Method for operating an internal combustion engine

Abstract

Method for operating an internal combustion engine, in particular for a motor vehicle, in which pre-injections of a respective, predeterminable pre-injection quantity of fuel are carried out into a corresponding combustion chamber of the internal combustion engine by means of at least one injection element prior to the respective main injections following the pre-injections, wherein the pre-injection quantity is varied starting from one of the pre-injections in comparison to a subsequent pre-injection, characterized in that the pre-injection quantity is varied as a function of a signal characterizing a temperature of the internal combustion engine.

Description

[0001] The invention relates to a method for operating an internal combustion engine according to the preamble of patent claim 1.

[0002] It is known from the general state of the art to operate internal combustion engines, in particular diesel engines, for motor vehicles in such a way that fuel is injected into a corresponding combustion chamber of the internal combustion engine by means of at least one injection element, which is usually also referred to as an injector.

[0003] If the internal combustion engines have high compression pressures, as is particularly the case with diesel engines, this can lead to combustion noises - if no appropriate countermeasures are taken - which can be heard by vehicle occupants, for example, in the interior of the vehicle and are perceived as unpleasant.

[0004] In order to minimize this combustion noise, it is known to use the injection element during operation of the internal combustion engine to pre-inject a predetermined amount of fuel into the corresponding combustion chamber prior to the respective main injections following the pre-injections. Such a pre-injection is also commonly referred to as a pilot injection.

[0005] Such a method for operating an internal combustion engine, in which such pre-injections are carried out, is known, for example, from DE 103 15 817 A1. In this method, the pre-injection quantity is varied starting from a first of the pre-injections in comparison to a subsequent one of the pre-injections. In other words, during a first of the pre-injections, which is carried out before a corresponding first of the main injections, a first pre-injection quantity of fuel is injected into the combustion chamber. During a second pre-injection following the first pre-injection, which is carried out after the first main injection and before a corresponding second of the main injections, a second pre-injection quantity of fuel is injected into the combustion chamber. The first and second pre-injection quantities are different from one another.This means that during the second pre-injection a smaller or larger pre-injection quantity is injected into the combustion chamber than during the first pre-injection.

[0006] It has been shown that such methods for operating internal combustion engines - if no appropriate measures are taken - can lead to misfiring due to delayed or missing combustion in the combustion chamber, increased emissions, speed fluctuations, particularly when the internal combustion engine is idling, and unpleasant noises, particularly when idling. Furthermore, it has been determined that what is known as stalling of the internal combustion engine can occur because it can no longer overcome the frictional power and / or applied loads. Furthermore, poor combustion can lead to undesirably high main injection quantities of fuel during the main injections. This is caused by a control unit for regulating and / or controlling the internal combustion engine and results in deterioration of combustion.

[0007] DE 199 45 618 A1 describes a method for controlling a fuel metering system of an internal combustion engine. The amount of fuel to be injected is determined based on the actuation duration of at least one electrically actuated valve. In certain operating states, a minimum actuation duration is determined during which fuel is injected. Starting from a starting value, the actuation duration is increased or decreased. The actuation duration during which a change in a signal occurs is stored as the minimum actuation duration. A variable characterizing the rotational uniformity, an output signal from a lambda sensor, or an output signal from an ion current sensor is used as the signal.

[0008] DE 10 2011 105 110 A1 describes the operation of an internal combustion engine, in which an injection control module is provided for controlling the injection of fuel into the cylinders. The injection control module determines a fuel injection quantity per cylinder at least according to a load. It performs a main injection and at least one pilot injection. The injection control module also executes a cylinder deactivation actuation mode, in which the fuel supply to the cylinder or cylinders is interrupted when the engine is in a low-load state.

[0009] It is an object of the present invention to further develop a method for operating an internal combustion engine of the type mentioned at the outset in such a way that the problems mentioned in the prior art can be avoided.

[0010] This object is achieved by a method for operating an internal combustion engine having the features of patent claim 1. Advantageous embodiments with expedient and non-trivial further developments of the invention are specified in the remaining claims.

[0011] In order to create a method for operating an internal combustion engine, in particular for a motor vehicle, of the type specified in the preamble of claim 1, by which the aforementioned problems can be avoided, the invention provides that the pre-injection quantity is varied depending on a signal characterizing a temperature of the internal combustion engine. In other words, the pre-injection quantity, which can also be referred to as the pilot injection quantity, is varied depending on the temperature, which is referred to as temperature-dependent adaptation of the pre-injection quantity.This allows the pilot injection quantity to be adjusted to the temperature of the internal combustion engine, preventing the problems described above, such as misfiring, increased emissions, engine speed fluctuations, unwanted noise, spontaneous engine shutdown, and undesirably high main injection quantities caused by pilot injection quantities not being adjusted to the temperature. It has been shown that the problems described above occur particularly at cold temperatures in the internal combustion engine and thus during a cold start. These problems can now be avoided thanks to the temperature-dependent adjustment of the pilot injection quantity.

[0012] In an advantageous embodiment of the invention, a movement signal characterizing a movement, in particular an acceleration, of a piston of the internal combustion engine accommodated in the combustion chamber is used as the signal. The variation or adaptation of the pre-injection quantity is preferably carried out such that - if the internal combustion engine has multiple combustion chambers - the movement signal and thus the movement or acceleration behavior of the piston is evaluated in a combustion chamber-selective manner. For this purpose, for example, a change in the speed of a shaft of the internal combustion engine coupled to the piston in a region of a bottom dead center of the piston is compared with a change in the speed in a region of a top dead center of the piston, in particular during one rotation of the shaft.Based on this comparison, the combustion quality in the combustion chamber can be determined, whereby, for example, the pre-injection quantity is varied depending on the combustion quality. The combustion quality can depend on the temperature in the combustion chamber.

[0013] If this comparison detects incorrect movement behavior of the piston in the corresponding combustion chamber, the pre-injection quantity can subsequently be varied in order to achieve a desired movement or acceleration behavior of the piston.

[0014] For this purpose, one embodiment of the invention provides that, based on the comparison of the speed changes, an actual value characterizing the piston movement is determined, which is then compared with a predeterminable target value characterizing the movement. The pre-injection quantity is varied depending on the comparison of the actual value with the target value. The predeterminable target value can be calculated, for example, using a model.

[0015] In a particularly advantageous embodiment of the invention, provision is made for a larger pre-injection quantity of fuel to be injected at at least one first temperature value than at at least one second temperature value that is higher than the first value. In other words, higher pre-injection quantities of fuel are injected at lower temperatures of the internal combustion engine than at higher temperatures. As a result, the movement behavior of the piston and the behavior of the internal combustion engine can be adapted to a desired target behavior in line with the temperature. As a result, misfires can be avoided and hydrocarbon emissions from the internal combustion engine can be kept particularly low. Furthermore, it is possible to avoid fluctuations in the speed of the shaft, particularly during cold operation, i.e. during a warm-up phase of the internal combustion engine.In addition, the internal combustion engine exhibits advantageous noise characteristics even during cold operation due to only low combustion noise and quiet running, especially when idling.

[0016] The internal combustion engine can, in particular, be designed as a reciprocating piston internal combustion engine and comprise a crankshaft as a shaft, to which the piston is coupled via a connecting rod. The combustion chamber is a cylinder of the reciprocating piston internal combustion engine.

[0017] Further advantages, features, and details of the invention will become apparent from the following description of a preferred embodiment and from the drawings. The features and combinations of features mentioned above in the description, as well as the features and combinations of features mentioned below in the description of the figures and / or shown alone in the figures, can be used not only in the respective specified combinations, but also in other combinations or on their own, without departing from the scope of the invention.

[0018] The following serve Fig. 1 and Fig. 2 to explain the background of the invention.

[0019] The drawing shows: Fig. 1 a diagram with six curves illustrating respective accelerations of pistons in respective combustion chambers of an internal combustion engine, whereby undesirable combustion behavior occurs in two of the six combustion chambers; Fig. 2 another diagram to illustrate the respective combustion and pressure curves in the respective combustion chambers; Fig. 3 the diagram according to Fig. 1, wherein the undesirable combustion behavior in the two combustion chambers due to a temperature-dependent variation of a pre-injection quantity of pre-injections is avoided; and Fig. 4 a diagram illustrating the respective combustion and pressure curves in the combustion chambers according to Fig. 3.

[0020] Based on Fig. 1 to Fig. 4, a method for operating an internal combustion engine for a motor vehicle, in particular a passenger car, is explained below. The internal combustion engine is designed as a reciprocating piston internal combustion engine and comprises six combustion chambers in the form of cylinders in which combustion takes place during fired operation of the internal combustion engine. To do this, the internal combustion engine draws in air, which flows into the cylinders. Each cylinder is assigned an injection element. The injection element is usually also referred to as an injector and serves to inject a liquid fuel into the respective associated cylinders. The internal combustion engine is, for example, a diesel engine, so the liquid fuel is present as diesel fuel.

[0021] By injecting fuel into the cylinders, a fuel-air mixture is formed, which combusts as a result of ignition. In a diesel engine, this ignition can occur without a spark plug and as a result of compression of the fuel-air mixture in the cylinder.

[0022] Each cylinder contains a piston that can move translationally relative to the corresponding cylinder. The piston's primary function is to compress the respective fuel-air mixture and to transfer forces resulting from combustion and the associated expansion of the fuel-air mixture to an output shaft in the form of a crankshaft of the internal combustion engine. For this purpose, the pistons are articulated to the crankshaft via a connecting rod, so that the translational movements of the pistons in the cylinders can be converted into a rotational movement of the crankshaft.

[0023] In order to provide a torque specified by the driver of the motor vehicle via the crankshaft, a corresponding amount of fuel is injected into the respective cylinder by means of the respective injection element. To keep combustion noise particularly low, pre-injections of fuel are carried out into the cylinders during the fired operation of the internal combustion engine before the respective main injections of fuel into the cylinders. During the main injections, a respective main injection quantity of fuel is injected into the corresponding cylinder by means of the respective injection element. The main injections serve the specific purpose of achieving the torque specified by the driver.

[0024] During pre-injection, a pre-injection quantity of fuel is injected into the corresponding cylinder via the respective injection element. The primary purpose of pre-injection is to keep combustion noise to a minimum.

[0025] Fig. Figure 1 now shows a diagram 10, on whose abscissa 12 a control variable is plotted, above which the respective accelerations of the pistons in the cylinders, illustrated by curves 14a-f in diagram 10, are plotted. This control variable is, for example, time. The control variable can also be degrees of crank angle, which characterizes the rotational movement of the crankshaft. A further variable characterizing the combustion in the cylinders can be plotted on the ordinate 16 of diagram 10.

[0026] As can be seen from curves 14a-f, if no appropriate countermeasures are taken, at a point in time indicated by point 18 in diagram 10, despite the performance of a pilot injection and a subsequent main injection into the respective cylinders, no or only sporadic combustion occurs in two of the six cylinders. Curve 14a characterizes the combustion in a first cylinder via the corresponding acceleration, while curve 14b characterizes the combustion in a second cylinder via the acceleration.Accordingly, curve 14c characterizes the combustion in a third cylinder via the acceleration of the corresponding piston, curve 14d characterizes the combustion in a fourth cylinder via the acceleration of the corresponding piston, curve 14e characterizes the combustion in a fifth cylinder via the acceleration of the corresponding piston, and curve 14f characterizes the combustion in the sixth cylinder via the acceleration of the corresponding piston. As can be seen from curves 14b and 14c, no combustion occurs at point 18 in the second cylinder and only sporadic combustion occurs in the third cylinder. In the remaining cylinders, combustion proceeds as desired.

[0027] The curves 14a-f illustrate the accelerations in the cylinders during a so-called cold operation of the internal combustion engine, ie during a warm-up after a cold start, with the internal combustion engine at idle.

[0028] This undesirable acceleration and thus combustion behavior can also be seen on the basis of Fig. 2 illustrates. Fig. Figure 2 shows a diagram 20, with the crankshaft angle plotted on the abscissa 22. The pressure in the respective cylinders, measured in bar, as well as values ​​for the respective combustion process in the cylinders, are plotted on the ordinate 24 of diagram 20.

[0029] Curves 26 in diagram 20 illustrate a respective combustion curve in the first, fourth, fifth, and sixth cylinders, while a curve 28 illustrates the evaporation of the main injection. Accordingly, curves 30 in diagram 20 illustrate a respective pressure curve in the first, fourth, fifth, and sixth cylinders, while curves 32 illustrate a respective pressure curve in the second and third cylinders. As shown in Fig. As illustrated in Figure 2 by a straight line 34, the non-existent or only sporadic combustion in the second and third cylinders results in a respective combustion and pressure curve that differs significantly from the combustion and pressure curves in the first, fourth, fifth, and sixth cylinders. In other words, the second and third cylinders do not burn, while the remaining cylinders burn cleanly, i.e., in the desired manner.

[0030] This undesirable combustion behavior in the second and third cylinders can lead to increased emissions, engine speed fluctuations, undesirable noise, and increased fuel consumption, especially during cold operation. Furthermore, excessive friction and excessive external loads can cause the internal combustion engine to stall.

[0031] To avoid these problems, the pre-injection quantity is varied depending on the temperature. In other words, the pre-injection quantity is varied starting from a first of the pre-injections compared to a subsequent pre-injection, depending on a signal characterizing the temperature of the internal combustion engine.

[0032] This means, for example, that during a first of the pre-injections, which is performed before a corresponding first of the main injections, a first pre-injection quantity of fuel is injected into the cylinders. During a second pre-injection following the first pre-injection, which is performed after the first main injection and before a corresponding second of the main injections, a second pre-injection quantity of fuel is injected into the combustion chamber. The first and second pre-injection quantities are different from one another, with the different pre-injection quantities being adjusted depending on the temperature.

[0033] Thus, if the internal combustion engine is at a lower temperature, the pre-injection quantity is increased compared to higher temperatures. This variation in the pre-injection quantity occurs cylinder-selectively, i.e., for each cylinder individually or independently of the corresponding other cylinders. This allows for the individual combustion behavior in the respective cylinder to be taken into account.

[0034] Fig. 3 shows diagram 10 according to Fig. 1 with the gradients 14a-f, where now, in contrast to Fig. 1 a temperature-dependent variation, ie adaptation, of the pre-injection quantities is carried out. Fig. As can be seen from Figures 14a-f, the undesirable and unfavorable combustion behavior of the second and third cylinders can be avoided.

[0035] This is also based on Fig. 4, which shows the diagram 20 with the curves 26, 28, 30, 32, whereby the curves 26, 28, 30, 32 are now combined with the curves 14a-f according to Fig. 3 correspond. How Fig. As can be seen in Figure 4, the curves 28 of the second and third cylinders now correspond at least substantially to the curves 26 of the first, fourth, fifth, and sixth cylinders. Accordingly, the curves 32 of the second and third cylinders also correspond at least substantially to the curves 30 of the first, fourth, fifth, and sixth cylinders. In other words, all cylinders burn cleanly.

[0036] The pilot injection quantity is varied, for example, by correcting a pilot injection output quantity stored, for example, in a characteristic map or the like by a correction quantity. This is done, for example, such that a factor, such as 1, corresponds to the pilot injection output quantity. To increase the pilot injection quantity, the factor is increased. To reduce the pilot injection quantity based on the pilot injection output quantity, for example, the factor is decreased.

[0037] Alternatively or additionally, the variation, i.e., the adaptation of the pilot injection quantity, can proceed as described below: in a first step, a correction value for the pilot injection quantity is learned. In a second step, this correction value is stored as a function of temperature and used as the pilot control value for the pilot injection quantity when the internal combustion engine is restarted. New correction values ​​are permanently learned as long as at least one predeterminable condition for this learning is met. This means that multiple learned correction values ​​or correction quantities are used, and a single learned correction value is not permanently used for adaptation. This enables particularly advantageous adaptation.

Claims

[1] Method for operating an internal combustion engine, in particular for a motor vehicle, in which pre-injections of a respective predeterminable pre-injection quantity of fuel are carried out by means of at least one injection element into a corresponding combustion chamber of the internal combustion engine prior to respective main injections following the pre-injections, wherein the pre-injection quantity is varied starting from one of the pre-injections in comparison to a subsequent pre-injection, characterized by that the pre-injection quantity is varied depending on a signal characterizing a temperature of the internal combustion engine. [2] Method according to claim 1, characterized by that a movement signal characterizing a movement, in particular an acceleration, of a piston of the internal combustion engine accommodated in the combustion chamber is used as the signal. [3] Method according to claim 2, characterized by that, to generate the movement signal, a speed change of a shaft of the internal combustion engine coupled to the piston in a region of a bottom dead center of the piston is compared with a speed change in a region of a top dead center of the piston. [4] Method according to claim 3, characterized by that an actual value is determined on the basis of the comparison, which is compared with a predeterminable target value, whereby the pre-injection quantity is varied depending on the comparison of the actual value with the target value. [5] Method according to one of the preceding claims, characterized by that at least one first temperature value a larger pre-injection quantity of fuel is injected than at least one second temperature value which is higher than the first value.

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