JP2005188341A - Exhaust emission control device of diesel engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To surely perform NOx removal and clean-up of a NOx trap catalyst by rich spike when a fuel property deteriorates (specific gravity increases). <P>SOLUTION: The specific gravity of fuel is detected from an intake air amount, a fuel injection quantity, an actual air-fuel ratio and the like and set as a parameter representative of the fuel property. When the fuel specific gravity Gn is higher than a specific gravity Gn-1 previously detected, it is judged that the fuel property deteriorates, and when exhaust gas temperature T is lower than a predetermined temperature Tg, as NOx removal and clean-up performance at the NOx trap catalyst is lowered, an air amount is increased at the time of rich spike. When the exhaust gas temperature is higher than the predetermined temperature Tg, a fuel quantity is reduced at the time of rich spike. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an exhaust emission control device for a diesel engine equipped with a NOx trap catalyst, and more particularly to a technique for more appropriately reducing NOx in accordance with the fuel properties such as the cetane number of the fuel being used.

  For example, in order to reduce NOx (nitrogen oxide), which is a problem in diesel engines, a function of trapping NOx when the exhaust air-fuel ratio is lean, and desorbing and purifying the trapped NOx when the exhaust air-fuel ratio is rich A NOx trap catalyst having the above has been conventionally known.

  On the other hand, Patent Literature 1 discloses a cetane number sensor as a technique for determining the fuel property of a fuel used in an internal combustion engine. In this conventional example, since the cetane number of light oil fuel in the market is not always constant and stable operation is not possible, the cetane number of fuel oil supplied is judged, and fuel is injected at a timing according to the judgment value It is an object.

  The determination method of the cetane number in the cetane number sensor of Patent Document 1 is based on the premise that there is a proportional relationship between the viscosity of light oil and the cetane number. In addition to providing a pendulum that falls due to gravity, a mechanism for measuring the drop time of the pendulum that depends on the viscosity is provided, and the viscosity is obtained from the drop time. And it is the structure which adds the correction | amendment based on the fuel temperature at that time with respect to the detected viscosity, and determines a cetane number.

  However, in this research conducted by the present applicant, the correlation between the viscosity of the light oil and the cetane number is low, and both are inversely proportional, and the higher the viscosity, the lower the cetane number tends to be. . Therefore, regardless of whether the relationship between the viscosity of the light oil and the cetane number is proportional or inversely proportional, the cetane number cannot be accurately detected by detecting the viscosity. it is obvious.

As another conventional example, Patent Literature 2 discloses a combustion control device for a diesel engine. This combustion control device has an ignition timing detection means using an in-cylinder pressure sensor. When the target ignition timing differs from the actual ignition timing due to variations in the cetane number of fuel in the market, etc. The injection timing, EGR rate, etc. are changed so that the ignition timing is reached.
Japanese Utility Model Publication 3-45181 Japanese Patent Laid-Open No. 11-107820

  The relationship between the standard specific gravity of light oil (specific gravity at a standard temperature of 20 ° C.) and fuel properties in the market investigated by the present applicant will be described based on the characteristic diagrams shown in FIGS.

  In light oil, as shown in FIG. 2, the cetane number decreases in inverse proportion to the standard specific gravity (hereinafter simply referred to as density). The reason for this is that, as shown in FIG. 3, the higher the density, the more aromatic components having a benzene ring structure having a lower cetane number and lower evaporability are contained. In addition, aromatic hydrocarbons are generally linked to a polycyclic structure with a straight chain or a side chain structure, and are generally heavy with a large number of carbons. For this reason, aromatic hydrocarbon components As the amount increases, the viscosity increases. Accordingly, since the viscosity increases in proportion to the density, the cetane number decreases in inverse proportion to the viscosity.

  FIG. 5 shows the relationship between the aromatic hydrocarbon content and the exhaust gas temperature. The higher the aromatic hydrocarbon content, the lower the exhaust gas temperature.

  When the engine temperature of the diesel engine is low and the specific gravity of the fuel is high, hydrocarbons (HC) in the exhaust gas exhausted from the engine increase, and unburned HC containing a large amount of aromatic hydrocarbons is discharged. Since the air-fuel ratio in the exhaust gas decreases, it is desirable to provide an air-fuel ratio sensor in the engine exhaust passage to correct the air-fuel ratio in the engine to the lean side.

  On the other hand, in a diesel engine equipped with a NOx trap catalyst in the exhaust passage, when the exhaust air-fuel ratio is lean, NOx is trapped in the catalyst, and this trapped NOx forcibly enriches the exhaust air-fuel ratio, Although desorption and purification are performed, when the specific gravity of the fuel is high, the air-fuel ratio correction is performed, so that the target rich degree becomes shallow as shown in FIG. There is a problem that a reducing gas component such as carbon oxide (CO) is insufficient, and NOx desorbed from the catalyst is released into the atmosphere without being purified.

  Further, since the exhaust gas temperature is lowered, there is a problem that not only the NOx desorption / purification ability is lowered, but also the ability to oxidize HC and CO is reduced, and these are released into the atmosphere.

  The exhaust gas purification apparatus for a diesel engine according to the present invention traps NOx (nitrogen oxide) in the exhaust passage when the exhaust air-fuel ratio is lean, and desorbs and purifies the trapped NOx when the exhaust air-fuel ratio is rich. A functioning NOx trap catalyst is provided. Further, in order to desorb and purify NOx from the NOx trap catalyst, there is provided enriching means for forcibly enriching the exhaust air-fuel ratio. In the present invention, the fuel property detecting means for detecting the fuel property of the fuel being used is provided. Based on the detected fuel property, the exhaust air at the time of enrichment of the exhaust air / fuel ratio by the enrichment means. The fuel ratio target value is corrected.

  As the fuel property detection means, for example, a device that estimates the specific gravity of the fuel, a device that estimates a parameter indicating the ignitability of the fuel, a device that estimates the ratio of aromatic hydrocarbons in the fuel, and the like it can.

  According to the present invention, even if the combustion state changes due to a change in fuel properties and the exhaust air / fuel ratio changes, the exhaust air / fuel ratio is forcibly enriched by the enrichment means to desorb and purify NOx from the NOx trap catalyst. In doing so, the exhaust air-fuel ratio required for NOx desorption purification can be maintained, and NOx desorption purification can be performed reliably.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a system configuration diagram of a diesel engine equipped with an exhaust emission control device of the present invention, and this diesel engine uses light oil as fuel.

  In FIG. 1, 1 indicates a diesel engine (hereinafter simply referred to as an engine), and 3 indicates an exhaust passage of the engine 1.

  An exhaust outlet passage 3a constituting an upstream portion of the exhaust passage 3 of the engine 1 is connected to a turbine 3b of a supercharger, and a casing 20 containing a NOx trap catalyst therein is arranged in series downstream thereof. ing. An air-fuel ratio sensor 37 serving as an actual air-fuel ratio detection unit is provided at the inlet of the casing 20. The air-fuel ratio sensor 37 detects the oxygen concentration in the exhaust gas using, for example, an oxygen ion conductive solid electrolyte, and obtains the air-fuel ratio from the oxygen concentration. The NOx trap catalyst has a function of trapping NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the trapped NOx and purifying it when the oxygen concentration of the inflowing exhaust gas is reduced. ing.

  As an exhaust gas recirculation device, an EGR passage 4 for recirculating a part of the exhaust gas is provided between the intake collector 2c and the exhaust outlet passage 3a of the intake passage 2, and the opening is opened by a stepping motor. Is provided with an EGR valve 5 that can be continuously controlled.

  The intake passage 2 includes an air cleaner 2a at an upstream position, and an airflow meter 7 serving as an intake air amount detection means is provided on the outlet side thereof. A turbocharger compressor 2b is disposed downstream of the air flow meter 7, and an intake throttle valve 6 that is opened and closed by an actuator (for example, a stepping motor type) between the compressor 2b and the intake collector 2c. Is intervening.

  The fuel supply system of the engine 1 includes a fuel tank 60 that stores diesel oil as diesel fuel, a fuel supply passage 16 that supplies fuel to the fuel injection device 10 of the engine 1, and a fuel supply device 16 of the engine 1. And a fuel return passage 19 for returning the return fuel (spill fuel) to the fuel tank 60.

  The fuel injection device 10 of the engine 1 is a known common rail type fuel injection device, and generally includes a supply pump 11, a common rail (accumulation chamber) 14, and a fuel injection valve 15 provided for each cylinder. After the fuel pressurized by the supply pump 11 is temporarily stored in the common rail 14 via the fuel supply passage 12, the high-pressure fuel in the common rail 14 is distributed to the fuel injection valves 15 of each cylinder.

  The common rail 14 is provided with a pressure sensor 34 and a temperature sensor 35 in order to detect the pressure and temperature of the fuel in the common rail 14. Further, in order to control the fuel pressure in the common rail 14, a part of the fuel discharged from the supply pump 11 is returned to the fuel supply passage 16 via the overflow passage 17 having the one-way valve 18. Yes. Specifically, a pressure control valve 13 that changes the flow passage area of the overflow passage 17 is provided, and the pressure control valve 13 changes the flow passage area of the overflow passage 17 in accordance with a duty signal from the engine control unit 30. Thereby, the substantial fuel discharge amount from the supply pump 11 to the common rail 14 is adjusted, and the fuel pressure in the common rail 14 is controlled.

  The fuel injection valve 15 is an electronic injection valve that is opened and closed by an ON-OFF signal from the engine control unit 30, and injects fuel into the combustion chamber by the ON signal and stops injection by the OFF signal. The fuel injection amount increases as the period of the ON signal applied to the fuel injection valve 15 increases, and the fuel injection amount increases as the fuel pressure of the common rail 14 increases.

  A water temperature sensor 31 for detecting the cooling water temperature is attached to an appropriate position of the engine 1 as representative of the temperature of the internal combustion engine.

  The engine control unit 30 includes a signal (Qa) from the air flow meter 7 that detects the intake air amount, a signal from the water temperature sensor 31 (cooling water temperature Tw), and a signal from the crank angle sensor 32 for crank angle detection (of the engine speed Ne). Basic crank angle signal), cylinder discrimination crank angle sensor 33 signal (cylinder discrimination signal Cyl), pressure sensor 34 signal for detecting fuel pressure in common rail 14 (common rail pressure PCR), temperature sensor for detecting fuel temperature 35 (fuel temperature TF), an accelerator opening sensor 36 signal (accelerator opening (load) L) for detecting an accelerator pedal depression amount corresponding to a load, and an air-fuel ratio sensor 37 signal (O2). Entered.

  Next, the contents of the control of this embodiment executed by the engine control unit 30 will be described based on the flowcharts of FIGS.

  FIG. 9 is a basic control routine relating to control of the entire diesel engine 1.

  In this engine basic control routine, in step 100, the coolant temperature Tw, the engine speed Ne, the cylinder discrimination signal Cyl, the common rail pressure PCR, the signal Qa of the air flow meter 7, the fuel temperature TF, the accelerator opening L, the air-fuel ratio sensor. Each of the signals O2 is read, and the process proceeds to Step 200.

  In step 200, common rail pressure control is performed. In the present invention, the common rail pressure control itself is not an essential part and will be described briefly. That is, the common rail pressure control calculates the target reference pressure PCR0 of the common rail 14 by searching a predetermined map stored in advance in the ROM of the control unit 30 using the engine speed Ne and the load L as parameters. Feedback control of the pressure control valve 13 is executed so that the reference pressure PCR0 is obtained.

  Then, the process proceeds from step 200 to step 300, fuel specific gravity detection control is performed, engine exhaust control is further performed in step 400, and the process returns.

  FIG. 10 is a flowchart showing details of the control subroutine for detecting the specific gravity of fuel in step 300. With this control, the specific gravity of the fuel being used is accurately detected.

  The fuel specific gravity detection control routine will be described below. In step 310, based on the signal Qa of the air flow meter 7 for detecting the intake air amount, the table data of the predetermined intake air amount Qair stored in advance in the ROM of the control unit 30 is retrieved using the value of the signal Qa as a parameter. To do. Then, the process proceeds to step 320.

  In step 320, the main fuel injection amount (fuel supply amount) Q main set with the engine speed Ne and the load L as parameters is obtained by searching a predetermined map stored in the ROM of the control unit 30 in advance. . Then, the process proceeds to step 330.

  Note that the main fuel injection amount (fuel supply amount) Q main is not limited to the above-described method, but the fuel injection period M period of the fuel injection device that is set with the engine speed Ne and the load L as parameters. The main fuel injection amount (fuel supply amount) Q main which is obtained by searching a predetermined map stored in advance in the ROM of 30 and set with the fuel injection period M period and the common rail pressure PCR as parameters, A predetermined map stored in advance in 30 ROMs may be searched for.

  In step 330, based on the signal O2 of the air-fuel ratio sensor 37, the table data of the actual air-fuel ratio AF real stored in advance in the ROM of the control unit 30 is retrieved using the value of the signal O2 as a parameter. Then, the process proceeds to Step 340.

  In step 340, it is determined whether or not the conditions are suitable for detecting the fuel property.

  For example, in general, an automobile engine is generally provided with an exhaust gas recirculation device including an EGR valve 5 and the like for NOx reduction. Since the fuel ratio shifts to the rich side, it is necessary to correct the exhaust gas recirculation in order to accurately obtain the actual air fuel ratio. Accordingly, since there is a concern that the detection accuracy of the actual air-fuel ratio deteriorates due to the correction, it is desirable to issue the actual air-fuel ratio detection command only in a region where exhaust gas recirculation is stopped.

  If the detection condition is not suitable in step 340, the process proceeds to step 400 without detecting the fuel property.

  If the detection condition is suitable in step 340, the process proceeds to step 350, and the actual fuel supply weight G main is obtained based on the intake air flow rate Q air obtained in step 310 and the actual air-fuel ratio AF real obtained in step 330. . Specifically, the intake air flow rate Q air is divided by the actual air-fuel ratio AF real to obtain the actual fuel supply weight G main (G main = Q air ÷ AF real). Then, the actual specific gravity G fuel is obtained based on the obtained actual fuel supply weight G main and the main fuel injection amount (fuel supply amount) Q main obtained in step 320. Specifically, the actual fuel supply weight Gmain is divided by the main fuel injection amount (fuel supply amount) Qmain to obtain an actual specific gravity Gfuel (Gfuel = Gmain ÷ Qmain). Then, the process proceeds to step 360.

  In step 360, a standard specific gravity (a specific gravity at a reference temperature, for example, a standard temperature of 20 ° C.) G std is obtained from the actual specific gravity G fuel and the fuel temperature TF. Specifically, a map of the standard specific gravity G std stored in advance in the ROM of the control unit 30 using the actual specific gravity G fuel and the fuel temperature TF as parameters is searched to obtain a corresponding value. Then, the process proceeds to Step 400.

  Next, FIG. 11 is a subroutine related to engine exhaust control performed in step 400 of FIG. 9. Here, fuel injection timing control is performed in step 410 so as to obtain a predetermined engine exhaust emission performance, and step EGR control is performed at 420, and exhaust aftertreatment control is performed at step 430.

  FIG. 12 is a subroutine of the injection timing control in step 410 described above.

  In step 411, the main fuel injection amount Q main is obtained by searching a predetermined map stored in advance in the ROM of the control unit 30 with the engine speed Ne and the load L as parameters. Then, the process proceeds to Step 412.

  In step 412, based on the main fuel injection amount Q main and the common rail pressure PCR, a predetermined map stored in advance in the ROM of the control unit 30 is searched using these as parameters to obtain the main injection period M period. Then, the process proceeds to step 413.

  Here, the main injection period M period is set in units of m sec (milliseconds). As shown in FIG. 7, if the main fuel injection amount Q main is the same, the higher the common rail pressure PCR, the higher the common rail pressure PCR. If the main injection period M period becomes shorter and the common rail pressure PCR is the same, the main injection period M period becomes longer as the main fuel injection amount Q main increases.

  In step 413, a predetermined map stored in advance in the ROM of the control unit 30 is searched using the engine speed Ne and the main fuel injection amount Q main as parameters, and the main injection start timing M start that becomes the reference fuel injection timing is determined. Ask. Then, the process proceeds to step 414.

  In step 414, it is determined whether the specific gravity of the fuel being used has been detected.

  If the fuel specific gravity has not been detected yet, the process proceeds from step 414 to step 415, and the table data of the predetermined injection timing correction coefficient KTWINJ stored in advance in the ROM of the control unit 30 is searched using the coolant temperature as a parameter. Then, the process proceeds to step 417. The injection timing correction coefficient KTWINJ based on the coolant temperature is set so that the injection timing is advanced as the coolant temperature is lower.

  In step 417, the main injection start timing M start is corrected by multiplying the injection timing correction coefficient KTWINJ (M start × K TWINJ → M start). In step 418, the main injection is performed based on the crank angle signal of the crank angle detection crank angle sensor 32 and the cylinder discrimination signal Cyl of the cylinder discrimination crank angle sensor 33 so that the main fuel injection amount Q main is supplied. During the period of M period from the start timing M start, the fuel injection valve 15 of the cylinder to be injected is driven to open.

  In the above example, the correction for the cooling water temperature is performed by multiplying the correction coefficient. However, an injection timing correction value is obtained based on the cooling water temperature, and this is added to the main injection start timing M start. May be.

  If the fuel specific gravity has been detected in step 414, the process proceeds to step 416, where the predetermined injection timing correction coefficient K DINJ stored in advance in the ROM of the control unit 30 is set using the cooling water temperature and the fuel specific gravity as parameters. The map data is searched and the process proceeds to step 417. In step 417, as described above, the main injection start timing M start is corrected by multiplying this injection timing correction coefficient K DINJ (M start × K DINJ → M start).

  Here, as shown in FIG. 6, the injection timing correction coefficient K DINJ based on the coolant temperature and the fuel specific gravity is set so that the injection timing is advanced as the coolant temperature is lower and the fuel specific gravity is larger. It is desirable to improve the startability and improve the fuel efficiency and exhaust (particularly HC) during warm-up.

  In the above example, the correction for the cooling water temperature and the fuel specific gravity is performed by multiplying the correction coefficient. However, the injection timing correction value is obtained based on the cooling water temperature and the fuel specific gravity, and this is used as the main injection start. You may make it add to time Mstart.

  Next, FIG. 13 is a subroutine relating to EGR control of the diesel engine performed in step 420 of FIG.

  In this EGR control routine, in step 421, the main fuel injection amount Q main set with the engine speed Ne and the load L as parameters is obtained by searching a predetermined map stored in the ROM of the control unit 30 in advance. The process proceeds to step 422.

  In step 422, a predetermined map stored in advance in the ROM of the control unit 30 is retrieved using the main fuel injection amount Q main and the engine speed Ne as parameters, and an intake throttle valve drive signal (intake throttle valve 6) is searched. TH duty is calculated, and the process proceeds to step 423.

  The opening degree of the intake throttle valve 6 is desirably set so that the opening degree is increased (the intake throttle degree is decreased) as the engine load L is higher and the engine speed Ne is higher.

  In step 423, a predetermined map stored in advance in the ROM of the control unit 30 is searched using the engine speed Ne and the main fuel injection amount Q main as parameters, and an EGR drive signal (EGR) serving as a reference EGR control signal is searched. Valve 5 opening signal) EGR duty is obtained. Then, the process proceeds to step 424.

  It is desirable that the opening degree of the EGR valve 5 is set so that the opening degree increases as the engine load L is lower (the amount of EGR is increased).

  In step 424, it is determined whether the specific gravity of the fuel being used has been detected.

  If the fuel specific gravity has not been detected yet, the routine proceeds from step 424 to step 425, where the correction coefficient KTWTH and EGR of the predetermined intake throttle valve drive signal stored in advance in the ROM of the control unit 30 using the coolant temperature as a parameter. The table data of the correction coefficient KTWEGR for the valve drive signal is searched for, and the process proceeds to step 427.

  Here, the correction coefficient KTWTH of the intake throttle valve drive signal based on the coolant temperature is desirably set so that the intake throttle degree becomes weaker as the coolant temperature is lower. The correction coefficient KTWEGR of the EGR valve drive signal is It is desirable to set so that the EGR opening is decreased and the amount of EGR is decreased as the cooling water temperature is lower.

  In step 427, the intake throttle valve drive signal THduty is corrected by multiplying the intake throttle valve drive signal correction coefficient KTWTH (THduty × KTWTH → THduty) and multiplied by the correction coefficient KTWEGR of the EGR valve drive signal. To correct the EGR valve drive signal EGRduty (EGRduty × KTWEGR → EGRduty). Then, the process proceeds to step 428.

  In the above example, the correction for the cooling water temperature is performed by multiplying the correction coefficient. However, the respective correction values are obtained based on the cooling water temperature, and these values are obtained as the intake throttle valve drive signal THduty and the EGR valve drive signal. You may make it add to EGRduty.

  In step 428, the intake throttle valve 6 and the EGR valve 5 are driven based on the respective drive signals.

  If the fuel specific gravity has been detected in step 424, the process proceeds to step 426, and the predetermined intake throttle valve drive signal stored in advance in the ROM of the control unit 30 is corrected using the cooling water temperature and the fuel specific gravity as parameters. The map data of the coefficient KDTH and the correction coefficient KDEGR of the EGR valve drive signal are respectively retrieved, and the process proceeds to step 427. In step 427, as described above, the intake throttle valve drive signal THduty and the EGR valve drive signal EGRduty are corrected by multiplying these correction coefficient KDTH and correction coefficient KDEGR, respectively.

  Here, the correction coefficient KDTH of the intake throttle valve drive signal based on the coolant temperature and the fuel specific gravity and the correction coefficient KDEGR of the EGR valve drive signal are higher as the coolant temperature is lower and the fuel specific gravity is larger as shown in FIG. It is desirable to reduce the exhaust gas recirculation amount by decreasing the intake throttle degree and decreasing the EGR opening degree. Thus, EGR control suitable for the engine temperature and fuel characteristics can be performed. Note that the exhaust gas recirculation amount may become 0 as a result of the reduction by correction.

  In the above example, the correction for the cooling water temperature and the specific gravity of the fuel is performed by multiplying the correction coefficient. However, the respective correction values are obtained based on the cooling water temperature and the specific gravity of the fuel, and these are obtained as intake throttle valves. You may make it add to drive signal THduty and EGR valve drive signal EGRduty.

  In step 430 of FIG. 11, exhaust aftertreatment control is performed. Here, at the regeneration time of the NOx trap catalyst disposed in the casing 20, the intake throttle is strengthened (the intake throttle valve 6 is small), the exhaust gas recirculation is strengthened, or the post-injection (a small amount of fuel is performed after the main injection). Injection) is performed alone or in combination, so that the air-fuel ratio of the exhaust gas exhausted by the engine is made rich to perform NOx regeneration.

  In the above embodiment, the actual fuel supply weight G main is obtained by dividing the intake air flow rate Q air by the actual air-fuel ratio AF real, and this actual fuel supply weight G main is determined as the main fuel injection amount (fuel supply amount) Q main. The actual specific gravity G fuel is obtained by dividing by this, and this is corrected by the fuel temperature TF to obtain the standard specific gravity G std. Instead of this method, the fuel injection in the fuel injection device 10 of the engine 1 is performed. The fuel injection amount Q main is obtained from the period M period and the common rail pressure PCR, the reference fuel injection weight G main is obtained from the fuel injection amount Q main, the preset reference fuel specific gravity γstd and the fuel temperature TF, and the reference fuel The reference air-fuel ratio AF std is obtained from the injection weight G main and the air weight G air detected by the air flow meter 7, and the reference air-fuel ratio is calculated. The actual fuel specific gravity γ real can be obtained by comparing AF std and the actual air-fuel ratio AF real detected by the air-fuel ratio sensor 37.

  Next, FIG. 14 shows a flowchart for correcting the exhaust air-fuel ratio according to the fuel property, which is a main part of the present invention, when the engine is forcibly rich. That is, the routine shown in this flowchart is a part of the exhaust aftertreatment control in step 430 described above, and it is determined from the operation history or the like that the NOx trapped in the NOx trap catalyst has reached a predetermined amount. It is executed when regeneration processing execution by forced rich operation is permitted, and specifically, it is executed on condition that the rich operation flag is ON.

  First, in step 1, the specific gravity of the fuel detected as described above is read. Next, in step 2, it is determined whether or not the specific gravity Gn of the fuel detected this time is higher than the specific gravity Gn-1 detected previously. When the specific gravity Gn of the fuel is equal to or lower than the specific gravity Gn−1 detected previously, the routine proceeds to Steps 6 and 7, and the normal regeneration process is executed as it is. In other words, the exhaust air-fuel ratio is set to the normal target rich air-fuel ratio, and in step 7, forcible enrichment (so-called rich spike) is performed by the above-described post injection or the like, and the NOx trap catalyst is regenerated. .

  On the other hand, when the specific gravity Gn of the fuel is higher than the previously detected specific gravity Gn-1, it is determined that the fuel property has deteriorated, and the target exhaust air-fuel ratio at the time of regeneration of the NOx trap catalyst is corrected to the rich side. . Specifically, the process proceeds from step 2 to step 3, where it is determined whether or not the exhaust gas temperature T is lower than a predetermined temperature Tg, and the exhaust air / fuel ratio is corrected by means optimal for each. The exhaust temperature T can be estimated based on operating conditions and fuel properties.

  When the exhaust gas temperature T is lower than the predetermined temperature Tg, it is determined that the NOx desorption / purification ability of the NOx trap catalyst is low even if the same rich spike is used, and the routine proceeds to step 4 where the exhaust air / fuel ratio during rich operation is determined. Is corrected by controlling the amount of air supplied to the engine 1. Specifically, the opening degree of the intake throttle valve 6 is decreased and the air amount is corrected to be small. On the other hand, if the exhaust temperature T is equal to or higher than the predetermined temperature Tg, the routine proceeds to step 5 where the exhaust air / fuel ratio is corrected by controlling the amount of fuel supplied to the engine 1 during the rich operation. Specifically, an increase in the amount of fuel injected from the fuel injection valve 15 or an increase in the fuel pressure in the common rail 14 (common rail pressure PCR) is executed.

The system block diagram of the diesel engine provided with the exhaust gas purification device of this invention. The characteristic view which shows the relationship between the density of light oil, and a cetane number. The characteristic view which shows the relationship between the density of light oil, and aromatic hydrocarbon component content. The figure which shows the air fuel ratio change at the time of rich operation when air fuel ratio correction | amendment is carried out by deterioration of a fuel property. The characteristic view which shows the relationship between exhaust temperature and aromatic hydrocarbon content. Fig. 4 is a characteristic diagram of a fuel injection timing correction coefficient using light oil specific gravity and cooling water temperature as parameters. The characteristic view of the fuel injection quantity by common rail pressure and fuel injection period. FIG. 5 is a characteristic diagram of an intake throttle valve correction coefficient and an EGR valve correction coefficient using light oil specific gravity and cooling water temperature as parameters. The flowchart which shows the basic control routine of a diesel engine. The flowchart which shows the control routine for fuel specific gravity detection. The flowchart which shows an engine exhaust control routine. The flowchart which shows an injection timing control routine. The flowchart which shows an EGR control routine. The flowchart which shows the correction | amendment routine of the exhaust air fuel ratio at the time of the enrichment according to fuel specific gravity.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Diesel engine 6 ... Intake throttle valve 10 ... Fuel injection apparatus 20 ... Casing (NOx trap catalyst)
30 ... Engine control unit

Claims (9)

A NOx trap catalyst having a function of trapping NOx (nitrogen oxide) when the exhaust air-fuel ratio is lean and desorbing and purifying the trapped NOx when the exhaust air-fuel ratio is rich is disposed in the exhaust passage. In addition, in an exhaust emission control device for a diesel engine provided with enriching means for forcibly enriching the exhaust air-fuel ratio,
A fuel property detecting means for detecting the fuel property of the fuel being used, and correcting the exhaust air / fuel ratio target value when the exhaust air / fuel ratio is enriched by the rich means based on the detected fuel property; Diesel engine exhaust gas purification device. 2. The exhaust emission control device for a diesel engine according to claim 1, wherein the fuel property detecting means calculates or estimates a parameter representing the weight per unit volume of the fuel as the fuel property. 2. The exhaust emission control device for a diesel engine according to claim 1, wherein the fuel property detecting means calculates or estimates a parameter representing the ignitability of the fuel as the fuel property. 2. The exhaust emission control device for a diesel engine according to claim 1, wherein the fuel property detecting means calculates or estimates a ratio of aromatic hydrocarbons contained in the fuel as the fuel property. The exhaust emission control device for a diesel engine according to any one of claims 1 to 4, wherein the correction of the exhaust air-fuel ratio target value is realized by adjusting a fuel injection amount supplied to the engine. The exhaust emission control device for a diesel engine according to any one of claims 1 to 4, wherein the correction of the exhaust air-fuel ratio target value is realized by adjusting the amount of air flowing into the engine. The temperature detecting means for detecting the temperature of the NOx trap catalyst is further provided, and means for realizing correction of the exhaust air / fuel ratio target value is selected based on the temperature of the NOx trap catalyst. 6. An exhaust emission control device for a diesel engine according to any one of claims 6 to 10. As means for realizing the correction of the exhaust air-fuel ratio target value, the fuel injection amount supplied to the engine and the air amount flowing into the engine are adjusted, and both contributions based on the temperature of the NOx trap catalyst. The exhaust purification device for a diesel engine according to claim 7, wherein the ratio is variably controlled. The fuel property detection means includes
An intake air amount detecting means for detecting an intake air amount;
Fuel supply amount detection means for detecting the fuel supply amount;
An actual air-fuel ratio detecting means for detecting the actual air-fuel ratio;
Specific gravity detection means for detecting the specific gravity of the fuel being used based on the detected intake air amount, fuel supply amount, and actual air-fuel ratio;
The exhaust emission control device for a diesel engine according to claim 1, comprising:

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