Abstrict
The invention relates to a method and an arrangement for checking
the operability of an electric heater in motor vehicles and especially
the heater of an oxygen probe which is mounted in the exhaust gas
channel of an internal combustion engine. It is necessary to determine
the temperature-dependent electrical resistance and the temperature
of the oxygen probe heater for a precise evaluation of the operational
state of the oxygen probe heater. The basic principle of the invention
is based upon the consideration that the measurement of the electrical
resistance of the oxygen probe heater is then carried out when the
oxygen probe heater has cooled down to the ambient temperature.
Whether this cool down has taken place can be determined in various
ways in accordance with the invention, for example, from the cool
down of the engine block or by a comparison of the engine block
temperature to the intake-air temperature. The operating state of
the oxygen probe heater so determined is displayed to the driver
by the activation of a corresponding control device and/or, if required,
read into a fault memory.
Claims
What is claimed is:
1. An arrangement for checking the operability of an electric heater
in a motor vehicle such as the heater of an oxygen probe in the
exhaust gas pipe of an internal combustion engine, the arrangement
comprising:
temperature sensor means for detecting a temperature at a location
remote from said heater;
means for determining a time point at which an assumption can be
made that the temperature of said heater and the temperature measured
by said temperature sensor means are approximately equal;
means for determining the electric resistance of said heater and
means for determining the temperature of said heater utilizing said
temperature sensor means having made the assumption that said temperature
of said heater and said measured temperature are approximately equal;
means for comparing said electric resistance to desired values
in the manner of a plausibility check, said desired values being
stored in a characteristic field in dependence upon temperature;
means for entering the result of the check of operability in a
fault memory;
supply voltage means for supplying voltage to the electric heater;
and,
means for adapting said supply voltage means in dependence upon
said result.
2. An arrangement for checking the operability of an electric heater
in a motor vehicle such as the heater of an oxygen probe in the
exhaust gas pipe of an internal combustion engine, the arrangement
comprising:
temperature sensor means for detecting a temperature at a location
remote from said heater;
means for determining a time point at which an assumption can be
made that the temperature of said heater and the temperature measured
by said temperature sensor means are approximately equal;
means for determining the electric resistance of said heater and
means for determining the temperature of said heater utilizing said
temperature sensor means having made the assumption that said temperature
of said heater and said measured temperature are approximately equal;
means for comparing said temperature of said heater to desired
values in the manner of a plausibility check, said desired values
being stored in a characteristic field in dependence upon resistance;
means for entering the result of the check of operability in a
fault memory;
supply voltage means for supplying voltage to the electric heater;
and,
means for adapting said supply voltage means in dependence upon
said result.
3. A method for checking the operability of an electric heater
in a motor vehicle such as the heater of an oxygen probe in the
exhaust gas pipe of an internal combustion engine, the method comprising
the steps of:
determining an operating state of the engine for which the temperature
at the mounting location of the heater and the temperature of a
temperature sensor are approximately equal to each other;
carrying out a temperature measurement with said temperature sensor;
determining the resistance of the heater; and,
carrying out a plausibility check of said resistance at the particular
temperature.
4. The method of claim 3, the method comprising the further steps
of entering the result of the operability check in a fault memory;
and, utilizing the result of the operability check for adapting
the supply voltage of the electric heater.
5. The method of claim 3, wherein said temperature sensor is an
engine temperature sensor disposed at a location remote from said
mounting location.
6. The method of claim 5, wherein the approximate temperature equality
of said heater and said temperature sensor can be assumed when the
engine block has cooled down by more than a pregiven temperature
since the last operation of the engine.
7. The method of claim 5, wherein the approximate temperature equality
of said heater and said temperature sensor can be assumed when the
temperature of the engine block differs by less than a pregiven
value from the temperature of the intake air while the engine is
running.
8. The method of claim 5, wherein the temperature measured with
said temperature sensor has the measured temperature value (Tmot),
said plausibility check including the steps of:
determining a stored resistance value (R=f(Tmot)) starting from
said measured temperature value (Tmost); and,
concluding the presence of a malfunctioning heater in the event
that the measured resistance deviates from the stored resistance
value by more than a permissible tolerance.
9. The method of claim 5, said plausibility check comprising the
steps of:
determining a stored temperature value (T=f(R)) starting from said
measured resistance value; and,
concluding the presence of a malfunctioning heater in the event
that the measured temperature differs from the stored temperature
value by more than a permissible tolerance.
Electric heater descriptionFIELD OF THE INVENTION
The method of the invention and the arrangement for carrying out
the method relate to the checking of the operability of a heater
in a motor vehicle and especially the heater of an oxygen probe
which is mounted in the exhaust gas channel of an internal combustion
engine. The method and arrangement of the invention also relate
to checking the supply lines of the heater.
BACKGROUND OF THE INVENTION
The operating principle of the invention is explained with respect
to the heater of an oxygen probe. The application of the invention
is however not limited to the application in connection with the
oxygen probe but is always then applicable when heaters having a
temperature-dependent resistance are to be checked with the aid
of a temperature sensor mounted at a remote location.
The oxygen content of the exhaust gas is determined with the oxygen
probe and the value determined in this manner is supplied to a control
arrangement which operates to adjust a pregiven air/fuel ratio.
The oxygen probe is only operationally ready above a minimum operating
temperature. In this way, the control of the air/fuel mixture by
means of the oxygen probe is only then possible when the oxygen
probe has reached its operating temperature. Only then can a control
to an optimal air/fuel mixture take place, for example, with respect
to a low emission of toxic material. The operating temperature of
the oxygen probe should be reached as rapidly as possible after
the internal combustion engine is started in order to hold the emission
values low. The heat-up of the oxygen probe takes place by means
of the exhaust gases of the engine and is accelerated by the electric
oxygen probe heater for the reasons delineated above. The electric
oxygen probe heater is also then necessary when, for example, the
heating capacity of the exhaust gas is inadequate such as during
idle in order to maintain the oxygen probe at the operating temperature
or for overrun operation which takes a longer time.
It is necessary to check the operability of the oxygen probe heater
in the context of providing a low emission of toxic materials. Many
methods are known in order to recognize one or several fault conditions
such as interruptions, short circuits and shunts. The test of the
operability of the oxygen probe heater takes place in a number of
ways, for example: via the current flow through the oxygen probe
heater detected by means of the measuring resistance as disclosed
in United States patent application Ser. No. 07/862,567, filed Jun.
22, 1992, still pending; via the output signals of the oxygen probe
as disclosed in U.S. Pat. Nos. 4,170,967 and 5,054,452; via the
heat-up performance of the oxygen probe as disclosed in U.S. Pat.
No. 5,090,387; or, via the oxygen probe temperature which can be
determined in various ways such as from the internal resistance
of the oxygen probe as disclosed in U.S. Pat. No. 4,419,190 or with
a temperature sensor as disclosed in U.S. Pat. No. 3,915,828.
The methods recited above have the disadvantage that only a severe
malfunction of the oxygen probe heater can be detected. The precise
value of the electrical resistance of the oxygen probe heater is
usually not included in the check of operability since the electrical
resistance varies considerably in dependence upon temperature even
for an oxygen probe heater which is fully operational and, for this
reason, a relatively large desired value interval must be pregiven.
In this way, only such deviations of the resistance from the desired
value can be reliably detected which are greater than the resistance
changes caused by temperature fluctuations and occurring during
normal operation. A changed resistance can however lead to a lower
heating capacity of the oxygen probe heater and therefore lead to
a longer heat-up time of the oxygen probe. In this way, the toxic
material emission of the engine increases since the time duration
becomes longer until the oxygen probe can take over the control
function for which it is provided. The reverse case, namely an increased
heating capacity, can lead to damage of the oxygen probe and/or
of the oxygen probe heater by exceeding the maximum permissible
operating temperature.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method of checking
as to whether the oxygen probe heater is in a trouble-free condition.
It is another object of the invention to carry out this check with
components and means which are already in place and available.
The method of the invention affords the advantage that the actual
value of the electric resistance of the oxygen probe heater determined
according to the method can be compared to desired values pregiven
in a narrow tolerance range since the influence of the temperature
on the electrical resistance is considered for the determination
of the actual value as well when making the comparison with the
desired value interval. In this way, already a slight deviation
of the electric resistance from the desired value is detected and
suitable measures can be taken such as warning the driver and/or
adapting the supply voltage to the changed resistance value. A further
advantage is that this improvement can be realized with respect
to known monitoring arrangements without too much difficulty. The
determination of the temperature of the oxygen probe heater is carried
out with the temperature sensors already available. Only very modest
additional circuitry is required for use in combination with a central
control apparatus. If the control takes place via a computer then
the program must only the changed and a few additional program parts
installed.
The use of the monitoring operation provided by the invention is
especially necessary because a change of the electric resistance
of the oxygen probe heater is not an exception; rather, it is the
rule because of the effects of aging. It is necessary to ensure
that an optimal control of the air/fuel ratio can be provided under
as many operating conditions as possible because of a planned reduction
in the statutory limit values for the emission of toxic materials
from internal combustion engines.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described with reference to the drawings
wherein:
FIG. 1 is a schematic representation of an internal combustion
engine shown with the components essential in the context of the
method and arrangement of the invention;
FIG. 2 is a schematic of an embodiment of the arrangement of the
invention;
FIG. 3 is a flowchart of an embodiment of the method of the invention
with which a determination can be made as to whether the engine
block and the oxygen probe heater are at approximately the same
temperature;
FIG. 4 is a flowchart of a method for checking the oxygen probe
heater at a known temperature; and,
FIG. 5 is a portion of a flowchart of an embodiment of the method
as an alternative to the embodiment shown in FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
The invention relates to a method and an arrangement for checking
the operability of a heater in motor vehicles and especially the
heater of an oxygen probe. The operability is then ensured when
the electric resistance of the oxygen probe heater lies at a pregiven
temperature within a pregiven interval. In this case, it is noted
that the electric resistance of the oxygen probe heater is dependent
upon temperature. For this reason, it is necessary that the temperature
of the oxygen probe heater be detected in addition to the electric
resistance of the heater. The desired value interval to which the
measured resistance of the oxygen probe heater is compared is likewise
dependent upon temperature. For this comparison, resistance values
are applied which have been determined for the same temperature.
The resistance measurement is then carried out when the temperature
of the oxygen probe heater is approximately equal to the engine
temperature. The engine temperature is determined with a sensor
for cooling water temperature or a sensor for oil temperature which
is already in place.
FIG. 1 shows a schematic representation of an internal combustion
engine together with components essential for the invention. An
oxygen probe 10 and its heater 11 are mounted in the exhaust gas
channel 12 of the engine and are connected to a central control
apparatus 14. The central control apparatus 14 receives data from
additional sensors such as a temperature sensor 16 in the intake
pipe 18 or a sensor 20 for the temperature of the engine block 22
and controls a device 24 for metering fuel.
FIG. 2 shows a schematic of an embodiment of the arrangement according
to the invention. The oxygen probe heater 11 is connected between
the collector of a transistor 30 and the plus pole of a battery
32. The emitter of the transistor is connected via a measuring resistor
34 to the minus pole of the battery. Furthermore, a connection is
provided between the emitter of the transistor and a non-inverting
input of an operational amplifier 36. An analog/digital converter
38 is connected to the output of the operational amplifier. The
converter 38 conducts the digitalized signal to a microprocessor
40. A control output of the microprocessor 40 is connected to the
base of the transistor 30. Further connecting lines to the microprocessor
40 are provided for transmitting the detected fault conditions to
a display device (not shown) and for the data exchange with other
electronic devices. The microprocessor 40 and the analog/digital
converter 38 receive their supply voltage from the battery 32. The
components identified by reference numerals 30, 34, 36, 38 and 40
are components of the control apparatus 14.
A flowchart of an embodiment of the method is shown in FIG. 3.
With this embodiment, a determination is made as to whether the
engine block and the oxygen probe heater are at approximately the
same temperature.
In a first step 44, the inquiry is made as to whether the ignition
is switched on. If the ignition is switched on, the temperature
Tmot of the engine block is measured in the next step 46; otherwise,
the inquiry 44 is repeated.
Steps 48 to 52 follow step 46 and can be replaced by the flowchart
shown in FIG. 5 which is described further below. This alternate
embodiment is represented by the reference letter C between the
steps 46 and 48 and reference D at the yes-output of step 50.
After step 46, the method continues with step 48 wherein the difference
Tdiff1 is formed from the engine block temperature Toff at the last
switch-off of the engine and the actual engine block temperature
Tmot. The determination of Toff is not shown and takes place in
the following manner: the engine block temperature measured in step
46 is stored in a RAM-cell. The content of the RAM-cell is written
into a second RAM-cell when the ignition is switched off and, with
the next switch-on of the ignition, the value Toff is read out of
this second RAM-cell. Suitable measures ensure that the memory content
of the second RAM-cell is maintained even when the ignition is switched
off.
Step 50 follows step 48 and the inquiry is made in step 50 as to
whether Tdiff1 is greater than a pregiven value Tlimit1. If this
is the case, then the assumption can be made that the engine block
has cooled down to the ambient temperature. The oxygen probe including
the heater has then also cooled down to the ambient temperature
because of its low thermal mass so that the oxygen probe now is
at the same temperature as the engine block.
In this way, the temperature of the oxygen probe heater is known
and the operational check can be continued with the sequence shown
in FIG. 4. The connecting location is identified by reference character
A. If the condition in step 50 is not fulfilled, then a determination
is made in step 52 that the function check of the oxygen probe heater
cannot be continued since the temperature of the oxygen probe heater
is not known.
In FIG. 4, a flowchart for checking the function of the oxygen
probe heater at a known temperature is shown. The symbol A at the
start of the flowchart indicates the connection to the flowchart
of FIG. 3 which is identified in a like manner.
Step 62 follows the connecting point A and a check is made in this
step as to whether the engine speed at the end of starting has been
reached. Only when this is the case, can the method continue to
the next step. In this next step 64, the oxygen probe heater is
switched on. Thereafter, in step 66, the battery voltage Ubatt1
is measured. Thereafter, in step 68, the measurement of the current
I which flows through the oxygen probe heater is measured and, thereafter,
the battery voltage is measured again in step 70 and is stored as
value Ubatt2. In the next step 72, the amount of the difference
dU of the two measured battery voltages Ubatt1 and Ubatt2 is formed.
In step 74, an inquiry is made as to whether dU is less than a pregiven
value dUlimit. If this condition is fulfilled, then, in step 76,
the resistance R of the series circuit of the oxygen probe heater
11, collector-emitter path of the transistor 30 and the measuring
resistance 34 is determined in that the mean value of the voltage
Ubatt1 and Ubatt2 is divided by the current I measured in step 68.
The series circuit is shown in FIG. 3 and was explained further
above.
Thereafter, in step 78, the desired value interval (Rmin, Rmax)
of the resistance of the oxygen probe heater including the resistors
connected in series therewith for the measured temperature Tmot
is determined from a temperature-dependent characteristic field.
In the next inquiry step 80, a check is made as to whether the actual
value R determined in step 76 lies within the desired value interval
(Rmin, Rmax). If this is the case, then, in the next step 82, a
conclusion is drawn as to the operability of the oxygen probe heater.
If the condition 80 is not fulfilled, then in step 84, which follows
the condition 80, a malfunction of the heater is determined. The
reactions to the malfunction so detected are not shown in FIG. 4
and can, for example, be an entry into the fault memory and/or a
warning to the driver. The entry in the fault memory can be evaluated
during the next visit to the service center and the fault eliminated.
If the determination is made in step 74 that the fluctuation of
the battery voltage dU does not lie within the pregiven range dUlimit,
then the step 86 is carried out next wherein a check is made as
to whether more than three measurements for determining R have been
carried out since the engine has been started. If this is the case,
then step 88 follows and the operational check is discontinued.
Otherwise, the method proceeds with step 66.
In the embodiment described here, the measured resistance of the
heater is compared to desired values listed in dependence upon the
temperature. The measured temperature of the heater can also be
compared to desired values listed in dependence upon resistance
as an alternative.
In a further embodiment, the content of the fault memory can be
used additionally for adapting the supply voltage of the oxygen
probe heater to the resistance of the oxygen probe heater.
FIG. 5 shows a portion of the flowchart of an embodiment which
can replace the portion between points C and D of the embodiment
of FIG. 3.
At point C of the flowchart of FIG. 5, a step 54 follows wherein
the temperature Tair of the intake air in the intake pipe is measured
with a temperature sensor. In the next step 56, the difference Tdiff2
between the engine block temperature Tmot and the temperature Tair
of the intake air is formed. An inquiry follows in step 58 as to
whether the magnitude of Tdiff2 is less than a pregiven value Tlimit2.
If yes, then for a running engine, the assumption can be made of
a new start and a cooling-down phase and correspondingly that the
temperature of the oxygen probe heater and the air temperature are
approximately equal and the functional check can be continued at
point D. If no, then a step 60 follows inquiry 58 and the function
check is terminated.
It is understood that the foregoing description is that of the
preferred embodiments of the invention and that various changes
and modifications may be made thereto without departing from the
spirit and scope of the invention as defined in the appended claims.
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