JP2001159310A - Exhaust emission control device for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve an NOx purifying rate by a catalyst. SOLUTION: When an exhaust gas flow rate Vex is below a given value Vexo (a step S6) and a catalyst temperature Tcal exceeds a given value Tcato (a step S7), by varying a fuel injection form (a step S8), an HC amount in exhaust gas is increased or an exhaust gas temperature is lowered.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an engine, and more particularly to an apparatus for suppressing the emission of NOx (nitrogen oxide).

[0002]

2. Description of the Related Art As a catalyst for purifying an exhaust gas of an engine, HC (hydrocarbon), CO (carbon monoxide) and the NOx in the exhaust gas are simultaneously and very effectively purified in the vicinity of a stoichiometric air-fuel ratio. Possible three-way catalysts are known.
However, since the diesel engine is operated at a fairly lean air-fuel ratio (for example, A / F ≧ 18 and the oxygen concentration is 4% or more), the three-way catalyst cannot reduce NOx in the exhaust gas. When the air-fuel ratio is lean, the concentration of oxygen in the exhaust gas becomes considerably high, so that NOx can be sufficiently reduced and purified even with a NOx purification catalyst (for example, a catalyst in which zeolite carries, for example, Pt as a catalytically active metal). Difficult to do. This point is the same in a gasoline engine having an operating range where the air-fuel ratio is lean.

[0003] On the other hand, Japanese Patent Application Laid-Open No. 8-261502 discloses a system in which main fuel is injected near the top dead center of a compression stroke of an engine and post-fuel injection is performed in an expansion stroke or an exhaust stroke. It describes that when the temperature of the reducing catalyst is high, the post-injection timing is delayed as compared with when the temperature is low. That is, by controlling the post-injection timing, the HC having a small carbon number in the exhaust gas is increased when the catalyst temperature is low, and the HC having a large carbon number in the exhaust gas is increased when the catalyst temperature is high. That is. This is based on the finding that HC having a small carbon number effectively works as a reducing agent in purifying NOx on the low temperature side, and HC having a large carbon number works effectively as a reducing agent in purifying NOx on the high temperature side.

[0004]

By the way, it is known that the purification characteristics of a NOx reduction purification catalyst are related not only to the temperature of the exhaust gas or the catalyst, but also to the flow rate of the exhaust gas passing through the catalyst. That is, when the flow rate of the exhaust gas is small, the contact between the exhaust gas and the catalyst is performed well, so that the NOx can be efficiently reduced and purified by the catalyst.

However, according to experiments performed by the present inventor, a temperature suitable for the catalyst to efficiently purify NOx (for example, NOx
Even when the exhaust gas flow rate is low, the NOx purification efficiency does not always increase when the exhaust gas flow rate is low, but the NOx purification rate increases at lower temperatures. (This point will be described later based on data.) This is originally high N
This is a state in which NOx reduction purification has not progressed efficiently even though an Ox purification rate should be obtained.

An object of the present invention is to solve the problem that the reduction and purification of NOx do not proceed efficiently when the exhaust gas flow rate is low despite the temperature at which the catalyst is active. .

[0007]

SUMMARY OF THE INVENTION The present inventor has found that the state where the reduction and purification of NOx does not proceed efficiently when the flow rate of exhaust gas is small is not a problem with the catalyst itself, but rather as a problem with NO.
The inventors have found that there is a problem with the reducing agent component for x purification and completed the present invention. Hereinafter, a specific description will be given.

The invention of this application is directed to a catalyst disposed in an exhaust passage of an engine for reducing and purifying NOx in exhaust gas by a reducing agent component in the exhaust gas, and a flow rate of the exhaust gas passing through the catalyst. Flow rate detecting means for detecting, temperature detecting means for detecting the temperature of the catalyst, and at least the exhaust gas based on the exhaust gas flow rate detected by the flow rate detecting means and the catalyst temperature detected by the temperature detecting means. When the flow rate is equal to or less than a predetermined value and the catalyst temperature is equal to or more than a predetermined value, the state of the exhaust gas is changed to a NOx reduction reaction by the catalyst as compared with when the exhaust gas flow rate is equal to or more than a predetermined value and the catalyst temperature is equal to or more than a predetermined value. And an exhaust gas state changing means for changing the state of the exhaust gas to a state in which the vehicle easily advances.

That is, the fact that the reduction reaction of NOx occurs on the catalyst means that the reducing agent component is oxidized by NOx. However, NO in the exhaust gas
In addition to x, oxygen is present, and a reaction between the oxygen and the reducing agent component occurs on the catalyst. When the exhaust gas flow rate is low, the reaction between NOx and the reducing agent component on the catalyst easily proceeds, while the reaction between the reducing agent component and oxygen also easily proceeds.

[0010] Accordingly, the reason why the reduction and purification of NOx does not proceed efficiently even though the catalyst temperature is high is that the reducing agent component required for the reduction is oxidized by oxygen in the exhaust gas and the NOx is reduced. It is recognized that they are not working effectively.

[0011] Therefore, the present invention provides a fuel cell system, which comprises:
The state changing means is operated to change the state of the exhaust gas to a state in which the NOx reduction reaction by the catalyst proceeds more easily than when the exhaust gas flow rate is equal to or higher than a predetermined value and the catalyst temperature is equal to or higher than a predetermined value. It was done.

Specifically, when the exhaust gas flow rate is equal to or lower than a predetermined value and the catalyst temperature is equal to or higher than a predetermined value, the state of the exhaust gas is changed by the state changing means to increase the amount of the reducing agent component in the exhaust gas. It is preferable to change it. As a result, even when the flow rate of the exhaust gas is small and the oxidation of the reducing agent component is likely to proceed, the shortage of the reducing agent component is eliminated by the change in the state of the exhaust gas, and the NO by the catalyst is reduced.
The reduction reaction of x proceeds easily.

The amount of the reducing agent component can be increased by increasing the proportion of the reducing agent component in the exhaust gas.
Preference is given to NOx in exhaust gas based on operating conditions.
The purpose is to estimate the amount and increase the ratio of the reducing agent component when compared with this NOx amount.

Further, even when the catalyst temperature is equal to or lower than a predetermined value (regardless of the amount of exhaust gas flow), the catalyst temperature is equal to or higher than the predetermined value and the exhaust gas flow is equal to or lower than the predetermined value. The state of the exhaust gas may be changed so that the NOx reduction reaction easily proceeds.

As the reducing agent component to be increased by the operation of the state changing means, HC is preferable. That is, in a direct injection type engine in which a fuel injection valve is made to face a combustion chamber of the engine and fuel is directly injected and supplied to the combustion chamber,
If the state changing means changes the exhaust gas state by changing the fuel injection mode by the fuel injection valve, it is relatively easy to increase the amount of HC in the exhaust gas by changing the injection mode. Because it is.

For example, normally, only the main injection for injecting fuel near the top dead center of the compression stroke of the engine is performed, and when the state of the exhaust gas is to be changed, a small amount of fuel is supplied in the intake stroke or the compression stroke before the main injection. By performing pre-injection before injection, or post-injection in which a small amount of fuel is injected in the expansion stroke or exhaust stroke after the main injection, the HC amount in the exhaust gas can be increased by the pre-injection or post-injection. Even if the main injection is a batch injection mode in which scheduled fuel is injected at one time, the scheduled fuel is set at an injection pause interval (the time from the end of the previous injection to the start of the next injection) of 50 near the top dead center of the compression process.
It may be a divided injection mode in which the injection is performed in a plurality of times with about 1000 μs.

Usually, the main injection and the post-injection are performed, and when the state of the exhaust gas is to be changed, the amount of the injected fuel is increased or the post-injection timing is delayed. Can be increased.

When the main injection is in the split injection mode, the amount of HC in the exhaust gas can be increased by changing the injection mode so as to lengthen the injection pause interval.

When the exhaust gas flow rate is equal to or lower than a predetermined value and the catalyst temperature is equal to or higher than a predetermined value, the state of the exhaust gas is changed by operating the state changing means to change the ratio of the aromatic hydrocarbon in the exhaust gas. May be changed so as to increase.

That is, although the exhaust gas of the engine contains paraffin, olefins and aromatic hydrocarbons, among these HCs, aromatic hydrocarbons are the least oxidatively decomposed. Therefore, when the flow rate of the exhaust gas is small and the oxidation of the reducing agent component is easy to proceed, the aromatic hydrocarbon is rarely completely decomposed quickly on the catalyst, but is partially decomposed to reduce NOx. It is easy to become an olefin that works effectively. Therefore, even if the total amount of HC does not change much, if the proportion of the aromatic hydrocarbon increases, the shortage of the reducing agent component is eliminated, and the reduction reaction of NOx by the catalyst proceeds easily.

In order to change the state of the exhaust gas so that the proportion of the aromatic hydrocarbons in the exhaust gas increases, the state changing means may be arranged such that the post-injection is considerably delayed from the main injection (for example, around 90 degrees ATDC). 2) that changes the fuel injection mode so as to perform it. That is, the fuel before combustion of the engine contains a relatively large amount of aromatic hydrocarbons, and if the post-injection is performed at a later time, the post-injection fuel is discharged from the combustion chamber without burning much. Therefore, the proportion of the aromatic hydrocarbons in the exhaust gas increases accordingly. Even in this case, the ratio of the aromatic hydrocarbons in the exhaust gas may be increased, and it is preferable that the ratio of N in the exhaust gas be determined based on the operating state.
The purpose is to estimate the amount of Ox and increase the proportion of aromatic hydrocarbons when compared with the amount of NOx.

When the proportion of the aromatic hydrocarbon is increased, the amount of the reducing agent component is also increased at the same time, which is more advantageous in eliminating the shortage of the reducing agent component.

When the exhaust gas flow rate is equal to or lower than a predetermined value and the catalyst temperature is equal to or higher than a predetermined value, the state of the exhaust gas is changed by operating the state changing means so as to lower the temperature of the exhaust gas. May be used.

That is, when the exhaust gas flow rate is small, N
The lack of the reducing agent component effective for Ox reduction is due to the fact that the reducing agent component in the exhaust gas is easily oxidized by oxygen in the exhaust gas on the catalyst. The oxidation reaction of the reducing agent component at the step is suppressed, and waste of the reducing agent component can be avoided.

In order to change the state of the exhaust gas so that the temperature of the exhaust gas decreases, the state changing means includes:
For example, a configuration may be adopted in which the fuel injection mode is changed so that the post-injection is performed with a considerable delay from the main injection (for example, after ATDC 90 degrees). As a result, the post-injection fuel does not contribute much to the combustion, and acts in a direction to lower the exhaust gas temperature by the latent heat. Alternatively, when the main injection and the post-injection at around 30 to 90 degrees ATDC are performed, the exhaust gas temperature can be lowered by changing the fuel injection mode so as to inhibit the post-injection, or When the above-mentioned split injection mode is adopted, the split injection is stopped and changed to the collective injection, whereby the exhaust gas temperature can be lowered.

[0026]

As described above, according to the present invention, at least when the exhaust gas flow rate is equal to or lower than a predetermined value and the catalyst temperature is equal to or higher than a predetermined value, the exhaust gas flow rate is equal to or higher than the predetermined value and the catalyst temperature is equal to or lower than the predetermined value. Since the state of the exhaust gas is changed to a state in which the NOx reduction reaction by the catalyst proceeds more easily than the above case, for example, the amount of the reducing agent component in the exhaust gas is increased, or Since the state of the exhaust gas is changed so as to increase the proportion of aromatic hydrocarbons or to lower the exhaust gas temperature, the exhaust gas is exhausted even though the catalyst is at a temperature suitable for NOx reduction purification. It is possible to avoid a situation in which there is a shortage of a reducing agent component effective for NOx purification due to a small flow rate, and increase the NOx purification rate.

[0027]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows the overall configuration of a diesel engine control device A according to an embodiment of the present invention, and 1 is a multi-cylinder diesel engine mounted on a vehicle. The engine 1 has a plurality of cylinders 2 (only one is shown), and a piston 3 is inserted into each cylinder 2 in a reciprocating manner. A combustion chamber 4 is formed. An injector (fuel injection valve) 5 is disposed substantially at the center of the upper surface of the combustion chamber 4 with the injection hole at the tip end facing the combustion chamber 4, and the injection hole is provided at a predetermined injection timing for each cylinder. Is opened and closed to inject fuel directly into the combustion chamber 4.

Each of the injectors 5 is connected to a common common rail (accumulator) 6 for storing high-pressure fuel.
A high-pressure supply pump 8 driven by a crankshaft 7 is connected to the common rail 6. The high-pressure supply pump 8 operates so that the fuel pressure in the common rail 6 detected by the pressure sensor 6a is maintained at a predetermined value or more. A crank angle sensor 9 for detecting a rotation angle of the crank shaft 7 is provided.
Is composed of a plate to be detected (not shown) provided at the end of the crankshaft 7 and an electromagnetic pickup arranged to face the outer periphery of the plate, and the electromagnetic pickup is provided around the entire periphery of the plate to be detected. A pulse signal is output in response to the passage of the projections formed at predetermined angles.

Reference numeral 10 denotes an intake passage for supplying intake air (air) filtered by an air cleaner (not shown) to the combustion chamber 4 of the engine 1. At a downstream end of the intake passage 10, a surge tank (not shown) is provided. Each passage branched from the surge tank is connected to a combustion chamber 4 of each cylinder 2 by an intake port. An intake pressure sensor 10a for detecting a supercharging pressure supplied to each cylinder 2 is provided in the surge tank.
Is provided. A hot film type air flow sensor 11 for detecting the flow rate of intake air taken into the engine 1 in order from the upstream side to the downstream side in the intake passage 10;
A blower 12 driven by a turbine 21 to be described later to compress the intake air, an intercooler 13 to cool the intake air compressed by the blower 12, and an intake throttle valve (intake amount adjusting means) 14 to reduce the cross-sectional area of the intake passage 10. Are provided respectively. The intake throttle valve 14 is a butterfly valve provided with a notch so that intake air can flow even in a fully closed state. Like the EGR valve 24 described later, the magnitude of the negative pressure acting on the diaphragm 15 is negative pressure. Solenoid valve for control 1
The adjustment by 6 controls the opening of the valve. Further, the intake throttle valve 14 is provided with a sensor (not shown) for detecting its opening.

An exhaust passage 20 discharges exhaust gas from the combustion chamber 4 of each cylinder 2 and is connected to the combustion chamber 4 of each cylinder 2 via an exhaust manifold. The exhaust passage 20 includes, in order from the upstream side to the downstream side, an O2 sensor 17 for detecting the oxygen concentration in the exhaust gas, a turbine 21 rotated by the exhaust gas, HC, CO, and N in the exhaust gas.
A catalytic converter 22 capable of purifying Ox is provided.

As shown in FIG. 2, the catalytic converter 22 has two honeycomb catalysts 22a, 22a arranged in the axial direction (the exhaust gas flow direction). This honeycomb catalyst 22a has a structure in which a catalyst layer is formed on the wall surface of each through-hole of a honeycomb-structured cordierite carrier having a large number of through-holes extending parallel to the axial direction, and when the exhaust gas temperature (catalyst temperature) is low. While exhausting NOx, when the exhaust gas temperature increases, the absorbed NOx is released,
The released NOx and the NOx in the inflowing exhaust gas
And has the property of reducing and purifying the same.

That is, the catalyst layer of the honeycomb catalyst comprises:
The catalyst is formed by coating a support comprising Pt on a zeolite in a dry state with a binder (hydrated alumina) on a support. This catalyst functions not only as an oxidation catalyst for oxidizing HC, but also in the exhaust gas atmosphere burned at the stoichiometric air-fuel ratio, as well as the air-fuel ratio A / F of the exhaust gas.
When (the ratio of oxygen to fuel (HC and CO) contained in the exhaust gas) is leaner than the stoichiometric air-fuel ratio (for example, A /
F> 18), it has a function as a NOx reduction catalyst for reducing and purifying NOx, and also functions as a three-way catalyst near the stoichiometric air-fuel ratio.

An exhaust gas recirculation passage (hereinafter, referred to as an EGR passage) 23 for recirculating a part of the exhaust gas to the intake side branches from a portion of the exhaust passage 20 upstream of the turbine 21, and a portion downstream of the EGR passage 23. The end is connected to the intake passage 10 downstream of the intake throttle valve 14. Near the downstream end of the EGR passage 23, an exhaust gas recirculation amount control valve (exhaust gas recirculation amount adjusting means: hereinafter referred to as an EGR valve) whose opening degree can be adjusted.
A part of the exhaust gas is recirculated to the intake passage 10 while adjusting the flow rate of the exhaust gas in the exhaust passage 20 by the EGR valve 24.

The EGR valve 24 is of a negative pressure responsive type, and a negative pressure passage 27 is connected to a negative pressure chamber of the valve box. The negative pressure passage 27 is connected to a vacuum pump (negative pressure source) 29 via a negative pressure control electromagnetic valve 28. The negative pressure passage 27 is controlled by a control signal (current) from an ECU 35 described later. By opening and closing 27, the negative pressure for driving the EGR valve in the negative pressure chamber is adjusted, whereby the opening of the EGR passage 23 is linearly adjusted.

The turbocharger 25 is a VGT (Variable Geometry Turbo), to which a diaphragm 30 is attached, and a solenoid valve 3 for negative pressure control.
By adjusting the negative pressure acting on the diaphragm 30 by 1, the cross-sectional area of the exhaust gas passage is adjusted.

Each injector 5, high-pressure supply pump 8, intake throttle valve 14, EGR valve 24, turbocharger 25
And the like are configured to be operated by a control signal from a control unit (Engine Control Unit: hereinafter referred to as ECU) 35. On the other hand, the ECU 35 receives an output signal from the pressure sensor 6a, an output signal from the crank angle sensor 9, an output signal from the pressure sensor 10a, an output signal from the air flow sensor 11, and an output signal from the O2 sensor 17. At least an output signal, an output signal from a lift sensor 26 of the EGR valve 24, and an output signal from an accelerator opening sensor 32 for detecting an operation amount (accelerator opening) of an accelerator pedal (not shown) by a driver of the vehicle are input. Have been.

Then, the fuel injection control by the operation of the injector 5 is performed, the fuel injection amount and the fuel injection timing are controlled according to the operation state of the engine 1, and the common rail pressure by the operation of the high-pressure supply pump 8, ie, the common rail pressure, The fuel injection pressure is controlled. In addition, the intake air amount is controlled by operating the intake throttle valve 14, the exhaust gas recirculation amount is controlled by operating the EGR valve 24, and the operation control of the turbocharger 25 is performed. (VGT control).

The ECU 35 stores a fuel injection amount map in which an optimum fuel injection amount Q experimentally determined according to changes in the target torque and the rotation speed of the engine 1 is electronically stored in a memory. Provided. Then, based on the target torque determined based on the output signal from the accelerator opening sensor 32 and the engine speed determined based on the output signal from the crank angle sensor 9, the basic fuel injection amount is calculated from the fuel injection amount map. Qbase is read, and the excitation time (valve opening time) of each injector 5 is determined based on the basic fuel injection amount Qbase and the common rail pressure detected by the pressure sensor 6a. By this basic fuel injection control, an amount of fuel corresponding to the target torque of the engine 1 is supplied, and the engine 1 is in a state where the average air-fuel ratio in the combustion chamber 4 is considerably lean (A / F).
≧ 18).

During a steady operation (when the change in the accelerator opening is small), the sub-injection of the fuel is periodically performed to supply a reducing agent component to the catalyst of the catalytic converter 22 to promote the reduction and purification of NOx. It is done on a regular basis. That is, post-injection for injecting a small amount of fuel (for example, several percent to ten and several percent of the main fuel injection amount) is performed in the expansion stroke or the exhaust stroke after the main injection. This sub-injection (in this case, post-injection) can be performed every time only for a specific one of the plurality of cylinders, or can be performed every several times, or the state where the sub-injection is performed every time is determined. The state in which the sub-injection is not performed may be alternately continued several times.

(Change Control of Exhaust Gas State) A feature of the present invention is to change the state of exhaust gas by changing the fuel injection mode according to the flow rate of exhaust gas passing through the catalytic converter 22 and the temperature of the catalyst. That's the point.

-Relationship between exhaust gas flow rate and catalyst temperature (exhaust gas temperature) and NOx purification rate-First, the relationship between exhaust gas flow rate and catalyst temperature and NOx purification rate will be described. Fig. 3 shows a diesel engine with a turbocharger (number of cylinders;
NOx purification rate when the vehicle equipped with the catalytic converter 22 (the amount of Pt carried per 1 L of carrier; 0.35 g, the base material of Pt; ZSM5) and the above-described catalytic converter 22 was run in the ECE mode. FIG. 4 shows the HC purification rate data at that time. However, the temperature of the exhaust gas at the inlet of the catalytic converter is used as the catalyst temperature.

In FIGS. 3 and 4, among the symbols attached to the respective purification rate characteristics, “ID” indicates “32 →
“0” is “50” when the vehicle speed is reduced from 32 km / h to zero vehicle speed.
"→ 32" is for deceleration from 50 km / h to 32 km / h, "32" is for steady operation at 32 km / h, and "5"
“0” is “0 → 32” during steady operation at a vehicle speed of 50 km / h.
Is "0 → 5" during acceleration operation from zero vehicle speed to 32 km / h.
"0" indicates an acceleration operation from a vehicle speed of zero to 50 km / h.

In FIG. 3 and FIG. 4, "32" and "50" appear to overlap with each other, and eight of them, which are provided with lead lines and are marked with "о", relate to "50". It is.

The sub-injection is performed every time only for a specific one of the four cylinders.
The post-injection is such that fuel is injected at A (a crank angle of 90 degrees after the top dead center of the compression stroke). By this sub-injection, an amount of reducing agent component (HC) necessary for reducing NOx discharged in each operation state of the engine is supplied to the catalyst.

According to FIG. 3, the temperature range where the NOx purification rate peaks changes to a higher temperature side as the exhaust gas flow rate increases, as indicated by the chain line in FIG. And 50 degrees.

On the other hand, according to FIG. 4, the temperature range in which the HC purification rate reaches its peak, as indicated by the dashed line in FIG. 4, increases as the flow rate of the exhaust gas increases as in the case of the peak temperature range of the NOx purification rate. The peak temperature range of the HC purification rate and the peak temperature range of the NOx purification rate substantially coincide with each other in a region where the exhaust gas flow rate is high, while the peak temperature range of the HC purification rate is substantially equal in a region where the exhaust gas flow rate is small. The region is higher by 20 to 30 degrees than the peak temperature region of the NOx purification rate.

As described above, in the region where the flow rate of the exhaust gas is small, the peak temperature range of the HC purification rate is higher by 20 to 30 degrees than the peak temperature range of the NOx purification rate. It is considered that the reason why the temperature range is lower than that of the HC purification rate is that HC in the exhaust gas easily burns when the flow rate of the exhaust gas is small, and does not work effectively as a reducing agent for NOx purification.

That is, when the catalyst temperature is high, HC in the exhaust gas is quickly oxidized and decomposed, and HC effective for NOx purification is removed.
Is insufficient and the catalyst temperature is lower than when the catalyst temperature is lower.
x becomes difficult to purify, and therefore, the peak temperature range of the NOx purification rate appears on the low temperature side.

-Change of Fuel Injection Mode- The processing operation of the ECU 35 will be described below with reference to the flowchart of FIG. This control is executed at every predetermined crank angle.

First, in step S1 after the start, a crank angle signal, an air flow sensor output, an accelerator opening, and the like are read. In the following step S2, the basic fuel injection amount Qbase is read from the fuel injection amount map based on the target torque obtained from the accelerator opening and the engine speed obtained from the crank angle signal. As illustrated in FIG. 6, the fuel injection amount map records an optimum fuel injection amount Qbase experimentally determined according to changes in the accelerator opening and the engine speed. In this map,
The basic fuel injection amount Qbase increases as the accelerator opening increases.
The setting is made to increase as the engine speed increases.

The main injection timing Ibase is set near the top dead center of the compression stroke. For example, with reference to BTDC 5 ° CA (crank angle), the more the injection amount Qbase is, the more the advance is made, and conversely, the smaller the injection amount Qbase, the more the retard is Is done.

In the following step S3, the sub-injection amount Qsub and the sub-injection timing Isub are set. This sub-injection is for increasing the amount of HC in the exhaust gas and supplying this HC to the catalytic converter 22 as a reducing agent component for NOx purification.

The sub-injection condition is satisfied every predetermined time when the engine is in a steady operation state (when the change in the accelerator opening is small). This is to periodically increase the HC concentration of the exhaust gas supplied to the catalytic converter 22, thereby improving the NOx purification performance of the catalyst. That is, when the HC concentration increases, the NO
x Although the purification rate rises, if the state of high HC concentration continues,
The NOx purification rate decreases. On the other hand, when the increase and decrease of the HC concentration are periodically repeated, a state where the NOx purification rate is high can be intermittently formed, and as a whole, the NOx purification rate increases.

Specifically, for example, in a four-cylinder engine, during a steady operation, the main injection and the sub-injection are always executed for a specific cylinder, and only the main injection is executed for the remaining three cylinders. The sub-injection may be executed for the specific cylinder every other time or several times.

The sub-injection amount Qsub is read from the fuel injection amount map. The map records the optimum Qsub experimentally determined according to changes in the accelerator opening and the engine speed. The injection amount Qsub is determined as the accelerator opening increases and the engine speed increases. , Is set to be more.

The sub-injection timing Isub is a period from the completion of the main injection to the first half of the expansion stroke (for example, ATDC 60 ° CA
(Crank angle)), and is advanced as the engine load is higher, and is retarded as the engine load is lower.

In step S4, the flow rate Vex of the exhaust gas passing through the catalytic converter 22 is estimated. This estimation is performed by reading the exhaust gas flow rate Vex from the flow rate map based on the engine speed and the accelerator opening. As illustrated in FIG. 7, the flow rate map is obtained by recording an exhaust gas flow rate Vex experimentally obtained according to changes in the accelerator opening and the engine speed. In this map,
The exhaust gas flow rate Vex is set to increase as the accelerator opening increases and as the engine speed increases. However, during the EGR control, since the amount of exhaust gas flowing to the catalytic converter 22 decreases, the decrease correction of the exhaust gas flow rate Vex is performed based on the EGR rate. This step S4 constitutes an exhaust gas flow rate detecting means.

In step S5, the catalyst temperature Tcat of the catalytic converter 22 is estimated. The catalyst temperature Tcat is estimated based on the history of the engine operation state such as the total fuel injection amount since the start of the engine operation, the history of the traveling speed of the vehicle equipped with the engine, the traveling distance, and the like. A temperature sensor may be provided in an exhaust passage or the like to detect the exhaust gas temperature, and this exhaust gas temperature may be used as a catalyst temperature. This step S
Reference numeral 5 denotes a catalyst temperature detecting means.

Then, the exhaust gas flow rate Vex is smaller than the predetermined value Vexo and the catalyst temperature Tcat is lower than the predetermined value Texo.
When it is higher than cato, the fuel injection mode is changed (steps S6 to S8). That is, step S6 is an operation region where the exhaust gas flow rate Vex is small (NO in FIGS. 3 and 4).
This is for determining whether the peak temperature range of the x purification rate and the peak temperature range of the HC purification rate are different from each other), and for example, 75000 L / Hr is given as the predetermined value Vexo. Step S7 is for judging whether or not the catalyst is in a temperature range in which HC in the exhaust gas is easily combusted. For example, 165 ° C. is given as the predetermined value Tcato. Step S8 constitutes an exhaust gas state changing means.

The fuel injection mode is changed by changing the HC in the exhaust gas.
In order to increase the amount, the sub-injection amount Qsub for the specific cylinder that performs the above-described sub-injection is increased from the set value in step S3. Increase the sub-injection amount for
If the sub-injection is not being executed, the sub-injection is executed. However, without increasing the sub-injection amount for the specific cylinder, the same sub-injection as for the specific cylinder is executed for one or more other cylinders so as to increase the overall sub-injection amount. Another modification of the injection mode may be to increase the amount of HC in the exhaust gas by delaying the sub-injection timing Isub for the specific cylinder from the set value in step S3 to, for example, around 90 ° CA at ATDC. It may be to do. In this case, the proportion of aromatic hydrocarbons in HC contained in the exhaust gas increases. Further, the retard may be increased to set the sub-injection timing Isub after ATDC 90 ° CA to suppress afterburning and lower the exhaust gas temperature.

When the exhaust gas flow rate Vex is equal to or higher than a predetermined value Vexo or the catalyst temperature Tcat is equal to or lower than a predetermined value Tcato, the fuel injection mode is not changed.

Next, when it is determined in step S9 that it is the main injection timing Ibase, the main injection is executed in step S10, and when it is determined in step S11 that it is the sub-injection timing Isub, the sub-injection is performed in step S12. Be executed.

Therefore, with the above fuel injection control, when the exhaust gas flow rate Vex is lower than the predetermined value Vexo and the catalyst temperature Tcat is higher than the predetermined value Tcato, the amount of HC in the exhaust gas increases. Alternatively, the shortage of the NOx reducing agent in the catalyst of the catalytic converter 22 due to an increase in the amount of aromatic hydrocarbons is eliminated, or the exhaust gas temperature is reduced and the combustion of HC is suppressed. The state is prevented. Therefore,
The NOx purification rate can be improved.

<Other Embodiment 1> The above-described embodiment is an example in which the post-injection is performed as the sub-injection after the main injection, but the pre-injection for injecting the fuel during the period from the beginning of the intake stroke to before the main injection is performed. Even when the amount of HC in the exhaust gas is increased, the same effect can be obtained by performing the pre-injection increase processing in accordance with the case of the post-injection.

The injection timing of the main injection may be slightly changed to the retard side in order to increase the HC amount.

<Other Embodiment 2> The above-described embodiment is a case where the form of the sub-injection is changed, but when the exhaust gas flow rate Vex is lower than a predetermined value Vexo and the catalyst temperature Tcat is higher than a predetermined value Tcato. Alternatively, the mode of the main injection (fuel injection performed near the top dead center of the compression stroke, and it does not matter whether the pre-injection or the post-injection exists) may be changed. That is, the main injection mode of the embodiment is a batch injection in which all of the main injection amount is injected at a time at the main injection timing, but employs multi-stage injection as the main injection, and sets at least one of the number of injections and the injection timing. It may be changed.

In the multi-stage injection, the main injection fuel is divided into a plurality of injections so as to continue the combustion of the fuel in the combustion chamber near the top dead center of the compression stroke as illustrated in FIG. It is. The valve opening time of each injection is 800μ
It is preferable that the injection suspension time (the time from when the injection hole of the injector 5 is closed to when it is next opened) Δt be 50 to 1000 μs or less. The second injection is preferably performed after the top dead center of the compression stroke. The basic operation of this split injection is as follows.

The fuel injected from the injection hole of the injector 5 spreads into the combustion chamber 4 while forming a spray having a conical shape as a whole, and also repeatedly breaks up into small oil droplets due to friction with air. The fuel evaporates from the surface of the fuel cell to generate fuel vapor. At that time, since the fuel is divided and injected, the ratio of the premixed combustion by the initially injected fuel is relatively reduced, so that the combustion pressure and the combustion temperature do not excessively increase in the early stage of the combustion. , NOx generation is reduced.

Since the injection suspension time Δt is set to 50 μs or more, the fuel oil droplets injected later hardly catch up with the fuel oil droplets injected earlier. In particular, if the second injection is performed after the top dead center of the compression stroke, the fuel injected in the second injection immediately burns, the pressure in the combustion chamber 4 increases greatly, and the viscosity of the compressed air increases. The oil droplets of the injected fuel are immediately decelerated and do not catch up with the oil droplets of the previously injected fuel. Since the valve opening time of each time is set to approximately 800 μs or less, the fuel injection amount of each time is small,
Since the recombination of oil droplets during the fuel spray is also minimized, for example, by increasing the fuel pressure to increase the ejection speed of the fuel, the atomization of the fuel and, consequently, the vaporization and atomization are sufficiently promoted. Thus, the mixing state of the fuel vapor and the air can be greatly improved. Since the injection stop time Δt is set to 1000 μs or less, the fuel injected by each injection is not interrupted so that the next injected fuel starts burning before the combustion of the previously injected fuel ends. Burned.

In short, by performing the main injection in a divided manner, the combustion state of the injected fuel can be made very good, and the improvement of fuel consumption and suppression of smoke generation can be realized. Further, although the injection end timing is relatively late, the fuel intermittently injected during that period is well vaporized and atomized and diffusely burns as described above. The combustion state is not deteriorated, but rather, the pressure of the combustion chamber 4 is maintained at a high level for a relatively long time, so that the expansion force of the combustion gas is transmitted to the piston 3 very effectively. Fuel efficiency can also be improved by improving efficiency.

Thus, in the case of the multi-stage injection, since the fuel is satisfactorily burned, the amount of HC in the exhaust gas is reduced as compared with the case of collectively injecting the fuel. In any case where the injection suspension time Δt is increased, the end time of combustion is delayed, and the amount of HC in the exhaust gas increases.

FIG. 9 shows the effects of the number of divisions of the main injection and the injection suspension time Δt on the amount of HC in the exhaust gas. This is the case where the fuel of the basic injection amount corresponding to the target torque of the engine 1 is collectively injected from the top dead center of the approximately compression stroke (hereinafter referred to as collective injection), and the fuel is divided into two equal injections (hereinafter referred to as the collective injection). For each of three equal injections (hereinafter referred to as three-split injection), the injection pause time Δt is changed to change the crank angle at the end of the injection, and The relationship between the amount of HC in exhaust gas and the amount of HC was examined. In two-split injection, Δt = 35
At 0, 400, 700, and 900 μsec, Δt = 400, 550, 700, and 900 μ for the three-split injection.
The sec was examined.

According to the figure, the collective injection has a larger HC amount in the exhaust gas than the split injection, the three-split injection has a larger HC amount than the two-split injection, and the two-split injection. And the three-split injection, the longer the injection pause time Δt
It can be seen that the amount of C increases. Therefore, the exhaust gas flow rate V
ex is less than a predetermined value Vexo and the catalyst temperature Tcat
Is higher than the predetermined value Tcato, the main injection mode is changed, that is, for example, during two-split or three-split injection,
By increasing the amount of HC in the exhaust gas by performing batch injection, increasing the injection suspension time Δt, or increasing the number of injection divisions, the catalyst of the catalytic converter 22
A shortage can be prevented.

FIG. 10 shows the results of an investigation on the effects of the number of divisions of the main injection and the injection pause time Δt on the exhaust gas temperature in the same manner as in the case of the HC amount. According to the drawing, the exhaust gas temperature is lower in the batch injection than in the split injection, the exhaust gas temperature is lower in the two-split injection than in the three-split injection, and in the two-split injection and the three-split injection, It can be seen that the shorter the injection suspension time Δt, the lower the exhaust gas temperature. Therefore, the exhaust gas flow rate Vex becomes equal to the predetermined value Vex.
o and the catalyst temperature Tcat is lower than a predetermined value Tcato
When the main injection mode is higher than the above, the main injection mode is changed, that is, for example, during two-split or three-split injection, batch injection is performed or the injection pause time Δt is shortened. If so, by performing two-split injection to lower the exhaust gas temperature, it is possible to prevent the catalyst of the catalytic converter 22 from becoming in a state of insufficient HC.

<Other Embodiment 3> Further, when the sub-injection is executed during the steady operation as described above, the exhaust gas flow rate Vex is smaller than the predetermined value Vexo and the catalyst temperature Tcat is lower than the predetermined value Tcato. When the temperature is high, the sub-injection may be stopped to lower the exhaust gas temperature.

<Other Embodiment 4> In each of the above embodiments, the fuel injection mode is changed when the exhaust gas flow rate Vex is lower than the predetermined value Vexo and the catalyst temperature Tcat is higher than the predetermined value Tcato. However, the exhaust gas flow rate Vex
Alternatively, the mode of changing the injection mode may be changed according to the catalyst temperature Tcat.

That is, FIG. 11 is a region diagram showing the state of the exhaust gas in the relationship between the exhaust gas flow rate Vex and the catalyst temperature Tcat, where the exhaust gas flow rate Vex is smaller than the predetermined value Vexo and the catalyst temperature Tcat is lower than the predetermined value Tcato. A region higher than
When the exhaust gas flow rate Vex is a predetermined value Vexo to Vex1, the catalyst temperature Tca
t is Tcat = K × Vex + C (where K is positive and Vex
> Vexo, Tcat> Tcato) B higher than line L
It is divided into an area and another area C.

In the area C, the fuel injection is controlled according to the injection mode set in steps S2 and S3 shown in FIG. 5, and in the area A, the injection mode is described in each of the above embodiments. In the B region, the injection mode is changed in the same manner as in the A region, but the degree of change is reduced, so that the amount of HC in the exhaust gas increases or the exhaust gas temperature decreases. That is, in the B region, the degree of increase of HC in the exhaust gas due to the change of the injection mode is made smaller than that in the A region, or the degree of decrease in the exhaust gas temperature is made smaller than that in the A region. It is.

Although the above embodiments relate to a diesel engine, the present invention can also be applied to a gasoline engine. For example, the HC amount in the exhaust gas is increased by retarding the ignition timing from a normal time. Alternatively, the exhaust gas temperature can be reduced. Further, an injector may be disposed in the exhaust passage, and the reducing agent may be supplied by the injector to increase the amount of the reducing agent component in the exhaust gas.

[Brief description of the drawings]

FIG. 1 is a diagram showing an overall configuration of an exhaust gas purification device for a diesel engine according to an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a catalytic converter.

FIG. 3 is a graph showing NOx purification rate data when the vehicle is running in the ECE mode.

FIG. 4 is a graph showing HC purification rate data when the vehicle is running in the ECE mode.

FIG. 5 is a flowchart illustrating a processing procedure of fuel injection control.

FIG. 6 is a diagram showing an example of a map for setting a basic fuel injection amount Qbase.

FIG. 7 is a diagram showing an example of a map for estimating an exhaust gas flow rate.

FIG. 8 is a time chart showing injection timings of fuel batch injection and multi-stage injection.

FIG. 9 is a graph showing the influence of the number of divisions of fuel injection and the suspension time of injection on the HC concentration of exhaust gas.

FIG. 10 is a graph showing the influence of the number of divisions of fuel injection and the injection suspension time on exhaust gas temperature.

FIG. 11 is a region diagram showing an exhaust gas state by an exhaust gas flow rate Vex and a catalyst temperature Tcat.

[Explanation of symbols]

 A Exhaust gas purification device 1 Diesel engine 2 Cylinder 4 Combustion chamber 5 Injector (fuel injection valve) 20 Exhaust passage 22 Catalytic converter 23 EGR passage (Exhaust recirculation passage) 24 EGR valve (Exhaust recirculation amount adjusting means) 35 ECU (Control unit)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/28 301 F02D 21/08 311B F02D 21/08 301 41/40 P 311 45/00 312R 41/40 B01D 53/36 ZAB 45/00 312 101A (72) Inventor Hiroshi Hayashibara 3-1, Fuchu-cho, Shinchu, Aki-gun, Hiroshima Mazda Co., Ltd. (72) Inventor Satoshi Ichikawa 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima F-term in Mazda Motor Corporation (reference) 3G084 AA01 BA00 BA13 BA15 CA02 DA10 EA11 EB08 EB12 FA00 FA10 FA33 3G091 AA02 AA10 AA11 AA17 AA18 AA28 AB05 BA14 BA15 BA19 BA32 CA13 CA18 CB02 CB03 DA05 CB07 DA03 DA03 DA03 EA06 EA07 EA17 EA18 EA21 EA30 EA31 EA34 FA06 FA12 FA17 FA18 FA19 GA06 GB01X GB06W GB09X GB10X GB17X HA08 HA36 HA47 HB03 HB05 HB06 3G092 AA02 AA13 AA17 AA18 BB13 BB15 DB03 DC03 DC09 DE18S DF04 DF08 DG06 EA08 EA12 EA28 EA29 FA18 FA24 HA01Z HA05Z HA06X HA06Z HA16Z HB03Z HD02Z HD05Z HD07X HD07Z HE03Z0104 MA04 MA03 HA04 4D048 AA06 AB02 AC02 BB02 CC32 CC61 DA01 DA02 DA03 DA05 DA06 DA09 DA10 DA13

Claims (5)

[Claims] 1. A catalyst disposed in an exhaust passage of an engine for reducing and purifying NOx in exhaust gas by a reducing agent component in the exhaust gas, and a flow rate detection for detecting a flow rate of the exhaust gas passing through the catalyst. Means, temperature detection means for detecting the temperature of the catalyst, and at least the exhaust gas flow rate is a predetermined value based on the exhaust gas flow rate detected by the flow rate detection means and the catalyst temperature detected by the temperature detection means. When the catalyst temperature is equal to or less than a predetermined value, the exhaust gas flow is more easily changed than when the exhaust gas flow rate is equal to or higher than a predetermined value and the catalyst temperature is equal to or higher than a predetermined value. An exhaust gas purifying apparatus for an engine, comprising: 2. The exhaust gas purifying apparatus for an engine according to claim 1, wherein said exhaust gas state changing means is configured to output the exhaust gas state when the exhaust gas flow rate is at least a predetermined value and the catalyst temperature is at least a predetermined value. An exhaust gas purifying apparatus for an engine, wherein an amount of a reducing agent component in the exhaust gas is increased as compared with a case where an exhaust gas flow rate is a predetermined value or more and a catalyst temperature is a predetermined value or more. 3. The exhaust gas purifying apparatus for an engine according to claim 2, wherein the reducing agent component increased by the operation of the exhaust gas state changing means is HC. 4. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the exhaust gas state changing means is configured such that at least the exhaust gas flow rate is equal to or lower than a predetermined value and the catalyst temperature is equal to or higher than a predetermined value. An exhaust gas purifying apparatus for an engine, wherein the ratio of the aromatic hydrocarbons in the exhaust gas is higher than when the exhaust gas flow rate is equal to or higher than a predetermined value and the catalyst temperature is equal to or higher than a predetermined value. 5. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the exhaust gas state changing means is configured so that at least the exhaust gas flow rate is equal to or lower than a predetermined value and the catalyst temperature is equal to or higher than a predetermined value. An exhaust gas purifying device for an engine, wherein the temperature of the exhaust gas is reduced when the exhaust gas flow rate is equal to or higher than a predetermined value and the catalyst temperature is equal to or higher than a predetermined value.