JPH084599A - Combustion control method for engine - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a combustion control method for an engine that can obtain an optimum EGR amount according to a load in an engine that performs lean burn at least in a predetermined operating state. A combustion control method for an engine, comprising a plurality of spark plugs and controlling operation with a lean air-fuel ratio in at least a predetermined operation region, wherein the exhaust valve closing timing and intake valve The opening timing is overlapped by the maximum amount, and the exhaust valve closing timing is changed so that the overlapping amount of the exhaust valve closing timing and the intake valve opening timing decreases as the load increases and / or the rotational speed increases and decreases. Control.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a combustion control method for an engine in which the amount of residual gas in a cylinder is controlled by controlling the opening / closing timing of intake / exhaust valves.

[0002]

2. Description of the Related Art Conventionally, in a vehicle engine, a method of diluting an air-fuel mixture introduced into a cylinder to improve fuel consumption (so-called lean burn) has been researched. As an engine, for example, one disclosed in Japanese Patent Laid-Open No. 5-86866 is known. In this conventional technique, it is considered that three to four spark plugs are arranged in the cylinder of the engine along the diametrical direction thereof to improve both the ignitability of the air-fuel mixture and the propagating property of the flame.

In the meantime, in the lean burn engine, in addition to the above problems of ignitability and flame propagation, there is also a problem of exhaust gas purifying property. In lean burn engines, it is difficult to reduce NOx with a catalyst, so
It is necessary to significantly reduce the NOx emitted from the engine. On the other hand, the conventional EGR method has the effect of reducing NOx in the internal combustion engine. Therefore, performing EGR when lean burn is established is very effective for purifying exhaust gas. is there. However, in the lean region, the combustion speed decreases, so the air-fuel mixture is gas-phase quenched, and unburned H
The amount of C emission greatly increases. If EGR is performed under such conditions, the combustion speed is further reduced, and HC
It will spur the increase.

Known EGR techniques are an external EGR that introduces exhaust gas from the outside of the engine and an internal EGR that keeps combustion gas inside the engine. The former is E and the latter is E
The temperature of the GR gas can be kept high, and since the unburned gas flows back into the intake port at the beginning of the intake stroke, the evaporation of fuel droplets and liquid film in the intake port can be promoted. Further, such vaporization promotion can prevent the process of inflow of a large amount of liquid fuel → adhesion to the cylinder wall → uneven distribution of fuel, so that the air-fuel mixture distribution can be made uniform. Due to these effects, when the internal EGR is performed, it is possible to reduce NOx even in a lean region without increasing HC.
When such an internal EGR method is used, it is desirable to reduce the amount of recirculated gas in the internal EGR particularly in a light load region where combustion is likely to become unstable, and increase it in a high load region where NOx generation amount is large. .

[0005]

However, the internal E
Since the GR amount is determined by the intake negative pressure, contrary to the above desirable condition, the EGR becomes excessive due to the large amount of unburned gas flowing back to the intake port due to the large intake negative pressure at light load, and conversely at high load. There will be a shortage on the side.

Therefore, the present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an optimum EGR amount according to the load in an engine that performs lean burn at least in a predetermined operating state. It is to provide a combustion control method of an engine that can be obtained.

[0007]

Means for Solving the Problems The above-mentioned problems are solved,
In order to achieve the object, a combustion control method for an engine of the present invention is a combustion control method for an engine that includes a plurality of spark plugs, and that controls operation with a lean air-fuel ratio in at least a predetermined operation region, and a medium-high load, In the medium speed range, the exhaust valve closing timing and the intake valve opening timing are overlapped by the maximum amount, and the exhaust valve closing timing and the intake valve opening timing are increased or decreased as the load increases and / or the rotational speed increases and decreases. It is characterized in that the closing timing of the exhaust valve is variably controlled so that the overlap amount is reduced.

Further, the engine combustion control method of the present invention is an engine combustion control method which comprises a plurality of spark plugs and which controls the operation at a lean air-fuel ratio in at least a predetermined operation region, and which has a medium to high load and a medium speed. In the region, the exhaust valve closing timing and the intake valve opening timing overlap each other by the maximum amount, and the exhaust valve closing timing and the intake valve opening timing overlap as the load increases and / or the rotational speed increases and decreases. It is characterized in that the opening timing of the intake valve is variably controlled substantially before the top dead center so that the amount becomes small.

Further, the engine combustion control method of the present invention is a combustion control method for an engine, which comprises a plurality of spark plugs and which controls the operation at a lean air-fuel ratio in at least a predetermined operation region, and which has a medium to high load and a medium rotation speed. In the region, the opening timing of the intake valve is set to the latest timing after the top dead center, and the opening timing of the intake valve is advanced as the load increases and / or the rotational speed increases and decreases. .

Further, the engine combustion control method according to the present invention is characterized in that the exhaust valve closing timing is variably controlled together with the advance control of the intake valve opening timing.

Further, the engine combustion control method of the present invention has two intake ports, a first intake port and a second intake port.
A combustion control method for an engine having a plurality of ignition plugs for performing a lean air-fuel ratio operation in at least a predetermined operation region, in which a fuel injection valve is provided in the intake port of the engine and a swirl control valve is provided in the second intake port. The feature is that the intake valve is opened by a small amount near the opening timing of the exhaust valve.

Further, the engine combustion control method according to the present invention is characterized in that the opening operation of a small amount of the intake valve is stopped in a high load region of a predetermined value or more.

Further, the engine combustion control method of the present invention has a plurality of spark plugs and exhaust gas recirculation means, operates at a predetermined lean air-fuel ratio in a low load range, and operates in a lean load range above a predetermined high load range. A combustion control method for an engine that operates at a predetermined air-fuel ratio richer than the fuel ratio, in which the recirculation exhaust gas amount is increased as the load increases in the low load range, and the recirculation exhaust gas amount is gradually reduced in the rich air-fuel ratio operation range. It is characterized by controlling.

Further, in the engine combustion control method according to the present invention, in a low load range including an idling state and an idling state, the air-fuel ratio is controlled to be richer than the lean air-fuel ratio and the exhaust gas recirculation is stopped. There is.

[0015]

Since the engine combustion control method according to the present invention is configured as described above, in the medium and high load region where the EGR amount is insufficient, the amount of recirculated gas introduced into the cylinder is increased and the EGR amount is excessively increased. In the low load range, where the amount of reflux gas introduced into the cylinder is reduced,
It is possible to provide a combustion control method for an engine that can obtain an optimum EGR amount according to the load.

[0016]

The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. (First Embodiment) FIG. 1 is a plan view mainly showing the configuration of a cylinder portion of an engine to which a combustion control method according to the present invention is applied. FIG. 2 is a side sectional view of FIG. 1 viewed from the side.

1 and 2, the upper portion of the cylinder 12 of the four-cycle engine, that is, the cylinder head 14
A pent roof type combustion chamber 16 is formed therein. In this combustion chamber 16, along the ridge of the pent roof,
One in the center of the combustion chamber and two in the periphery with this in between, a total of 3
A book spark plug 17 is provided. The first intake port 1 is provided on both sides of the pent roof ridge of the combustion chamber 16.
8, second intake port 20 and first exhaust port 22, second
The exhaust port 24 is formed. First intake port 18
The first and second intake ports 20 have substantially the same diameter.
The intake pipes 26 and 28 are connected. A fuel injection valve 30 having a single injection hole is provided in the first intake pipe 26 of the first and second intake pipes 26, 28, and a swirl control valve 32 is provided in the second intake pipe 28. . First intake pipe 26
From the second intake pipe 28, the air is always taken into the cylinder 12 (however, depending on the opening of the throttle valve and the intake valve (not shown)). Air is introduced according to the magnitude of the swirl flow. That is, when the maximum swirl is required, the swirl control valve 32 is fully closed, air is not taken in from the second intake pipe 28, and air is introduced into the cylinder 12 only from the first intake pipe 26. When the swirl is not required, the swirl control valve is fully opened and the swirl ratio is set to about 0.

Also, the first and second intake ports 18, 20
The intake valves 34 for opening and closing these ports, respectively.
And the first and second exhaust ports 22,
24 are exhaust valves 3 for opening and closing these ports, respectively.
6 is provided. The intake valve 34 and the exhaust valve 36 are opened and closed by an intake valve driving cam shaft and an exhaust valve driving cam shaft, which are not shown. The intake valve driving camshaft and the exhaust valve driving camshaft are rotationally driven by a crankshaft via a chain, a timing belt, a gear and the like.

Next, a combustion control method in the engine configured as described above will be described.

In the combustion control method according to the present embodiment, lean combustion is performed in a predetermined operating state. In this lean burn state, internal EGR is performed and NO is increased without increasing HC.
We try to reduce the occurrence of x. When the internal EGR is used in lean burn as described above, as already described in the section of the prior art, the recirculation gas amount of the internal EGR is reduced in the light load region where combustion is likely to become unstable, and N
It is desirable to increase the amount in the high load range where a large amount of Ox is generated. Therefore, in the first embodiment, the opening / closing phase of the exhaust valve is controlled, and the overlap amount in the vicinity of the top dead center of the closing timing of the exhaust valve 36 and the opening timing of the intake valve 34 is controlled. In controlling the phase of the opening / closing timing of the exhaust valve 36, the rotational phase difference between the exhaust valve driving camshaft and the crankshaft is changed by using, for example, a cam pulley.

FIG. 3 is a diagram showing the relationship between the valve lift of the intake valve 34 and the exhaust valve 36 with respect to the rotation angle of the crankshaft. Top dead center of crankshaft (T
The left side shows the valve lift amount of the exhaust valve 36, and the right side shows the valve lift amount of the intake valve 34 with DC in between.

In FIG. 3, the opening / closing phase of the exhaust valve 36 is
The position is adjusted from the position shown by the solid line to the position shown by the broken line. Then, the exhaust valve 36 and the intake valve 34 shown by A in the figure
During the overlap period, which is the period when the fuel cells are open simultaneously, the burnt gas in the exhaust ports 22 and 24 and the cylinder 12 recirculates to the cylinder 12 and the intake ports 18 and 20, respectively, and is burned as internal EGR gas in the next cycle. Reduces temperature and suppresses NOx production.

On the other hand, a decrease in the temperature of the combustion gas makes the combustion unstable, so that high NOx can be maintained without impairing the drivability of the engine.
In order to obtain the reduction effect, it is necessary to optimally control the EGR amount according to the operating conditions. Therefore, in the first embodiment, as shown in FIG. 4, the closing timing of the exhaust valve 36 is set to the latest timing in the medium rotation speed and medium / high load regions (that is, the overlap period is maximized), and this region is set. The other parts are controlled as follows. (1) First, on the low rotation speed side, in order to prevent the recirculation time of burnt gas from increasing and the EGR amount to become excessive, the closing timing of the exhaust valve 36 is advanced as the rotation speed decreases, and the EGR ratio is substantially reduced. It is kept constant (indicated by arrow A in FIG. 4). (2) On the high rotation speed side, particularly in the case of a plug arranged in the peripheral portion away from the center of the cylinder, blown-off of the flame kernel and cooling loss due to the flow of air increase and the ignitability deteriorates significantly. In this region, the EGR amount is reduced as the rotation speed increases, and the exhaust valve closing timing is advanced (indicated by arrow B in FIG. 4) in order to secure ignitability. (3) On the low load side, the intake negative pressure becomes large and the EGR amount becomes excessive, resulting in deterioration of combustion stability. Therefore, as the load decreases, the closing timing of the exhaust valve 36 is advanced to reduce the EGR amount (shown by an arrow C in FIG. 4). (4) On the high load side, the output decreases due to the decrease in combustion speed and the decrease in volume efficiency due to EGR. In order to prevent these, the exhaust valve closing timing is advanced as the load increases (indicated by arrow D in FIG. 4).

As described above, by controlling the opening / closing timing of the exhaust valve 36, it becomes possible to obtain the optimum EGR amount according to the load.

The change of the overlap period of the exhaust valve and the intake valve as described above may be combined with the control of the intake valve opening / closing timing, or the opening timing may be fixed and only the closing timing may be controlled. . (Second Embodiment) In the first embodiment, the closing timing of the exhaust valve 36 is controlled when the overlap period of the exhaust valve and the intake valve is changed. In the embodiment, the opening timing of the intake valve is mainly controlled in the exhaust stroke (before TDC).

FIG. 5 shows the relationship between the rotation angle of the crankshaft and the valve lift amount in the second embodiment. In the second embodiment, the closing timing of the intake valve is fixed, and only the opening timing is controlled by, for example, an actuator for hydraulic control. Then, as shown in FIG. 6, the intake valve opening timing is set to the earliest timing with a medium rotation speed and a high load (that is, the overlap period is maximized), and control is performed as follows in a portion other than this region. . (1) On the low speed side, the recirculation time of burnt gas increases and EG
In order to prevent the R amount from becoming excessive, the intake valve opening timing is delayed as the rotational speed decreases (indicated by arrow A in FIG. 6). (2) On the high engine speed side, the intake valve opening timing is delayed as the engine speed increases (indicated by arrow B in FIG. 6) in order to prevent the ignition performance from decreasing due to EGR. (3) On the low load side, in order to prevent the intake negative pressure from becoming large and the EGR amount from becoming excessive, the intake valve opening timing is delayed as the load decreases (shown by arrow C in FIG. 6). (4) On the high load side, the intake valve opening timing is delayed as the load increases in order to prevent a decrease in combustion speed due to EGR and a decrease in output due to a decrease in volumetric efficiency. However, at full load on the high rotation side, conversely, the overlap is lengthened to improve the scavenging property to improve the volumetric efficiency (indicated by arrow D in FIG. 6).

In this way, by controlling the opening timing of the intake valve 34, it becomes possible to obtain the optimum EGR amount according to the load.

The expansion of the overlap amount (improvement of the scavenging property) on the high load / high rotation side as described above may be applied to the above-described first embodiment. (Third Embodiment) In the above-mentioned first and second embodiments, the burnt gas is circulated from the exhaust port to the cylinder and the intake port. The burnt gas is recirculated only inside. Therefore, the opening timing of the intake valve mainly depends on the intake stroke (TDC
It will be controlled later). That is, in the early stage of the intake stroke, the intake valve is not opened, the exhaust valve is opened, and the negative pressure generated in the piston is used to recirculate the burnt gas from the exhaust port into the cylinder.

FIG. 7 shows the relationship between the rotation angle of the crankshaft and the valve lift amount in this third embodiment. In the third embodiment, similarly to the second embodiment, the opening timing and the valve lift amount are variable by the hydraulic actuator while the intake valve closing timing is fixed. In addition, the exhaust valve also adopts a method of switching the phase in two steps.

When the exhaust gas is recirculated from the exhaust port to the cylinder, the phase of the exhaust valve is delayed so that the exhaust valve remains open for a relatively long time even in the early stage of intake. In this state, the opening timing of the intake valve is controlled as shown in FIG. That is, the intake valve opening timing is set to the latest timing in the medium rotation speed and medium / high load regions, and the control is performed as follows in portions other than this region. (1) In order to prevent the EGR amount from becoming excessive by increasing the recirculation time on the low rotation speed side, the intake valve opening timing is advanced as the rotation speed decreases (indicated by arrow A in FIG. 8). (2) On the high engine speed side, the intake valve opening timing is advanced as the engine speed increases (indicated by arrow B in FIG. 8) in order to prevent the ignition performance from decreasing due to EGR. (3) In order to prevent the intake negative pressure from increasing and the EGR amount to become excessive on the low load side, the intake valve opening timing is advanced as the load decreases (indicated by arrow C in FIG. 8). (4) On the high load side, the intake valve opening timing is advanced as the load increases in order to prevent a decrease in combustion speed due to EGR and a decrease in output due to a decrease in volumetric efficiency. At that time, the exhaust phase is set to the lead side at a low rotation speed, and is set to the delay side at a high rotation speed range to obtain high volume efficiency.

The phase on the exhaust side may be fixed to the above intermediate value, or may be continuously variable so that the optimum overlap period is always obtained.

In this way, the optimum EG according to the load
The amount of R can be obtained. (Fourth Embodiment) FIG. 9 shows the relationship between the rotation angle of the crankshaft and the valve lift amount in the fourth embodiment. In the fourth embodiment, the intake valve 34 that opens and closes the first intake port 18 in FIG. 2 is lifted by a small amount before opening the exhaust valve as shown in FIG. At that time, the cylinder internal pressure before blowdown has a value according to the load, so if this is used to recirculate the burnt gas in the intake port, the valve phase etc. can be changed even in the middle and high load regions. Therefore, the EGR amount can be secured.

On the high load side, a decrease in combustion speed due to EGR and a decrease in output due to a decrease in volumetric efficiency occur. Therefore, when the load is high, the above-mentioned minute lift may be stopped by selecting a cam or the like. (Fifth Embodiment) In the fifth embodiment, an exhaust pressure control valve for controlling the exhaust pressure is provided in the exhaust path (for example, immediately upstream of the main silencer) to eliminate exhaust in a high load region where the internal EGR amount is insufficient. The pressure control valve is throttled to increase the exhaust pressure and increase the difference with the intake port internal pressure to prevent a decrease in the recirculation amount of burnt gas.

Specifically, as shown in FIG. 10, when the absolute pressure in the intake pipe is used as the back pressure, the exhaust pressure control valve is opened on the low load side where the exhaust pressure exceeds the critical pressure, and the back pressure There is no control. When the load (absolute pressure in the intake pipe) increases and the exhaust pressure becomes lower than the critical pressure at that time, the control valve is throttled to increase the exhaust pressure until the critical pressure is reached. Further, when the EGR becomes unnecessary near the full load, the exhaust pressure control valve is gradually opened according to the increase of the load to reduce the exhaust pressure. (Sixth Embodiment) In a lean burn engine, when lean burn (for example, A / F = 22) is continuously performed, NOx increases as the load increases. When is increased, the lean burn state is switched to the normal stoichiometric air-fuel ratio. In the sixth embodiment, the control of the EGR amount is performed in synchronization with the timing of switching the air-fuel ratio from the lean burn state to the stoichiometric air-fuel ratio. That is, as shown in FIG. 11, the timing for switching the stoichiometric air-fuel ratio operation from the lean air-fuel ratio (for example, A / F = 22) is set to a load that provides the maximum EGR amount, and on the higher load side, the EGR amount is changed. Gradually decrease the EGR amount to "0" at the time of switching to the output air-fuel ratio side.
To By doing so, the NOx reduction effect of EGR can be maximized in a lean region where NOx purification by exhaust aftertreatment (catalyst) is difficult. Also, in the stoichiometric air-fuel ratio range, a high NOx purification action of the three-way catalyst is obtained, and EGR is not required. Aggravate. Therefore, after shifting to the stoichiometric air-fuel ratio, EGR gradually
Reduce the amount.

As described above, according to the above embodiment, the optimum EGR amount can be obtained according to the load, and the NOx generation can be highly suppressed even in the lean burn engine.

The present invention can be applied to modifications and variations of the above embodiments without departing from the spirit of the invention.

[0037]

As described above, according to the combustion control method for an engine of the present invention, the amount of recirculated gas introduced into the cylinder is increased in the medium and high load range where the EGR amount is insufficient.
In a low load region where the EGR amount becomes excessive, it is possible to provide an engine combustion control method that can obtain an optimum EGR amount according to the load by reducing the amount of recirculated gas introduced into the cylinder. Becomes

[Brief description of drawings]

FIG. 1 is a plan view mainly showing a configuration of a cylinder portion of an engine to which a combustion control method according to the present invention is applied.

FIG. 2 is a side sectional view of FIG. 1 viewed from the side.

FIG. 3 is a diagram showing a relationship between a valve lift amount of an intake valve and an exhaust valve with respect to a rotation angle of a crankshaft in the first embodiment.

FIG. 4 is a diagram showing a relationship between a load and a rotation speed and an opening / closing timing of an exhaust valve.

FIG. 5 is a diagram showing a relationship between a valve lift amount of an intake valve and an exhaust valve with respect to a rotation angle of a crankshaft in a second embodiment.

FIG. 6 is a diagram showing a relationship between a load and a rotation speed and an intake valve opening timing.

FIG. 7 is a diagram showing a relationship of valve lift amounts of an intake valve and an exhaust valve with respect to a rotation angle of a crankshaft in a third embodiment.

FIG. 8 is a diagram showing a relationship between a load and a rotation speed and an intake valve opening timing.

FIG. 9 is a diagram showing a relationship between a valve lift amount of an intake valve and an exhaust valve with respect to a rotation angle of a crankshaft in a fourth embodiment.

FIG. 10 is a diagram showing an adjustment state of exhaust pressure with respect to a load.

FIG. 11 is a diagram showing the relationship between the air-fuel ratio switching timing and the EGR amount.

[Explanation of symbols]

 12 cylinder 14 cylinder head 16 combustion chamber 17 spark plug 18 first intake port 20 second intake port 22 first exhaust port 24 second exhaust port 26 first intake pipe 28 second intake pipe 30 fuel injection valve 32 swirl control valve 34 Intake valve 36 Exhaust valve

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display area F02D 45/00 301 G

Claims (8)

[Claims] 1. A combustion control method for an engine, comprising a plurality of ignition plugs, wherein the operation is controlled at a lean air-fuel ratio in at least a predetermined operating range, wherein the exhaust valve closing timing and the intake air are controlled in a medium to high load range. In addition to overlapping the valve opening timing by the maximum amount, the exhaust valve is closed so that the overlap amount between the exhaust valve closing timing and the intake valve opening timing decreases as the load increases and / or the rotational speed increases and decreases. Combustion control method for an engine, characterized by variably controlling the closing timing of the engine. 2. A combustion control method for an engine, comprising a plurality of ignition plugs, wherein the operation is controlled at a lean air-fuel ratio in at least a predetermined operating range, wherein the exhaust valve closing timing and the intake air are controlled in a medium to high load range. In addition to overlapping the valve opening timing by the maximum amount, the intake valve is closed so that the overlap amount between the exhaust valve closing timing and the intake valve opening timing decreases as the load increases and / or the rotational speed increases and decreases. A combustion control method for an engine, wherein the opening timing of the engine is variably controlled before the top dead center. 3. A combustion control method for an engine, comprising a plurality of spark plugs, wherein the operation is controlled at a lean air-fuel ratio in at least a predetermined operating range, wherein the intake valve opening timing is substantially constant in the middle and high load ranges and the middle speed range. A combustion control method for an engine, which is set at a latest time after top dead center, and advances the opening timing of the intake valve in accordance with an increase / decrease in load and / or an increase / decrease in rotational speed. 4. The combustion control method for an engine according to claim 3, wherein the exhaust valve closing timing is variably controlled together with the advance control of the opening timing of the intake valve. 5. A fuel injection valve is provided in the first intake port, and a swirl control valve is provided in the second intake port. A combustion control method for an engine having a plurality of ignition plugs for performing a lean air-fuel ratio operation in a predetermined operation region, the combustion control method for an engine, characterized by opening a small amount of an intake valve near the opening timing of an exhaust valve. Method. 6. The combustion control method for an engine according to claim 5, wherein the opening operation of a small amount of the intake valve is stopped in a high load range equal to or higher than a predetermined value. 7. A plurality of spark plugs and exhaust gas recirculation means are provided, which operate at a predetermined lean air-fuel ratio in a low load range and operate at a predetermined air-fuel ratio richer than the lean air-fuel ratio in a high load range above a predetermined level. A combustion control method for an engine, comprising: increasing the recirculation exhaust gas amount with an increase in the load in a low load region, and gradually reducing the recirculation exhaust gas amount in the rich air-fuel ratio operation region. Control method. 8. The combustion control of the engine according to claim 7, wherein in a low load range including an idling state and an idling state, the air-fuel ratio is controlled to be richer than the lean air-fuel ratio and the exhaust gas recirculation is stopped. Method.