JP4049057B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine that is particularly suitable for a diesel engine.

  As the combustion temperature rises, the amount of NOx generated increases. Therefore, an EGR device that recirculates (recirculates) part of the exhaust gas to the intake system is widely known. In other words, when the exhaust gas is recirculated to the intake system, the amount of fresh air in the intake air is reduced accordingly, so that the oxygen concentration in the intake air is reduced. Since the recirculation gas (hereinafter referred to as “EGR gas” as appropriate) is an inert gas, the maximum combustion temperature is lowered and the generation of NOx can be suppressed.

In Patent Document 1, the EGR passage is provided with a heat dissipation function. By doing in this way, combustion temperature can further fall and generation | occurrence | production of NOx can further be suppressed.
JP 2001-73741 A

  By the way, the amount of carbon dioxide contained in the exhaust gas is small in the low-speed medium load region. Therefore, the partial pressure of carbon dioxide in the EGR gas is reduced, and the oxygen partial pressure is relatively increased. When the oxygen concentration increases, combustion is activated, the combustion temperature rises, and the generation of NOx increases (see FIG. 2A).

  Therefore, in such a case, reduce the amount of fresh air introduced by using a VNT (Ventilation Nozzle Turbo) nozzle throttle and a throttle in the intake pipe (fresh air introduction amount adjusting throttle) to reduce the amount of oxygen introduced. The amount is reduced so as not to lower the EGR rate (the ratio of EGR gas to the total intake air). Also, when deposits accumulate on the EGR device and clogging occurs, correction is made to increase the EGR valve opening, but the correction reaches almost the upper limit and increases the EGR amount even if the EGR valve is opened. Even when it is not possible, the EGR rate is not lowered by reducing the amount of fresh air introduced by restricting the intake air. However, if the intake air is throttled too much, the amount of oxygen in the intake air decreases, and the excess air ratio decreases too much. Then, the unburned fuel that cannot be burned increases and the amount of smoke generated deteriorates (increases) (see FIG. 2B). Thus, it was difficult to reduce both NOx and smoke.

  The present invention has been made paying attention to such conventional problems, and an object thereof is to provide an exhaust purification device for an internal combustion engine that can reduce both NOx and smoke.

  The present invention solves the above problems by the following means. In addition, in order to make an understanding easy, although the code | symbol corresponding to embodiment of this invention is attached | subjected, it is not limited to this.

The present invention provides a filter (7) that is provided in an exhaust system of a diesel engine, collects particulates, extracts exhaust gas from the downstream side of the filter, and returns a flow rate required according to operating conditions to the intake system. And an exhaust gas recirculation control means (9, 10) for performing the filter regeneration control (20; step S3) for performing regeneration control of the filter when the required exhaust gas recirculation amount is greater than a reference value. And

  According to the present invention, the regeneration gas that flows out when the filter is regenerated is returned to the intake system from the downstream side of the filter as EGR gas. This regeneration gas contains a large amount of carbon dioxide. Since carbon dioxide is an inert gas, if this regeneration gas is recirculated to the intake system and used as the EGR gas, the EGR performance equivalent to the conventional one can be achieved with a smaller amount of EGR gas than the conventional one.

  In particular, when the EGR valve or the EGR passage is clogged, correction is made to increase the EGR valve opening, but the correction almost reaches the upper limit, and the EGR amount can be increased even if the EGR valve is opened. In such a situation, the DPF regeneration is performed to increase the concentration of the inert gas, so that the combustion temperature can be lowered and the NOx can be reduced even if the absolute amount of the EGR gas itself cannot be increased. it can. On the other hand, since sufficient oxygen can be supplied, smoke does not deteriorate.

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

  FIG. 1 is a diagram showing an embodiment of a filter regeneration control device for a diesel engine according to the present invention.

  Air flowing from the intake port passes through an air filter 2, an air flow meter (hereinafter abbreviated as “AFM” as appropriate) 3, a fresh air introduction amount adjustment throttle 4, a turbocharger 5, and an intercooler 6, and is sucked into the diesel engine 1. . An EGR passage 10 is connected between the fresh air introduction amount adjusting throttle 4 and the turbocharger 5, and a part of the exhaust gas is recirculated from the downstream of the DPF 7.

  Fuel is supplied from the injection nozzle 11 to the air drawn into the diesel engine 1, and the combustion gas flows through the exhaust system to a diesel particulate filter (hereinafter abbreviated as “DPF”) 7. The DPF 7 is a filter that collects particulates (hereinafter referred to as “PM”) in the exhaust gas discharged from the diesel engine 1. For example, a ceramic porous filter or the like can be used. If the DPF 7 continues to collect PM, clogging occurs. Therefore, if PM accumulates to some extent, the exhaust gas temperature is raised, and the accumulated PM is burned and removed to regenerate the DPF. A differential pressure sensor 8 is provided in the DPF 7. This differential pressure sensor 8 is a pressure difference detection means for detecting the differential pressure between the pressure on the inlet side and the pressure on the outlet side of the DPF 7. The PM trapping state of the DPF 7 is estimated based on the magnitude of the differential pressure, and the regeneration of the DPF 7 Make it possible to determine when. The differential pressure sensor 8 outputs the detected differential pressure signal to the control unit 20.

  An EGR valve 9 is provided downstream of the DPF 7, and a part of the exhaust gas is passed through the EGR passage 10 to be returned to the intake system.

  When the DPF 7 is regenerated, that is, when the PM collected by the DPF 7 is burned, a large amount of carbon dioxide is contained in the gas flowing out from the DPF 7 (hereinafter referred to as “regenerated gas”) during DPF regeneration. Since carbon dioxide is an inert gas, if this regeneration gas is recirculated to the intake system and used as the EGR gas, the EGR performance equivalent to the conventional one can be achieved with a smaller amount of EGR gas than the conventional one. For example, when deposits accumulate on the EGR valve, EGR passage, etc., and clogging occurs, the EGR valve opening is corrected to be increased. However, even if the correction reaches almost the upper limit and the EGR valve is opened, EGR When the amount cannot be increased, the EGR amount becomes insufficient and the NOx amount increases (see FIG. 2A). In such a case, conventionally, the intake air is throttled so as not to lower the EGR rate, but the amount of smoke generated may deteriorate (increase) (see FIG. 2B). However, if the concentration of the inert gas can be increased by performing DPF regeneration, the combustion temperature can be lowered and the NOx can be reduced even if the absolute amount of EGR gas itself cannot be increased. On the other hand, since sufficient oxygen can be supplied, smoke is not deteriorated (see FIG. 2C).

  Therefore, in the present invention, the DPF regeneration process is performed when the amount of EGR gas cannot be increased. By doing in this way, the regeneration gas containing many carbon dioxides can be utilized as EGR gas.

  Further, when the same amount of gas as the conventional gas is refluxed, more inert gas can be introduced into the intake system than before. When this is combined with a low temperature premixed combustion type diesel engine, the effect becomes very large. The low-temperature premixed combustion method is a method in which a large amount of inert gas is introduced into the intake system and the fuel injection timing is delayed, thereby making the ignition delay time longer than the fuel injection time. By doing so, the fuel is dispersed as much as possible around the oxygen during the ignition delay time, and the premixed combustion reduces the generation of smoke and also reduces the NOx due to the decrease in the combustion temperature. is there. Thus, according to the low temperature premixed combustion method, it is possible to simultaneously reduce both NOx and smoke. Thus, in low temperature premixed combustion, a large amount of inert gas is recirculated, so that its performance is easily affected by EGR gas. Therefore, the effect of the present invention that can increase the ratio of the inert gas is great.

  In this low-temperature premixed combustion, the generation of smoke is suppressed even if the excess air ratio is close to the theoretical air-fuel ratio. However, in order to avoid deterioration in fuel consumption due to deviation from the air cycle, the oxygen excess (excess ratio of oxygen to fuel) is secured while suppressing the oxygen concentration (with respect to the total intake air (including EGR)), and the excess air ratio is set to 1. It is effective to keep 3 or more (Kimura et al., Automobile Engineering Society Proceedings Vol.30, No.3 (1999)).

  Conventionally, when the amount of EGR gas cannot be increased to an amount that sufficiently reduces the oxygen concentration, it has been necessary to reduce the total intake air amount by lowering the supercharging or restricting the intake air. The excess air ratio decreased and the fuel consumption deteriorated for the above reasons. However, in the present invention, when the required amount of EGR gas is larger than the reference value, the filter regeneration control is performed, so that the oxygen concentration of the EGR gas is lowered and the carbon dioxide (inert gas) concentration is raised. Since the oxygen concentration of all the intake air can be made sufficiently low without reducing the total intake air amount with a small amount of EGR gas, the oxygen amount is maintained while maintaining the oxygen amount to keep the excess air ratio at 1.3 or more. The concentration can be reduced, and it becomes possible to avoid deterioration in fuel consumption in low temperature premixed combustion.

  Hereinafter, specific control logic of the control unit 20 will be described with reference to the flowchart of FIG.

In step S1, determine the deposition amount of soot DPF7 (S _ cont). Specifically, the pressure difference ΔP of the inlet portion and outlet portion of DPF7 detected by the differential pressure sensor 8, an air flow meter 2 provided in the fresh air introduction passage (AFM) to estimate the exhaust gas flow rate Q _ air (exhaust gas Based on the map shown in FIG. 4, the accumulation amount of soot accumulated in the DPF is determined (deposition amount estimation unit).

In step S2, the soot deposition amount (S _ cont) is determined whether exceeds a reference value (Soot _ th), when exceeded, performing DPF regeneration control at step S3. Specifically, the DPF regeneration control is a known control method such as post-injection in which fuel is further injected after normal fuel injection or IT retard in which the exhaust gas temperature is increased by delaying the injection timing. In this way, the exhaust gas temperature is raised to regenerate the DPF.

On the other hand, in step S2, if less than soot deposition amount (S _ cont) the reference value (Soot _ th), the process proceeds to subsequent step S4.

In step S4, the oxygen concentration in the exhaust gas is estimated. The specific contents will be described with reference to FIG. Soot deposition amount in the DPF is estimated (S _ cont (n)) ( step S401), the soot amount estimated to have been generated by the combustion chamber therewith (S _ comb (n)) of seeking the map operating conditions (step S402), subtracts to obtain the S _ total (n) (step S403). Then, a time change amount of the soot deposited amount from a comparison of the soot deposition amount of the previous (S _ cont (n-1 )), soot amount which has been reproduced at any time (S_reg) determining (step S405). (Ie from HC conversion value of soot, calculates the amount of oxygen required for complete combustion) the reproduction soot amount (S _ reg) converted into oxygen consumed amount to (S406). Also drawn from the fuel consumption (Q _ Fuel) (i.e. required for complete combustion) is calculated oxygen amount (step S407). Converted into a concentration based on their total value AFM signal (Q _ air) (step S408). And the oxygen concentration in exhaust gas is calculated | required by subtracting the density | concentration from the oxygen concentration calculated from atmospheric pressure (step S409) (step S410).

  Returning again to FIG. In step S5, the EGR valve and the fresh air introduction amount adjustment throttle are controlled so that the target intake oxygen concentration and the target excess air ratio are obtained in accordance with the exhaust gas oxygen concentration determined above. The specific contents will be described with reference to FIG.

  First, based on the engine speed Ne and the target torque T, the target oxygen concentration is obtained from the map of FIG. This oxygen concentration corresponds to C / (C + D) in FIG. A target EGR rate is obtained based on the value and the oxygen concentration in the exhaust gas obtained as described above (step S51). The oxygen concentration in the exhaust gas corresponds to E / F in FIG. 7, and the EGR rate corresponds to G / H in FIG.

Further, based on the engine speed Ne and the target torque T, the target excess air ratio is obtained from the map of FIG. For example, as described above, if low temperature premixed combustion control is performed, the value is set to 1.3 or more. Determining its value, fuel consumption (Q _ Fuel) required oxygen amount based on (corresponding to B in FIG. 7) (step S52).

  Then, the EGR valve opening is obtained from the target EGR rate based on FIG. 10, and the opening of the fresh air introduction amount adjustment throttle is obtained from the required oxygen amount based on FIG. 11 and controlled (step S53).

Returning again to FIG. In step S6, performs the DPF regeneration control proceeds to step S3, when the EGR valve opening (EGR _ position) exceeds the reference value (EGR _ th), there is no need to perform DPF regeneration When used below (step S7 ). By performing DPF regeneration in this way, the EGR gas can contain a large amount of inert gas. Therefore, even if the absolute amount of EGR gas is small, the amount of inert gas can be increased, and both NOx and smoke can be reduced.

(effect)
When the EGR gas is recirculated by the conventional method, since the EGR gas contains a large amount of oxygen, the oxygen concentration with respect to the entire intake air becomes high, the combustion temperature rises, and NOx may not be reduced sufficiently. (FIG. 2 (A)). Therefore, conventionally, it has been necessary to throttle the intake air. However, if this is done, the excess air ratio is reduced, and there is a possibility that the amount of unburned fuel that cannot be combusted during diffusion combustion increases and the amount of smoke generated increases (see FIG. 2B). Therefore, in the present embodiment, DPF regeneration is performed to increase the concentration of the inert gas, and the gas is recirculated to the intake system as EGR gas. By doing in this way, even if the absolute amount of EGR gas itself cannot be increased, the combustion temperature can be lowered and NOx can be reduced. On the other hand, since sufficient oxygen can be supplied, smoke is not deteriorated (see FIG. 2C).

  Further, when performing low temperature premixed combustion in which the required fuel is injected within the ignition delay period by increasing the exhaust gas recirculation amount and delaying the fuel injection timing, the oxygen amount (fuel) is suppressed while suppressing the oxygen concentration with respect to the total intake air (including EGR). By ensuring the excess ratio of oxygen with respect to the air and maintaining the excess air ratio at 1.3 or more, fuel consumption deterioration due to deviation from the air cycle can be avoided. According to the present embodiment, when the required amount of EGR gas is larger than the reference value, the filter regeneration control is performed, whereby the oxygen concentration of the EGR gas is lowered and the carbon dioxide (inert gas) concentration is raised. Since it is possible to sufficiently reduce the oxygen concentration of all intake air without reducing the total intake air amount with a smaller EGR gas amount, while ensuring an oxygen amount that keeps the excess air ratio at 1.3 or more The oxygen concentration can be reduced, and it becomes possible to avoid fuel consumption deterioration in low temperature premixed combustion.

  Further, since the opening degree of the EGR valve 9 and the fresh air introduction amount adjusting throttle 4 is controlled based on the oxygen concentration, it is possible to prevent the torque from changing suddenly depending on the presence or absence of DPF regeneration.

  The present invention is not limited to the embodiment described above, and various modifications and changes can be made within the scope of the technical idea, and it is obvious that these are equivalent to the present invention. Further, it is obvious that the present invention can be applied not only to low-temperature premixed combustion but also to normal combustion control.

It is a figure which shows one Embodiment of the filter regeneration control apparatus of the diesel engine of this invention. It is a figure explaining the conventional problem and the effect of this invention. It is a flowchart of a control logic. It is a map which calculates the accumulation amount of the soot accumulated in DPF. It is a figure which shows the estimation method of the oxygen concentration in exhaust_gas | exhaustion. It is a figure which shows the calculation method of the opening degree of a fresh air introduction amount adjustment throttle and an EGR valve. It is a figure which shows the component ratio in inhalation air. It is a calculation map of target oxygen concentration. It is a calculation map of a target excess air ratio. It is a figure for calculating the opening degree of an EGR valve. It is a figure for calculating the opening degree of the fresh air introduction amount adjustment throttle.

Explanation of symbols

1 Diesel Engine 2 Air Filter 3 Air Flow Meter (AFM)
4 New air introduction amount adjustment throttle (intake throttle valve)
7 Diesel particulate filter (DPF)
8 Differential pressure sensor 9 EGR valve (exhaust gas recirculation control valve)
10 EGR passage (exhaust gas recirculation passage)
20 Control unit Step S3 Filter regeneration control means Step S4 Oxygen concentration estimation means Step S5 EGR valve opening degree detection means Step S53 EGR valve and fresh air introduction amount adjustment throttle control means

Claims (8)

A filter provided in the exhaust system of a diesel engine for collecting particulates;
Exhaust gas recirculation control means for extracting exhaust gas from the downstream side of the filter and recirculating the flow rate required according to operating conditions to the intake system;
Filter regeneration control means for performing regeneration control of the filter when the required exhaust gas recirculation amount is greater than a reference value ;
An exhaust emission control device for an internal combustion engine, comprising: The filter regeneration control means increases unburned components in the exhaust gas and increases carbon dioxide in the exhaust gas downstream of the filter by combustion in the filter .
The exhaust emission control device for an internal combustion engine according to claim 1. The exhaust gas recirculation control means includes
An exhaust gas recirculation passage connecting an exhaust system and an intake system on the downstream side of the filter;
An exhaust gas recirculation control valve interposed in the exhaust gas recirculation passage;
With
The filter regeneration control means performs regeneration control of the filter when the opening of the exhaust gas recirculation control valve exceeds a reference opening.
3. An exhaust emission control device for an internal combustion engine according to claim 1, wherein the exhaust gas purification device is an internal combustion engine. Combustion control means for increasing the exhaust gas recirculation amount and delaying the fuel injection timing to prolong the ignition delay period and performing low-temperature premixed combustion for injecting the required fuel within the ignition delay period,
When the exhaust gas recirculation amount exceeds a reference value , regeneration control of the filter is performed.
The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein the exhaust gas purification apparatus is an internal combustion engine. An intake throttle valve arranged in the intake system upstream of the connection site of the exhaust gas recirculation passage;
Estimating means for estimating the oxygen concentration in the exhaust gas downstream of the filter;
With
Valve control means for controlling the opening degree of the exhaust gas recirculation control valve and the intake throttle valve according to the estimated oxygen concentration;
An exhaust purification system of an internal combustion engine according to claim 3 or claim 4, characterized in that it comprises a. The oxygen concentration estimating means estimates the oxygen concentration based on a change value of the soot accumulation amount of the filter,
The exhaust emission control device for an internal combustion engine according to claim 5 . As the estimated oxygen concentration decreases, the opening degree of the exhaust gas recirculation control valve is relatively decreased, and the opening degree of the intake throttle valve is relatively increased.
The exhaust emission control device for an internal combustion engine according to claim 5 or 6, characterized by comprising : The valve control means relatively increases the opening of the intake throttle valve during regeneration of the filter ;
The exhaust emission control device for an internal combustion engine according to claim 5 .

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