DE102008013254B4 - Method for operating an internal combustion engine with exhaust gas recirculation - Google Patents

Abstract

Method for operating an internal combustion engine (1) with at least one exhaust gas turbocharger (6) and an intake line (2) for supplying fresh air or fresh mixture and an exhaust line (4) for removing the exhaust gas, with a line (8) for exhaust gas recirculation (7) is provided, which branches off from the exhaust line (4) and opens into the intake line (2) and in which a compressor (9) is provided for conveying the amount of exhaust gas to be recirculated, a bypass line (10) forming a branch upstream of the compressor (9 ) branches off from the return line (8) and opens again into the return line (8) with the formation of a confluence downstream of the compressor (9) and the at least one exhaust gas turbocharger (6) has a turbine (6a) arranged in the exhaust gas line (4) and one in the suction line (2) arranged compressor (6b), in which • the compressor (9) in the line (8) for exhaust gas recirculation (7) is activated as soon as the pressure drop between the A gas line (4) and the suction line (2) for conveying a specifiable ...

Description

The invention relates to a method for operating an internal combustion engine having at least one exhaust gas turbocharger and a suction line for supplying fresh air or fresh mixture and an exhaust pipe for discharging the exhaust gas, wherein a line is provided for exhaust gas recirculation, which branches off from the exhaust pipe and opens into the intake and in which a compressor is provided for promoting the amount of exhaust gas to be recirculated, wherein a bypass line branches off to form a branch upstream of the compressor from the return line and opens to form a junction downstream of the compressor back into the return line and the at least one exhaust gas turbocharger arranged in the exhaust pipe turbine and comprising a compressor arranged in the intake line, in which the compressor in the line for exhaust gas recirculation is activated as soon as the pressure gradient between the exhaust pipe and the intake line to the delivery g a predetermined recirculating exhaust gas amount is not large enough.

The German patent application DE 100 35 375 A1 describes a method for controlling exhaust gas recirculation of a heavy duty diesel engine equipped with a bypass line and a bypass valve arranged in the bypass line.

In the context of the present invention, the term internal combustion engine includes both diesel engines and gasoline engines.

Due to the increasingly restrictive legislation regarding the limit values for pollutant emissions from motor vehicles, a fundamental aim is to reduce the relevant pollutant emissions. In particular, the nitrogen dioxide emissions (NO _x ) are of increasing relevance. Since carbon dioxide emissions correlate directly with fuel consumption, the development of vehicle drives is constantly striving to minimize fuel consumption.

In internal combustion engines, therefore, the development of consumption-optimized combustion processes is in the foreground of efforts. Often these are hybrid combustion processes which attempt to exploit both the advantages of the diesel engine method and the advantages of the Otto engine method, the conventional Otto engine method being characterized by mixture compression, homogeneous mixture, spark ignition, and quantity control whereas the traditional diesel engine process is characterized by air compression, inhomogeneous mixture, auto-ignition and quality control.

An example of a hybrid combustion process is the direct-injection Otto engine combustion method, since the direct injection of fuel was originally made only in diesel engines.

The so-called HCCI method for operating a gasoline engine is also such a hybrid combustion method. This method is based on a controlled auto-ignition of the fuel supplied to the cylinder. Here, the fuel - as in a diesel engine - in excess of air, that is superstoichiometric, burned. The lean-burn gasoline engine has comparatively low nitrogen oxide (NO _x ) emissions due to the low combustion temperatures and also no soot emissions due to the lean mixture.

In addition, the HCCI process leads to a high thermal efficiency. The fuel can be introduced both directly into the cylinder and into the intake manifold. The HCCI method and an internal combustion engine using this method of combustion of the fuel are disclosed in U.S.P. US 6,390,054 B1 described.

The main disadvantage of the HCCI method is that this prior art method can not be used in all operating points of the internal combustion engine, so that the - mentioned above - advantages of this combustion process only in a limited range of the engine map (load over speed ). This is also the reason why, as soon as the HCCI method is to be used, basically a hybrid internal combustion engine is required, which can be operated at the operating points where the HCCI method fails by means of another combustion method. The necessary use of different combustion processes for different operating ranges of the internal combustion engine leads to a very complex to be controlled and thus to a costly and fault-prone internal combustion engine.

While innovative combustion processes - as explained above - attempt to prevent the formation of pollutants a priori, exhaust aftertreatment systems are used to reduce existing pollutant emissions.

In the gasoline engines, catalytic reactors are used in the prior art which, using catalytic materials, ensure oxidation of the unburned hydrocarbons (HC) and of the carbon monoxide (CO). If additional nitrogen oxides (NO _x ) can be reduced, can This can be achieved by the use of a three-way catalyst, but this requires a running within narrow limits stoichiometric operation (λ ≈ 1) of the gasoline engine.

In this case, the nitrogen oxides (NO _x ) are reduced by means of the existing unoxidized exhaust gas components, namely the carbon monoxide and the unburned hydrocarbons, wherein at the same time these exhaust gas components are oxidized.

In internal combustion engines, which are operated with an excess of air, ie for example in lean-burn gasoline engines, but especially direct-injection diesel engines, but also direct injection gasoline engines, the nitrogen oxides located in the exhaust gas may be due to the principle -. H. due to the lack of reducing agents - can not be reduced. For oxidation of the unburned hydrocarbons (HC) and carbon monoxide (CO), an oxidation catalyst is provided in the exhaust system.

To reduce the nitrogen oxides selective catalysts - so-called SCR catalysts - are used, in which targeted reducing agent is introduced into the exhaust gas to selectively reduce the nitrogen oxides. As a reducing agent, not only ammonia and urea but also unburned hydrocarbons are used.

In principle, the nitrogen oxide emissions can also be reduced with so-called nitrogen oxide storage catalysts (LNT). The nitrogen oxides are first - during a lean operation of the internal combustion engine - absorbed in the catalyst d. H. collected and stored, to then be reduced during a regeneration phase, for example by means of a substoichiometric operation (for example, λ <0.95) of the internal combustion engine with lack of oxygen, the unburned hydrocarbons serve as a reducing agent.

The sulfur contained in the exhaust gas, which is also absorbed in the LNT, must be removed regularly in the context of a so-called desulfurization. For this purpose, the LNT must be heated to high temperatures, usually between 600 ° C and 700 ° C, and supplied with a reducing agent.

The equipment of the internal combustion engine with exhaust aftertreatment systems, in particular the provision of exhaust aftertreatment systems for the reduction of nitrogen oxides, leads to higher manufacturing costs of motor vehicles. In addition, exhaust aftertreatment systems - at least temporarily - unwanted or unwanted influence on the operation of the internal combustion engine, since the method for operating the exhaust aftertreatment often require a certain operation of the internal combustion engine itself, for example, a substoichiometric operation for the regeneration of a storage catalytic converter. In this respect, exhaust aftertreatment systems can prevent consumption-optimized operation of the internal combustion engine.

Since the formation of nitrogen oxides requires not only an excess of air but also high temperatures, a concept for reducing nitrogen oxide emissions is to develop combustion processes with lower combustion temperatures.

In this case, the exhaust gas recirculation ie the return of combustion gases from the exhaust pipe in the intake line is expedient in which with increasing exhaust gas recirculation rate, the nitrogen oxide emissions can be significantly reduced. The exhaust gas recirculation rate x _{EGR is} determined as follows: x _AGR = m _EGR / m _EGR + m _{fresh air} ) wherein m _{AGR refers to} the mass of recirculated exhaust gas and m _{fresh air,} the supplied - optionally guided by a compressor and compressed - fresh air or combustion air.

Exhaust gas recirculation is also suitable for reducing emissions of unburned hydrocarbons in the partial load range.

To achieve a significant reduction in nitrogen oxide emissions, high exhaust gas recirculation rates are required, which can be on the order of x _EGR 60% to 70%.

In order to realize these high return rates, in turn, a correspondingly large pressure gradient between the exhaust gas side and intake side is required, which serves as a driving force for the recirculating exhaust gas amount. In principle, two measures can be expedient to increase the pressure gradient, namely the increase of the exhaust backpressure and / or the reduction of the pressure in the intake line.

The former can be achieved in that the exhaust gas is stowed in the exhaust pipe, for example by means of a throttle valve or the arrangement of a turbine in the exhaust pipe. However, the damming up of the exhaust gas basically has a detrimental effect on the charge exchange and the damming by means of a turbine ie the removal of the exhaust gas upstream of the turbine inevitably entails that the exhaust gas to be recirculated does not flow through the turbine and the exhaust gas enthalpy thereof Exhaust partial flow in the turbine is not used or can not be used.

The arrangement of a throttle valve in the intake passage, in which the pressure in the intake fresh air is reduced across the throttle and opens the line for exhaust gas recirculation downstream of the throttle valve in the intake, is considered to be disadvantageous from an energetic point of view, taking into account that just Entschrosselung in gasoline engines is a goal to reduce fuel consumption. The throttling of the intake fresh air inevitably leads to lower efficiencies.

Against this background, it is the object of the present invention to provide a method according to the preamble of claim 1, d. H. of the generic type to show, with the known disadvantages of the prior art are overcome and can be realized with the particular high exhaust gas recirculation rates under all operating conditions of the internal combustion engine.

• – in einem ersten Verfahrensschritt (S1) ausgehend von einem stationären Betrieb der Brennkraftmaschine und einem deaktivierten Verdichter eine Rückführrate AGR-Rate_stat bestimmt wird, wobei die zu erzielende Verringerung ΔNO_x der Stickoxidemissionen (NO_x), das Druckverhältnis p₁/p₂ am Verdichter des Abgasturboladers und der durch den Verdichter hindurchgeführte Frischluftmassenstrom dm_L/dt als Eingangsgrößen berücksichtigt werden,
• – in einem zweiten Verfahrensschritt (S2) ausgehend von einem aktivierten Verdichter eine zusätzliche Rückführrate AGR-Rate_korr bestimmt wird, wobei wiederum die zu erzielende Verringerung ΔNO_x an Stickoxidemissionen (NO_x), das Druckverhältnis p₁/p₂ am Verdichter des Abgasturboladers und der durch den Verdichter hindurchgeführte Frischluftmassenstrom dm_L/dt als Eingangsgrößen berücksichtigt werden, und
• – in einem dritten Verfahrensschritt (S3) die Rückführrate AGR-Rate_stat und die Rückführrate AGR-Rate_korr zu einer Gesamtrückführrate AGR-Rate_tat zusammengeführt werden.

This object is achieved by a method for operating an internal combustion engine having at least one exhaust gas turbocharger and an intake for supplying fresh air or fresh mixture and an exhaust pipe for discharging the exhaust gas, wherein a line is provided for exhaust gas recirculation, which branches off from the exhaust pipe and into the intake opens and in which a compressor is provided for promoting the amount of exhaust gas to be recirculated, wherein a bypass line branches off to form a branch upstream of the compressor from the return line and opens to form a junction downstream of the compressor back into the return line and the at least one exhaust gas turbocharger in the exhaust pipe arranged turbine and a compressor arranged in the intake line, in which the compressor is activated in the line for exhaust gas recirculation, as soon as the pressure gradient between the exhaust pipe and the intake pipe to Promote a predetermined recirculating exhaust gas amount is not large enough and which is characterized in that the predetermined recirculating exhaust gas amount taking into account a reduction to be achieved nitric oxide emissions .DELTA.NO _x , the pressure ratio p ₁ / p ₂ on the compressor of the exhaust gas turbocharger and passed through this compressor fresh air mass flow dm _L / dt is set, in the way that

• - In a first method step (S1), starting from a steady state operation of the internal combustion engine and a deactivated compressor, a return rate EGR rate _{stat is} determined, wherein the target reduction ΔNO _{x of} the nitrogen oxide emissions (NO _x ), the pressure ratio p ₁ / p ₂ am Compressor of the exhaust gas turbocharger and the guided through the compressor fresh air mass flow dm _L / dt are considered as input variables,
• - In a second step (S2) starting from an activated compressor, an additional EGR rate _corr rate is determined, in turn, the target reduction ΔNO _x nitrogen oxide emissions (NO _x ), the pressure ratio p ₁ / p ₂ on the compressor of the exhaust gas turbocharger and the fresh air mass flow dm _L / dt passed through the compressor is taken into account as input variables, and
• - in a third method step (S3), the recirculation rate EGR rate _stat and the recirculation rate EGR rate _{did corr} to a total recirculation rate EGR rate are merged.

The internal combustion engine, which is the subject of the method according to the invention, is equipped with at least one exhaust gas turbocharger, which comprises a turbine arranged in the exhaust gas line and a compressor arranged in the intake line.

The compressor and the turbine of an exhaust gas turbocharger are arranged on the same shaft. The hot exhaust gas flow is supplied to the turbine, in which it relaxes with release of energy, whereby the shaft is rotated. The energy emitted by the exhaust gas flow to the turbine and finally to the shaft is used to drive the compressor, which is also arranged on the shaft. The compressor promotes and compresses the charge air supplied to it, whereby a charge of the internal combustion engine or cylinder is achieved.

The advantage of the exhaust gas turbocharger - for example compared to a mechanical supercharger - is that there is no mechanical connection to the power transmission between the supercharger and the internal combustion engine or is required. While a mechanical supercharger obtains the energy required for its drive completely from the internal combustion engine and thus reduces the power provided and thus adversely affects the efficiency, the exhaust gas turbocharger uses the exhaust gas energy of the hot exhaust gases.

The charge is used primarily to increase the performance of the internal combustion engine. The air required for the combustion process is compressed, which allows each cylinder per working cycle, a larger air mass can be supplied. As a result, the fuel mass and thus the mean pressure p _{me can be} increased.

The charge is a suitable means to increase the capacity of an internal combustion engine with unchanged displacement, or to reduce the displacement at the same power. In any case, the leads Charging to an increase in space performance and a lower power mass. At the same vehicle boundary conditions, the load collective can thus be shifted to higher loads, where the specific fuel consumption is lower. The latter is also referred to as downsizing.

Charging therefore also supports the fundamental objective of reducing pollutant emissions. The emissions of carbon dioxide, which correlate directly with fuel consumption, also decrease with decreasing fuel consumption. With targeted design of the charge and the nitrogen oxide emissions can be reduced without sacrificing efficiency. At the same time, the hydrocarbon emissions can be favorably influenced.

In accordance with the method according to the invention, the amount of exhaust gas to be recirculated and thus the recirculation rate can be determined independently of the currently present pressure gradient between the diversion of the return line from the exhaust line and the connection into the intake line. By activating the arranged in the return line compressor, regardless of the pressure gradient and thus regardless of the current operating point of the internal combustion engine, an arbitrarily large return rate can be realized, even if the pressure gradient to achieve the desired, d. H. specifiable recirculating exhaust gas quantity or recirculation rate, is not sufficiently large.

For controlling the return rate embodiments are advantageous in which upstream of the branch of the bypass line, a first shut-off element is provided in the return line.

The internal combustion engine, which is the subject of the method according to the invention, is equipped with a compressor for conveying the amount of exhaust gas to be recirculated. At operating points of the internal combustion engine, in which the present pressure gradient between the diversion of the return line from the exhaust pipe and the inlet into the intake manifold is not large enough to realize the desired recirculation rate, the amount of recirculated exhaust gas by using the arranged in the return line compressor increase.

If, on the other hand, the compressor is not required to convey the exhaust gas or to achieve the desired recirculation rate, the recirculated exhaust gas can be bypassed via the bypass line, for which purpose the bypass line branches off from the recirculation line upstream of the compressor and returns to the recirculation line downstream of the compressor.

In this connection, embodiments are advantageous in which upstream of the compressor and downstream of the branch of the bypass line d. H. a second shut-off element in the return line is provided between the compressor and branch of the bypass line. By opening and closing this shut-off the compressor is switched on and off to promote the exhaust gas. When the second shut-off element is closed, the exhaust gas is recirculated using the bypass line.

When the compressor is switched off, the closed shut-off element also prevents the exhaust gas to be returned from flowing into the shut-off compressor.

The procedure according to the invention, namely the equipment of the exhaust gas recirculation with a compressor for conveying the recirculated exhaust gas, ensures that arbitrarily high return rates can be realized under all operating conditions, whereby the object underlying the invention is achieved.

Further advantageous process variants are discussed in connection with the subclaims.

Advantageous embodiments of the method in which the first and / or the second shut-off element is electrically, hydraulically, pneumatically, mechanically or magnetically controllable, preferably by means of motor control.

Embodiments of the method in which a cooler in the return line is provided downstream of the junction of the bypass line are advantageous. This cooler lowers the temperature in the hot exhaust gas flow and thus increases the density of the recirculated exhaust gases. The temperature of the cylinder fresh charge, which occurs in the mixture of fresh air with the recirculated exhaust gases, is consequently also lowered, which is why the cooler contributes to a better filling of the combustion chamber.

Embodiments in which the compressor is an electrically operated compressor are advantageous. In contrast to a mechanically operated compressor can be dispensed with an electrically operated compressor on a mechanical coupling with the internal combustion engine.

With regard to the use of the exhaust gas turbocharger embodiments are advantageous in which the line for exhaust gas recirculation branches off from the exhaust pipe upstream of the turbine. This embodiment of the exhaust gas recirculation may also be referred to as high pressure EGR, since the exhaust pressure in the exhaust pipe upstream of the Turbine is comparatively high, ie compared to the low downstream of the turbine prevailing exhaust pressure.

The variant in question is advantageous because the pressure gradient between the exhaust side and intake due to the removal of the exhaust gas upstream of the turbine in a variety of operating points to realize the desired return rate is large enough and thus provided in the return line compressor must be used less frequently.

In addition to this high-pressure EGR but also embodiments may be advantageous in which the line branches off to the exhaust gas recirculation downstream of the turbine from the exhaust pipe. In this so-called low-pressure EGR, the entire exhaust gas is first passed through the turbine and used to charge the internal combustion engine, before a portion of the exhaust gas is introduced via return line into the intake.

Embodiments in which the turbine has a variable turbine geometry are advantageous. A variable turbine geometry increases the flexibility of charging. It allows by adjusting the turbine blades a stepless adjustment of the turbine geometry to the current exhaust gas mass flow. Optionally, can be dispensed with an energetically disadvantageous Abgasabblasung, as is done in waste-gate turbines.

But are also advantageous embodiments in which the turbine has a fixed, unchangeable turbine geometry. This embodiment has particular cost advantages. On the one hand, the complex and costly adjustable turbine geometry or mechanics is eliminated in this turbine design. On the other hand, due to the principle, no control of the turbine is necessary.

Embodiments in which a charge air cooler is arranged in the intake line downstream of the compressor arranged in the intake line are advantageous. The charge air cooler lowers the air temperature and thus increases the density of the air, whereby the radiator - such as the charge - contributes to a better filling of the combustion chamber.

Embodiments in which the line for exhaust gas recirculation opens downstream of the charge air cooler into the intake line are advantageous. In this way, the exhaust gas flow is not passed through the intercooler and consequently can not pollute this radiator by deposits of pollutants contained in the exhaust stream, in particular soot particles and oil.

• – das zweite Absperrelement geöffnet wird, wenn der Verdichter in der Leitung zur Abgasrückführung aktiviert wird, und dieses zweite Absperrelement geschlossen wird, wenn der Verdichter deaktiviert wird.

In internal combustion engines in which a second shut-off element is provided in the return line upstream of the compressor and downstream of the branch of the bypass line, ie between the compressor and branch of the bypass line, process variants are advantageous in which

• - The second shut-off is opened when the compressor is activated in the line for exhaust gas recirculation, and this second shut-off is closed when the compressor is deactivated.

Advantageously, process variants in which a momentary driver request for the acceleration and / or the torque of the internal combustion engine is taken into account in determining the specifiable exhaust gas quantity to be recirculated.

The latter method variant is used in conjunction with the description of 3 explained in more detail.

In the following the invention is based on an embodiment according to the 1 to 3 described in more detail. Hereby shows:

1 schematically a first embodiment of the internal combustion engine,

2 schematically an embodiment of the method in the form of a flow chart, and

3 schematically a compressor map of a compressor of an exhaust gas turbocharger.

1 shows a first embodiment of the internal combustion engine 1 using the example of a six-cylinder V-engine. The cylinders 3 the internal combustion engine 1 are distributed on two cylinder banks and form in this way two groups of cylinders, each via an exhaust pipe 4a . 4b have to dissipate the hot combustion gases, which to a common exhaust pipe 4 merge, causing the exhaust pipes 4 . 4a . 4b communicate with each other and in the exhaust pipes 4 . 4a . 4b the same exhaust pressure prevails. Furthermore, the internal combustion engine has 1 via a suction line 2 to supply the internal combustion engine 1 with fresh air or fresh mixture.

The internal combustion engine 1 is with an exhaust gas turbocharger 6 equipped, one in the exhaust pipe 4 arranged turbine 6a and one in the suction line 2 arranged compressor 6b includes. That of the internal combustion engine 1 supplied fresh air is thus by means of compressor 6b compressed and downstream of this compressor 6b one in the intake pipe 2 arranged intercooler 5 fed. The intercooler 5 lowers the air temperature and thus increases the density of the air, causing it to a better filling of the cylinder 3 contributes with air.

The one from the cylinders 3 discharged exhaust gas mass flow is the turbine 6a fed. The exhaust gas relaxes in the turbine 6a , wherein the exhaust enthalpy for driving the compressor 6b and thus for charging the internal combustion engine 1 is being used.

The internal combustion engine 1 is also with an exhaust gas recirculation 7 equipped, with a line 8th for exhaust gas recirculation 7 is provided by the exhaust pipe 4 upstream of the turbine 6a branches off and into the intake pipe 2 opens or with the intake pipe 2 is merged.

In the line 8th for exhaust gas recirculation 7 is a compressor 9 intended to promote the amount of exhaust gas to be recirculated, bypassing the compressor 9 a bypass line 10 upstream of this compressor 9 from the return line 8th branches off and downstream of the compressor 9 back into the return line 8th opens.

To control the return rate is upstream of the bypass line 10 a first shut-off element 11 in the return line 8th intended. Between the compressor 9 and the bypass line 10 is another, second shut-off 12 in the return line 8th arranged. By opening and closing this shut-off element 12 becomes the compressor 9 switched on or off to promote the exhaust gas. Is the second shut-off element 12 closed, the exhaust gas via bypass line 10 at the compressor 9 passed back over. Closing the second shut-off element 12 Prevents the recirculating exhaust gas when the compressor is deactivated 9 in the compressor 9 into flows.

Downstream of the junction of the bypass line 10 in the return line 8th is a cooler 13 in the return line 8th intended. This cooler 13 lowers the temperature in the hot exhaust gas flow and thus increases the density of the recirculated exhaust gases. The temperature of the fresh cylinder charge, resulting in the mixture of via intake 2 supplied fresh air with the via return line 8th recirculated exhaust gases, is thereby further reduced, which is why this cooler 13 to a better filling of the cylinder 3 contributes.

2 schematically shows an embodiment of the method in the form of a flow chart.

In a first method step (S1), a recirculating exhaust gas _quantity m _AGRstat or the corresponding ie corresponding recirculation rate EGR rate _{stat is} determined, wherein a reduction ΔNO _{x of} the nitrogen oxide emissions (NO _x ) to be achieved, the pressure ratio p ₁ / p ₂ at the compressor of the Exhaust gas turbocharger and the guided through the compressor fresh air mass flow dm _L / dt be considered as input variables. It is assumed that a stationary operation of the internal combustion engine or the compressor.

In a second method step (S2), an additional amount of exhaust gas m _{AGRkorr to be recirculated} or the corresponding ie corresponding recirculation rate AGR rate _{corr is} determined, again a reduction ΔNO _x of nitrogen oxide emissions (NO _x ) to be achieved, the pressure ratio p ₁ / p ₂ am Be considered compressor of the exhaust gas turbocharger and passed through the compressor fresh air mass flow dm _L / dt as input variables.

The recirculation rate EGR rate _corr determined in the second method step (S2) takes account of the fact that more exhaust gas can be recirculated than the recirculation rate EGR rate _stat would permit, without the surge limit of the compressor being exceeded, ie without the danger in that the compressor of the exhaust-gas turbocharger is brought into the operating state of the pumping, which is to be avoided. In connection with the description of the 3 , which represents a compressor map, this will be explained in more detail.

In a third step (S3), the recirculation rate EGR rate are _stat and the recirculation rate EGR rate _corr to a total recirculation rate EGR rate _did merged.

When setting this EGR rate feedback rate _, an instantaneous driver request for the acceleration dn / dt and / or the torque dT / dt of the internal combustion engine is taken into account, so that the return rate may be limited and replaced by a limited EGR rate _lim must (S4).

3 schematically shows a compressor map of a compressor of an exhaust gas turbocharger as the method of the invention is based. The map is for better understanding and to make the relevant relationships clearly visible, not shown as a look-up table or in tabular form, but graphically represented in the form of a diagram.

The pressure ratio p ₁ / p ₂ on the ordinate and the air mass flow dm _L / dt through the compressor are plotted on the abscissa. The curve A indicates the surge limit of the compressor.

In addition, three more curves are entered, the curve B represents the full load curve during acceleration and the curve C exemplifies a compressor characteristic at partial load.

The curve D shows the compressor characteristic at partial load when using the method according to the invention d. H. in the event that the arranged in the return line compressor was switched on and activated to promote the amount of exhaust gas to be recirculated.

The distance of the curve C from the surge line (curve A) is sufficiently large, so that the actually traceable exhaust gas amount can be selected to be larger. In the present case, the recirculation rate is increased using the method _{according to} the invention, ie the amount of recirculated exhaust gas is increased or corrected by an additional amount of exhaust gas m _AGRkorr or an additional exhaust gas mass flow Δm _AGR , which is indicated by arrows. The fresh air mass flow dm _L / dt is reduced accordingly ie reduced.

However, this takes place only as long as the pressure ratio p ₁ / p _{2 is} smaller than a specifiable limit pressure ratio (p ₁ / p ₂ ) _LIMIT .

The pressure ratio should be limited, so that the driving characteristics do not noticeably worsen, especially the driver always the desired d. H. requested service can be provided d. H. is available. Namely, the pressure ratio can be regarded as a measure of the exhaust gas mass recirculated by the compressor, with the exhaust gas mass guided through the turbine of the exhaust gas turbocharger decreasing as the EGR rate increases, which in principle reduces the power.

LIST OF REFERENCE NUMBERS

1

Internal combustion engine

2

suction

3

cylinder

4

exhaust pipe

4a

exhaust pipe

4b

exhaust pipe

5

Intercooler

6

turbocharger

6a

turbine

6b

compressor

7

Exhaust gas recirculation

8th

Line for exhaust gas recirculation, return line

9

compressor

10

bypass line

11

first shut-off element

12

second shut-off element

13

cooler

• AGR

  Exhaust gas recirculation

  EGR rate

  _corr corrected return rate

  EGR rate _lim

  limited return rate

  EGR rate _stat

  static return rate

  EGR rate _did

  actual return rate

  dm _L / dt

  Fresh air mass flow

  Δm _EGR

  additional exhaust gas mass flow

  m _AGRcorr

  additional amount of exhaust gas

  dn / dt

  Speed change, acceleration

  dT / dt

  torque change

  ΔNO _x

  Reduction of nitrogen oxide emissions to be achieved

  m _AGR

  Mass of recirculated exhaust gas

  m _AGRstat

  amount of exhaust gas to be recirculated (stationary operation of the internal combustion engine)

  m _AGRcorr

  additional recirculating exhaust gas quantity

  _{Fresh air}

  Mass of supplied fresh air or combustion air

  p ₁ / p ₂

  Pressure ratio at the compressor of the exhaust gas turbocharger

  (p ₁ / p ₂ ) _LIMIT

  Limit pressure ratio

  X _AGR

  Exhaust gas recirculation rate

Claims (8)

Method for operating an internal combustion engine ( 1 ) with at least one exhaust gas turbocharger ( 6 ) and a suction line ( 2 ) for the supply of fresh air or fresh mixture and an exhaust pipe ( 4 ) for the discharge of the exhaust gas, wherein a line ( 8th ) for exhaust gas recirculation ( 7 ) provided by the exhaust pipe ( 4 ) branches off and into the intake line ( 2 ) and in which a compressor ( 9 ) is provided for the promotion of the amount of exhaust gas to be recirculated, wherein a bypass line ( 10 ) forming a branch upstream of the compressor ( 9 ) from the return line ( 8th ) and forming an opening downstream of the compressor ( 9 ) back into the return line ( 8th ) and the at least one exhaust gas turbocharger ( 6 ) one in the exhaust pipe ( 4 ) arranged turbine ( 6a ) and one in the intake line ( 2 ) arranged compressors ( 6b ), in which • the compressor ( 9 ) in the line ( 8th ) for exhaust gas recirculation ( 7 ) is activated as soon as the pressure gradient between the exhaust pipe ( 4 ) and the suction line ( 2 ) is not large enough to promote a predetermined recirculating exhaust gas amount, characterized in that The predefinable amount of exhaust gas to be recirculated taking into account a reduction of nitrogen oxide emissions ΔNO _x to be achieved, of the pressure ratio p ₁ / p ₂ at the compressor ( 6b ) of the exhaust gas turbocharger ( 6 ) and by this compressor ( 6b ) fresh air mass flow dm _L / dt is determined, in the manner that • in a first method step (S1) starting from a stationary operation of the internal combustion engine ( 1 ) and a deactivated compressor ( 9 ) a rate EGR rate _{stat is} determined, wherein the reduction ΔNO _{x of} the nitrogen oxide emissions (NO _x ) to be achieved, the pressure ratio p ₁ / p ₂ at the compressor ( 6b ) of the exhaust gas turbocharger ( 6 ) and through the compressor ( 6b ) fresh air mass flow dm _L / dt passed through are taken into account as input variables, • in a second method step (S2) starting from an activated compressor ( 9 ) an additional return rate EGR rate _{corr is} determined, wherein in turn the reduction ΔNO _x of nitrogen oxide emissions (NO _x ) to be achieved, the pressure ratio p ₁ / p ₂ at the compressor ( 6b ) of the exhaust gas turbocharger ( 6 ) and through the compressor ( 6b ) Therethrough guided fresh air mass flow are taken into account as input variables dm _L / dt, and • (in a third method step S3), the recirculation rate EGR rate are _stat and the recirculation rate EGR rate _{did corr} to a total recirculation rate EGR rate merged. A method according to claim 1, characterized in that upstream of the branch of the bypass line ( 10 ) a first shut-off element ( 11 ) in the return line ( 8th ) is provided. Method according to claim 1 or 2, characterized in that upstream of the compressor ( 9 ) and downstream of the branch of the bypass line ( 10 ) a second shut-off element ( 12 ) in the return line ( 8th ) is provided. Method according to one of the preceding claims, characterized in that downstream of the confluence a cooler ( 13 ) in the return line ( 8th ) is provided. Method according to one of the preceding claims, characterized in that the compressor ( 9 ) is an electrically operated compressor. Method according to claim 5, characterized in that the line ( 8th ) for exhaust gas recirculation ( 7 ) upstream of the turbine ( 6a ) from the exhaust pipe ( 4 ) branches off. Method according to one of claims 3 to 6, characterized in that • the second shut-off element ( 12 ) is opened when the compressor ( 9 ) in the line ( 8th ) for exhaust gas recirculation ( 7 ), and this second shut-off element ( 12 ) is closed when the compressor ( 9 ) is deactivated. Method according to one of the preceding claims, characterized in that • in the definition of the predefinable amount of exhaust gas to be recirculated a momentary driver request to the acceleration and / or the torque of the internal combustion engine ( 1 ) is taken into account.

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