JP2003138970A - Diesel engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diesel engine capable of emitting and reducing NOx from an NOx storage catalyst without increasing the smoke emission quantity by increasing the upper limit value of the EGR rate for controlling the emission of smoke. SOLUTION: The NOx purge combustion mode, when the fuel injection timing IT is set at 36 deg.BTDC which is widely advanced compared with a general 10 deg.BTDC, is executed. At the fuel injection timing IT, the smoke emission quantity is controlled to a very low value even if the EGR rate Regr is increased to about 56%, while the excess air ratio of 1.0 or lower is achieved because of the large EGR, so that NOx can be emitted and reduced with the increased HC and CO.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diesel engine, and more particularly to a diesel engine equipped with a storage type NOx catalyst that stores and releases and reduces NOx in exhaust gas.

[0002]

Related Background Art As is well known, an occlusion-type NOx catalyst, which is one of the measures for reducing NOx, occludes NOx in the exhaust gas when the exhaust air-fuel ratio is lean, and the occluded NOx is It functions to release and reduce when rich or stoichiometric. For adjusting the exhaust air-fuel ratio, a dedicated reducing agent supply means may be provided, but if the excess air ratio of the engine is controlled, the adjustment of the exhaust air-fuel ratio can be realized without adding a structure such as reducing agent supply means. Therefore, this method may be implemented.

However, the above method has a problem that it is difficult to apply it to a diesel engine. That is, the diesel engine is operated at an air excess ratio on the lean side, which is significantly larger than 1.0, which corresponds to the theoretical air-fuel ratio, and the combustion mode at this time is combustible mixing at the boundary between the injected fuel and the surrounding compressed air. It is mainly composed of diffusion combustion that causes combustion in the air layer. Therefore, even if the fuel injection amount is increased to change the excess air ratio to the rich side, most of the increased fuel remains unburned and does not contribute to the enrichment of the excess air ratio. Will be.

Therefore, in Japanese Unexamined Patent Publication No. 8-218920, fuel is injected in the intake stroke and a large amount of EGR is introduced to reduce the excess air ratio to 1.0 or less.
Techniques for releasing and reducing Ox have been proposed. The combustion state at this time is switched to premixed combustion in which the injected fuel is diffused / vaporized by the compression top dead center and burned in a state of being mixed with air in advance, so that the unburned residue of the fuel hardly occurs and the EGR rate It is presumed that the upper limit value of is increased, and as a result, an excess air ratio of 1.0 or less can be achieved.

On the other hand, Japanese Patent No. 3116876 discloses EG
Paying attention to the characteristic that if the R rate is increased beyond the above upper limit value, the smoke emission amount increases sharply and then exceeds the peak, and then decreases, and the EGR rate when the smoke emission amount decreases There is disclosed a technique for controlling both smoke and NOx by controlling the above. Therefore, when NOx purging is required, it is conceivable to use this technique to control the excess air ratio to 1.0 or less to release and reduce NOx.

[0006]

However, in the former technique, since the period from the fuel injection in the intake stroke to the ignition in the compression stroke is long, there are variations in the ignition timing, and there are premature ignition and ignition delay. There is a problem that ignition failure is likely to occur and stability is poor. Further, a part of the fuel injected in the intake stroke diffuses into the cylinder and adheres to the cylinder wall, which causes a problem of oil dilution.

Further, in the latter-stage technique, when the control mode is switched between the normal operation and the NOx purge, the peak of the smoke emission amount is passed, so that the smoke is inevitably transitional every mode switching. There is a problem that the amount of emissions will increase rapidly. The object of the present invention is to prevent the deterioration of stability due to poor ignition and the occurrence of oil dilution and the like, and to raise the upper limit value of the EGR rate at which smoke emission can be suppressed, thereby increasing the smoke emission amount. An object of the present invention is to provide a diesel engine that can release and reduce NOx from a NOx storage catalyst without increasing it.

[0008]

In order to achieve the above object, the invention of claim 1 is directed to a fuel injection means for injecting fuel into a combustion chamber of an engine, and a recirculation amount of exhaust gas discharged from the engine to an intake system. And an EGR rate adjusting means for controlling the NGR in the exhaust passage of the engine when the exhaust air-fuel ratio is lean.
A NOx storage catalyst that stores Ox and releases and reduces the stored NOx when the exhaust air-fuel ratio is rich or stoichiometric, and when NOx is to be released from the NOx storage catalyst, the fuel injection timing by the fuel injection means is set to EGR. The smoke emission characteristics with respect to the increase of the fuel injection rate show an increasing tendency and then a decreasing tendency, and are on the advance side of the injection timing and later than the injection timing when the injected fuel leaves the cavity formed in the piston and reaches the cylinder wall It is provided with first control means for executing a first control mode for controlling the operation of the EGR rate adjusting means so that the excess air ratio is set to 1.0 or less while being set to the corner side.

The characteristic of the smoke emission amount with respect to the EGR rate changes depending on the injection timing. FIG. 5 shows an example of the test results in which the fuel injection timing is changed in a certain operating region. When the angle is advanced to 20 ° BTDC compared to 10 ° BTDC applied to a general diesel engine, the EGR rate The smoke emission amount with respect to the increase has a characteristic that after showing an increasing tendency, it goes beyond the peak and shows a decreasing tendency. In this case, if the excess air ratio is changed together with the EGR rate, the smoke emission amount exceeds the peak region, so that a transient surge increase in the smoke emission amount cannot be avoided. On the other hand, in the 36 ° BTDC in which the injection timing is further advanced, the smoke emission amount is suppressed to a low value even if the EGR rate is increased, and the NOx emission amount is also suppressed due to the high EGR rate. Can be reduced, and since the peak of the smoke emission amount is not formed, the transient increase in smoke is also suppressed.

The reason why the smoke emission amount is suppressed by advancing the injection timing is that the period from fuel injection to ignition is extended and premixing of the injected fuel and intake air is promoted during that period. This is probably because the injection timing is not advanced so much as in the intake stroke injection, so the injected fuel is ignited reliably at a predetermined timing near the compression top dead center and stable without ignition failure. It is possible to drive the car.

On the other hand, FIG. 6 shows the test results of measuring the THC emission amount and the diluted fuel amount by changing the injection timing in the same operating range as in FIG. 5, and when the injection timing is advanced,
The amount of fuel mixed in the engine oil increases from around 40 ° BTDC, which may cause oil dilution. Based on the above factors, the smoke emission characteristic with respect to the increase of the EGR rate shows an increasing tendency and then a decreasing tendency, and is on the advance side of the injection timing, and the injected fuel is out of the cavity formed in the piston. If the injection timing is set to a retard angle side relative to the injection timing that reaches the wall surface, the upper limit of the EGR rate that can suppress the discharge of smoke is prevented while preventing the harmful effects such as oil dilution due to excessive advance. Be lifted.

Under the above injection timing, the EGR rate is set to a value that achieves an excess air ratio of 1.0 or less, and the EGR rate adjusting means controls the recirculation amount based on this EGR rate. Fig. 4 shows the test results in which the injection timing was further subdivided in the same operating range as in Fig. 5, and the ● mark in the figure shows the characteristics when the injection timing was set to 8 ° BTDC, and so on. , ○ indicates 10 ° BTDC, ▲ indicates 12 ° BTDC, Δ indicates 14 ° BTDC, × indicates 16 ° BTDC, * indicates 20 ° BTDC
, ◆ indicates 24 ° BTDC, ◇ indicates 28 ° BTDC, ■ indicates 3
2 ° BTDC, □ indicates characteristics at 36 ° BTDC. As is clear from the THC characteristics in this figure, the injection timing is 36
At BTDC, HC and CO emissions increase when the excess air ratio λ is 1.0 or less. Therefore, the NOx stored in the NOx storage catalyst is efficiently released and reduced in the reducing atmosphere.

In a preferred mode, it is desirable that the first control mode is executed when the engine operating region is in the low load region, and even if the air amount is reduced due to a large amount of EGR, the low fuel region requiring a small amount of fuel is required. If so, the control can be executed without any trouble. The invention of claim 2 is provided in a fuel injection means for injecting fuel into a combustion chamber of an engine, an EGR rate adjusting means for controlling a recirculation amount of exhaust gas discharged from the engine to an intake system, and an exhaust passage of the engine. NO that stores NOx in the exhaust gas when the exhaust air-fuel ratio is lean and releases and stores the stored NOx when the exhaust air-fuel ratio is rich or stoichiometric
x The storage catalyst and the fuel injection timing by the fuel injection means are
On the advance side of the injection timing at which the smoke emission characteristic with respect to the increase of the GR rate shows an increasing tendency and then on the decreasing tendency, rather than the injection timing at which the injected fuel leaves the cavity formed in the piston and reaches the cylinder wall surface. A first control mode is set in which the retard angle side is set and a first control mode in which the operation of the EGR rate adjusting means is controlled so that the excess air ratio becomes 1.0 or less.
A control means and a second control mode in which the injection timing is retarded from the control state of the first control mode to set it near the compression top dead center and the EGR rate is reduced from the control state of the first control mode. The two control means and the NOx storage catalyst are switched to the first control mode when NOx is to be released, and are switched to the second control mode when NOx is not to be released, and the EGR rate is gradually increased when the control mode is switched. And a mode switching means for instantaneously switching the injection timing in the middle of the changing EGR rate changing period.

In the first control mode, as described in claim 1, the harmful effect such as oil dilution is prevented, and the upper limit value of the EGR rate at which smoke emission can be suppressed is raised to 1.0. NOx due to the following excess air ratio
NOx is efficiently released and reduced from the storage catalyst. In the second control mode as compared with the above first control mode, the injection timing is retarded and the EGR rate is reduced. Therefore, in the operating region where the output demand of the driver is high, the mode switching means is used. By switching to the second control mode, it becomes possible to meet the output request.

On the other hand, when the control mode is switched by the mode switching means, the EGR rate changes only gradually due to the exhaust gas recirculation, but if the injection timing is gradually changed according to the changing characteristics, smoke will be generated. In some cases, the smoke may be temporarily increased by passing through a state where the injection timing and the EGR rate satisfy the condition of the region where the abrupt increase occurs.

For example, in FIG. 4, the second control mode is indicated by a large circle, and the injection timing is 8 °.
The BTDC is set, the EGR rate is set to 45%, and the excess air ratio λ is controlled to 1.8. On the other hand, the first control mode is shown by a large □ mark, and the injection timing is set to 36 ° BTDC which is far more advanced, and the EGR rate Regr is set to 56%. The excess ratio λ is controlled to 1.0 or less.

In the transient region where the excess air ratio λ = 1.0 to 1.8, when the injection timing is set to any of 14 to 32 ° BTDC, a phenomenon that noise mainly increases on the lean side, or It can be seen that noise and smoke cannot be compatible because the phenomenon that smoke increases mainly on the rich side occurs. Therefore, when switching between the first control mode and the second control mode, if the injection timing is gradually changed according to the broken line in the figure, it will inevitably pass through this 14 to 32 ° BTDC region. Occurrence of areas where noise and smoke increase sharply is inevitable.

On the other hand, the injection timing is set to 8 or 10
When ° BTDC is set, both noise and smoke can be reduced in the lean side region from the excess air ratio λ of around 1.3, while when the injection timing is set to 36 ° BTDC, It can be seen that both noise and smoke can be reduced in the region where the excess ratio λ is near the rich side from around 1.3. Therefore, in this example, as shown by the solid line in the figure, the injection timing may be instantaneously switched with the excess air ratio λ = 1.3 as the boundary.

According to the above-mentioned example, in the invention of this claim, the injection timing is instantaneously switched in the middle of the EGR rate changing period in which the EGR rate gradually changes. Therefore, the control mode is switched over the area where the smoke rapidly increases. As a result, the temporary increase in smoke that occurs during the EGR rate change period is avoided in advance. According to a third aspect of the present invention, the fuel injection timing by the fuel injection means is set in the same manner as in the control state of the first control mode, and the operation of the EGR rate adjusting means is performed so as to simultaneously reduce the smoke emission amount and the NOx emission amount. A third control means for executing the third control mode for controlling is provided, and NOx should not be released, and the third control mode is switched to when the operating region of the engine is in the low load region.

For example, in FIG. 4, the calibration points in the third control mode are indicated by the large □ marks in the broken line, and the injection timing is the same as in the first control mode, 36 ° BTDC.
And the excess air ratio is set slightly leaner than 1.0. In this third control mode, HC and CO emissions are reduced as compared with the first control mode as the excess oxygen increases, while smoke and NOx are reduced as in the first control mode. According to HC and C
It is possible to operate with both smoke and NOx reduced while suppressing an increase in O.

In a fourth aspect of the invention, the first control means is the first
The injection amount of the fuel injection means is increased so as to compensate for the decrease in engine torque when the control mode is executed. Therefore, since the decrease in the engine torque is compensated by the increase in the fuel injection amount, the control mode can be switched without causing the torque fluctuation. In the invention of claim 5, the first control means is
The injection timing is advanced as the engine speed increases.

When the piston speed increases with the engine rotation speed, the time at which the injected fuel reaches the cavity is relatively advanced, so that the piston position is still low in order to make the injected fuel reach the cavity at an appropriate time. It is necessary to accelerate fuel injection until the timing. Therefore, if the injection timing is advanced as the engine speed increases, the injected fuel always arrives in the cavity of the piston at an appropriate time regardless of the engine speed, and the first control mode is set to a stable combustion state. It becomes feasible.

According to the sixth aspect of the present invention, the first control means keeps the injection timing substantially constant with respect to the load change. Therefore, the dependency of the injection timing on the engine load is low compared to the engine speed, so that if the injection timing is kept almost constant with respect to the load change, the first control will not be affected by the fluctuation of the engine load. The mode can be realized in a stable combustion state.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION [First Embodiment] The first embodiment of a common rail type diesel engine according to the present invention will be described below. FIG. 1 is an overall configuration diagram showing a diesel engine of this embodiment. This figure shows one cylinder of a diesel engine, and a piston 2 arranged in the cylinder block 1 is connected to a crankshaft (not shown) via a connecting rod 3. A fuel injection valve 5 is arranged in the cylinder head 4 of the engine so as to face the inside of the cylinder, and the fuel injection valve 5 is connected to a common rail 6 common to each cylinder.
A fuel pump 7 is connected to the common rail 6, and high-pressure fuel supplied from the fuel pump 7 is stored in the common rail 6.

The fuel injection valve 5 is opened at a predetermined timing in the vicinity of the compression top dead center, and the high-pressure fuel in the common rail 6 is injected from the fuel injection valve 5 toward the cavity 2a at the piston head, and in the compressed air. The piston 2 is pushed down by igniting and burning (fuel injection means). Although the configuration of the common rail system is well known, it will not be described in detail, but the fuel injection valve 5
It is possible to freely set the fuel injection amount and the injection timing by controlling the valve open state of.

On the other hand, inside the cylinder of each cylinder, the cylinder head 4
Is connected to a common intake passage 9 through an intake port 8 formed in the air cleaner 1.
0, a compressor 11a of the turbocharger 11, and an intake throttle valve 12 are provided. Further, the inside of each cylinder is connected to a common exhaust passage 14 via an exhaust port 13 of the cylinder head 4, and the exhaust passage 14 is provided coaxially with the exhaust throttle valve 15 and the compressor 11a from the upstream side. Turbine 11b of the stored turbocharger 11, storage type N
An Ox catalyst 16, an oxidation catalyst 17, and a silencer (not shown) are provided. The NOx catalyst 16 stores the NOx in the exhaust when the exhaust air-fuel ratio of the engine is lean, and when the exhausted air-fuel ratio is rich or stoichiometric (when HC or CO exists in the exhaust gas), the stored NOx is stored. The oxidation catalyst 17 has a function of releasing and reducing, and the oxidation catalyst 17 contains HC, CO
It has a function to oxidize and purify.

The intake air introduced into the intake passage 9 through the air cleaner 10 is supercharged by the compressor 11a and then introduced into the cylinder when the intake valve 18 of each cylinder is opened, and the piston 2 rises. Is compressed and used for combustion as described above. The exhaust gas after combustion is discharged to the exhaust passage 14 with the opening of the exhaust valve 19 to drive the turbine 11b, then passes through the NOx catalyst 16 and the oxidation catalyst 17, passes through the silencer, and is discharged into the atmosphere.

On the other hand, one end of the EGR passage 23 is connected to the intake passage 9 at a position downstream of the intake throttle valve 12, and the E
An EGR valve 21 and an EGR cooler 25 are provided in the GR passage 23, and the other end of the EGR passage 23 is connected to an upstream position of the exhaust throttle valve 15 in the exhaust passage 14.
Recirculation of EGR from the exhaust passage 14 to the intake passage 9 is 2 EG
It is performed through the R passage 23, and when the EGR valve 21 is opened,
The exhaust gas cooled by the EGR cooler 25 is recirculated through the EGR passage 23. The EGR rate at this time is
Opening of the valve 21, restriction of intake air by the intake throttle valve 12,
It is appropriately adjusted according to the restriction of exhaust gas by the exhaust throttle valve 15 (EGR rate adjusting means).

On the other hand, in the passenger compartment, an input / output device (not shown), a storage device (ROM, RAM, etc.) for storing control programs and control maps, and a central processing unit (CP) are provided.
U), an ECU (electronic control unit) 31 including a timer counter and the like is installed. An accelerator sensor 3 for detecting the accelerator operation amount APS is provided on the input side of the ECU 31.
2, rotation speed sensor 3 for detecting engine rotation speed Ne
Various sensors such as 3 are connected, and the fuel injection valve 5, the intake throttle valve 12, the exhaust throttle valve 15, the EGR valve 1 are provided on the output side.
Various devices such as 2 are connected.

Then, the ECU 31 determines the accelerator operation amount AP
The fuel injection amount Q is calculated from a map (not shown) based on S and the engine rotation speed Ne, while the fuel injection timing IT is calculated from a map (not shown) based on the engine rotation speed Ne and the fuel injection amount Q, and these calculated values are calculated. The fuel injection valve 5 is drive-controlled based on the above. Also, the engine speed N
Based on the calculated value of e and the fuel injection amount Q, a target excess air ratio λtgt is calculated from a map (not shown), and the target excess air ratio λtgt and the actual excess air ratio λ (for example, intake air obtained from the output of an air flow sensor (not shown)). Quantity, EGR
It is possible to calculate from the fresh air amount and the fuel injection amount Q to which the estimated value of the residual oxygen amount in the gas is added, or to provide the linear air-fuel ratio sensor in the exhaust passage 14 and obtain it from the sensor output). The opening degree of the EGR valve 21 (that is, the EGR rate Reg is set so that the excess air rate λ becomes the target excess air rate tgtλ).
feedback control of r).

Here, in the diesel engine of the present embodiment, the combustion mode is switched according to the operating state of the engine to greatly change the fuel injection timing IT. The switching of the combustion mode will be described in detail below. FIG. 2 is a flowchart showing a combustion mode switching routine executed by the ECU 31. First, the ECU 31 determines in step S2 whether or not the engine has been warmed up based on the cooling water temperature and the like, and in subsequent step S4, the fuel injection amount Q and the engine rotation speed Ne.
Based on the above, it is determined whether or not the current operation area is the preset specific area according to the map shown in FIG. Here, as the specific region, the fuel injection amount Q (correlated with the engine load)
And the engine rotation speed Ne is set to a low load and low rotation speed region below a predetermined value Q0, Ne0.

Next, at step S6, it is judged whether or not it is the NOx purge timing at which the NOx purge should be executed. There are various concrete determination methods. For example, when the detection value of the NOx sensor (not shown) is equal to or higher than a predetermined value, the NOx catalyst 16
When the leakage of NOx from the engine is estimated, or the NOx emission amount estimated from the operating region of the engine is sequentially integrated to obtain the current NOx storage amount on the NOx catalyst 16, and the value exceeds a predetermined value. In this case, the NOx purge timing is considered.

When the determination is NO (negative) in any of the above steps S2 to S6, the routine proceeds to step S8, the normal combustion mode (second control mode) is executed, and then the routine ends (second control). means). Also, from step S2
When the determination of YES (affirmative) is made in all of 6, the process proceeds to step S10 and the NOx purge combustion mode (first
After executing the control mode), the routine ends (first
Control means).

The above-mentioned normal combustion mode is a control mode similar to the control contents implemented in a general diesel engine, and the NOx purge combustion mode is a combustion mode peculiar to the present invention. FIG. 4 is a control situation of the fuel injection timing IT and the EGR rate Regr in the normal combustion mode and the NOx purge combustion mode, and THC emission amount, NOx emission amount, noise when the fuel injection timing IT is set to 8 to 36 ° BTDC, It is a characteristic diagram showing the test result of measuring the amount of smoke discharged, and ● in the figure shows the characteristic when IT = 8 ° BTDC is set. In the same manner, ○ is 10 ° BTDC and ▲ is 12 ° BTDC
, △ indicates 14 ° BTDC, × indicates 16 ° BTDC, * indicates 2
0 ° BTDC, ◆ indicates 24 ° BTDC, ◇ indicates 28 ° BTDC,
The solid squares show the characteristics at 32 ° BTDC, and the solid squares show the characteristics at 36 ° BTDC. The test results are obtained when the operating region of the engine is set to the target average effective pressure Pe = 0.2 MPa (correlated with the engine load) and the engine rotation speed Ne = 2000 rpm.

The calibration point in the normal combustion mode is large. The position is indicated by ●, and the fuel injection timing IT is 8 ° BTDC.
Then, the EGR rate Regr is set to 45% and the excess air rate is controlled to around 1.8. On the other hand, the calibration point of the NOx purge combustion mode is set at a large □ position, and the fuel injection timing IT is far advanced by 36.
The temperature is set to BTDC, the EGR rate Regr is set to 56%, and as a result, the excess air ratio λ is controlled to 1.0 (theoretical air-fuel ratio) or slightly rich side (λ ≦ 1.0).

Fuel injection timing I in NOx purge combustion mode
T = 36 ° BTDC is set as follows. FIG. 5 shows that fuel injection timing IT is 10, 2 from FIG.
It is a characteristic diagram that extracts test results at 0,36 ° BTDC, and IT = 10 ° BTDC shown by a circle in the figure is the fuel injection timing IT applied to a general diesel engine (the above normal combustion mode). IT = 20 indicated by * mark
° BTDC is described in Japanese Patent No. 311687
It corresponds to the fuel injection timing IT applied to No. 6, and IT = 36 ° BTDC shown by a square corresponds to the fuel injection timing IT in the NOx purge combustion mode of the present embodiment.

At 10 ° BTDC, although the NOx emission amount decreases as the EGR rate Regr increases, although not shown in the figure, since the smoke increases rapidly when λ = 1.8, the smoke emission increases. The upper limit value of the EGR rate Regr that can suppress the above is considerably low. Also, at 20 ° BTDC, the amount of smoke emission shows a tendency to increase with an increase in the EGR rate Regr, and then shows a tendency to decrease after exceeding the peak, but even at the minimum value of λ = 1.05, it is sufficient. It is presumed that the smoke emission amount transiently sharply increases because the smoke emission amount exceeds the peak every time the excess air ratio λ changes with the switching of the combustion mode.

On the other hand, in the case of 36 ° BTDC in which the fuel injection timing IT is further advanced, the smoke emission amount is suppressed to an extremely low value even if the EGR rate Regr is increased to about 56%.
Since the high EGR rate Regr also suppresses the NOx emission amount, the smoke excess and the NOx are both reduced, and the achievable excess air ratio λ is expanded to a region near 1.0. Moreover, since the peak of the smoke emission amount is not formed, the transient smoke increase is also suppressed. And, as is clear from the THC characteristics of FIG. 4,
When the excess air ratio λ is 1.0 or less, the emission amount of HC and CO increases. Therefore, when the NOx purge combustion mode of step S10 is executed, the NOx stored in the NOx catalyst 16 is reduced in the reducing atmosphere. Is efficiently released and reduced.

The reason why the smoke emission amount is suppressed by the advance of the fuel injection timing IT is that the period from the fuel injection to the ignition is extended and the premixing of the injected fuel and the intake air is performed during that period. Is believed to be promoted. However,
Unlike Japanese Patent Application Laid-Open No. 8-218920 described as the prior art, since the fuel injection timing IT is not advanced to the intake stroke, the injected fuel is near the compression top dead center even in this NOx purge combustion mode. Is reliably ignited at a predetermined timing, and stable operation is performed without causing ignition failure.

On the other hand, FIG. 6 is a characteristic diagram showing the test results of measuring the THC emission amount and the diluted fuel amount by changing the fuel injection timing IT in the same operating range as in FIGS. As shown in this figure, when the fuel injection timing IT is advanced, 40
° From around BTDC, the amount of diluted fuel (the amount of fuel that mixes with engine oil and exerts a diluting effect) increases, which may cause oil dilution. This is because a part of the fuel injected by the early fuel injection diffuses and adheres to the cylinder wall surface without being trapped in the cavity 2a of the piston 2, and the adhered fuel passes through the piston clearance to the oil. It is caused by mixing with the oil in bread.
Further, if the mixing action of the fuel spray and the intake air due to the cavity lip or the reverse squish flow is not sufficiently obtained, it will lead to an increase in smoke, and from this point too, an excessive advance angle is not desirable.

Further, when the fuel injection timing IT is excessively advanced, the fuel injection is performed in the intake stroke.
As in the prior art of Japanese Patent No. 218920, the ignition timing in the vicinity of the compression top dead center of the fuel varies, which easily causes an ignition failure such as premature ignition or ignition delay. However, there is a limit to the advance angle. Due to the above factors, the fuel injection timing IT is 40 ° BT on the advance side.
Limited to DC, resulting in NOx as above
The fuel injection timing IT in the purge combustion mode is set to 36 ° BTDC.

As described above, in the NOx purge combustion mode, since a large amount of EGR is recirculated to enrich the excess air ratio λ, the engine load and the engine rotation speed Ne are reduced.
In the high range, the amount of air for burning a large amount of fuel is insufficient, and the engine load and the engine speed Ne are limited, and the implementation is limited to the specific region of the map of FIG. However, in the present embodiment, since a relatively large amount of air can be supplied even in the NOx purge combustion mode by supercharging the turbocharger 11, the upper limit value of the specific region is significantly expanded as compared with the naturally aspirated diesel engine. Has been done.

On the other hand, in the specific region, the optimum fuel injection timing IT changes according to the operating region of the engine, but the dependency on the fuel injection amount Q (engine load) is low, but on the other hand, with respect to the engine rotation speed Ne. The optimum fuel injection timing IT changes greatly. That is, when the piston speed increases together with the engine rotation speed Ne, the timing at which the injected fuel reaches the cavity 2a is relatively advanced. Therefore, in order to make the injected fuel reach the cavity 2a at an appropriate time, the piston position is This is because it is necessary to accelerate fuel injection until the time is still low. Therefore, within the specific region of the map of FIG. 3, the fuel injection timing IT is set so as to change to the advance side as the engine speed Ne increases, and it hardly changes as the fuel injection amount Q increases or decreases. Is set to be almost constant.

On the other hand, as described with reference to FIG. 4, when the combustion mode is switched with the change of the operating region of the engine,
The fuel injection timing IT is switched between 8 ° BTDC and 36 ° BTDC, and the excess air ratio λ is between 1.8 and 1.0 (E
The GR rate Regr is switched between 45% and 56%). Here, the fuel injection timing IT can be changed stepwise in the fuel injection control of the common rail type with high responsiveness, but in the EGR control, the EGR rate R is caused by exhaust gas recirculation.
Since egr changes gently, the EGR rate Regr at the time of mode switching inevitably has a transient region. At the time of mode switching, the fuel injection timing IT is not tailed according to the change in the EGR rate Regr as shown by the broken line in FIG.
The fuel injection timing IT is changed stepwise with a predetermined point in the GR rate change period as a boundary (mode switching means), and the necessity of control during the mode switching will be described below.

As shown in FIG. 4, the excess air ratio λ = 1.0
In the transitional region of ~ 1.8, the fuel injection timing IT is set to 14
When set to any of ˜32 ° BTDC, it can be seen that noise and smoke cannot be compatible because the phenomenon that noise increases mainly on the lean side or the phenomenon that smoke increases mainly on the rich side occurs. If the fuel injection timing IT is tailed according to the broken line in FIG. 4, this 14 to 32 ° is inevitably generated.
Since it passes through the BTDC area, it is inevitable that noise and smoke will increase rapidly.

On the other hand, when the fuel injection timing IT is set to 8 or 10 ° BTDC, both noise and smoke can be reduced in the region where the excess air ratio λ is leaner than around 1.3. Meanwhile, the fuel injection timing IT is set to 36 °
When set to BTDC, it can be seen that both noise and smoke can be reduced in a region where the excess air ratio λ is near the rich side from around 1.3.

Therefore, when the normal combustion mode is switched to the NOx purge combustion mode, the excess air ratio λ is 1.3.
In the above range, after gradually increasing the fuel injection timing IT to 8 to 10 ° BTDC, the fuel injection timing IT is gradually increased to 3 at a boundary of λ = 1.3.
Switch to 6 ° BTDC. When switching from the NOx purge combustion mode to the normal combustion mode, control may be performed in the reverse procedure.

On the other hand, in the NOx purge combustion mode in which the excess air ratio λ is set to the rich side with respect to the normal combustion mode,
A torque drop occurs under the same injection amount. Therefore, in the present embodiment, the required fuel amount is calculated in advance for each excess air ratio λ, and the actual fuel injection amount Q is corrected according to the excess air ratio λ, whereby the torque fluctuation at the time of switching the combustion mode is corrected. To prevent.

As described above, in the diesel engine of this embodiment, the fuel injection timing I is compared with the value generally applied.
Paying attention to the fact that the upper limit value of the EGR rate Regr capable of suppressing smoke emission increases in a region where T is greatly advanced,
When the NOx catalyst 16 reaches the NOx purge timing, the NOx purge combustion mode in which the fuel injection timing IT is set to the advanced side is executed. Then, in this NOx purge combustion mode, an excess air ratio λ of 1.0 or less is realized by a large amount of EGR, and the amount of HC and CO emissions increases, so that NOx does not increase as a result, and NOx increases. NOx can be released and reduced extremely efficiently from the catalyst 16.

Moreover, in this embodiment, the air amount is secured by supercharging the turbocharger 11, and the upper limit of the specific region in which the NOx purge combustion mode is executed is expanded thereby, so that the NOx purge combustion is performed in a wide operating region. The mode can be executed, and when the NOx purge timing is reached, NOx release reduction can be started immediately. In addition, the fuel injection timing IT in the NOx purge combustion mode is
Since it is set in consideration of the advance angle limit caused by the deterioration of stability due to poor ignition, the occurrence of oil dilution, etc., it is possible to obtain the above-mentioned effects after preventing these adverse effects. .

Further, the fuel injection amount Q is increased and corrected in the NOx purge combustion mode so as to compensate for the decrease in the engine torque due to the enrichment of the excess air ratio λ, so that the torque fluctuation at the time of switching the combustion mode is prevented. As a result, a good driving feeling can be maintained. Further, since the fuel injection timing IT is controlled to the advance side as the engine speed Ne increases, the injected fuel should always reach the cavity 2a of the piston 2 at an appropriate time regardless of the engine speed Ne. As a result, the NOx purge combustion mode can be realized in a stable combustion state.

On the other hand, it is considered that the increase or decrease of the fuel injection amount Q (engine load) is less dependent on the fuel injection timing IT, and the increase or decrease of the fuel injection amount Q is controlled to be substantially constant. The NOx purge combustion mode can be realized in a stable combustion state without being affected by changes in the engine load. or,
At the NOx purge timing, the NOx purge combustion mode is executed, and in other normal times, it is switched to the normal combustion mode similar to a general diesel engine, so that it is possible to reliably meet the output required by the driver. Good traveling characteristics of the vehicle can be realized. Particularly, in the present embodiment, since the exhaust gas cooled by the EGR cooler 25 is recirculated, the achievable engine torque is increased and the running performance of the vehicle can be further improved.

Moreover, when switching the combustion mode, the EGR
The fuel injection timing IT is not tailed according to the change of the rate Regr, but is switched stepwise with a predetermined point (λ = 1.3) as a boundary. Therefore, the EGR rate Reg
The mode switching is performed by skipping over the area (IT = 14 to 32 ° BTDC) where the noise and smoke present in the transient region of r rapidly increase, and it is possible to avoid the temporary increase in noise and smoke due to the mode switching. .

In the NOx purge combustion mode, the fuel injection timing IT is set to 36 ° BTDC, but as can be seen from FIG. 4, noise and smoke characteristics are hardly deteriorated even at 32 ° BTDC or 28 ° BTDC. Therefore, in the NOx purge combustion mode based on this operating range, the lower limit of the fuel injection timing IT is 28 ° BTDC, and the upper limit is 40 ° BTDC which is limited by the oil dilution, etc., and is arbitrarily set between them. May be.

[Second Embodiment] Next, a second embodiment of a diesel engine embodying the present invention will be described. The diesel engine according to the present embodiment is different from that described in the first embodiment in that a low NOx combustion mode is executed in addition to the normal combustion mode when the NOx purge timing is not set. Have in common. Therefore, the description of the common parts will be omitted, and the differences will be mainly described.

FIG. 7 is a flow chart showing a combustion mode switching routine executed by the ECU 31, which is step S6.
It is determined whether or not it is the NOx purge timing. If YES, the process proceeds to step S10 to execute the NOx purge combustion mode. When the determination in step S6 is NO, the process proceeds to step S12, and the low NOx combustion mode (third control mode) is executed (third control means).

In FIG. 4, the calibration points in the low NOx combustion mode are indicated by the large square □ marks, and the fuel injection timing IT is set to the same value as in the NOx purge combustion mode (36 ° BTDC in FIG. 4). The air excess ratio λ is set slightly leaner than 1.0 (λ> 1.0) unlike the NOx purge combustion mode. That is, as is clear from the characteristics of FIG. 4, in the low NOx combustion mode, the HC is higher than that in the NOx purge combustion mode as the excess oxygen increases.
While the CO and CO emissions are reduced, the feature that smoke and NOx are reduced as in the NOx purge combustion mode is maintained.

The switching between the low NOx combustion mode and the normal combustion mode is similar to the case of the NOx purge combustion mode described in the first embodiment, and the predetermined point (λ = 1.3).
The fuel injection timing IT is changed stepwise at the boundary of
This prevents temporary noise and smoke from increasing in the transient region. As described above, in the diesel engine of the present embodiment, in addition to the effects of the first embodiment, when the NOx purge timing is not set and the operating region of the engine is in the specific region, the fuel is compared with the normal combustion mode. Since the low NOx combustion mode in which the injection timing IT is set to the advanced side is executed, smoke and NOx can be reduced at a very high level.

On the other hand, the HC and CO emissions in the low NOx combustion mode slightly increase as compared with those in the normal combustion mode.
HC and CO are easier to post-process compared to smoke,
The oxidation catalyst 17 in the exhaust passage 14 can surely perform oxidative purification. Further, when the oxidation catalyst 17 has not been activated yet, the determination in step S2 of FIG. 7 becomes NO, and low NOx
Since the execution of the combustion mode is prohibited and the normal combustion mode in which the emission amount of HC and CO is smaller is switched to, the emission of HC and CO can be reliably prevented in any operating region as a result.

Although the embodiment has been described above, the aspect of the present invention is not limited to this embodiment. For example, in the first and second embodiments, the fuel injection timing IT
The diesel engine is configured as a common rail type for precise control of the engine, supercharged by the turbocharger 11 to expand the specific region of the low NOx combustion mode, and the EGR cooler 25 is operated to increase the engine torque. However, the present invention is not limited to these modes, and is embodied in a general diesel engine in which fuel injection is controlled by a governor, or the turbocharger 11 is a variable nozzle type,
The turbocharger 11 and the EGR cooler 25 may be omitted.

Further, in the first and second embodiments, the fuel injection timings IT and EG are based on the characteristics of FIGS. 4 to 6 corresponding to the predetermined operation range (Pe = 0.2 MPa, Ne = 2000 rpm).
Although the control states of the R rate Regr and the excess air rate λ are illustrated, as described above, these settings are different depending on the operating region and are different if the engine specifications are changed. Therefore, it is needless to say that the setting is not limited to the above setting, and may be set based on the characteristics corresponding to the operating region, the specification of the engine, and the like.

[0062]

As described above, according to the diesel engine of the first aspect of the present invention, smoke emission is suppressed while preventing the deterioration of stability due to poor ignition and the occurrence of oil dilution. It is possible to raise the upper limit of the possible EGR rate, and thereby release and reduce NOx from the NOx storage catalyst without increasing the smoke emission amount.

According to the diesel engine of the invention of claim 2, in addition to the invention of claim 1, by executing the second control mode, good running characteristics of the vehicle can be realized in accordance with the output demand of the driver. Moreover, when the control mode is switched, the injection timing is instantaneously switched to jump over the area where the smoke increases rapidly, and thus a temporary increase in smoke can be avoided.

According to the diesel engine of the third aspect of the present invention, in addition to the first or second aspect of the invention, in the low load range where NOx should not be emitted, the control mode is switched to the third control mode.
It is possible to achieve both smoke and NOx reduction at an extremely high level. According to the diesel engine of the invention of claim 4, in addition to the invention of claim 1, it is possible to prevent torque fluctuation at the time of switching the control mode and maintain a good traveling feeling.

According to the diesel engine of the fifth aspect of the present invention, in addition to the first to third aspects of the present invention, the injected fuel always reaches the cavity of the piston at an appropriate time regardless of the engine speed. NOx can be released and reduced in a stable combustion state. According to the diesel engine of the invention of claim 6, in addition to the invention of claim 5, NOx can be released and reduced in a stable combustion state without being affected by changes in engine load.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram showing a diesel engine of a first embodiment.

FIG. 2 is a flowchart showing a combustion mode switching routine executed by the ECU of the first embodiment.

FIG. 3 is an explanatory diagram showing a map for setting a combustion mode.

FIG. 4 is a control situation of fuel injection timing IT and EGR rate Regr in a normal combustion mode and a NOx purge combustion mode,
FIG. 6 is a characteristic diagram showing test results of measuring THC emission amount, NOx emission amount, noise, and smoke emission amount when the fuel injection timing IT is set to 8 to 36 ° BTDC.

FIG. 5 shows that fuel injection timing IT is 10, 20, 36 from FIG.
It is the characteristic figure which extracted the test result at the time of ° BTDC.

FIG. 6 is a characteristic diagram showing test results obtained by measuring the THC emission amount and the diluted fuel amount by changing the fuel injection timing IT.

FIG. 7 is a flowchart showing a combustion mode switching routine executed by the ECU of the second embodiment.

[Explanation of symbols]

5 Fuel injection valve (fuel injection means) 17 Oxidation catalyst 21 EGR valve (EGR rate adjustment means) 31 ECU (first control means, second control means, third control means, mode switching means)

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) F01N 3/28 301 F01N 3/28 301D F02D 21/08 301 F02D 21/08 301D 41/02 380 41/02 380E 385 385 385 41/04 355 41/04 355 385 385Z F02M 25/07 570 F02M 25/07 570J (72) Inventor Riki Morita No. 33-8 Shiba 5-chome, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Inventor Koga ▲ Toku ▼ Sachi 5-3 8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Industrial Co., Ltd. (72) Inventor Yuji Yanagawa 5-3-33 8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation F-term (Reference) 3G062 AA01 AA03 AA05 BA04 BA05 BA06 CA06 DA01 DA02 EA10 ED01 ED04 ED08 ED10 FA02 FA05 FA06 FA13 FA23 GA01 GA04 G A06 GA08 GA15 3G084 AA01 BA09 BA13 BA15 BA20 CA04 CA09 DA10 EB12 EC03 FA07 FA10 FA33 3G091 AA02 AA10 AA11 AA18 AA28 AB02 AB06 BA14 CB02 CB03 DA02 DC03 EA01 EA05 EA07 FA08 FA13 FB12 GA06 HA09 HB05 3G092 AA02 AA17 AA18 AB03 BA04 BA07 BB01 BB04 DC03 DC09 DC12 DE03S EA01 EA03 EA05 EC01 FA17 GA05 GA17 HA01Z HE01Z HF08Z 3G301 HA02 HA11 HA13 JA25 KA08 KA24 LA03 LB11 MA01 MA14 MA18 NA08 NC02 ND02 NE01 NE11 NE13 PA01Z PE01Z PF03Z

Claims (6)

[Claims] 1. A fuel injection means for injecting fuel into a combustion chamber of an engine, an EGR rate adjusting means for controlling a recirculation amount of exhaust gas discharged from the engine to an intake system, and an exhaust passage provided for the engine. NOx stored in the exhaust gas when the exhaust air-fuel ratio is lean, and the stored NOx
The NOx storage catalyst that releases and reduces the exhaust air-fuel ratio when the exhaust air-fuel ratio is rich or stoichiometric, and the fuel injection timing by the fuel injection means when the NOx storage catalyst should release NOx It is set to the advance side of the injection timing at which the quantity characteristic shows an increasing tendency and then to the decrease tendency, and to the retard side of the injection timing at which the injected fuel leaves the cavity formed in the piston and reaches the cylinder wall surface. And a first control means for executing a first control mode for controlling the operation of the EGR rate adjusting means so that the excess air ratio becomes 1.0 or less. 2. A fuel injection means for injecting fuel into a combustion chamber of an engine, an EGR rate adjusting means for controlling a recirculation amount of exhaust gas discharged from the engine to an intake system, and an exhaust passage provided for the engine. NOx stored in the exhaust gas when the exhaust air-fuel ratio is lean, and the stored NOx
The NOx storage catalyst that releases and reduces the exhaust air-fuel ratio when the exhaust air-fuel ratio is rich or stoichiometric, and the fuel injection timing by the fuel injection means shows a decreasing tendency after the smoke emission amount characteristic shows an increasing tendency with respect to the increase of the EGR rate. On the advance side of the injection timing, the injection fuel is set on the retard side of the injection timing when it leaves the cavity formed in the piston and reaches the cylinder wall surface.
First control means for executing a first control mode for controlling the operation of the EGR rate adjusting means so that the excess air ratio becomes 1.0 or less; and the injection timing is retarded from the control state of the first control mode. Is set near the compression top dead center, and second control means for executing a second control mode in which the EGR rate is reduced from the control state of the first control mode, and when NOx is to be released from the NOx storage catalyst Switching to the first control mode, switching to the second control mode when the NOx should not be released, and switching the control mode, the injection timing is set in the middle of the EGR rate changing period in which the EGR rate gradually changes. A diesel engine having a mode switching means for instantaneously switching. 3. The fuel injection timing of the fuel injection means is set in the same manner as in the control state of the first control mode, and the EGR rate adjusting means operates so as to simultaneously reduce the smoke emission amount and the NOx emission amount. A third control means for executing a third control mode for controlling the engine, wherein the NOx should not be released and the operating range of the engine is switched to the third control mode when the operating range is a low load range. The diesel engine according to claim 1 or 2. 4. The first control means increases the injection amount of the fuel injection means so as to compensate for a decrease in engine torque during execution of the first control mode. The diesel engine according to any of the above. 5. The first control means advances the injection timing as the engine speed increases.
4. The diesel engine according to any one of 3 to 3. 6. The diesel engine according to claim 5, wherein the first control means keeps the injection timing substantially constant with respect to a load change.