DE102008044013A1 - Method for determining combustion pressure of combustion chamber of internal combustion engine of motor vehicle, involves determining combustion pressure depending on ignition delay, maximum value of combustion pressure and rise time - Google Patents

Abstract

The method involves determining a combustion pressure (Pdiff) in model based depending on an ignition delay, a maximum value (Prise) of the combustion pressure and a rise time between a beginning of combustion and attaining the maximum value for the combustion pressure. A time dependency of the combustion pressure is sectionally approximated by a mathematical function e.g. parabola. The parabola depends on the maximum value of the combustion pressure and the rise time, and is represented by a preset equation. An independent claim is also included for a control device for an internal combustion engine of a motor vehicle.

Description

State of the art

The The invention relates to a method for determining a combustion pressure a combustion chamber of an internal combustion engine.

The The invention further relates to a control device for execution such a method.

It is known that the emergence or proportion of pollutants in the exhaust of an internal combustion engine by optimizing the fuel combustion can be reduced. For targeted optimization of individual steps Fuel combustion is an accurate knowledge of the time History of the combustion pressure necessary. The combustion pressure changes inter alia, depending on a stroke of a Piston of the internal combustion engine, the heat loss properties the internal combustion engine, the gas composition in the combustion chamber and depending on other influences such as the rail pressure, that is a fuel pressure in one High-pressure fuel storage (common rail) and the injection quantity.

For the calculation of wall heat losses already exists various approaches. To calculate the combustion pressure depending on the aforementioned influences However, there are currently no fast algorithms suitable approaches known.

From the EP 1 477 651 A1 already a method is known which is used to determine a cylinder pressure of an internal combustion engine. However, the known method is disadvantageously based on the evaluation of a plurality of differential equations, which excludes an application for real-time purposes in currently used control units for internal combustion engines due to the high demands on the resources of the controller, in particular the computing power.

Disclosure of the invention

Accordingly It is an object of the present invention, a method and a control unit of the type mentioned in the further develop that an efficient determination of the combustion pressure possible is, in particular under real-time conditions in conventional Control units for internal combustion engines running can be.

These The object is achieved in a method of the type mentioned in the present invention, that the combustion pressure is model-based in dependence an ignition delay, a maximum value of the combustion pressure and a rise time between a start of the combustion and the Reaching the maximum value for the combustion pressure determined becomes. The principle of the invention allows a particularly simple and therefore real-time capable determination the combustion pressure, because in contrast to the conventional method evaluated only the three aforementioned sizes must be, which influence the combustion pressure. In particular, it is unnecessary in the inventive Process the evaluation known from conventional methods of differential equations which is a real time computation of the Precludes combustion pressure.

repräsentiert ist. Hierbei gibt φ einen Kurbelwellenwinkel der Brennkraftmaschine an, und φ_max entspricht dem Kurbelwellenwinkel bei einem maximalen Verbrennungsdruck in dem Brennraum der Brennkraftmaschine. Der Kurbelwellenwinkel φ korrespondiert in bekannter Weise mit einem entsprechenden Zeitpunkt, so dass die vorstehende Form der erfindungsgemäßen Parabelgleichung äquivalent ist zu dem zeitlichen Verlauf des Verbrennungsdrucks.In a particularly advantageous embodiment of the method according to the invention, it is provided that a temporal profile of the combustion pressure p _{diff is} approximated in sections via a first mathematical function, in particular a parabola, the parabola being dependent on the maximum value p _{rise of} the combustion pressure and the rise time t _rise , and being the parabola in particular by the equation

is represented. Here, φ indicates a crankshaft angle of the internal combustion engine, and φ _max corresponds to the crankshaft angle at a maximum combustion pressure in the combustion chamber of the internal combustion engine. The crankshaft angle φ corresponds in a known manner with a corresponding time, so that the above form of the parabola equation according to the invention is equivalent to the time profile of the combustion pressure.

The use of the invention described above Parabola for approximating the combustion pressure allows advantageously a particularly simple and therefore in particular also real-time capable Calculation of the combustion pressure.

A particularly high precision in the invention Approximation of the time course of the combustion pressure is According to another very advantageous variant of the invention, then given when the first mathematical function for approximation the time course of the combustion pressure before or is used until reaching the maximum combustion pressure. Investigations by the applicant have shown that in this period or crankshaft angle range a particularly good match between the parabola selected according to the invention and the actual course of the combustion pressure.

In another very advantageous Ausfüh tion form of the method according to the invention, a temporal profile of the combustion pressure is partially approximated via a second mathematical function, which is dependent on the maximum value p _{rise of} the combustion pressure p _diff , of a cylinder volume V _φ , V _rise assigned to the combustion chamber and of the polytropic exponent κ of a gas charge in the combustion chamber, the second mathematical function being in particular:

The cylinder volume Vrise is that volume of the considered cylinder of the internal combustion engine that is present when the maximum value for the combustion pressure is reached, while the cylinder volume V _φ generally indicates the volume of the cylinder under consideration at the crankshaft angle φ or a corresponding point in time.

The second mathematical function will be a further advantageous Embodiment of the invention According to a method for approximating the time course of the Combustion pressure from or after reaching the maximum Combustion pressure used. That is, according to the invention the time course of the combustion pressure is particularly preferred in a first time range or crankshaft angle range by the first mathematical function, preferably the one above described parabola, approximated while subsequently, that is, in a subsequent second time range or Crankshaft angle range, another mathematical function used becomes.

investigations According to the Applicant, the inventively chosen complement each other and mathematical functions described above are ideal for precise approximation of an actual temporal Course of the combustion pressure in the combustion chamber of the internal combustion engine.

In a further very advantageous embodiment of the operating method according to the invention, it is proposed that the ignition delay τ is determined as a function of a first linear combination of a plurality of operating variables of the internal combustion engine, wherein the ignition delay of a particularly preferred variant of the invention is a linear combination τ = a ₁ * p _rail + a ₂ * t _inj + a ₃ · p _inj + a ₄ · n + a ₅ · r _EGR as a function of a rail pressure p _rail and / or a combustion chamber _temperature t _inj at the time of a control start and / or a combustion chamber _pressure p _inj at the time of the start of control and / or a rotational speed n of the internal combustion engine and / or an exhaust gas recirculation rate r _EGR and / or the coefficients a ₁ , a ₂ , a ₃ , a ₄ , a _{5 is} determined.

The inventive formation of ignition delay τ, which is needed to evaluate the mathematical functions which is the approximation of the time course of the combustion pressure serve, by means of the first linear combination according to the invention represents a particularly simple computational method, which resource-saving and therefore especially real-time capable be performed in a control unit of an internal combustion engine can.

Of the according to the invention first linear combination the realization that the rail pressure is a statement about provides the atomization of the fuel in the combustion chamber, the fuel being at a higher rail pressure during the injection in the combustion chamber in particular fine atomized. The inventive first linear combination further takes into account the combustion chamber temperature and the combustion chamber pressure in each case at the time of a control start, because these two operating variables evaporate of the fuel in the combustion chamber. For a larger one Combustion chamber temperature results, for example, a faster evaporation of the fuel in the combustion chamber.

Of the Influence of the speed of the internal combustion engine on the ignition delay considers the speed-dependent compression speed the considered cylinder of the internal combustion engine and thus also the heat losses, which occur in the area of the cylinder wall.

The also within the first invention Linear combination considered exhaust gas recirculation rate corresponds to a ratio between air and exhaust gas in the combustion chamber and serves to illustrate the influence of the chemical Properties of the combustion chamber gas on the ignition delay. For example becomes at an increasing proportion of the exhaust gas to the combustion chamber gas a temperature of the prevailing in the combustion chamber gas mixture or a corresponding combustion reduced.

The previously described first linear combination of usually already present in a control unit of the internal combustion engine Operating sizes allows advantageous a precise and at the same time less computationally intensive investigation the ignition delay τ in the operation of the internal combustion engine. The simplified in the present invention determined ignition delay is for the invention Approximation of the time course of the combustion pressure by means of the two mathematical functions usable.

The also for the evaluation of the for the approximation of the time course of burning According to another variant of the invention, the rise time required for the mathematical functions required by the pressure pressure can advantageously be determined as a function of a second linear combination of a plurality of operating variables of the internal combustion engine. The rise time t _{rise is} particularly preferably used as a linear combination t _rise = b ₁ * τ + b ₂ * q _inj + b ₃ * p _inj + b ₄ * n + b ₅ * p _rail as a function of the ignition delay τ and / or an injected fuel quantity q _inj and / or the combustion chamber pressure p _inj at the time of control start and / or the speed of the internal combustion engine and / or of the rail pressure p _rail and / or the coefficient b _1, b _{2 3 4} n, b, b, b ₅ determined.

In a comparable manner, the maximum value p _{rise of} the combustion pressure can also be determined as a function of a third linear combination of a plurality of operating _{variables of} the internal combustion engine, in particular as a linear combination p _rise = c ₁ * q _inj + c ₂ * n, where advantageously the injected fuel quantity q _inj and the Speed n of the internal combustion engine is considered, as well as the coefficients c ₁ , c ₂ .

All coefficients a ₁ , a ₂ , a ₃ , a ₄ , a ₅ , b ₁ , b ₂ , b ₃ , b ₄ , b ₅ , c ₁ , c _{2 of} the inventively provided linear combinations can be determined experimentally according to another advantageous variant of the invention , For this purpose, a specific internal combustion engine can be operated, for example, under standard conditions, and the sought-after coefficients are modified until a sufficiently good agreement between the quantities modeled according to the invention, inter alia by means of the linear combinations, and actual values recorded for reference purposes is obtained. The experimental determination of the coefficients used can advantageously also be carried out using statistical optimization methods, in particular genetic algorithms or also methods of the simulated annealing type. The determination of the coefficients according to the invention advantageously only has to be carried out once for a specific configuration of the internal combustion engine, and coefficients determined therefrom can be stored, for example, in a non-volatile memory of a control unit of the internal combustion engine for normal operation.

Next for the linear combinations according to the invention above operating sizes can also further operating variables of the internal combustion engine be involved in the formation of linear combinations, without that is a significant increase in the computational effort for evaluation the corresponding linear combination results.

It is also advantageously possible, the coefficients themselves depending on an operating condition of the internal combustion engine to choose or modify. For example, for a cold start of the engine another set of coefficients be used as for an idling operation. If necessary or for further simplification can - at least temporarily - even individual coefficients set to zero which is particularly useful in such operating areas in which the size of the business associated with the coefficient in question is only a small influence on the way by the linear combination has to be determined size.

Especially Advantageously, the coefficients for the inventive Linear combinations even during normal operation of the Internal combustion engine to be modified, for example, changing Operating conditions or a continuous adjustment to allow the coefficients.

The inventive method requires in contrast to the conventional methods advantageous alone the evaluation the described linear combinations as a function of present anyway in a control unit of the internal combustion engine Farm sizes and the coefficients held and furthermore the evaluation of the mathematical function (s) for approximating the time course of the combustion pressure. These Calculations can be very efficient, especially in Real-time, from conventional control devices for Internal combustion engines are performed and therefore allow a real-time view of the combustion pressure.

Under Use of the inventively determined in real time Combustion pressure, the operation of the internal combustion engine advantageous be controlled and / or regulated, in particular with the aim of Optimization of fuel combustion for the purpose of emission reduction.

Of the According to the invention determined combustion pressure and a combustion chamber temperature derivable therefrom can be advantageous For example, be used to a torque efficiency of a Partial injection, in particular a post-injection to calculate or an ignition angle for a post-injection.

Especially the method according to the invention is also suitable for a predictive combustion optimization by a dynamic adaptation of the operating variables of the Internal combustion engine, that means especially during a duty cycle of the internal combustion engine.

A further increase in the efficiency of the method according to the invention in determining the combustion pressure can advantageously be achieved in that at least one of the Linearkombinatio NEN is formed at least partially using a table of values. That is, in particular frequently used products from certain coefficients and corresponding operating variables of the internal combustion engine can be advantageously stored in a lookup table or the like, so that the product in question does not need to be constantly evaluated in real time. As a result, an arithmetic unit of a control device implementing the method according to the invention can be further relieved. The inventively considered mathematical functions for approximating the time profile of the combustion pressure can also be evaluated according to a further advantageous variant of the invention at least partially using a value table.

When another solution of the object of the present invention is a control device according to claim 20 indicated.

Further Features, applications and advantages of the invention result from the following description of exemplary embodiments of the invention, which are illustrated in the figures of the drawing. All described or illustrated features form for itself or in any combination the subject of the invention, independently from its summary in the claims or their relationship and regardless of theirs Formulation or presentation in the description or in the drawing.

In the drawing shows:

1 a profile of different operating variables of a cylinder of an internal combustion engine plotted over a crankshaft angle,

2 a temporal course of a model-based determined according to the invention combustion pressure in a combustion chamber of an internal combustion engine, and

3 a schematic functional diagram of an embodiment of the method according to the invention.

1 is a graph showing the cylinder pressure p _cyl applied over a crankshaft angle φ, as they appear during the operation of an internal combustion engine. The cylinder pressure p _cyl is composed here of a compression pressure _curve p _compr , as it is z. B. results in a towed operation of the internal combustion engine, and from the combustion pressure _curve p _diff , which results from combustion of an in the combustion chamber of the internal combustion engine located air / fuel mixture.

In order to simplify the determination of the time course of the combustion pressure p _{diff, it} is proposed to use a model which determines the time profile of the combustion pressure p _diff as a function of the ignition delay τ, the maximum value p _{rise of} the combustion pressure p _diff and the rise time t _rise between a start of the combustion time Combustion in the considered cylinder and reaching the maximum value p _rise for combustion pressure.

According to the invention, the three above-mentioned operating variables τ, p _rise , t _{rise of} the internal combustion engine can advantageously be determined very efficiently in a control unit controlling the internal combustion engine, so that in particular the evaluation of differential equations known from conventional methods can be dispensed with. Thus, the inventive method allows a real-time capable determination of the time course of the combustion pressure p _diff .

at the self-igniting example considered here by way of example Internal combustion engine describes the ignition delay τ the Time duration between the fuel injection into the combustion chamber and an actual start of combustion in the combustion chamber of the cylinder.

The time duration between the start of combustion and the attainment of the maximum combustion pressure p _rise in the combustion chamber of the cylinder defines the rise time t _rise considered in accordance with the invention.

The maximum value p _{rise of} the combustion pressure p _diff is the maximum value of the combustion pressure p _diff , as it adjusts to the combustion chamber of the cylinder of the internal combustion engine after the end of the rise time t _rise .

According to the invention, a temporal profile of the combustion pressure p _{diff is} approximated in sections via relatively simple mathematical functions that can be evaluated. This situation is due to the schematic functional diagram of 3 illustrates, in which serve to implement the method of the invention function blocks 21 . 22 in the control unit 20 the internal combustion engine 10 are arranged.

The inventive method can essentially, for example, by a not shown computing unit of the controller 20 such as a microcontroller or a digital signal processor (DSP).

The input variables are the function block 21 later described operating parameters of the internal combustion engine 10 fed, as z. B. also be detected, modeled or otherwise obtained and used in conventional internal combustion engines or corresponding operating methods.

According to the invention, two mathematical functions M1, M2 are advantageously considered in order to approximate different sections of the time profile of the combustion pressure p _diff . The mathematical functions M1, M2 are as in 3 indicated by the function block 22 of the control unit 20 evaluated, which provides at its output according to the invention determined temporal profile the combustion pressure P _diff.

Particularly advantageously, the first mathematical function M1 is used for approximating the time profile of the combustion pressure p _diff before or until the maximum combustion pressure p _rise is reached. This section of the time course of the combustion pressure p _diff is shown in the graph of FIG 2 provided with the reference symbol M1 '.

The in 2 shown crankshaft angle φ ₀ corresponds to the time of the start of combustion, and the crankshaft angle φ _max the time at which the combustion pressure p _diff assumes its maximum value p _rise .

The time duration between the start of combustion and the attainment of the maximum combustion pressure p _rise corresponds, as already described, to the rise time t _rise considered in accordance with the invention and accordingly corresponds to the crankshaft angle difference φ _max -φ ₀ .

repräsentiert ist.According to the invention, the first mathematical function M1 is advantageously a parabola, which is given by the equation

is represented.

Due to the equivalence of the crankshaft angle φ with time, the above mathematical function also indicates in a corresponding manner the time profile of the combustion pressure p _diff in the considered interval.

The second mathematical function M2 ( 3 ) is advantageously used to approximate a further portion M2 'of the combustion pressure p _diff , which is in accordance with 2 for rising crankshaft angle φ to the crankshaft angle φ _max connects.

wodurch sich für den entsprechenden Abschnitt M2' in 2 ein hyperbelähnlicher Verlauf ergibt.The second mathematical function M2 is preferred:

resulting in the corresponding section M2 'in 2 gives a hyperbolaous course.

The mathematical functions M1, M2 considered according to the invention are, according to investigations by the Applicant, particularly well suited to efficiently and nevertheless precisely approximate the actual course of the combustion pressure p _diff . The good agreement of the inventively obtained approximation by the graphs M1 ', M2' according to 2 is from a comparison with the metrologically obtained pressure curves according to 1 seen.

According to the invention, the quantities required for evaluation of the mathematical functions M1, M2 are ignition delay τ, rise time t _rise and maximum value p _{rise of} the combustion pressure as a function of different linear combinations L1, L2, L3 of further operating variables of the internal combustion engine 10 obtained, whereby a very efficient calculation of the operating variables τ, t _rise , p _{rise is} possible.

The linear combinations L1, L2, L3 according to the invention by the function block 21 ( 2 ), which is also preferably in the control unit 20 is realized. For example, a corresponding program that is based on a computing unit of the control unit 20 expires, the calculation of the linear combination L1, L2, L3 provide, as well as the evaluation of the mathematical functions M1, M2.

The ignition delay τ is particularly preferably determined by the first linear combination L1 according to the invention ( 3 ) too τ = a 1 · p rail + a 2 · t inj + a 3 · p inj + a 4 · N + a 5 · r EGR calculated. The coefficients a ₁ , a ₂ , a ₃ , a ₄ , a ₅ are preferably determined experimentally and, together with the operating _variables p _rail , t _inj , p _inj , n, r _EGR weighted by them, take into account the functional relationships of the internal combustion engine 10 , which affect the ignition delay τ.

The rail pressure p _rail provides information about the atomization of the fuel in the combustion chamber of the internal combustion engine 10 , The higher the rail pressure p _rail , the finer the fuel will be in the combustion chamber of the internal combustion engine 10 injected.

The combustion chamber _temperature t _inj the combustion chamber _pressure p _inj at the time of the start of the drive influence the evaporation of the fuel in the combustion chamber. For example, a higher leads Combustion chamber _temperature t _inj a faster evaporation of the fuel in the combustion chamber of the internal combustion engine 10 ,

The speed n corresponds to the speed of the piston of the considered cylinder of the internal combustion engine 10 and allows conclusions about the compression speed of the air / fuel mixture or gas mixture contained in the combustion chamber. Furthermore, the consideration of the speed n allows conclusions about the heat losses in the area of the cylinder wall.

The exhaust gas recirculation rate r _EGR is used to determine the mixture ratio of air and exhaust gas in the combustion chamber of the considered cylinder of the internal combustion engine 10 , The term containing the exhaust gas recirculation rate r _{EGR represents} the chemical properties of the gas contained in the combustion chamber for the linear combination L1 according to the invention. With increasing proportion of the exhaust gas in the mixture, the temperature of the mixture or a corresponding combustion is reduced.

According to the invention, the rise time t _rise is advantageously also dependent on a second linear combination L2 (FIG. 3 ).

The rise time t _rise is preferably increased t rise = b 1 · Τ + b 2 · q inj + b 3 · p inj + b 4 · N + b 5 · p rail receive. It can be seen from the above equation that the second linear combination L2 according to the invention advantageously takes into account the ignition delay τ in determining the rise time t _rise as well as the consideration of an injected fuel quantity q, because both the ignition delay τ and the injected fuel quantity q _inj influence the _rise rise time t since by increasing the injected fuel quantity q _inj particular the duration of the ignition delay increases.

The combustion of already vaporized fuel begins while the remaining fuel still evaporates, resulting in an extension of the rise time t _rise. The pressure at the start of control, the rotational speed n and the rail pressure p _rail have a similar effect on the rise time t _rise as on the ignition delay τ obtained by means of the first linear combination L1 according to the invention ( 3 ) is determined.

The third linear combination L3 according to the invention is advantageously provided for enabling an efficient determination of the maximum value p _{rise of} the combustion pressure: p rise = c 1 · q inj + c 2 · N.

The inventively determined maximum combustion pressure p _rise thus takes into account the in the combustion chamber of the considered cylinder of the internal combustion engine 10 injected fuel quantity q _inj and the speed n internal combustion engine 10 ,

Like the coefficients a ₁ , a ₂ , a ₃ , a ₄ , a _{5 of} the first linear combination L1, the coefficients b ₁ , b ₂ , b ₃ , b ₄ , b ₅ and c ₁ , c ₂ can also be determined experimentally, for example using statistical optimization methods, in particular genetic algorithms or also methods of the simulated annealing type.

In addition to the operating variables mentioned above for the linear combinations L1, L2, L3 according to the invention, other operating variables of the internal combustion engine can also be used 10 be included in the formation of linear combinations, without resulting in a significant increase in the computational effort to evaluate the relevant linear combination.

It is also advantageously possible, one or more coefficients even in dependence on an operating condition of the internal combustion engine 10 to choose or modify. For example, for a cold start of the internal combustion engine 10 a different set of coefficients are used than for idling. If required or for further simplification, individual coefficients can also be set to zero, at least temporarily, which is particularly useful in those operating areas in which the operating variable associated with the relevant coefficient has only a small influence on the quantity to be determined by means of the linear combination ,

The coefficients for the linear combinations L1, L2, L3 according to the invention can also be particularly advantageous during normal operation of the internal combustion engine 10 be modified, for example, to take into account changing operating conditions or to allow continuous adjustment of the coefficients.

A further increase in the efficiency in the inventive determination of the time profile of the combustion pressure p _diff is given by the fact that at least one of the linear combinations L1, L2, L3 is formed at least partially using a table of values.

A Comparable optimization is likewise in the case of the invention Evaluation of the mathematical function M1, M2 possible.

Under the knowledge of the inventively determined in real time over time course of the combustion pressure p _diff , the internal combustion engine 10 be be particularly advantageously controlled and / or regulated, in particular with regard to a minimization of the internal combustion engine 10 emitted pollutant emissions.

In particular, by evaluating corresponding thermodynamic relationships in a manner known to the person skilled in the art, a combustion chamber temperature can also be determined from the time curve of the combustion pressure p _diff determined according to the invention in real time.

With knowledge of the variables combustion temperature p _diff , combustion chamber temperature determined according to the invention in real time, a torque injection efficiency of a partial injection, in particular a post-injection, can advantageously be determined according to a further variant of the invention. The torque efficiency of the partial injection indicates what contribution the partial injection makes to a generated torque of the internal combustion engine. Since the variables combustion pressure p _diff , combustion chamber temperature are determined in real time according to the invention, a particularly precise determination of the torque efficiency of a partial injection can be made by taking the current values of the combustion pressure p _diff and the combustion chamber temperature at the time of commencement of the relevant partial injection into the torque efficiency to determine from this. In contrast to conventional methods, in which the torque efficiency is determined based on a map - and thus is valid only for those environmental conditions (eg, temperature of the engine block, atmospheric pressure) that were also present during the creation or application of the map - this provides inventive method always accurate values even under changing environmental conditions. Furthermore, advantageously no more maps in the controller 20 be stored, since the torque efficiency is determined if necessary in real time.

With knowledge of the torque efficiency determined according to the invention with high precision, the adjustment of a desired total fuel quantity, ie for one main injection and possibly one or more partial injections, can be made more accurately relative to the desired torque than with the conventional methods. Finally, the overall efficiency, ie the ratio of torque and injected total fuel amount, using the method according to the invention can be given with greater precision, which for many other functions in the context of the control of the internal combustion engine 10 is beneficial.

With knowledge of the variables combustion temperature p _diff , combustion chamber temperature determined according to the invention in real time, a firing angle for a post-injection can advantageously be determined in a further variant of the invention. For example, a limit value for the ignition angle can advantageously be determined, starting from the ignition conditions in the combustion chamber of the internal combustion engine 10 are no longer fulfilled. Accordingly, fuel can be injected by means of a post injection from this critical angle, without there being any ignition in the combustion chamber. The introduced with this post-injection fuel mass may, for example, in one of the internal combustion engine 10 downstream oxidation catalyst are reacted to regenerate this in a conventional manner.

at known method of this kind is the critical angle for the post-injection usually determined map-based and therefore has the previously discussed several times Inaccuracies (eg due to changed environmental conditions) on. To be able to make sure that the heating the amount of fuel provided by the oxidation catalyst is not already ignited in the combustion chamber, choose conventional Method a very late crankshaft angle for the corresponding post-injection, for example, by a corresponding safety margin on the map-based value is obtained. The security surcharge can already be found in the Map should be included. The conventionally very late Selected crankshaft angle leads adversely an increased fuel input into the engine oil, because part of the fuel quantity of the post-injection to the already condensed significantly cooled cylinder walls.

These disadvantages are avoided by using the principle according to the invention, because the optimum ignition angle for a post-injection to be used for catalyst heating can be determined very precisely with knowledge of the variables real-time capable of combustion pressure p _diff , combustion chamber temperature. That is to say, using the principle according to the invention, the optimum ignition angle for a post-injection to be used for the catalyst heating can be determined so accurately that no undesired ignition in the combustion chamber takes place with certainty, and that the combustion chamber has not cooled down so much yet, that the fuel input into the engine oil is increased by the post-injection.

Especially in a low load range of the internal combustion engine 10 In the variant of the invention described above, the reliability of the method for catalyst heating is improved.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• EP 1477651 A1 [0005]

Claims (20)

Method for determining a combustion pressure of a combustion chamber of an internal combustion engine ( 10 ), characterized in that the combustion pressure (p _diff ) is model based on an ignition delay (τ), a maximum value (p _rise ) of the combustion pressure (p _diff ) and a rise time (t _rise ) between a start of the combustion and the reaching of the maximum value is determined for the combustion pressure. A method according to claim 1, characterized in that a temporal profile of the combustion pressure (p _diff ) is partially approximated by a first mathematical function (M1), in particular a parabola, the parabola being dependent on the maximum value (p _rise ) of the combustion pressure ( p _diff ) and the rise time (t _rise ), and where the parabola in particular by the equation

where φ is a crankshaft angle of the internal combustion engine ( 10 ), and where φ _max corresponds to the crankshaft angle at maximum combustion pressure. A method according to claim 2, characterized in that the first mathematical function (M1) is used to approximate the time course of the combustion pressure (p _diff ) before or until reaching the maximum combustion pressure (p _rise ). Method according to one of the preceding claims, characterized in that a temporal profile of the combustion pressure (p _diff ) is approximated in sections via a second mathematical function (M2), which is dependent on the maximum value p _{rise of} the combustion pressure p _diff , of a combustion chamber associated cylinder volume V _φ, V _rise, and of the polytropic exponent κ a charge of gas in the combustion chamber, wherein the second mathematical function (M2) in particular are:

A method according to claim 4, characterized in that the second mathematical function (M2) is used for approximating the time profile of the combustion pressure (p _diff ) from or after reaching the maximum combustion pressure (p _rise ). Method according to one of the preceding claims, characterized in that the ignition delay (τ) in dependence of a first linear combination (L1) of a plurality of operating variables of the internal combustion engine ( 10 ) is determined. Method according to Claim 6, characterized in that the ignition delay (τ) is a linear combination τ = a ₁ * p _rail + a ₂ * t _inj + a ₃ * p _inj + a ₄ * n + a ₅ * r _EGR as a function of a rail pressure p _rail (and / or a combustion chamber temperature t _inj at the time of control start and / or a combustion chamber pressure p _inj at the time of control start and / or a rotational speed of the engine n 10 ) and / or an exhaust gas recirculation rate r _EGR and / or the coefficients a ₁ , a ₂ , a ₃ , a ₄ , a _{5 is} determined. Method according to one of the preceding claims, characterized in that the rise time (t _rise ) as a function of a second linear combination (L2) of a plurality of operating variables of the internal combustion engine ( 10 ) is determined. A method according to claim 8, characterized in that the rise time (t _rise ) as linear combination t _rise = b ₁ · τ + b ₂ · q _inj + b ₃ · p _inj + b ₄ · n + b ₅ · p _rail depending on the _Ignition delay τ and / or an injected fuel quantity q _inj and / or a combustion chamber _pressure p _inj at the time of / a control start and / or a rotational speed n of the internal combustion engine ( 10 ) and / or a rail pressure p _rail and / or the coefficients b ₁ , b ₂ , b ₃ , b ₄ , b _{5 is} determined. Method according to one of the preceding claims, characterized in that the maximum value (p _rise ) of the combustion pressure in dependence of a third linear combination (L3) of a plurality of operating variables of the internal combustion engine ( 10 ) is determined. A method according to claim 10, characterized in that the maximum value (p _rise ) of the combustion pressure as a linear combination p _rise = c ₁ · q _inj + c ₂ · n as a function of an injected fuel quantity q _inj and / or a rotational speed n of the internal combustion engine ( 10 ) and / or the coefficients c ₁ , c _{2 is} determined. Method according to one of Claims 7 to 11, characterized in that the coefficients a ₁ , a ₂ , a ₃ , a ₄ , a ₅ , b ₁ , b ₂ , b ₃ , b ₄ , b ₅ , c ₁ , c ₂ be determined experimentally. Method according to one of claims 7 to 12, characterized in that the coefficients a ₁ , a ₂ , a ₄ , a ₅ , b ₁ , b ₂ , b ₃ , b ₄ , b ₅ , c ₁ , c ₂ during normal operation the internal combustion engine ( 10 ) are modified. Method according to one of the preceding claims, characterized in that the combustion pressure (p _diff ) is determined in real time. Method according to one of claims 6 to 14, characterized in that at least one of the linear combinations (L1, L2, L3) is formed at least partially using a value table. Method according to one of claims 2 to 15, characterized in that at least one of the mathematical Functions (M1, M2) at least partially using a value table is evaluated. Method according to one of the preceding claims, characterized in that an operation of the internal combustion engine ( 10 ) is controlled and / or regulated as a function of the combustion pressure (p _diff ). A method according to claim 17, characterized in that a torque efficiency of a partial injection, in particular a post-injection, depending on the combustion pressure (p _diff ) and / or one of the combustion pressure (p _diff ) derived combustion chamber temperature is determined. Method according to one of Claims 17 to 18, characterized in that an ignition angle for a post-injection is determined as a function of the combustion pressure (p _diff ) and / or a combustion chamber temperature derived from the combustion pressure (p _diff ). Control unit ( 20 ) for an internal combustion engine ( 10 ), in particular of a motor vehicle, characterized in that the control unit ( 20 ) is designed for carrying out the method according to one of the preceding claims.