GB1568679A

GB1568679A - Methods and systems for regulating fuelair mixtures fed toan internal combuston engine

Info

Publication number: GB1568679A
Application number: GB4245776A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1975-10-13
Filing date: 1976-10-13
Publication date: 1980-06-04
Also published as: FR2328113A2; DE2545759A1; SE435861B; SE7611268L; JPS6044504B2; DE2545759C2; JPS5248738A; FR2328113B2

Description

(54) IMPROVEMENTS IN OR RELATING TO METHODS AND SYSTEMS FOR REGULATING THE FUEL-AIR MIXTURES FED TO INTERNAL COMBUSTION ENGINES (71) We, ROBERT BOSCH GMBH, a German Company, of Postfach 50, 7 Stuttgart 1, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to methods and systems for regulating the mass ratio of the fuel-air mixtures fed to internal combustion engines.

It is known that the mass ratio, i.e. the air number A of the fuel-air mixture fed to an internal combustion engine can be influenced as a function of the composition of the exhaust gas by arranging in the exhaust gas stream of the internal combustion engine an oxygen probe which is capable of producing, as a function of the composition of the exhaust gas, an output voltage, the shape and switching action of which will be discussed later. This output voltage of the oxygen probe is fed to a regulating device which is preferably in the form of an integrating action controller and which, as a function of its output signal, induces a corresponding increase or decrease of the instantaneously metered quantity of fuel.Such a variation of the air number of the fuel-air mixture may be carried out in internal combustion engines provided with carburettors as well as in engines provided with fuel injection systems which normally are capable of metering more precisely the fuel quantity fed to the internal combustion engine over the working range of the same. In such a system the internal combustion engine itself therefore forms the controlled process, and there is an inherent dead time for the control system which is hereinafter referred to as engine dead-time or engine transit time Tt, and which constantly changes in road operation, mainly as a function of the speed of the internal combustion engine.

Of particular importance in a control which makes use of the output voltage of an oxygen probe is the characteristic of the probe which is shown schematically in Fig. I of the accompanying drawings and which is able in the steady state (i.e. when appropriately heated) to assume two different switching states. The first switching state corresponds to an output voltage of for example approximately 900 mV and is obtained when the oxygen probe in the exhaust pipe detects a rich fuel-air mixture, the other output voltage is at approximately 100 mV and corresponds to a lean fuel-air mixture originally fed to the internal combustion engine.The transition between these two probe voltages occurs virtually suddenly at an air number of approximately ;t=l. In practice a finite slope exists, but the curved course of the probe characteristic at A=1 allows regulation to slightly rich air numbers only if a correspondingly high voltage threshold value is pre-set. Apart from this disadvantage the working in the curved, and hence less steep, part of the probe characteristic, is disadvantageous because it is just this part which is temperaturedependent and susceptible to ageing. An almost stable point in the probe characteristic exists in present-day probes at a probe voltage of Us~300 to 350 mV and corresponds approximately to point P in the characteristic shown in Fig. 1.On the other hand, if in fact point P of the probe characteristic is to be used, it means a limitation to a certain value of the air number. It is desirable, however, that there should be a range of variation of at least approximately +5 /n around the air number A=l, so that the internal combustion engine can be regulated within a freely selectable range between approximately A=0.95 and A= 1.05 .

There is a need for a method and a system for regulating the mass ratio of the fuel-air mixture fed to an internal combustion engine with an oxygen probe in the exhaust gas path, in which a stable point of the probe characteristic can be selected as a threshold value and suitable variations of the air number are nevertheless possible.

According to one feature of the present invention a method of regulating the mass ratio of the fuel-air mixture fed to an internal combustion engine comprises feeding to an integrating action controller for adjusting the fuel-air mixture signals derived from the output of an oxygen probe arranged in the exhaust gas stream of the internal combustion engine, integrating the derived signal at a first rate and in one direction after a step change in the derived signal in one direction, integrating the derived signal at a second rate and in a second direction after a step change in the derived signal in the other direction, and with each step change in the derived signal in either direction integrating in the same direction for a predetermined period of time and at a third rate which is steeper than the first rate and the second rate, whereby, in order that the mass ratio of the fuel-air mixture can be regulated to an air number other than that at which the output of the oxygen probe changes, the mean value of the curve of the output voltage of the integrating action controller against time can be displaced independently of engine transit time in a desired direction and to a desired extent relative to a value which it would normally have based on the signals derived from the output of the oxygen probe.

According to another feature of the present invention a system for regulating the mass ratio of a fuel-air mixture fed to an internal combustion engine comprises an oxygen probe adapted to be arranged in the exhaust gas path of the internal combustion engine, a threshold value circuit for deriving step signals from the output of the oxygen probe, an integrating action controller for adjusting the fuel-air mixture and responsive to the derived signals to integrate the derived signal in one direction and at a first rate after a step change in the derived signal in one direction and to integrate the derived signal in the opposite direction and at a second rate after a step change in the derived signal in the other direction, and delay circuit means responsive to each step change in the derived signal in either direction to enable the integrating action controller to integrate for a predetermined period of time in the same direction and at a third rate which is steeper than the first and second rates.

By the arbitrarily adjustable variation of the shape of the output voltage of the integrating action controller, which is transmitted for example to a final control element for fuel metering, it is possible to influence the characteristic of the integrating action controller in such a manner that by using an oxygen probe regulation to a mean A optionally on the rich or in the lean side can take place, so that lean mixtures with a predetermined A and also mixtures with A < 1 can be realized.

Quite generally, it is desirable in the operation of an internal combustion engine to be able to vary the air number either in the basic setting or possibly also during operation. The invention makes it possible to vary, if desired, the operating point for the air number also as a function of the speed of the internal combustion engine, for example by acting upon adjustment controls for the integrating action controller output voltage as a function of the speed.

The present invention will be further described by way of example with reference to the accompanying drawings in which: Figure 1 shows the output voltage Us of an oxygen probe as a function of time with varying fuel/air mixture, Figs. 2a and 2b show the output voltage UR of a controller as a function of time dependent upon the probe voltage U5, and Fig. 3 is a circuit diagram of one embodiment designed to influence the controller characteristic.

Before describing in detail the invention and its embodiments, it should be pointed out that the invention is applicable virtually to all systems which are capable of bringing about an adjustment of the fuel-air proportions fed to an internal combustion engine from a predetermined value, for example with the help of a final control element which is acted upon by the output voltage of a controller. In this manner it is of course also possible to vary the adjustment of different models of carburettor in that by mechanical control elements, for example by magnetically controlled valves or the like, the fuel feed can be varied as a function of the probe voltage.

In a particualrly advantageous manner, however, the system can be applied to an electronic fuel injection system, which for example may be arranged so that injection valve opening pulses which are variable in their duration are transmitted to injection valves assigned to an internal combustion engine and controllable on an electromagnetic basis. The fuel to be injected is fed to these injection valves through pipe lines under an appropriate constant pressure, with the length of time of the opening pulses determining the quantity of fuel fed to the internal combustion engine at each stroke or continuously.Such an electronic fuel injection system may be designed so that an electronic control device is arranged to supply a final power stage directly controlling the injection valves and generates output pulses whose duration determines the duration of the control commands finally transmitted to the injection valves. The control device may comprise a monostable trigger stage which includes a time-determining capacitor in a feedback circuit. The upset time of the monostable multivibrator is determined by the recharging of the capacitor, whose recharging time in turn is determined by a discharge current source and a charging current source.The discharge current is related to of the air quantity fed to the internal combustion engine which can be sensed and converted in an optional manner; the charging current is related to the particular speed of the internal combustion engine, that is, it is speed-related. It is not necessary to describe in detail the specific set-up of the electronic part of the fuel injection system, since the important part is merely that the fuel injection system should be designed so that a variable voltage transmitted to it should be capable of influencing in an appropriate manner the quantity of the fuel fed to the internal combustion engine.

Regulation to a desired air number A can be achieved for example in an electronic fuel injection system by locating in the exhaust pipe of the internal combustion engine an oxygen probe which supplies a probe voltage Us as a function of the exhaust gas composition. A curve of the probe voltage Us as a function of time is shown in heavy lines in Fig. 1 and designated A. For the sake of simplicity, in the same diagram of Fig. 1, an effective curve of the probe voltage as a function of the air number A is also shown in broken lines and designated B, whilst curve C illustrates the probe characteristic as susceptible to temperature and ageing.The curve of the probe characteristic is generally steepest at A=l and less steep at other values of A; in principle it can be said that all the curves have approximately one stable point of the characteristic in common, i.e. the point P of the curves of Fig. 1 which, as will be explained further hereinafter, is used as a threshold value. To this end there is provided a probe threshold value circuit which will be described later; the output voltage of the probe threshold value circuit then passes to an integrating action controller which then presents on its output a varying voltage which is transmitted to the electronic fuel injection system for adjustment of the fuel quantity fed.

Before entering into detail, the basic concept of the invention will be explained with the help of the illustrations of Fig. 2a and Fig. 2b. At the instant tl of Fig. 1 the fuel-air mixture, in the case of a customary A-regulation by means of an oxygen probe and integrating action controller; just passes through a particular A value which the probe can indicate by virtue of its steep characteristic, namely A=l. This mixture is sucked in by the internal combustion engine and processed, and reaches the probe only after the engine transit time Tt, and the probe then, at the instant tl+Tt, signals the attainment of A=l by alteration of its output voltage.

The integrating action controller, which up to this instant tl+Tt has continued to adjust the mixture in one direction, will as from this instant adjust the mixture in the reverse direction and at the instant t2 reaches once more the value where the mixture is at A=l. It is again only at the instant t2+Tt that this state of the mixture fed fo the internal combustion engine is picked up by the oxygen probe, so that a constant oscillation of the fuel-air mixture about the mean value, which lies at the air number A=l, results, as will be readily understood.

To allow regulation of the fuel-air mixture to a A-value as required for example for a one-bed catalyst for the reduction of the harmful susbtances in the exhaust gas, preferably to a value of A~0.99, a shifting of this mean A-value takes place by reshaping the controller characteristic in such a manner that the curve shape of the integrating action controller output signal is transformed.

A first embodiment illustrating the possibility of a transformation is represented in Fig. 2a, which shows the course of the controller output voltage UR as a function of time. At the instant tl the air number of the fuel-air mixture fed to the internal combustion engine has the value A=l, but the oxygen-probe cannot yet respond to this, since it can sense this value only after expiration of the engine transit time Tt. At the instant tl+Tt the integrating action controller, whose output signal is represented in Fig. 2a, is then influenced in such a way that in the first place there occurs a sudden shift of the controller output signal UR by an amount AU in the direction in which the mean A-value is to be shifted. After performing this jump function, the controller then integrates in the opposite direction and crosses the zero line at the instant t2.At the instant t2+Tt the oxygen-probe once more signals A 1 and the controller output voltage once more shifts suddenly (upwards in the present instance), so that, as can be seen, a shift Hm of the mean controller output voltage by a value of AU/2 results. This shift is independent of the engine transmit time Tt.

However, the technical execution of a jump function with infinite steepness is difficult to realize, although it is quite feasible optionally to approach the desired function. From the illustration of Fig. 2b the course of the controller output voltage curve with finite steepness of slope k2 can be seen. Each change of voltage of the oxygen probe induces in the curve of Fig. 2b for a fixed time increment tz a rise in the output voltage UR of the integrating action controller with increased slope in the direction in which the mean value is to be shifted. As a mean shifting we obtain here the quantity k2tz Hm= 2 which is likewise independent of Tt. Thus by a choice of k2 and tz any desired shifting can be achieved.It is understood that the shifting may also take place in reverse direction if operation is to take place for example in the direction of a lean fuel-air mixture.

The circuit of Fig. 3 of one embodiment of the invention illustrates the circuit elements which are necessary when using an oxygen probe with jump action at A=l, to produce a mixture which differs from this air number. The circuit of Fig. 3 comprises first a probe threshold value circuit 5 which need not be described in detail and which is provided in order to transmit a probe switching voltage to an integrating action controller, which voltage changes step-wise from one state to another each time the actual probe output voltage Us passes through the stable point P of the characteristic of Fig. 1.For this purpose a comparator 6 is provided, to one input of which is fed a predetermined fixed voltage from an adjustable voltage divider comprising resistors 7 and 8, and to the other input of which is fed preferably via a transistor 9, the probe output voltage Us fed to terminal P1 of the circuit. No further details need be given regarding the probe threshold value circuit 5; the output of the comparator 6 transmits to the point P2 of the circuit a square-wave signal which at A=l suddenly changes its state.The integrating action controller itself is in the form of an operational amplifier 15, to the non-inverting input of which is fed a constant voltage from a voltage divider comprising resistors 16 and 17, and to the inverting input of which is fed through a voltage divider comprising resistors 19 and 20 and a transistor 18, a voltage which can be varied as a function of the probe threshold voltage. To this end, the collector of the transistor 18 is connected through a resistor 21 to the junction of the two resistors 19 and 20, which junction in turn is connected through a resistor 22 to the inverting input of the operational amplifier 15. A capacitor 23 is connected between the output of the operational amplifier 15 and its inverting input to give it an integrating characteristic and the voltage UR is obtained at its output.The output voltage originating from the probe threshold circuit 5 is fed through a resistor 24 to the base of the transistor 18.

To allow a variation of the integrating action of the operational amplifier or integrator 15 of the control device in accordance with the curves of Fig. 2a and Fig. 2b, a reversing circuit 25 comprising the transistors 26 and 27 together with a trigger circuit 28 is provided; these circuits are arranged and operate as follows:- When the transistor 26 of a first reversing stage is made conductive through a resistor 30 from the point P2 by a negative-going jump of the output voltage of the probe threshold circuit 5, a positive-going voltage jump is transmitted through the capacitor 31 a to the base of transistor 31 or, to put it more accurately, the diode 32 upstream of the base of the transistor 31 is cut off and the transistor 31 is also cut off.The transistor 31, the coupling capacitor 31 a and a variable resistance leakage resistor 33, form a mono-stable trigger stage whose upset time will be determined by capacitance and resistance of the capacitor 31a and the resistor 33 respectively. Hence it is possible to set the length of time during which the transistor 31 is in its nonconducting state. As soon as the transistor 31 cuts off, a diode 34 connected to its collector becomes conductive so that a current flows from the inverting input of the operational amplifier or integrator 15 through the resistor 37 connected to the diode 34 and through a resistor 35 to the negative lead 36.This current flowing through the resistor 37 causes the output voltage of the operational amplifier 15 to change very rapidly in a positive direction at a rate depending on the magnitude of this current so that approximately the action illustrated by the curve of Fig. 2a, but eased as illustrated by the curve of Fig. 2b, results.

Since in the circuit of Fig. 3 this process is to be made independent of the normal slope of the integration process of the operational amplifier or integrator 15, a further transistor 39 is provided which is biassed by the transistor 31. When the transistor 31 is non-conducting, a diode 40 connected to its collector is also non-conducting and the junction between this diode and a further diode 41 moves in a negative direction since this junction is connected through a resistor 42 to the negative lead 36. The transistor 39 is rendered conducting through a diode 43 which is connected to its base and effectively short-circuits or diverts the input signal on the base of the transistor 18 since the collector of the transistor 39 is connected to the base of the transistor 18.In this manner it is ensured that the integrating action of the operational amplifier integrator 15 during the upset time of the mono-stable trigger circuit comprising substantially the transistor 31, is determined exclusively by the value of the resistor 37 (and naturally of the resistor 35 leading to the negative lead 36).

In the formula given earlier for the shift, Hm, it is possible therefore to adjust the slope K2 by selecting the value of the resistor 37 and to adjust the duration of the delay (corresponding to the time increment tr) before the integrator integrates in the other direction by suitably selecting the upset time of the mono-stable trigger circuit.

A suitable correction or transformation of the integrator output signal should also occur when the integrator 15 integrates in the other direction, and this does occur in the embodiment of Fig. 3 when the input voltage at point P2 jumps in a positive-going direction. The transistor 26 will then become non-conducting and the transistor 27 will become conducting, since its base is connected through a diode 45 and a resistor 46a to the negative lead 36.The voltage jump is transmitted from the collector of the transistor 27 through a capacitor 46 and a diode 48 to the base of a transistor 49, which forms a second monostable trigger circuit and now blocks for this direction of integration, so that, since its collector is connected through diodes 50 and 51 to the same parts of the circuit which have been mentioned previously, the previously described process is repeated and the output voltage UR follows the curve of Fig. 2b.

It should be pointed out in this connection, that the whole process can of course be used in reverse direction, to which end substantially, in view of the complementary set-up of the circuit, only the inputs of the integrator 15 have to be reversed. Moreover, it will be understood that the type of transistors used and the polarity of the supply voltage leads have been chosen arbitrarily, and the circuit will also operate perfectly if other voltage polarities and correspondingly other types of transistors are used.

The making ineffective of the "normal slope", as given by the transistor 18, is desirable also in order that the slopes at the upper and lower inversion points, for example the slope k2 according to Fig. 2b, should be identical.

Thus it is possible to achieve a stable regulation, when owing to ageing of the oxygen probe and to possibly constantly altering exhaust gas temperatures, a desired probe voltage threshold corresponding to optimum exhaust gas composition cannot be kept steady. The stable point of the probe voltage characteristic occurring at approx. 300 mV is ulitized, and it is also possible to regulate to a desired value which deviates from this 300 mV point of the characteristic.

WHAT WE CLAIM IS: 1. A method of regulating the mass ratio of the fuel-air mixture fed to an internal combustion engine, which comprises feeding to an integrating action controller for adjusting the fuel-air mixture signals derived from the output of an oxygen probe arranged in the exhaust gas stream of the internal combustion engine, integrating the derived signal at a first rate and in one direction after a step change in the derived signal in one direction, integrating the derived signal at a second rate and in a second direction after a step change in the derived signal in the other direction, and with each step change in the derived signal in either direction integrating in the same direction for a predetermined period of time and at a third rate which is steeper than the first rate and the second rate, whereby, in order that the mass ratio of the fuel-air mixture can be regulated to an air number other than that at which the output of the oxygen probe changes, the mean value of the curve of the output voltage of the integrating action controller against time can be displaced independently of engine transit time in a desired direction and to a desired extent relative to a value which it would normally have based on the signals derived from the output of the oxygen probe.

2. A system for regulating the mass ratio of a fuel-air mixture fed to an internal combustion engine, comprising an oxygen probe adapted to be arranged in the exhaust gas path of the internal combustion engine, a threshold value circuit for deriving step signals from the output of the oxygen probe, an integrating action controller for adjusting the fuel-air mixture and responsive to the derived signals to integrate the derived signal in one direction and at a first rate after a step change in the derived signal in one direction and to integrate the derived signal in the opposite direction and at a second rate after a step change in the derived signal in the other direction, and delay circuit means responsive to each step change in the derived signal in either direction to enable the integrating action controller to integrate for a predetermined period of time in the same direction and at a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. effectively short-circuits or diverts the input signal on the base of the transistor 18 since the collector of the transistor 39 is connected to the base of the transistor 18. In this manner it is ensured that the integrating action of the operational amplifier integrator 15 during the upset time of the mono-stable trigger circuit comprising substantially the transistor 31, is determined exclusively by the value of the resistor 37 (and naturally of the resistor 35 leading to the negative lead 36). In the formula given earlier for the shift, Hm, it is possible therefore to adjust the slope K2 by selecting the value of the resistor 37 and to adjust the duration of the delay (corresponding to the time increment tr) before the integrator integrates in the other direction by suitably selecting the upset time of the mono-stable trigger circuit. A suitable correction or transformation of the integrator output signal should also occur when the integrator 15 integrates in the other direction, and this does occur in the embodiment of Fig. 3 when the input voltage at point P2 jumps in a positive-going direction. The transistor 26 will then become non-conducting and the transistor 27 will become conducting, since its base is connected through a diode 45 and a resistor 46a to the negative lead 36.The voltage jump is transmitted from the collector of the transistor 27 through a capacitor 46 and a diode 48 to the base of a transistor 49, which forms a second monostable trigger circuit and now blocks for this direction of integration, so that, since its collector is connected through diodes 50 and 51 to the same parts of the circuit which have been mentioned previously, the previously described process is repeated and the output voltage UR follows the curve of Fig. 2b. It should be pointed out in this connection, that the whole process can of course be used in reverse direction, to which end substantially, in view of the complementary set-up of the circuit, only the inputs of the integrator 15 have to be reversed. Moreover, it will be understood that the type of transistors used and the polarity of the supply voltage leads have been chosen arbitrarily, and the circuit will also operate perfectly if other voltage polarities and correspondingly other types of transistors are used. The making ineffective of the "normal slope", as given by the transistor 18, is desirable also in order that the slopes at the upper and lower inversion points, for example the slope k2 according to Fig. 2b, should be identical. Thus it is possible to achieve a stable regulation, when owing to ageing of the oxygen probe and to possibly constantly altering exhaust gas temperatures, a desired probe voltage threshold corresponding to optimum exhaust gas composition cannot be kept steady. The stable point of the probe voltage characteristic occurring at approx. 300 mV is ulitized, and it is also possible to regulate to a desired value which deviates from this 300 mV point of the characteristic. WHAT WE CLAIM IS:

1. A method of regulating the mass ratio of the fuel-air mixture fed to an internal combustion engine, which comprises feeding to an integrating action controller for adjusting the fuel-air mixture signals derived from the output of an oxygen probe arranged in the exhaust gas stream of the internal combustion engine, integrating the derived signal at a first rate and in one direction after a step change in the derived signal in one direction, integrating the derived signal at a second rate and in a second direction after a step change in the derived signal in the other direction, and with each step change in the derived signal in either direction integrating in the same direction for a predetermined period of time and at a third rate which is steeper than the first rate and the second rate, whereby, in order that the mass ratio of the fuel-air mixture can be regulated to an air number other than that at which the output of the oxygen probe changes, the mean value of the curve of the output voltage of the integrating action controller against time can be displaced independently of engine transit time in a desired direction and to a desired extent relative to a value which it would normally have based on the signals derived from the output of the oxygen probe.

third rate which is steeper than the first and second rates.

3. A system as claimed in claim 2, in which the integrating action controller comprises an operational amplifier with feedback through a capacitor, and to a first input of which can be fed a fixed potential and to a second input of which can be fed a potential which is variable in response to the signals derived from the probe output and controlled by a switching transistor connected in parallel with an input voltage divider and which can be switched into its conducting or its blocking state in response to the derived step signals, and a further transistor whose emitter to collector path is connected in the base to emitter circuit of the switching transistor and is adapted to switch the switching transistor to render the input controlled thereby ineffective during each said predetermined period of time.

4. A system as claimed in claim 3, in which the threshold value circuit comprises a comparator to which the output voltage of the oxygen probe is fed and whose threshold value corresponds to a stable point of the characteristic of the oxygen probe.

5. A system as claimed in claim 4, in which the delay circuit means comprises two monostable trigger stages whose inputs are coupled to respective reversing stages responsive to output signals from the comparator to trigger the monostable trigger stages which in both switching states of the output of the comparator act in the same direction such that a resistor connected to the second input of the operational amplifier to which the variable potential is applied passes such a current away from this input that both in negativegoing and in positive-going changes of output voltage of the comparator the rate of integration by the integrating action controller is in the same direction and is at the third rate.

6. A method of regulating the mass ratio of the fuel-air mixture fed to an internal combustion engine, substantially as hereinbefore particularly described with reference to and as illustrated in the accompanying drawings.

7. A system for regulating the mass ratio of the fuel-air mixture fed to an internal combustion engine, constructed and arranged and adapted to operate substantially as hereinbefore particularly described with reference to and as illustrated in the accompanying drawings.