EP1079090B1 - Method for calibration of a wideband 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.

For detecting a ratio of an oxygen content and a fuel fraction in an air-fuel mixture (lambda value), it is known to arrange lambda probes in an exhaust gas passage of an internal combustion engine. Such lambda probes provide 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 used to control a composition of the fuel-air mixture (lambda control).

In practice, essentially two different types of lambda sensors are used. On the one hand, so-called jump lambda probes provide the signal in the form of an electrical voltage which adjusts according to an equilibrium oxygen concentration between two catalytically active electrodes of the probe. Since the equilibrium oxygen concentration in the range at λ = 1 (stoichiometric operation) changes by several orders of magnitude, such a jump lambda probe shows 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, a regulation of a working mode of the internal combustion engine in a lean operation (λ> 1) or rich operation (λ <1) is greatly difficult or impossible.

Alternatively, in practice so-called broadband lambda probes, for example two-cell current limit probes, are used. In this case, the exhaust gas must first overcome a diffusion barrier before it enters a measuring chamber. In the measuring chamber, the catalytically active electrodes are arranged as a concentration cell analogous to the jump lambda probe. An output voltage of this Concentration cell is supplied to a controller in the broadband lambda probes and compared with a voltage which usually corresponds to the equilibrium oxygen concentration at λ = 1. An output signal of this regulator controls a current through a second cell of the probe, a so-called pump cell. In the lean mode, this current brings about an oxygen transport out of the measuring chamber, and this corresponds to a diffusion current through the diffusion barrier after equilibrium has been established at the electrodes. However, this also provides an output signal of the probe in the form of a measuring current which 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 increasing extent through the diffusion barrier into the measuring chamber and react there on the catalytically active electrodes with the oxygen now brought up by the pump cell. The flowing measurement stream is a function of a sum of the partial pressures of the reducing agents multiplied by their respective diffusion coefficients. Such broadband lambda probes allow a measurement of the lambda value in a range of λ = 0.7 to ∞.

A disadvantage of such broadband lambda probes is that essential, a height of the measuring current influencing parameters are insufficient or not considered. It is thus known that the measuring current, apart from the exhaust gas composition, also depends on a geometry of the probe, a diffusion barrier porosity, a gas pressure and a temperature which prevails in the region of the probe. It is known to compensate for production-related tolerances, the output signal with a predetermined correction value to multiply (calibration). However, the parameters affecting the sensitivity of the probe will change due to aging effects or contamination during operation of the internal combustion engine.

According to US Pat. No. 4,751,907 A, a correction value for a broadband lambda probe is determined by first detecting an air mass flow in a stoichiometric operating point with known fuel mass. Subsequently, an additional air mass of the internal combustion engine is supplied via a bypass line while keeping the fuel mass constant. The correction value is dependent formed by the air mass flows, the measurement signals of the two operating points and the constant fuel mass. In order to mitigate the effects of the inevitable torque fluctuation, the method is carried out in steady-state operation, in particular at idle.

The object of the present invention is to provide a method which makes it possible to determine the lambda value of the exhaust gas of the internal combustion engine with long-term stability and with high accuracy and which determines the correction value of a broadband lambda probe at any operating point, in particular also in dynamic operation. allowed without unwanted torque fluctuations. The predefinable correction value should also largely compensate for the operational tolerances.

• (a) in einem ersten, stöchiometrischen Betriebspunkt p₁ der Verbrennungskraftmaschine, an der die Lambdasonde ein erstes Messsignal I, entsprechend einem Lambdawert λ = 1 anzeigt, eine zugeführte Kraftstoffmasse m_K1 und ein Luftmassenstrom m_L1 erfasst wird,
• (b) nachfolgend ein zweiter, magerer oder fetter Betriebspunkt p₂ der Verbrennungskraftmaschine mit λ ≠ 1 im Wesentlichen durch Änderung des Luftmassenstroms m_L eingestellt wird, wobei die zugeführte Kraftstoffmasse m_K beim Betriebspunktwechsel im Wesentlichen so verändert wird, dass eine Leistungsänderung der Verbrennungskraftmaschine kompensiert wird,
• (c) in dem zweiten Betriebspunkt p₂ eine Kraftstoffmasse m_K2 und ein Luftmassenstrom m_L2 erfasst wird, und
• (d) in Abhängigkeit von den Luftmassenströmen m_L1, m_L2 und den Kraftstoffmassen m_K1, m_K2des ersten und zweiten Betriebspunkts p₁, p₂ der Korrekturwert k_w für den Lambdawert des Betriebspunktes p₂ gebildet wird,

ist es möglich, eine genaue Bestimmung des Lambdawertes langzeitstabildurchzuführen und die Kalibrierung der Lambdasonde in beliebigen Betriebspunkten unter weitgehender Unterdrückung von Momentenschwankungen durchzuführen.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. In that, to determine the correction value

• (a) in a first, stoichiometric operating point p _{1 of} the internal combustion engine, at which the lambda probe indicates a first measurement signal I, corresponding to a lambda value λ = 1, an added fuel mass m _K1 and an air mass flow m _{L1 is} detected,
• (b) subsequently a second, lean or rich operating point p _{2 of} the internal combustion engine with λ ≠ 1 is set essentially by changing the air mass flow m _L , wherein the supplied fuel mass m _{K is} substantially changed during the operating point change, that compensates for a power change of the internal combustion engine becomes,
• (C) in the second operating point p _2, a fuel mass m _K2 and an air mass flow m _{L2 is} detected, and
• (d) the correction value k _w for the lambda value of the operating point p _{2 is} formed as a function of the air mass flows m _L1 , m _L2 and the fuel masses m _K1 , m _{K2 of} the first and second operating point p ₁ , p ₂

For example, it is possible to perform an accurate long-term determination of the lambda value and to carry out the calibration of the lambda probe at any desired operating points while largely suppressing torque fluctuations.

The determination of the correction value advantageously takes place as a function of selected calibration parameters. Thus, 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. In the same way can be moved when exceeding a predetermined threshold for the water content of the intake air, a pipe wall temperature or an exhaust gas temperature. These measures subsequently lead to an influence on a water gas content of the exhaust gas (CO and H ₂ content). Of course, the water gas content can also be detected directly and thus a disturbing influence on the calibration of the lambda probe can be excluded.

It is also advantageous to take into account the position of the measuring signal or of the predefinable measuring signal range during the calibration. Thus, it makes sense to use different correction values in lean operation or rich operation of the internal combustion engine for the determination of the lambda value. In addition, calibration parameters, such as the temperature or the predeterminable temperature range of the lambda probe, can be taken into account when calibrating the lambda probe.

A change from the operating point p ₁ into the operating point p ₂ with λ> 1 of the internal combustion engine should preferably be effected by a measure which essentially influences the air mass flow, since an efficiency of the internal combustion engine changes only to a relatively small extent and the air mass flows are detected particularly accurately can. Advantageously, a change to an operating point p ₂ with λ <1 (rich operation) exclusively by the Change the fuel mass m _{K1 be} forced 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 an operating point with equivalent power as in stoichiometric operation with λ = 1. Overall, can be carried out in this way, the calibration of the lambda probe with very low tolerances.

The determination of the correction value can be initiated periodically after a predeterminable period of time or takes place during a dynamic operation of the internal combustion engine, if two successive suitable operating points are reached by chance.

Further preferred embodiments of the invention will become apparent from the remaining, mentioned in the dependent claims characteristics.

The invention will be explained below in an embodiment.

For detecting a mixture composition of an air-fuel mixture, which serves by combustion a drive of an internal combustion engine, it is known to arrange lambda probes in an exhaust passage of the internal combustion engine. Location and shape of such lambda probes are known. The mode of operation will be explained briefly by way of example with reference to a two-cell limit current probe, a so-called broadband lambda probe.

The two-cell limit current probe essentially consists of a concentration cell and a pumping cell. Both cells are formed by partly catalytically active electrodes, wherein the concentration cell is associated with a measuring chamber. The exhaust gas enters the measuring chamber through a porous diffusion barrier. In this case, an output signal of the concentration cell is set in the form of an electrical voltage as a function of an equilibrium oxygen concentration. This output voltage of the concentration cell is fed to a controller and compared in this with a voltage of usually 450 mV, which corresponds to the equilibrium oxygen concentration at λ = 1.

The equilibrium oxygen concentration changes in a transition from a lambda value of just over 1 to a lambda value just below 1 and vice versa by several orders of magnitude, so that the resulting measurement signal in the concentration cell changes greatly. Due to this, the lambda probe has a very high accuracy in the range around λ = 1.

An output signal of the controller controls a current through the pumping cell in such a way that in a lean operation of the internal combustion engine (λ> 1) an oxygen transport takes place out of the measuring chamber. After equilibration of the oxygen concentration at the catalytically active electrodes, 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 to an oxygen partial pressure in the exhaust gas.

In rich operation (λ <1), reducing agents such as CO, HC or H ₂ additionally diffuse to an increased extent through the diffusion barrier into the measuring chamber. At the catalytically active electrodes is an oxidation of the reducing agent by the oxygen brought from the pump cell instead. The flowing stream 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 allows a measurement of the lambda value of the exhaust gas of an internal combustion engine in a wide range of λ = 0.7 to ∞.

However, an additional calibration of the lambda probe is necessary to prevent interfering influences, such as 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 compensate for manufacturing tolerances by multiplying the measurement signal with an adjustable correction value k _w . Not taken into account, however, remains that contamination or aging effects can lead to a drift of the measured signal and operating tolerances are disregarded.

The internal combustion engine are usually associated with means which allow detection of an air mass flow and a supplied fuel mass within a predetermined injection time. The air mass flow can be measured by an air mass meter or calculated 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 determination of the correction value k _w in the lean operation takes place taking into account the following conditions:

First, at an operating point p ₁ with λ ₁ = 1 within an injection time t _1, a fuel mass m _K1 and an air mass flow m _{L1 is} detected. For the measuring current I _{1 of} the two-cell limiting current probe, the following applies: $$I_1=k_w\cdot X{\left(O_2\right)}_1$$

X (O ₂ ) ₁ indicates a residual oxygen content of the exhaust gas at the operating point p ₁ . The residual oxygen content may indicate an excess of oxygen or an oxygen deficiency with respect to a stoichiometric ratio after the catalytic reaction at the electrodes. Under stoichiometric conditions, ie 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 results in the lambda value λ ₁ at the operating point p ₁ . $${\lambda }_1=\frac{m_{L1}}{m_{K1}\cdot k_{st}}$$

Furthermore, the supplied fuel mass m _K1 during the injection time t ₁ at the operating point p ₁ can be expressed as a product of the injection time t ₁ and a proportionality factor kin. $$m_{K1}=k_{\mathrm{in}}\cdot t_1$$

With the injection time t _1, the fuel mass m _K1 is adjusted via the lambda control so that the lambda probe indicates a lambda value of λ = 1. The accuracy of the lambda probe is particularly high at λ = 1, because after catalytic reaction, no excess oxygen or oxygen deficiency exists. Errors in the sensitivity, which are to be compensated by the correction value k _w play no role in the operating point p ₁ , so that it can be assumed that the lambda value can be set with λ ₁ = 1 with high accuracy. The proportionality factor k _in can be determined from the existing measured values with good accuracy and results from the equations (II) and (III). $$k_{\mathrm{in}}=\frac{m_{L1}}{t_1\cdot k_{st}}$$

wobei der Restsauerstoffgehalt X(O₂)₂ im Abgas im Betriebspunkt p₂ unter der Annahme, daß ein Verhältnis von Wasserstoff zu Kohlenstoff im Kraftstoff etwa 2 : 1 beträgt, näherungsweise durch die Gleichung $$X{\left(O_2\right)}_2=\frac{{\lambda }_2-1}{4,76{\lambda }_2+0,33}$$

gegeben ist. Diese Gleichung wird beispielsweise von Pischinger et al. in "Thermodynamik der Verbrennungskraftmaschine", Springer Verlag, angegeben. Dabei ist der Lambdawert λ₂ für den Betriebspunkt p₂ über die Gleichung $${\lambda }_2=\frac{m_{L2}}{m_{K2}\cdot k_{st}}$$

wiederum als ein Verhältnis eines Luftmassenstroms m_L2 zu einer über eine Einspritzzeit t₂ zugeführten Kraftstoffmasse m_K2 definiert. Die innerhalb der Einspritzzeit t₂ am Betriebspunkt p₂ zugeführte Kraftstoffmasse m_K2 ist gegeben über $$m_{K2}=k_{\mathrm{in}}\cdot t_2$$

Subsequently, a change takes place in 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 be effected as far as possible by means of 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. At the same time, an optionally necessary change in the supplied fuel mass m _K1 essentially serves to compensate for a power change of the internal combustion engine. The following applies to the measuring current I ₂ : $$I_2=k_w\cdot X{\left(O_2\right)}_2$$

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 about 2: 1, approximately by the equation $$X{\left(O_2\right)}_2=\frac{{\lambda }_2-1}{4.76{\lambda }_2+0.33}$$

given is. This equation is described, for example, by Pischinger et al. in "Thermodynamics of the internal combustion engine", Springer Verlag, stated. Here, the lambda value λ ₂ for the operating point p ₂ via the equation $${\lambda }_2=\frac{m_{L2}}{m_{K2}\cdot k_{st}}$$

again defined as a ratio of an air mass flow m _L2 to a fuel mass m _K2 supplied over an injection time t ₂ . The within the injection time t ₂ at the operating point p ₂ supplied fuel mass m _K2 is given over $$m_{K2}=k_{\mathrm{in}}\cdot t_2$$

wobei $$k_{21}=\frac{m_{L2}}{m_{L1}}\cdot \frac{t_1}{t_2}$$

ist.By substituting the equations (II) to (VIII) for the two operating points p ₁ , p _{2, the following} equation is obtained for the measuring current I ₂ at the operating point p ₂ : $$I_2=k_w\frac{k_{21}-1}{4.76\cdot k_{21}+0.33}$$

in which $$k_{21}=\frac{m_{L2}}{m_{L1}}\cdot \frac{t_1}{t_2}$$

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 small 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: $$k_w=I_2\cdot \frac{4.76\cdot k_{21}+0.33}{k_{21}-1}$$

mit $$X{\left(O_2\right)}_i=\frac{I_i}{k_w}$$

With the thus determined correction value k _w for the output signal of the lambda probe, the lambda value for the remaining operating points with λ ≠ 1, in particular λ> 1, can now be determined: $${\lambda }_t=\frac{1+0.33\cdot X{\left(O_2\right)}_i}{1-4.76\cdot X{\left(O_2\right)}_i}$$

With $$X{\left(O_2\right)}_i=\frac{I_i}{k_w}$$

A special case of the operating point p2 is the overrun without fuel injection. In this case, the equation (XI) simplifies too $$k_w=I_2\cdot 4.76$$

Overall, lambdageregelt can also be performed by such a calibration of the lambda probe and a lean operation of the internal combustion engine. Furthermore, known monitoring functions which detect, for example, a conversion rate of a catalytic converter arranged in the exhaust gas duct in the internal combustion engine can be carried out much more accurately.

To avoid incorrect calibration, it is useful to determine the correction value k _W taking into account calibration parameters such as a position of the measurement signal, a predeterminable Meßsignalbereich, a temperature or water content of an intake air, a temperature or a predetermined temperature range of the lambda probe, a water gas content or temperature of the exhaust gas or a combination thereof.

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

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

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

The correction values determined such k _w can be redefined to take into account aging or contamination of the lambda probe periodically after a predeterminable period of time. It is also conceivable that the determination of the correction values k _{w takes place} during a dynamic operation of the internal combustion engine as a result of two randomly successive, suitable operating points.

Furthermore, the temperature of the intake air during calibration should not be above a predefinable limit temperature. Advantageously, the limit temperature is 35 ° C, since below this temperature, the water gas content of the intake air is negligible. In addition, the determination of the correction value can be aborted if the water content of the intake air is above a predefinable threshold value.

The calibration should also only take place if the exhaust gas temperature in the region of the lambda probe during the determination of the correction value k _{w is} above a predefinable threshold value. The exhaust gas temperature may be detected directly with an exhaust gas temperature sensor or calculated from the engine operating data via a model. A pipe wall temperature between the exhaust valves of the internal combustion engine and the installation location of the lambda probe should also be above a threshold value. The threshold value for the exhaust gas temperature and the pipe wall temperature are preferably chosen such that the calibration is carried out 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 safely 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 probe corresponds to that of the engine. In this way, the influence of condensation or evaporation processes can be avoided within the exhaust passage.

Claims (12)

1. Method for determining a lambda value of an exhaust gas from an internal combustion engine having a lambda sensor arranged in an exhaust-gas duct of the internal combustion engine, in which a measurement signal (I) from the lambda sensor delivers the lambda value (λ) as a function of a predeterminable correction value (k_W), and the internal combustion engine is assigned means which allow an air mass flow (m_L) and a fuel mass (m_K) supplied to be recorded, and in which method, to establish the correction value (k_W)

   (a) in a first, stoichiometric operating point (p₁) of the internal combustion engine, at which the lambda sensor indicates a first measurement signal (I₁) corresponding to a lambda value λ = 1, a fuel mass (m_K1) supplied and an air mass flow (m_L1) are recorded,

   (b) then a second, lean or rich operating point (p₂) of the internal combustion engine with λ ≠ 1 is set substantially by changing the air mass flow (m_L), with the fuel mass (m_K) supplied, at the change in operating point, substantially being altered in such a way as to compensate for a change in power of the internal combustion engine,

   (c) in the second operating point (p₂), a fuel mass (m_K2) and an air mass flow (m_L2) are recorded, and

   (d) the correction value (k_W) for the lambda value of the operating point (p₂) is formed as a function of the air mass flows (m_L1, m_L2) and the fuel masses (m_K1, m_K2) of the first and second operating points (p₁, p₂).
2. Method according to Claim 1, characterized in that the correction value (k_W) is established 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 predeterminable temperature range of the lambda sensor, a water gas content or a temperature of the exhaust gas or a combination thereof.
3. Method according to Claim 1 or 2, characterized in that a correction value (k_W) is determined for the calibration in the lean-burn operating mode.
4. Method according to Claim 1 or 2, characterized in that a correction value (k_W) is determined for the calibration in the rich-burn operating mode.
5. Method according to Claim 2, characterized in that the temperature of the intake air is below a predeterminable limit temperature while the correction value (k_W) is being established.
6. Method according to Claim 5, characterized in that the limit temperature is 35°C.
7. Method according to Claim 2, characterized in that the water content of the intake air is below a predeterminable threshold value while the correction value (k_W) is being established.
8. Method according to Claim 2, characterized in that the temperature of the exhaust gas and/or of a pipe wall of the exhaust-gas system in the region of the lambda sensor is above a predeterminable threshold value while the correction value (k_W) is being established.
9. Method according to Claim 8, characterized in that the threshold value is above 60°C, in particular 100°C.
10. Method according to one of the preceding claims, characterized in that the establishing of the correction value (k_W) is initiated periodically after a predeterminable period of time has elapsed.
11. Method according to one of Claims 1 to 9, characterized in that the correction value (k_W) is established during dynamic operation of the internal combustion engine resulting from two randomly successive, suitable operating points.
12. Method according to one of the preceding claims, characterized in that the lambda sensor is a wide-band lambda sensor.