DE102019219647A1 - Measurement of the shunt resistance of a lambda probe and correction of its influence - Google Patents

Abstract

The invention relates to a method (100; 200) for calculating a lambda value using a broadband lambda probe (L). At least one shunt resistor (RN1; RN2; RN3) and, if necessary, a mass offset ( Uλ.0) determined. During normal operation of the broadband lambda probe (L), which takes place after the start phase, a pump current (lP, t) of a pump cell (P) of the broadband lambda probe (L) taking into account the at least one shunt resistor (RN1; RN2; RN3) and / or of the mass offset (Uλ.0) is determined. The lambda value is then calculated from the determined pump current (lP, t).

Description

The present invention relates to a method for calculating a lambda value using a broadband lambda probe as well as a computing unit and a computer program for its implementation.

State of the art

In many vehicles with internal combustion engines, lambda probes or lambda sensors are used to regulate the ratio of fuel and combustion air in an injection mixture that is fed to the internal combustion engine. For this purpose, the electrical voltage of a Nernst cell or the electrical resistance of a resistance jump probe can be measured in the lambda probe.

The lambda value corresponds to a stoichiometric ratio between combustion oxygen and fuel molecules, where a value of 1 means that there is as much oxygen in the combustion chamber as is theoretically required for complete combustion of the fuel. Lambda values above 1 indicate an excess of oxygen (lean mixture), values below 1 an oxygen deficiency (rich mixture) in the combustion chamber.

A wide variety of probes are used in today's vehicles. In addition to jump probes, there are also so-called broadband lambda probes with two cells, such as those from the DE 10 2007 057 707 A1 emerge. These probes are used to measure the concentration of a gas component in the exhaust gas of the internal combustion engine. Broadband lambda probes are often used, which function on the basis of a conventional galvanic Nernst cell combined with an oxygen pump cell. A measuring gas is located in a measuring gap between the two cells. A pump current that is applied to the oxygen pump cell, which is essentially of the same type as the Nernst cell, causes oxygen ions to be transported into or out of the measurement gas through the membrane of the pump cell. The measurement gas is also connected to the exhaust gas to be tested via diffusion openings, independently of the pump current, so that the composition of the measurement gas changes depending on the composition of the exhaust gas if no pump current is applied. To determine the lambda value, a lambda value of 1 is set in the measurement gas via the pump current. This is achieved by keeping the potential generated at the Nernst cell constant via an electronic control circuit that regulates the pump current. If there is excess air in the exhaust gas (lean area), oxygen is pumped out of the measuring gap. If the residual oxygen content of the exhaust gas is low (rich area), oxygen is fed into the measuring gap by reversing the pump voltage. The respective pump current forms the output signal. The output signal line of such broadband lambda probes is connected to an evaluation unit.

The lambda value of the exhaust gas to be tested can then be calculated from the pump current. The pump current can be measured directly as the current that flows from the lambda probe into a measuring connection of the evaluation unit, or it can be calculated on the basis of the currents that leave the control connections of the evaluation unit for setting the pump current.

Disclosure of the invention

According to the invention, a method for calculating a lambda value using a broadband lambda probe as well as a computing unit and a computer program for its implementation are proposed with the features of the independent claims. Advantageous refinements are the subject of the subclaims and the description below.

In the case of lambda probes for use in vehicles, the connections of evaluation electronics are fundamentally electrically isolated from the metallic probe body and from a built-in heating element. If the insulation is faulty, a shunt resistance will build up from the connections to the probe housing or to the heating element. Depending on the probe, age, etc., this resistance can become relatively small and thus cause significant leakage currents.

In addition, the ground potential of the metallic probe housing, which is connected to the vehicle ground via the exhaust, can differ from the ground potential of the evaluation electronics in the control unit. In the case of cars, a potential difference, also known as a ground offset, of ± 1 V is realistic.

Since the evaluation unit for calculating the lambda value, in particular the control unit of a vehicle, reacts very sensitively to disruptive influences, the invention provides for at least one leakage current or shunt resistance to be measured and taken into account when calculating the lambda value, so that only the actually caused by the Pump current flowing through the pump cell, and thus “oxygen-effective”, influences the result. As a result, an essentially constant evaluation quality can be achieved over the entire service life of the lambda probe.

If the pump current is determined by measuring the current that flows from the lambda probe into the evaluation unit, a first shunt resistance interferes, through which current flows from a current source through the measuring connection of the evaluation unit without having flowed through the pump cell, e.g. a current from a Vehicle battery via a probe heater to the probe control unit.

According to advantageous embodiments of the invention, this first shunt resistance can be determined by measuring the current flow into the measuring connection as the first leakage current in the start phase of the lambda probe, while current neither flows through the control connections of the evaluation unit nor through the probe heater. At the same time, the voltage of the current source is measured and the first shunt resistance is calculated from the voltage of the current source and the measured first leakage current. In later normal operation, the current first leakage current can then be calculated from the current measured voltage of the current source and the initially calculated first shunt resistance. This current first leakage current is then subtracted from the measured current flow in order to determine a corrected pump current.

If, on the other hand, the pump current is calculated on the basis of the control currents that leave the control connections of the evaluation unit for setting the pump current, the first leakage current does not interfere because it does not flow through the cells. In this case, however, second leakage currents are advantageously taken into account, which flow from the setting connections of the evaluation unit directly to ground without having flowed via the cell connected to the respective setting connection. Determination is more complicated here, since no current measuring points are provided in the current path between the setting connections of the evaluation unit and the probe or within the lambda probe. Therefore, the probe connection to be measured is advantageously set to the highest possible input resistance when it is still cold, and a first defined output current intensity is set at the control connection to be measured. After the system has settled, the potential of the probe connection is measured against the ground connection of the evaluation device. This procedure is repeated with a second defined output current that is different from the first. The second shunt resistance of the relevant control connection can be calculated from the difference between the two measured potentials and the difference between the two set currents.

The mass offset of the lambda probe is advantageously also taken into account. This mass offset of the probe mass in relation to the mass of the evaluation device results as the difference between the measured potential and the associated, set current strength multiplied by the calculated second shunt resistance.

In normal operation, the pump current is corrected by measuring the current probe connection potential against the ground connection of the evaluation device and calculating the second leakage current from the current probe connection potential corrected for mass offset and the calculated second shunt resistance.

The lambda value determined in this way is preferably used to set or optimize an air-fuel mixture for a combustion process, in particular in an internal combustion engine of a vehicle.

A computing unit according to the invention, for example a control unit of a motor vehicle, is set up, in particular in terms of programming, to carry out a method according to the invention.

The implementation of a method according to the invention in the form of a computer program or computer program product with program code for performing all method steps is advantageous, since this causes particularly low costs, in particular if an executing control device is also used for other tasks and is therefore available anyway. Suitable data carriers for providing the computer program are, in particular, magnetic, optical and electrical memories, such as hard drives, flash memories, EEPROMs, DVDs, etc. A program can also be downloaded via computer networks (Internet, intranet, etc.).

Further advantages and embodiments of the invention emerge from the description and the accompanying drawings.

The invention is shown schematically in the drawings using exemplary embodiments and is described below with reference to the drawings.

Figure list

• 1 shows a schematic representation of an advantageous interconnection of a broadband lambda probe, with possible shunt resistances also being shown.
• 2 shows an advantageous embodiment of a method according to the invention, wherein a pump current is determined by measuring a measuring current and correcting the measured measuring current.
• 3 shows a further advantageous embodiment of a method according to the invention, wherein a pump current is determined by setting a control current and correcting the set control current.

Embodiments of the invention

For purposes of illustration, assume the procedures described herein 100 , 200 and circuits 50 are used in connection with the operation of an internal combustion engine of a vehicle.

In 1 is an advantageous interconnection 50 a broadband lambda probe L with an engine control unit M is shown schematically. Control connections S2 , S3 of the engine control unit M are connected to inputs of the lambda probe L. In particular, there is an adjusting connection S2 at the input of a pump cell P and an adjusting connection S3 connected to the input of a Nernst cell N of the lambda probe L. A measuring connection S1 of the engine control unit M is connected to a signal output of the lambda probe L.

The lambda probe L has a probe heater H which is set up to heat the lambda probe L to operating temperature as quickly as possible during normal operation. The probe heater is connected to a vehicle battery B for this purpose.

With increasing age of the circuit 50 are electrical insulation between the individual components of the circuit 50 permeable, so that currents can flow in places not intended for this purpose. _{Shunt resistances R N1} are particularly important for the determination of a lambda value, RN2 , R _N3 , the currents in the test port S1 of the engine control unit M and of the setting connections S2 , S3 of the engine control unit M to the lambda probe L, since they _{make it more difficult to determine a pumping current I P, t} that flows through the pumping cell P of the broadband lambda probe L.

Depending on the configuration of the evaluation, the current that is transmitted via the signal line of the lambda probe L into the measurement connection S1 of the engine control unit M flows, measured or control currents that are supplied to the control connections S2 , S3 of the engine control unit M are set, evaluated.

A first shunt resistor R _N1 , which produces an electrically conductive connection from the vehicle battery B, in the example shown via the probe heater H, to the signal output of the lambda probe L, increases the current flow into the measurement input S1 of the engine control unit M.

By the shunt resistors R _N2 and R _N3 can be part of the adjustment ports S2 and S3 The set actuating currents flow off directly to ground.

In 2 an advantageous embodiment of a method according to the invention for calculating a lambda value is shown schematically on the basis of a flowchart and denoted overall by 100. In the process 100 a broadband lambda probe L is used.

In the process 100 a first shunt resistance R _N1 is first determined. During a start phase of the lambda probe L, a voltage U _{B, 0 of} a vehicle battery B to a ground connection G of an evaluation device, in the example described, an engine control unit M, and a _{measurement current I M, 0 are} fed into a measurement connection S1 of the engine control unit M measured. The measuring connection S1 of the engine control unit M is connected to a signal connection of the lambda probe L. During the start phase, the lambda probe L is in particular still cold and in this embodiment of the method it flows 100 no control current from control connections S2 , S3 of the engine control unit M. In the absence of regularly flowing currents, the measurement current I _{M, 0} can therefore only be a leakage current that flows from the vehicle battery via the first shunt _{resistor R N1} into the measurement connection S1 of the engine control unit M flows. The first shunt resistance R _{N1 is} calculated by dividing the battery voltage U _{B, 0} by the leakage current I _{M, 0.} This first shunt resistance R _N1 is advantageously stored in the engine control device M in order to be able to fall back on it during normal operation following the start phase.

During normal operation, control currents are applied to the control connections S2 , S3 of the engine control unit M is set in order to set a gas composition with a lambda value of 1 in a measuring gap of the broadband lambda probe L. At least some of these actuating currents flow into the measurement connection via a pump cell P and / or via a Nernst cell N of the lambda probe L S1 of the engine control unit M. At the same time, a current leakage current I _{N1, t , which is} dependent on the current battery voltage U B, t, also flows during normal operation. The current leakage current I _{N1, t} can therefore be calculated in each case by dividing the currently measured battery voltage U _{B, t} by the first shunt resistance R _N , which is assumed to be constant.

The actual current pump current Ip _.t , which flows through the pump cell P of the lambda probe L, is consequently obtained by subtracting the calculated current leakage current I _{N1, t} from a current in the Measuring connection S1 flowing current measurement current I _{M, t is} calculated.

The lambda value is calculated by the engine control unit M from this actual pump current I _P , _t corrected with regard to the leakage current I _{N1, t} flowing through the shunt resistor R _N1.

This current lambda value can then advantageously be used to set an optimized air-fuel mixture at the input of the internal combustion engine.

Another advantageous embodiment of a method according to the invention is shown in FIG 3 shown schematically and designated as a whole by 200. In the in 3 presented procedure 200 the pump current I _{P, t is} out at the control connections S2 , S3 set actuating currents determined. Since the in relation to the in 2 illustrated procedure 100 Leakage current I _{M, 0 described has} no influence on the through the control connections S2 , S3 has flowing actuating currents, the first shunt resistor needs R _N1 in the process 200 not to be considered. However, similar to the procedure explained above 100 , initially at least a second shunt resistor R _N2 or third shunt resistor R _N3 determined over which a part of the through the respective setting connection S2 , S3 flowing current flows directly to the ground connection of the vehicle battery B without flowing through the lambda probe L.

Since there is no measurement option for the currents actually flowing in the current path within the lambda probe L, the second or third shunt resistor must R _N2 , R _N3 are calculated by, in the starting phase, when the lambda probe L is still cold, _{actuating currents I S, 1} , I _{S , 2} which are different from one another several times in succession in the respective actuating connection S2 , S3 of the engine control unit M is set and, after the circuit has settled, the corresponding potential U _{λ, 1} , U _{λ, 2} at the input of the lambda probe L against the ground connection G of the engine control unit M is measured. The resistance R _{λ, 0 of} the respective input of the lambda probe L is set as high as possible. The second and third shunt resistance results from the differences in the probe potentials U _{λ, 1} , U _{λ, 2} in combination with the differences in the associated actuating currents l _{S, 1} , l _{S, 2} R _N2 , R _N3 as a quotient. If more than two different _{actuating currents I S, 1} , I _{S, 2} with the associated probe potentials U _{λ, 1} , U _{λ, 2} are recorded, the reliability can be increased by averaging.

A ground offset U _{λ, 0 of} the ground connection of the lambda probe L with respect to the ground connection G of the engine control unit M can be calculated from values for actuating current l _{S, 1} , l _{S, 2} and probe potential U _{λ, 1} , U _{λ, 2 that are} calculated using the second or third shunt resistor R _N2 , R _N3 be calculated. The ground offset results as the difference between a probe potential U _{λ, 1,} U _{λ, 2,} and the product of the associated actuating current I _{S, 1} , I _{S, 2} with the calculated second or third shunt resistance R _N2 , R _N3

In normal operation, the input of the lambda probe L is switched without additional resistance and the pump current I _{P, t} can be measured by measuring the current probe potential U _{λ, t} , correcting the mass _{offset U λ, 0} and dividing by the calculated second shunt resistance R _N2 be calculated.

The engine control unit M calculates the lambda value on the basis of this corrected pump current I _{P, t} and preferably uses the calculated lambda value to adjust the air-fuel mixture at the input of the internal combustion engine.

It should be expressly pointed out that the methods described here can not only be used in connection with vehicles, but are also advantageous for all other applications in which the composition of an air-fuel or exhaust gas mixture is to be analyzed.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 102007057707 A1 [0004]

Claims (12)

Method (100; 200) for calculating a lambda value using a broadband lambda probe (L), wherein by means of an evaluation device (M) which has a measuring connection (S1) and at least one setting connection (S2; S3), in a start phase of the broadband lambda probe (L ) at least one shunt resistor (R _N1; R _N2 ; R _N3 ) is determined; During normal operation of the broadband lambda probe (L), which takes place after the start phase, a pump current (l _{P, t} ) of a pump cell (P) of the broadband lambda probe (L) taking into account the at least one shunt resistor (R _N1; R _N2 ; R _N3 ) is determined; and the lambda value is calculated from the determined pump current (I _{P, t} ). Method (100) according to Claim 1 , the pump current (I _{P, t} ) being determined by measuring a measuring current (I _{M, t} ) flowing into the measuring connection (S1), the determined shunt resistance (R _N1; R _N2 ; R _N3 ) being a first shunt resistance (R _N1 ), through which current can flow from a current source (B) into the measuring connection (S1), bypassing the pump cell (P). Method (100) according to Claim 2 wherein the determination of the first shunt _{resistance (R N1} ) comprises measuring a measurement current in the measurement connection (S1) as a first leakage current (I _{M, 0} ) while no control current flows through the at least one control connection (S2; S3); simultaneously measuring a voltage (U _{B, 0} ) of the current source (B) against a ground connection (G) of the evaluation device (M); and to calculate the first shunt _{resistance (R N1} ) as the quotient of the voltage (U B.0) of the current source (B) and the first leakage current (I _{M, 0).} Method (100) according to Claim 2 or 3 , wherein the determination of the pump current (I _{P, t} ) comprises measuring a current voltage (U _{B, t} ) of the current source (B); to calculate a current leakage current (l _{N1, t} ) as the quotient of the first shunt resistance (R _N1 ) and the current voltage (U _Bt ) of the current source (B); measure the current measurement current (l _{M, t} ); and subtract the current leakage current (l _{N1, t} ) from the current measurement current (l _{M, t} ). Method (200) according to Claim 1 , the pump current (I _{P, t} ) being determined by detecting a control current (Is, t) which flows via the at least one control connection (S2; S3) of the evaluation device (M), the determined shunt resistance being a second shunt resistance (R _N2 ), through which current can flow from the at least one setting connection (S2; S3) to a ground connection of a current source (B), bypassing a connection of the broadband lambda probe (L); and a ground offset (U _{λ, 0} ) is determined and taken into account as the difference between a ground potential of the broadband lambda probe (L) and a ground potential of the evaluation device (M). Method (200) according to Claim 5 , wherein determining the second shunt resistance (R _N2 ) comprises setting the connection of the broadband lambda probe (L) to a high input resistance (R _{λ, 0} ; a first defined control current (l _{S, 1} ) at the control connection to be measured (S2; S3 ); after the system has settled, a first potential (U _{λ, 1} ) of the connection of the broadband lambda probe (L) to the ground connection (G) of the evaluation device (M) to be measured; a second defined control current (l _{s, 2} ), which is differs from the first one (l _{S, 1} ), to be set at the adjusting connection (S2; S3) to be measured; after the system has settled, a second potential (U _{λ, 2} ) of the connection of the broadband lambda probe (L) to the ground connection (G) of the To measure the evaluation device (M); and the second shunt _{resistance (R N2} ) of the relevant control connection (S2; S3) as the quotient of the difference between the two measured potentials (U _{λ, 1} ; U _λ.2 ) and the difference between the two set control currents me (l _{S, 1} ; l _{S, 2} ). Method (200) according to Claim 6 , where the mass offset (U _{λ, 0} ) is the difference between the measured first or second potential (U _{λ, 1} ; U _{λ, 2} ) and the product of the respective associated actuating current (l _{S, 1} ; l _{S, 2} ) with the calculated second shunt resistance (R _N2 ). Method (200) according to one of the Claims 5 to 7th wherein the determination of the pump current (I _{P, t} ) comprises setting a current actuating current (I _{S, t} ); to measure a current potential (U _{λ, t} ) of the connection of the broadband lambda probe (L) to the ground connection (G) of the evaluation device (M); to calculate a second leakage current (I _{N2, t} ) as the quotient of the difference between the current potential (U _{λ, t} ) and the offset (U _{λ, 0} ) with the second shunt _{resistance (R N2);} and to subtract the second leakage current (I _{N2, t} ) from the current actuating current (I _{S, t).} Method according to one of the preceding claims, wherein the lambda value is used to set an air-fuel mixture for a combustion process. Computing unit (M) which is set up to carry out all method steps of a method (100; 200) according to one of the preceding claims. Computer program which causes a processing unit (M) to perform all method steps of a method (100; 200) according to one of the Claims 1 to 9 to be carried out when it is executed on the computing unit (M). Machine-readable storage medium with a computer program stored thereon Claim 11 .

Images