EP3155250B1 - Mehtod and apparatus for controlling an exhaust gas sensor - Google Patents

Description

The invention relates to a method for operating an exhaust gas probe in the exhaust gas duct of an internal combustion engine, wherein the exhaust gas probe has at least one heating element for reaching a nominal temperature of a measuring cell in the exhaust gas probe. The invention also relates to a computer program which is set up to carry out each step of the method and a machine-readable storage medium on which the computer program is stored. The invention further relates to a device for carrying out the method.

State of the art

To reduce emissions in cars with gasoline engines, 3-way catalytic converters are usually used as exhaust gas purification systems, which only convert sufficient exhaust gases if the air-fuel ratio λ is regulated with high precision. For this purpose, the air-fuel ratio λ is measured using an exhaust gas probe located upstream of the exhaust gas purification system. The oxygen storage capacity of such an exhaust gas purification system is used to absorb oxygen in lean phases and release it again in rich phases. This ensures that oxidizable harmful gas components of the exhaust gas can be converted. An exhaust gas probe connected downstream of the exhaust gas purification system is used to monitor the oxygen storage capacity of the exhaust gas purification system. The oxygen storage capacity must be monitored as part of on-board diagnostics (OBD) as it represents a measure of the conversion ability of the exhaust gas purification system. To determine the oxygen storage capacity, either the exhaust gas purification system is first installed in a Lean phase filled with oxygen and then emptied in a rich phase with a lambda value known in the exhaust gas, taking into account the amount of exhaust gas passing through, or the exhaust gas purification system is first emptied of oxygen in a rich phase and then filled in a lean phase with a lambda value known in the exhaust gas, taking into account the amount of exhaust gas passing through. The lean phase is ended when the exhaust gas probe connected downstream of the exhaust gas purification system detects the oxygen that can no longer be stored by the exhaust gas purification system. Likewise, a rich phase is ended when the exhaust gas probe detects the passage of rich exhaust gas. The oxygen storage capacity of the exhaust gas purification system corresponds to the amount of reducing agent supplied for emptying during the rich phase or the amount of oxygen supplied for filling during the lean phase. The exact quantities are calculated from the signal from the upstream exhaust gas sensor and the exhaust gas mass flow determined from other sensor signals.

In today's engine control systems, lambda sensors are used to detect the oxygen concentration in the exhaust gas and to regulate the lambda of the engine. A distinction is made between a continuous probe or broadband lambda probe and a two-point lambda probe or jump probe. A lambda sensor is based on a galvanic oxygen concentration cell with a solid electrolyte. The solid electrolyte typically becomes conductive for oxygen ions at an activation temperature of approximately 350°C. The nominal temperature of the probe is usually significantly higher, typically between 650°C and 850°C. The temperature at which the oxygen sensor becomes operational and meets the requirements in an engine control system is between the activation temperature and the nominal temperature of the sensor. A broadband lambda sensor according to the state of the art and its structure is, for example, in DE 10 2008 042 268 A1 described.

Once the probe is ready for operation, the probe signal can be used for control and diagnostic purposes. In particular, the lambda control can only be activated when the probe is ready for operation. Since active lambda control leads to a reduction in pollutant emissions, the lambda sensor must be ready for operation as quickly as possible after starting the engine.

For this reason, the probe is usually actively heated electrically. For this purpose, the lambda sensor has an electrical heating element that is controlled by a control unit. A sensor element made of zirconium dioxide with an integrated platinum heating element is common.

The operating principle of a two-point lambda sensor is based on comparing the oxygen partial pressures at an exhaust gas electrode and a reference electrode. A probe voltage that is dependent on the oxygen partial pressure in the exhaust gas is established.

The engine control unit usually has a voltage source that is connected in parallel to the two-point lambda sensor. This voltage source generates a constant counter voltage and has a low internal resistance compared to a cold probe. Since the probe itself also represents a voltage source, it is a parallel connection of two voltage sources. The output voltage of this parallel connection can be measured and is a superposition of the probe voltage and the counter voltage, whereby the voltage of the voltage source with the low internal resistance predominates.

The nominal value of the counter voltage is used in numerous places in the ECU software. In common systems, for example, a temperature-independent probe signal is calculated using the counter voltage. Diagnostics also use counter voltage. For example, to detect an interruption in the probe signal circuit, the measured probe voltage is compared with the nominal value of the counter voltage.

In practice, the counter tension is subject to tolerance. Their actual value may differ from the nominal value. The reason for this is tolerances in the components that are used in the control unit to generate the counter voltage. Due to these deviations of the actual counter voltage from its nominal value, derived variables, for example the temperature-independent probe signal, are falsified. In addition, the tolerances of the counter voltage must be taken into account in all comparisons, for example to avoid incorrect diagnoses.

Another method for improving the accuracy of the lambda determination is described in the DE 10 2011 077353 A1 described.

Disclosure of the invention

1. a) Auswählen (11) eines Betriebspunktes der Abgassonde (1), bei dem die Temperatur der Abgassonde/Zweipunkt-Lambdasonde unter 100°C beträgt
2. b) Auslesen (12) der Ausgangsspannung der Parallelschaltung bei dem in Schritt a) ausgewählten Betriebspunkt und Speicherung als tatsächliche Gegenspannung im Steuergerät
3. c) Verwenden (13) der tatsächlichen Gegenspannung zur Lambda Bestimmung mit der Zweipunkt-Lambdasonde (1).

The invention comprises a method for determining lambda using a two-point lambda probe (1) in the exhaust duct (2) of an internal combustion engine (3), wherein the two-point lambda probe (1) has at least one heating element for reaching a nominal temperature of a measuring cell in the exhaust gas probe (1). has, wherein during the lambda determination on the two-point lambda probe, a probe voltage that is dependent on the oxygen partial pressure in the exhaust gas is established, with a voltage source which is located in a control unit and which has an internal resistance that is smaller than the internal resistance of the two-point lambda probe 100 ° C, is connected in parallel to the two-point lambda sensor, an output voltage resulting from the parallel connection of the voltage source with the two-point lambda sensor, comprising the steps:

1. a) Selecting (11) an operating point of the exhaust gas probe (1) at which the temperature of the exhaust gas probe/two-point lambda probe is below 100°C
2. b) Reading out (12) the output voltage of the parallel connection at the operating point selected in step a) and storing it as the actual counter voltage in the control unit
3. c) Using (13) the actual counter voltage to determine lambda with the two-point lambda sensor (1).

The expression “an output voltage corresponds to a counter voltage” means in particular that the possible range of values of the output voltage overlaps with the possible range of values of the counter voltage. In particular, in this context, similar or completely identical value ranges are and therefore similar or identical Tensions can be determined.

This means that the actual counter voltage is measured at a defined operating point in which there is no influence from the connected probe. The measured counter voltage is stored in the control unit and used in the control unit software instead of the nominal value. Advantageously, the signal quality of quantities that are calculated using the measured counter voltage is better than when using the nominal value of the counter voltage. The accuracy of the calculated probe signal is further improved, particularly in low-temperature operation of the probe, where the influence of the counter voltage is particularly large. This improves the quality of the lambda control as well as fuel consumption and emissions are reduced. The robustness of the diagnoses is also increased. For diagnoses that involve a comparison of the probe voltage with the countervoltage, a higher selectivity is achieved if the measured countervoltage is used instead of its nominal value because the tolerances of the electrical circuitry do not have to be taken into account, or at least not to the same extent.

One idea of the invention is that regulation of the heating output is not necessary in every case. In principle, the method according to the invention also works with pure pre-control of the heating output. An unheated probe can also advantageously be used in the method according to the invention. According to the invention, a possibility for operating an exhaust gas probe is provided in which the actual countervoltage can be determined and possible tolerances are correctly taken into account. At the same time, the signal quality of derived variables and the selectivity of diagnoses of the probe signal are improved.

According to a preferred embodiment of the invention, the operating point is selected at a predetermined operating condition, the predetermined operating condition comprising at least one state of unconnected exhaust gas probe and cold exhaust gas probe. The term “cold exhaust gas probe” is used below in particular for the operation of an exhaust gas probe at temperatures of less than 100 ° C.

According to a preferred embodiment of the invention, the probe voltage at the output of the electrical circuit of the measuring cell is determined by an analog-digital converter (ADC). The output voltage of a parallel connection of two voltage sources, the voltage source, which generates a counter voltage and can be integrated in a control unit, and the voltage source of the measuring cell of the exhaust gas probe, is measured. Using the ADC, the signals from the exhaust gas probe can be converted into corresponding digital signals for further processing.

According to a preferred embodiment of the invention, the actual countervoltage is used to determine lambda and/or to regulate the exhaust gas probe. This allows for more precise lambda control be made possible. Switch-on conditions for the lambda control can therefore be designed to be less restrictive. The lambda control can therefore be activated more frequently, which helps reduce fuel consumption and pollutant emissions.

According to a preferred embodiment of the invention, the actual counter voltage is used in the calculation of a temperature-independent probe signal. The probe signal can be a probe voltage. The measured and stored counter voltage is used in the calculation to improve the accuracy of the temperature-independent probe signal. Advantageously, voltage thresholds that are used in diagnoses to compare the probe voltage with the counter voltage can be adapted to the measured counter voltage in order to improve the selectivity of these diagnoses.

According to a preferred embodiment of the invention, a broadband lambda sensor, a two-point lambda sensor, another exhaust gas sensor or a gas sensor is used as the exhaust gas sensor. With the exhaust gas probes mentioned, it is particularly important that they are heated very quickly to their optimal operating temperature for optimal function. It can be provided that each of the lambda sensors installed in the exhaust duct of the internal combustion engine is operated with the method presented and its variants. In principle, the method can also be applied to other exhaust gas sensors, for example NO _x sensors or gas sensors with a temperature-dependent output signal. Such gas sensors can also be installed elsewhere, for example in the supply air duct. Particularly in the case of exhaust gas sensors with a TSP (thermal shock protection) protective layer, the method according to the invention can be used to achieve a significant reduction in the cold emissions of an internal combustion engine.

Brief description of the drawings

Fig. 1

zeigt in schematischer Darstellung das technische Umfeld, in dem das erfindungsgemäße Verfahren gemäß einem ersten bevorzugten Ausführungsbeispiel der Erfindung angewendet wird; und

Fig. 2

illustriert schematisch die Verfahrensschritte des erfindungsgemäßen Verfahrens gemäß dem ersten bevorzugten Ausführungsbeispiel der Erfindung.

The invention is explained in further detail below using preferred exemplary embodiments with reference to the drawings.

Fig. 1

shows a schematic representation of the technical environment in which the method according to the invention is used according to a first preferred embodiment of the invention; and

Fig. 2

schematically illustrates the process steps of the method according to the invention according to the first preferred embodiment of the invention.

Embodiments of the invention

Fig. 1 shows schematically, using an example of a gasoline engine, the technical environment in which the method according to the invention can be used according to a first preferred embodiment of the invention for signal processing of an exhaust gas probe 1, 5. Air is supplied to an internal combustion engine 3 via an air supply 6 and its mass is determined with an air mass meter 7. The air mass meter 7 can be designed as a hot film air mass meter. The exhaust gas from the internal combustion engine 3 is discharged via an exhaust gas duct 2, with an exhaust gas purification system 9 being provided behind the internal combustion engine 3 in the direction of flow of the exhaust gas. The exhaust gas purification system 9 usually includes at least one catalytic converter.

To control the internal combustion engine 3, an engine control 4 is provided, which, on the one hand, supplies fuel to the internal combustion engine 3 via a fuel metering 8 and, on the other hand, the signals from the air mass meter 7 and the exhaust gas probe 5 arranged in the exhaust gas duct 2 as well as a further exhaust gas probe arranged in the exhaust gas discharge line 2 1 are supplied. In the first preferred exemplary embodiment, the exhaust gas probe 5 determines an actual lambda value of a fuel-air mixture supplied to the internal combustion engine 3. It can be designed as a broadband lambda probe or a continuous lambda probe. The exhaust gas probe 1 determines the exhaust gas composition after the exhaust gas purification system 9. The exhaust gas probe 1 can be designed as a jump probe or as a two-point lambda probe.

The exhaust gas probe 5 has as its main component a measuring cell with an integrated heating element, which depends on the oxygen content in the exhaust gas duct 2 provides a dependent output signal, which serves as an input signal for a lambda control. The measuring cell can be designed as a Nernst cell. The lambda control is usually part of the engine control 4. Accordingly, instead of the exhaust gas probe 5 or in addition to it, the exhaust gas probe 1 with its heating element and its measuring cell can be connected to the engine control 4.

The method according to the invention is explained below using the example of the exhaust gas probe 1 designed as a two-point lambda sensor. This can also be applied to other exhaust gas sensors with a temperature-dependent output signal.

There is usually a voltage source in the control unit or in the engine control 4, which is connected in parallel to the exhaust gas probe 1. This voltage source generates a constant counter-voltage and has a low internal resistance compared to a cold exhaust gas probe 1. Since the exhaust gas probe 1 itself also represents a voltage source, it is a parallel connection of two voltage sources. The output voltage of this parallel circuit is measured using an ADC and is a superposition of the Nernst voltage of the exhaust gas probe 1 and the counter voltage, whereby the voltage of the voltage source with the smaller internal resistance predominates. When the probe is cold, it has a high internal resistance, so that the counter voltage dominates. With a hot probe, however, the internal resistance is very small, so that the Nernst voltage of the probe dominates.

The voltage source in the control unit generates a constant counter voltage. This counter voltage is, for example, 1.6 V in the case of a two-point lambda probe with a pumped oxygen reference and 0.45 V in the case of a probe without a pumped oxygen reference. The counter voltage is typically generated from a fixed voltage available in the control unit, for example 5 V or 3.3 V, via a voltage divider. Both the fixed voltage and the resistances of the voltage divider are usually subject to tolerances. In the first preferred embodiment of the invention, the actual counter voltage is in the range, for example, between 1.3 V and 1.9 V.

Fig. 2 schematically illustrates the process steps of the method according to the invention according to the first preferred embodiment of the invention. After the start 10 of the method, in a first step 11 an operating point of the exhaust gas probe 1 is selected, at which an output voltage of a parallel connection consisting of a probe voltage of the exhaust gas probe 1 and a countervoltage of a voltage source integrated in a control unit 4 is the countervoltage of the voltage source integrated in the control unit 4 corresponds. For this purpose, the continuous operating points are observed, if necessary, until an operating point fulfills the aforementioned condition. In a second step 12, the output voltage of the parallel connection is then read out at the operating point selected in the first step. In a third step 13, the output voltage read out in the second step 12 is used as the actual countervoltage before the method is terminated 14.

According to other preferred embodiments of the invention, the measurement of the counter tension is carried out after vehicle assembly at the end of the line before the first engine start. Either the probe is connected or the probe remains unconnected. Such a measurement can be very easily integrated into the process at the end of the belt, as it only requires a short time for a voltage measurement, usually less than a second. According to other preferred embodiments of the invention, a measurement of the countervoltage is also repeated later in the life of the vehicle when appropriate switch-on conditions ensure that the probe is cold, for example after the internal combustion engine has been switched off for a sufficiently long time. Long-term drift in the counter voltage is detected and taken into account through regular repeated measurements. Advantageously, the measured value of the counter voltage is permanently stored in the control device and used instead of the nominal value in the control device software.

Claims (3)

1. Method for lambda determination with a two-point lambda probe (1) in the exhaust gas duct (2) of an internal combustion engine (3), wherein the two-point lambda probe (1) has at least one heating element for reaching a nominal temperature of a measurement cell in the exhaust gas probe (1), wherein a probe voltage that is dependent on the oxygen partial pressure in the exhaust gas is established at the two-point lambda probe during the lambda determination, wherein a voltage source that is located in a control device and that has an internal resistance that is lower than the internal resistance of the two-point lambda probe at 100°C is connected in parallel with the two-point lambda probe, wherein an output voltage results from the parallel connection of the voltage source with the two-point lambda probe, comprising the steps of:

   a) selecting (11) an operating point of the exhaust gas probe (1) at which the temperature of the exhaust gas probe/two-point lambda probe is below 100°C,

   b) reading (12) the output voltage of the parallel connection at the operating point selected in step a) and storing same as actual back-EMF in the control device,

   c) using (13) the actual back-EMF for the lambda determination with the two-point lambda probe (1).
2. Method according to Claim 1, wherein the probe voltage at the output of the electrical circuit of the measurement cell is determined by an analogue-to-digital converter.
3. Method according to either of the preceding claims, wherein the actual back-EMF is used in the calculation of a temperature-independent probe signal.

Images