Method and apparatus for fuel mixture control

The fuel mixture ratio of an internal combustion engine is adjusted by metering out a fuel quantity in relation to the air flow rate through the induction tube. The air flow rate is measured indirectly by monitoring the engine speed (rpm) with an electrical transducer driving a frequency-voltage converter, thus providing a first voltage, while a throttle plate position transducer generates a second voltage. The two voltages are applied to a logical circuit including parallel diodes which selects the lower of the applied signals and presents the resultant voltage as the primary control signal for fuel metering. Various compensating networks may be added to provide additional smoothing and adaptation to the operating characteristics of a particular engine and state of operation. Various fuel metering devices to be used in conjunction with the control circuit are also described.

BACKGROUND OF THE INVENTION 
The invention relates to a method and an apparatus for controlling the 
composition of a fuel-air mixture supplied to an internal combustion 
engine. More particularly, the invention relates to a method and an 
apparatus for generating control signals from the position of the main 
throttle valve in the induction tube as well as from engine rpm and to use 
these voltages in generating a control current for actuating a fuel 
metering device in accordance with the requirements of the engine 
operation. 
Customarily, the composition of the operational mixture of an engine is 
governed by means of a carburetor in which the venturi cross section 
defines a local vacuum which determines the amount of fuel delivered to 
the air on the basis of the pressure difference with respect to 
atmospheric pressure. In fuel injection systems, the air flow is generally 
sensed with great precision by means of air flaps and a resulting control 
variable is used to meter out fuel. It should be noted that both of these 
air and fuel metering systems can be embodied with the required precision 
only with substantial effort and cost. 
It has also been proposed to use simply available variables, namely the 
engine speed (rpm) and the throttle valve position to generate a control 
variable which corresponds to the air flow rate at any moment and which 
can be used to meter out the required fuel. This method introduces the 
difficulty that the connection between the fuel quantity per unit time on 
the one hand and the throttle valve position and engine speed on the other 
hand is a relatively complicated function. In a known circuit for 
controlling fuel injection valves, a monostable multivibrator receives 
pulses of rpm-dependent frequency and the unstable time constant is 
changed in dependence on a voltage related to the throttle valve position. 
The unstable time constant in the circuit determines the fuel injection 
period. This system starts with a linear rpm dependence and is able to 
provide only a relatively coarse adaptation of the injection time to the 
characteristic curves of injection timing which are specific to a 
particular internal combustion engine. In another known system, there is 
generated a voltage which depends on the rpm and on the position of the 
gas pedal for adjusting the position of a three dimensional cam via 
electromechanical transducers. The three dimensional cam which represents 
the operating characteristics of the engine is then followed by a second 
mechanical-electrical transducer which generates a suitable control 
variable for influencing the final control element of a fuel injection 
pump. If sufficient precision is required, this latter known system is 
very expensive because the attainable precision is adversely affected by 
the several transducing steps and by the mechanical following of the cam. 
OBJECT AND SUMMARY OF THE INVENTION 
It is therefore a principal object of the invention to provide a method and 
apparatus for controlling the composition of the combustible mixture of an 
engine in which the given connection between the aspirated air quantity of 
an engine on the one hand and the rpm and throttle valve position on the 
other hand can be stored precisely in an electronic circuit and in 
relatively inexpensive manner. 
It is a further object of the invention to use as electronic computer 
circuitry a relatively simple logical AND circuit which, in spite of being 
very inexpensive, permits a sufficient adaptation of the control signal to 
the actual conditions in which the engine operates. 
Yet another object of the invention is to provide means for obtaining a 
linear and unfalsified translation of the control variable into a 
metered-out fuel quantity. 
These and other objects of the invention are attained by generating an 
rpm-dependent voltage and a throttle valve position-dependent voltage and 
applying these two voltages to a logical AND circuit so as to obtain a 
voltage which is in turn dependent on the air flow rate through the 
engine. This signal is used for adjusting the composition of the fuel-air 
mixture by engaging an appropriate fuel metering system either as an 
analog signal or a derived cyclic signal. In this way, two simply measured 
variables are used with little expense for obtaining a precise fuel 
mixture control. 
The invention will be better understood as well as further objects and 
advantages thereof become more apparent from the ensuing detailed 
description of preferred exemplary embodiments taken in conjunction with 
the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It is well known that the quantity of combustible mixture required by an 
engine depends on the operational state of the engine. The operational 
mixture is generally obtained by measuring the air quantity aspirated per 
unit time and supplying thereto a requisite amount of fuel. A very precise 
metering of fuel is required for optimum operation especially with a view 
to rules and laws relating to the composition of exhaust gases. In 
general, a substantially stoichiometric mixture in which the air factor 
.lambda. = 1 is desired. The amount of air taken in by the engine depends 
in the first instance on rpm and also on the throttle valve position but, 
in addition, depends on particular flow conditions and oscillatory states 
within the induction tubes of particular engines and is normally measured 
with the aid of mechanisms such as venturis. However, it is possible to 
store the characteristic curves which relate the aspirated air quantity 
and the engine speed and throttle valve position in a particular engine 
and use these data for determining the metered-out fuel quantity. A set of 
characteristic curves of this type is shown in FIG. 1 where there are 
plotted curves for constant throttle valve position .alpha..sub.D which 
relate the aspirated air flow rate to the requisite fuel quantity for a 
constant air factor .lambda.. It will be seen that when the throttle valve 
angle is 90.degree., the functional relationship between the engine speed 
(rpm) and the required fuel quantity is substantially linear. It may also 
be observed that, beginning with a straight line at .alpha..sub.D = 
90.degree., the other curves, in which .alpha..sub.D is a constant less 
than 90.degree., become progressively more parallel to the abscissa at 
increasing rpm. These characteristic curves may be generated by an 
electronic circuit which will be explained in detail below with the aid of 
FIG. 2 without making direct measurements of the air flow rate through the 
engine. The signal finally obtained by the circuit to be described is 
proportional to the fuel quantity per unit time. 
Turning now to FIG. 2 there will be seen the circuit diagram of an 
apparatus for generating curves such as illustrated in FIG. 1. The circuit 
includes a pulse generator 1 which is associated with a rotational member 
of the engine and generates a pulse train at a frequency equal to the 
engine speed. Such a generator may be for example the ignition distributor 
of the engine. The pulses generated are fed to a frequency-to-voltage 
transducer 3 for generating a continuous voltage proportional to the input 
frequency, i.e., the engine speed. The output of the F-V converter 3 is 
connected via a resistor 5 to an input junction 6 of a logical AND circuit 
defined by the dashed border 7. In known manner, the circuit 7 includes a 
diode 9 and a diode 10 the anodes of which are joined and connected 
through a common resistor 12 to the positive voltage supply line 13 of the 
circuit. The cathodes define inputs 6 and 11, respectively. The joined 
anodes represent an output junction 15. 
A voltage divider consisting of series-connected resistors 18 and 19 is 
connected between the cathode of the diode 10 and ground. The junction of 
the two resistors 18 and 19 is connected through a further diode 17 to the 
input junction 6. The throttle valve position is transduced by a linear 
potentiometer 29 which provides a voltage related to the throttle valve 
angle and is mechanically coupled to the throttle plate 22. The throttle 
plate 22 rotates in a portion of the engines induction tube 23, which is 
not further illustrated. The potentiometer tap is connected to the 
non-inverting input of an impedance converter embodied as an operational 
amplifier 24, the output of which is connected to the throttle plate 
signal input point 11 and is also connected via a feedback line 25 to the 
inverting input 26. The voltage obtained at the output 15 is connected to 
the non-inverting input of a further operational amplifier 28 whose output 
is connected to the base of an NPN transistor 29. The collector of the 
transistor 29 is connected to a coil 30 which lies in series with the 
positive voltage supply line of the circuit while the emitter of the 
transistor 29 is connected to ground via an emitter resistor 31. The coil 
30 is an operational part of a solenoid which is used in a mechanism to be 
further described below for metering out fuel in proportion to the applied 
signal. 
The emitter of the transistor 29 is connected through a feedback path 
including a resistor 32 to the inverting input of the operational 
amplifier 28. The inverting input 33 of the operational amplifier 28 is 
further connected through a resistor 34 with the output of a comparator 35 
also embodied as an operational amplifier. The inverting input of the 
comparator 35 receives a sawtooth voltage of fixed frequency while the 
non-inverting input receives a correcting signal. The sawtooth signal is 
taken from the inverting input of another operational amplifier 37 
connected as a multivibrator 38. The multivibrator is constructed in know 
manner by connecting its inverting input 37 to a capacitor 39 which is 
grounded and to connect it further through a resistor 40 to its output 
thereby providing a feedback path. The non-inverting input of the 
operational amplifier 37 is connected to a voltage divider consisting of 
resistors 41 and 42 and via a resistor 43 to the output. The inverting 
input of the operational amplifier 35 receives a control signal which in 
this particular embodiment is shown to be the output signal of a control 
circuit 44 which receives the signal of a known oxygen sensor 45 located 
in an exhaust channel 46 of the engine and uses it to provide an 
appropriate control signal. A control circuit 44 which may be used is 
described for example in U.S. Pat. No. 3,903,853. 
The output of the operational amplifier 37 is connected via a resistor 47 
to the inverting input of a further operational amplifier 48 whose output 
is connected to the base of a second NPN transistor 49. The collector of 
the transistor 49 is also connected to the coil 30 in parallel with that 
of the transistor 29. The emitter of the transistor 49 is connected 
through a resistor 50 to ground and through a resistor 51 to its inverting 
input, thereby closing the feedback path of the operational amplifier 48. 
The non-inverting input of the operational amplifier 48 receives a voltage 
from a potentiometer 53 through a resistor 52 and is also connected 
through a resistor 54 and a series-connected capacitor 55 to the inverting 
input 26 of the previously described operational amplifier 24. 
The circuit described above operates as follows. The signal from the pulse 
generator 1 which may be, for example, an inductive pulse generator of 
known construction, is fed to the rpm input 6 through the resistor 5. 
Depending on the angular position of the throttle valve 22 and the 
consequent displacement of the potentiometer 21, the input junction 11 
receives a voltage related to the throttle valve position. In known 
manner, the voltage at the junction 15 follows the smaller of the two 
voltages present at inputs 6 and 11 respectively. In a chosen throttle 
valve position, one may therefore obtain the curve labeled "a" in FIG. 3. 
Beginning at a very low rpm, the rpm voltage signal is smaller than the 
throttle valve voltage so that the increase of the voltage at the junction 
15 at first follows the approximate straight line .alpha..sub.D = 
90.degree. with increasing rpm. As soon as the voltage at the junction 6 
is equal in magnitude to that at the junction 11, and for any subsequent 
increase in rpm, i.e., an increase of the rpm-dependent voltage, the 
output voltage 15 follows the voltage at the input 11. During any further 
rpm increase and for a fixed throttle valve position, no change in the 
voltage takes place. If the throttle valve opening is greater, one obtains 
a curve "b" parallel to the curve "a". In this manner a coarse adaptation 
of the signal to the family of curves .alpha..sub.D = constant of FIG. 1 
is obtained. 
In order to obtain an adaptation to the actual curves of FIG. 1 in the 
transition region between the proportional voltage increase obtained by 
the logical AND circuit 7 and the domain of constant voltage during 
further rpm increases, there is provided a network 16 consisting of the 
voltage divider resistors 18 and 19, a diode 17 and the resistor 5. The 
voltage divider resistors 18 and 19 define a voltage which is stepped down 
from that present at the throttle valve input 11. As soon as the voltage 
at the input contact 6 is larger than this stepped-down voltage, the diode 
17 begins to conduct. If the frequency-dependent voltage is further 
increased, the voltage at the output contact 15 no longer follows the 
proportional curve .alpha..sub.D = 90.degree. but follows instead a curve 
of lesser slope defined by the ratios of the resistors 5, 18 and 19. In 
this manner the input 6 reaches the level of the input 11 only at a higher 
rpm. The curve obtained in this manner is labeled "c" in FIG. 3. The point 
at which the curve "c" branches off from the curve .alpha..sub.D is 
determined by the ratio of the voltage dividing resistors. 
A further degree of adaptation to the actual curves may be obtained by 
connecting other parallel diode-resistor networks similar to that of the 
network 16. In this manner one may obtain the portion of the curve "d" 
within the curve "a". Thus the characteristic family of curves of FIG. 1 
is seen to be capable of simulation with considerable precision. The 
voltage occurring at the contact 15 is then used in proportionality to the 
metered-out fuel quantity. 
The fuel metering is performed by metering system which includes a coil 30 
which is part of the final control element of the fuel metering system. 
The coil 30 is actuated by rectangular pulses generated by a multivibrator 
38 which feeds the inverting input of the operational amplifier 48. 
Accordingly, the transistor 49 conducts cyclically and a pulsating current 
flows over its collector emitter path through the coil 30. Superimposed 
thereon is a constant current defined by the voltage present at the 
non-inverting input of the operational amplifier 48. Thus the total 
current through the coil 30 due to the operational amplifier 48 is a 
pulsating current the constant component of which is raised by a specific 
amount as long as the voltage at the non-inverting input of the 
operational amplifier remains constant. This voltage may be used for 
adjusting the idling state by means of the potentiometer 53. As is well 
known, the inverting input of an operational amplifier connected as a 
multivibrator exhibits a triangular voltage of constant frequency. Based 
on this triangular voltage and the correcting signal applied to the 
non-inverting input of the operational amplifier 35, the input 33 of the 
amplifier 28 receives a rectangular voltage of constant frequency and of a 
pulse width which varies according to the correcting signal. Here too, the 
operational amplifier 28 determines the current flowing through the 
transistor 29. As already described with respect to the transistor 49, a 
pulsating current flows through the coil 30 and the collector-emitter path 
of the transistor 29. This pulsating current is raised by a constant 
component according to the voltage applied to the inverting input of the 
operational amplifier 28. 
Thus the total current flowing through the coil 30 is composed of a first 
uniformly pulsating current and a second current which also pulses but 
which varies according to the output signal at the circuit point 15 and 
due to a control influence. In the illustrated example, the control 
influence is due to a circuit 44 which processes signals of an oxygen 
sensor but the invention is not limited to this example and any suitable 
control signal related to exhaust gas composition may be applied to the 
non-inverting input of the comparator 35. Such a signal may be, for 
example, from an engine roughness controller. However, yet other 
parameters of the engine may be used, for example the engine temperature, 
etc., for purposes of correction. The circuit of FIG. 2 further includes 
an RC member to improve the dynamic operation of the circuit. This RC 
member consists of a capacitor 55 connected in series with a resistor 54 
between the inverting input of the amplifier 24 and the non-inverting 
input of the amplifier 48. If the throttle plate 22 changes position and 
the inverting input 26 of the amplifier 24 experiences a suddenly 
increasing or decreasing voltage, the voltage at the non-inverting input 
of the amplifier 48 is changed for a short period of time in such a manner 
as to also change the collector current flowing through the coil 30 in the 
same sense as the throttle valve position change. 
It will be appreciated that the circuit illustrated in FIG. 2 and described 
above represents merely a preferred exemplary embodiment. The final 
control element of a fuel metering system may also be addressed in some 
other way based on the output voltage at the point 15 of the logical AND 
circuit 7. For example, a known circuit may generate a pulse train of 
constant frequency and use the voltage at the point 15 for changing the 
pulse width. This derived signal may then be used for actuating 
electromagnetic injection valves. The cycling frequency must be chosen 
large enough so that even when the engine runs at full rpm, each cylinder 
receives fuel at least once for each set of cycles. Instead of addressing 
a final control element in cyclic manner, for example by means of the coil 
30, it would be possible to provide a final control element which responds 
to analog control on the basis of the voltage at the point 15 of the AND 
circuit 7. Pulsating, i.e., cyclic control is preferred, however, if 
mechanical hysteresis due to friction is to be prevented. 
The fuel metering system which may be used in association with the circuit 
described so far is embodied in a first variant in FIG. 4. In that 
embodiment, there is shown an internal combustion engine 6 with an exhaust 
system 46 and an induction tube 23. The induction tube contains a throttle 
valve 22 which is coupled to the linear potentiometer 21. Associated with 
the crankshaft 61 is the pulse generator 1 which may be for example a part 
of the ignition distributor of the engine. As described with respect to 
FIG. 2, the pulse generator is connected to the frequency-to-voltage 
converter 3 the output of which leads to a control circuit 62 described in 
detail above in connection with FIG. 2. This circuit 62 is shown as a 
symbolic box in FIG. 4 and it receives the signal from the potentiometer 
21 and from the correcting circuit 44 which receives its information from 
an oxygen sensor 45 in the exhaust system 46. The output of the circuit 62 
is connected to the coil 30 which is part of the final control element 64 
of a fuel metering system 65. The fuel metering device 65 may be a known 
mechanism including a metering piston 66 which is sealingly guided in a 
bore 67 and which has an upper annular groove 68 and a lower annular 
groove 69 which communicate through radial bores 70 and 71 as well as 
through an axial bore 72. In the blind bore 67, the metering piston 66 
defines a space 73 into which extends an actuating pin 75 which is guided 
in a bore 74 which is coaxial with the bore 67. The actuating pin is the 
armature of a solenoid 76 which is used as the final control element 64 
and which employs the previously mentioned coil 30 to provide an actuating 
magnetic field. The solenoid further includes a return spring 77 which 
engages the piston at its opposite end and which is supported on a set 
screw 78 in a bracket 79 coupled to the housing of the metering system. A 
relatively weak holding spring 80 also acts on the metering piston 66 via 
the pin 75. 
The fuel metering system 75 further includes differential pressure valves 
81 the number of which corresponds to the number of fuel injection 
locations 82 in the induction tube and which are located around the 
metering piston 66. Each of the differential pressure valves includes a 
diaphragm 83 which defines a control chamber 84 and a reference chamber 
85. The control chamber receives the terminus of an injection line 86 
coupled to an injection location 82 which is the valve seat whose opening 
is adjusted by the position of the diaphragm 83. The control chamber also 
includes a compression spring 93 tending to open the valve seat. A bore 87 
connects each of the control chambers 84 in the individual differential 
pressure valves 81 with the cylindrical bore 67 and defines fuel metering 
apertures 88 the cross section of which can be altered by the lower edge 
of the upper annular groove 68 depending on the axial position of the 
metering piston 66. The reference chambers 85 of the various differential 
pressure valves 81 are in constant communication via bores 89 extending 
radially from the bore 67 and the lower annular groove 69. Fuel is 
supplied to the reference chambers 85 via a fuel line 90 coming from a 
fuel supply pump 92 which pumps fuel from a fuel container 91. A pressure 
control valve 94 in parallel with the pump 92 regulates the return flow of 
fuel through a return line 96 with the aid of a spring loaded piston 95. 
The pressure control valve 94 thus maintains a constant fuel pressure in 
the reference chambers 85 and this pressure is communicated through the 
lower annular groove 69 and the axial bore 72 to the metering apertures 
88. The operation of the just described fuel metering system is as 
follows. Depending on the degree of actuation of the coil 30 by the 
circuit 62, a pulsating current of variable magnitude flows through the 
coil and results in an appropriate displacement of the actuating pin 75. 
As a result, the metering piston 66 and thus the free aperture of the 
metering openings 88 is adjusted. The differential pressure valves 81 
always maintain a constant pressure difference across each of the metering 
openings and this differential pressure is determined substantially by the 
tension of the compression springs 93 and the constant reference pressure. 
Thus the quantity of fuel is metered out only in dependence on the axial 
position of the piston 66 and not on any of the pressure conditions 
prevailing at the injection locations 82. If the metering apertures 88 are 
suitably dimensioned, it is possible to obtain a linear dependence of the 
fuel quantity supplied to the various injection lines on the axial 
displacement of the metering piston 66. It is also possible to so 
construct the magnet 76 as to obtain a linear displacement of the metering 
piston 66 as a function of the control voltage at the point 15 of the 
logical AND circuit. 
Inasmuch as the end face of the metering piston 66 is exposed to 
atmospheric pressure as well as to the force of the spring 77, the 
metering system is provided with an adjustment which responds to the 
altitude of operation. An altitude adjustment can also be provided by a 
barometric device connected ahead of the spring 77. The idling position 
may be adjusted by means of the set screw 78. An advantage of the 
construction of the present embodiment is that the metering piston 66 is 
pushed into its topmost position in which the metering apertures 88 are 
closed whenever the current through the coil 30 fails. Only when the 
current through the coil 30 increases is the metering piston 66 displaced 
downwardly and according to the voltage present at the output 15 of the 
AND circuit 7 so that an increasing cross section of the metering 
apertures 88 is opened. 
A second embodiment of a fuel metering device which may be used under the 
control of the circuit of FIG. 2 is illustrated in FIG. 5. In most 
respects the apparatus of FIG. 5 is similar to that of FIG. 4 and 
identical parts will retain the same reference numerals. Their description 
may be obtained from the foregoing example. The embodiment of FIG. 5 
differs from the previous example by the provision of a magnetic valve 98 
with a coil 99 controlled by the circuit 62. The magnetic valve 98 lies in 
a return line 100 leading back to the fuel container 91. The line 100 is 
connected to the chamber 73 defined at the top of the metering piston 66 
and this chamber 73 is constantly connected via a throttle 101 with the 
axial bore 72 and hence with the reference chambers 85. Located between 
the electromagnetic valve 98 and the space 73 is an adaptation throttle 
102 for those cases in which the valve is cyclically actuated. The 
actuation signal, as already described, is based on the control voltage at 
the output contact 15 of the logical AND circuit and is transformed in 
known manner into a pulse train having constant frequency and variable 
pulse width depending on the control voltage. 
However the electromagnetic valve 98 may also be controlled by using the 
circuit 62 to modulate the control signal in which its coil 99 is 
equivalent to the coil 30 of FIG. 2. 
Depending on the actuation of the electromagnetic valve 98, the pressure in 
the chamber 73 is altered. The pressurized fluid displaces the metering 
piston 66 to varying degrees in the direction of the spring 77 and thus 
changes the flow cross section through the apertures 88. 
A third embodiment of the fuel metering system to be used with the control 
circuit 62 is illustrated in FIG. 6. This example is substantially similar 
to that of FIG. 5 except for the insertion of a pressure controller 103 to 
take the place of the electromagnetic valve 98 in the return line 100. In 
this case, the previously described throttle 102 is not present because 
the pressure controller itself provides the required throttling. This 
pressure controller 103 is actuated by the circuit 62 in a manner similar 
to that described in the exemplary embodiment of FIG. 4 by passing through 
a coil 30' a pulsating current superimposed on a constant current of a 
magnitude depending on the control signal at the output 15 of the AND 
circuit 7. 
FIG. 7 illustrates the pressure controller 103 of FIG. 6 in detail. In this 
exemplary case, the pressure controller includes a pressure cell 105 in 
which a diaphragm 106 defines a reference chamber 107 and a pressure 
chamber 108. The reference chamber has a relief bore 109 and a compression 
spring 110 which urges the diaphragm 106 to close a valve seat provided at 
the terminus of a line 100b extending into the pressure chamber 108. The 
pressure chamber 108 is always connected through a conduit 100a belonging 
to the return line 100 with the chamber 73. The conduit 100b connects the 
pressure chamber 108 with the fuel container 91. The terminus of the line 
100b and thus the flow cross section is determined by the diaphragm 106. 
On the side of the pressure chamber 108, the coil 30' is fixedly attached 
to the diaphragm. This coil enters an annular depression 112 of an annular 
magnet 113 which constitutes the bottom of the pressure cell. The annular 
depression provides a magnetic core 111 which is concentric with the coil 
30' and which receives the concentric partial conduit 100b of the return 
line 100. The magnetic core 111 and the coil 30' are embodied to 
preferably provide a setting magnet having linear characteristics. 
Electrical lines 114 and 115 connect the coils to the circuit 62 and 
ground. 
The foregoing construction permits an adjustment of the force exerted on 
the diaphragm in addition to that of the spring 110. Depending on the 
magnitude of the current which passes through the coil 30', the diaphragm 
106 is lifted from the terminus of the conduit 100b in opposition to the 
force of the spring 110 at a relatively higher or lower pressure in the 
pressure chamber 108. Thus depending on the current through the coil, 
there is obtained in the pressure chamber 108 and hence also in the 
chamber 73 a variable fuel pressure. The metering piston 66 is displaced 
by this variable pressure in a manner described in connection with FIG. 5. 
Instead of employing the above-described pressure control valve, it is 
also possible to use a pressure control valve of different construction 
which may be engaged electromagnetically. A significant aspect of the 
various pressure controllers is that the pressure in the chamber 73 is 
made linearly dependent on the control voltage present at the output 15 of 
the logical AND circuit 7. If a pressure controller is used and if a 
piston is employed instead of the diaphragm 106, it is advantageous to 
employ a pulsating control for the coil 30' in order to prevent one sided 
frictional effects and undesirable hysteresis. 
The method and the various embodiments of the apparatus described above 
make possible a sufficiently precise control of the composition of a 
fuel-air mixture of an engine in relatively simple manner. A particular 
advantage of the method and apparatus of the invention is to make 
unnecessary a separate air flow metering device. This omission not only 
results in reduction of cost but provides the additional advantage that 
the throttling losses in the induction system of the engine are kept low, 
resulting in a relative increase of power. It is further possible to 
provide multiplicative adjustment depending on the exhaust gas composition 
or the engine roughness. Inasmuch as the control voltage at the output of 
the logical AND circuit 7 is proportional to the fuel quantity (if a 
constant air factor .lambda. is assumed) this control voltage may be used 
advantageously for a direct indication of the fuel consumption per unit 
time. A known dividing device may be employed to provide an indication of 
the fuel consumption in liters per 100 kilometers or miles per gallon. 
The foregoing relates to preferred exemplary embodiments of the invention, 
it being understood that other embodiments and variants thereof are 
possible within the spirit and scope of the invention, the latter being 
defined by the appended claims.