Variable reference mixture control with current supplied exhaust gas sensor

Mixture ratio control for internal combustion engines supplies a constant current into an exhaust gas sensor to develop a voltage of a substantial magnitude in proportion to the initial high value of the gas sensor internal impedance during low temperature conditions. The voltage so developed decreases as a function of time corresponding to the decrease of the internal impedance with temperature. A voltage detector is provided to trigger the control system to operate in a closed loop mode when the gas sensor voltage is reduced to a level below a first threshold level. Responsive to the output of the voltage detector the supplied current is momentarily interrupted to allow the gas sensor voltage to drop rapidly to a level which is higher or lower than a second threshold level depending on the concentration of the sensed gas in the exhaust system. A second detector senses this voltage drop relative to the second threshold to determine whether the gas sensor output represents rich or lean condition. The reference point of the closed loop is raised or lowered in response to the output of the second detector and further decreased as a function of time such that the reference point lies within the range between maximum and minimum peak values of the gas sensor output signal.

BACKGROUND OF THE INVENTION 
The present invention relates to fuel control system for internal 
combustion engines, and in particular to a method and system for 
controlling the air-fuel ratio of mixture supplied to the engine in a 
closed loop operational mode during warm-up periods to thereby reduce the 
harmful components of the emission during such periods. 
In conventional closed loop fuel control systems the air-fuel ratio of 
mixture supplied to the engine is corrected in response to a feedback 
signal which represents the deviation of the concentration of oxygen in 
the exhaust emissions detected by a zirconia dioxide oxygen sensor from a 
reference point which usually corresponds to the stoichiometric air-fuel 
ratio. The internal impedance of the oxygen however exhibits a 
considerably high impedance value when temperature within the exhaust 
system is low during warm-up periods. This impedance decreases as a 
function of temperature to a low or normally operating value when the 
engine has warmed up. Therefore, the signal provided by the gas sensor 
having a high internal impedance value cannot be used as a valid feedback 
signal and the conventional practice is to suspend the closed loop mode 
until the engine has warmed up, tending to produce a considerable amount 
of noxious emissions during warm-up periods. 
SUMMARY OF THE INVENTION 
According, it is an object of the invention to allow closed loop fuel 
control operation to commence during warm-up periods to decrease the 
noxious emissions. 
According to the invention, this object is achieved by supplying a 
substantially constant current to the gas sensor to allow it to develop a 
corresponding voltage across its high impedance during warm-up periods. 
Since the internal impedance decreases as a function of temperature, the 
voltage so developed decreases accordingly. However, the voltage has 
different value depending on the initial concentration of oxygen gas 
within the emissions. If the concentration represents a rich mixture 
condition, a higher voltage output will be delivered from the gas sensor 
than that generated during the lean mixture condition. 
A first voltage detector or comparator is provided to detect when the 
voltage delivered from the gas sensor reuduces to a level below a first 
threshold level to briefly interrupt the current to the gas sensor and to 
allow the system to commence closed loop operation. In response to this 
current suspension, the voltage output from the gas sensor rapidly reduces 
to a level which is higher or lower than a second threshold level 
depending on the initial voltage level of the gas sensor; the second 
threshold level being lower than the first threshold level and 
corresponding to the constant reference point of the closed loop operation 
which is effected when the gas sensor is operating above its normally 
operating temperature. If the initial gas sensor output represents a rich 
mixture the voltage will reduce to a level higher than the second 
threshold and conversely, under lean initial condition, the voltage will 
reduce to a level lower than the second threshold. 
A second detector is provided to sense the extent of the voltage drop with 
respect to the second threshold to detect the initial condition of the gas 
sensor. The reference point of the system is initially set at a point 
corresponding to the first threshold level with which the system commences 
feedback operation and varied in different directions depending on the 
output from the second threshold detector. The varied reference point is 
then allowed to decrease as a function of time until it reaches the second 
threshold level.

DETAILED DESCRIPTION 
In FIG. 1, an air-fuel mixture control system embodying the invention 
comprises an exhaust gas sensor 10 provided in the exhaust conduit of an 
internal combustion engine 12 upstream from a catalytic converter 13 to 
generate a gas sensor output signal for application to the noninverting 
input of a comparator 14 through a buffer amplifier 16. The gas sensor 10 
is of a zirconia dioxide type which detects the concentration of oxygen 
gas in the exhaust emissions and generates a corresponding electrical 
signal. This oxygen gas sensor has a very large internal impedance when 
ambient temperature is very low and has a small internal impedance when 
the temperature is high. Therefore, gas sensor signals are usually valid 
only when the gas sensor is above its normally operating temperature, and 
closed loop fuel control is conventionally effected in response to such 
valid gas sensor signals. The comparator 14 compares the gas sensor output 
signal with a reference voltage supplied to its inverting input to develop 
a signal representative of the deviation of the concentration of the 
sensed exhaust composition from the reference point which is usually set 
at a point at or near the stoichiometric air-fuel ratio. The deviation 
signal from the comparator 14 is coupled by a normally open switch 15 to a 
proportional/integral controller 17 and thence to an air-fuel correction 
means 68 such as electronic carburetor or fuel injection control unit. In 
the initial period of engine start, the switch 15 remains off, so that the 
mixture is controlled in an open loop mode. 
A constant current source 18 is provided to supply electric current of a 
substantially constant magnitude into the gas sensor 10 developing a 
voltage of a substantial magnitude across the internal impedance 11 of the 
gas sensor 10, since the gas sensor internal impedance is considerably 
high during warm-up periods. 
Since the gas sensor internal impedance reduces as a function of 
temperature, the voltage so developed also decreases correspondingly. 
Therefore, as engine warm-up operation progresses the gas sensor voltage 
decreases with time. The voltage developed in response to the current also 
depends on the concentration of oxygen gas within the exhaust system, as a 
results it adopts one of curves X and Y illustrated in FIG. 2 depending on 
the sensed concentration representing rich or lean mixture condition, 
respectively. Curves X and Y are also representative of a plot of maximum 
and minimum peak values of the gas sensor 10. During the time prior to 
closed loop operation, the gas sensor output tends to remain on one of the 
plotted curves depending on its initial condition, and during the closed 
loop operation the gas sensor output fluctuates between curves X and Y 
depending on the relative value of the gas sensor output to the reference 
point of closed loop control system. With no injection current, the gas 
sensor exhibits an output of low voltage level as indicated by curve X' or 
Y' corresponding to rich or lean mixture condition, respectively. 
The voltage developed by the gas sensor 10 is applied to the inverting 
input of a comparator 20 for comparison with a fixed threshold voltage 
V.sub.H (=1.2 volts) supplied from terminal 22 to provide a high voltage 
output to a monostable multivibrator 24 and concurrently to a delay 
circuit 26 when the gas sensor output voltage reduces to a level lower 
than the threshold V.sub.H. The monostable 24 generates an inhibit pulse 
for disabling the constant current source for a short interval. This 
results in a rapid reduction of the gas sensor output to the level of one 
of the curves X' and Y' depending on the previous condition of the gas 
sensor 10. For example, if the gas sensor output adopts curve X and 
crosses the threshold V.sub.H at point a in FIG. 2, the voltage will 
reduce to a point b on curve X' which lies above a second or lower 
threshold level V.sub.L which corresponds to the stoichiometric point of 
the mixture ratio, and if it crosses V.sub.H at point c the voltage will 
reduce to a point d on curve Y' which lies below the lower threshold 
V.sub.L. Therefore, it is appreciated that whether the gas sensor 
indication is rich or lean can be determined by sensing the reduced 
voltage level relative to threshold V.sub.L. This is accomplished by a 
comparator 28 which receives the amplified gas sensor output on its 
noninverting input for comparison with a reference voltage corresponding 
to the threshold value V.sub.L supplied from terminal 30. This voltage 
reduction manifests itself in a delayed interval from the time of the 
disablement. A delay circuit 26 is connected to the output of the 
comparator 20 to introduce a delay to trigger a monostable multivibrator 
32 to allow it to generate a sampling or enabling pulse for sampling AND 
gates 36 and 38. AND gate 36 has an inverted input connected to the output 
of the comparator 28 and AND gate 38 has a noninverted input connected to 
the output of this comparator. Therefore, AND gate 36 produces a logic "1" 
when the comparator 28 output is low in the presence of the sampling 
pulse, and AND gate 38 produces a logic "1" when the comparator 28 output 
is high in the presence of said sampling pulse. Flip-flop circuits 40 and 
42 are provided to receive output signals from sampling gates 36 and 38, 
respectively. These flip-flops are initially reset in response to a low 
voltage output from the comparator 14. 
Assuming that the initial output condition of the gas sensor 10 indicates a 
lean condition adopting curve Y as illustrated in FIG. 3. The comparator 
20 will be switched to a high output state in response to the voltage on 
curve Y crossing the threshold level V.sub.H at time t.sub.1 causing 
monostable 24 to produce a pulse 24-1 which is applied to the injection 
current source 18. During this pulse period, the injection current is 
inhibited to cause the voltage across the internal impedance 11 of the gas 
sensor 10 to drop sharply to a level corresponding to the curve Y' after a 
delay interval T. In response to the high voltage output from the 
comparator 20, the monostable 32 is triggered after the delay interval 
introduced by the delay circuit 26 to produce a sampling pulse 32-1 for 
application to the AND gates 36, 38. Since the potential at the 
noninverting input of the comparator 28 is higher than the threshold 
V.sub.L during the time prior to time t.sub.2, the comparator 28 remains 
in the high output state until that time and then switches to a low output 
state in response to the gas sensor output reducing to a level below 
V.sub.L. The gas sensor output then adopts the curve Y' during the 
interval the injection current is inhibited until time t.sub.3 at which 
the monostable 24 output terminates and returns to the curve Y. 
Simultaneously, the comparator 28 output returns to the high voltage 
level. 
The low voltage output from the comparator 28 is sampled by AND gate 36 in 
response to the sampling pulse 32-1 and triggers the flip-flop 40 into a 
set condition producing therefrom a signal indicating that the gas sensor 
10 is in a lean condition. This signal is applied to the control terminal 
of a switch 44 which is provided with a home position H and lean and rich 
positions L and R. In the absence of a control signal, the switch 44 is in 
the position H to couple the threshold voltage V.sub.H from terminal 22 to 
the noninverting input of an integral operational amplifier 48 and 
activated in response to the lean condition signal from flip-flop 40 to 
switch to the lean position L to connect a higher threshold voltage 
V.sub.HH' whereby the output of the integrator 48 and hence the potential 
at the noninverting input of the comparator 14 is raised from V.sub.H to 
V.sub.HH as indicated by broken lines 50 in FIG. 3. The integrator 48 
includes a resistor 52 and a capacitor 54 which are connected in the known 
integrator circuit configuration with the operational amplifier and is 
arranged to receive a positive polarity input voltage B+ of a suitable 
value from a terminal 56 via switch 58 and resistor 52 at the inverting 
input thereof. The output from the delay circuit 26 is also coupled to 
switches 60 and 15 to enable them to pass the output of comparator 14 to 
the control gate of switch 58 and to a proportional/integral controller 
17, respectively. The controller 17 modifies the output of the comparator 
14 in accordance with predetermined control characteristics and supplies 
its output signal to an air-fuel correction means 68 such as electronic 
carburetor or fuel injection circuit in order to correct the mixture ratio 
in accordance with the deviation of the gas sensor output from the 
variable reference voltage applied to the comparator 14. The fuel control 
system of the invention is thus switched from the initial open loop mode 
to a closed control mode at time t.sub. 2 ' in response to the closure of 
switch 15. As a result, the gas sensor output begins to fluctuate between 
chain-dot curves X and Y as indicated in FIG. 3. More specifically, the 
comparator 14, which is initially at low output state, switches to a high 
voltage output state at time t.sub.1 when the gas sensor output falls 
below the reference level V.sub.H. Thus the high voltage signal from the 
comparator 14 is coupled through switch 60 to the control terminal of 
switch 58 to apply the positive potential to the inverting input of the 
integrator 48 through resistor 52 to permit it to allow integration of the 
input voltage in the negative direction with respect to the polarity of 
the potential at the noninverting input thereof, resulting in a gradual 
reduction of the reference voltage at the noninverting input of the 
comparator 14 as shown in FIG. 3 until the comparator 14 switches to a low 
voltage state in response to the gas sensor output becoming higher than 
the reference potential supplied from the integrator 48 at time t.sub.4. 
Thus, during the subsequent period between times t.sub.4 and t.sub.5, the 
comparator 14 remains in the low voltage condition and the switch 58 is 
thus inhibited. The integrator 48 suspends integration during the time 
interval t.sub.4 to t.sub.5 and holds its output voltage constant. 
It is thus appreciated that the reference potential for the comparator 14 
is increased in response to the gas sensor output reducing to a level 
below the higher reference point V.sub.H and then decreased in step with 
the change in output state of the comparator 14 if the initial condition 
of the gas sensor indicates a lean condition, and the reference voltage 
adopts a curve which lies between curves X and Y. 
A clamping circuit 66 is connected between terminal 30 and the noninverting 
input of comparator 14 to clamp the variable reference potential at the 
level of the low threshold V.sub.L after the output of integrator 48 
reaches V.sub.L. 
During the closed loop operation, the comparator 14 is fed with the 
constant reference voltage V.sub.L with which the gas sensor output is 
compared to develop a signal representative of the deviation of air-fuel 
ratio from the reference point. The catalytic converter 13 is exposed thus 
to the controlled exhaust gases and operates at maximum efficiency to 
convert the hamful emissions into harmless products. 
Conversely, if the gas sensor is initially indicative of a rich condition, 
the output voltage therefrom adopts the curve X as shown in FIG. 4 which 
decreases with time to a point where it crosses the high threshold 
V.sub.H. This is detected by the comparator 20 producing a high voltage 
signal which triggers the monostable 24 and delay circuit 26, the latter 
subsequently triggering the monostable 32 in the same manner as described 
in connection with FIG. 3. The gas sensor output on curve X thus rapidly 
drops to the corresponding point on curve X'. Since the level of the 
reduced gas sensor output is still higher than the threshold V.sub.L, the 
comparator 28 remains in the high output state which is sampled in 
response to the monostable 32 output to activate AND 38 triggering a 
flip-flop 42 into the set condition to indicate that the initial condition 
of the gas sensor 10 represents enriched mixture. The switch 44 is 
activated to couple a lower threshold voltage V.sub.HL, so that the 
noninverting input of the comparator 14 is lowered from V.sub.H to 
V.sub.HL as indicated by broken lines 51 in FIG. 4. Since the comparator 
14 is switched to the low output state at time t.sub.1 ' in response to 
the threshold V.sub.H reducing to V.sub.HL, the switch 58 is held open and 
the integrator 48 thus maintains its output constant until time t.sub.2 
when the gas sensor output falls below the lower threshold V.sub.HL. 
During time interval between t.sub.2 and t.sub.3 the gas sensor output is 
reduced to the minimum voltage level on curve Y, permitting the comparator 
14 to generate a high voltage output pulse 14-1. The pulse 14-1 is coupled 
via switch 60 to the control terminal of switch 58 to apply B+ potential 
to the inverting input of the integrator 48, so that the latter provides 
integration of the input voltage in the negative direction as mentioned 
previously, reducing the threshold potential at the noninverting input of 
the comparator 14. In a subsequent interval between times t.sub.3 and 
t.sub.4 the integrator 48 suspends integration and maintains its output 
voltage constant. 
It will be understood from the foregoing that the reference point of the 
fuel control system of the invention is first raised or lowered by a 
predetermined amount depending on the initial condition of the gas sensor 
10 and then decreased in step with variations in the gas sensor output 
voltage with respect to the reference point as the system commences closed 
loop operation until the reference value reaches V.sub.L, whereupon the 
reference potential is held at this value by means of the clamping circuit 
66. The variable reference point thus lies within the range between curves 
X and Y during the initial stage of the gas sensor operation and the 
system is switched to closed loop operational mode earlier than the prior 
art closed loop fuel control system, thereby reducing the amount of 
noxious emissions during engine warm-up periods.