Abstrict
A novel apparatus, for determining the concentration of gas components
in exhaust gas from a motor vehicle, features a lambda sensor element,
an electric heater thermally coupled to the sensor element, a control
unit regulating application of voltage to the heater, and at least
two voltage sources which can be alternatively applied to the heater,
as commanded by the control unit.
Claims
What is claimed is:
1. An apparatus for determining the concentration of gas components
in exhaust gas from a motor vehicle, comprising
a sensor element;
an electric heater thermally coupled to said sensor element;
a voltage source supplying power to said electric heater; and a
control unit controlling supply of power to said heater;
wherein
said voltage source includes at least a first voltage source and
a second voltage source, said control unit controlling which one
of said first and second voltage sources is applied to said heater
at each instant; and one of said first and second voltage sources
being directly connected from an alternator/generator.
2. The apparatus of claim 1, wherein the second voltage source
is the on-board network voltage of the motor vehicle.
3. The apparatus of claim 2, wherein said on-board network voltage
is in a nominal range of about 12 to 14 volts.
4. The apparatus of claim 2, wherein said on-board network voltage
is in a nominal range of about 24 to 28 volts.
5. The apparatus of claim 1, wherein said first voltage source
has a voltage at least 30% higher than a nominal voltage of the
second voltage source.
6. The apparatus of claim 5, wherein the first voltage source is
a generator/alternator of the motor vehicle.
7. The apparatus of claim 6, wherein, immediately after a cold
start, said first voltage source is applied to said heater.
8. The apparatus of claim 1, wherein, after said sensor element
has reached a predetermined minimum operating temperature, as indicated
by current drawn by said heater, said second voltage source is applied
to said heater.
9. The apparatus of claim 1, wherein a switchover, from application
of said first voltage source to said heater to application of said
second voltage source to said heater, is carried out upon expiration
of a predetermined period of time after a cold start.
10. The apparatus of claim 1, wherein a switchover, from application
of said first voltage source to said heater to application of said
second voltage source to said heater, is carried out, depending
upon temperature of said sensor element.
11. The apparatus of claim 1, wherein a switchover, from application
of said first voltage source to said heater to application of said
second voltage source to said heater, is carried out, depending
upon temperature of said heater, as indicated by current drawn by
said heater.
12. The apparatus of claim 1, wherein a switchover, from application
of said first voltage source to said heater to application of said
second voltage source to said heater, is carried out, depending
upon current drawn by said sensor element.
13. The apparatus of claim 1, wherein said heater includes a heated
conductor whose electrical resistance rises as its temperature rises.
14. The apparatus of claim 13, wherein said conductor consists
essentially of a metal selected from the group consisting of platinum,
ruthenium, rhodium, palladium, osmium, iridium, and alloys thereof.
15. The apparatus of claim 1, wherein said sensor element and said
heater are located on a common substrate.
16. A method of operating an exhaust gas sensor having an associated
heater in a motor vehicle equipped with a combustion engine and
first and second voltage sources, said first source having a nominal
voltage higher than that of said second source,
comprising the steps of
applying said first voltage source to said heater for a limited
period of time after starting said combustion engine; and one of
said first and second voltage sources being directly connected from
an alternator/generator.
17. The method of claim 16, wherein, after expiration of said limited
period of time, said second voltage source, having a lower nominal
voltage, is applied to said heater.
18. The method of claim 17, wherein switchover from said first
voltage source to said second voltage source is carried out between
2.5 seconds and 20 seconds after a cold start.
19. The method of claim 17, wherein switchover from said first
voltage source to said second voltage source is carried out between
1 second and 2.5 seconds after a cold start.
20. The method of claim 17, wherein switchover from said first
voltage source to said second voltage source is carried out between
3 seconds and 10 seconds after a cold start.
21. The method of claim 20, wherein switchover from said first
voltage source to said second voltage source is carried out between
5 seconds and 10 seconds after a cold start.
22. The method of claim 16, wherein switchover from said first
voltage source to said second voltage source is carried out at a
time which is a function of current drawn by said heater as measured
by a control unit which senses said current drawn.
Description CROSS-REFERENCE TO RELATED LITERATURE
Automotive Handbook, 3rd English Ed., Robert Bosch GmbH, Stuttgart,
Germany, 1993, pages 425 & 482.
FIELD OF THE INVENTION
The present invention relates generally to an apparatus for determining
the concentration of gas components in exhaust gases from motor
vehicles, including an electrically heated lambda sensor, and a
method of operating an exhaust gas sensor including a faster warm-up
to operating temperature than in the prior art.
BACKGROUND
Gas sensors, especially lambda sensors, have long been known. For
example, German published patent application DE-OS 36 10 363 and
corresponding U.S. Pat No. 4,985,126 show a lambda sensor with a
tubular solid electrolyte, which has a separate electric heater.
This heating is necessary to bring the solid electrolyte up to its
operating temperature. Further, German utility model DE 91 03 547
U1 discloses a gas sensor in which an electrical resistance heater
and an active sensor element are arranged on a common carrier or
substrate, and the overall mass of the active region is kept small,
in order to assure a fast warm-up. The warm-up of this sensor is
about 10 seconds faster than that of the older lambda sensors, but
the time required for warm-up even with this sensor is up to 20
seconds.
In the context of efforts to minimize the overall pollutant output
of a motor vehicle during a predetermined test cycle, the cold-start
phase assumes great importance. There are, indeed, electrically
heatable catalytic converters which become effective immediately
after a cold start. Trouble-free functioning of the three-way catalytic
converter, however, does not presuppose a controlled warm-up phase
with a rich or lean mixture, but rather a controlled warm-up phase
in which the mixture fed to the combustion engine has a .lambda.-value
in the area of 1. In the time interval before effective operation
of the exhaust gas sensor, the exhaust gas cleaning apparatus finds
itself in an ineffective operating state, because the signals necessary
for the control process are not yet available. Whenever this time
is longer than a few seconds, the pollutant output in the first
seconds of the cold-running phase of the combustion engine can exceed
the complete permitted pollutant output for the test cycle, so that
the fully operational exhaust gas cleaner can no longer lead to
staying within the predetermined limit values. It is therefore necessary,
to make the installed exhaust gas sensors operational within a fewer
number of seconds.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention, to provide
an apparatus and a method for determining the concentration of gas
components in exhaust gas of a motor vehicle, which already offers
usable measurement results, about 5 seconds after a cold start.
Briefly, this is achieved by heating up the lambda sensor from
ambient temperature to operating temperature quickly by applying
a relatively high-voltage source to the sensor's associated electric
heater, and subsequently maintaining operating temperature by application
of a second, lower-voltage, source to the heater.
Due to the fact that, in an apparatus with two voltage sources,
the heater can be supplied from a first voltage source or from a
second voltage source, as specified by a control device, the warm-up
of the exhaust gas sensor during the cold-start phase can be supplied
from a different voltage source than the heating during the steady
operation of the exhaust gas sensor. The first voltage source can
be the unregulated generator/alternator voltage of the dynamo, tapped
before the voltage regulator, and the second voltage source can
be the on-board network of the motor vehicle tapped downstream of
the voltage regulator.
It is also possible, upon reaching the operating temperature of
the sensor, to limit the applied heating power, using a clocked
voltage feed, to the value which is adequate to maintain the operating
temperature of the exhaust gas sensor in a warm state. The nominal
voltage of the second voltage source approximates, depending on
the model of motor vehicle, for example, 13.6 V (12-volt installations)
or 27.2 V (24-volt installations). In the future, other on-board
networks with other nominal voltages are of course possible.
If the voltage of the first voltage source is at least 30% above
the nominal voltage of the second voltage source, a particularly
quick heating of the sensor element is possible, since the preheating
electrical power applied rises quadratically with the increase in
voltage. One can advantageously use, as the first voltage source,
the generator/alternator of the motor vehicle, since tapping upstream
of the voltage regulator results in about double the usual on-board
network voltage of the motor vehicle.
Immediately after the cold start, the heating device of the exhaust
gas sensor is connected to the first voltage source, in order to
supply the heating device with sufficient electrical power. Upon
reaching operating temperature, the heating device is preferably
connected to the second voltage source, which, with lower electrical
power, supplies the voltage necessary to maintain operating temperature.
One could also provide that the first voltage source is continued
to be used for heating, but that heating power applied is reduced
by clocking or pulsing the current fed.
The switchover from the first voltage source to the second voltage
source can be carried out, in particularly simple fashion, as a
function of time elapsed since the cold start. A reliable switchover,
particularly in connection with very high electrical power, results
if the switchover is carried out as a function of the temperature
of the heating device. One can employ, as an indicator for the temperature
of the sensor element, the current drawn by the heating device,
since its electrical resistance depends upon the temperature reached.
A good temperature tolerance, and a reproducible dependence of
electrical resistance upon temperature, are achievable if the heating
device has a heating conductor which consists essentially of a platinum
group metal, i.e. platinum, ruthenium, rhodium, palladium, osmium,
iridium, or an alloy of the foregoing "Group VIII" metals.
The warm-up of the sensor element occurs particularly quickly if
the heating device and the sensor element are arranged on a common
substrate. A heating device with a heating conductor, whose electrical
resistance climbs as temperature increases, has the advantage that,
at cold ambient temperature, and thus low starting temperature,
the electrical heating power applied is high. The colder the sensor
is at the start, the more strongly it is heated up.
Equally, the object of fast warm-up is achieved, in a motor vehicle
having a combustion engine and an exhaust gas sensor with an associated
heater, by initially applying a high voltage source to the heater
to bring the sensor up to operating temperature, and subsequently
applying a lower-voltage source, to maintain the sensor at operating
temperature.
Since, after the start of the engine, the heating device of the
exhaust gas sensor can be supplied initially from the first voltage
source having higher nominal voltage and subsequently from the second
voltage source having lower nominal voltage, a fast warm-up of the
sensor element can be achieved initially, while the electrical power
required to maintain the operating temperature is lower and can
be applied from the second voltage source having lower nominal voltage.
Depending upon the particular installation, one may be able to entirely
omit a further electrical heating of the sensor, e.g. when the exhaust
gas temperature is so high that, during operation, the sensor maintains
a sufficient operating temperature without electrical heating.
It is particularly advantageous if the switchover from the first
voltage source to the second voltage source is carried out between
2.5 and 20 seconds after a cold start. In the case of particularly
high electrical heating power, it can even be advantageous for the
switchover to be carried out between 1 second and 2.5 seconds after
a cold start. According to another configuration, the switchover
can be carried out between 3 seconds and 10 seconds after a cold
start. The method can be advantageously arranged so that the determination
of the instant for the switchover from the first voltage source
to the second voltage source is a function of the current drawn
by the heater.
BRIEF FIGURE DESCRIPTION
In the following, a preferred embodiment of the apparatus of the
invention, and one of the method of the invention, are explained
with reference to the drawings, in which:
FIG. 1 shows an exhaust gas sensor for use in the apparatus of
the invention, including a sensor element and a heating element
arranged commonly on a single substrate; and
FIG. 2 is a schematic circuit diagram of the electrical circuitry
of the exhaust gas sensor of FIG. 1.
DETAILED DESCRIPTION
FIG. 1 illustrates an exhaust gas sensor I without a housing or
attachments. The sensor comprises, in the conventional manner, a
substrate 2, e.g. a carrier plate made of aluminum oxide. Onto the
substrate is deposited a pad 3, with sensor material (strontium
titanate etc.) applied by screen printing, and two electrodes 4
and 5 are provided. Via leads 6, electrodes 4 and 5 are connected
to respective terminal pads 7 and 8, which serve for connection
to an external electrical evaluation circuit (not shown).
Sensor material 3 is surrounded by a heating conductor 10, which
in turn is connected via leads 11 to terminal pads 12 for electrical
heating. Terminal pads 12 also serve for external electrical connection
of the exhaust gas sensor.
FIG. 2 illustrates a simplified equivalent circuit diagram of an
exhaust gas sensor. Such a circuit is suitable for use in the sensor
of the present invention.
At the terminal pads 7 and 8 of the exhaust gas sensor, with resistive
sensor element 3, during operation, one can sense a resistance value
R.sub.S which is characteristic of the concentration of a particular
gas. At the terminal pads 12 of the electrical resistance heater
10, the electrical supply voltage for the heating of the sensor
is to be applied. The electrical resistance R.sub.H of heating conductor
10 is dependent upon the temperature.
Upon a cold start of a motor vehicle, the apparatus of the invention
operates as follows.
Immediately after starting the engine, the unregulated voltage
of the generator/alternator (up to 30 volts in the case of a 12-volt
on-board voltage) is applied to the heater 10 of the sensor. The
heater's electrical resistance is small at low temperatures, so
a high current flows. The heating power applied is high, and heats
the sensor up correspondingly quickly. Upon reaching operating temperature,
the sensor emits evaluatable signals about the exhaust gas composition.
The engine control can then transition to a control mode in which
the mixture composition (fuel/air) is kept in a target or command
range. The exhaust gas catalytic converters can operate effectively
in this range.
The time elapsed until reaching of operating temperature of the
sensor is, in the planar sensor example, approximately 1 to 3 seconds.
The applied heating power, at the time operating temperature is
reached, has already dropped about 40%, since the resistance value
R.sub.H of the heating conductor has already risen.
Then the heating device 10 is connected to the on-board network
voltage of the vehicle. The applied electrical heating power is
sufficient to maintain the sensor at operating temperature. The
point in time for the switchover of the electrical heating applied
can be determined approximately from the current drawn by heating
conductor 10. The current drawn drops, when operating temperature
is reached, due to the climbing resistance R.sub.H, below a predetermined
threshold. Alternatively, the point in time can be simply specified
by lapse of a predetermined time interval after the cold start.
The more temperature-independent the sensor signal is from the actual
temperature of the sensor, the less critical it is to maintain a
particular temperature. Correspondingly less investment for the
control or regulation of the heating is therefore necessary.
The thus-far-described apparatus can lead, when used with quick-responding,
electrically heated metal catalytic converters, to a substantial
reduction in output of pollutants from a motor vehicle during a
cold start.
In addition to use with the planar sensors of the embodiment described,
the invention can also be advantageously used with conventional
lambda sensors having a tubular solid electrolyte and a differently
arranged electrical heater, even though the time required to reach
operating temperature in such an embodiment would not be as short
as that in a planar sensor having a printed heating conductor.
The second voltage source also need not operate with a lower voltage.
A transition from a continuous-current mode to a pulsed-current
mode would equally well reduce the heating power applied, in a suitable
manner.
Those of ordinary skill in the art will appreciate that various
changes and modifications are possible within the scope of the invention
concept. For example, features of one embodiment could be used together
with features of another embodiment. Therefore, the present invention
is not limited to the particular embodiments shown and described,
but rather is defined by the following claims.
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