JP2855966B2 - Air-fuel ratio control device for LPG internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a liquefied petroleum gas (LP)
The present invention relates to an LPG internal combustion engine using G) as fuel, and more particularly, to an air-fuel ratio control device for controlling the air-fuel ratio.

[0002]

2. Description of the Related Art Conventionally, as this kind of technology, for example, an "air-fuel ratio control device for an LPG internal combustion engine" disclosed in Japanese Patent Application Laid-Open No. 1-200053 is already known. In the technique of this publication, main fuel is supplied to a venturi formed in an intake passage of an LPG internal combustion engine via a normal fuel passage (main fuel passage). Then, the main fuel and the intake air are mixed in the venturi, and the air-fuel mixture is supplied to each cylinder of the internal combustion engine body. In the middle of the main fuel passage, a control valve whose opening is variable using a stepping motor as a drive source is provided. By controlling the opening and closing of the control valve, the supply amount of the main fuel is controlled. Further, an injector is provided in an intake passage downstream of the venturi. Then, a part of the idle slow fuel is injected and supplied as auxiliary fuel from the injector to the intake passage. The fuel injection amount of this injector is controlled by controlling the injection time.

In the technique disclosed in this publication, when the LPG internal combustion engine is in a medium-to-light load state, first, the control valve of the main fuel passage is opened and closed so that the air-fuel ratio becomes lean, and the main fuel is supplied. Further, in order to make the air-fuel ratio lean by the main fuel close to the stoichiometric air-fuel ratio, the injection amount of the auxiliary fuel from the injector is corrected, and the air-fuel ratio feedback control is performed. At this time, the oxygen concentration in the exhaust gas detected by the oxygen sensor in the exhaust passage has been used to bring the air-fuel ratio closer to the stoichiometric air-fuel ratio. Then, an air-fuel ratio correction coefficient (FAF) is calculated from the oxygen concentration, and the air-fuel ratio feedback control is performed based on the air-fuel ratio correction coefficient.

On the other hand, when the LPG internal combustion engine is in a high load state, the control valve of the main fuel passage is opened and closed by a step motor so that the air-fuel ratio approaches the stoichiometric air-fuel ratio, and the supply amount of the main fuel is corrected. Fuel ratio feedback control has been executed. At this time, in order to bring the air-fuel ratio closer to the stoichiometric air-fuel ratio, the air-fuel ratio feedback control has been executed based on the air-fuel ratio correction coefficient calculated from the oxygen concentration as described above.

By the way, in the technology of this publication, the mode of controlling the supply of the auxiliary fuel by the injector in a medium-to-light load state is taken, but the following problems are considered. That is, even when the load of the LPG internal combustion engine is cold, particularly at the time of first idle-up, the fuel injected from the injector may cause the air-fuel ratio to become over-rich, leading to rough idle even in the medium-to-light load state. In view of this, in a normal state, even when the load is low even in a medium-to-light load state, the control mode of the main fuel supply by the step motor is used instead of the control mode of the auxiliary fuel supply by the injector. Then, when a transition is made from cold to warm, the control mode of the main fuel supply by the step motor is switched to the control mode of the auxiliary fuel supply by the injector.

[0006]

However, in the prior art described above, when the control mode is switched along with the transition from cold to warm, the same value is obtained before and after the switching. The air-fuel ratio correction coefficient has been continuously used for the air-fuel ratio feedback control in each mode.
Therefore, in this case, since the correction amount per unit of the air-fuel ratio correction coefficient differs between the two control modes, the air-fuel ratio obtained by the feedback control immediately after the control mode is switched due to the deviation of the correction amount. There was a risk that the fuel ratio would be significantly out of order. Further, as a result, there is a fear that rough idle or engine stall may be caused.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a control mode in which air-fuel ratio feedback control is performed mainly by main fuel supply with the transition from cold to warm. When switching from the control mode to the control mode mainly using the auxiliary fuel, it is possible to prevent the occurrence of a large deviation in the air-fuel ratio and to prevent the occurrence of rough idle and engine stall in the LPG internal combustion engine. It is to provide a fuel ratio control device.

[0008]

In order to achieve the above object, according to the present invention, as shown in FIG. 1, a venturi M3 formed in an intake passage M2 of an internal combustion engine M1 using liquefied petroleum gas as a fuel. And a main fuel mixing means M4 for mixing the main fuel and intake air through an internal combustion engine M1 to supply the air-fuel mixture to the internal combustion engine M1, and an intake passage M2 provided at a position different from the main fuel mixing means M4. An auxiliary fuel injection means M5 for injecting auxiliary fuel into M2; and an exhaust passage M of the internal combustion engine M1.
6, an air-fuel ratio detecting means M7 for detecting the air-fuel ratio of the internal combustion engine M1 from the oxygen concentration in the exhaust gas, an air-fuel ratio supplied by the main fuel mixing means M4 and an auxiliary fuel injection amount from the auxiliary fuel injection means M5. A correction coefficient calculating means M8 for calculating an air-fuel ratio correction coefficient based on the air-fuel ratio detected by the air-fuel ratio detecting means M7, and an internal combustion engine M1 for each correction.
Temperature state detecting means M9 for detecting the temperature state of the above, and when the detection result of the temperature state detecting means M9 is cold,
A cold fuel control unit M10 for correcting the air-fuel ratio supplied by the main fuel mixing unit M4 based on the air-fuel ratio correction coefficient calculated by the correction coefficient calculation unit M8 to perform air-fuel ratio feedback control; When the detection result of M9 is warm, the auxiliary fuel injection amount by the auxiliary fuel injection means M5 is corrected based on the air-fuel ratio correction coefficient calculated by the correction coefficient calculation means M8, and the air-fuel ratio feedback control is performed. In the air-fuel ratio control device for an LPG internal combustion engine provided with the warm-time fuel control means M11, when the detection result of the temperature state detecting means M9 is a transition period from cold to warm, the fuel during cold In order to switch the mode from the air-fuel ratio feedback control by the control means M10 to the air-fuel ratio feedback control by the hot fuel control means M11, the cold fuel control means M10 When the fuel control unit M11 is operated, the correction ratio for the air-fuel ratio feedback control performed based on the air-fuel ratio correction coefficient is gradually reduced by the cold fuel control unit M10. There is provided a mode switching control unit M12 for performing control so as to increase gradually.

[0009]

According to the above construction, as shown in FIG. 1, the main fuel mixing means M4 allows the venturi M3 of the intake passage M2 to be opened.
The main fuel of liquefied petroleum gas (LPG) and the intake air are mixed through the liquefied petroleum gas (LPG), and the mixture is supplied to the internal combustion engine M1.
The auxiliary fuel of the LPG is injected into the intake passage M2 at a position different from the main fuel mixing means M4 by the auxiliary fuel injection means M5. Then, in the internal combustion engine M1, the mixture or the auxiliary fuel is taken in, exploded and burned, and after the driving force is obtained, the exhaust gas is discharged through the exhaust passage M6.

During operation of the internal combustion engine M1, the air-fuel ratio detection means M7 detects the air-fuel ratio of the internal combustion engine M1 from the oxygen concentration in the exhaust gas. The correction coefficient calculation means M8 corrects the air-fuel ratio supplied by the main fuel mixing means M4 and the auxiliary fuel injection quantity from the auxiliary fuel injection means M5, respectively, in order to correct the air-fuel ratio detected by the air-fuel ratio detection means M7. The air-fuel ratio correction coefficient is calculated based on Further, the temperature state detecting means M9 detects the temperature state of the internal combustion engine M1.

Here, in the cold fuel control means M10,
When the detection result of the temperature state detecting means M9 is in the cold state, the air-fuel ratio is corrected based on the air-fuel ratio correction coefficient calculated by the correction coefficient calculating means M8 and the air-fuel ratio is corrected by the main fuel mixing means M4. Feedback control is performed. On the other hand, in the warm fuel control means M11, when the detection result of the temperature state detecting means M9 is warm, the auxiliary fuel injection amount from the auxiliary fuel injection means M5 is similarly corrected based on the air-fuel ratio correction coefficient. Thus, the air-fuel ratio feedback control is performed.

In the mode switching control means M12, when the detection result of the temperature state detection means M9 is a transition period from a cold state to a warm state, the mode switching control means M12 changes the air-fuel ratio feedback control by the cold fuel control means M10. Warm time fuel control means M1
In order to perform the mode switching to the air-fuel ratio feedback control according to No. 1, both the cold fuel control unit M10 and the warm fuel control unit M11 are operated. At the same time, the correction ratio for the air-fuel ratio feedback control performed based on the air-fuel ratio correction coefficient is gradually reduced by the cold fuel control unit M10 and gradually increased by the warm fuel control unit M11.

Therefore, at the transition from cold to warm, the correction rate by the cold fuel control means M10 is gradually reduced, and conversely, the correction rate by the hot fuel control means M11 is gradually reduced. Therefore, the mode is gradually shifted from the air-fuel ratio feedback control mainly using the main fuel by the main fuel mixing means M4 to the air-fuel ratio feedback control mainly using the auxiliary fuel by the auxiliary fuel injection means M5. Therefore, even if the correction amount per unit related to the air-fuel ratio correction coefficient differs between the two control modes, the air-fuel ratio does not suddenly change immediately after the control mode switching.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of an air-fuel ratio control apparatus for an LPG internal combustion engine according to the present invention will be described in detail with reference to FIGS.

FIG. 2 is a schematic configuration diagram showing an LPG engine system to which an air-fuel ratio control device for an LPG internal combustion engine in this embodiment is applied. An LPG internal combustion engine (LPG engine) 1 using liquefied petroleum gas (LPG) as a fuel includes a plurality of cylinders (only one cylinder is shown in FIG. 2). An intake passage 2 communicating with each cylinder is connected to the LPG engine 1, and an air cleaner 3 is provided at an inlet side of the intake passage 2. A venturi 4 is provided in the middle of the intake passage 2. On the downstream side of the venturi 4 in the intake passage 2, there is provided a throttle valve 5 that opens and closes in response to operation of an accelerator pedal (not shown). Then, by opening and closing the throttle valve 5, the amount of outside air sucked from the air cleaner 3 into the intake passage 2, that is, the amount of intake air is adjusted. Furthermore,
Each cylinder of the LPG engine 1 is provided with a spark plug 6. On the other hand, the LPG engine 1 is provided with an exhaust passage 7 communicating with each cylinder.
Is provided with a three-way catalyst 8. A well-known exhaust gas recirculation device 9 is provided between the exhaust passage 7 and the intake passage 2.

An ignition signal distributed by a distributor 10 is applied to each ignition plug 6. The distributor 10 distributes the high voltage output from the igniter 11 to each spark plug 6 in synchronization with the crank angle of the LPG engine 1. The ignition timing of each ignition plug 6 is determined by the high voltage output timing from the igniter 11. The distributor 10 is provided with a rotation speed sensor 31 for detecting the rotation speed (engine speed) NE of the LPG engine 1 from the rotation of the rotor.

The venturi 4 of the intake passage 2 has an LP
One end of a main fuel passage 12 for supplying G as a main fuel is in communication. The other end of the main fuel passage 12 is provided with an LPG regulator 1 for adjusting LPG vaporization.
3 is connected.

A fuel throttle valve 15 driven by a step motor 14 is provided in the main fuel passage 12.
In this embodiment, the main fuel passage 12 and the fuel throttle valve 15 constitute a main fuel mixing means. Then, by controlling the driving of the fuel throttle valve 15, the opening of the main fuel passage 12 is adjusted.
The amount of main fuel supplied to the engine 1 is adjusted.

On the upstream side of the fuel throttle valve 15, the base end of the main fuel passage 12 is connected to a secondary pressure reducing chamber 13 b of the LPG regulator 13. Also, the main fuel passage 12
In the vicinity of the base end side, a slow fuel passage 16 is provided which is connected to the primary pressure reducing chamber 13a of the LPG regulator 13 and supplies slow fuel for idling. In the middle of the slow fuel passage 16, near the junction with the main fuel passage 12, an idle adjust screw 13 c for slow fuel adjustment is provided.

When the LPG engine 1 is operating,
Outside air is sucked from the air cleaner 3 through the intake passage 2. Also, at the time of inhalation of the outside air, Venturi 4 has LPG
Main fuel from the regulator 13 is led out through the main fuel passage 12. Then, the main fuel and the intake air are mixed, and the main fuel mixture is taken into the LPG engine 1. The amount of the main fuel mixture taken into the LPG engine 1 is determined by the opening of the throttle valve 5.

On the other hand, an injector 17 as auxiliary fuel injection means is provided in the intake passage 2 on the downstream side of the throttle valve 5. The injector 17 is connected to an auxiliary fuel passage 18 communicating with the slow fuel passage 16. The driving of the injector 17 is controlled, so that the primary pressure reducing chamber 13 of the LPG regulator 13 is controlled.
A portion of the idle slow fuel sent from a through the auxiliary fuel passage 18 is injected into the intake passage 2 as auxiliary fuel. The injected auxiliary fuel and intake air are taken into the LPG engine 1. Further, the LPG regulator 13 has a slow lock valve 20 driven by a solenoid 19 to open and close the slow fuel passage 16.
Is provided. The drive control of the slow lock valve 20 performs a fuel cut or the like at the time of deceleration.

Therefore, in the LPG engine 1, the driving of each ignition plug 6 causes the taken-in main fuel mixture or auxiliary fuel to explode and burn, and after the driving force is obtained, the exhaust gas is exhausted. The gas is discharged to the exhaust passage 7. The exhaust gas discharged to the exhaust passage 7 is purified while passing through the three-way catalyst 8 and then discharged to the outside. At the same time, part of the exhaust gas passing through the exhaust passage 7 is recirculated to the intake system by the exhaust gas recirculation device 9.

In order to detect the operating state of the LPG engine 1, the intake passage 2 is provided with an intake air temperature sensor 32 for detecting the temperature (intake air temperature) THA of the outside air sucked from the air cleaner 3. A throttle sensor 33 for detecting the opening (throttle opening) VL is provided near the throttle valve 5. Similarly, the intake passage 2 is provided with an intake pressure sensor 34 for detecting the pressure (intake pressure) PM of the intake air. Further, the LPG engine 1 is provided with a water temperature sensor 35 as temperature state detecting means for detecting the temperature (cooling water temperature) THW of the cooling water representing the temperature state. On the other hand, the exhaust passage 7
Is provided with an oxygen sensor 36 as an air-fuel ratio detecting means for detecting the air-fuel ratio of the LPG engine 1 from the oxygen concentration in the exhaust gas discharged from the passage 7. Similarly, the exhaust passage 7 is provided with an exhaust gas temperature sensor 37 for detecting the temperature of the exhaust gas (exhaust gas temperature).

The step motor 14 of the fuel throttle valve 15, the solenoid 19 of the slow lock valve 20,
The injector 17 and the igniter 11 are electrically connected to an electronic control unit (hereinafter simply referred to as “ECU”) 38. The ECU 38 includes a rotation speed sensor 31,
An intake temperature sensor 32, a throttle sensor 33, an intake pressure sensor 34, a water temperature sensor 35, an oxygen sensor 36, an exhaust temperature sensor 37, and the like are connected to each other. In this embodiment, the ECU 38 constitutes a correction coefficient calculation unit, a cold fuel control unit, a warm fuel control unit, and a mode switching control unit. The ECU 38 includes sensors 31 to 3
Step motor 1 based on the detection signal output from
4. The solenoid 19, the injector 17, the igniter 11, and the like are suitably controlled.

Next, the electrical configuration of the ECU 38 will be described with reference to the block diagram shown in FIG. The ECU 38 includes a central processing unit (CPU) 41, a read-only memory (ROM) 42 in which a predetermined control program and the like are stored in advance,
A random access memory (RAM) 43 for temporarily storing the operation results and the like of the PU 41, a backup RAM 44 for storing previously stored data, and the like, and these units, an external input circuit 45, an external output circuit 46, and the like are connected by a bus 47. It is configured as a logical operation circuit.

The external input circuit 45 includes a rotational speed sensor 31, an intake air temperature sensor 32, a throttle sensor 33,
An intake pressure sensor 34, a water temperature sensor 35, an oxygen sensor 36, an exhaust temperature sensor 37, and the like are connected to each other. or,
The external output circuit 46 includes the igniter 11, the step motor 14, the injector 17, and the solenoid 19 described above.
Etc. are connected respectively. Then, the CPU 41 reads detection signals input from the sensors 31 to 37 via the external input circuit 45 as input values. Also, CPU4
Reference numeral 1 denotes an igniter 11, a step motor 14, and an injector 1 based on these input values via an external output circuit 46.
7 and the solenoid 19 are suitably controlled.

Next, the processing operation for controlling the air-fuel ratio of the LPG engine 1 executed by the ECU 38 will be described with reference to FIGS. FIG.
"Main Routine" related to the drive control of the fuel throttle valve 15 and the injector 17 among the processes executed by
5 is a flowchart showing the processing of FIG.

When the process proceeds to this routine, first, at step 100, the process of "ST calculation routine" for calculating the target number ST of the step motor 14 in the fuel throttle valve 15 is executed.

Next, in step 200, a process of "FAF calculation routine" for calculating an air-fuel ratio correction coefficient FAF for air-fuel ratio feedback control is executed.
Subsequently, in step 300, a process of a “learning routine” for further improving the control accuracy to the stoichiometric air-fuel ratio in the air-fuel ratio feedback control is executed.

Further, in step 400, the processing of a "TAU calculation routine" for calculating the injection time TAU of the injector 17 is executed, and thereafter the processing of the "main routine" is temporarily terminated.

Next, each processing of steps 100, 200, 300, and 400 in the above-mentioned "main routine" will be described in detail. First, step 10
The processing of the “ST calculation routine” at step 0
The subroutine is called and executed at 00, and is shown in the flowchart of FIG.

When this processing is started, first, in step 1
At 10, the basic step number S of the step motor 14 in the fuel throttle valve 15 is calculated. The basic step number S is obtained by referring to a three-dimensional map (not shown) stored in the ROM 42 in advance based on the engine speed NE and the intake pressure PM obtained by the detection of the rotation speed sensor 31 and the intake pressure sensor 34. The basic step number S of the three-dimensional map is set in advance so that the air-fuel ratio becomes leaner by the main fuel supplied through the main fuel passage 12.

Next, at step 120, a learning correction value KG for correcting the basic step number S is loaded. This learning correction value KG is obtained in the processing of a “learning routine” described later.

In step 130, an intake air temperature correction coefficient FTHA is calculated. This calculation is based on the intake air temperature sensor 3
2 based on the intake air temperature THA obtained by the detection of
This is performed by referring to a two-dimensional map stored in advance in 42.

Further, at step 140, a water temperature correction coefficient FTHW is calculated. This calculation is based on the water temperature sensor 35
Based on the cooling water temperature THW obtained by detecting the
This is performed with reference to a two-dimensional map as shown in FIG. In the map of FIG. 6, in a cold temperature range where the cooling water temperature THW is lower than a certain reference temperature α, the water temperature correction coefficient FTHW is uniformly set to a certain reference value A larger than “1.0”. In the same map, the coolant temperature correction coefficient FTHW is uniformly set to “1.0” in a temperature range in a warm state where the coolant temperature THW is higher than a certain reference temperature β. Further, in the same map, between the reference temperature α and the reference temperature β, the cooling water temperature TH
The water temperature correction coefficient FTHW is set to linearly decrease from the reference value A to “1.0” in accordance with the change of W.

Next, at step 150, a feedback correction coefficient FAFST for performing feedback control of the step motor 14 of the fuel throttle valve 15 is loaded. This feedback correction coefficient FAFST is obtained in the processing of a “FAFST calculation routine” described later.

In step 160, the learning correction value KG and the intake air temperature correction coefficient FTH are added to the basic step number S.
A, the basic step number S is corrected by multiplying each of the water temperature correction coefficient FTHW and the feedback correction coefficient FAFST. Further, the calculation result is set as the target step number ST for the step motor 14, and the subsequent processing is temporarily ended.

Here, the "FAFST calculation routine" for the feedback correction coefficient FAFST will be described with reference to the flowchart shown in FIG. The processing of the “FAFST calculation routine” is executed by a periodic interruption every predetermined time.

When the process proceeds to this routine, first, in step 151, the air-fuel ratio correction coefficient FAF is loaded. The air-fuel ratio correction coefficient FAF is obtained by a process of a “FAF calculation routine” described later.

Next, at step 152, the cold correction coefficient FCLD is calculated. This calculation is based on the water temperature sensor 3
5 based on the cooling water temperature THW obtained by the detection of
This is performed with reference to a two-dimensional map as shown in FIG. In the map of FIG. 8, in the cold temperature range where the cooling water temperature THW is lower than a certain reference temperature α, the cold correction coefficient FCLD is uniformly set to “1.0”. In the same map, the cooling water temperature THW
In a temperature range in a warm state higher than a certain reference temperature β, the cold correction coefficient FCLD is uniformly set to “0.0”. Further, in the same map, between the reference temperature α and the reference temperature β, the cold-time correction coefficient FCLD linearly decreases from “1.0” to “0.0” in accordance with the change of the cooling water temperature THW. Is set to

In step 153, the result obtained by subtracting "1.0" from the air-fuel ratio correction coefficient FAF is multiplied by a cold correction coefficient FCLD.
"0" is added. Further, the calculation result is set as the feedback correction coefficient FAFST, and the subsequent processing is temporarily terminated.

Next, the "FAF calculation routine" executed in step 200 in the "main routine" described above.
Will be described. This “FAF calculation routine”
Is executed by calling a subroutine in step 200, and is shown in the flowchart of FIG.

First, in step 201, it is determined whether a feedback (F / B) control condition is satisfied. In this embodiment, based on the detection signals of the water temperature sensor 35 and the rotation speed sensor 31, it is determined whether or not the condition that the cooling water temperature THW is sufficiently high and the engine rotation speed NE is not higher than necessary is satisfied. .

Here, if the condition of the feedback control is satisfied, in step 202, the oxygen sensor 3
It is determined whether the air-fuel ratio (A / F) is rich based on the detection signal of No. 6. Then, when the air-fuel ratio is rich, the processing of the following steps 203 to 208 is executed.

That is, in step 203, when the processing of this "FAF calculation routine" is executed in the previous control cycle, it is determined whether or not the air-fuel ratio is lean by the air-fuel ratio flag YOX. Here, the air-fuel ratio flag Y
If OX is “0”, the process proceeds to step 204 assuming that the air-fuel ratio has changed from lean to rich based on the determinations in steps 202 and 203.

In step 204, the average air-fuel ratio correction coefficient FA during the air-fuel ratio feedback control is determined.
Calculate FAV. This average air-fuel ratio correction coefficient FAFAV
Is calculated based on the current air-fuel ratio correction coefficient FAF and the old air-fuel ratio correction coefficient FAF obtained when the previous time was changed from rich to lean.
This is performed by calculating an arithmetic mean with zero.

Next, at step 205, the air-fuel ratio correction coefficient FAF is set as the previous old air-fuel ratio correction coefficient FAF0. Subsequently, in step 206, a result obtained by subtracting the predetermined skip amount a from the air-fuel ratio correction coefficient FAF is set as a new air-fuel ratio correction coefficient FAF.

In step 207, since the current air-fuel ratio is rich, the learning timing flag YK
Set G to "1". This learning timing flag Y
KG is used to determine whether or not it is time to learn the learning correction value KG, which will be described later.

Further, in step 208, since the air-fuel ratio is rich, the air-fuel ratio flag YOX is set to "1", and the subsequent processing is temporarily terminated. On the other hand, if it is determined in step 203 that the air-fuel ratio flag YOX is not “0”, the processing in steps 209 to 211 is executed. Here, when the process proceeds to step 209 according to the determination in steps 202 and 203, it indicates that the air-fuel ratio is maintaining a rich state.

Then, in step 209, it is determined whether or not the timer counter CNT1 exceeds a constant c. The timer counter CNT1 is a value that has a shorter cycle than the processing of the “FAF calculation routine” and is added in the processing of a “compare interrupt routine” described later. Here, when the timer counter CNT1 exceeds the constant c, in step 210, the result obtained by subtracting the constant b from the air-fuel ratio correction coefficient FAF is used as a new air-fuel ratio correction coefficient FAF.
Set as

Then, in step 211, the timer counter CNT1 is cleared to "0", and the subsequent processing is temporarily terminated. When the timer counter CNT1 is equal to or smaller than the constant c in step 209, the subsequent processing is temporarily terminated.

That is, steps 209 to 211
In this process, the value of the air-fuel ratio correction coefficient FAF is subtracted by a constant b every predetermined time. Note that the processing in steps 203 to 211 described above is processing when the air-fuel ratio is rich, and is processing for reducing the air-fuel ratio correction coefficient FAF. In contrast to this processing, the processing in steps 212 to 220 described below is processing when the air-fuel ratio is lean, and is processing for increasing the air-fuel ratio correction coefficient FAF.

That is, if the air-fuel ratio is lean in step 202, the process proceeds to step 212. In step 212, the air-fuel ratio flag YOX is set to "1".
Is determined. Air-fuel ratio flag YOX is "1"
If, the process proceeds to step 213 assuming that the air-fuel ratio has been switched from rich to lean. Then, in step 213, the average air-fuel ratio correction coefficient FAFAV during the air-fuel ratio feedback control is calculated in the same manner as in the processing in step 204 described above.

Subsequently, in step 214, the value of the air-fuel ratio correction coefficient FAF is set as the old air-fuel ratio correction coefficient FAF0. In step 215, the result of adding the skip amount a to the air-fuel ratio correction coefficient FAF is set as a new air-fuel ratio correction coefficient FAF.

Further, in step 216, the learning timing flag YKG is set to "1". And
In step 217, the air-fuel ratio flag YOX is reset to "0" assuming that the air-fuel ratio is lean, and the subsequent processing is temporarily ended.

If it is determined in step 212 that the air-fuel ratio flag YOX is not "1", the processing of steps 218 to 220 is executed. Here, step 21
When the process proceeds from step 2 to step 218, it indicates that the air-fuel ratio is maintaining a lean state.

In step 218, it is determined whether or not the above-mentioned timer counter CNT1 exceeds a constant c. Here, when the timer counter CNT1 exceeds the constant c, in step 219, the air-fuel ratio correction coefficient F
The result of adding the constant b to AF is used as a new air-fuel ratio correction coefficient F
Set as AF.

Then, in step 220, the timer counter CNT1 is cleared to "0", and the subsequent processing is temporarily terminated. If it is determined in step 218 that the timer counter CNT1 is equal to or smaller than the constant c, the subsequent processing is temporarily terminated.

That is, steps 218 to 220
Is a process opposite to the above-described steps 209 to 211, and the air-fuel ratio correction coefficient FA
The value of F is increased by a constant b.

If the condition of the feedback control is not satisfied in step 201, step 2
Move to 21. Then, in step 221,
The value of the air-fuel ratio correction coefficient FAF and the value of the old air-fuel ratio correction coefficient FAF0 are set to “1”, respectively, and the subsequent processing is temporarily terminated.

Next, the processing of the "learning routine" executed in step 300 in the "main routine" will be described. The processing of the "learning routine" is executed by calling a subroutine in step 300, and is shown in the flowchart of FIG.

When the processing of this routine is started, first, at step 310, the learning timing flag YKG
Is determined to be “1”. If the learning timing flag YKG is not "1", the process directly proceeds to step 350, where the learning timing flag YKG is set to "0", and the subsequent processing is temporarily terminated.

Then, the following processing is executed only when the learning timing flag YKG is "1", that is, only when the air-fuel ratio is switched from rich to lean or from lean to rich. That is, in step 310,
If the learning timing flag YKG is “1”, in step 320, the magnitude of the average air-fuel ratio correction coefficient FAFAV obtained in the processing of the “FAF calculation routine” is determined.

If the average air-fuel ratio correction coefficient FAFAV is "1" in step 320, it is determined that the actual air-fuel ratio has become the stoichiometric air-fuel ratio, and the learning timing flag YKG is directly determined in step 350.
Is reset to "0", and the subsequent processing is once ended.

If the average air-fuel ratio correction coefficient FAFAV is larger than "1" in step 320, the result obtained by adding the learning correction value KG corresponding to the intake pressure PM at that time by a constant i is determined in step 330. It is set as a new learning correction value KG. And step 350
, The learning timing flag YKG is reset to “0”, and the subsequent processing is temporarily terminated.

Further, if the average air-fuel ratio correction coefficient FAFAV is smaller than "1" in step 320,
In step 340, a result obtained by subtracting a constant i from the learning correction value KG is set as a new learning correction value KG. Then, in step 350, the learning timing flag YKG is reset to "0", and the subsequent processing is temporarily ended.

As described above, each of the aforementioned steps 320,
By executing a series of processes at 330, 340, and 350 each time the air-fuel ratio switches between lean and rich,
The learning correction value KG is increased or decreased by a constant i, and eventually becomes an optimal value for the intake pressure PM at that time. Then, by using the learning correction value KG, in step 160 of the “ST calculation routine”, the target step number ST according to the current average air-fuel ratio correction coefficient FAFAV is calculated.
At the same time, in step 460 of the “TAU calculation routine” described later, the average air-fuel ratio correction coefficient FA
A target injection time TAU according to the FAV is calculated.

Next, the processing of the “TAU calculation routine” executed in step 400 in the processing of the aforementioned “main routine” will be described. The processing of this “TAU calculation routine” is a processing executed by a subroutine call in step 400, and is shown in the flowchart of FIG.

When the processing of this routine is started, first, at step 410, the basic injection time TAUBSE is calculated. The basic injection time TAUBSE is obtained by referring to a three-dimensional map stored in the ROM 42 in advance based on the values of the engine speed NE and the intake pressure PM detected by the rotation speed sensor 31 and the intake pressure sensor 34.

Next, in step 420, the air-fuel ratio correction coefficient FAF calculated in the above-mentioned "FAF calculation routine" is loaded. In step 430, the intake air temperature TH obtained by the detection of the intake air temperature sensor 32
Based on A, an intake air temperature correction coefficient FTHAI is calculated with reference to a two-dimensional map stored in the ROM 42 in advance.

Further, at step 440, based on the cooling water temperature THW obtained by the detection of the water temperature sensor 35,
As shown in FIG.
The water temperature correction coefficient FTHWI is calculated with reference to the dimensional map. In the map of FIG. 12, the coolant temperature correction coefficient FTHWI is uniformly set to “0.0” in a cold temperature range where the coolant temperature THW is lower than a certain reference temperature α. or,
In the same map, in a temperature range in a warm state in which the cooling water temperature THW is higher than a certain reference temperature β, the water temperature correction coefficient FTHW
I is uniformly set to “1.0”. Further, in the same map, between the reference temperature α and the reference temperature β, the water temperature correction coefficient FTHWI is set to increase linearly from “0.0” to “1.0” in accordance with the change of the cooling water temperature THW. Have been.

Subsequently, in step 450, the learning correction value K calculated in the above-described "learning routine" is calculated.
Load G. Then, in step 460,
The intake temperature correction coefficient FTHA is added to the basic injection time TAUBSE.
The basic injection time TAUBSE is corrected by multiplying each of I, a water temperature correction coefficient FTHWI, an air-fuel ratio correction coefficient FAF, and a learning correction value KG. Also, the calculation result is
Target injection time TA for drive control of injector 17
U is set, and the subsequent processing is temporarily ended.

Using the target step number ST and the target injection time TAU obtained as described above, how to control the drive of the step motor 14 and the injector 17 will be described with reference to the flowcharts of FIGS. Will be described.

FIG. 13 shows the "capture interrupt routine".
This is the process. When the process of this routine is started, first, at step 500, the engine speed NE is calculated based on the detection signal of the speed sensor 31.

Next, at step 510, the injector 17 is operated based on the calculated engine speed NE.
It is determined whether or not it is the injection timing. Here, if it is not the injection timing of the injector 17, the subsequent processing is once ended as it is.

On the other hand, if it is determined in step 510 that the injection timing of the injector 17 has been reached, step 5
At 20, the power supply to the injector 17 is started to open the injector 17.

Further, at step 530, the energization end time of the injector 17 is set based on the above-described target injection time TAU, and the subsequent processing is temporarily ended. FIG. 14 shows the processing of the "compare interrupt routine", which is executed at a relatively short predetermined time interval.

When the processing of this routine is started, first, in step 600, the timing of the energization end time set based on the target injection time TAU in step 530 in the processing of the aforementioned “capture interruption routine”. It is determined whether or not. Here, if it is not the timing, the process directly proceeds to step 620.

If it is determined in step 600 that the current has come to the end of energization, in step 610 the energization of the injector 17 is terminated and the
7 is closed, and the routine proceeds to step 620.

Here, as the target injection time TAU is longer, the injection amount of the auxiliary fuel injected from the injector 17 into the intake passage 2 is increased. Step 600
Alternatively, the process proceeds from step 610 and in step 620, it is determined whether or not the control timing of the step motor 14 is determined in the processing described later. Here, if the control timing is not reached, the subsequent processing is once ended as it is.

On the other hand, if it is the control timing of the step motor 14 in step 620, the process of the "step motor control routine" is executed in step 630.

The processing of this "step motor control routine" is shown in the flowchart of FIG.
In step 630, a subroutine is called and executed.

When the processing of this routine is started, first, in step 631, the current step number SNOW representing the current step number of the step motor 14 is loaded. The current step number SNOW is a value written in the backup RAM 44 when the CPU 41 outputs a rotation command value to the step motor 14 via the external output circuit 46.

In step 632, "ST
Target step number ST obtained in the process of "calculation routine"
To load. Then, in step 633, the loaded current step number SNOW and the target step number ST
Compare with. In steps 633 to 637, a process of converging the current step number SNOW indicating the step number of the step motor 14 to the target step number ST is performed.

That is, in step 633, if the target step number ST is equal to the current step number SNOW, there is no need to drive the step motor 14, and the subsequent processing is temporarily terminated.

If the target step number ST is larger than the current step number SNOW in step 633, a forward rotation command is output to the step motor 14 to increase the step number of the step motor 14 in step 634. The motor 14 is rotated forward by one step.

Subsequently, in step 635, the current step number SNOW is incremented by "1", the result is set as a new current step number SNOW, and the subsequent processing is temporarily terminated.

Further, if the target step number ST is smaller than the current step number SNOW in step 633, a reverse rotation command is output to the step motor 14 to reduce the step number of the step motor 14 in step 636. The motor 14 is rotated reversely by one step.

Subsequently, in step 637, the current step number SNOW is decremented by "1", the result is set as a new current step number SNOW, and the subsequent processing is temporarily terminated.

Here, as the target step number ST is larger, the main fuel passage 12 is opened by the fuel throttle valve 15 to be larger, and the supply amount of the main fuel from the venturi 4 to the LPG engine 1 is increased.

Then, by repeatedly executing the processing of this routine, the number of steps of the step motor 14 converges to the target number of steps ST. After the processing of the “step motor control routine” is completed as described above, the processing returns to the flowchart of FIG.
At 40, the next control timing is set. This control timing is used for the determination in step 620, and is, for example, a time obtained by adding a certain time to the current time.

Thereafter, at step 650, the value of the timer counter CNT1 is incremented by "1", the result is set as a new value of the timer counter CNT1, and the subsequent processing is temporarily terminated.

As described above, according to the air-fuel ratio control apparatus for an LPG internal combustion engine in this embodiment, the "ST" for calculating the target step number ST of the step motor 14 is determined.
In the process of the "calculation routine", the cooling water temperature correction coefficient FTHW is obtained according to the change of the cooling water temperature THW. That is, FIG.
As shown in the two-dimensional map, the water temperature correction coefficient FTHW
Is a reference value A larger than "1.0" when the cooling water temperature THW is lower than a certain reference temperature α, and the cooling water temperature TW
It becomes “1.0” when the HW is higher than the reference temperature β. Further, the cooling water temperature THW is changed from the reference temperature α to the reference temperature β.
, That is, during the transition period from cold to warm, the water temperature correction coefficient FTHW is determined to linearly decrease from the reference value A to “1.0”.

In addition, the feedback correction coefficient FAFST used in the calculation of the target number of steps ST in the processing of the “ST calculation routine” is determined during the cold state determined in accordance with the cooling water temperature THW in the processing of the “FAFST calculation routine”. It is obtained by correcting the air-fuel ratio correction coefficient FAF based on the correction coefficient FCLD. Further, as shown in the two-dimensional map of FIG. 8, the cold-time correction coefficient FCLD becomes “1.0” when the cooling water temperature THW is lower than a certain reference temperature α, and the cooling water temperature THW is lower than the reference temperature β. Is "0.
0 ". Further, when the cooling water temperature THW is between the reference temperature α and the reference temperature β, that is, in a transition period from cold to warm, the cold correction coefficient FCLD is changed from “1.0” to “0.
It is required to linearly decrease to "0".

Accordingly, the target number of steps ST is obtained based on the air-fuel ratio correction coefficient FAF or the like in a cold state.
At the time of warming, it is obtained as a relatively small value regardless of the air-fuel ratio correction coefficient FAF. Further, during the transition period from cold to warm, it is required to be gradually reduced based on the air-fuel ratio correction coefficient FAF and the like and as the cooling water temperature THW increases.

Therefore, when the engine is cold, the opening of the main fuel passage 12 by the fuel throttle valve 15 is controlled based on the air-fuel ratio correction coefficient FAF and the like, and the amount of main fuel supplied from the venturi 4 to the LPG engine 1 is controlled. . That is, the air-fuel ratio feedback control is executed based on the main fuel amount to be supplied from the venturi 4 to the LPG engine 1. Further, at the time of warming, the opening degree of the main fuel passage 12 is controlled to be relatively small irrespective of the air-fuel ratio correction coefficient FAF, and the amount of main fuel supplied from the venturi 4 to the LPG engine 1 is controlled to be relatively small. Further, during the transition period from cold to warm, the opening degree of the main fuel passage 12 is controlled based on the air-fuel ratio correction coefficient FAF and the like so as to be gradually narrowed as the cooling water temperature THW increases. That is, the correction ratio for the air-fuel ratio feedback control performed based on the air-fuel ratio correction coefficient FAF is gradually reduced so that the main fuel amount to be supplied from the venturi 4 to the LPG engine 1 is gradually reduced.

On the other hand, the target injection time T of the injector 17
In the processing of the “TAU calculation routine” for calculating the AU, the cooling water temperature correction coefficient F according to the change of the cooling water temperature THW.
THWI is required. That is, as shown in the two-dimensional map of FIG. 12, the water temperature correction coefficient FTHWI is equal to the cooling water temperature TH.
W becomes "0.0" in a cold state lower than a certain reference temperature α, and becomes "1.0" in a cold state in which the cooling water temperature THW is higher than the reference temperature β. Further, the cooling water temperature THW is equal to the reference temperature α.
During the transition from cold to warm, the water temperature correction coefficient FTHWI is determined to increase linearly from "0.0" to "1.0".

Therefore, the target injection time TAU is "0.0" when cold and the air-fuel ratio correction coefficient F when warm.
It is determined based on AF or the like. In the transition period from cold to warm, the target injection time TAU is determined by the air-fuel ratio correction coefficient F.
Based on AF etc. and with the increase of the cooling water temperature THW,
It is required to gradually increase from "0.0".

Therefore, in the cold state, the auxiliary fuel is not injected from the injector 17. In the warm state, the injection amount of the auxiliary fuel from the injector 17 is controlled based on the air-fuel ratio correction coefficient FAF and the like. That is, the air-fuel ratio feedback control is executed based on the amount of auxiliary fuel to be supplied from the injector 17 to the LPG engine 1. Further, during the transition period from cold to warm, the injector 17
Is controlled based on the air-fuel ratio correction coefficient FAF and the like so that the amount of auxiliary fuel injected from the engine is gradually increased as the coolant temperature THW increases. That is, LP from the injector 17
The correction ratio for the air-fuel ratio feedback control performed based on the air-fuel ratio correction coefficient FAF is gradually increased so that the amount of auxiliary fuel to be supplied to the G engine 1 is gradually increased.

That is, in this embodiment, the air-fuel ratio feedback control mainly using the main fuel supplied from the venturi 4 is executed instead of the control mode mainly using the auxiliary fuel injected from the injector 17 at the time of cold. Then, the injection of the auxiliary fuel by the injector 17 is cut off. Therefore, at the time of cold, especially at the time of first idle up,
It is possible to prevent the air-fuel ratio from being over-rich due to the fuel injected from the injector 17 beforehand.
It is possible to prevent the engine 1 from becoming a rough idle.

In the warm state, the amount of main fuel supplied from the venturi 4 is controlled to be relatively small, and the air-fuel ratio feedback control of the LPG engine 1 is executed mainly with the amount of auxiliary fuel injected from the injector 17. You. Therefore, the air-fuel ratio leaned by the main fuel supplied from the venturi 4 is corrected by the amount of auxiliary fuel injected from the injector 17, and the air-fuel ratio approaches the stoichiometric air-fuel ratio.

Further, in the transition period from cold to warm,
The air-fuel ratio feedback control is executed so that the amount of main fuel supplied from the venturi 4 is gradually reduced. At the same time, the air-fuel ratio feedback control is executed so that the amount of auxiliary fuel injected from the injector 17 is gradually increased.

Therefore, at the transition from the cold state to the warm state, the air-fuel ratio feedback control mainly using the main fuel supplied from the venturi 4 by using the step motor 14 is performed, and the auxiliary fuel by the injector 17 is mainly used. The mode is gradually shifted to the adjusted air-fuel ratio feedback control. Therefore, the air-fuel ratio correction coefficient FAF for the step motor 14 and the injector 17 is changed between the two control modes.
Even if the correction amount per unit is different,
Due to the deviation of the correction amount, the air-fuel ratio does not suddenly change immediately after the switching of the control mode.

FIG. 16 shows a change in the target step number ST for controlling the step motor 14 and a change in the target injection time TAU for controlling the injector 17 with respect to the behavior of the air-fuel ratio correction coefficient FAF in comparison with the prior art. It is a time chart.

As is clear from this time chart, in the present embodiment, in the cold state lower than the reference temperature α, the air-fuel ratio correction coefficient FAF is reflected in the target number of steps ST, and the temperature higher than the reference temperature β is used. It can be seen that in the interim period, the air-fuel ratio correction coefficient FAF is reflected in the target injection time TAU. In the present embodiment, in the transition period from when the temperature reaches the reference temperature α to when the temperature reaches the reference temperature β, the behavior of the target number of steps ST indicated by a solid line is gentler than that of the conventional technology indicated by a broken line. . At the same time, the behavior of the target injection time TAU shown by the solid line is gentler than that of the conventional technique shown by the broken line. Further, at the point in time when the reference temperature β is reached, it can be seen that the changes in the target step number ST and the target injection time TAU of the present embodiment are smaller than those of the prior art. In other words, when the mode is switched from the air-fuel ratio feedback control by the step motor 14 to the air-fuel ratio feedback control by the injector 17 during the transition period from cold to warm, the air-fuel ratio correction coefficient FA
It can be seen that the deviation of the correction amount based on F is kept small.

As a result, the air-fuel ratio feedback control is switched from the control mode mainly based on the supply of the main fuel to the control mode mainly based on the supply of the auxiliary fuel during the transition period from the cold state to the warm state. In addition, it is possible to prevent a large deviation in the air-fuel ratio from occurring. Also, as a result,
The occurrence of rough idle or stall of the LPG engine 1 can be prevented.

Note that the present invention is not limited to the above-described embodiment, and may be implemented by appropriately changing a part of the configuration without departing from the spirit of the invention. For example, in the above-described embodiment, the main fuel amount is gradually reduced as the cooling water temperature THW increases in the transition period from the cold state to the warm state between the reference temperature α and the reference temperature β. In addition, the air-fuel ratio feedback control is executed such that the auxiliary fuel amount is gradually increased with the increase of the cooling water temperature THW. On the other hand, cold time and warm time are distinguished by a certain reference temperature, and in the transition period from cold to warm, the time after the main fuel amount reaches the reference temperature The air-fuel ratio feedback control may be executed so that the auxiliary fuel amount is gradually decreased with the passage of time and the auxiliary fuel amount is also gradually increased with the passage of time.

[0108]

As described above in detail, according to the present invention, the auxiliary fuel supply is changed from the control mode mainly based on the main fuel supply to the air-fuel ratio feedback control with the transition from the cold state to the warm state. When switching to the main control mode, the correction ratio for the air-fuel ratio feedback control performed based on the air-fuel ratio correction coefficient is gradually reduced in the control mode mainly using the main fuel supply, and the auxiliary fuel supply is mainly used. The control mode is gradually increased. for that reason,
The transition between the two control modes is gradually performed, so that the air-fuel ratio does not suddenly change immediately after the switching of the control modes, so that it is possible to prevent a large deviation in the air-fuel ratio from occurring. And an excellent effect of preventing the occurrence of engine stalls.

[Brief description of the drawings]

FIG. 1 is a conceptual configuration diagram illustrating a basic conceptual configuration of the present invention.

FIG. 2 is an LPG according to an embodiment of the present invention;
It is a schematic structure figure showing an engine system.

FIG. 3 is a block diagram illustrating an electrical configuration of an electronic control unit according to one embodiment.

FIG. 4 is a flowchart illustrating a “main routine” executed by the electronic control unit in one embodiment.

FIG. 5 is a flowchart illustrating an “ST calculation routine” executed by an electronic control unit in one embodiment.

FIG. 6 is a map in which a relationship between a cooling water temperature and a water temperature correction coefficient is determined in advance in one embodiment.

FIG. 7 is a flowchart illustrating a “FAFST calculation routine” executed by the electronic control unit in one embodiment.

FIG. 8 is a map in which a relationship between a cold water correction coefficient and a cooling water temperature is determined in advance in one embodiment.

FIG. 9 is a flowchart illustrating a “FAF calculation routine” executed by the electronic control unit in one embodiment.

FIG. 10 is a flowchart illustrating a “learning routine” executed by the electronic control unit in one embodiment.

FIG. 11 is a flowchart illustrating a “TAU calculation routine” executed by the electronic control unit in one embodiment.

FIG. 12 is a map in which a relationship between a cooling water temperature and a water temperature correction coefficient is determined in one embodiment.

FIG. 13 is a flowchart illustrating a “capture interruption routine” executed by the electronic control unit in one embodiment.

FIG. 14 is a flowchart illustrating a “compare interrupt routine” executed by the electronic control unit in one embodiment.

FIG. 15 is a flowchart illustrating a “step motor control routine” executed by the electronic control unit in one embodiment.

FIG. 16 is a time chart showing a relationship between a target step number for step motor control and a target injection time for injector control with respect to an air-fuel ratio correction coefficient in one embodiment.

[Explanation of symbols]

1 LPG engine, 2 intake passage, 4 venturi,
7 ... Exhaust passage, 12 ... Main fuel passage, 14 ... Step motor, 15 ... Fuel throttle valve (12 and 15 constitute main fuel mixing means), 17 ... Injector as auxiliary fuel injection means, 35 ... Temperature A water temperature sensor as a state detecting means; 36, an oxygen sensor as an air-fuel ratio detecting means;
The ECU (38 constitutes a correction coefficient calculating means, a cold fuel control means, a warm fuel control means and a mode switching control means).

Continuation of the front page (51) Int.Cl. ⁶ identification code FI F02M 21/02 301 F02M 21/02 301R 311 311B (56) References JP-A-58-65943 (JP, A) JP-A-61-286546 ( JP, A) JP-A-63-36057 (JP, A) JP-A-56-52559 (JP, A) JP-A-58-206858 (JP, A) JP-A-2-218851 (JP, A) JP-A-62-291446 (JP, A) JP-A-63-36058 (JP, A) JP-A-1-200053 (JP, A) JP-A-5-44525 (JP, A) (58) Fields investigated (Int) .Cl. ⁶ , DB name) F02D 41/14 310 F02D 41/02 325 F02D 41/06 325 F02M 21/02 301 F02M 21/02 311

Claims (1)

(57) [Claims] 1. A main fuel mixing means for mixing main fuel and intake air through a venturi formed in an intake passage of an internal combustion engine using liquefied petroleum gas as fuel, and supplying the mixture to the internal combustion engine. , Provided in the intake passage at a position different from the main fuel mixing means,
Auxiliary fuel injection means for injecting auxiliary fuel into the intake passage; air-fuel ratio detection means provided in an exhaust passage of the internal combustion engine for detecting an air-fuel ratio of the internal combustion engine from oxygen concentration in exhaust gas; Correction coefficient calculation for calculating an air-fuel ratio correction coefficient based on the air-fuel ratio detected by the air-fuel ratio detection means in order to correct the mixture supply amount by the means and the auxiliary fuel injection amount from the auxiliary fuel injection means, respectively. Means, a temperature state detecting means for detecting a temperature state of the internal combustion engine, and when the detection result of the temperature state detecting means is a cold state, the correction coefficient calculation of the mixture supply amount by the main fuel mixing means. A cold fuel control unit that performs air-fuel ratio feedback control by correcting based on an air-fuel ratio correction coefficient calculated by the unit; and when the detection result of the temperature state detection unit is a warm state. An LPG internal combustion engine comprising: a warm-time fuel control unit for performing an air-fuel ratio feedback control by correcting an auxiliary fuel injection amount by the auxiliary fuel injection unit based on an air-fuel ratio correction coefficient calculated by the correction coefficient calculation unit. The air-fuel ratio control device according to claim 1, wherein when the detection result of the temperature state detecting means is a transition period from cold to warm, the cold-fuel control is performed by the cold-fuel feedback control by the cold fuel control. In order to switch the mode to the air-fuel ratio feedback control by the control unit, both the cold fuel control unit and the warm fuel control unit are operated, and the air-fuel ratio feedback performed based on the air-fuel ratio correction coefficient is performed. The correction ratio for control is gradually reduced by the cold fuel control means and gradually increased by the warm fuel control means. Air-fuel ratio control system for an LPG engine, characterized in that a mode switching control means for controlling.