JPS6122904B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in an air-fuel ratio control device using a membrane structure sensor with a heater that indirectly detects the air-fuel ratio from the oxygen concentration of exhaust gas.

In the case where a three-way catalyst is installed in the exhaust passage of the engine to remove harmful substances from the exhaust gas, the conversion efficiency of the three-way catalyst is the best for the exhaust gas at the stoichiometric air-fuel ratio. It is preferable to control the air-fuel mixture so that it is always near the stoichiometric air-fuel ratio.

Furthermore, the stoichiometric air-fuel ratio control of the air-fuel mixture is also advantageous in terms of engine output, that is, fuel efficiency.

Therefore, a sensor that detects oxygen concentration is installed in the exhaust passage upstream of the catalyst, and from this detection signal, the air-fuel ratio of the exhaust gas before combustion is indirectly detected, and the fuel supply amount is controlled according to this detection signal. Engines have heretofore been known in which feedback control is performed so that the air-fuel mixture always maintains the stoichiometric air-fuel ratio.

Various sensors have been proposed for use in this engine, one of which is, for example, a membrane structure oxygen sensor with a heater as shown in FIG.
(Unexamined Japanese Patent Publication No. 53-39789) That is, the platinum wire heater 3 is embedded between the upper alumina substrate 1 and the lower alumina substrate 2,
A substrate 4 of a sensor element section with a heater installed therein is formed.

On this substrate 4, an inner platinum electrode 6, a solid electrolyte 7, and a detection gas side platinum electrode 8, which constitute the sensor element main body 5, are sequentially stacked in a thick film shape (approximately 20 microns thick). ing.

The substrate 4 and the element main body 5 constitute a sensor element, the periphery of which is covered with a protective layer 9.

In addition, lead wires 1 are connected to the platinum electrodes 6 and 8, respectively.
0 and 11 are connected, and two lead wires for supplying electricity to the platinum wire heater 3 are also connected.

Among these lead wires, lead wire 11 and heater 3
is internally connected to one lead wire, and taken out by a lead wire 14 as a common ground wire.

The other lead wire of the heater 3 is taken out as a lead wire 12, and therefore the lead wires 10 and 14 are used as sensor terminals, and the terminal system for taking out the sensor signal is taken out, and the lead wires 12 and 14 are used as heater terminals.
Terminal systems for supplying electricity to the heaters are configured respectively.

In the oxygen sensor having the above configuration, the output terminal, that is, the lead wire 10 ,14
An electromotive force is generated between the two.

By the way, the surrounding protective layer 9 has countless holes that allow gas molecules to pass through without any problem. Therefore, when this sensor is exposed to a gas to be detected, there are no holes near the platinum electrode 8 on the detection gas side. A gas to be detected is introduced.

Therefore, if the oxygen concentration near the inner platinum electrode 6 is always kept constant as a reference value, an electromotive force corresponding to the oxygen concentration in the gas to be detected can be obtained, and the oxygen concentration can be detected.

To achieve this, a constant current is passed from the inner platinum electrode 6 through the solid electrolyte 7 to the detection gas side platinum electrode 8 via the lead wires 10 and 14, and the anions trapped in the platinum electrode 8 are A certain oxygen ion is sent to the inner platinum electrode 6 through the solid electrolyte 7 at a constant rate, and depending on the degree of activity of the solid electrolyte 7 (determined by the temperature of the solid electrolyte 7), it naturally flows from the vicinity of the platinum electrode 6 to the outside of the sensor, etc. The oxygen concentration near the inner platinum electrode 6 is always maintained at a constant reference value by offsetting and compensating for the amount of oxygen that diffuses and escapes.

Note that in order to operate this sensor well, it is necessary to maintain the sensor element main body 5 at a relatively high predetermined temperature, and for this reason, the heater 3 is usually
The element main body 5 is heated by this.

As a device for supplying a constant current between the inner platinum electrode and the detection gas side platinum electrodes 6 and 8 of this sensor, as well as a heater current for the heater 3, a device as shown in FIG. 2 has conventionally been used.

That is, a constant current is supplied to the sensor element section 15 from a power source 18 through a well-known constant current circuit composed of a field effect transistor 16 and a resistor 17, and a constant current is supplied to the sensor element section 15 from a power source 18. Heater current is supplied directly from the power supply 18 to the heater current.

By the way, when the air-fuel ratio is subjected to feed-back control, the air-fuel ratio periodically fluctuates above and below the target value under the influence of the feed-back control.
The sensor output signals generated at both ends of the sensor also fluctuate in level with a slow period of about several hertz.

This sensor output signal is then compared with a predetermined reference level B in a comparator 20,
For example, after being converted into a pulse signal, it is sent as an air-fuel ratio detection pulse signal to a control unit for controlling the air-fuel ratio.

Generally, the internal resistance of the sensor element section 15 changes according to its temperature, so the so-called DC level of the sensor output signal excluding the fluctuation component of the voltage value also changes according to the sensor temperature.

For example, in the case of a cold start, while the exhaust temperature gradually rises, the heating by the heater 19 is also gradual;
As shown in FIG. 3, the DC level of the sensor output signal A1 gradually decreases from engine startup C1.

For example, by momentarily cutting off the fuel supply,
When the air-fuel ratio is made leaner, the sensor output signal A
1 crosses the reference level B1 and the sensor output signal drops to a point where at least some kind of air-fuel ratio detection pulse signal is output from the comparator 20, at which point the air-fuel ratio control can be started for the first time. .

Normally, it takes a relatively long time of about 2 minutes from start-up C1 to control start point D1, and during this time open control is performed with a slightly richer air-fuel ratio, but this is disadvantageous in terms of exhaust emissions and fuel efficiency. becomes.

The present invention has been made in view of these conventional problems, and an object of the present invention is to provide a device that shortens the time from activation to the point at which control can be started.

This will be explained below with reference to the drawings. FIG. 4 is a circuit diagram showing essential parts of an embodiment of the present invention.

In the figure, 31, 32, 33 are switches;
4 and 35 are comparators, 36 is a diode, 37 is a capacitor, 38 is a resistor, and 39 is an amplifier.

The other parts have the same configuration as those in FIG. 2, and are given the same reference numerals.

The comparator 34 detects a sensor temperature (for example, 200° C.) at which the sensor works to a certain extent and closes the switch 31.

Specifically, taking advantage of the fact that the internal resistance of the heater 19 inside the sensor changes simultaneously according to the sensor temperature, a resistor 38 is inserted between the power supply 18 and the heater 19, and the internal resistance changes according to the internal resistance. The voltage across the heater 19 is made to vary, and the sensor temperature is detected by comparing this voltage with a predetermined reference voltage E.

The comparator 35 detects a state in which an air-fuel ratio detection pulse signal is obtained and feedback control of the air-fuel ratio is possible even if the sensor operates sufficiently and sends the sensor output signal as it is to the comparator 20, and switches the switch 3.
2 is opened, while switch 33 is closed.

Specifically, when the DC level of the sensor output signal obtained via the amplifier 39, the diode 36, and the capacitor 37 falls below a predetermined reference voltage F that provides a guideline for starting feedback control, the switch is activated. Switch the contacts 32 and 33.

Next, the overall operation will be explained with reference to FIG.

When the engine is started during a cold start, the sensor temperature gradually rises, so the sensor output signal A2 starts to gradually drop from the starting point C2.

Then, at time G2 when the sensor temperature rises to a point where the sensor is working to some extent, the comparator 34 closes the switch 31.

Naturally, at this point in time G2, the sensor is not operating sufficiently, so the comparator 35 switches to the switch 32.
is closed and switch 33 is opened.

Therefore, an air-fuel ratio setting signal H having a pulse shape (or a sawtooth wave shape or a sine wave shape) resembling the air-fuel ratio detection pulse signal is applied to the switches 31 and 3 in advance.
2 to an air-fuel ratio control unit (not shown).

By the way, from starting point C2 to this point G2, open control is performed to provide a slightly richer air-fuel ratio in order to ensure a good start.
Further, from this point in time G2, the air-fuel ratio is controlled in accordance with the air-fuel ratio setting signal H.

As a result, the air-fuel ratio fluctuates up and down at a period corresponding to the signal H, and the sensor output signal A2 starts to fluctuate periodically in accordance with this.

This fluctuation causes the sensor output signal A2 to change to the comparator 2.
The time point D2 at which the value falls below the reference level B2 to 0 is earlier than in the conventional case (see FIG. 3). At this time D2, it is possible for the comparator 20 to output some kind of air-fuel ratio detection pulse signal, and in consideration of this, the comparator 35 switches the switch 32,
The value of the reference voltage F is set in advance to switch the 33 contacts.

Therefore, almost at the same time as point D2, switch 3
2 is opened, while the switch 33 is closed and the air-fuel ratio setting signal H is cut off, while the sensor output signal A2 is sent to the comparator 20, and the air-fuel ratio detection pulse signal is output to the control unit.

As a result, feedback control of the air-fuel ratio is started, and as a result, the feedback control of the air-fuel ratio is increased by an amount corresponding to the periodic fluctuation of the sensor output signal A2 caused by the air-fuel ratio setting signal H compared to the conventional method (see Fig. 3). Control can be started earlier.

As described above, according to the present invention, once the temperature rises to a point where the sensor can function normally,
Since an air-fuel ratio setting signal that periodically changes the air-fuel ratio up and down in a manner similar to the air-fuel ratio detection signal is output in advance, the sensor output periodically pulsates up and down in response to fluctuations in the air-fuel ratio. Compared to the conventional system where the sensor output changes statically, the time when the sensor output value crosses the reference level of the comparator, which is the comparison standard for the air-fuel ratio, will be earlier due to the downward pulsation, and the sensor output value will start up during a cold start. The time from the start to the start of air-fuel ratio feedback control can be shortened compared to the conventional method, and therefore fuel efficiency and exhaust purification performance can be improved.

Further, when shifting to air-fuel ratio feedback control, operations such as cutting off fuel supply are not required.

Furthermore, in other words, the period of time when the air-fuel ratio reaches a slightly richer air-fuel ratio before switching to feedback control is shortened, so that a large amount of oxygen escapes from the inner platinum electrode inside the sensor to the exhaust gas with a low oxygen concentration. Also, it is possible to prevent the oxygen concentration near the electrode from falling below the reference value.

[Brief explanation of the drawing]

Fig. 1 is a perspective view showing an oxygen concentration sensor, Fig. 2 is a circuit diagram showing a conventional device that supplies inflow current and heater current to the sensor, and Fig. 3 is a timing diagram showing the operation of a conventional air-fuel ratio control device. FIG. 4 is a circuit diagram showing essential parts of an embodiment of the present invention, and FIG. 5 is a time chart showing its operation. 3...Platinum wire heater, 5...Sensor element section main body, 15...Sensor element section, 16...Field effect transistor, 17...Resistor, 18...Power source, 1
9... Heater, 20... Comparator, 31, 32, 3
3...Switch, 34, 35...Comparator, 37...
...Capacitor, 38...Resistor, E, F...Reference voltage, H...Air-fuel ratio setting pulse signal, A2...Air-fuel ratio detection pulse signal, B2...Reference level.

Claims (1)

[Scope of Claims] 1. A membrane structure sensor with a heater that indirectly detects the air-fuel ratio from the oxygen concentration in the exhaust gas of an engine, and an air-fuel ratio detection pulse signal by comparing the sensor output signal with a predetermined reference level. and means for controlling the air-fuel ratio in an open manner until the sensor output signal drops to a predetermined level at the time of starting, and means for controlling the air-fuel ratio in an open manner until the sensor output signal drops to a predetermined level at startup, In a device that performs feedback control of an air-fuel ratio, means for detecting a state in which the sensor temperature exceeds a predetermined value, and a predetermined air-fuel ratio setting signal that periodically raises and lowers the air-fuel ratio in place of the open control signal when the state is detected. and a means for sending the information to the control unit. means for detecting a state in which feedback control based on an air-fuel ratio detection signal is possible;
An air-fuel ratio control device comprising means for sending an air-fuel ratio detection signal to a control unit in place of the air-fuel ratio setting signal when detecting the condition. 2. The air-fuel ratio control device according to claim 1, wherein the sensor temperature detection means is means for detecting a change in internal resistance of a heater of the sensor. 3. The air-fuel ratio control device according to claim 1 or 2, wherein the air-fuel ratio setting signal has a waveform of any one of a pulse shape, a sawtooth wave shape, and a sine wave shape.