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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a method of improving the exhaust-gas
behavior of mixture-comprising internal combustion engines, in which the
output signal of an oxygen measurement probe arranged in the exhaust gas
channel of the internal combustion engine is fed to a controller the
output voltage of which represents a setting variable for controlling the
fuel-air ratio.
The invention furthermore relates to a device for the carrying out of the
method.
In the art of preparing the fuel-air mixture for mixture-compressing
internal combustion engines, it has been known for a long time to control
the composition of the mixure as a function of the output signal of an
oxygen measurement probe arranged in the stream of exhaust-gas in order to
achieve the best possible elimination of noxious exhaust gases. In this
case, the output signal of the oxygen measurement probe is fed to a
controller, and the output voltage of the controller forms a setting
variable for controlling the fuel-air ratio.
By the control of the fuel-air ratio of internal combustion engines, it is
desired primarily to reduce noxious contents of the exhaust-gas emissions
of internal combustion engines. For this purpose, a method is known, for
instance, which employs an oxygen measurement probe arranged in the
exhaust-gas stream of the internal combustion engine and controls an
integrating device, the output signal of the exhaust-gas measurement probe
being applied to a threshold switch, and switching the latter when the
threshold valve is reached. The direction of integration of the
integrating device is changed upon the switching of the threshold switch.
The time constant of the integrating device is changed as a function of at
least one operating parameter of the internal combustion engine, and
particularly as a function of the amount of intake air of the internal
combustion engine. The integrating device of variable time constant
proposed in the case of the known method, however, does not satisfy all
requirements with respect to precise and adaptable control. In particular,
in the known method the setting variable does not follow sudden changes in
loads sufficiently rapidly.
The voltage signal of an oxygen measurement probe typically has a steep
course, in the region of lambda=1, which passes in each case into a flat
branch directly adjoining this region. The voltage signal of the oxygen
measurement probe acts on the means for forming the mixture in the manner
that upon a signal from the probe which corresponds to a lambda of less
than 1, the mixture is made leaner, while with a probe signal of more than
1, the mixture is made richer. As a result of the steep course of the
characteristic curve, the control passes in quasi-steady state
continuously through the region of the characteristic curve between the
two points of inflection. On the average, the compostion of the mixture is
such that it corresponds to the stoichiometric value and the exhaust gas
can be substantially freed of undesired noxious portions by a catalytic
exhaust-gas purification device.
Upon a sudden change in load, such as occurs frequently in the normal
travel of an automotive vehicle, the value of the fuel-air ratio, lambda,
can differ considerably from the desired value 1, primarily as a result of
condensation processes in the intake region of the internal combustion
engine. As a result of the flat course of the characteristic curve of the
voltage value of the oxygen measurement probe with lambda values above or
below a value of 1, however, only a relatively small disturbance signal is
produced so that the control device only inadequately recognizes the
actual deviation of the lambda value from the desired condition.
Accordingly, the return of the control to the value of 1 takes an
undesiredly long period of time and, during this time, the exhaust gases
contain a high percentage of noxious substances. Furthermore, the
continuous variation of the fuel-air ratio within relatively wide limits
around a value of 1 results in a periodic change in the composition of the
exhaust gas which must be counteracted in the following manner, namely,
that both the exhaust-gas measurement probe and the catalytic
after-treatment device for the purification of the exhaust gas are
arranged at a minimum distance from the outlet of the internal combustion
engine.
A method is already known in which a regulation in accordance with a stored
typical field of characteristic curves is superimposed on the control of
the fuel-air ratio. However, this method does not eliminate the
difficulties inherent in the nonlinear course of the characteristic curve
of the oxygen measurement probe. Furthermore, there is a considerable
expense for memory in the electronic regulating device.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method of improving the
exhaust-gas behavior of mixture-comprising internal combustion engines
which, upon quasi-steady operation, permits a stable control of the excess
fuel-air ratio, lambda, at the value 1. Furthermore, the fuel-air-mixture
forming device in question is to be capable of rapidly counteractig
disturbances in the composition of the mixure due to changes in the
operation conditions of the internal combustion engine so that the time
span within which the mixture-forming device permits exhaust gases having
a high proportion of noxious substances is shorter than in devices of the
prior art.
According to the invention, the voltage range of the output signal of the
oxygen measurement probe is divided into a fixed number of individual
steps, a corrected voltage value is stored in a memory for each individual
step, and the corrected voltage values are taken as basis for the further
processing of the signal.
Further according to the invention, there is provided for the carrying out
of the method a device which has a read-only memory to the input of which
the analog-digital converted output signal of the oxygen measurement probe
is fed, and which experiences a change by a correction table stored in the
read-only memory. At the output of the memory there is obtained the
changed probe signal which is fed then to the input of a controller for
controlling the composition of the fuel-air mixture, possibly after
digitial-analog conversion.
In this connection, there is the advantage that the stability of the
control of the mixture-forming measurement device on the whole becomes
better. There is also the advantage that disturbances due to changes in
the operating parameters of the internal combustion engine can be
counteracted faster and that, furthrmore, the brief deviations in cotrol
in this connection are less than in the case of the known devices.
The method of the invention is suitable for different fuel-injection
systems such as, for instance, continuously or intermittently injecting
systems with central or cylinder injection. Accordingly, the setting
variable given off by the controller can regulate the pressure or the
duration of injection of the fuel in the injection system.
According to a feature of the invention, the analog output signal of the
oxygen measurement probe is converted by an analog-digital conversion into
a digital signal, a correction table for each detectable value of the
probe output signal is stored in a read-only memory, and the corrected
signal from the output of the read-only memory is used for contolling the
gas mixture.
According to another feature of the invention, the corrected voltage values
result referred to the actual fuel-air ratio, lambda, in a selectable
course of the characteristic curve.
Still further according to the invention, the corrected voltage values
referred to the actual fuel-air ratio, lambda, give a linear course of the
characteristic curves.
Yet further according to the invention, the corrected voltage values
referred to the actual fuel-air ratio, lambda, give a progressive course
of the characteristic curve on both sides of the fuel-air ratio, lambda=1.
BRIEF DESCRIPTION OF THE DRAWINGS
With the above and other objects and advantages in view, the present
invention will become more clearly understood in connection with the
detailed description of a preferred embodiment, when considered with the
accompanying drawings, in which:
FIG. 1 is a graph of voltage vs. fuel-air ratio of an oxygen measurement
probe;
FIG. 2 is a circuit diagram showing the basic circuit for varying the
characteristic curve of an oxygen measurement probe, insofar as this is
essential for the invention;
FIGS. 3a, 3b, 3c, 3d and 3e are numerical examples of a change of this
characteristic curve in tabular form;
FIG. 4 is, for instance, a corrected voltage vs. fuel-air ratio graph of an
oxygen measurement probe;
FIG. 5 is a graph showing the jump response of a known fuel-air mixture
forming device; and
FIG. 6 is a graph showing the jump response of the fuel-air-mixture forming
device of the invention, in each case for the same change in load of the
internal combustion engine.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a graph of the output signal of an oxygen measurement probe
such as is arranged in the exhaust-gas channel of an internal combustion
engine in order to detect the composition of the exhaust gas. The oxygen
measurement probe is typically a zirconium oxide probe the output signal
of which is between a few millivolts for a fuel-air ratio, lambda, which
is definitely greater than 1 and about 900 millivolts for a fuel-air ratio
which is definitely smaller than 1. In this connection, there is obtained
a flat course of the characteristic curve in the regions above and below
the stoichiometric value 1, while in a very narrow range around lambda=1,
the voltage changes by several hundred millivolts. Since for the proper
operation of a catalytic exhaust-gas aftertreatment device, it is
necessary to keep the value of the fuel-air ratio close to 1, relatively
large variations in the probe voltage are produced upon merely slight
changes in the value of the number lambda.
Accordingly, continuous control cannot be obtained, but a sort of two-point
control is developed, the lambda value oscillating rapidly around the
value 1 so that the composition of the exhaust gas is, on the average, in
accord with the requirements. In this connection, the control slope cannot
be made of just any size since, in such case, the system becomes unstable.
On the other hand, in case of changes in load, the control takes place
only slowly and the fuel-air mixture deviates temporarily from the
stoichiometric value. In this way, the catalytic aftertreatment takes
place incompletely and the proportion of noxious comonents in the exhaust
gas increases in undesired fashion. This delayed control is furthermore
effected by the flat course of the two sections 3 and 4 of the
characteristic curve since, even upon considerable deviations from the
desired value of 1 of the number lambda, the output voltage of the probe
changes only slightly, so that the recognition of large changes affords
difficulties.
FIG. 2, the output of the oxygen measurement probe is designated 21. The
signal present there is amplified in an input amplifier 22 and converted
in a A/D converter 23 for the conversion of the analog signal value into
digital data. The latter are fed in parallel to a memory device 24 in
which correction values which change the characteristic curve of the
oxygen measurement probe in desired fashion are stored in a read-only
memory. For example, it may be desired to linearize the characteristic
curve in order to obtain a proportional relationship between the fuel-air
ratio, lambda, and the signal voltage. However, by simple change of the
correction values it is also possible to impart to the characteristic
curve a progressive course starting from the value lambda=1 in order to
improve the quality of the control as a whole. Thus, with increasing
deviation of the fuel-air ratio, lambda, from the desired value, a
considerably greater increase in the proportional share of the return
takes place and automatically declines after a counteracting of the
disturbance. In this way, the stability of the control is generally
increased. The probe signal which is processed in the device 24 then
passes for further processing into a digital control device 25, from where
it is passed in suitable form to a setting member 26 which controls the
composition of the mixture of the internal combustion engine. The electric
components 23, 24, 25 can also be combined in a single electric component
20.
Below the dash-dot line 27 in FIG. 2, there is shown, as alternative, an
analog signal processing of the probe signal from which the distortion is
removed in the device 24. The signal is first of all fed to a
D/A-converter 28 for reconversion of the digital signal into an analog
signal, and the latter is raised, via another amplifier 29, to a voltage
level suitable for processing by a following contoller 30. The controller
30 furthermore acts on a setting member 31 which, as shown in the upper
half of FIG. 2, acts on the composition of the mixture of the internal
combustion engine in such a manner that the fuel-air ratio, lambda, is
kept as close as possible to a value of 1.
FIG. 3 shows in tubular form the appearance of stored values in the event
that the probe voltage is to be brought into a substantially linear
relationship to the value of the measured air number lambda. In this
connection, the first numerical column gives the probe voltage which is
used to address the memory device 24. The probe voltage is generated at
the lambda value which is printed horizontally alongside of it in the
second column. The mumerical columns 36 and 37 show, opposite each other,
the desired linear relationship between the processed probe signal and the
fuel-air ratio lambda. The numerical column 35 shows the corresponding
voltage of the oxygen measurement probe.
FIG. 4 shows, by way of example, two possible ways in which the output
signal of the oxygen measurement probe can advantageously be modified. The
curve 41 represents a linear relationship between the value of the number
lambda and the voltage value, while the curve 42 shows a progressive
course towards both sides, starting from the lambda equal to 1.
FIG. 5 shows how lambda responds to a change in load of an internal
combustion engine. The change in load is characterized by the change in
the intake pipe pressure P.sub.sro. As a result of the relationships
described at the start, the deviation of lambda from the value of 1 is
counteracted only slowly. The build-up process for the current at the
setting member (I.sub.driver) takes a correspondingly long period of time.
At the lower edge of FIG. 5, the proportions of noxious substances in the
exhaust gas as shown in the form of curves. Both the proportion of
unburned hydrocdarbons HC and of carbon monoxide CO differ for a lengthy
period of time from the desired value of zero.
In contradistinction to this, FIG. 6, which shows the recording for the
method of the invention with the use of the device of the invention, the
deviation from lambda with a similar change of the intake pipe pressure
P.sub.sro is considerably less than in the known arrangement corresponding
to FIG. 5a. Similarly, the build-up time for the setting current
I.sub.driver is substantially shorter and the deviations of the curves for
the noxious substances HC and CO in the exhaust gas less.
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