US3916848A

US3916848A - Automotive-type internal combustion engine exhaust gas emission control system

Info

Publication number: US3916848A
Application number: US424464A
Authority: US
Inventors: Peter-Jurgen Schmidt
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1973-01-12
Filing date: 1973-12-13
Publication date: 1975-11-04
Anticipated expiration: 1992-11-04
Also published as: NL7400418A; JPS5629100B2; FR2214046B1; DE2301354C3; JPS49108429A; DE2301354B2; DE2301354A1; FR2214046A1; GB1410463A; IT1006851B

Abstract

A timing circuit is connected between a threshold switch providing alternately voltages representative of excess, or absence of oxygen in the exhaust emission from the engine, the timing circuit measuring the time interval between change-over of the threshold switch as determined by changes in the composition of the exhaust emission; if the change-over is within a predetermined timing range, a control switch is held in one position; if the change-over rate drops, however, indicative of malfunction in the exhaust emission sensor, or associated connected equipment, the switch is placed into another state whereupon it will command a fuel-air control amplifier to provide a control ratio of a predetermined value and, if desired, additionally provide a sensed output to control an alarm or other trouble indicator. Switching change of the control switch can be overriden by other signals available within the automotive vehicle.

Description

United States Patent [191 Schmidt 51 Nov. 4, 1975 Peter-Jiirgen Schmidt, Schwieberdingen, Germany [73] Assignee: Robert Bosch GmbH,

Gerlingen-Schillerhohe, Germany [22] Filed: Dec. 13, 1973 [21] Appl. No.: 424,464

[75] lnventor:

[51] Int. Cl. F02B 3/00 [58] Field of Search l23/32 EA [56] References Cited UNITED STATES PATENTS l/l974 Schmidt 123/32 EA Primary ExaminerCharles J. Myhre Assistant ExaminerJoseph A. Cangelosi Attorney, Agent, or Firm-Flynn & Frishauf ABSTRACT A timing circuit is connected between a threshold switch providing alternately voltages representative of excess, or absence of oxygen in the exhaust emission from the engine, the timing circuit measuring the time interval between change-over of the threshold switch as determined by changes in the composition of the exhaust emission; if the change-over is within a predetermined timing range, a control switch is held in one position; if the change-over rate drops, however, indicative of malfunction in the exhaust emission sensor, or associated connected equipment, the switch is placed into another state whereupon it will command a fuel-air control amplifier to provide a control ratio of a predetermined value and, if desired, additionally provide a sensed output to control an alarm or other trouble indicator. Switching change of the control switch can be overriden by other signals available within the automotive vehicle.

11 Claims, 2 Drawing Figures U.S. Patent Nov. 4; 1975 AUTOMOTIVE-TYPE INTERNAL COMBUSTION ENGINE EXHAUST GAS EMISSION CONTROL SYSTEM The present invention relates to a system to decrease the noxious components in the emission from internal combustion engines, and more particularly to such a system in which the mass ratio of fuel and air of the mixture applied to the internal combustion engine is controlled by means of an integrating controller which, in turn, has input signals applied thereto representative of compliance with, or deviation from sensed standards in the exhaust from the engine.

Exhaust sensors located to be responsive to the composition of the exhaust gases from an internal combustion engine, and providing electrical output signals, have previously been proposed. To control a mass ratio of air-fuel being applied to the internal combustion engine, also called A control, fuel and air, in suitable relationships, are applied by means of a carburetor or an electronically controlled fuel injection system to the engine. The control system maintains the mass ratio of fuel and air being supplied at such a value that the exhaust from the engine has a minimum of noxious components. This is the case if the exhaust is just slightly reducing, that is, if the air-fuel mixture being applied .is maintained at a value which is just below stoichiometric ratio. At this just slightly under stoichiometric value, this exhaust gases will have a composition which will have a minimum of noxious components, or can be limited to a minimum of noxious components, if the exhaust system of the internal combustion engine has a catalytic reactor to treat the exhaust gases.

The exhaust gas sensor may fail. This sensor, located in gas-sensing relationship to the exhaust gases of the engine, typically in the exhaust system thereof, will then no longer provide an output signal. Failure of an output signal from a typical sensor is, electrically, the equivalent of an indication that the air-fuel mixture is lean. A control system connected thereto will then control the input to the engine to compensate for this erroneous lean mixture and command a rich fuelair mixture being applied to the engine.

It is an object of the present invention to provide a control system in which failure of the sensing device, or at least a portion of the control circuitry will provide an output indication to the operator by commanding an alarm system, for example, and/or changing the operating conditions of the internal combustion engine to such an extent that the operator of the engine, typically of a vehicle, cannot but fail to notice, and thus is forced to repair, or have repaired the vehicle and particularly the exhaust system, for example by replacing the sensor.

Control systems in which exhaust sensors are used and also employing integrating amplifiers have output signals derived from the sensors which jump between extreme values, indicative of presence or absence of oxygen in the exhaust system. The integrating amplifier then controls the system in such a manner that, on the average, the oxygen being sensed is a minimum. The system includes a threshold switch, which is responsive to changes in output signals from the sensor.

SUBJECT MATTER OF THE PRESENT INVENTION Briefly, a timing circuit is connected to the threshold switch, and controls a switch which is held in a certain position when the threshold switch has a change-over repetition rate indicative of normal or ordinary operation, but permits, or causes change-over of the switch if there is trouble, or malfunction in the sensor, or the circuit connected thereto. The timing switch, then sensing a different repetition rate of change-over of the threshold switch, commands change of the switching position which, in turn, provides an output signal which can be directly applied to the control circuit of the integrating amplifier, to an alarm or warning device, or to other apparatus to influence the operation of the internal combustion engine.

The invention will be described by way of example with reference to the accompanying drawings, wherein:

FIG. 1 is a general, highly schematic circuit diagram and illustrating an example of a A-control circuit and supervisory circuit; and

FIG. 2 is a fragmentary portion of another embodiment of the circuit of FIG. 1.

An oxygen sensor 1 is located in sensing relation to the exhaust gases of an internal combustion engine (not shown). The output sensor 1 provides an electrical signal which is a characteristic of the composition of the gases within the exhaust from the internal combustion engine. The oxygen sensor is, preferably, enclosed in a shield, schematically indicated at l. The sensor, as well as the control circuit, are powered from

supply buses

3, 10, connected, for example, to the battery circuit of an internal combustion engine, or any other suitable d-c circuit which may be subject to voltage fluctuations.

The electrical signal from sensor 1 is applied to the base of a transistor 2 connected as an emitter follower. The collector of transistor 2 is directly connected to bus 3; the emitter of the transistor 2 is connected over a resistor 4 to a junction J1. The collector-emitter path of transistor 2 and the series connected resistor 4 form one leg of a first branch of a bridge 5. A second leg of the branch ofi-the bridge 5 is formed by the series circuit of the collector-emitter path of a second transistor 6 and a resistor 7. The base of transistor 6 is connected to a voltage divider formed by resistors 8, 9, connected over a common resistor 12 between

supply buses

10 and 3. Resistor 9 is adjustable. The two resistors 8, 9 have a Zener diode 11 connected in parallel thereto.

The third and fourth legs of the bridge 5, forming the second branch, are formed by a resistor 13, connected to the common bus 3 and a resistor 14 connected to the common bus '10, both being connected to junction .12, which forms, together with junction J l, the diagonal connection of the bridge.

The diagonal of the bridge is connected to an operational amplifier I5, junction J2 being connected over coupling resistor 17 with the non-inverting input, and junction J1 being connected over coupling resistor 16 to the inverting input of the operational amplifier 15. An output resistor 18 is connected to the output of the operational amplifier l5, and connected to the common supply 10. The output of the operational amplifier 15 is further connected to a junction point of two

base resistors

19, 20 for

transistors

21, 22, respectively.

Transistors

21, 22 have their emitters connected to respective tap points of two serially connected voltage dividers formed of

resistors

23, 24, 26, respectively. The

transistors

21, 22 vary the transfer of the voltage swing derived from operational amplifier l5, and thus can change the voltage difference of the output signal being derived from the operational amplifier 15 and further connected to an integrating amplifier. The emitter of transistor 21 is connected to the junction between

resistors

23, 24; the emitter of transistor 22 is connected to the junction between

resistors

25, 26. The collectors of the two

transistors

21, 22 are connected together. The jointly connected collectors of

transistors

21, 22 are connected over a transfer switch 27 with an input or coupling resistor 28 of an operational amplifier 29, which functions as an integrator, having an integrating capacitor 30 connected between its output and its inverting input, to which the coupling resistor 28 is likewise connected. Switch 27 can selectively connect

resistor

31, 32 as coupling resistors, rather than the resistor 28 to the inverting input of operational amplifier 29. The non-inverting input of operational amplifier 29 is connected over acoupling resistor 33 with the junction of

resistors

24, 25. The output of the operational amplifier 29 is connected over a resistor 34 to the common supply bus and, further, to an output terminal 29'. The output terminal 29 can be used to control, if necessary over an additional power amplifier, the airfuel mixture of an internal combustion engine, for example by providing a position output which influences supply of fuel, or air to the internal combustion engine, or by modifying the injection period of fuel injection valves in the fuel injection system.

Operational amplifier is further connected by line 15 to a timing circuit 36. The output line 15' is connected to a monostable flip-flop (FF) which triggers the timing circuit 36. The monostable FF includes a transistor 37, coupled over a coupling capacitor 39 and a diode 38, the cathode of diode 38 being connected to the base of transistor 37. A resistor 40' connects to the common supply bus 3. The junction point of the anode of the diode 38 and capacitor 39 is connected to a resistor 41 which is connected to the common supply bus 10. The emitter of transistor 37 is connected to the common supply bus 3; the collector of transistor 37 is connected over resistor 42 to supply bus 10; a diode 43 connects from the collector of transistor 37 to one terminal of a capacitor 44, the second terminal of which is connected to supply bus 3. Resistor 45, of compara- I tively high value, is connected in parallel to the capacitor 44. A coupling resistor 46 connects from the common connection of capacitor 44, diode 43 and resistor 45 to the base of an input transistor 47 of a control change-over switch 46. Transistor 47 is connected as an emitter-follower, and has its collector connected to bus 10 and its emitter to bus 3, if necessary over an emitter-resistor (not shown). The emitter of transistor 47 is further connected over a resistor 48 to the base of a switching transistor 49. A resistor 50 connects the base to common line 3. The junction between resistor 48, resistor 50, and base 49 has one or

more resistors

51, 51 connected thereto, which, in turn, are connected to respective terminals 52, 52'. Further resistors similar to

resistors

51, 51 and having further terminals similar to

terminals

52, 52, may be connected to the same junction, and therefore the connection to resistor 51' and terminal 52 is shown in dashed lines. The

terminals

52, 52 may have .various electrical signals applied thereto corresponding to operating or operation parameters of the internal combustion engine, and

4 to influence the state of the switching transistor 49, as will appear in detail below. .The emitter of the switching transistor 49 is connected to supply bus 3. The collector of transistor 49 is connected to an output terminal 49; further, the collector is connected over a resistor 53 to common bus 10. A connection line, including a diode 54, connects from the collector of transistor 49, and hence output terminal 49', to the common line connecting the collectors of

transistors

21, 22, and further forming the common switch terminal of switch 27.

OPERATION The output sensing signal from sensor 1 is connected into the first branch of the bridge 5 over transistor 2. This electrical signal, which is representative of the composition of the exhaust gases of the internal combustion engine, forms the actual value for the control circuit. The sensors 1 have a high internal resistance when the temperature in the exhaust gases is still low. Thus, the control circuit must have high input resistance and, therefore, transistor 2 isoperated as an emitter-follower and, together with its load resistor 4, provides the necessary input resistance for the circuit.

The command value, that is, the value which the air number A should have, and to which the air-fuel mixture is to. be controlled so that the exhaust gases from the engine will have a predetermined composition, is formed by means of Zener diode 11. The resulting signal is applied over the voltage divider formed of resistors 8, 9 and transistor 6 into the second arm of the branch of bridge 5. The command value can be adjusted by adjusting resistor 9.

Transistors

2 and 6, in the first branch of the bridge, are of opposite conductivity type, and so connected that they are temperature-compensating.

The comparison circuit for the command value and actual value is connected as a bridge, and thus is essentially independent of variations in supply voltage, supplied over the

common lines

3, 10. The bridge is in balance, that is, command and actual value are equal, if the collector-emitter paths of

transistors

2, 6 have the same voltage thereacross. This is the case if the output signal from sensor 1, and the commanded value which the signal should have are equal. If the command value and actual value deviate, a voltage will appear across the diagonal terminals J1, J2 of the bridge. Let it be assumed that the output voltage of the sensing signal from sensor 1, that is, the actual value is greater than the commanded value. The inverting input to the operational amplifier 15 will have a higher voltage applied than at the non-inverting input. Operational amplifier 15, functioning as a comparator circuit, provides an output signal which will be approximately at the voltage of the supply line 3. In the opposite case, that is if the actual value is smaller than the commanded value, the output signal of operational amplifier 15 will jump to a voltage which corresponds approximately to that of supply line 10.

The voltage swing or voltage jump at the input to integrator 29, 30 is decreased by an intermediately connected circuit 40, which includes

transistors

21, 22. The voltage jump is decreased and, therefore, the value of integrating capacitor 30 can be decreased. The output signal of operational amplifier 15 controls either the one or the other of

transistors

21, 22 to be in conductive condition, the other, or the one transistor, then, being blocked. A signal will be applied over the respectively conductive transistor to the input network of the integrator 29, 30 which has a smaller voltage swing than that of the output signal of the operational amplifier.

Switch 27 is provided in order to change the time constant of the integrator 29, 30. The output signal of the integrator 29, 30 then controls a suitable control element in such a manner that the air-fuel mixture applied to the internal combustion engine is so changed that it corresponds to a predetermined commanded ratio, resulting in exhaust of minimum noxious components. Switch 27 is shown schematically; as referred to in US. Pat. Nos. 3,782,347 and 3,831,564, both assigned to the assignee of this application, the actual construction of the switch can be different. The switch 27 can be a controlled semiconductor switch, for example transistors in series with resistors corresponding to schematically indicated

resistors

28, 31, 32. The transistors are selectively rendered conductive in accordance with desired integration time constants, as explained in the cross-referenced applications.

Let it be assumed that the sensor 1 and the connected circuit are functioning properly. Operational amplifier 15 will change over, that is, will change state in relatively short time intervals, the output voltage of the operational amplifier 15 switching between the voltage of supply line and supply line 3, periodically. The output of the operational amplifier thus provides, in short intervals, positive and negative pulses. Upon each negative jump, monostable FF, that is transistor 37, is triggered by transfer of a pulse over capacitor 39, that is, transistor 37 is blocked by a period of time determined by capacitor 39. When transistor 37 blocks, resistor 42 and diode 43 will have no current flowing thereover, and capacitor 44 can charge. The time, determined by capacitor 39, is so selected that the capacitor 44 can be fully charged. After this time has elapsed, and when transistor 37 again becomes conductive, capacitor 44 can discharge over the high-resistance resistor 45 and the input of transistor 47, operated as an emitter-follower. The discharge time of the storage capacitor 44 is so selected that, assuming proper operation of the control system and of the sensor 1, a residual charge will always remain in capacitor 44 and capacitor 44 will never completely discharge, but always retain a certain minimum charge. Thus, the switch 46, formed by

transistors

47, 49, is not changed over. Switching transistor 49 is held in conductive state by transistor Let it be assumed that sensor 1 fails, or that the control system fails, that is, that no output signal will be available at the output of operational amplifier 15. Capacitor 44 can now discharge completely. This blocks transistor 49. The air-fuel mixture which is applied to the internal combustion engine is then controlled in such a manner, by output from integrating amplifier 29, 30, that the mixture will go to the lean direction. This is obtained by transferring a signal over diode 54 which applies its signal to the resistors 28 (or 31, 32, respectively, if connected) in such a direction that the integrator 29, 30 will integrate in the direction to command a lean mixture. The operating performance of the internal combustion engine will become worse, forcing the operator of the engine, usually a motor vehicle, to en gage in maintenance and repair, that is, typically to re place sensor 1.

The output signal available at the collector of transistor 49, and hence the terminal 49', can be used in other ways than to connect a signal over diode 54, as shown.

It may be applied, for example, at other points in a fuelair mixture supply system, or elsewhere in the vehicle, for example by interfering with proper electrical operation of the electrical components of the system, in order to inhibit normal operation of the engine and to provide an indication to the operator. The output signal can also be used to control a warning device, which informs the driver that the sensor 1, or a portion of the control system, has failed.

Referring to FIG. 2, the resistor 53 can be omitted and the circuit, otherwise identical to that shown in FIG. 1, can be broken at terminals X-X'. Instead of resistor 53, the coil 53 of a relay is connected between terminals X-X, which, when energized, pulls in a relay contact 60 to energize an alarm 61, for example an alarm lamp, connected with the switch 60 between

buses

3 and 10. Alarm system 61 can be used in lieu of, or in combination with diode 54, or to interfere in any other way with the operation of the motor vehicle electrical system, or, by control of other parameters, with any other element, in addition to providing a warning signal.

The monostable FF is controlled by the negative going flank, due to the presence of diode 38, and uses a single transistor. It can be replaced by the well-known monostable FF circuits with two transistors, so that the positive as well as the negative switching flank from the output of the operationalamplifier 15 will trigger the timing circuit to charge capacitor 44 twice as often. The size of capacitor 44 can thus be decreased, at the cost of an additional transistor.

The control systems to influence the operation of the internal combustion engine, that is, to control the airfuel mixture, should not be effective under some operating conditions of the internal combustion engine. Some of such typical conditions are: when the exhaust sensor 1 is still cold, that is, upon starting, upon warmup of the internal combustion engine; or upon full-load operation of the engine; or upon idling operation of the engine. In all these instances, electrical signals can be applied over terminals 52, 52' to the junction of

resistors

48, 50, over appropriate coupling resistors 51, 51' which electrical signals are characteristic of the one or the other operating condition under which the transistor 49 is to be held in conductive condition, so that the control circuit and a warning device (FIG. 2), if used, is disabled; in other words, the normal operation of transistor 49 is overriden, so that no output signal will be available from transistor 49 at terminal 49'. The operating conditions of the internal combustion engines will, under such override conditions, not be affected, since, under those operating conditions, the control system is not operating but, due to the special conditions then pertaining, normal operation of the internal combustion engine, without malfunction or trouble indication, is desired.

Various changes and modifications may be made in accordance with the inventive concept.

I claim:

1. In an internal combustion engine exhaust emission control system having means (1) sensing the composition of the exhaust gases and providing a sensed electrical signal,

an integrating control amplifier (29, 30) connected to said sensing means (1) and providing an output signal (29) which is available to command the relative mass ratio (A) of air-fuel applied to the engme;

and a threshold switch connecting the sensing means (1) to the integrating control amplifier and operative to change switching state above, and below its threshold level, said threshold switch, upon normal operation of the system, cyclically switching between its states,

timing circuit (36) connected to the threshold switch (15) and measuring the time intervals between change-over of switching state of the threshold switch (15), as determined by changing output from the sensing means (1) the timing circuit sensing the intervals between change-over of the threshold switch (15); and

a change-over switch (46; 47, 49) controlled by said timing circuit (36) the timing circuit controlling said change-over switch to be in a first state when the duration of the intervals between change-over of the threshold switch are below a predetermined timing interval, indicative of proper operation of the sensing means and of the threshold switch, the timing circuit controlling said change-over switch (46; 47, 49) to be in another state when the switching interval since the last change-over of the threshold switch is above said predetermined interval, said change-over switch, when in the second state, providing an output signal (49) indicative of excess time lapse between change-over of the threshold switch (15) and representative of system malfunction.

2. System according to claim 1, further comprising a monostable flip-flop (37, 39) connected between the threshold switch (15) and the timing circuit (36) and triggered into unstable, state by change of state of the threshold switch (15);

and wherein said timing circuit comprises a capacitor (44) connected to be charged during one state of the monostable flip-flop (37, 39) and a discharge circuit (45) permitting discharge of the capacitor during the other state of the monostable flip-flop, said discharge circuit having a time constant which prevents complete discharge of the capacitor when the switching interval of the threshold switch (15) is within said predetermined value to provide a residual charge on the capacitor (44), said residual charge holding said change-over switch (46) in said state indicative of proper operation.

3. System according to claim 2, wherein the capacitor (44) is charged during the unstable state of the monostable flip-flop (37, 39) and discharged during the stable state thereof and said discharge circuit has a high time constant and is connected to and controlling 8 said change-over switch (46) to change state indicative of trouble operation upon discharge of the capacitor below a predetermined level.

4. System according to claim 1, wherein the integrating control amplifier comprises an operational amplifier (29);

and a coupling circuit (54) is provided between the output (49') of the change-over switch (46) and the inverting input of the operational amplifier (29) to modify the input signal applied thereto.

5. System according to claim 4, wherein the coupling circuit comprises a diode (54).

6. System according to claim 4, further comprising selectively variable input resistance means (28, 31, 32) connected to the coupling network and the operational amplifier (29, 30).

7. System according to claim 2, wherein the monostable flip-flop (37, 39) comprises a transistor (37) having its emitter-collector path connected in parallel to the capacitor (44) of the timing circuit;

and a coupling capacitor (39) and a diode (38) coupling network connecting the output of the threshold switch (15) to the control electrode of the transistor (37).

8. System according to claim 1, wherein the timing circuit comprises a capacitor (44);

and the change-over switch (46) comprises an emitter-follower input transistor (47) connected to the timing capacitor (44), and a switching output transistor (49) connected to and controlled by said input transistor (47).

9. System according to claim 1, further comprising alarm circuit means (53 60, 61) connected to the output (49') of the change-over switch (46) to provide an alarm output signal upon sensing of trouble operation.

10. System according to claim 1, further comprising at least one override connection terminal (52, 52') connected to the change-over switch (46) to hold said switch in a position indicating proper operation of the sensing means to override control of the integrating control amplifier (29, 30) by said change-over switch (46) upon energization of at least one of said override terminals.

1 1. System according to claim 1, wherein the relative mass ratio (A) of the air-fuel applied to the engine is controlled to apply a lean mixture to the engine when the timing circuit has sensed a time interval from the last change-over of the threshold switch (15) which is above the time duration of the predetermined interval.

Claims

1. In an internal combustion engine exhaust emission control system having means (1) sensing the composition of the exhaust gases and providing a sensed electrical signal, an integrating control amplifier (29, 30) connected to said sensing means (1) and providing an output signal (29'') which is available to command the relative mass ratio ( lambda ) of airfuel applied to the engine; and a threshold switch (15), connecting the sensing means (1) to the integrating control amplifier and operative to change switching state above, and below its threshold level, said threshold switch, upon normal operation of the system, cyclically switching between its states, a timing circuit (36) connected to the threshold switch (15) and measuring the time intervals between change-over of switching state of the threshold switch (15), as determined by changing output from the sensing means (1) the timing circuit sensing the intervals between change-over of the threshold switch (15''); and a change-over switch (46; 47, 49) controlled by said timing circuit (36) the timing circuit controlling said change-over switch to be in a first state when the duration of the intervals between change-over of the threshold switch are below a predetermined timing interval, indicative of proper operation of the sensing means and of the threshold switch, the timing circuit controlling said change-over switch (46; 47, 49) to be in another state when the switching interval since the last change-over of the threshold switch is above said predetermined interval, said change-over switch, when in the second state, providing an output signal (49'') indicative of excess time lapse between change-over of the threshold switch (15) and representative of system malfunction.

2. System according to claim 1, further comprising a monostable flip-flop (37, 39) connected between the Threshold switch (15) and the timing circuit (36) and triggered into unstable state by change of state of the threshold switch (15); and wherein said timing circuit comprises a capacitor (44) connected to be charged during one state of the monostable flip-flop (37, 39) and a discharge circuit (45) permitting discharge of the capacitor during the other state of the monostable flip-flop, said discharge circuit having a time constant which prevents complete discharge of the capacitor when the switching interval of the threshold switch (15) is within said predetermined value to provide a residual charge on the capacitor (44), said residual charge holding said change-over switch (46) in said state indicative of proper operation.

3. System according to claim 2, wherein the capacitor (44) is charged during the unstable state of the monostable flip-flop (37, 39) and discharged during the stable state thereof and said discharge circuit has a high time constant and is connected to and controlling said change-over switch (46) to change state indicative of ''''trouble'''' operation upon discharge of the capacitor below a predetermined level.

4. System according to claim 1, wherein the integrating control amplifier comprises an operational amplifier (29); and a coupling circuit (54) is provided between the output (49'') of the change-over switch (46) and the inverting input of the operational amplifier (29) to modify the input signal applied thereto.

7. System according to claim 2, wherein the monostable flip-flop (37, 39) comprises a transistor (37) having its emitter-collector path connected in parallel to the capacitor (44) of the timing circuit; and a coupling capacitor (39) and a diode (38) coupling network connecting the output of the threshold switch (15) to the control electrode of the transistor (37).

8. System according to claim 1, wherein the timing circuit comprises a capacitor (44); and the change-over switch (46) comprises an emitter-follower input transistor (47) connected to the timing capacitor (44), and a switching output transistor (49) connected to and controlled by said input transistor (47).

9. System according to claim 1, further comprising alarm circuit means (53'', 60, 61) connected to the output (49'') of the change-over switch (46) to provide an alarm output signal upon sensing of ''''trouble'''' operation.

10. System according to claim 1, further comprising at least one override connection terminal (52, 52'') connected to the change-over switch (46) to hold said switch in a position indicating proper operation of the sensing means to override control of the integrating control amplifier (29, 30) by said change-over switch (46) upon energization of at least one of said override terminals.

11. System according to claim 1, wherein the relative mass ratio ( lambda ) of the air-fuel applied to the engine is controlled to apply a lean mixture to the engine when the timing circuit has sensed a time interval from the last change-over of the threshold switch (15) which is above the time duration of the predetermined interval.