JPH0213150B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an air-fuel ratio control device for a gas engine that switches the air-fuel ratio according to the load condition of the engine. First, the combustion characteristics of the lean burn system and the stoichiometric air-fuel ratio combustion system in gas engines will be explained. Figure 1 shows the case of excess air ratio λ = 1.4 and λ
This shows an example of the relationship between thermal efficiency and output when λ = 1.0, and the solid line shows the case when λ = 1.4.
The case of λ=1.0 is shown by a broken line. When compared at the same rotation speed, both have contradictory advantages and disadvantages: when λ = 1.4, the thermal efficiency is high but the maximum output is low, and when λ = 1.0, the thermal efficiency is low but the maximum output is high. I understand that. In addition, Figure 2 shows an example of the relationship between the excess air ratio and the exhaust emission, thermal efficiency, etc.
This figure shows that exhaust emissions are lower in lean burn, and higher near λ=1.0. However, even around λ=1.0, if a three-way catalyst is used and it works effectively, it is possible to keep the exhaust emissions low, so it can be considered that the exhaust emissions are the same for both types. The present invention focuses on this point, and makes it possible to select between the lean burn method and the stoichiometric air-fuel ratio combustion method depending on the load amount, and switches to the lean burn method with high thermal efficiency at low loads when a small output is required, and when a large output is required. This was done with the aim of effectively operating the gas engine by switching to the stoichiometric air-fuel ratio combustion method, which has a higher maximum output, at times of high load, taking advantage of the advantages of each method. That is, the air-fuel ratio control device for a gas engine according to the present invention includes a means for detecting the engine speed, a means for detecting the throttle valve opening, a mixer adjusted in advance to perform lean combustion, and a mixer for performing EGR control.
an EGR valve, a stoichiometric air-fuel ratio control means for controlling fuel additionally supplied from a gas injector according to the oxygen concentration in exhaust gas under the effective action of a three-way catalyst to perform stoichiometric air-fuel ratio combustion; a storage means for storing each load amount when switching from lean burn mode to stoichiometric air-fuel ratio combustion mode and vice versa in the form of a numerical table of the relationship between throttle valve opening and engine speed; the throttle valve opening is compared with the numerical value of the throttle valve opening corresponding to the engine speed at that time according to the numerical table, and the combustion method is switched using the numerical value as a threshold value. It is characterized by FIG. 6 is a diagram showing the configuration of the present invention. A is the engine, B is the rotation speed detection means, C is the throttle valve, D
is the opening detection means, E is the mixer, F is the EGR valve,
G is a gas injector, H is an O ₂ sensor, I is a three-way catalyst, J is a stoichiometric air-fuel ratio control means, K is a memory means, and L is a switching means. The broken line arrow indicates the flow of fuel and exhaust gas, and the solid line arrow indicates the signal. Each flow is shown below. At low loads when switching to lean burn mode, the stoichiometric air-fuel ratio control is turned off and the mixer, which has been pre-adjusted to perform lean burn, controls, for example, λ = 1.4.
lean combustion is performed, and at the same time the EGR valve is opened to apply EGR control to reduce NO _x , and when the load increases and high output is required,
By closing the EGR valve, stopping EGR control, and turning on the stoichiometric air-fuel ratio control, detecting the oxygen concentration in the exhaust gas and feeding it back, and operating the gas injector to additionally supply gas fuel, λ=1 This means that combustion is switched to combustion at a nearby stoichiometric air-fuel ratio. As is clear from Figure 1, if the engine speed differs, the maximum output point at λ = 1.4, for example, will also differ, so the lean burn method and the stoichiometric air-fuel ratio combustion method can be effectively used together at all engine speeds. In order to do this, the load amount to be changed according to the engine speed, and therefore the throttle valve opening degree which is a substitute characteristic thereof, are set in advance and stored in the form of a numerical table. Hereinafter, the present invention will be specifically explained with reference to an illustrated embodiment. Figure 3 is a conceptual system diagram, where 1 is a gas engine, 2
is mixer, 3 is throttle valve, 4 is governor, 5
is an opening detector of the throttle valve 3, 6 is a gas injector, 7 is an exhaust pipe, 8 is a three-way catalyst, 9 is an O ₂ sensor, 10 is an exhaust gas circulation passage 11 for EGR control
12 is a rotation speed sensor, 1 is an EGR valve installed in
3 is a microcomputer. After the fuel gas 15 and air 16 are mixed in the mixer 2, they are supplied to the engine 1 through the throttle valve 3, and the exhaust gas 17 discharged into the exhaust pipe 7 is exhausted through the three-way catalyst 8. Throttle valve 3 is governor 4
The opening degree is controlled by the opening degree detector 5, and the valve opening degree is detected by the opening degree detector 5, the output of which is sent to the microcomputer 13. The O ₂ sensor 9 detects the oxygen concentration in the exhaust gas 17 , and its output is sent to the microcomputer 13 , which controls the gas injector 6 to additionally supply fuel gas 18 to the mixer 2 .
The rotation speed sensor 12 detects the rotation speed of the engine 1, and its output is sent to the microcomputer 13. The microcomputer 13 includes a CPU 21,
It is equipped with a ROM 22, a RAM 23, an I/O interface 24, a system bus line 25, etc., and the output of the opening detector 5 using a potentiometer, for example, is sent to the multiplexer A/D converter 2.
6, it is converted into a digital quantity and inputted to the microcomputer 13, and the output of the rotation speed sensor 12, which consists of a pulse detector, for example, is counted by a counter 27 and inputted to the microcomputer 13.
3 is input. The ROM 22 stores arithmetic control programs and the throttle valve opening relative to the engine speed in the form of a numerical table, and the CPU 21 stores the throttle valve opening corresponding to the engine speed detected by the rotation speed sensor 12. Read out the numerical value θn of
It is compared with the detected value θm of the throttle valve opening detected by the opening detector 5. When the detected value θm is smaller than the numerical value θn, it is determined that the load is small and lean combustion is desirable, and an output is output from the I/O interface 24 and a signal from the power transistor array 28 is output. Tsute EGR valve 10
open. The air-fuel ratio of the mixer 2 is adjusted in advance to perform lean combustion, and the engine 1 is operated in a lean combustion mode with λ=1.4, for example, and under EGR control. Further, when the detected value θm is larger than the numerical value θn, it is determined that the load amount is large and stoichiometric air-fuel ratio combustion is desirable, and a signal from the power transistor array 28 is used to control the EGR valve 1.
The I/O interface 24 outputs an output corresponding to the detected value of the O ₂ sensor 9 which is input via the multiplexer A/D converter 26, and the gas injector is closed by the signal from the power transistor array 28. 6 to control the air-fuel ratio so that λ=1, and the engine 1 is operated in the stoichiometric air-fuel ratio combustion mode. At this time, the exhaust emissions tend to increase as described above, but the action of the three-way catalyst 8 suppresses the exhaust emissions contained in the actual exhaust gas to a low level. FIG. 4 shows a control flowchart for the above-mentioned operation. By the way, the above explanation is a theoretical explanation,
When actually controlled according to the flowchart shown in Fig. 4, even if the load increases and the combustion mode is switched to the stoichiometric air-fuel ratio combustion mode, the throttle valve 3
The pulsation of the fuel may cause the fuel to return to the lean burn mode, resulting in a cycling phenomenon. Therefore, in reality, the numerical value θn ₁ of the throttle opening is the threshold when switching from the lean burn system to the stoichiometric air-fuel ratio combustion system, and the numerical value θn ₂ is the threshold when switching vice versa. With a slight difference between θn ₁ > θn ₂
It is desirable to set this so that the switching operation has hysteresis. Figure 5 is a control flowchart in this case, and in step 1 on the left, the detected value θm and the numerical table value _θn1 are compared to determine whether to switch to the stoichiometric air-fuel ratio combustion method and to perform the switching operation. and in step 2 on the right,
The detected value θm and the numerical table value _θn2 are compared to determine whether to switch to the lean burn system and to perform the switching operation. Therefore, the detected value θm is θn ₁ > θm
> θn ₂ , switching is not performed,
Since the current state is maintained, cycling phenomenon is prevented and stable operation is achieved. Note that step 1 and step 2 are
For example, the steps can be performed alternately at fixed intervals using a software timer, or by waiting for an interrupt signal and performing appropriate steps. The attached table shown below shows an example of a numerical table when performing the above-mentioned control, and the throttle opening is a dimensionless number.

[Table] As is clear from the description of the embodiments above, the present invention switches between the lean burn method and the stoichiometric air-fuel ratio combustion method according to the load amount, and the lean burn method has the advantage of high thermal efficiency and high maximum output. By taking advantage of both the advantages of the stoichiometric air-fuel ratio combustion method, it is possible to operate while ensuring high thermal efficiency and maximum output over the entire operating load range, and the advantages include reducing exhaust emissions and reducing engine capacity. There is.

[Brief explanation of drawings]

FIG. 1 is an example of a characteristic diagram showing the relationship between thermal efficiency and output of the gas engine according to the present invention, FIG. 2 is an example of a characteristic diagram showing the relationship between exhaust emission and the excess air ratio, and FIG. FIG. 4 is a conceptual system diagram of one embodiment, FIG. 4 is a control flowchart, FIG. 5 is a control flowchart of another modification, and FIG. 6 is a diagram showing the configuration of the present invention. 1... Gas engine, 2... Mixer, 3... Throttle valve, 5... Opening degree detector, 6... Gas injector, 8... Three-way catalyst, 9... O ₂ sensor, 1
0...EGR valve, 11...Exhaust gas circulation passage, 12
...Rotation speed sensor, 13...Microcomputer, 21...CPU, 22...ROM.

Claims (1)

[Scope of Claims] 1. A rotation speed detection means for detecting the engine rotation speed, an opening detection means for detecting the throttle valve opening as a substitute characteristic for the load amount, and a mixer adjusted in advance to perform lean combustion. Under the effective action of the EGR valve that opens and closes the exhaust gas circulation passage for EGR control and the three-way catalyst, additional fuel is controlled from the gas injector according to the oxygen concentration in the exhaust gas to achieve stoichiometric air-fuel ratio combustion. A stoichiometric air-fuel ratio control means for controlling the air-fuel ratio, and a load amount when switching the air-fuel ratio from a lean burn system to a stoichiometric air-fuel ratio combustion system, and when switching from a stoichiometric air-fuel ratio combustion system to a lean combustion system, respectively, with respect to the engine rotation speed. A storage means for storing the relationship between throttle valve opening degrees in the form of a numerical table, and comparing the detected throttle valve opening degree with the numerical table value of the throttle valve opening degree corresponding to the engine speed at that time according to the numerical table. However, the lean combustion method uses the throttle valve opening according to this numerical table as a threshold value to open the EGR valve and turn off the gas injector control.
An air-fuel ratio control device for a gas engine, comprising: switching means for switching between a stoichiometric air-fuel ratio combustion method in which an EGR valve is closed and gas injector control is turned on;