DE102011087300A1 - Method for operating an internal combustion engine and for the execution of the method set up control device - Google Patents

Abstract

The invention relates to a method for operating an internal combustion engine (10). According to the method, an exhaust gas produced by the internal combustion engine (10) is guided via a 3-way catalytic converter (20) arranged in the exhaust duct (16). A lambda probe (26) detects a characteristic of an exhaust lambda size before the 3-way catalytic converter (20) and forwards them to an engine control unit (28) with integrated controller on. With the controller of the engine control unit (28) by setting a lambda setpoint, a middle Abgaslambda λm set and the average Abgaslambda λm with a predetermined periodic setpoint variation alternately in the direction of a lean lambda and a lambda lambda value deflected (lambda modulation). During the transition in the direction of the lean lambda value or during the transition in the direction of the rich lambda value at a point in time t1 at which a probe signal of the lambda probe (26) reaches a predetermined threshold value λs, a controller deviation ΔR between a current controller actual value R1 and a current controller Setpoint value and a new controller setpoint value R2 is determined by adding the 2-fold controller deviation ΔR to the current controller actual value R1.

Description

The invention relates to a method for operating an internal combustion engine, wherein an exhaust gas generated by the internal combustion engine is guided over a arranged in an exhaust passage 3-way catalyst.

State of the art and technological background

Processes for lambda control in internal combustion engines can be used to reduce the emissions of harmful exhaust gases into the environment. For this purpose, at least one catalyst can be arranged in the exhaust system of the internal combustion engine. In order to keep the catalytic converter in an optimal operating point, it is necessary to control the mixture preparation of the internal combustion engine with the aid of a lambda control in such a way that a controlled lambda value, which is approximately close to 1.0 at least in the mean value (hereinafter also referred to as mean Exhaust lambda λ _m denotes). To generate a measurement signal, a lambda probe can be arranged in the exhaust system of the internal combustion engine.

In the so-called lambda modulation, the mixture is operated in stoichiometric operation with a fixed modulation amplitude and frequency alternately slightly rich or slightly lean to increase the cleaning effect of a downstream catalyst. Catalysts according to the state of the art thus achieve their maximum exhaust gas purification efficiency, if they are applied on average with a lambda value which is slightly less than 1, see 3 , However, in order to apply a lambda value on average to a downstream catalytic converter, with which the catalytic converter can reach its maximum conversion efficiency, the control therefore has to offer the possibility of predetermined shifts in the mean lambda nominal value, for example by preset delta lambda values or also by setting interventions of a possible trim control behind the catalyst to be able to adjust. According to the prior art, this is done via the imprint of so-called delay or dead times.

Jump probes which generate a signal according to the Nernst principle provide a signal as exemplified in US Pat 2 is shown. With such probes, a control method is usually used for the control, in which only the so-called jump range is considered, that is, it is only evaluated whether the signal is above or below a defined threshold, which is oriented at the jump range by lambda = 1. Typically, this may be, for example, a voltage threshold of about 450 mV. In the richer and lean mixture range, the probe signal is too inaccurate, and the characteristic too flat and superimposed by additional cross influences, so that it is not used in the prior art directly to influence the scheme.

State of the art is inter alia the application of a control method described below with a two-position controller and using a jump lambda probe. 4 shows in the upper part of the course of the probe signal against time, with the x-axis was set to a threshold value of the jump probe for the stoichiometric point (for example, 450 mV). The lower area of the 4 shows a course of the controller intervention over time. The x-axis is set to the controller value 1, ie a value which should ideally lead to the exhaust gas produced by the internal combustion engine having an exhaust lambda of lambda = 1. Below the x-axes, the internal combustion engine is consequently in a rich operating mode and above the x-axes in a lean operating mode. Basically, the switchover of the controller, if a defined switching point is reached, for example, the 450 mV threshold of the probe signal, as is the case at the times t1, t3 and t4. It can also be seen how at the time t1, the controller is stopped, and only at time t2, the switching takes place. The same is the case at the time t4, at which the controller is stopped until the time t5. The time difference between t1 and t2, or t4 and t5, is set so that on average the desired lambda deviation should result from the lambda = 1 point for the mean exhaust lambda λ _m .

A disadvantage of this method is that the setting of the desired lambda deviation from the lambda = 1 point over a dead time is fundamentally inaccurate, since it is not known which lambda value the mixture actually has at all. An average lambda deviation in the catalyst only sets in correctly if the oxygen input and output into the catalyst are the same on average. In addition, this is determined not only by the lambda value of the mixture before the catalyst, but also by the exhaust gas mass flow, since the oxygen mass flow results from lambda and exhaust gas mass flow. If the controller remains at a fixed value for the given dead time, dynamic deviations in the lambda result in additional deviations from the desired average lambda.

Summary of the invention

One or more of the mentioned problems of the prior art can be eliminated or at least reduced with the aid of the method according to the invention for operating an internal combustion engine. According to the method becomes Exhaust gas generated by the internal combustion engine is guided via a 3-way catalyst arranged in the exhaust duct. A lambda probe detects a variable characteristic of an exhaust lambda before the 3-way catalytic converter and forwards them to an engine control unit with integrated controller. With the controller of the engine control unit, a mean exhaust lambda λ _{m is} set by specifying a desired lambda value and the mean exhaust lambda λ _{m is} alternately deflected in the direction of a lean lambda value and a lambda lambda value with predetermined periodic desired value variation (lambda modulation). During the transition in the direction of lean lambda value, or in the transition in the direction of the fat lambda value at a time t1 at which a probe signal of the lambda probe reaches a predetermined threshold λ _s, a controller deviation AR of a current controller value R1 and a current controller setpoint formed and a new controller setpoint R2 determined by adding the 2-fold controller deviation .DELTA.R to the current controller actual value R1. The average exhaust lambda λ _m is preferably in the range from 0.98 to 0.998, and the new controller setpoint value R2 is determined during the transition in the direction of the lean lambda value.

The invention is based on the finding that an adjustment of the average exhaust lambda λ _m to a value slightly below 1 takes much better account of dynamic deviations in the lambda, if the displacement of the mean exhaust lambda λ _m not over the specification of a dead time, but as an offset over the controller is set directly. To determine this offset, the actual controller actual value R1 is recorded at a point in time t1 at which the lambda probe, in particular a leap lambda probe, crosses a threshold value λ _s toward the lean mode of operation. The remaining lambda deviation Δλ to the current lambda desired value is detected, that is to say the correlating controller deviation ΔR is determined. The latter value is multiplied by 2 and added to the current controller actual value R1 to provide a new controller reference value R2. The threshold value λ _s corresponds, in particular in an analogous manner, a control intervention in direction can be made thin, for example, when at Trim control behind the 3-way catalyst has an average exhaust lambda λ _m to be determined> 1 a probe signal for lambda = 1.

The controller will now continue to run with the parameters provided until the new controller setpoint R2 has been reached. A mean lambda of the setpoint variation of a cycle then results as the sum over the lean and rich modulation half-wave. Due to the shift of 2 · Δλ at each lean half-wave, on average exactly the desired deviation from the stoichiometric point for the average exhaust lambda λ _{m will be} established. In this case, preset lambda shifts, control adjustments (for example trim control based on an additional probe behind the catalytic converter) or determined deviations of the jump point from the stoichiometric point can be taken into account. With this method, therefore, the mixture supplied to the catalyst can be adjusted to be emission-optimized with much greater accuracy than hitherto when using a two-point control. In an analogous manner, a shift in the direction of a middle Abgaslambda λ _m can be done with lambda> 1. Here, the new controller setpoint R2 is determined during the transition in the direction of the rich lambda value. Accordingly, there is a shift of 2 · Δλ at each rich half-wave, and on average, exactly the desired deviation from the stoichiometric point to the rich operating mode for the average exhaust lambda λ _{m will} set.

Another aspect of the present invention relates to a control device for controlling an operation of an internal combustion engine, which is set up for carrying out the method according to the invention. For this purpose, the controller may include a computer-readable control algorithm for performing the method. In an advantageous embodiment, the control unit is an integral part of the engine control unit.

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

The invention will be explained below in embodiments with reference to the accompanying drawings. Show it:

1 a schematic structure of an internal combustion engine with an exhaust system and 3-way catalytic converter;

2 a characteristic of a jump lambda probe;

3 Conversion rates of a 3-way catalyst for HC, CO and NO _x as a function of exhaust lambda;

4 a progression of the exhaust lambda and regulator intervention in a two-point regulator according to the prior art for achieving a lambda lambda modulation slightly below λ = 1 average exhaust lambda λ _m ; and

5 a profile of Abgaslambda and regulator intervention according to the inventive method to achieve a lying at Lambdamodulation slightly below λ = 1 average Abgaslambda λ _m .

1 schematically shows the structure of an internal combustion engine 10 with a downstream exhaust system. The internal combustion engine 10 can be a spark ignition engine (gasoline engine). With regard to their fuel supply, they can have a direct injection fuel supply, so working with internal mixture formation, or have a pilot fuel injection and thus work with external mixture formation. In addition, the internal combustion engine 10 be operated homogeneously, wherein in the entire combustion chamber of a cylinder a homogeneous air-fuel mixture is present at the ignition timing, or in an inhomogeneous mode (stratified charge mode), in which at the ignition a comparatively rich air-fuel mixture, in particular in the range of a spark plug, is present, which is surrounded by a very lean mixture in the remaining combustion chamber. Important in the context of the present invention is that the internal combustion engine 10 can be operated with different air-fuel mixtures whose composition can be varied in particular in a range around the stoichiometric point around.

The exhaust system comprises an exhaust manifold, which is the exhaust gas of the individual cylinders of the internal combustion engine 10 in an exhaust duct 16 merges. In the exhaust duct 16 There may be various emission control components. Essential in the context of the present invention is a in the exhaust duct 16 arranged 3-way catalyst 20 ,

The 3-way catalyst 20 has a coating of catalytically active components, such as platinum, rhodium and / or palladium, which are on a porous catalyst support, for example, Al ₂ O ₃ , applied. The coating further comprises an oxygen storage component, such as ceria (CeO ₂ ) and / or zirconia (ZrO ₂ ), which has the oxygen storage capacity (OSC) of the 3-way catalyst 20 certainly. In a stoichiometric or slightly rich exhaust gas atmosphere, the 3-way catalyst 20 Nitrogen oxides NO _x to reduce nitrogen N ₂ and oxygen O ₂ . In stoichiometric or slightly lean operation, unburned hydrocarbons HC and carbon monoxide CO are oxidized to carbon dioxide CO ₂ and water H ₂ O.

With a Abgaslambda easily maintain stoichiometric exhaust gas atmosphere, that is at a λ between 0.98 and 0.998, these conversions run almost completely. Such catalytic coatings are known and customary in the prior art from the exhaust aftertreatment of gasoline engines. Structure and operation of 3-way catalysts are thus well known in the art and require no further explanation.

The exhaust duct 16 may contain various sensors, in particular gas and temperature sensors. Shown here is a lambda probe 26 at a position close to the engine in the exhaust duct 16 is arranged. The lambda probe 26 can be configured as a jump lambda probe and allows in a known manner the lambda control of the engine 10 for what it measures the oxygen content of the exhaust gas.

The signals detected by the various sensors, in particular those with the lambda probe 26 measured exhaust lambda, go into an engine control unit 28 one. Likewise, various parameters of the internal combustion engine 10 , in particular the engine speed and the engine load of the engine control unit 28 read. Depending on the various signals, one controls the engine control unit 28 implemented controller thus the operation of the internal combustion engine 10 , In particular, the fuel supply and the air supply are controlled so that a desired fuel mass and a desired air mass are supplied to represent a desired air-fuel mixture (the desired exhaust lambda). The air-fuel mixture becomes dependent on the operating point of the internal combustion engine 10 , in particular the engine speed and the engine load determined from maps.

To improve the cleaning effect of the 3-way catalyst 20 is provided that the internal combustion engine 10 is operated continuously with a mean exhaust lambda λ _m slightly below the stoichiometric composition, that of the internal combustion engine 10 supplied air-fuel ratio with a predetermined oscillation frequency and a predetermined oscillation amplitude is deflected by this average lambda value periodically alternately in the direction of a lean lambda value and a rich lambda value (so-called lambda modulation). The oscillation frequency and the oscillation amplitude are chosen so that the 3-way catalyst 20 is regenerated quasi-continuously.

Under quasi-continuous regeneration of the 3-way catalyst 20 In the present case, it is understood that its loading state remains essentially constant and in particular at an extremely low level. This means that the time average during a time interval in the size range less lambda oscillations no increasing loading of the 3-way catalyst 20 takes place. Preferably, a limit of at most 50% of the maximum loading of the 3-way catalyst 20 not exceeded.

The oscillation frequency and the oscillation amplitude are further selected such that at the 3-way catalytic coating 22 a minimum conversion rate of unburned hydrocarbons (HC) and / or carbon monoxide (CO) and / or nitrogen oxides (NO _x ) is present, wherein minimum conversion rate can be based on legal limits.

In most cases, the oscillation frequency is dependent on a current operating point of the internal combustion engine 10 , in particular depending on the engine load and / or engine speed determined. The oscillation amplitude can also be determined as a function of the OSC.

Depending on the various signals on the engine control unit 28 accumulate, regulates into the engine control unit 28 implemented controller therefore the operation of the internal combustion engine 10 to represent a desired exhaust target lambda.

Controllers automatically influence one or more physical variables to a predetermined level while reducing disturbing influences. For this purpose, controllers within a control loop constantly compare the signal of the setpoint with the measured and returned actual value of the controlled variable and determine from the difference between the two variables - the control deviation (control deviation) - a manipulated variable which influences the controlled system in such a way that the control deviation becomes a minimum , Because the individual control circuit elements have a time response, the controller must increase the value of the control deviation and at the same time compensate for the time behavior of the path so that the controlled variable reaches the desired value in the desired manner. Incorrectly set controllers make the control loop too slow, lead to a large control deviation or to undamped oscillations of the controlled variable and thus possibly to the destruction of the controlled system. In general, the controllers are distinguished according to continuous and unsteady behavior. Among the best-known continuous controllers are the "standard controllers" with P, PI, PD and PID behavior. For the purposes of the present invention, a continuous proportional, integral, and optionally differential (PI or PID) controller is typically used. A PID controller therefore consists of the proportions of the P-element, the I-element and the D-element. The P element provides a contribution to the manipulated variable, which is proportional to the control deviation. The I-element acts by temporal integration of the control deviation on the manipulated variable with a weighting by the reset time. The D-element is a differentiator, which is only used in conjunction with regulators with P and / or I behavior as a controller. He does not react to the level of the control deviation, but only to the rate of change.

According to the lambda modulation is carried out as in 5 exemplified. In the upper area of the 5 For example, a trace of the probe signal vs. time is shown with the x-axis set at a stoichiometric point jump probe threshold (eg, 450 mV). The lower area of the 5 shows a course of the controller intervention over time. The x-axis is set to the controller value 1, ie a value which should ideally lead to the exhaust gas produced by the internal combustion engine having an exhaust lambda of λ = 1. The controller value is multiplied by the amount of fuel supplied, ie it is directly correlated to the lambda value.

If the lambda probe crosses the threshold value λ _s at time t 1, the controller actual value R1 is recorded according to the invention. At time t1, a lambda deviation Δλ of the desired lambda value from the lambda actual value (here = 1) at the lambda probe or a controller deviation ΔR of the controller setpoint from controller actual value R1 is determined. The controller deviation ΔR is now multiplied by 2 and added to the controller actual value R1. This results in the target of the new controller setpoint R2. The controller then continues to run at the parameters intended for the new operating state, until the controller setpoint R2 has been reached. Likewise, at time t4 proceed. The mean lambda in one cycle of lambda modulation now results as the sum over the lean and rich modulation half wave, that is, in this example, as a mean between times t2 and t3. An average value of the mean lambda of the successive cycles should then correspond to the desired mean exhaust lambda λ _m . Due to the shift of 2 · Δλ in the one half-wave, on average exactly the desired deviation Δλ results.

LIST OF REFERENCE NUMBERS

10

Internal combustion engine

16

exhaust duct

20

3-way catalyst

26

lambda probe

28

Engine control unit

λ _m

medium exhaust lambda

λ _s

Threshold for the probe signal

Δλ

Lambda deviation between lambda actual value and lambda nominal value

R1

Controller value

R2

Controller setpoint

.DELTA.R

Controller deviation between controller actual value R1 and controller reference value R2

Claims (5)

Method for operating an internal combustion engine ( 10 ), in which one of the internal combustion engine ( 10 ) generated exhaust gas via a in the exhaust passage ( 16 ) arranged 3-way catalyst ( 20 ) is guided and a lambda probe ( 26 ) a characteristic of an exhaust lambda size before the 3-way catalyst ( 20 ) and to an engine control unit ( 28 ) with integrated controller, whereby with the controller of the engine control unit ( 28 ) is set by specifying a lambda setpoint, a mean Abgaslambda λ _m and the average Abgaslambda λ _m with a predetermined periodic setpoint variation alternately in the direction of a lean lambda and a lambda lambda value is deflected (Lambdamodulation), characterized in that the transition towards the Magerlambda value or at Transition in the direction of the rich lambda value at a time t1 at which a probe signal of the lambda probe ( 26 ) reaches a predetermined threshold λ _s , a controller deviation ΔR between a current controller actual value R1 and a current controller setpoint value is formed and a new controller setpoint value R2 is determined by adding the 2-fold controller deviation ΔR to the current controller actual value R1 , A method according to claim 1, characterized in that the threshold λ _s corresponds to a probe signal for λ = 1. Method according to one of the preceding claims, characterized in that the lambda probe ( 26 ) is a jump lambda probe. Method according to one of the preceding claims, characterized in that the average Abgaslambda λ _{m is} in the range of 0.98 to 0.998 and the new controller setpoint R2 is determined at the transition in the direction of the lean lambda value. Engine control unit ( 20 ) for controlling an operation of an internal combustion engine ( 10 ) arranged to carry out the method according to one of claims 1 to 4.

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