GB2059643A - Temperature control system for oxygen sensor disposed in engine exhaust gas - Google Patents

Abstract

An internal combustion engine fuel control system includes a known oxygen sensor (18) which is provided with a heater (24) to ensure proper function and is disposed in an exhaust passage to provide a feedback signal indicative of the air/fuel ratio of the mixture supplied to the engine. Typically, the oxygen sensor (18) is of the concentration cell type utilizing a solid electrolyte. To avoid excessive and detrimental heating of the oxygen sensor (18) under a very high exhaust temperature condition such as full-throttle accelerating condition, a circuit to supply a heating current to the heater (24) is provided with a control system which comprises an operating condition detecting means (40) for detecting such a condition and then producing a command signal and an on-off type switching means (30) for breaking the current-supplying circuit in response to the command signal. <IMAGE>

Description

SPECIFICATION Temperature control system for oxygen sensor disposed in engine exhaust gas This invention relates to an internal combustion engine system. including an oxygen sen sor which is disposed in an exhaust passage to provide a feedback signal indicative of the air/fuel ratio of a mixture supplied to the engine and provided with an electric heater to maintain the sensor in an appropriately heated state during operation, and more particularly to a sub-system to control the supply of a heating current to the heater.

In recent internal combustion engines and particularly in automotive engines, it has become popular to perform feedback control of the air/fuel ratio with the objects of improving the efficiencies of the engines and reducing the emission of noxious or harmful substances contained in the exhaust gas by utilizing an oxygen sensor as a device that provides an electrical feedback signal indicative of the air/fuel ratio of a mixture actually supplied to an engine when disposed in the exhaust gas discharged from the engine.

At present, oxygen sensors practicaliy useful for this purpose are almost exclusively of the oxygen concentration cell type having a layer of an oxygen ion conductive solid electrolyte, as typified by zirconia stabilized with calcia or yttria, with two electrode layers (a measurement electrode layer and a reference electrode layer, both usually made of platinum) formed on the two opposite sides of the solid electrolyte layer. Where there is a difference in oxygen partial pressure between the measurement electrode side and the reference electrode side of the solid electromotive force of which the magnitude depends on the magnitude of the oxygen partial pressure difference.As an air/fuel ratio detector for the above mentioned purpose, the measurement electrode is exposed to an engine exhaust gas, while a reference oxygen partial pressure is maintained on the reference electrode side.

The most familiar method for the establishment of such a reference oxygen partial pressure is to expose the reference electrode layer to atmospheric air. Also it has been proposed to incorporate a metal-metal oxide mixture such as nickel and nickel oxide in the oxygen sensor as a source of a reference oxygen partial pressure. Furthermore, U.S. Patent No.

4,207,159 discloses the possibility of establishing a reference oxygen partial pressure without using any extra source material by forcing an externally supplied DC current to flow through the solid electrolyte layer between the two electrode layers so as to cause adequate migration of oxygen ions through the solid electrolyte layer toward a selected electrolyte layer.

In practice it is usual to provide an oxygen sensor of the solid electrolyte concentration cell type with an electric heater, irrespective of the manner of establishing a reference oxygen partial pressure, because the activity of the solid electrolyte represented by the level and stableness of the generated electromotive force remains unsatisfactorily low while the temperature of the solid electrolyte layer is relatively low, e.g. is below about 400"C, so that the sensor installed in an engine exhaust system becomes ineffective as an air/fuel ratio detector while the engine discharges a relatively low temperature exhaust gas, if the sensor should be heated solely by the heat of the exhaust gas. The electric heater is attached to or embedded in a structural member physically supporting the oxygen-sensitive part of the sensor.

In conventional engine systems including an oxygen sensor of the above described type provided with an electric heater, a heating current is continuously supplied to the heater of the sensor during operation of the engine, and in practice while a key switch for the ignition of the engine is in the on-state. We have recognized that such a heating method is unfavorable because there is a strong possibil ity of the oxygen sensor being heated to an excessively and detrimentaily high temperature while the engine discharges a considerably high temperature exhaust gas as in the case of operating under a full throttle accelerating condition.

Concerning an internal combustion engine system including an oxygen sensor which is provided with an electric heater to ensure proper function and disposed in an exhaust passage for the engine to provide an electrical signal indicative of the air/fuel ratio of an air/fuel mixture actually supplied to the engine and a current-supplying circuit to supply a heating current from a power source to the heater during operation of the engine system, it is an object of the present invention to provide an effective means for avoiding excessive and possibly detrimental heating of the oxygen sensor by the action of the heater while the engine discharges a very high temperature exhaust gas, e.g. during operation under a full-throttle accelerating condition, for thereby extending the service life of the oxygen sensor.

According to the present invention, the above stated engine system further comprises a current-supply control system which comprises an operating condition detecting means for producing and electrical command signal while the engine is operating under such a condition as causes a significant rise in the temperature of the exhaust gas and an on-off type switching means for breaking the aforementioned current-supplying circuit at least while the detection means produces the command signal.

Preferably, the operating condition detect ing means is embodied in throttle-position detector that provides the command signal while the degree of opening of a throttle valve for the engine is greater than a predetermined value, a fuel injection rate detector that produces the command signal while the rate of fuel injection is above a predetermined rate or an exhaust temperature detector that produces the command signal while the temperature of the exhaust gas is above a predetermined temperature.

The oxygen sensor in an engine system according to the invention is a known one and usually is of the oxygen concentration cell type having a layer of an oxygen ion conductive solid electrolyte, but there is no particular restriction on the design or construction of the concentration cell utilizing the solid electrolyte layer.

Figure 1 is a diagrammatic presentation of an internal combustion engine system including an oxygen sensor with which the present invention is concerned; Figure 2 is a circuit diagram showing a conventional method of supplying a current to an electric heater provided to an oxygen sensor employed in an engine system; Figure 3 is a circuit diagram showing an embodiment of a current supplying means according to the present invention for controlling the supply of a current to the heater of the aforementioned oxygen sensor; and Figure 4 is a circuit diagram showing another embodiment of a current supplying means according to the invention.

In Fig. 1, reference numeral 10 indicates an internal combustion engine, which may be an automotive engine, provided with an induction passage 1 2 and an exhaust passage 14.

Indicated at 16 is an electrically or electronically controlled fuel-supplying apparatus which may be either a carburetor or fuel injectors. To perform feedback control of the fuel-supplying apparatus 1 6 with the aim of supplying an air-fuel mixture of a predetermined optimum air/fuel ratio to the engine 10 during its normal operation, an oxygen sensor 18 is disposed in the exhaust passage 14 as a device to produce a feedback signal indicative of actual air/fuel ratio of the mixture supplied to the engine 10. This oxygen sensor 1 8 is of the concentration cell type having a solid electrolyte layer and is provided with an electric heater (though not shown in Fig. 1).An electronic control unit 20 receives the output of the oxygen sensor 1 8 and provides a control signal to the fuel-supplying apparatus 1 6 based on the magnitude of a deviation of the actual air/fuel ratio indicated by the output of the sensor 1 8 from the predetermined air/fuel ratio. Indicated at 22 is a circuit to supply a heating current of a predetermined intensity to the heater of the oxygen sensor 1 8. In practice, principal elments of this circuit 22 may be incorporated in the control unit 20.

Fig. 2 illustrates a typical example of conventional methods of supplying a heating current to an electric heater 24 of the oxygen sensor 1 8 in the engine system of Fig. 1. A current-supplying circuit to connect the heater 24 with a DC power source 26 such as a battery includes an electromagnetic relay 30 as an on-off type switching device, and the solenoid coil 30b of this relay is connected with the power source 26 via a key switch 28 for ignition of the engine 10 in Fig. 1. (Resistors in the heating current-supplying circuit and other various elements connected with the key switch 28 and power source 26 are omitted from the illustration.The output circuit for the oxygen sensor 1 8 is omitted, too.) When the key switch 28 is brought into the on-state to operate the engine 10, a pair of contacts 30a or the relay 30 are brought into the on-state to commence the supply of a heating current to the heater 24. Since the key switch 28 remains in the on-state during operation of the engine 10, the relay contact 30a too remain in the on-state regardless of the operating conditions of the engine 10.

Therefore, there is a possibility of overheating the oxygen sensor 1 8 under certain operating conditions where the exhaust gas temperature becomes very high.

Fig. 3 shows a modification of the circuit of Fig. 2, as an embodiment of the present invention. As in the case of Fig. 2, the heater 24 of the oxygen sensor 1 8 is connectable to the power source 26 via the contacts 30a of the relay 30. In Fig. 3, however, the connection of the coil 30b of this relay 30 with the power source 26 is not directly governed by the key switch as shown in Fig, 2 or an analogous switch. In the circuit of Fig, 3 interposed between the power source 26 and the coil 30b of the relay 30 is a logic switch 32 which has first and second input terminals 33 and 35 and selectively takes on-state and off-state depending on the combination of two kinds of input signals respectively supplied to the first and second input terminals 33 and 35. A first input signal S1 supplied to the first input terminal 33 is an on-off signal which represents the on-state or off-state of an engine ignition switch (not shown) corresponding to the key switch 28 in Fig. 2, and in this case a second input signal S2 supplied to the second input terminal 35 is an on-off signal which is produced by a throttle-position sensor (not shown) and assUmes on-state only when the engine 10 is operated under a full throttle condition, or a nearly full-throttle condition defined by a throttle-opening degree greater than a predetermined value.

The logic switch 32 is so constructed as to take on-state and connect the coil 30b to the power source 26 only while the first signal Si is in the on-state but the second signal S2 is in the off-state. Accordingly the relay contacts 30a remain in the on-state, allowing continued supply of a heating current to the heater 24, So long as the engine 10 is operated under conditions other than full-throttle or nearly full-throttle condition. When the throttle is opened to such a degree that the second signal S2 shifts to the on-state, the logic switch 32 shifts to the off-state to cause deenergization of the coil 30b. Then the relay contacts 30a shift to the off-state, although the engine ignition switch still remains in the on-state, with resultant interruption of the supply of the heating current to the heater 24.Therefore, the fear of excessive and detrimental heating of the oxygen sensor 1 8 during its exposure to a very high temperature exhaust gas can effectively be precluded. Of course the oxygen sensor 1 8 exhibits a proper output characteristic in such a high temperature exhaust gas, without the need of heat supply from the heater 24.

When the second signal S2 reverts to the off-state as the result of a decrease in the opening degree of the throttle while the first signal S1 is still in the on-state, the logic switch 32 resumes the on-state to allow resumption of the supply of the heating current to the heater 24 via the relay contacts 30a.

The logic switch 32 shifts to the off-state to disconnect the heater 24 from the power source 26 when the first signal S, shifts to the off-state, irrespective of the state of the second signal S2.

Where the fuel-supplying apparatus 1 6 in Fig. 1 is a fuel injection system, the second input signal S2 in Fig. 3 will be replaced by an on-off signal that represents the rate of fuel injection and takes the on-state while the fuel injection rate is above a predetermined rate.

Fig. 4 shows a modification of the circuit of Fig. 3, as another embodiment of the present invention. The arrangement of the relay 30 between the power source 26 and the heater 24 is similar to that in Fig. 3. Also in this case a logic switch 38 is interposed between the power source 26 and the coil 30b of the relay 30. This logic switch 38 has first and second input terminals 37 and 39. Supplied to the first input terminal 37 is the above described first signal S1 representing the onstate or off-state of the engine ignitioin switch.

The second input terminal 39 receives an onoff signal S3 which is produced by a temperature-sensitive element 40 incorporated in, or located close to, the oxygen sensor 1 8 in the exhaust passage 1 4 and takes on-state while the temperature detected by this element 40 is below a predetermined temperature. The logic switch 38 in Fig. 4 takes the on-state to establish electrical connection of the relay coil 30b with the power source 26 only while the two input signals S1 and S3 are respectively in on-state, so that the heater 24 is supplied with a heating current while the engine 10 is operated but the exhaust gas temperature is not very high.

When the exhaust gas temperature rises up to the predetermined temperature, the signal S3 shifts to off-state. Then the logic switch 38 takes the off-state, with resultant de-energization of the coil 30b and accordingly disconnection of the contacts 30a, so that the oxygen sensor 1 8 in a very high temperature exhaust gas is prevented from overheating by the action of the heater 24. When the exhaust gas temperature becomes below the predetermined temperature during operation of the engine 10, the signal S3 reverts to the onstate and causes the logic switch 38 to resume the on-state, so that the supply of the current to the heater 24 is resumed.The logic switch 38 shifts to the off-state and causes interruption of the current supply to the heater 24 also when the first signal S1 shifts to offstate, meaning that the engine ignition switch is turned off.

It will be understood that the electromagnetic relay 30 in Figs. 3 and 4 is no more than exemplary and can be replaced by a different type of switching device which may be a semiconductor relay or switch.

Also it will be understood that the present invention is effective irrespective of the construction of the oxygen-sensitive part of the oxygen sensor 1 8. That is, the solid electrolyte layer in the oxygen sensor 1 8 may have either a tubular form to introduce a reference gas such as air into the interior of the tube or a thin, film-like shape in the case of establishing a reference oxygen partial pressure by the provision of a metal-metal oxide layer or by the application of a DC current to cause controlled migration of oxygen ions through the solid state electrolyte.

Claims (9)

1. In an internal combustion engine system including an oxygen sensor which is provided with an electric heater and disposed in an exhaust passage for the engine to provide an electrical signal indicative of the air/ fuel ratio of an air-fuel mixture supplied to the engine and a current-supplying circuit to supply a heating current from a power source to said heater during operation of the engine system, the improvement comprising a current-supply control system which comprises an operating condition detecting means for producing an electrical command signal while the engine is operating under such a condition as causes a significant rise in the temperature of the exhaust gas and an on-off type switching means for breaking said current-supplying circuit at least while said detecting means produces said command signal.

2. An engine system according to Claim 1, wherein said operating condition detecting means has the function of detecting the rate of fuel feed to the engine and producing said command signal while the rate of fuel feed is greater than a predetermined rate.

3. An engine system according to Claim 2, wherein said operating condition detecting means has the function of detecting the degree of opening of a throttle valve provided in an induction passage through which said airfuel mixture is supplied to the engine.

4. An engine system according to Claim 2, wherein said operating condition detecting means has the function of detecting the rate of fuel injection into air being drawn into the engine.

5. An engine system according to Claim 1, wherein said operating condition detecting means comprises a temperature sensor which is disposed in said exhaust passage at a location close to said oxygen sensor and produces said command signal while the temperature of the exhaust gas is above a predetermined temperature.

6. An engine system according to Claims 1, 2 or 5, wherein said current-supply control means comprises an electrically controllable first switch means interposed between said power source and said heater and an electrically controllable second switch means for commanding said first switch means to disconnect said heater from said power source in response to said command signal.

7. An engine system according to Claim 6, wherein said second switch means is responsive also to an ignition signal which indicates whether the engine system is in operation or not and has the function of keeping said first switch means in the closed state to establish said current-supplying circuit only while said ignition signal indicates that the engine system is in operation but said command signal is not produced.

8. An engine system according to Claim 1, wherein said oxygen sensor is of the oxygen concentration cell type utilizing an oxygen ion conductive solid electrolyte.

9. An engine system according to Claim 1, substantially as hereinbefore described with reference to Figs. 1 and 3, ot Figs. 1 and 4 of the accompanying drawings.