Method for operating an internal combustion engine

Abstract

Various embodiments include a method for operating an internal combustion engine with a three-way catalytic converter with lambda control, comprising: monitoring a NO.sub.x sensor for a lambda value downstream of the converter; setting a threshold value determining a lambda setpoint value upstream of the converter using the difference between the setpoint value of the electrical signal and the measured electrical signal if the signal is below the threshold; if above the threshold value, determining the lambda setpoint value upstream of the converter using the difference between a NH.sub.3 setpoint value of the NO.sub.x sensor and the measured NH.sub.3 signal of the NO.sub.x sensor; and if the measured NH.sub.3 concentration is higher than the NH.sub.3 setpoint value, increasing the lambda setpoint value upstream of the converter and, if the measured NH.sub.3 concentration is lower than the NH.sub.3 setpoint value, reducing the lambda setpoint value upstream of the converter.

Claims

1. A method for operating an internal combustion engine with an exhaust system including a three-way catalytic converter with lambda control, the method comprising: employing an NOx sensor with integrated lambda probe downstream of the three-way catalytic converter; monitoring a signal of the NOx sensor representing a lambda value downstream of the three-way catalytic converter; setting a threshold value of the signal and determining a lambda setpoint value upstream of the three-way catalytic converter using a difference between the setpoint value of the signal downstream of the three-way catalytic converter and the measured signal if the measured signal is below the threshold value; if the measured signal is above the threshold value, determining the lambda setpoint value upstream of the three-way catalytic converter based on a difference between a NH3 setpoint value of the NOx sensor and a measured NH3 signal of the NOx sensor; and if the measured NH3 concentration is higher than the NH3 setpoint value, increasing the lambda setpoint value upstream of the three-way catalytic converter and, if the measured NH3 concentration is lower than the NH3 setpoint value, reducing the lambda setpoint value upstream of the three-way catalytic converter.

2. The method as claimed in claim 1, further comprising adapting the NH3 setpoint value.

3. The method as claimed in claim 2, wherein adapting the NH3 setpoint value includes: reducing the setpoint value of the signal over time from the measured voltage value in the direction of a low voltage value under quasi-static conditions; and adjusting the lambda setpoint value upstream of the three-way catalytic converter based at least in part on the difference between the setpoint value of the signal and the actual signal.

4. The method as claimed in claim 3, further comprising: measuring the signal of the NOx sensor as it reduces continuously to a minimum owing to the reduction in the NH3 concentration; and measuring the NOx signal as it increases as a result of the probability of a breakdown of the NOx concentration through the catalytic converter; wherein the minimum of the NOx signal is used for the adaptation and also for diagnosis of the three-way catalytic converter.

5. The method as claimed in claim 1, wherein: an offset of the NOx sensor is adapted during a shut-off of the fuel supply or an engine stop; during this phase the signal of the NOx sensor is observed continuously until a stable minimum value is reached; and the stable minimum value is used to adapt a NOx signal characteristic diagram to match a NOx output signal of 0.

6. An internal combustion engine comprising: a three-way catalytic converter with lambda control in an exhaust system branch; an NOx sensor with an integrated lambda probe is arranged downstream of the three-way catalytic converter; wherein the sensor generates an electrical signal representing a lambda value downstream of the three-way catalytic converter and an NH3 signal representing an NH3 concentration in the exhaust gas; and a control device modifying operation of the internal combustion engine based on the signals generated by the sensor, the control device programmed to: employ an NOx sensor with integrated lambda probe downstream of the three-way catalytic converter; monitor a signal of the NOx sensor representing a lambda value downstream of the three-way catalytic converter; set a threshold value of the signal and determining a lambda setpoint value upstream of the three-way catalytic converter using a difference between the setpoint value of the signal downstream of the three-way catalytic converter and the measured signal if the measured signal is below the threshold value; if the measured signal is above the threshold value, determine the lambda setpoint value upstream of the three-way catalytic converter based on a difference between a NH3 setpoint value of the NOx sensor and a measured NH3 signal of the NOx sensor; and if the measured NH3 concentration is higher than the NH3 setpoint value, increase the lambda setpoint value upstream of the three-way catalytic converter and, if the measured NH3 concentration is lower than the NH3 setpoint.

7. The internal combustion engine as claimed in claim 6, wherein the NOx sensor arranged downstream of the three-way catalytic converter generates an NOx signal representing a NOx concentration in the exhaust gas.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The single FIGURE shows in a diagram the NO.sub.x signal and the binary lambda signal of an NO.sub.x sensor with an integrated lambda probe which is arranged downstream of the three-way catalytic converter in the exhaust system branch of a spark ignition engine.

DETAILED DESCRIPTION

(2) In some embodiments, a lambda setpoint value which is important for the emission control is determined or set upstream of a three-way catalytic converter by combined measurement of a lambda value and NH.sub.3 value by means of an NO.sub.x sensor with an integrated lambda probe downstream of the three-way catalytic converter. By accurately setting this lambda setpoint value upstream of the three-way catalytic converter, it is possible to keep lambda downstream of the catalytic converter in an accurately defined range, in order to minimize the NO.sub.x and CO.sub.2/HC emissions.

(3) In some embodiments, below a threshold value of the electrical signal (binary signal) which represents the lambda value and which is set, for example, to 650 mV, the lambda setpoint value upstream of the three-way catalytic converter is determined by the difference between the setpoint value of the electrical signal for the lambda value and the measured lambda value (binary signal). However, above a threshold value of the corresponding lambda signal (binary signal), i.e. for example above 650 mV, the lambda setpoint value upstream of the catalytic converter is determined in another way, specifically using the difference between an NH.sub.3 setpoint value of the NO.sub.x sensor which is set, for example, at 10 ppm, as a function of the catalytic converter temperature, and the measured NH.sub.3 signal of the NO.sub.x sensor. The quantity of NH.sub.3 which occurs downstream of the three-way catalytic converter is therefore used according to the invention for control purposes, since in a rich mixture state NH.sub.3 is generated by the three-way catalytic converter and the NH.sub.3 signal is very sensitive with respect to the lambda value downstream of the three-way catalytic converter. NH.sub.3 can also be measured with the NO.sub.x sensor.

(4) In this range, the lambda setpoint value upstream of the three-way catalytic converter is now varied as a function of the above-mentioned difference, specifically the lambda setpoint value upstream of the three-way catalytic converter is increased toward lean if the measured NH.sub.3 concentration is higher than the NH.sub.3 setpoint value. If, in contrast to this, the measured NH.sub.3 concentration is lower than the NH.sub.3 setpoint value, the lambda setpoint value upstream of the three-way catalytic converter is reduced to rich.

(5) In some embodiments, the NH.sub.3 setpoint value is adapted, since, owing to aging of the catalytic converter, the generation of NH.sub.3 can decline over the service life of the three-way catalytic converter with the same lambda value, and the NO.sub.x probability of a breakdown can rise. The NH.sub.3 setpoint value can be adapted here, for example, in the following way: The setpoint value of the electrical signal is slowly reduced from the actual voltage value in the direction of a low voltage value under quasi-static conditions, and the lambda setpoint value upstream of the three-way catalytic converter is adjusted by means of the difference between the setpoint value of the electrical signal and the actual signal. The speed of the reduction can be here, for example, 40 mV per second in the direction of the low voltage (for example 400 mV).

(6) At the same time, the NO.sub.x signal can be measured by the NO.sub.x sensor and reduced continuously to a minimum owing to the reduction in the NH.sub.3 concentration, and then is increased again as a result of the relatively high probability of a breakdown of the NO.sub.x concentration through the catalytic converter, wherein the minimum value of the NO.sub.x signal can be used for the adaptation and also for the diagnosis of the three-way catalytic converter. The NH.sub.3 setpoint value corresponds here to the minimum value and to a difference (delta, for example 10 ppm). In a new catalytic converter the minimum value should be 0.

(7) If the minimum value is above a threshold value, for example 70 ppm (as a function of the temperature), the catalytic converter is diagnosed as being defective.

(8) In some embodiments, in order to increase the accuracy of the NO.sub.x sensor at a low concentration, the offset of the NO.sub.x sensor may be adapted during a shut-off of the fuel supply or an engine stop, wherein during this phase the NO.sub.x signal is observed continuously until a stable minimum value is reached, and wherein this value is used to adapt an NO.sub.x signal characteristic diagram, since in this case the NO.sub.x output signal should be 0. The NO.sub.x signal characteristic diagram corresponds here to the correlation between the current of the NO.sub.x sensor and the NO.sub.x concentration output signal.

(9) The single FIGURE shows in a diagram the NO.sub.x signal and the binary lambda signal of an NO.sub.x sensor with an integrated lambda probe which is arranged downstream of the three-way catalytic converter in the exhaust system branch of a spark ignition engine. In this context, the lambda value downstream of the three-way catalytic converter is plotted on the abscissa. The ordinate represents the NO.sub.x signal in ppm and the binary lambda sensor signal in mV. In the rich region of the lambda signal, a threshold value of the lambda signal of 750 mV is specified. Below this threshold value, i.e. below 750 mV, the lambda setpoint value upstream of the catalytic converter is determined by the difference between the setpoint value of the binary signal and the measured binary signal. Above this threshold value of 750 mV, the lambda setpoint value upstream of the catalytic converter is determined by the difference between a NH.sub.3 setpoint value, which is specified here as 10 ppm, of the NO.sub.x sensor and the measured NH.sub.3 signal of the NO.sub.x sensor. If, the measured NH.sub.3 concentration is higher than the NH.sub.3 setpoint value, the lambda setpoint value upstream of the catalytic converter is increased to lean. If the concentration is lower than the NH.sub.3 setpoint value, the lambda setpoint value is reduced to rich.

(10) Through a combined measurement of a lambda value and of an NH value by means of an NO.sub.x sensor with an integrated lambda probe downstream of the three-way catalytic converter it is therefore possible to carry out particularly accurate setting of the lambda setpoint value upstream of the catalytic converter.

(11) In some embodiments, an internal combustion engine in the exhaust system branch of which three-way catalytic converter with lambda control is arranged, includes an NO.sub.x sensor with an integrated lambda probe is arranged downstream of the three-way catalytic converter, which sensor is designed to generate an electrical signal which represents a lambda value downstream of the three-way catalytic converter, and to generate an NH.sub.3 signal which represents the NH.sub.3 concentration in the exhaust gas, and is designed to pass on these signals to a control device.

(12) In the internal combustion engine, the NO.sub.x sensor arranged downstream of the three-way catalytic converter may be designed to generate an NO.sub.x signal which represents the NO.sub.x concentration in the exhaust gas and to pass on this signal to the control device.