DE19939555A1 - Method for calibrating a broadband lambda probe used in internal combustion engines - Google Patents

Abstract

To calibrate the sensor and compensate for manufacturing tolerances a correction value is determined as follows. With the engine running with l = 1 a fuel mass mK1 and air mass flow mL1 are determined. Then, with l = 1 (lean or fat mixture) values mK2 and mL2 are measured. Based on these values the correction value for the condition l = 1 is derived

Description

The invention relates to a method for determining a lambda value with the The preamble of claim 1 mentioned features.

To detect a ratio of an oxygen content and a fuel content in an air-fuel mixture (lambda value), it is known in an exhaust duct Arrange internal combustion engine lambda probes. Such lambda sensors stel len a signal (corresponding to the lambda value of the exhaust gas. This signal is usually forwarded to an engine control unit, processed by this and for Control of a composition of the fuel-air mixture used (Lambda regulation).

In practice, there are essentially two different types of lambda sensors Application. On the one hand, so-called jump lambda probes put the signal into shape an electrical voltage available, which corresponds to an equilibrium sets the oxygen concentration between two catalytically active electrodes of the probe. Since the equilibrium oxygen concentration in the range at λ = 1 (stö chiometric operation) changes by several powers of ten, such a step Lambda probe a very steep and stable characteristic curve for the stoichiometric range. On the other hand, it is disadvantageous that the characteristic curve in Areas with λ ≠ 1 runs very flat. Thus, regulation of a work mode is the Internal combustion engine in a lean operation (λ <1) or rich operation (λ <1) strong difficult or not possible.

Alternatively, so-called broadband lambda probes are found in practice, for example two-cell limit current probes, application. Here, the exhaust gas first overcome a diffusion barrier before it enters a measuring chamber. In the  Analogous to the step lambda probe, the measuring chamber are the catalytically active electrodes a concentration cell is arranged. An output voltage of this Concentration cell is fed to a controller in the broadband lambda sensors and compared to a voltage that is usually the Equilibrium oxygen concentration at λ = 1 corresponds. An output signal from this Regulator controls a current through a second cell of the probe, a so-called Pumping cell. In lean operation, this current causes oxygen transport from the Measuring chamber out, this after an equilibrium to the Electrodes corresponds to a diffusion current through the diffusion barrier. With that stands but also an output signal of the probe in the form of a measuring current is available is proportional to the oxygen partial pressure in the exhaust gas.

In rich operation, reducing agents such as CO, HC or H ₂ diffuse to an increased extent through the diffusion barrier into the measuring chamber and react there at the catalytically active electrodes with the oxygen now brought in by the pump cell. The flowing measuring current is a function of a sum of the partial pressures of the reducing agents multiplied by their respective diffusion coefficients. Such broadband lambda probes enable measurement of the lambda value in a range from λ = 0.7 to ∞.

A disadvantage of such broadband lambda sensors is that an essential amount of Parameters influencing the measuring current are only considered insufficiently or not at all become. So it is known that the measuring current in addition to the exhaust gas composition a geometry of the probe, a porosity of the diffusion barrier, a gas pressure and a temperature that prevails in the area of the probe. It is known to Compensation of manufacturing-related tolerances with an output signal multiply predefinable correction value (calibration). However, they are changing parameters influencing the sensitivity of the probe due to aging effects or due to contamination during operation of the internal combustion engine.

The object of the present invention is to provide a method which it enables the lambda value of the exhaust gas of the internal combustion engine to be stable over the long term  and to determine with high accuracy. The preselectable correction value should be used also largely compensate for operational tolerances.

According to the invention, this object is achieved by the method for determining a lambda value of a lambda probe with the features mentioned in claim 1. Because that to determine the correction value

• a) in a first operating point p _{1 of} the internal combustion engine with λ = 1 (stoichiometric operation), a fuel mass m _K1 and an air mass flow m _L1 are recorded,
• b) a fuel mass m _K2 and an air mass flow m _{L2 are} subsequently detected in a second operating point p _{2 of} the internal combustion engine with λ ≠ 1 (lean or rich operation),
• c) as a function of the air mass flows m _L1 , m _L2 and the fuel masses m _K1 , m _{K2 of} the operating points p ₁ , p _2, the correction value k _w for the lambda value of the operating point p _{2 is} formed,

it is possible to determine the lambda value more precisely over the long term perform.

The correction value is advantageously determined as a function of selected calibration parameters. It is conceivable to take into account a temperature and / or a water content of an intake air of the internal combustion engine when determining the correction value. If, for example, the temperature of the intake air exceeds a limit temperature during the determination of the correction value, the calibration is aborted. The same procedure can be followed if a predeterminable threshold value for the water content of the intake air, a pipe wall temperature or an exhaust gas temperature is used. These measures subsequently influence the water gas content of the exhaust gas (CO and H ₂ content). Of course, the water gas content can also be recorded directly, thus preventing the influence on the calibration of the lambda sensor from being disrupted.

The position of the measurement signal or of the predeterminable measurement signal range is also advantageous to be taken into account during calibration. So it makes sense to have different ones Correction values in lean operation or rich operation of the internal combustion engine for the Determine the lambda value to use. In addition, calibration parameters, such as the temperature or the predefinable temperature range of the lambda probe at which Calibration of the lambda probe are taken into account.

A change from the operating point p ₁ to the operating point p ₂ with λ <1 of the internal combustion engine should preferably take place by a measure which essentially influences the air mass flow, since the efficiency of the internal combustion engine changes only to a relatively small extent and the air mass flows are particularly precise can be detected. It is also advantageous if a change in the supplied fuel mass m _K1 essentially serves to compensate for a change in output of the internal combustion engine when changing from operating point p ₁ to operating point p ₂ . Advantageously, a change to an operating point p ₂ with λ <1 (rich operation) can only be forced by changing the fuel mass m _K1 if the operating point p _{2 is} in a lambda range of λ = 0.8 to 0.9. Experience has shown that in this lambda range there is an operating point with equivalent performances as in stoichiometric operation with λ = 1. Overall, the lambda probe can be calibrated in this way with particularly small tolerances.

The correction value can be specified periodically after a predefinable one Period of time to be initiated or takes place during dynamic operation of the Internal combustion engine, if at random two successive suitable ones Operating points can be reached.

Further preferred refinements of the invention result from the others in the Characteristics mentioned subclaims.

The invention is explained below in an exemplary embodiment.  

To detect a mixture composition of an air-fuel mixture, the serves to drive an internal combustion engine by combustion, it is known Arrange lambda sensors in an exhaust duct of the internal combustion engine. location and the shape of such lambda probes are known. The mode of operation is intended to be exemplary briefly using a two-cell limit current probe, a so-called broadband lambda probe, are explained.

The two-cell limit current probe essentially consists of a concentration cell and a pump cell. Both cells are activated by catalytically active electrodes formed, wherein the concentration cell is assigned a measuring chamber. By a porous diffusion barrier, the exhaust gas enters the measuring chamber. In doing so, a Output signal of the concentration cell in the form of an electrical voltage in Dependence on an equilibrium oxygen concentration set. This Output voltage of the concentration cell is fed to and in a regulator with a voltage of typically 450 mV compared to that of equilibrium corresponds to oxygen concentration at λ = 1.

The equilibrium oxygen concentration changes with a transition from one Lambda value from just over 1 to a lambda value just below 1 and vice versa several powers of ten, so that the resulting measurement signal is in the Concentration cell changes greatly. Because of this, the lambda sensor in the Range around λ = 1 very high accuracy.

An output signal from the controller controls a current through the pumping cell such that in a lean operation of the internal combustion engine (λ <1) Oxygen is transported out of the measuring chamber. After a Setting the equilibrium of the oxygen concentration at the catalytically active elec This current is equal to a diffusion current through the diffusion barrier and serves as the output signal of the probe (measuring current). The measuring current is proportional an oxygen partial pressure in the exhaust gas.

In rich operation (λ <1), reducing agents such as CO, HC or H ₂ additionally diffuse through the diffusion barrier into the measuring chamber to an increased extent. Oxidation of the reducing agents by the oxygen brought up by the pump cell takes place on the catalytically active electrodes. The flowing current is thus a function of the sums of the partial pressures of the reducing agents, multiplied by their respective diffusion coefficients. With suitable characteristics and under the simplifying assumption that the influence of the reducing agent is essentially due to an equilibrium water gas (CO and H ₂ content), a lambda value can be determined in this way. Overall, such a two-cell limit current probe enables a measurement of the lambda value of the exhaust gas of an internal combustion engine in a wide range from λ = 0.7 to ∞.

However, an additional calibration of the lambda probe is necessary in order to prevent disturbing influences, such as, for example, geometric properties, a porosity of the diffusion barrier, a gas pressure or a temperature of the probes on the measuring current. It is therefore known to multiply the measurement signal by an adjustable correction value k _w to compensate for manufacturing-related tolerances. However, this does not take into account that contamination or aging effects can lead to a drift of the measurement signal and operational tolerances are not taken into account.

Means are usually assigned to the internal combustion engine for recording an air mass flow and a supplied fuel mass within one allow predefinable injection time. The air mass flow can be from one Air mass meter measured or based on an existing load signal, for example an intake manifold pressure. An accuracy of the available Air mass meter is better than 3% of the measured value as long as the pulsation amplitudes of an intake air are sufficiently small.

In the method according to the invention, the correction value k _{w is determined} in lean operation, taking into account the following conditions:
First, at an operating point p ₁ with λ ₁ = _1, a fuel mass m _K1 and an air mass flow m _{L1 are} detected within an injection time t ₁ . The following applies to the measuring current I _{1 of} the two-cell limit current probe:

I ₁ = k _w .X (O ₂ ) ₁ (I)

X (O ₂ ) ₁ indicates a residual oxygen content of the exhaust gas at the operating point p ₁ . The residual oxygen content can indicate an excess of oxygen or an oxygen deficiency in relation to a stoichiometric ratio after the catalytic reaction at the electrodes. Under stoichiometric conditions, i.e. at λ = 1, X (O ₂ ) ₁ is negligible. Taking into account a stoichiometric factor k _st , a ratio of the air mass flow m _L1 to the fuel mass m _K1 supplied within the injection time t ₁ gives the lambda value λ ₁ at the operating point p ₁ .

Furthermore, the supplied fuel mass m _K1 during the injection time t ₁ at the operating point p _{1 can be expressed} as the product of the injection time t ₁ and a proportionality factor k _in .

m _K1 = k _in .t ₁ (III)

The fuel mass m _K1 is set with the injection time t ₁ via the lambda control so that the lambda probe displays a lambda value of λ = 1. The accuracy of the lambda probe is particularly high at λ = 1, since there is no residual oxygen excess or lack of oxygen after a catalytic reaction. Errors in the sensitivity that are to be compensated for via the correction value k _w therefore do not play any role at the operating point p ₁ , so that it can be assumed that the lambda value can be set with high accuracy with λ ₁ = 1. The proportionality factor k _in can be determined with good accuracy from the available measured values and results from equations (II) and (III).

Subsequently, there is a change to a second operating point p _{2 of} the internal combustion engine with λ ≠ 1, for example with λ ₂ = 2 (lean operation). The change from the operating point p ₁ to the operating point p _{2 of} the internal combustion engine should take place as far as possible by a measure which essentially influences the air mass flow m _L1 , since a change in the efficiency of the internal combustion engine is relatively small here. At the same time, a change in the supplied fuel mass m _{K1 which} may be necessary essentially serves to compensate for a change in output of the internal combustion engine. The following applies to the measuring current I ₂ :

I ₂ = k _w .X (O ₂ ) ₂ (V)

wherein the residual oxygen content X (O ₂ ) ₂ in the exhaust gas at the operating point p ₂ assuming that a ratio of hydrogen to carbon in the fuel is approximately 2: 1, approximately by the equation

given is. This equation is described, for example, by Pischinger et al. in. "Thermodynamics of the internal combustion engine", Springer Verlag. The lambda value λ2 for the operating point p _{2 is} via the equation

again defined as a ratio of an air mass flow m _L2 to a fuel mass m _K2 supplied via an injection time t ₂ . The fuel mass m _K2 supplied at the operating point p ₂ within the injection time t ₂ is given by

m _K2 = k _in .t ₂ (VIII)

By using equations (II) to (VIII) for the two operating points p ₁ , p _{2, the} equation for the measuring current I ₂ at the operating point p ₂ is as follows:

in which

is.

The correction value k _w for the measuring current can thus be determined from the air mass flows m _L1 , m _L2 and the injection times t ₁ , t ₂ at the operating points p ₁ and p ₂ . Assuming a high accuracy of the air mass measuring system and a linearity of an injection system, which is the case with only slight changes in the injection time in most operating points of the internal combustion engine, this correction value k _{w can} be determined with high accuracy and results in:

With the correction value k _w determined in this way for the output signal of the lambda probe, the lambda value for the other operating points can now be determined with λ ≠ 1, in particular λ <1:

With

A special case of the operating point p ₂ is the overrun situation without fuel injection. In this case, the equation (XI) simplifies too

k _w = I ₂ .4.76 (XIV)

Overall, such a calibration of the lambda probe can also be used Lean operation of the internal combustion engine can be carried out under lambda control. Wei Furthermore, known monitoring functions, such as a Conversion rate one arranged in the exhaust duct in the internal combustion engine Detect neten catalyst, be carried out much more accurately.

To avoid incorrect calibrations, it makes sense to determine the correction value k _w taking into account calibration parameters such as a position of the measurement signal, a predeterminable measurement signal range, a temperature or a water content of an intake air, a temperature or a predefinable temperature range of the lambda probe, a water gas content or a temperature of the exhaust gas or a combination thereof.

By taking the measurement signal or the predeterminable measurement signal range into account, various correction values k _{w can be} defined, for example, for the lean operation and the rich operation of the internal combustion engine. This makes sense because in fat operation, the relevant diffusion coefficients are less determined by the mechanism of a pore diffusion because of a higher hydrogen content. To determine the correction value k _w , an operating point p ₂ is preferably set in a lambda range of λ = 0.8 to 0.9 by changing the supplied fuel mass m ₁ . It can be exploited that, with a constant air mass flow, an operating point p ₂ exists in this lambda range, in which an output of the internal combustion engine approximately corresponds to the output of the internal combustion engine at the operating point p ₁ .

When determining the correction value k _w in rich operation, the equations previously established in connection with the determination of the correction value k _w for lean operation also apply. Only the residual oxygen content according to equation (VI) has to be adjusted accordingly, since it is known that there is an excess of oxygen in lean operation and, on the other hand, there is a lack of oxygen in rich operation. This can be calculated in a known manner, taking into account a water gas equilibrium for the proportions in the exhaust gas of CO, H ₂ , H ₂ O and CO ₂ .

In the rich mode, the reducing agents - as already explained - diffuse through the diffusion barrier to the catalytically active electrodes of the lambda sensor. There they react with the oxygen brought in by the pump cell, an oxygen flow required for the oxygen equilibrium concentration corresponding to λ = 1 representing the measured value. The height of the oxygen flow corresponds to the diffusion flow of CO and H ₂ , so that there is ultimately a measuring current I ₂ which corresponds to the exhaust gas components of CO and H ₂ multiplied by their respective diffusion coefficients, and from which a correction value k _w for the rich operation can be calculated.

The correction values k _{w determined in this way} can be periodically redefined to take aging processes or soiling of the lambda probe into account after a predefinable period of time has elapsed. It is also conceivable that the correction values k _{w are determined} during dynamic operation of the internal combustion engine as a result of two random successive, suitable operating points.

Furthermore, the temperature of the intake air should not be above during calibration a predetermined limit temperature. This is advantageously Limit temperature 35 ° C, because below this temperature the water gas content of the Intake air is negligible. In addition, the correction value can be determined be stopped if the water content of the intake air is above a predefinable threshold.  

The calibration should also only be carried out if the exhaust gas temperature in the area of the lambda probe is above a predefinable threshold value during the determination of the correction value k _w . The exhaust gas temperature can be recorded directly with an exhaust gas temperature sensor or calculated from the engine operating data using a model. A pipe wall temperature between the exhaust valves of the internal combustion engine and the installation location of the lambda sensor should also be above a threshold value. The threshold value for the exhaust gas temperature and the pipe wall temperature are preferably selected such that the calibration takes place only from a temperature above 60 ° C., in particular 100 ° C. At a temperature of <60 ° C of the exhaust gas, the dew point of the exhaust gas is surely exceeded. At a temperature of <100 ° C, all evaporation processes of condensed water are completed, so that the water gas content of the exhaust gas at the location of the lambda sensor corresponds to that of the engine. In this way, the influence of condensation or evaporation processes within the exhaust gas duct can be avoided.

Claims (13)

1. Method for determining a lambda value of an exhaust gas of an internal combustion engine with a lambda probe, in particular a broadband lambda probe, the lambda probe being arranged in an exhaust gas duct of the internal combustion engine and a measurement signal of the lambda probe depending on a predefinable correction value k _w providing the lambda value (calibration ) and the internal combustion engine are associated with means which enable the detection of an air mass flow and a supplied fuel mass, characterized in that to determine the correction value (k _w )

• a) in a first operating point (p ₁ ) of the internal combustion engine with λ = 1 (stoichiometric operation), a fuel mass (m _K1 ) and an air mass flow (m _L1 ) are recorded,
• b) a fuel mass (m _K2 ) and an air mass flow (m _L2 ) are subsequently detected in a second operating point (p ₂ ) of the internal combustion engine with λ ≠ 1 (lean or rich operation),
• c) depending on the air mass flows (m _L1 , m _L2 ) and the fuel masses (m _K1 , m _K2 ) of the operating points (p ₁ , p ₂ ), the correction value (k _w ) for the lambda value of the operating point (p ₂ ) is formed .

2. The method according to claim 1, characterized in that the determination of the correction value (k _w ) on the basis of calibration parameters, such as a position of the measurement signal, a predeterminable measurement signal range, a temperature or a water content of an intake air, a temperature or a predefinable temperature range of the lambda probe , a water gas content or a temperature of the exhaust gas or a combination thereof. 3. The method according to claim 1 or 2, characterized in that a correction value (k _w ) for the calibration in the lean operation is determined. 4. The method according to claim 1 or 2, characterized in that a correction value (k _w ) for the calibration in the rich mode is determined. 5. The method according to claim 2, characterized in that the temperature of the intake air during the determination of the correction value (k _w ) is below a predetermined limit temperature. 6. The method according to claim 5, characterized in that the limit temperature Is 35 ° C. 7. The method according to claim 2, characterized in that the water content of the intake air during the determination of the correction value (k _w ) is below a predetermined threshold. 8. The method according to claim 2, characterized in that the temperature of the exhaust gas and / or a pipe wall of the exhaust system in the region of the lambda probe during the determination of the correction value (k _w ) is above a predeterminable threshold value. 9. The method according to claim 8, characterized in that the threshold value is above 60 ° C, in particular 100 ° C. 10. The method according to claims 1 and 3, characterized in that a change from the operating point (p ₁ ) to the operating point (p ₂ ) of the internal combustion engine by a measure influencing the air mass flow (m _L1 ) takes place. 11. The method according to any one of the preceding claims, characterized in that a change in the supplied fuel mass (m _K1 ) when changing from the operating point (p ₁ ) to the operating point (p ₂ ) is used essentially to compensate for a change in performance of the internal combustion engine. 12. The method according to any one of the preceding claims, characterized in that the determination of the correction value (k _w ) is initiated periodically after a predetermined period of time has elapsed. 13. The method according to any one of claims 1 to 11, characterized in that the correction value (k _w ) is determined during dynamic operation of the internal combustion engine as a result of two randomly successive, suitable operating points.