JP4626774B2 - Diesel engine - Google Patents

Description

  The present invention relates to a diesel engine, and more particularly to control of fuel injection timing and EGR recirculation amount.

  In a diesel engine that performs lean operation mainly with diffusion combustion, NOx is likely to be generated due to a high excess air ratio. As a countermeasure, part of the exhaust gas is recirculated to the intake system, and NOx is generated by lowering the combustion temperature. EGR control is performed to suppress this. As the EGR rate increases, the NOx suppression action increases. However, in the diffusion combustion that causes combustion in the combustible air-fuel mixture layer at the boundary between the injected fuel and the compressed air, if the EGR rate is excessively increased, fuel remains unburned. As a result, there is a harmful effect that the smoke emission increases rapidly. Therefore, in order to avoid this, the upper limit value of the EGR rate is limited, and as a result, sufficient NOx suppression cannot be realized.

  For example, in Patent Document 1, as a technique for releasing NOx from a NOx occlusion material provided in an exhaust system of a diesel engine, fuel is injected in an intake stroke and a large amount of EGR is introduced. A substance that releases NOx by reducing it to 0 or less is disclosed. The combustion state at this time is switched to premixed combustion in which the injected fuel diffuses and vaporizes up to the compression top dead center and burns in a state where it is mixed with air in advance. As a result, it is presumed that both smoke and NOx reduction can be achieved.

On the other hand, Patent Document 2 pays attention to the fact that when the EGR rate is increased beyond the above-described upper limit value, the smoke emission amount suddenly increases and then shifts to a decreasing tendency beyond the peak. In the area, a technique for controlling the EGR rate when the smoke emission amount starts to decrease is disclosed, thereby achieving both smoke and NOx.
JP-A-8-218920 Patent No. 3116876

  However, in the technique of Patent Document 1, since the period from the fuel injection in the intake stroke to the ignition in the compression stroke is long, the ignition timing varies, and it tends to cause poor ignition such as premature ignition or ignition delay. There is a problem of lack of stability. In addition, a part of the fuel injected in the intake stroke diffuses in the cylinder, adheres to the cylinder wall, causes oil dilution, and seems to be injected at a timing near the normal compression top dead center. In addition, since it is not trapped in the cavity of the piston, there is a problem that it causes a rapid increase in hydrocarbon (HC) and carbon monoxide (CO).

Further, in the technique of Patent Document 2, when the control mode is switched between the predetermined region and the other region, the smoke discharge amount peak is passed, so that it is inevitably transient every time the mode is switched. However, there is a problem that smoke emission increases rapidly.
The object of the present invention is to raise the upper limit of the EGR rate that can suppress smoke emission, while preventing the deterioration of stability due to poor ignition, the occurrence of oil dilution and the increase of HC and CO. An object of the present invention is to provide a diesel engine capable of achieving both smoke and NOx reduction.

In order to achieve the above object, the invention of claim 1 includes 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, The fuel injection timing by the fuel injection means is advanced from the specific fuel injection timing in the operating range where the smoke emission characteristic with respect to the decrease in the excess air ratio can be operated at the specific fuel injection timing that shows a downward trend after the tendency to increase. On the other hand, the injection fuel is set to be retarded from the injection timing when the fuel is released from the cavity formed in the piston and reaches the cylinder wall surface, and the operation of the EGR control means is controlled at the set injection timing to obtain the smoke discharge amount. First control means for executing a first control mode for simultaneously controlling the NOx emission amount, and retarding the injection timing from the control state of the first control mode to retard the specific fuel injection timing And sets the compression top dead center near near side, and a second control means for executing the second control mode with reduced EGR rate from the control state of the first control mode, the first control mode based on the operating region of the engine Between the first control mode and the second control mode, and when the control mode is switched, the fuel injection in both the first control mode and the second control mode exists in the middle of the EGR rate change period in which the EGR rate changes gradually. And mode switching means for instantaneously switching the injection timing at an EGR rate that can reduce both noise and smoke at the timing.

  The characteristic of the smoke emission amount with respect to the EGR rate changes according to the injection timing. FIG. 5 shows an example of a test result in which the fuel injection timing is changed in a certain operation region. When the angle is advanced to 20 ° BTDC compared to 10 ° BTDC applied to a general diesel engine, the EGR rate is increased. The smoke emission amount with respect to the increase has a characteristic of showing an increasing tendency and then a decreasing tendency beyond the peak. In this case, if the excess air ratio is changed together with the EGR ratio, the smoke emission amount exceeds the peak region, so a transient increase in the smoke emission amount cannot be avoided. On the other hand, at 36 ° BTDC in which the injection timing is further advanced, the smoke emission amount is suppressed to a low value even when the EGR rate is increased, and the NOx emission amount is also suppressed by the high EGR rate. Since both the smoke emission peak is not formed, a transient increase in smoke is also suppressed.

  The reason why the smoke emission amount is suppressed by the advance of the injection timing in this way is thought to be that the period from fuel injection to ignition is extended and premixing of the injected fuel and intake air is promoted during that period. However, since the injection timing is not extremely advanced as in the intake stroke injection, for example, the injected fuel is reliably ignited at a predetermined timing in the vicinity of the compression top dead center, and a stable operation without causing an ignition failure is achieved. Is possible.

On the other hand, FIG. 6 shows a test result in which the THC emission amount and the diluted fuel amount are measured by changing the injection timing in the same operation region as in FIG. 5, and when the injection timing is advanced, around 40 ° BTDC As THC increases, the amount of fuel mixed in the engine oil increases and there is a risk of oil dilution.
Based on the above factors, in the first control mode, the smoke emission characteristic with respect to the increase in the EGR rate is on the advance side with respect to the injection timing that shows a decreasing trend and then the injected fuel is formed in the piston. If the injection timing is set to the retarded angle side relative to the injection timing to reach the cylinder wall after leaving the cavity, it is possible to prevent harmful effects such as increase in THC due to excessive advance and oil dilution, etc. The upper limit of the EGR rate that can suppress discharge is raised. And since the recirculation amount is controlled by the EGR rate adjusting means based on this EGR rate, in the first control mode, the smoke discharge amount and the NOx discharge amount can be simultaneously reduced.

Since it is reduced EGR rate with the first compared to the control mode in the second control mode is set to the compression top dead center near vicinity of the retarded angle side than a specified injection timing retarded injection timing In the driving region where the driver's output request is high, it is possible to respond to the output request by switching to the second control mode by the mode switching means.
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 change characteristics, the smoke increases rapidly. There is a case where smoke is temporarily increased by passing through an injection timing and an EGR rate that satisfy the above condition.

  For example, FIG. 4 shows a test result in which the injection timing is further subdivided in the same operation region as in FIG. 5. The ● mark in the figure shows the characteristics when the injection timing is set to 8 ° BTDC. Similarly, ○ mark is 10 ° BTDC, ▽ mark is 12 ° BTDC, △ mark is 14 ° BTDC, X mark is 16 ° BTDC, * mark is 20 ° BTDC, ◆ mark is 24 ° BTDC, ◇ mark is 28 ° BTDC, ★ mark indicates characteristics at 32 ° BTDC, and □ mark indicates characteristics at 36 ° BTDC.

  In the figure, the second control mode is indicated by a large ● mark, 8 ° BTDC is set as the injection timing, 45% is set as the EGR rate, and the excess air ratio λ is controlled to 1.8. Is done. On the other hand, the first control mode is indicated by a large □ mark, and the injection timing is set to 36 ° BTDC far ahead, and the EGR rate Regr is set to 56%. The excess ratio λ is controlled to be slightly leaner than 1.0, for example.

  When the injection timing is set to any of 14 to 32 ° BTDC in the transient region where the excess air ratio λ is 1.0 to 1.8, the phenomenon is that the noise increases mainly on the lean side or is mainly rich. It can be seen that noise and smoke cannot be achieved at the same time because smoke increases. 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, the region inevitably passes through the region of 14 to 32 ° BTDC. Generation of areas where noise and smoke increase rapidly is inevitable.

  On the other hand, when the injection timing is set to 8 or 10 ° BTDC, both the noise and smoke can be reduced in the region where the excess air ratio λ is leaner than around 1.3, while the injection timing is When it is set to 36 ° BTDC, it can be seen that both noise and smoke can be reduced in the region where the excess air ratio λ is richer than around 1.3. Therefore, in this example, as indicated by the solid line in the figure, the injection timing may be switched instantaneously with the excess air ratio λ = 1.3 as a boundary.

In accordance with the above examples, in the invention of this claim, since the injection timing is instantaneously switched in the middle of the EGR rate change period in which the EGR rate changes gradually, the control mode can be switched over the region where noise and smoke rapidly increase. As a result, a temporary increase in noise and smoke occurring during the EGR rate change period is obviated.
According to a second aspect of the present invention, in the first aspect, the first control means advances the injection timing as the engine speed increases.

  When the piston speed increases with the engine speed, the time at which the injected fuel reaches the cavity is relatively advanced. Therefore, in order for the injected fuel to reach the cavity at an appropriate time, the fuel must be fueled until the piston position is still low. It is necessary to accelerate the injection. Therefore, if the injection timing is advanced as the engine rotation speed increases, the injected fuel always reaches the piston cavity at an appropriate time regardless of the engine rotation speed, so that the combustion state can achieve both smoke and NOx. Can be realized stably.

According to a third aspect of the present invention, in the second aspect, the first control means keeps the injection timing substantially constant with respect to the load change.
Therefore, since the dependence of the injection timing on the engine load is low compared to the engine rotation speed, if the injection timing is kept substantially constant with respect to the load change, smoke and NOx are not affected by fluctuations in the engine load. It is possible to stably realize a combustion state that can achieve both.

According to a fourth aspect of the present invention, in the first aspect, the first control means controls the operation of the EGR rate adjusting means so that the EGR rate is 50% or more and the excess air ratio is 1.0 or more. .
Therefore, by setting the EGR rate to 50% or more, it is possible to efficiently reduce NOx by the effect of large-volume EGR, and by setting the excess air ratio to 1.0 or more, it is possible to effectively discharge HC and CO. Can be suppressed.

According to a fifth aspect of the present invention, in the first aspect, the engine exhaust system is provided with a catalyst having an oxidation function, and the first control means prohibits execution of the first control mode when the catalyst is in an inactive state. It is.
Therefore, a large amount of EGR by the control of the first control means leads to a decrease in surplus oxygen, and thus the HC and CO emissions tend to increase, but the discharged HC and CO are reliably purified by the oxidation function of the catalyst. Is done. On the other hand, when the catalyst is in an inactive state, execution of the first control mode by the first control means is prohibited, and HC and CO discharged from the engine are reduced as the EGR amount is reduced. HC and CO emissions are also prevented.

According to a sixth aspect of the present invention, in the first aspect, the first control means sets the fuel injection timing in the first control mode to the advance side with respect to the specific fuel injection timing, and the injected fuel deviates from the cavity formed in the piston. Thus, it is set on the retard side from the vicinity of 40 ° before compression top dead center, which is the injection timing to reach the cylinder wall surface.
Therefore, as described with reference to FIG. 6 in claim 1, when the injection timing is advanced, THC increases from around 40 ° BTDC, and the amount of fuel mixed in the engine oil increases, resulting in oil dilution. However, by setting the fuel injection timing in the first control mode to the retard side from the vicinity of 40 ° before the compression top dead center, there are harmful effects such as an increase in THC due to excessive advance and oil dilution. Is reliably prevented.

As described above, according to the diesel engine of the first aspect of the present invention, in the first control mode, the deterioration of stability due to poor ignition, the occurrence of oil dilution and the increase of HC and CO are prevented. Thus, the upper limit of the EGR rate that can suppress smoke emission can be raised to reduce both smoke and NOx. In the second control mode, it is possible to achieve good vehicle running characteristics that meet the driver's output requirements. Moreover, at the time of switching between the first control mode and the second control mode, the injection timing is switched instantaneously to jump over a region where noise and smoke rapidly increase, thereby avoiding temporary increase in noise and smoke in advance. be able to.

According to the diesel engine of the second aspect of the invention, in addition to the invention of the first aspect, regardless of the engine speed, the injected fuel always reaches the piston cavity at an appropriate time, and smoke and NOx are produced. A compatible combustion state can be stably realized.
According to the diesel engine of the invention of claim 3, in addition to the invention of claim 2, it is possible to stably realize a combustion state in which both smoke and NOx can be achieved without being affected by fluctuations in engine load. it can.

According to the diesel engine of the fourth aspect of the invention, in addition to the invention of the first aspect, NOx is efficiently reduced by the effect of a large amount of EGR, and the exhaust of CO and HC is effective by the excess air ratio of 1.0 or more. As a result, and overall emission emission characteristics can be improved.
According to the diesel engine of the fifth aspect of the invention, in addition to the invention of the first aspect, the HC and CO are reliably purified by the oxidation function of the catalyst, and when the catalyst is in an inactive state, the first control means Since the execution of the first control mode is prohibited and the HC and CO discharged from the engine are reduced, the discharge of HC and CO can be reliably prevented in any operating region.

  According to the diesel engine of the sixth aspect of the invention, in addition to the invention of the first aspect, by appropriately limiting the fuel injection timing to the advance side, an increase in THC due to excessive advance, oil dilution, etc. Can be surely prevented.

[First Embodiment]
Hereinafter, a first embodiment of a common rail type diesel engine will be described.
FIG. 1 is an overall configuration diagram showing a diesel engine of the present embodiment. In this figure, one cylinder of a diesel engine is shown, and a piston 2 disposed in the cylinder block 1 is connected to a crankshaft (not shown) via a connecting rod 3. A fuel injection valve 5 is disposed on the cylinder head 4 of the engine so as to face the inside of the cylinder, and this 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 near 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 of the piston head, and is ignited and burned in compressed air. Then, the piston 2 is pushed down (fuel injection means). Although the configuration of the common rail system is well known and will not be described in detail, the fuel injection amount and the injection timing can be freely set by controlling the open state of the fuel injection valve 5.

  On the other hand, the cylinder of each cylinder is connected to a common intake passage 9 via an intake port 8 formed in the cylinder head 4, and the intake passage 9 is connected to the air cleaner 10, the compressor 11 a of the turbocharger 11 from the upstream side, An intake throttle valve 12 is provided. The cylinders of the cylinders are connected to a common exhaust passage 14 via an exhaust port 13 of the cylinder head 4. The exhaust passage 14 is provided coaxially with the exhaust throttle valve 15 and the compressor 11a from the upstream side. A turbine 11 b of the turbocharger 11, an occlusion-type NOx catalyst 16, an oxidation catalyst 17, and a silencer (not shown) are provided. The NOx catalyst 16 stores NOx in the exhaust when the exhaust air-fuel ratio of the engine is lean, and stores the stored NOx when the exhaust air-fuel ratio is rich or stoichiometric (when HC or CO exists in the exhaust gas). The oxidation catalyst 17 has a function of oxidizing and purifying HC and CO in the exhaust gas.

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

  On the other hand, one end of an EGR passage 23 is connected to the intake passage 9 downstream of the intake throttle valve 12, and an EGR valve 21 and an EGR cooler 25 are provided in the EGR passage 23, and the other end of the EGR passage 23 is The exhaust passage 14 is connected to an upstream position of the exhaust throttle valve 15. The recirculation of EGR from the exhaust passage 14 to the intake passage 9 is performed via the 2EGR passage 23. 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 appropriately adjusted according to the opening degree of the EGR valve 21, the restriction of the intake air by the intake throttle valve 12, and the restriction of the exhaust gas by the exhaust throttle valve 15 (EGR rate adjusting means).

  On the other hand, an input / output device (not shown), a storage device (ROM, RAM, etc.) used for storage of a control program, a control map, etc., a central processing unit (CPU), a timer counter, etc. Control unit) 31 is installed. Various sensors such as an accelerator sensor 32 for detecting the accelerator operation amount APS and a rotation speed sensor 33 for detecting the engine rotation speed Ne are connected to the input side of the ECU 31, and the fuel injection valve 5 and the intake throttle are connected to the output side. Various devices such as the valve 12, the exhaust throttle valve 15, and the EGR valve 21 are connected.

The ECU 31 calculates the fuel injection amount Q from a map (not shown) based on the accelerator operation amount APS and the engine rotational speed Ne, and calculates the fuel injection timing IT from the map (not shown) based on the engine rotational speed Ne and the fuel injection amount Q. The fuel injection valve 5 is driven and controlled based on these calculated values.
Further, based on the calculated values of the engine rotational speed Ne and the fuel injection amount Q, the 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, an airflow sensor not shown) It is calculated from the fresh air amount and the fuel injection amount Q obtained by adding the estimated value of the residual oxygen amount in the EGR gas to the intake air amount obtained from the output, or a linear air-fuel ratio sensor is provided in the exhaust passage 14, and The opening degree of the EGR valve 21 (that is, the EGR rate Regr) is feedback-controlled so that the actual excess air ratio λ becomes the target excess air ratio tgtλ.

Here, in the diesel engine of the present embodiment, the fuel injection timing IT is significantly changed by switching the combustion mode according to the operating state of the engine, and 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, in step S2, the ECU 31 determines whether or not the engine has been warmed up based on the coolant temperature or the like, and in the subsequent step S4, based on the fuel injection amount Q and the engine rotational speed Ne, according to the map shown in FIG. It is determined whether or not the current operation region is a predetermined specific region. Here, as the specific region, the fuel injection amount Q (correlated with the engine load) and the engine rotational speed Ne are set to a low load low rotational region where the predetermined values Q0 and Ne0 are equal to or less.

  When a negative (NO) determination is made in either of steps S2 and S4, the routine proceeds to step S6, and after executing the normal combustion mode (second control mode), the routine is terminated (second control means). If YES in both steps S2 and S4, the routine proceeds to step S8, the low NOx combustion mode (first control mode) is executed, and the routine is terminated (first control means). .

The above-mentioned normal combustion mode is a control mode similar to the control content implemented in a general diesel engine, and the low NOx combustion mode is a combustion mode unique to the present invention.
FIG. 4 shows the control status of the fuel injection timing IT and the EGR rate Regr in the normal combustion mode and the low NOx combustion mode, and the 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 results of measuring smoke emissions. The ● mark in the figure indicates the characteristic when IT = 8 ° BTDC is set. Similarly, the ◯ mark is 10 ° BTDC, the ▽ mark is 12 ° BTDC, △ mark is 14 ° BTDC, x mark is 16 ° BTDC, * mark is 20 ° BTDC, ◆ mark is 24 ° BTDC, ◇ mark is 28 ° BTDC, ★ mark is 32 ° BTDC, □ mark is 36 ° The characteristics at BTDC are shown. This test result is obtained when the engine operating range is a target average effective pressure Pe = 0.2 MPa (correlation with the engine load) and the engine rotational speed Ne = 2000 rpm.

  The calibration point in the normal combustion mode is a large mark position, the fuel injection timing IT is set to 8 ° BTDC, 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 for the low NOx combustion mode is set to a large □ mark position, the fuel injection timing IT is set to 36 ° BTDC far ahead, and the EGR rate Regr is set to 56%. As a result, the excess air ratio λ is controlled to 1.0 (theoretical air-fuel ratio) or slightly lean side (λ ≧ 1.0).

The fuel injection timing IT = 36 ° BTDC in the low NOx combustion mode is set as follows.
FIG. 5 is a characteristic diagram obtained by extracting test results when the fuel injection timing IT is 10, 20, and 36 ° BTDC from FIG. 4 above. IT = 10 ° BTDC indicated by a circle in the figure is a general diesel engine. It corresponds to the fuel injection timing IT applied to the engine (close to the normal combustion mode), and IT = 20 ° BTDC indicated by * is the fuel injection timing IT applied to Japanese Patent No. 31168776 described as the prior art. IT = 36 ° BTDC indicated by □ corresponds to the fuel injection timing IT in the low NOx combustion mode of the present embodiment.

At 10 ° BTDC, although the NOx emission amount decreases as the EGR rate Regr increases, it is omitted in the figure, but smoke emission increases rapidly when λ = 1.8 is exceeded, so smoke emission can be suppressed. The upper limit value of the EGR rate Regr is quite low.
Further, at 20 ° BTDC, the smoke emission amount once showed an increasing tendency with respect to the increase in the EGR rate Regr, and showed a decreasing tendency after exceeding the peak. However, even when λ = 1.05, which is the minimum value, is sufficient. Smoke reduction cannot be achieved, and the smoke emission amount exceeds the peak every time the excess air ratio λ changes with the switching of the combustion mode, so that it is estimated that the smoke emission amount increases transiently.

  On the other hand, at 36 ° BTDC in which the fuel injection timing IT is further advanced, the smoke emission amount is suppressed to a very low value even if the EGR rate Regr is increased to about 56%, and as a result, the air excess rate λ that can be realized. Is expanded to an area around 1.0. Since the NOx emission amount is also suppressed by the high EGR rate Regr, both the smoke and NOx can be reduced according to the fuel injection timing IT, and the peak of the smoke emission amount is not formed. Smoke increase is also suppressed.

  Note that the smoke emission amount is suppressed by the advance of the fuel injection timing IT in this way, because the period from fuel injection to ignition is extended, and during that time, premixing of the injected fuel and intake air is promoted. It is thought to be for this purpose. However, as disclosed in Japanese Patent Laid-Open No. 8-218920 described as the prior art, the fuel injection timing IT is not extremely advanced to the intake stroke, so the injected fuel is compressed top dead center even in this low NOx combustion mode. It is reliably ignited at a predetermined timing in the vicinity, and stable operation is performed without causing poor ignition.

Even in this 36 ° BTDC, if the excess air ratio λ is set to a rich side exceeding 1.0, smoke is increased and HC and CO emissions are also increased with a decrease in surplus oxygen ( In order to avoid these problems, the excess air ratio λ in the low NOx combustion mode is set slightly leaner than 1.0.
On the other hand, 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 in the same operation region as in FIGS. As shown in this figure, when the fuel injection timing IT is advanced, THC increases from around 40 ° BTDC, and the amount of diluted fuel (the amount of fuel mixed in engine oil and exerting a dilution effect) increases. There is a risk of oil dilution. These factors are because a part of the fuel injected by the early fuel injection diffuses and adheres to the cylinder wall surface without being captured in the cavity 2a of the piston 2, and the attached fuel is discharged as THC. Or mixed into the oil in the oil pan through the piston clearance. Further, if the mixing action of the fuel spray and the intake air by the cavity lip or the reverse squish flow is not sufficiently obtained, it leads to an increase in smoke. Therefore, an excessive advance is not desirable from this point.

Further, when the fuel injection timing IT is excessively advanced, the ignition timing in the vicinity of the compression top dead center of the fuel as in the prior art disclosed in Japanese Patent Laid-Open No. 8-218920 which performs fuel injection in the intake stroke. In order to avoid this problem, there is a limit to the advance angle.
Due to the above factors, the fuel injection timing IT is limited to 40 ° BTDC on the advance side, and as a result, the fuel injection timing IT in the low NOx combustion mode is set to 36 ° BTDC as described above.

  In such a low NOx combustion mode, a large amount of EGR is recirculated in order to suppress NOx. Therefore, in an area where the engine load and the engine speed Ne are high, the amount of air for burning a large amount of fuel is insufficient. The engine load and the engine speed Ne are limited, and the implementation is limited to a specific area of the map of FIG. However, in the present embodiment, since the turbocharger 11 can be supercharged to supply a relatively large amount of air even in the low NOx combustion mode, the upper limit value in the specific region is significantly increased compared to a naturally aspirated diesel engine. Has been.

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

On the other hand, as explained based on FIG. 4, when the combustion mode is switched in accordance with the change of the engine operating region, the fuel injection timing IT is switched between 8 ° BTDC and 36 ° BTDC, and the excess air ratio λ is switched between 1.8 and 1.0 (between 45% and 56% in EGR rate Regr).
Here, in the common rail type fuel injection control with high responsiveness, the fuel injection timing IT can be switched in steps. However, in the EGR control, the EGR rate Regr gradually changes due to exhaust gas recirculation, which is inevitable. In addition, a transition region exists in the EGR rate Regr at the time of mode switching. At the time of mode switching, a predetermined point during the EGR rate change period is shown as a boundary as shown by a solid line without tailing the fuel injection timing IT in accordance with a change in the EGR rate Regr as shown by a broken line in FIG. The fuel injection timing IT is changed stepwise (mode switching means), and the necessity of control at the time of mode switching will be described below.

  As shown in FIG. 4, when the fuel injection timing IT is set to any of 14 to 32 ° BTDC in the transient region where the excess air ratio λ is 1.0 to 1.8, noise is mainly generated on the lean side. It can be seen that noise and smoke cannot be achieved at the same time because a phenomenon of increasing the smoke or a phenomenon of increasing the smoke mainly on the rich side occurs. When tailing the fuel injection timing IT according to the broken line in FIG. 4, the fuel injection timing IT inevitably passes through the region of 14 to 32 ° BTDC, so that it is inevitable that a region where noise and smoke rapidly increase is generated.

On the other hand, when the fuel injection timing IT is set to 8 or 10 ° BTDC, both the noise and smoke can be reduced in the region where the excess air ratio λ is leaner than around 1.3, When the injection timing IT is set to 36 ° BTDC, it can be seen that both noise and smoke can be reduced in a region where the excess air ratio λ is richer than around 1.3.
Therefore, when switching from the normal combustion mode to the low NOx combustion mode, after gradually increasing the fuel injection timing IT to 8 to 10 ° BTDC in the region where the excess air ratio λ is 1.3 or more, λ = 1.3 is set. Switch to 36 ° BTDC stepwise as a boundary. Further, when switching from the low NOx combustion mode to the normal combustion mode, the control may be performed in the reverse order.

On the other hand, in the low NOx combustion mode in which the excess air ratio λ is set to the rich side with respect to the normal combustion mode, the torque decreases under the same injection amount. Therefore, in the present embodiment, the required fuel amount is obtained in advance for each excess air ratio λ, and the actual fuel injection amount Q is corrected in accordance with the excess air ratio λ. It is preventing.
As described above, in the diesel engine of the present embodiment, the upper limit value of the EGR rate Regr that can suppress the emission of smoke in a region where the fuel injection timing IT is greatly advanced compared to a generally applied value. Focusing on the increase, the low NOx combustion mode in which the fuel injection timing IT is set to the advance side is executed in a specific region of low rotation and low load, so both smoke and NOx reduction are achieved at a very high level. can do.

Moreover, in the present embodiment, the amount of air is secured by supercharging the turbocharger 11, thereby expanding the upper limit of the specific region in which the low NOx combustion mode is executed. Can be obtained at
In addition, the fuel injection timing IT in the low NOx combustion mode is set in consideration of the advance angle limit caused by a decrease in stability due to poor ignition, the occurrence of oil dilution, an increase in THC, and the like. Thus, the above-described effects can be obtained after preventing these harmful effects.

Further, since the fuel injection timing IT is controlled to be advanced as the engine rotation speed Ne increases, the injected fuel always reaches the cavity 2a of the piston 2 at an appropriate time regardless of the engine rotation speed Ne. As a result, it is possible to stably realize a combustion state in which both the smoke and NOx can be achieved.
Further, since it is assumed that the fuel injection timing IT is less dependent on the increase / decrease in the fuel injection amount Q (engine load), the engine load is controlled so as to be substantially constant with respect to the increase / decrease in the fuel injection amount Q. Without being affected by fluctuations, it is possible to stably realize a combustion state in which both the smoke and NOx can be achieved.

  On the other hand, as described in the example based on FIG. 5 (in the operation region of Pe = 0.2 MPa and Ne = 2000 rpm), in the low NOx combustion mode, it is close to the upper limit (56%) at which smoke emission can be suppressed. While controlling the EGR rate Regr, the excess air ratio λ is controlled to be slightly leaner than 1.0, and in the other operation regions, the same EGR rate Regr and excess air ratio λ are controlled. By increasing the EGR rate Regr as much as possible in this way, the maximum NOx reduction action can be obtained, and the excess air ratio λ is suppressed from 1.0 to the lean side, thereby reducing HC and CO emissions. Is effectively suppressed, and as a result, the overall emission emission characteristics can be improved.

  In addition, since a large amount of EGR in the low NOx combustion mode leads to a decrease in surplus oxygen, emissions of HC and CO slightly increase compared to the normal combustion mode. This phenomenon is based on the THC characteristics of FIG. Can also be guessed. However, HC and CO are easier to be post-treated than smoke, and can be reliably oxidized and purified by the oxidation catalyst 17 in the exhaust passage 14. On the other hand, when the oxidation catalyst 17 has not yet been activated, the determination in step S2 of FIG. 2 is NO, the execution of the low NOx combustion mode is prohibited, and the operation mode is switched to the normal combustion mode with less HC and CO emissions. Therefore, as a result, it is possible to reliably prevent HC and CO emissions in any operating region.

  On the other hand, the low NOx combustion mode is executed in the specific region, and when the fuel injection amount Q or the engine rotation speed Ne increases and the specific region is deviated, the normal combustion mode similar to that of a general diesel engine is switched. It is possible to reliably respond to the driver's required output, and as a result, good vehicle running characteristics can be realized. In particular, in the present embodiment, since the exhaust gas cooled by the EGR cooler 25 is recirculated, the realizable engine torque is increased, and the traveling performance of the vehicle can be further improved.

  Moreover, when the combustion mode is switched, the fuel injection timing IT is not tailed in accordance with the change in the EGR rate Regr, but is switched stepwise with a predetermined point (λ = 1.3) as a boundary. Therefore, the mode switching is performed by jumping over the region (IT = 14 to 32 ° BTDC) where the noise and smoke existing in the transition region of the EGR rate Regr rapidly increase, and the temporary noise and smoke accompanying the mode switching are increased. It can be avoided in advance.

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

[Second Embodiment]
Next, a second embodiment of the diesel engine embodying the present invention will be described. The diesel engine of the present embodiment is different from that described in the first embodiment in that a process of purging NOx stored in the NOx catalyst 16 is added, and other parts are common. . Therefore, the explanation of the common parts is omitted, and the differences are mainly explained.

  Although the NOx emission amount is reduced in the low NOx combustion mode, it is impossible to completely suppress the emission. In the normal combustion mode, the NOx emission characteristic is the same as that of a general diesel engine, and the exhausted NOx is a NOx catalyst. 16 is occluded and is prevented from being discharged into the atmosphere. Therefore, even when the low NOx combustion mode is executed, a process for releasing and reducing NOx stored in the NOx catalyst 16 is required. In this embodiment, the fuel injection timing IT is greatly advanced as in the low NOx combustion mode. The NOx purge combustion mode is executed to purge NOx.

  FIG. 7 is a flowchart showing a combustion mode switching routine executed by the ECU 31. In step S4, it is determined whether or not the current operation region is a specific region. If YES, the process proceeds to step S10 to execute NOx purge. It is determined whether or not the NOx purge time should be reached. Although there are various specific determination methods, for example, when a detected value of a NOx sensor (not shown) is not less than a predetermined value and NOx leakage from the NOx catalyst 16 is estimated, or an engine operating range The NOx emission amount estimated from the above is sequentially accumulated to obtain the current NOx storage amount on the NOx catalyst 16, and when the value exceeds a predetermined value, it is regarded as the NOx purge timing.

If NO determination is made in step S10 because it is not yet the NOx purge time, the process proceeds to step S8 to execute the low NOx combustion mode. On the other hand, when the NOx purge timing is determined as YES in step S10, the NOx purge combustion mode is executed in step S12.
In FIG. 4, the calibration point in the NOx purge combustion mode is indicated by a large □ marked with a broken line, and the fuel injection timing IT is set to the same value as in the low NOx combustion mode (36 ° BTDC in FIG. 4). Unlike the low NOx combustion mode, the excess rate λ is set slightly richer than 1.0 (λ <1.0). That is, as is apparent from the characteristics of FIG. 4, in a region where the fuel injection timing IT is greatly advanced, an increase in smoke due to a large amount of EGR is suppressed (of course, an increase in NOx is also suppressed). An operation in which the excess air ratio λ, which is impossible with a diesel engine, is controlled to the rich side is possible. Since the exhaust amount of HC and CO increases with the decrease of surplus oxygen (which can also be estimated from the THC characteristics), these HC and CO are supplied to the NOx catalyst 16 as a reducing agent, and the NOx release and reduction action is achieved. Play.

As described above, in the diesel engine of the present embodiment, in addition to the operational effects of the first embodiment, when the NOx purge time is reached, the excess air ratio λ is controlled to the rich side while suppressing the increase in smoke. Since the NOx purge combustion mode is executed, the NOx on the NOx catalyst 16 can be released and reduced without concern for an increase in smoke.
This is the end of the description of the embodiment, but the aspect of the present invention is not limited to this embodiment. For example, in the first and second embodiments, the diesel engine is configured as a common rail type for precise control of the fuel injection timing IT, and supercharging by the turbocharger 11 is performed to expand a specific region of the low NOx combustion mode. In order to increase the engine torque, the EGR cooler 25 is operated. However, the present invention is not limited to these modes, and the invention is embodied in a general diesel engine in which fuel injection is controlled by a governor, or the turbocharger 11. May be a variable nozzle type, or the turbocharger 11 and the EGR cooler 25 may be omitted.

  In the first and second embodiments, the fuel injection timing IT, the EGR rate Regr, and the excess air rate are based on the characteristics shown in FIGS. 4 to 6 corresponding to a predetermined operation range (Pe = 0.2 MPa, Ne = 2000 rpm). Although the control state of λ has been exemplified, as described above, these settings differ depending on the operation region, and differ if the engine specifications are changed. Accordingly, it is a matter of course that the setting is not limited to the above setting, and the setting may be made based on characteristics corresponding to the operation region, engine specifications, and the like.

  Further, in the first and second embodiments, the first control mode (low NOx combustion mode) and the other modes are switched depending on the situation. In the region, the operation in the first control mode may always be performed.

It is a whole lineblock diagram showing the diesel engine of a 1st embodiment. It is a flowchart which shows the combustion mode switching routine which ECU of 1st Embodiment performs. It is explanatory drawing which shows the map for setting a combustion mode. Control status of fuel injection timing IT and EGR rate Regr in normal combustion mode and low NOx combustion mode, and THC emissions, NOx emissions, noise, smoke emissions when fuel injection timing IT is set to 8-36 ° BTDC It is a characteristic view which shows the test result which measured. FIG. 5 is a characteristic diagram excerpting test results when the fuel injection timing IT is 10, 20, and 36 ° BTDC from FIG. 4. It is a characteristic view which shows the test result which changed the fuel injection timing IT, and measured the THC discharge | emission amount and the diluted fuel amount. It is a flowchart which shows the combustion mode switching routine which ECU of 2nd Embodiment performs.

Explanation of symbols

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

Claims (6)

Fuel injection means for injecting fuel into the combustion chamber of the engine;
EGR rate adjusting means for controlling the amount of exhaust gas exhausted from the engine to the intake system;
In the operating range in which the smoke emission characteristic with respect to the decrease in the excess air ratio shows an increasing trend and is operable at a specific fuel injection timing that shows a decreasing trend, the fuel injection timing by the fuel injection means is set to be higher than the specific fuel injection timing. On the advance side, the injection fuel is set to be retarded from the injection timing when the fuel is released from the cavity formed in the piston and reaches the cylinder wall surface, and the operation of the EGR control means is controlled at the set injection timing. First control means for executing a first control mode for simultaneously controlling the smoke emission amount and the NOx emission amount;
By retarding the injection timing from the control state of the first control mode and sets the compression top dead center near near the retard side of the specific fuel injection timing, the EGR rate by the control state of the first control mode Second control means for executing a reduced second control mode;
Switching between the first control mode and the second control mode is performed based on the operating range of the engine, and at the time of switching the control mode, the EGR rate is in the middle of the EGR rate changing period in which the EGR rate changes gradually. A diesel engine comprising: mode switching means for instantaneously switching the injection timing at an EGR rate that can reduce both noise and smoke at the fuel injection timing in both the first control mode and the second control mode.   The diesel engine according to claim 1, wherein the first control means advances the injection timing as the engine speed increases.   The diesel engine according to claim 2, wherein the first control means keeps the injection timing substantially constant with respect to a load change.   The diesel engine according to claim 1, wherein the first control means controls the operation of the EGR rate adjusting means so that the EGR rate is 50% or more and the excess air ratio is 1.0 or more.   The exhaust system of the engine is provided with a catalyst having an oxidation function, and the first control means prohibits execution of the first control mode when the catalyst is in an inactive state. The listed diesel engine.   The first control means sets the fuel injection timing in the first control mode to an advance side with respect to the specific fuel injection timing, and an injection timing at which the injected fuel arrives at the cylinder wall surface outside the cavity formed in the piston. 2. The diesel engine according to claim 1, wherein the diesel engine is set at a retarded angle side near 40 ° before a certain compression top dead center.

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