GB2071362A - Air-fuel ratio control system - Google Patents

Abstract

An air-fuel ratio control system for an internal combustion engine has on-off type electro-magnetic valve means 14, 15 for correcting the ratio of the mixture supplied to the engine, means 29 for producing a periodical dither signal, a shift control circuit 28 for shifting the level of the center of the dither signal, a driving circuit 30 for the on-off type electro-magnetic valve means, and an O2 sensor 19 for detecting the concentration of oxygen in exhaust gases passing through the exhaust passage. A timing circuit 23 is provided for detecting the period of the time when the output of the O<6> sensor is higher than a predetermined level and the period of the time when the output is lower than the predetermined level for producing an output signal in dependency on the difference between the periods. A shift signal generating circuit 27 is provided for generating a shift signal in dependency on the output of the timing circuit. The shift control circuit means is so arranged to control the air-fuel ratio of the mixture in such a direction that the difference is decreased. <IMAGE>

Description

SPECIFICATION Air-fuel ratio control system The present invention relates to a system for controlling the air-fuel ratio for an internal combustion engine emission control system suitably having a three-way catalyst, and more particularly to a system for controlling the air-fuel ratio to a value approximating to the stoichiometric air-fuel ratio so as to effectively operate such a three-way catalyst.

Such a system may be a feedback control system, in which an 02 sensor is provided to sense the oxygen content of the exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of the air-fuel mixture supplied by a carburettor. The control system comprises a comparator for comparing the output signal of the oxygen sensor with a predetermined value, an integrating circuit connected to the comparator, a driving circuit for producing square wave pulses from the output signal of the integrating circuit, and an on-off type electro-magnetic valve for correcting the air-fuel ratio of the mixture.The control system operates to judge whether the feedback signal from the O2 sensor is higher or lower than a predetermined reference value corresponding to the stoichiometric air-fuel ratio for producing an error signal for actuating the onoff type electro-magnetic valve to thereby control the air-fuel ratio of the mixture.

In the conventional system, as shown in Figs.

1 a and 1 b, the output waveform P, of the O2 sensor varies from the maximum output voltage thereof to the minimum one, because the O2 concentration in the exhaust gases exceeds values corresponding to the maximum and minimum outputs due to the control delay of the control system. As shown in Fig. 1 a, the output voltage of the 02 sensor varies steeply at a reference voltage VA which corresponds to the output voltage by exhaust gases when a mixture of the stoichiometric air-fuel ratio (St) is supplied to the engine and burned.Therefore, it may be regarded that a middle value M between the maximum and minimum values in each cycle of the output waveform of the 02 sensor is constant and substantially equal to the voltage VA corresponding to the stoichiometric air-fuel ratio.

Thus, in the conventional system, the middle value M is set as the reference value of the comparator for judging the air-fuel ratio of the mixture supplied to the engine.

On the other hand, Japanese Patent Application No. 54-98853 (British Patent App!ication No. 8025010) discloses a system intended for improvement of control delay in such a conventional system, in which the on-off electro magnetic valve is operated by a dither signal having a high frequency and a small amplitude.

However, it is not proper to use the middle value of each cycle in the output waveform P2 of the O2 sensor dependent on the dither signal D as a reference value, because the middle value of the output waveform of the 02 sensor does not always coincide with the reference voltage VA corresponding to the stoichiometric value, as shown in Fig. 1 b. Therefore, the middle value cannot be used as the reference value.

The present invention seeks to provide an airfuel ratio control system which controls the airfuel ratio to the stoichiometric air-fuel ratio without using a reference value as a stochiometric air-fuel ratio, whereby the air-fuel ratio may be exactly controlled to the stoichiometric air-fuel ratio.

According to the present invention there is provided an air-fuel ratio control system for an internal combustion engine having an intake passage, an exhaust passage, an air-fuel mixture supply means, on-off type electro-magnetic valve means for correcting the air-fuel ratio of the airfuel mixture supplied by the air-fuel mixture supply means, dither signal generating circuit means for producing a periodical dither signal, a shift control circuit means for shifting the level of the centre of the dither signal, driving circuit means for producing a driving output for the on-off type electro-magnetic valve means, and an O2 sensor for detecting the concentration of oxygen in exhaust gases passing through the exhaust passage, the improvement comprising first circuit means for producing a reference value, timing circuit means for detecting the period of the time when the output of said 02 sensor ;s higher than the reference value and the period of the time when the output of the 02 sensor is lower than the reference value for producing an output signal dependent on the difference between the periods, and shift signal generating circuit means for generating a shift signal dependent on the output of the timing circuit means, the shift control circuit means being so arranged to control the air-fuel ratio of the mixture in such a direction that the difference is decreased.

Other aspects and features of the present invention will be apparent from the following description of embodiments thereof with reference to the accompanying drawings, wherein Figures 1 a and 1 b are graphs showing output signals of the O2 sensor; Fig. 2 is a schematic view of a system according to the present invention Fig. 3 is a block diagram of an electronic control circuit of the system; Fig. 4 is a graph showing output waveforms of the 02 sensor; Fig. 5 shows an example of the relation between the output of a comparator and the shifting of the dither signal; Fig. 6 is a graph showing relation between the dither signal and the operation of the valve; Fig. 7 is a graph showing the operation of the system of the present invention; Fig. 8 shows an example of the electronic control circuit;; Fig. 9 shows a block diagram of another embodiment of the present invention; and Fig. 10 is a graph showing output waveform of the O2 sensor in the system of Fig. 9.

Now describing the principle of the present invention referring to Fig. 4, the figure shows the output waveform of the O2 sensor when the level of the output voltage is lower than the reference voltage VA, which means that lean air-fuel mixture is supplied to the engine. Because of the lower output voltage, the bottom of the waveform is limited to a low voltage owing to the characteristics of the O2 sensor. Therefore, an upper half and a lower half of the waveform of each cycle are different in shape. The reference M, shows a middle value of the height of the wave.By such deformation of the wave, each of times T1 to T5 when the output voltage of the O2 sensor is higher than the middle value M1 is smaller than each of times Te toT9 when the output voltage is lower than the middle value M1. If the middle value M, between the maximum voltage and the minimurn voltage of each cycle in the output waveform coincides with the reference voltage VA, times of higher and lower portions of each cycle are equal.

Thus, in accordance with the present invention, the air-fuel ratio of the mixture is controlled so that the times of wave portions higher and lower than the middle value M may be equal.

Referring to Fig. 2, a carburettor 1 communicates with an internal combustion engine 2. The carburettor 1 comprises a float chamber 3, a venturi 4 formed in an intake passage, a nozzle 5 communicating with the float chamber 3 through a main fuel passage 6, and a slow port 10 provided near a throttle valve 9 in the intake passage communicating with the float chamber 3 through a slow fuel passage 11. Air correcting passages 8 and 13 are disposed in parallel to a main air bleed 7 and a slow air bleed 12, respectively. On-off type electro-magnetic valves 14 and 15 are provided for the air correcting passages 8 and 13, respectively. Inlet ports of each on-off electro-magnetic valves 14 and 15 respectively communicates with the atmosphere through an air filter or air cleaner 1 6.An O2 sensor 1 9 is disposed in an exhaust pipe 1 7 which communicates with the internal combustion engine. The sensor 19 detects the oxygen content of the exhaust gases. A three-way catalytic converter 1 8 is provided in the exhaust pipe 1 7 downstream of the O2 sensor 1 9. The output signal of the 02 sensor 1 9 is applied to an electronic control circuit 20 of an electronic control system. The electronic control circuit 20 operates to correct the air-fuel ratio of the air-fuel mixture provided by the carburettor 1.

Fig. 3 shows the block diagram of the electronic control circuit 20.

The output of the 02 sensor 1 9 is connected to an output detecting circuit 21. One of outputs of the circuits 21 is connected to a timing circuit 23 through a wave height middle value detecting circuit 22 and the other is directly connected to the timing circuit 23. The output of the timing circuit 23 is connected to a high level time detecting circuit 24 and to a low level time detecting circuit 25. Outputs of both circuits 24 and 25 are connected to a comparator 26 for comparing both outputs. Output of the comparator 26 is connected to a shift signal generating circuit 27 which is adapted to generate a shift signal dependent on the output signal of the comparator.

The output of the shift signal generating circuit 27 is connected to the shift control circuit 28 which acts to shift the center of the dither signal fed from a dither signal generating circuit 29 in dependency on the output of the shift signal generating circuit 27. The output of the shift control circuit 28 is fed to on-off type electromagnetic values 14 and 1 5 through a driving circuit 30 for actuating the valves 14 and 1 5 so as to control the air-fuel ratio of the mixture.

In accordance with the present invention, the shift control circuit 28 operates to shift the center of the dither signal in such a direction that the difference between the outputs of circuits 24 and 25 is decreased. Thus, the air-fuel ratio of the mixture can be controlled to the stoichiometric airfuel ratio.

In such a case that there is a difference between the controlled air-fuel ratio and the stoichiometric air-fuel ratio due to an error of characteristic of the 02 sensor, the shift signal is modulated in accordance with a suitable function.

Fig. 5 shows an example of the modulation of the shift signal.

Fig. 6 shows the relation between the shifting of the dither signal and the duty ratio of the electro-magnetic valves. When the level of the dither signal is low, the duty ratio is small. The left half of Fig. 6 shows the condition when the dither signal deviates to the lower side and the right half shows when the dither signal is in a higher level.

From the figure, it will be seen that the air-fuel ratio of the mixture is controlled by shifting the dither signal.

For an engine having characteristics such that the exhaust gases contain a large amount of CO and a small amount of NOx and HC, the system should operate to reduce sufficiently the amount of CO. In order to reduce CO rather than NOx and HC, it is known that it is effective to control the air-fuel ratio to a slightly leaner side than the stoichiometric air-fuel ratio. On the contrary, in some cases, it is preferable to control the air-fuel ratio to the rich side. In the system of the present invention, it is easy to shift the air-fuel ratio of the mixture to either side. Fig. 7 shows an example of the lean side control. Dither variation X included in exhaust gases oscillates centering on a value corresponding to the stoichiometric value and the output of the 02 sensor is shown by X'. Reference Y shows a lean-controlled dither variation and Y' shows the output of the O2 sensor.

Fig. 8 shows an example of the electronic control circuit of the present invention. The same parts as in Fig. 3 are identified by the same numerals. In the circuit, operations of the output detecting circuit 21 ,timing circuit 23, and high and low level time detecting circuits 24 and 25 are included in other circuits 22 and 26.

Fig. 9 shows another embodiment of the present invention. In this system, a standard level circuit 31 is connected to the timing circuit 23 and a compensating circuit 32 is connected to the standard level in dependency on the temperature of the cooling water of the engine or opening degree of the throttle valve and the like. The standard level is applied to the timing circuit 23, so that the output of the 02 sensor 19 is compared with the standard level.

The high level time detecting circuit 24 and the low level time detecting circuit 25 detect the output of the timing circuit 23. Other operations are the same as the operations of the previous embodiment.

Fig. 10 shows the waveform of the O2 sensor.

The standard level is designated by S.

It will be understood that the system of the present invention may also be composed by digital circuit means.

Claims (5)

1. An air-fuel ratio control system for an internal combustion engine having an intake passage, an exhaust passage, an air-fuel mixture supply means, on-off type electro-magnetic valve means for correcting the air-fuel ratio of the airfuel mixture supplied by said air-fuel mixture supply means, dither signal generating circuit means for producing a periodical dither signal, a shift control circuit means for shifting the level of the centre of said dither signal, driving circuit means for producing a driving output for said onoff type electro-magnetic valve means, and an O2 sensor for detecting the concentration of oxygen in exhaust gases passing through said exhaust passage, the system further comprising first circuit means for producing a reference value; timing circuit means for detecting the period of the time when the output of said O2 sensor is higher than said reference value and the period of the time when the output of said 02 sensor is lower than said reference value for producing an output signal dependent on the difference between said periods; and shift signal generating circuit means for generating a shift signal dependent on the output of said timing circuit means; said shift control circuit means being so arranged to control the airfuel ratio of the mixture in such a direction that said difference is decreased.

2. An air-fuel ratio control system for an internal combustion engine according to claim 1, wherein said reference value is a middle value between the maximum value and the minimum value of the output voltage of said O2 sensor.

3. An air-fuel ratio control system for an internal combustion engine substantially as described herein with reference to Figures 2 to 8 or Figures 9 and 10.

4. An air-fuel ratio control system according to claim 1 substantially as described herein.

5. An internal combustion engine provided with an air-fuel ratio control system according to any one of the preceding claims.