JPS59215951A - Air-fuel ratio control device for gas engine - Google Patents

Abstract

PURPOSE:To change over smoothly and quickly air-fuel ratio by setting the increasing ratio of injector duty higher than the normal one in changing over a control device for changing over the air-fuel ratio according to load in the direction of decreasing the air-fuel ratio. CONSTITUTION:A gas engine 1 is supplied with a mixture of fuel gas 15 and air 16 mixed by a mixer 2 through a throttle valve 3 the opening of which is controlled by a governor 4. Also, O2 concentration of exhaust gas 17 in an exhaust pipe 7 is detected by an O2 sensor 19 by the output of which is controlled a gas injector 6 to supply additionally fuel gas 18 to the mixer 2. Further, an EGR valve 10 is opened and closed according to the output of an opening detector 5 to change over the lean combustion system to theoretical air-fuel ratio combustion system or vice versa. Thus, in changing over the air-fuel ratio to the direction of reducing said ratio, the increasing ratio of injector duty is set higher than the normal one while said duty is limited to a limit value determined by the rotational frequency of the engine.

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 in accordance with the load condition LK of the engine.

First, the combustion characteristics of the transverse motion combustion method and the bright air-fuel ratio combustion method in a gas engine will be explained. Figure 1 shows the air-fuel ratio, that is, the case of excess air ratio ^-1,4, and the case of λ=
This shows an example of the relationship between thermal efficiency and output in the case of λ-1,4, with the solid line showing the case of λ-1,0.
The case is shown by the broken line.

Comparing the same rotation speed, V! for λ-1 and 4 in both cases. :, thermal efficiency is high but maximum output is low, λ-1
, 0 has contradictory advantages and disadvantages, such as low thermal efficiency but high maximum output. In addition, Figure 2 shows an example of the relationship between exhaust emissions and thermal efficiency with respect to excess air ratio. From this figure, #exhaust emissions are lower in lean combustion, and higher near λ-1,0. I understand that. However, if you use a three-way catalyst and make it work effectively even around λ-1,0, it is possible to keep the exhaust emissions low, so the exhaust emissions can be considered to be the same for both types. I can do it.

Focusing on these points, we developed the lean burn system and Pl! The stoichiometric combustion method can be selected depending on the load. At low loads, where a small output is required, the combustion method switches to a lean combustion method with high thermal efficiency. At high loads, when a large output is required, the stoichiometric air-fuel ratio combustion method provides a high maximum output. It is conceivable that the gas engine could be operated effectively by switching to the two methods and taking advantage of the advantages of each method.

To do this, for example, an air-fuel ratio control means is provided that allows combustion to occur in a region where the air-fuel ratio is low by using a button that additionally supplies a predetermined amount of fuel from a gas injector depending on the oxygen concentration in the exhaust gas, and The air-fuel ratio may be switched according to the load by turning on/off the operation of the air-fuel ratio control means using the throttle valve opening set according to the number as a threshold value. In the case of λ-1,4 to λ-1,0 at low speed rotation.
In the worst case scenario, there is a possibility that the engine will stop and fail. This is because there is a delay in the output of the 0□ sensor that detects oxygen concentration, so a detection output indicating that the fuel is in a rich state cannot be obtained immediately after switching, and the air-fuel ratio control means overreacts. When the duty of the gas injector becomes 100% and the throttle valve opening is small and the engine rotates at low speeds, the amount of fuel added to the air-fuel mixture becomes relatively large, causing the actual air-fuel ratio to exceed 1.0 and become excessively rich. It's for a reason.

The present invention has focused on this point, and has been devised for the purpose of providing an air-fuel ratio control device for a gas engine that can smoothly and quickly switch the air-fuel ratio. In a gas engine that is equipped with a control means and an air-fuel ratio switching means for turning on and off the operation of the air-fuel ratio control means, the air-fuel ratio is switched according to the load, and the limit values for engine speed and injector duty are provided. storage means for storing the relationship between the two in the form of a numerical table; and a storage means for setting an increase rate of the injector duty higher than during steady operation when switching to a direction of decreasing air-fuel ratio, and storing the injector duty in the storage means. and arithmetic means for outputting a control output to limit the limit value to the specified limit value. The limit value of the injector duty may be determined by conducting a test in advance for each type of engine to find an appropriate value for each engine speed.

Therefore, according to the present invention, the injector duty is quickly increased when switching to the direction of decreasing the air-fuel ratio, and the air-fuel ratio is not increased unnecessarily beyond the appropriate value corresponding to the engine speed. By keeping the air-fuel ratio at an appropriate value, it is possible to smoothly and quickly switch the air-fuel ratio, reducing exhaust emissions over the entire operating load, and creating a gas engine that can ensure high B thermal efficiency and maximum output. The practicality of the system is greatly improved.

The present invention will be specifically explained below with reference to an illustrated example.〇@3 Figure is a conceptual system diagram, (!) is a gas engine, (2)
is the mixer, (3) is the throttle valve, (4) is the governor,
(5) is the opening detector of the throttle valve (3), (6) is the gas injector, (7) is the exhaust pipe, (8) is the three-way catalyst,
f91tj: Ox sensor, f+o) it EGR control] tl-gas circulation passage (! EGR valve installed in the EGR valve, (I2) is a rotation speed sensor, and θ□□□ is a microcomputer.

The fuel gas (15) and air (1G) are mixed in the mixer (2), then supplied to the engine f1+ via the throttle valve (3), and the exhaust gas (17) is discharged into the exhaust pipe (7).
is exhausted through a three-way catalyst (8). Throttle valve (3
) The valve opening is controlled by the valve governor (4), and the valve opening is detected by the opening detector (5), the output of which is sent to the microcomputer 03). The sensor (9) detects the oxygen concentration in the exhaust gas (I7), and its output is sent to the microcomputer O■, which controls the gas injector (6) to additionally supply fuel gas (18+) to the mixer (2). The rotation speed sensor 02) detects the rotation speed of the engine fi+, and its output is sent to the microcomputer 0.

Microcomputer 0 (6) is CPU 12]),
The system is equipped with RO8 ((Foundation), RAM (Transfer)), I10 interface (4), system bus line value (5), etc. Prec'/-A/D converter (26+
It is converted into a digital quantity by the microcomputer 03)
The output of a rotational speed sensor (I2) consisting of, for example, a pulse detector is inputted to the microcomputer 0'4 after the number of pulses is decremented by a decoder (27). The ROM (2'3) stores calculation control programs and the throttle valve opening relative to the engine speed in the form of a numerical table.The CPU 1) stores the engine speed detected by the engine speed sensor 02). Numerical value θ of throttle valve opening corresponding to rotation speed button
n is read out and compared with the detected value θm of the throttle valve opening detected by the opening detector (5). And the detected value θ.

is the numerical value θ. When it is smaller, it is determined that the load is small and lean combustion is desirable, an output is output from the I10 interface (241), and the EGR valve (10) is opened by a signal from the power transistor array (241).

The mixer (2) sets the air fuel ratio to S in advance to achieve lean combustion.
With +1 adjustment, the engine (1) is operated, for example, in a lean burn mode of λ-1.4, and under EGR control. If the detected value θm is larger than the numerical value θ, it is determined that the load is large and stoichiometric air-fuel ratio combustion is desirable, and the EGR valve (10) is activated by a signal from the power transistor array (c). The output corresponding to the detected value of the sensor (9), which is input via the multiplexer A/D comparator (6), is output from the I10 interface (4), and the gas is controlled by the signal from the power transistor array (2). Operate the injector (6) to control the air-fuel ratio so that the input is 1.0, and the engine full
d: Operated using stoichiometric air-fuel ratio combustion method. At this time, as described above, the exhaust emissions tend to increase, but the action of the three-way catalyst (8) keeps the exhaust emissions contained in the actual exhaust gas low. Figure wJ4 shows a control flowchart for the above-mentioned operation.

By the way, the above explanation is a theoretical explanation, and if the control is actually performed according to the flowchart shown in Fig. 4, for example, even if the load increases and the combustion mode is switched to the stoichiometric air-fuel ratio combustion method, the throttle valve There is a possibility that the pulsation in (3) causes the engine to return to the lean burn mode, resulting in a cycling phenomenon. Therefore, in reality, the numerical value θn1 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 θn2 is the threshold when switching vice versa. The setting is made such that θn1 and θl jump with a slight difference, thereby providing hysteresis in the switching operation. Fig. 5 is a control flowchart in this case, and in step 1 on the left side, the detected value θf and the numerical value θn1 are compared to determine whether to switch to the stoichiometric air-fuel ratio combustion method and to perform the switching operation. Do, step 2 on the right
In this case, the detected value θ is compared with the numerical value θl to determine whether to switch to the lean burn system and to perform the switching operation. Therefore, the detected value θ5 is 01111 >θ□
〉θl, no switching is performed and the state at that time remains '-j! Since this is maintained, cycling phenomenon is prevented and stable operation is obtained.

Furthermore, steps 1 and 2 can be performed alternately at regular intervals using a software timer, or by waiting for an interrupt signal and performing the appropriate steps.

The following Appendix Table 11d shows an example of a numerical table when the above-mentioned control is applied, and the throttle opening is a dimensionless number.

[Appendix Table 1] The air-fuel ratio control by additional supply of fuel from the gas injector (6) as described above is basically performed by following the portions of the subroutine control flowchart shown in Figure 6 that are not surrounded by broken lines. This is achieved by using it as an interrupt routine. That is, 0. Rich (RICI) obtained by comparing the detection signal from the sensor (9) with an appropriate reference value
/ Gas injector (6) driven by PWM (pulse width variable m) method based on the LEAN signal
In step 3, the duty of the applied voltage is controlled in such a way that it decreases in the case of richness and increases in the case of leanness. FIG. 7(a) shows that in such control,
This shows a general control time chart when switching from the lean burn method to the stoichiometric air-fuel ratio combustion method, and the time t
.

Switching is performed at , and the injector duty increases to a certain level. Then, increases and decreases within a certain range are repeated around this level, and the air-fuel ratio is macroscopically controlled to λ=1 while changing around λ-1 and 0.

There is no problem with the above operation in a steady state, but immediately after the air-fuel ratio is switched, that is, immediately after the stoichiometric air-twist ratio control is turned on, the injector duty is operated at the same rate of change as in the steady state. Therefore, at time t1, λ-
As shown in the figure, it takes -time for the values to become 1 and 0, and prompt switching is not performed. Therefore, in the present invention, the control within the broken line in FIG.
As shown as D+10, the rate of change in the injector duty immediately after switching, that is, the rate of increase, is greater than that during steady operation. As a result, the time tf at which the air-fuel ratio becomes λ-1,0 becomes earlier, and the required time -' is significantly shortened compared to the time jslc, as shown in FIG. 7(b).

On the other hand, if the increase rate of the injector duty is increased in the case of low speed rotation, the response delay of the 0□ sensor (9) will cause the actual detection signal to become rich before the detection signal of the 0□ sensor (9) becomes rich as described above. Since the air-fuel ratio becomes excessively rich, causing problems such as engine stoppage, the present invention limits the injector duty according to the engine speed. This limit is due to the fact that the actual air-fuel ratio is λ−
Check the maximum value of 1, 0 for each engine speed, and set this as the limit value ■ RO in the form of a numerical table for the engine speed.
M (24), and in step 4 in FIG. 6, this limit value ID is compared with the actual injector duty ID to limit the increase in the injector duty.This is done using the K button. Therefore, the injector duty increases rapidly up to the limit value IDn corresponding to the engine speed KM at that time, as shown in Fig. 7(b), and exceeds the limit value more than necessary as shown by the dashed arrow. This means that the engine speed will not increase, and situations such as engine stoppage will be prevented and smooth and prompt switching will be performed.

Attached Table 2 shown below is an example of a numerical table showing the relationship between engine speed 2 and the limit value of injector duty.

The control to smoothly and quickly perform the above-mentioned switching is carried out immediately after switching from the lean burn method to the stoichiometric air-fuel ratio combustion method.
A steady state flag is provided to determine that 0.3) is immediately after an air-fuel ratio switch, and in the main routine shown in FIG. The steady state flag is reset before the stoichiometric air-fuel ratio control by the sensor and gas injector is turned on, and the button steady state flag is reset immediately after the detection signal of the 0□ sensor (9) becomes rich, as shown within the broken line in Figure 6. It is recommended to preset it.

As is clear from the explanation of the above embodiments, zero inventions, for example, switch between lean burn mode and stoichiometric air-fuel ratio combustion mode depending on the load, and the advantage of lean burn mode is that it has high thermal efficiency, and the theory that the maximum output is high. The aim is to take advantage of both the advantages of the air-fuel ratio combustion system and operate while ensuring high thermal efficiency and maximum output over the entire operating load range, as well as reduce exhaust emissions and reduce engine capacity. In such a gas engine, the injector duty can be quickly increased to a limit value equal to the engine rotational speed VCL when switching in the direction of decreasing the air-fuel ratio, and the air-fuel ratio can be switched smoothly and quickly.

[Brief explanation of the drawing]

Figure 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, and Figure 2 is an example of a characteristic diagram showing the relationship between excess air ratio and exhaust emissions. Figure 3 and subsequent figures are a book showing one embodiment of the present invention, so Figure 3 is a conceptual system diagram, Figure 4 is a basic control flowchart, Figure 5 is a practical main routine control flowchart,
Figure 6 is a control flowchart of the subroutine, Figure 7 (
a) is a time chart for general control, Figure 7 (
b) is a time chart in the case of the improved control according to the present invention. fl)...Gas engine, (2)...Mixer, [3)
... Throttle valve, (5) ... Opening degree detector, (6)
...Gas injector, (8) ...Three-way catalyst, (9
)...02 sensor, (101...EGR valve, (11
)...Exhaust gas circulation passage, (I2)...Rotation speed sensor, Gate...Micro convenience store, (2t+...C
PtJ, (3)...ROM 0 (0n) jte
q4 Ruji ←

Claims (1)

[Claims] (11) An air-fuel ratio control means for performing combustion in a low air-fuel ratio region by additionally supplying a predetermined amount of fuel from a gas injector; and an air-fuel ratio switching means for switching the air-fuel ratio depending on the load by turning on and off the operation of the air-fuel ratio control means using the throttle valve range set as a threshold value, the engine speed and A storage means for storing the relationship with the limit value of the injector duty in the form of a numerical table; An air-fuel ratio control device for a gas engine, comprising: arithmetic means for outputting a control output to limit the amount of the fuel to a limit value stored in the storage means.