JP2013241905A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an air-fuel ratio control device for an internal combustion engine, ensuring stable driving during switching between liquid fuel and gas fuel.SOLUTION: A control device 41 mounted on an internal combustion engine 10 driven by selecting and switching gasoline or CNG to supply it to a combustion chamber 17 comprises a speed sensor 46 for detecting a driving speed of the internal combustion engine and an upstream side oxygen sensor 43 detecting oxygen concentration in exhaust gas. When executing overlap control for supplying both of the gasoline and the CNG in an overlapping manner or delay control for stopping the supply of both of them in response to fuel switching while extending the time corresponding to the length of a connection injector hose 51 of a CNG injector 32, the control is interrupted in a case below a set oxygen concentration when the internal combustion engine performing idle driving based on a detection driving speed of the speed sensor 46 and detection oxygen concentration of the upstream side oxygen sensor 43 is lowered to the driving speed or below at which a stall is likely to occur.

Description

  The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine, and more particularly to an apparatus for executing air-fuel ratio control at the time of switching of the fuel of an internal combustion engine that operates by selectively switching the fuel used to one of liquid fuel and gas fuel. .

The vehicle is equipped with an internal combustion engine that operates by supplying liquid fuel such as gasoline and gaseous fuel such as CNG (Compressed Natural Gas) and LPG (Liquefied Petroleum Gas) as fuel to be supplied to the combustion chamber. It is known to use as a drive source (for example, Patent Documents 1 and 2).
Patent Document 1 describes an internal combustion engine that is operated by supplying a mixture of gasoline and LPG. In this internal combustion engine, fuel is interrupted and injected during acceleration.
Patent Document 2 describes an internal combustion engine that operates by switching the fuel used to one of gasoline and hydrogen fuel. In this internal combustion engine, control is performed so as to obtain an output torque to be a target torque.

Since this type of internal combustion engine is based on a gasoline engine, it is difficult to directly install an injector for gaseous fuel adjacent to an intake manifold on the intake side, like an injector for gasoline. For this reason, the injector for gaseous fuel may be installed in an intake manifold via an injector hose, for example.
In this case, the gaseous fuel injector is separated from the intake manifold by the amount of the injector hose, and the timing for starting and stopping the supply of gaseous fuel is delayed.
For this reason, in the air-fuel ratio control apparatus mounted on the vehicle, in consideration of the delay period, a superposition period in which an operation for simultaneously injecting and supplying both gaseous fuel and gasoline is set, or a stop period in which both the injection operations are stopped May be set to optimize the air-fuel ratio control.

JP 2005-180187 A Japanese Patent Laid-Open No. 2008-14162

However, in such air-fuel ratio control, a superposition period in which both gasoline and gaseous fuel are injected simultaneously and a stop period in which both injection supplies are stopped simultaneously are fixed. For this reason, depending on the fuel injection situation and the driving situation of the internal combustion engine, there is a possibility that the air-fuel ratio becomes unsuitable and stalls.
Therefore, an object of the present invention is to provide an air-fuel ratio control device for an internal combustion engine that realizes stable driving even when switching between liquid fuel and gaseous fuel.

  A first aspect of the invention relating to an air-fuel ratio control device for an internal combustion engine that solves the above-mentioned problems is an air-fuel ratio of an internal combustion engine that is driven by selectively switching between one of liquid fuel and gaseous fuel and supplying it to a common combustion chamber An air-fuel ratio control unit that controls supply / stop of the liquid fuel and supply / stop of the gaseous fuel, a speed detection unit that detects a drive speed of the internal combustion engine, and a control device that is exhausted from the combustion chamber A combustion sensor for detecting a combustion state from the exhaust gas, wherein the air-fuel ratio control unit supplies the liquid fuel and the gaseous fuel after performing a superimposed supply operation or a supply stop operation during a set period. When the fuel is switched from one of the liquid fuel and the gaseous fuel to the other, the speed detection unit detects that the internal combustion engine has fallen to a driving speed at which the internal combustion engine is likely to stall during low speed driving of the internal combustion engine. To, and is characterized in that the combustion sensor to maintain or reduce the set period based on the combustion state of the internal combustion engine to detect from the exhaust gas.

  A second aspect of the invention relating to an air-fuel ratio control apparatus for an internal combustion engine that solves the above-described problem is a sensor that detects a residual oxygen concentration in the exhaust gas as the combustion sensor in addition to the specific matter of the first aspect. When the residual oxygen concentration is less than a set value when performing the superimposed supply operation during the set period in order to switch from the liquid fuel to the gaseous fuel, the air-fuel ratio control unit adopts Maintains the set period, and shortens the set period when the residual oxygen concentration is equal to or higher than a set value.

  According to a third aspect of the invention relating to an air-fuel ratio control device for an internal combustion engine that solves the above-mentioned problems, in addition to the specific matter of the first or second aspect, as a combustion sensor, a residual oxygen concentration in the exhaust gas can The air-fuel ratio control unit adopts a sensor to detect, and when the supply stop operation is executed during the set period in order to switch from the gaseous fuel to the liquid fuel, the residual oxygen concentration is equal to or higher than a set value. In this case, the set period is maintained, and when the residual oxygen concentration is less than a set value, the set period is shortened.

According to a fourth aspect of the invention relating to an air-fuel ratio control apparatus for an internal combustion engine that solves the above-mentioned problems, in addition to the specific matter of any one of the first to third aspects, the supply amount at the start of supply of the gaseous fuel is Gradual increase adjustment is made so that the supply amount is gradually reduced when the supply of the gaseous fuel is stopped.
According to a fifth aspect of the invention relating to an air-fuel ratio control apparatus for an internal combustion engine that solves the above-mentioned problems, in addition to the specific matter of any one of the first to fourth aspects, the air-fuel ratio control unit includes the internal combustion engine. In the idling driving, when the speed detecting unit detects a decrease in driving speed, the set period is maintained or shortened.

Thus, according to one aspect of the present invention, when the driving speed further decreases during the low-speed driving of the internal combustion engine and it is likely to stall, the superposition supply of liquid fuel and gaseous fuel according to the combustion state of the internal combustion engine The setting period of the operation or supply stop operation can be shortened. Therefore, without waiting until the set period elapses, it is possible to shift to injection supply of only the used fuel after switching and to correct (recover) the air-fuel ratio, thereby avoiding stalling the internal combustion engine. be able to. As a result, stable driving of the internal combustion engine can be continuously maintained even when the fuel is switched.
Also, stable adjustment of liquid fuel and gaseous fuel can be performed while maintaining a smooth drive of the internal combustion engine by adjusting the supply amount to gradually increase or decrease (gradual change) at the start or stop of supply of the gaseous fuel. Can do.

1 is a diagram showing an embodiment of an air-fuel ratio control apparatus for a bi-fuel type internal combustion engine according to the present invention, and is a partial cross-sectional configuration diagram showing a schematic overall structure thereof. It is a partial cross section top view which shows the principal part structure. It is a timing chart explaining the delay control in the air fuel ratio control at the time of the fuel switching. It is a timing chart explaining the overlap control in the air fuel ratio control at the time of the fuel switching. 4 is a flowchart illustrating a delay extension interruption control process in the delay control shown in FIG. 3. It is a timing chart explaining the effect by execution of interruption control processing of delay extension of the delay control. It is a timing chart explaining the inconvenience by the non-execution of the delay control interruption control process of the delay control. FIG. 5 is a flowchart for explaining an interruption control process for superposition and extension in the overlap control shown in FIG. 4. FIG. It is a timing chart explaining the effect by execution of the interruption control processing of superposition extension of the overlap control. It is a timing chart explaining the inconvenience by the non-execution of the interruption control process of the overlap extension of the overlap control.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 1 to 10 are diagrams for explaining an embodiment of an air-fuel ratio control apparatus for an internal combustion engine according to the present invention.
(Configuration of internal combustion engine)
In FIG. 1, an internal combustion engine 10 includes a mechanism for connecting a piston 11 to a piston rod 12 and accommodates it in a cylinder bore 15 so as to be movable up and down, and is mounted on a vehicle or the like as a drive source. In the internal combustion engine 10, a combustion chamber 17 is formed between the lower inner surface of the cylinder head 16 above the cylinder bore 15 and the upper surface of the piston 11. The internal combustion engine 10 moves the piston 11 up and down by repeating combustion expansion and exhaust by igniting a mixture of intake combustion air and injected fuel introduced into the combustion chamber 17. As a result, the internal combustion engine 10 transmits the driving force of the vertically moving piston 11 via the piston rod 12, the transmission, or the like, thereby rolling the wheels, for example, to realize traveling of the vehicle. Here, the internal combustion engine 10 ignites the air-fuel mixture in the combustion chamber 17 by applying a high voltage boosted by the ignition coil 19 to a spark plug (not shown) and causing it to spark.

  In the internal combustion engine 10, an intake valve 13 and an exhaust valve 14 are disposed in a cylinder head 16. The intake valve 13 and the exhaust valve 14 are respectively connected to the combustion chamber 17 when a drive cam (intake cam and exhaust cam) 28a that rotates integrally with the camshaft 28 moves (lifts) in the pressing direction in the figure. A gap is formed between the opening 21 and the opening edge of the exhaust port 22 to open and close.

  An air cleaner 25 is connected to the intake port 21 via a surge tank 26 so as to construct an intake side manifold 23 for intake of clean outside air. A catalytic converter 27 containing a catalyst 27a is connected to the exhaust port 22 so as to construct an exhaust side manifold 24 for exhausting the cleaned combustion gas. A throttle valve 29 that is linked to the accelerator pedal of the vehicle is disposed on the upstream side of the surge tank 26. The internal combustion engine 10 controls the intake air amount of the combustion air (outside air) mixed with the fuel by adjusting the opening of the throttle valve 29 so as to optimize the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 17. It has become.

In addition, the internal combustion engine 10 can selectively supply two types of fuel, that is, liquid fuel gasoline and gas fuel CNG (LPG may be used) as fuel to be injected into the combustion chamber 17. A gasoline injector (liquid fuel injector) 31 and a CNG injector (gaseous fuel injector) 32 are arranged in the vicinity.
The gasoline injector 31 is connected to a liquid fuel tank 33 via a liquid fuel supply pipe 34. The gasoline injector 31 injects gasoline stored in the liquid fuel tank 33 under normal pressure and supplies the gasoline into the combustion chamber 17 via the intake port 21.
The CNG injector 32 is connected to a gaseous fuel tank 36 via a gaseous fuel supply pipe 37. The CNG injector 32 adjusts the pressure of the CNG stored in the gaseous fuel tank 36 in a high pressure state, injects it, and supplies it to the combustion chamber 17 via the intake port 21. Here, in the gaseous fuel supply pipe 37, since it is difficult to cut off the supply of gaseous CNG (gaseous fuel) into the combustion chamber 17 only by the CNG injector 32, a stop valve 39 is arranged together with the pressure reducing valve 38. doing.

(Configuration of air-fuel ratio control device)
The internal combustion engine 10 includes a control device 41 constructed by a CPU, a memory, and the like. The control device 41 executes a control program stored in advance based on various parameters, sensor information, and the like, and performs overall control of the entire vehicle including air-fuel ratio control of fuel supplied to the internal combustion engine 10. For example, the control device 41 executes a fuel switching control process for the gasoline injector 31 and the CNG injector 32. That is, this control device 41 constitutes an air-fuel ratio control device including an air-fuel ratio control unit.

The control device 41 includes an intake air temperature sensor 42, an upstream oxygen sensor (combustion sensor) 43, a downstream oxygen sensor 44, an engine water temperature sensor 45, a crank angle sensor (speed detector) 46, and the like. The cam angle sensor 47 and the fuel changeover switch 48 are connected.
The intake air temperature sensor 42 is installed on the downstream side of the air cleaner 25 and detects the intake air temperature of the fuel air supplied to the combustion chamber 17. The upstream oxygen sensor 43 is installed on the upstream side of the catalytic converter 27, and detects the oxygen concentration in the exhaust gas after combustion exhausted from the combustion chamber 17 to detect the combustion state. The downstream oxygen sensor 44 is installed on the downstream side of the catalytic converter 27, and detects the purification status by detecting the oxygen concentration in the exhaust gas after the purification treatment by the catalyst 27a. The engine water temperature sensor 45 detects the engine coolant temperature. The crank angle sensor 46 detects the crank angle of a crankshaft (not shown) for the purpose of detecting the drive speed (drive speed), synchronizing the fuel injection timing, and the like. The cam angle sensor 47 detects the cam angle of the intake side camshaft 28 (intake cam 28a). The fuel changeover switch 48 performs a changeover operation of the fuel to be used manually according to the driving situation such as when the driver is warming up or accelerating.

  Since the control device 41 has a function of selectively switching between liquid fuel and gaseous fuel having different properties and supplying them to the internal combustion engine 10 (combustion chamber 17), the fuel supplied into the combustion chamber 17 is supplied from gasoline to CNG. Alternatively, the CNG is appropriately switched to gasoline. For example, the control device 41 may detect the internal combustion engine 10 during a manual switching operation by the fuel switch 48, when a fuel shortage is detected in the liquid fuel tank 33 or the gaseous fuel tank 36, or during a warm-up operation or a steady operation or acceleration. The fuel to be used is switched according to the operation mode. As the operation mode of the internal combustion engine 10, an inexpensive CNG can be selected during warm-up operation or steady operation, and gasoline can be selected during acceleration or climbing that requires torque.

  By the way, since it is difficult for the control device 41 to accurately switch between CNG supply and stop by the CNG injector 32, the flow path space (capacity) in the gaseous fuel supply pipe 37 from the stop valve 39 to the CNG injector 32 is taken into consideration. Thus, switching control between CNG and gasoline is executed.

In the present embodiment, the internal combustion engine 10 is based on a so-called gasoline engine that uses gasoline as fuel. For this reason, since the internal combustion engine 10 has a structure in which a CNG injector 32 is additionally provided in addition to the gasoline injector 31, the CNG injector 32 is installed in a narrow space adjacent to the gasoline injector 31 of the intake side manifold 23. Must. However, in the internal combustion engine 10, it is difficult to install the CNG injector 32 in a parallel state at a location adjacent to the gasoline injector 31.
Therefore, as shown in FIG. 2, the internal combustion engine 10 includes a pipe-shaped installation member 23 a that can be inserted into the intake-side manifold 23 by inserting the injection-side tip portion of the CNG injector 32 and the CNG injector. Both of the 32 injection-side tip portions are inserted into both end portions of the injector hose 51 so as to be connected to each other. In the case of this structure, when examined in detail, it is understood that there is a time difference between the fuel injector 31 and the CNG injector 32 until the fuel is supplied (arrived) in the combustion chamber 17 in a combustible manner.
For this reason, the control device 41 considers the time difference until the fuel injected from the gasoline injector 31 and the CNG injector 32 reaches the combustion chamber 17 when the fuel to be used is switched as the air-fuel ratio control unit. The injection stop timing is adjusted and controlled. In short, the controller 41 delays the supply of CNG from the CNG injector 32 to the intake side manifold 23 by the length of the injector hose 51, and therefore performs control for switching between CNG and gasoline in consideration of the delay time. It is supposed to be.

Specifically, for example, after the supply of CNG is stopped by the stop valve 39, the high-pressure CNG remaining in the gaseous fuel supply pipe 37 up to the CNG injector 32 is gradually released into the intake side manifold 23 (intake port 21). And supplied into the combustion chamber 17.
Therefore, as shown in the timing chart of FIG. 3, the control device 41 switches the fuel supplied into the combustion chamber 17 from CNG to gasoline in order to change the drive of the internal combustion engine 10 from the CNG mode to the gasoline mode. Performs the following switching control.
First, the control device 41 continues the supply of CNG from the gaseous fuel supply pipe 37 to the combustion chamber 17 while maintaining the supply state of the CNG injector 32 (supply signal ON) even after the stop valve 39 is shut off. Thereafter, the control device 41 delays the supply operation stop (supply signal OFF) of the CNG injector 32 and the supply operation start (supply signal ON) of the gasoline injector 31 in accordance with the decrease in the supply amount of the CNG. Execute control.

At this time, the control device 41 delays by the length of the injector hose 51 until the supply of CNG from the gaseous fuel supply pipe 37 to the intake side manifold 23 is completely completed. Considering this, the above delay control is extended. In short, the control device 41 sets a preset time for delay control for delaying the start of supply operation of the gasoline injector 31 (supply signal ON) even after the supply operation stop of the CNG injector 32 (supply signal OFF). The hose 51 is extended by a delay time corresponding to the length. The delay extension time in this delay control is obtained by calculating or experimenting the time required to stop the supply of CNG to the intake side manifold 23, which is delayed by the length of the injector hose 51, in advance in the control device 41. Set it.
Here, in the present embodiment, delay control delays in accordance with the reduction (gradual decrease) / stop of the supply amount according to the structure and shape of the supply path through which CNG in the gaseous fuel supply pipe 37 is supplied to the combustion chamber 17. The case where the extension time is set will be described as an example, but is not limited thereto. For example, the control device 41 performs gradual change control that finely adjusts the supply operation of the CNG injector 32 so as to reduce the supply amount of CNG at a desired reduction rate, thereby stabilizing the fuel supply at the time of switching. May be.

Further, after the start of CNG supply by the stop valve 39, the CNG concentration in the flow path space from the CNG injector 32 to the gaseous fuel supply pipe 37 is gradually increased and released into the intake side manifold 23 (intake port 21). Is supplied into the combustion chamber 17.
Therefore, as shown in the timing chart of FIG. 4, the control device 41 switches the fuel supplied into the combustion chamber 17 from gasoline to CNG in order to drive the internal combustion engine 10 from the gasoline mode to the CNG mode. Performs the following switching control.
First, the control device 41 starts the supply operation of the CNG injector 32 after the opening operation of the stop valve 39 (supply signal ON), and starts the supply of CNG from the gaseous fuel supply pipe 37 into the combustion chamber 17. . At the same time, the control device 41 executes overlap control for continuously superimposing the supply operation of the gasoline injector 31 (supply signal ON), and then performs the supply operation of the gasoline injector 31 in accordance with the increase in the supply amount of CNG. Stop (supply signal OFF) and end overlap control.

At this time, the control device 41 considers the delay time because a delay corresponding to the length of the injector hose 51 occurs from the inside of the gaseous fuel supply pipe 37 to the start of supply of a steady amount of CNG to the intake side manifold 23. Thus, the above overlap control is extended. In short, the control device 41 sets a preset time for the overlap control to maintain the supply operation (supply signal ON) of the gasoline injector 31 even after the supply operation start (supply signal ON) of the CNG injector 32. The hose 51 is extended by a delay time corresponding to the length. The overlap extension time in this overlap control is obtained by calculating or experimenting the delay time until the start of CNG supply to the intake side manifold 23, which is extended by the length of the injector hose 51. Set in advance.
Here, in the present embodiment, overlap extension of overlap control is performed in accordance with an increase (gradual increase) in supply amount according to the structure and shape of the supply path through which CNG in the gaseous fuel supply pipe 37 is supplied to the combustion chamber 17. The case of setting the time will be described as an example, but is not limited thereto. For example, the control device 41 performs gradual change control that finely adjusts the supply operation of the CNG injector 32 so as to gradually increase the supply amount of CNG at a desired increase rate, and stabilizes the fuel supply at the time of switching. Also good.

  As a result, the control device 41 can add a delay extension time to the delay control in the gasoline / CNG switching control, and a superposition extension time to the overlap control. For this reason, the control apparatus 41 covers the delay of the supply timing of CNG resulting from employ | adopting the installation structure of the CNG injector 32 which interposes the injector hose 51, and performs smoothly the switching control between gasoline. be able to. Therefore, it is possible to comfortably drive the so-called bi-fuel type internal combustion engine 10 that can use two types of fuels, liquid fuel gasoline and gas fuel CNG, having different properties, while effectively drawing out the advantages of gasoline and CNG. it can.

The control device 41 monitors the drive quality of the internal combustion engine 10 according to the state of fuel supply into the combustion chamber 17 at the time of switching between gasoline and CNG, and performs fine adjustment of the switching control to thereby make an internal combustion engine. Ten comfortable driving is realized.
Specifically, the control device 41 performs a switching control process (method) for supplying fuel into the combustion chamber 17 based on sensor information of a crank angle sensor (hereinafter also simply referred to as a speed sensor) 46 and an upstream oxygen sensor 43. ) And the extension time of the delay control and overlap control shown in the timing charts of FIGS. 3 and 4 is adjusted.

  Specifically, as shown in the flowchart of FIG. 5, the control device 41 closes the stop valve 39 in step S <b> 101 when executing a control process for switching the fuel supplied into the combustion chamber 17 from CNG to gasoline. Then, after stopping the supply of CNG, in step S102, a timer (not shown) is activated to start delay control.

  Next, in step S103, the drive rotational speed of the internal combustion engine 10 detected by the speed sensor 46 is subtracted from a preset low speed, for example, 50 rpm at which the possibility of stalling occurs from the target rotational speed for maintaining idling drive. Check if it has dropped below the set speed. In step S104, the oxygen concentration in the exhaust gas corresponding to the combustion state in the combustion chamber 17 detected by the upstream oxygen sensor 43 is lowered to a preset value that may cause a stall occurrence. Check if it exists. Further, in step S105, it is confirmed whether or not the delay control time including the delay extension time has elapsed with reference to the elapsed time from the start of the delay control timed by the timer.

At this time, after confirming in step S103 that it has not decreased below the set rotational speed of “idling target rotational speed−50 rpm”, the routine proceeds to step S105.
In step S103, after confirming that the engine speed has decreased below the set idling target engine speed of −50 rpm, the process proceeds to step S104 to confirm that the oxygen concentration in the exhaust gas has not decreased below the set value. In that case, it is determined that the proper air-fuel mixture for normal combustion is supplied in the combustion chamber 17, and the process directly proceeds to step S105.
When it is confirmed in step S105 that the delay control time including the delay extension time since the CNG injector 32 is stopped (OFF) has not elapsed, the process returns to step S103 and the same processing is repeated.
If it is confirmed in step S105 that the delay control time has elapsed, the delay control period taking into account the length of the injector hose 51 is terminated, and the timer is reset (step S106). The injector 31 is driven (ON) to start supplying gasoline (step S107), and the supply fuel switching control process is terminated.

On the other hand, in step S103, after confirming that the idling target rotational speed −50 rpm or less has been reduced, it is further confirmed in step S104 that the oxygen concentration in the exhaust gas has decreased below the set value. In such a case, the following control process for shortening the set time of delay control is executed.
That is, the control device 41 determines that an interruption condition has been established in which there is a high possibility that the internal combustion engine 10 will be stalled due to a lean state in which the fuel in the air-fuel mixture supplied into the combustion chamber 17 becomes insufficient. The process skips to step S106 as it is.
In step S106, the delay control period is shortened and then, in step S107, the gasoline supply from the gasoline injector 31 is started, and the supply fuel switching control process is ended.

  As a result, as shown in the timing chart of FIG. 6, the control device 41 can perform the CNG supply stop delay extension time considering the length of the injector hose 51 after stopping the CNG injector 32. Delay control can be terminated. In short, when the supply of CNG has substantially ended and the air-fuel mixture supplied to the combustion chamber 17 has reached a lean state where there is a high possibility of stalling the internal combustion engine 10 that is idling-driven (when the interruption condition is satisfied), Immediately, the delay control can be interrupted. Therefore, it is possible to start the gasoline supply from the gasoline injector 31 without causing the internal combustion engine 10 to stall and suppress the decrease in the driving speed to a small value, and to smoothly switch the fuel used to gasoline.

  Here, λ in FIG. 6 is a parameter for relative evaluation with λ = 1.0 when the fuel is supplied at the stoichiometric ratio (stoichiometry, so-called stoichiometry). This shows that the air-fuel mixture is in a lean state and the air-fuel mixture is in a lean state when 1.0 <λ. In FIG. 6, the output waveform of the upstream oxygen sensor 43 interlocked with the vertical movement of the piston 11 is illustrated. For example, it is illustrated at 0 V, MV (Middle): 0.45 V, and HV (High): 1 V. The output waveform of the sensor which amplifies with the output voltage to illustrate is shown.

  As a result, as shown in FIG. 6, it is determined that the interruption condition is satisfied without continuing the lean state (1.0 << λ) where λ is significantly lower than 1.0 for a long time, delay control is interrupted, and gasoline supply is stopped. You can see that it has started. In other words, for example, as shown in the timing chart of FIG. 7, it is possible to avoid inconvenience caused by executing delay control that expires until a set delay extension time. In brief, the supply of CNG is substantially terminated, and the lean state of 1.0 << λ continues for a long period of time, so that the engine speed is such that the internal combustion engine 10 is stalled and knocking occurs. It is possible to avoid a significant decrease.

  Further, as shown in the flowchart of FIG. 8, the control device 41 opens the stop valve 39 in step S201 when executing the control process for switching the fuel supplied into the combustion chamber 17 from gasoline to CNG. After starting the supply of CNG, in step S202, the timer is started to start overlap control. Here, since the same processing procedure corresponding to the above-described delay control is executed also in this overlap control, the same processing procedure will be briefly described using the same wording.

  Next, in step S203, it is confirmed whether or not the drive rotational speed of the internal combustion engine 10 detected by the speed sensor 46 has decreased below the set rotational speed of “idling target rotational speed−50 rpm”. Further, in step S204, it is confirmed whether or not the oxygen concentration in the exhaust gas detected by the upstream oxygen sensor 43 has risen to a preset value that causes a possibility of stall occurrence. In step S205, it is checked whether or not the overlap control time including the overlap extension time has elapsed with reference to the elapsed time from the start of the overlap control timed by the timer.

At this time, after confirming that it has not decreased below the set rotational speed of “idling target rotational speed−50 rpm” in step S203, the process proceeds to step S205.
In step S203, after confirming that the engine speed has decreased below the set idling target engine speed of −50 rpm, the process proceeds to step S204, where it is confirmed that the oxygen concentration in the exhaust gas has not increased to a set value or more. In that case, it is determined that the proper air-fuel mixture for normal combustion is supplied in the combustion chamber 17, and the process proceeds to step S205 as it is.
In step S205, when it is confirmed that the overlap control time including the overlap extension time since the CNG injector 32 is driven (ON) has not elapsed, the process returns to step S203 and the same processing is repeated. .
If it is confirmed in step S205 that the overlap control time has elapsed, the overlap control period taking into account the length of the injector hose 51 is expired and the timer is reset (step S206). Then, the gasoline injector 31 is stopped (OFF) to stop the supply of gasoline (step S207), and the supply fuel switching control process is ended.

On the other hand, in step S203, after confirming that it has decreased below the set idling speed of “idling target rpm−50 rpm”, in step S204, it is further confirmed that the oxygen concentration in the exhaust gas has risen to the set value or more. In such a case, the following control process for shortening the setting time of the overlap control is executed.
That is, the control device 41 determines that an interruption condition has been established in which there is a high possibility that the fuel in the air-fuel mixture supplied into the combustion chamber 17 will be in a rich state that causes the internal combustion engine 10 to stall. Then, the process skips to step S206 as it is.
In step S206, the overlap control period is shortened, and then in step S207, the supply of gasoline from the gasoline injector 31 is stopped, and the supply fuel switching control process is ended.

  As a result, as shown in the timing chart of FIG. 9, the control device 41 can perform the gasoline supply stop superposition extension time considering the length of the injector hose 51 after driving the CNG injector 32. Overlap control can be terminated. In short, when the supply of CNG has substantially started and the air-fuel mixture supplied to the combustion chamber 17 has reached a rich state where there is a high possibility of stalling the internal combustion engine 10 that is idling-driven (when the interruption condition is satisfied). The overlap control can be interrupted immediately. Therefore, it is possible to stop the supply of gasoline from the gasoline injector 31 without causing the internal combustion engine 10 to stall and to suppress the decrease in the driving speed, and to smoothly switch the fuel used to CNG.

  As a result, as shown in FIG. 9, a rich state in which λ is significantly greater than 1.0 (λ << 1.0 does not continue for a long time, it is determined that the interruption condition is satisfied, the overlap control is interrupted, and the gasoline supply is stopped. In other words, for example, as shown in the timing chart of Fig. 10, the inconvenience caused by executing overlap control that expires until the set overlap extension time is avoided. Briefly, the supply of CNG in addition to gasoline is substantially started, and the rich state of λ << 1.0 continues for a long period of time, so that the internal combustion engine 10 is stalled and knocking is performed. It is possible to avoid a significant decrease in the driving rotational speed that is generated.

As described above, in the present embodiment, when switching between CNG and gasoline during low speed driving such as during idling driving of the internal combustion engine 10, delay control and overlap control are uniformly extended by the length of the injector hose 51. Can be avoided. That is, the delay control and overlap control can be interrupted and shifted to the fuel supply after switching according to the drive rotational speed and combustion state of the internal combustion engine 10, and combustion can be driven at an appropriate air-fuel ratio. Therefore, the internal combustion engine 10 can be prevented from being stalled, and the fuel to be used can be switched smoothly while ensuring a stable drive rotation of the internal combustion engine 10.
Here, in this embodiment, a case where the extension time of delay control or overlap control is interrupted will be described as an example. However, the present invention is not limited to this, and it goes without saying that the control body itself before extension may be interrupted. Yes.

  The scope of the present invention is not limited to the illustrated and described exemplary embodiments, but includes all embodiments that provide the same effects as those intended by the present invention. Further, the scope of the invention is not limited to the combinations of features of the invention defined by the claims, but may be defined by any desired combination of particular features among all the disclosed features. .

  Although one embodiment of the present invention has been described so far, it is needless to say that the present invention is not limited to the above-described embodiment, and may be implemented in various forms within the scope of the technical idea.

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 11 Piston 13 Intake valve 14 Exhaust valve 17 Combustion chamber 21 Intake port 22 Exhaust port 23 Intake side manifold 24 Exhaust side manifold 29 Stop valve 31 Gasoline injector 32 CNG injector 33 Liquid fuel tank 36 Gas fuel tank 38 Pressure reducing valve 39 Stop Valve 41 Control device 42 Intake air temperature sensor 43 Upstream oxygen sensor 44 Downstream oxygen sensor 46 Crank angle sensor (speed sensor)
48 Fuel switch 51 Injector hose

Claims (5)

An air-fuel ratio control device for an internal combustion engine that is driven by selectively switching one of liquid fuel and gaseous fuel and supplying the same to a common combustion chamber,
An air-fuel ratio control unit that controls supply / stop of the liquid fuel and supply / stop of the gaseous fuel, a speed detection unit that detects a driving speed of the internal combustion engine, and a combustion state from exhaust gas exhausted from the combustion chamber A combustion sensor for detecting
The air-fuel ratio controller is
When the supply fuel is switched from one of the liquid fuel and the gaseous fuel to the other after the superimposed supply operation or supply stop operation of both the liquid fuel and the gaseous fuel is performed during a set period,
When the speed detection unit detects that the internal combustion engine has dropped to a drive speed at which the internal combustion engine is likely to stall during low speed driving of the internal combustion engine,
An air-fuel ratio control apparatus for an internal combustion engine, characterized in that the set period is maintained or shortened based on a combustion state of the internal combustion engine detected by the combustion sensor from the exhaust gas. As the combustion sensor, a sensor that detects the residual oxygen concentration in the exhaust gas is employed,
The air-fuel ratio control unit performs the setting period when the residual oxygen concentration is less than a set value when performing the superimposed supply operation during the set period to switch from the liquid fuel to the gaseous fuel. 2. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the set period is shortened when the residual oxygen concentration is equal to or higher than a set value. As the combustion sensor, a sensor that detects the residual oxygen concentration in the exhaust gas is employed,
The air-fuel ratio control unit, when performing the supply stop operation during the set period to switch from the gaseous fuel to the liquid fuel, sets the set period when the residual oxygen concentration is equal to or higher than a set value. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 or 2, wherein the set period is shortened when the residual oxygen concentration is less than a set value.   4. The gradual change adjustment is performed in which the supply amount is gradually increased at the start of the supply of the gaseous fuel, and the supply amount is reduced when the supply of the gaseous fuel is stopped. 5. An air-fuel ratio control apparatus for an internal combustion engine.   5. The air-fuel ratio control unit performs maintenance or shortening of the set period when the speed detection unit detects a decrease in driving speed during idling driving of the internal combustion engine. The air-fuel ratio control apparatus for an internal combustion engine according to any one of the above.

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