Note: Descriptions are shown in the official language in which they were submitted.
BACI~GROUND OF THE INVENTION
The present invention relates to fuel control system
for internal combustion engines, and in particùlar 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
~15 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 funGtion of temperature to a low or normally
operating value when the engine has warmed up. Therefore,
~0 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.
SUM~ Y 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 present invention, there is provided
a method for controlling the air-fuel ratio of mixture
supplied to an internal combustion engine having air-
fuel correcting means and an exhaust gas sensor f~r
generating a signal represen-tative of the concentration
of an exhaust composition of the emission from the engine,
the gas sensor having an internal impedance whi.ch varies
inversely as a function of temper~ture, and means for
generating a signal representative of the deviation of
the concentration representative signal from a reference
value, comprising the steps of: a) supplying a sub-
stantially constant current into the exhaust.gas sensorto develop a corresponding voltage across the internal
impedance; b) detecting when the voltage across the
internal impedance reduces to a level below a first
threshold level; c) applying the deviation representative
signal to the air-~uel correcting means substantially
in response to the step (b) to cause the mixture to be
controlled in a closed loop mode; d) interrupting the
current for a predetermined period of time in response
to the step (b); e) generating a first or a second
signal depending respectively on whether the concentration
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78
representative signal generated during the period of
time is above or below a second threshold level lower
than the first threshold level; f) varying the reference
value in different directions in response to the presence
of one of the first and second signals; and g) decreasing
the reference value in response to the step (f) as a
function of temperature until the second threshold level
is reached.
A first voltage detector or comparator is provided
to detect when the voltage delivered from the gas sensor
reduces to a level below a first threshold level to
brie1y 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 levei of the gas sensor; the second
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378
threshold level being lower than the first threshold
level and corresponding to the constant reference point
of the closed loop operation which is effec-ted 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 wil] 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
~15 the system commences feedback opera-tion 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.
BRIEF DESCRIPTION OF THE DR~WINGS
The invention will be further described by way of
example with reference to the accompanying drawings, in
- which:
Fig. 1 is a schematic block diagram of an embodiment
of the invention;
L8~7~3
~ ig. 2 is a graphic illustration of the operating
characteristics of an exhaust gas sensor into which a
constant current is injected;
Fig. 3 is a waveform diagram useful for describing
the operation of the embodiment when the gas sensor's
initial condition representents a lean mixture; and
Fig. 4 is a waveform diagram useful for describing
the operation of the embodiment when the gas sensor's
initial condition represents a rich mixture.
DET~ILED DESCRIPTION
In E'ig. 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
15 a gas sensor output signal for application to the non-
inv~orting 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 ambi~nt temperature is very low and has
a small internal impe~ance 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
effec-ted 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 6~ such as
electronic carburetor or fuel injection con-trol unit.
In the initial period of engine start, the switch 15
remains off, so that the mixture i!3 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 sub-
stantial magnitude across the internal impedance 11 of
the gas sensor 10, since the gas sensor internal impedance
- 20 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 opexation 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
:: 15 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 VH (= 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 VH. The
monostable 24 generates an inhibit pulse for disabling
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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 VH at point a in Fig. 2, the voltage
will reduce to a point _ on curve X' which lies above a
second or lower threshold level VL which corresponds to
the stoichiometric point of the mixture ratio, and if
it crosses VH at point c the voltage will reduce to
a point d on curve Y' which lies below the lower threshold
VL. Therefore, it is appreciated 'that whether the gas
sensor indication is rich or lean can be determined by
sensin~ the reduced voltage level :relative to threshold VL
This is accomplished by a comparator 28 which
receives the amplified gas sensor output on its non-
inverting input for comparison with a reference voltage
corresponding to the threshold value VL supplied from
terminal 30. This voltage reduction manifests itself
in a delayed interval from the time of the dlsablement.
A delay circuit 26 is connected to the output of the
comparator 20 to introduce a delay to trigger a mono-
stable 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
~L807~
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 ~s 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 a2 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 ~rom 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 cross.ing the threshold level VH at time tl
: 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 corres- .
ponding 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 a~ter 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 VL during the time prior
to time t2, 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 VL. The gas sensor output then adopts ~:
the curve Y' during the interval the injection current
is inhibited until time -t3 at which the monostable 24
output terminates and returns to t'he curve Y. Simultaneously,
the comparator 28 output returns to the high voltage level.
The low voltage output from t'he 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 VH 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 VHH,
whereby the output of the integrator 48 and hence the
potential at the noninverting input of the comparator 1
g _
'' ~'.
7~3
is raised from VH to VHH 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 circui.t con:Eiguration with the operational
amplifier and is arranged to receive a positive polarity
input voltage B+ of a suitable value from a terminal 5
: 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 t2 in response to the closure of switch 15. As
a resultr the gas sensor output begins to fluctuate between
chain-dot curves X and Y as indicated in E`ig. 3. More
specifically, the comparator 14, which is initially at
low output state, switches to a high voltage output state
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078
at time tl when the gas sensor output falls below the
reference level VH. 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 non-
inverting 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 t4. Thus, during
the subsequent period between times t4 and t5~ the
comparator 14 remains in the low voltage condition and -
the switch 58 is thus inhibited. The integrator 48
suspends integration during the tlme interval t4 to t5
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
reEerence point VH 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
be-tween curves X and Y.
A clamping cixcuit 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 VL after the output of integrator 4~ -
reaches VL.
During the closed loop operation, the comparator 14
is fed with the constant reference voltage VL with which
the gas sensor output is compared t:o 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 initialiy indicative
of a rich conditi~n, 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~. 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 VL~ the compara-tor 28 remains in the
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8~7~
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 VHL, so that the noninverting input
of the comparator 14 is lowered from VH to V~IL as indi-
cated by broken lines 51 in Fig. 4. Since the comparator
14 is switched to the low output st:ate at time tl in
response to the threshold VH reduci.ng to VHL, the switch
58 is held open and the integrator 48 thus maintains its
output constant until time t2 when the gas sensor output
falls below the lower threshold VH]. During time interval
between t2 and t3 the gas sensor output is reduced to the
minimum voltage level on curve Y, permitting the com-
parator 14 to generate a high voltage output pulse 14-1.
~he pulse 14-1 is coupled via switch 60 to the con-trol
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 rlegative
direction as mentioned previously, reducing the threshold
potential at the noninverting input of the comparator 140
In a subsequent interval between times t3 and t~ the
integrator 48 suspends integration and maintains its
output voltage constant.
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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 VL, 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
lS system, thereby reducing the amount of noxious emissions
during engine warm-up periods.
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