In step S203, it is determined whether or not the present temperature of the exhaust gas flowing into the NSR catalyst 3 is higher than the threshold temperature Tf read in step S202. In other words, it is determined in step S203 whether or not the exhaust gas temperature measured by the first temperature sensor 6 (i.e. the temperature of the exhaust gas flowing into the NSR catalyst 3) is appropriate for reduction of NO.sub.X by fuel in the NSR catalyst 3, in order to determine whether the NO.sub.X reduction process may be started. If an affirmative determination is made in step S203, the process proceeds to step S204. If a negative determination is made, the NO.sub.X reduction control is terminated.

(30) In step S204, the NO.sub.X reduction process is started, which is the process of supplying fuel to the exhaust gas in order to desorb and reduce NO.sub.X stored in the NSR catalyst 3. In the NO.sub.X reduction process, the quantity of fuel supplied through the fuel supply valve 5 is controlled so as to adjust the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 3 to an appropriate air-fuel ratio (which will be hereinafter referred to as the air-fuel ratio suitable for reduction) that enables well-balanced desorption and reduction of NO.sub.X stored in the NSR catalyst 3. This air-fuel ratio suitable for reduction is determined in advance on the basis of experiment or simulation. The quantity of supplied fuel is controlled so as to adjust the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 3 to the aforementioned air-fuel ratio suitable for reduction. For example, the quantity of supplied fuel is controlled on the basis of the difference between the air-fuel ratio of the air-fuel mixture burned in the cylinder of the internal combustion engine 1 and the air-fuel ratio suitable for reduction. Since an affirmative determination is made in step S203, the quantity of supplied fuel is determined on the assumption that the supplied fuel has appropriate volatility to contribute to reduction of NO.sub.X in the NSR catalyst 3. The fuel supplied to the exhaust gas by the fuel supply valve 5 flows into the NSR catalyst 3 to reduce desorbed NO.sub.X. After the completion of the processing of step S204, the process proceeds to step S205.

(31) In step S205, the quantity of heat Tc generated in the NSR catalyst 3 per unit time by the NO.sub.X reduction process started in step S204 is calculated. This heat quantity Tc represents the heat of reduction of NO.sub.X by fuel in the NSR catalyst 3. Specifically, the quantity of heat Tc generated in the NSR catalyst 3 per unit time is calculated from the change in the temperature of the exhaust gas flowing out of the NSR catalyst 3 per unit time, the temperature being measured by the second temperature sensor 7. After the completion of the processing of step S205, the process proceeds to step S206.

(32) In step S206, it is determined whether or not the heat quantity Tc calculated in step S205 is smaller than a specific value Tc0. The aforementioned specific value Tc0 is the heat quantity expected to be generated by reduction of NO.sub.X by fuel in the NSR catalyst 3 that is assumed to occur with the fuel supplied by the fuel supply valve 5 in the NO.sub.X reduction process started in step S204. More specifically, the specific value Tc0 is set equal to the heat of reaction that would be generated when a quantity of fuel supplied though the fuel supply valve 5 per unit time during the NO.sub.X reduction process reacts with NO.sub.X stored in the NSR catalyst 3 in the case where the fuel supplied through the fuel supply valve 5 has an assumed volatility. This heat of reaction may change depending on some factors such as the quantity of fuel supplied through the fuel supply valve 5 per unit time and the temperature of the NSR catalyst 3. Therefore, the specific value Tc0 may be changed according to these factors if necessary. If an affirmative determination is made in step S206, then it means that the volatility of fuel is insufficient contrary to the above assumption and the heat of reduction reaction of NO.sub.X is not generated satisfactorily in the NSR catalyst 3. Then, in step S207, the NO.sub.X reduction process in progress is suspended, namely the supply of fuel to the exhaust gas by the fuel supply valve 5 is stopped. Moreover, in step S208, since it is considered, as above, that the volatility of fuel is insufficient contrary to the above assumption and the heat of reduction reaction of NO.sub.X is not generated satisfactorily in the NSR catalyst 3, a request for adjusting the threshold temperature Tf upward (i.e. to a higher temperature) is made. In other words, it is requested that the threshold temperature Tf for starting the NO.sub.X reduction process be changed to a higher temperature in order to ensure sufficient volatility of fuel. In response to this request, an updated threshold temperature Tf is set in the memory of the ECU 20 by a threshold temperature setting process that will be described later. This request is maintained until the supply of fuel is performed in the internal combustion engine 1 next time and reset (or cancelled) upon the next supply of fuel.

(33) If a negative determination is made in step S206, then it means that the fuel exhibits sufficient volatility as assumed and the heat of reduction reaction of NO.sub.X is generated satisfactorily in the NSR catalyst 3. Then, the process proceeds from S206 to S209. In this case, the NO.sub.X reduction process is continued. In step S209, it is determined whether or not the NO.sub.X reduction process started in step S204 is to be terminated. Specifically, for example, it is determined whether or not a length of time long enough to desorb and reduce NO.sub.X stored in the NSR catalyst 3 by supply of fuel to the NSR catalyst 3 by the NO.sub.X reduction process has elapsed since the start of the NO.sub.X reduction process. If an affirmative determination is made in step S209, the process proceeds to step S210, where the NO.sub.X reduction process is stopped. If a negative determination is made in step S209, the processing of step S209 is executed again.

(34) Next, the threshold temperature setting process of setting the threshold temperature Tf as the threshold of the exhaust gas temperature at which the NO.sub.X reduction process is to be started will be described with reference to FIG. 5. The threshold temperature setting process is a control process that is performed repeatedly by execution of a certain control program in the ECU 20. This control process is performed independently from the above-described NO.sub.X reduction control. Firstly in step S301, it is determined whether or not the standard cetane number (cf. the processing of step S102 in the cetane number setting process) is set in the memory of the ECU 20 as the cetane number of the fuel.

(35) If an affirmative determination is made in step S301, the process proceeds to step S302. In step S302, on the basis of the relationship between the cetane number of the fuel and the threshold temperature Tf shown in FIG. 2, a value of the threshold temperature corresponding to the standard cetane number is set as the threshold temperature Tf.

(36) If a negative determination is made in step S301, the process proceeds to step S303. In step S303, it is determined whether or not a request for upward adjustment of the threshold temperature Tf is made in the processing of step S208 in the NO.sub.X reduction control. If an affirmative determination is made in step S303, the process proceeds to step S304, where a value of the threshold temperature adjusted to a higher temperature is set as the threshold temperature Tf in response to the request. The amount of change in the threshold temperature by this adjustment may be, for example, a predetermined value. After the above-described setting of the threshold temperature Tf is done, the request for upward adjustment of the threshold temperature is reset (or cancelled).

(37) If a negative determination is made in step S303, the process proceeds to step S305, where a value of the threshold temperature corresponding to the cetane number determined by the cetane number determination process in step S106 is set as the threshold temperature Tf on the basis of the relationship between the cetane number of the fuel and the threshold temperature Tf shown in FIG. 2.

(38) As above, by performing the NO.sub.X reduction control shown in FIG. 4 using the threshold temperature Tf set by the threshold temperature setting process, the timing of supplying fuel to the exhaust gas through the fuel supply valve 5 for the NO.sub.X reduction control is adjusted appropriately. Basically, the threshold of the exhaust gas temperature at which the supply of fuel for the NO.sub.X reduction process is to be started is adjusted taking account of the volatility of fuel that is estimated from the cetane number of the fuel. However, in cases where the volatility estimated on the basis of the cetane number of the fuel is lower than the actual volatility of the fuel, the supply of fuel for the NO.sub.X reduction process is suspended and the threshold of the exhaust gas temperature at which the supply of fuel is to be started is changed to a higher temperature. In that case, when the temperature of the exhaust gas flowing into the NSR catalyst 3 exceeds the upwardly adjusted threshold, the supply of fuel for the NO.sub.X reduction process is performed. Then, the fuel supplied to the exhaust gas in order to desorb and reduce NO.sub.X stored in the NSR catalyst 3 will be supplied in an environment in which the fuel exhibits excellent volatility. Therefore, the supply of fuel can be performed at as low a temperature as possible without causing a deterioration of the activity of the NSR catalyst 3 due to adhesion of supplied fuel to the NSR catalyst 3. In consequence, the chance of performing the NO.sub.X reduction process can be increased. Every time fuel is supplied to the fuel tank 30 of the internal combustion engine 1, the cetane number of fuel is determined and the threshold of the exhaust gas temperature is set again. Therefore, even if the volatility of the fuel used in the internal combustion engine 1 changes by refueling, the supply of fuel to the exhaust gas for the NO.sub.X reduction process can be carried out appropriately.

Second Embodiment

(39) Next, NO.sub.X reduction control according to a second embodiment of the present disclosure will be described with reference to flow charts shown in FIGS. 6 and 7. The processing steps in the NO.sub.X reduction control shown in FIGS. 6 and 7 that are the same as those in the NO.sub.X reduction control shown in FIG. 4 are denoted by the same reference signs and will not be described further.

(40) In the NO.sub.X reduction control shown in FIGS. 6 and 7, the processing of the steps S207 and S208 in the NO.sub.X reduction control shown in FIG. 4 is replaced by the processing of steps S401 to S409 in FIG. 7. Specifically, if an affirmative determination is made in step S206, the process proceeds to step S401.

(41) In step S401, it is determined whether or not the air-fuel ratio of the exhaust gas flowing out of the NSR catalyst 3 measured by the air-fuel ratio sensor 8 is higher than a specific air-fuel ratio Af0, namely whether or not this exhaust gas air-fuel ratio is leaner than the specific air-fuel ratio. The specific air-fuel ratio is an air-fuel ratio of the exhaust gas that is assumed to be achieved taking account of the supply of fuel to the exhaust gas through the fuel supply valve 5 by the NO.sub.X reduction process started in step S204. For example, this exhaust gas air-fuel ratio may be equal to the aforementioned air-fuel ratio suitable for reduction. As fuel is supplied by the fuel supply valve 5 for the NO.sub.X reduction process, the exhaust gas temperature is raised by the fuel, and with the addition of the supplied fuel to the exhaust gas, the air-fuel ratio of the exhaust gas is expected to change to a corresponding air-fuel ratio, which is the aforementioned specific air-fuel ratio Af0. However, if the fuel supply valve 5 is in some trouble, namely if a trouble prevents the fuel supply valve 5 from supplying a requested quantity of fuel in response to a fuel supply request from the ECU 20, the quantity of supplied fuel will be smaller than the requested quantity. Then, the quantity of heat generated in the NSR catalyst 3 can be smaller, and the air-fuel ratio of the exhaust gas can be leaner than the aforementioned specific air-fuel ratio Af0. To detect this, the determination of step S401 is performed, whereby it is determined whether or not the supply of fuel by the fuel supply valve 5 is in trouble. If an affirmative determination is made in step S401, the process proceeds to step S402. If a negative determination is made, the process proceeds to step S407. In cases where an air-fuel ratio sensor that measures the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 3 is provided upstream of the NSR catalyst 3, it may be determined in step S401 whether or not the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 3 is higher than the specific air-fuel ratio Af0.

(42) In step S402, since the air-fuel ratio of the exhaust gas flowing out of the NSR catalyst 3 is leaner than the specific air-fuel ratio Af0, a fuel-increasing process of increasing the quantity of fuel supplied through the fuel supply valve 5 in the NO.sub.X reduction process is started so as to adjust the air-fuel ratio of the exhaust gas to the specific air-fuel ratio Af0. This fuel-increasing process is a process of making up the shortfall from the quantity intended to be supplied in the NO.sub.X reduction process by making the quantity of fuel supplied by the fuel supply valve 5 in the NO.sub.X reduction process larger than that in the case where the fuel-increasing process is not performed. After the completion of the processing of step S402, the process proceeds to step S403, where the heat quantity Tc generated in the NSR catalyst 3 is calculated as in step S205 described above. Then, in step S404, it is determined whether or not the generated heat quantity Tc is smaller than a specific value Tc0 as in step S206. If an affirmative determination is made in step S404, the process proceeds to step S405. If a negative determination is made, the process proceeds to step S408.

(43) If an affirmative determination is made in step S404, then it means that the quantity of fuel supplied by the fuel supply valve 5 is corrected to a proper quantity but the reduction reaction of NO.sub.X in the NSR catalyst 3 does not progress satisfactorily. Then, it is confirmed with higher reliability that the volatility of fuel is not sufficient at the time when the fuel is supplied. Therefore, if an affirmative determination is made in step S404 and the process proceeds to step S405, the NO.sub.X reduction process and the fuel-increasing process in progress are stopped. Thereafter, in step S406, a request for upwardly adjusting the threshold temperature Tf is made in order to ensure sufficient volatility of the fuel as in step S208.

(44) If a negative determination is made in step S401, the processing of step S407 is executed. In step S407, the NO.sub.X reduction process in progress is stopped as in step S207. If a negative determination is made in step S401, then it means that although the reduction reaction of NO.sub.X in the NSR catalyst 3 does not progress satisfactorily, the quantity of fuel supplied by the fuel supply valve 5 is appropriate for the reduction reaction. In this case, it can be considered that it is not necessary to perform the fuel-increasing process in step S402 and the supply of fuel is performed in a condition in which the volatility of fuel is insufficient. Therefore, if a negative determination is made in step S401, the NO.sub.X reduction process is stopped immediately in step S407 without performing the fuel-increasing process, and thereafter the processing of step S406 is performed.

(45) If a negative determination is made in step S404, then it means that an appropriate quantity of fuel is supplied thanks to the fuel-increasing process and the fuel exhibits sufficient volatility, so that the heat of reduction reaction of NO.sub.X is generated satisfactorily in the NSR catalyst 3. Then, the process proceeds from step S404 to step S408. In this case, the NO.sub.X reduction process is performed while continuing the fuel-increasing process. In step S408, it is determined whether or not the NO.sub.X reduction process started in step S204 is to be terminated, as in step S209. If an affirmative determination is made in step S408, the process proceeds to step S409, where the NO.sub.X reduction process and the fuel-increasing process are stopped. If a negative determination is made in step S408, the processing of step S408 is executed again.

(46) As above, in the NO.sub.X reduction control according to the second embodiment, the upward adjustment of the threshold Tf of the exhaust gas temperature for starting the NO.sub.X reduction process is performed after determining the air-fuel ratio of the exhaust gas to which fuel has been supplied and performing the fuel-increasing process. Thus, the upward adjustment of the threshold of the exhaust gas temperature for starting the NO.sub.X reduction process is performed after excluding the possibility of insufficient reduction of NO.sub.X in the NSR catalyst 3 due to a trouble of the fuel supply valve 5. When the temperature of the exhaust gas flowing into the NSR catalyst 3 exceeds the adjusted threshold, the NO.sub.X reduction process is performed. Therefore, the supply of fuel for the NO.sub.X reduction process can be performed at as low a temperature as possible even if the fuel supply valve 5 is in trouble, and the chance of performing the NO.sub.X reduction process can be increased further.

Modification of Embodiments

(47) In the NO.sub.X reduction control according to FIGS. 6 and 7, the processing of steps S401 and S402 relating to the compensation of the quantity of fuel supplied through the fuel supply valve 5 is performed after starting the NO.sub.X reduction process and after making the determination as to whether or not reduction of NO.sub.X in the NSR catalyst 3 is carried out appropriately (in the processing of step S206). Alternatively, the processing of steps S401 and S402 relating to the compensation of the quantity of fuel supplied through the fuel supply valve 5 may be performed after starting the NO.sub.X reduction process but before making the determination as to whether reduction of NO.sub.X in the NSR catalyst 3 is carried out appropriately. In this case, the determination as to whether reduction of NO.sub.X in the NSR catalyst 3 is carried out appropriately (i.e. the processing of step S206 or S404) is made after confirming the absence of a trouble of the fuel supply valve 5 and performing the compensation of the quantity of fuel supplied by the fuel supply valve 5.

(48) Although the NSR catalyst 3 and the filter 4 are provided separately in the exhaust gas purification system for the internal combustion engine 1 shown in FIG. 1, the NSR catalyst 3 and the filter 4 may be integrated alternatively, namely the NSR catalyst 3 may be supported on the filter 4.

(49) While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.