Abstract:
A method for operating an internal combustion engine working according to the Otto principle, in which fuel, particularly gasoline, is injected directly into a combustion chamber and is inflamed by self-ignition. A characteristic quantity characterizing the stability of combustion of an air/fuel mixture located in the combustion chamber is ascertained, and, as a function of the characteristic quantity, a residual gas proportion in the cylinder associated with the combustion chamber is set, in particular minimized, the residual gas proportion being reduced, preferably iteratively, as long as the characteristic quantity does not fall below a specifiable stability boundary.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for operating an internal combustion engine. The present invention also relates to a computer program and a control unit for an internal combustion engine. 
       BACKGROUND INFORMATION 
       [0002]    Internal combustion engines having operating types also designated as HCCI (homogeneous charge compression ignition) are known, in which fuel is injected directly into a combustion chamber and ignites itself comparable to the Diesel principle. The HCCI combustion methods are used in internal combustion engines otherwise operated using externally supplied ignition, especially because of their high potential for reduction in fuel usage and reduction in emissions. Since the stability of HCCI combustion methods is generally very sensitive with respect to changed boundary conditions, such as environmental temperature, variances in a valve system of the internal combustion engine, environmental pressure, aging of components of the internal combustion engine and the like, usual HCCI combustion methods provide using a relatively large quantity of hot residual gas for a cylinder charge, this gas being introduced into the combustion chamber by way of internal or external exhaust-gas recirculation by an appropriate design and activation of gas exchange valves. In the known HCCI combustion methods, this achieves a sufficiently great cylinder temperature, which has a positive effect on the stability of the combustion process, and makes the combustion or ignition possible in the first place. Because of the relatively large proportion of residual gas of the cylinder charge in the usual HCCI combustion methods, the fresh air proportion in the corresponding cylinder is reduced, whereby the calorific properties of the air/fuel mixture, present in the combustion chamber of the cylinder, deteriorate, so that the fuel usage of the internal combustion engine is also increased. 
       SUMMARY OF THE INVENTION 
       [0003]    Accordingly, it is an object of the present invention to improve a method of the type mentioned above in such a way that it makes possible a lower fuel usage without impairment of the stability of the combustion. 
         [0004]    This object is attained by ascertaining the characteristic quantity that characterizes the stability of the combustion of an air/fuel mixture located in the combustion chamber and by setting and especially minimizing the residual gas proportion in the cylinder associated with the combustion chamber, the residual gas proportion being reduced, preferably iteratively, as long as the characteristic quantity does not fall below a specifiable stability boundary. 
         [0005]    According to the present invention, the formation of the characteristic quantity that characterizes the combustion, or rather its stability, makes possible a targeted optimization of parameters influencing the air/fuel ratio, particularly an optimization of the residual gas proportion, preferably its minimization, without impairing the stability of combustion at the same time. Using this method, the fuel consumption of the internal combustion engine can be optimized in a particularly efficient manner, even in an operating type that is based on HCCI combustion methods. An especially efficient reduction in fuel consumption is advantageously achieved, according to the present invention, by reducing the residual gas proportion, as long as the characteristic quantity does not fall below a specifiable stability boundary. 
         [0006]    A characteristic quantity, that particularly precisely characterizes the stability of combustion, is obtained as a result of an advantageous variant of the method according to the present invention if the characteristic quantity is ascertained as a function of the energy delivered by the internal combustion engine, especially per working cycle, and especially as a function of a variance of the energy delivered and/or as a function of the variance of the energy delivered referred to an average value of the energy delivered, formed over a number of work cycles. 
         [0007]    Alternatively, or in supplement, the characteristic quantity can advantageously also be ascertained as a function of a combustion position, particularly as a function of a variance of the combustion position. 
         [0008]    It is also advantageous to use the quotient of energy delivered by the internal combustion engine and the energy expected to be delivered based on the injected fuel quantity as the characteristic quantity characterizing the stability of combustion, or rather for the formation of the characteristic quantity. 
         [0009]    Particularly meaningful values for the characteristic quantity of the stability of combustion can also be obtained, according to the present invention, by ascertaining the characteristic quantity as a function of preceding, recorded incomplete combustions and/or misfires, especially as a function of their number. 
         [0010]    It is advantageously possible, according to the present invention, to form the characteristic quantity as a function of a plurality of sensor signals or a combination of them, especially as a function of a signal of an in-cylinder pressure sensor and/or an ion current sensor and/or a knock sensor and/or a crank angle sensor. 
         [0011]    As was mentioned before, an especially efficient reduction in fuel consumption is advantageously achieved, according to the present invention, by reducing the residual gas proportion, as long as the characteristic quantity does not fall below a specifiable stability boundary. According to the present invention, the reduction in the residual gas proportion advantageously takes place in an iterative method, in which the residual gas proportion is reduced in a plurality of cycles by a specifiable increment, as long as the characteristic quantity has not yet reached the specifiable stability boundary or hasn&#39;t already fallen below it. 
         [0012]    Accordingly, in response to reaching and falling below the stability boundary, the residual gas proportion can in turn be increased by a specifiable increment, in order to ensure sufficient stability of combustion. 
         [0013]    According to the present invention, the increment for raising or lowering the residual gas proportion is formed as a function of the characteristic quantity itself and/or as a function of additional operating variables of the internal combustion engine, whereby a particularly precise approximation of the residual gas proportion to a minimally possible value is enabled, particularly also as a function of an actually recorded operating state of the internal combustion engine. 
         [0014]    The residual gas proportion that is to be set, according to the present invention, is preferably set via a corresponding activation of gas exchange valves of the internal combustion engine. For instance, in a valve strategy having negative valve overlap, the residual gas proportion of the cylinder charge can be increased by the following interventions:
       The control times for the closing of an exhaust valve and the opening of an intake valve are shifted symmetrically in the direction of the upper dead center in the gas exchange cycle (GWOT),   the position in time, but not the duration of the opening of the exhaust valve is shifted in the direction GWOT while simultaneously maintaining the control times for the intake valve.       
 
         [0017]    Besides setting the residual gas proportion, it is also conceivable to set a fuel quantity to be injected into the combustion chamber as a function of the characteristic quantity, or, in general, to set further parameters of the internal combustion engine known to one skilled in the art, which influence the air/fuel ratio, especially parameters of the fuel system. When it comes to minimizing the fuel usage, the parameters are changed, analogously to the setting of the residual gas proportion, until the characteristic quantity according to the present invention reaches an appropriate stability boundary. A multidimensional optimization, while simultaneously taking into consideration several parameters, is likewise conceivable. 
         [0018]    Of particular importance is the implementation of the method according to the present invention in the form of a computer program that is able to be run on a computer or a processing unit, and is suitable for executing the method. The computer program may, for instance, be stored on an electronic storage medium, the storage medium, on its part, being included, for example, in the control unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  shows a schematic representation of an internal combustion engine according to the present invention. 
           [0020]      FIG. 2  shows a simplified flow chart of a first specific embodiment of the method According to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In  FIG. 1 , an internal combustion engine in its entirety bears reference numeral  10 . It is preferably used for driving a motor vehicle that is not shown. Internal combustion engine  10  includes a plurality of cylinders, of which only one is shown in  FIG. 1  in exemplary form, with its combustion chamber  12  and piston  14 . Piston  14  is connected to a crankshaft  16 . Combustion air arrives in combustion chamber  12  via an intake valve  18  and an intake port  20 . 
         [0022]    Intake port  20  has a throttle valve  22  for setting a fresh air mass flow which is supplied to combustion chamber  12  via intake port  20 . 
         [0023]    Fuel is injected directly into combustion chamber  12  by an injector  24  which, on its part, is supplied with fuel by a high-pressure fuel system  26 . Compared to a manifold injection, a direct injection of fuel into combustion chamber  12  makes possible a targeted influencing of HCCI-based operating types of internal combustion engine  10 . 
         [0024]    Hot combustion exhaust gases are carried off from combustion chamber  12  via an exhaust valve  28  in an exhaust port  30 . Intake valve  18  is operated by an adjustable camshaft  32  and exhaust valve  28  is operated by an adjustable camshaft  34 . 
         [0025]    As an alternative to adjustable camshafts  32 ,  34 , one may also use a fully variable valve control (not shown) in internal combustion engine  10 , in order to be able to utilize the maximum number of degrees of freedom in influencing the gas exchange process. 
         [0026]    A user of internal combustion engine  10  issues a torque request by operating an accelerator  36 . The operation of internal combustion engine  10  is controlled and regulated by a control and regulation device  38 , designated henceforth briefly as control unit, which processes the signals supplied by accelerator  36 , among other things. In addition, control unit  38  receives the signals of a crank angle sensor or rotary speed sensor  40 , which records a crank angle or the rotary speed of crankshaft  16 , a hot-film air mass (HFM) sensor  42  which records the air mass flowing through intake port  20 , and signals from an in-cylinder pressure sensor  44  which records the pressure present in combustion chamber  12 . 
         [0027]    Among other things, injector  24  and throttle valve  22  are activated for setting an operating point of internal combustion engine  10 . The settings of intake camshaft  32  and exhaust camshaft  34  or the state of a fully variable valve control are also influenced by appropriate signals from control unit  38 . 
         [0028]    In connection with the present invention, since only the homogeneous self igniting operation (HCCI) of internal combustion engine  10  is of interest, a spark plug, required for the further operating types of internal combustion engine  10 , that are based on the principle of externally supplied ignition, are not illustrated here. 
         [0029]    In the HCCI combustion method examined according to the present invention, the fuel injected by injector  24  into combustion chamber  12  is inflamed by itself based on a temperature increase of the air/fuel mixture enclosed in combustion chamber  12 , which comes about in response to its compression during the compression stroke of the cylinder. 
         [0030]    In order to make possible a particularly fuel-saving operation of internal combustion engine  10 , the method described with reference to the flow chart illustrated in  FIG. 2  is carried out, in which a characteristic quantity, that characterizes the stability of combustion of the air/fuel mixture located in combustion chamber  12 , is ascertained, and control of the operation of internal combustion engine  10  is carried out as a function of this characteristic quantity. 
         [0031]    In a first step  100 , the characteristic quantity described is ascertained, according to the present invention. 
         [0032]    The characteristic quantity is preferably ascertained as a function of the energy delivered by internal combustion engine  10 , particularly a variance of the energy delivered and/or a variance with reference to the average value of the energy delivered being examined so as to be able to draw conclusions on the change with time of the energy delivered by internal combustion engine  10 , and thus the stability of combustion in combustion chamber  12 . An additional criterion that can be used, either by itself or in combination with the above described energy delivered by internal combustion engine  10  to form the characteristic quantity according to the present invention, is the combustion position or its variance. Data concerning the combustion position can advantageously be obtained particularly from a crank angle signal or a rotary speed signal which indicates the rotary speed of crankshaft  16  of internal combustion engine  10 , or they can be obtained directly from a curve over time of the in-cylinder pressure, as it is obtained by in-cylinder pressure sensor  44  ( FIG. 1 ). 
         [0033]    It can further be provided, according to the present invention, that the characteristic quantity is ascertained as a function of the quotient of the energy delivered by internal combustion engine  10  and of the energy expected to be delivered based on the injected fuel quantity. 
         [0034]    In addition, it is also possible to ascertain the characteristic quantity as a function of previously recorded incomplete combustions and/or misfires, especially as a function of their number. 
         [0035]    A combination of the above described methods for forming the characteristic quantity is likewise conceivable. 
         [0036]    Besides the evaluation of signals from in-cylinder pressure sensor  44  and/or crank angle sensor or rotary speed sensor  40 , in particular, signals from ion current sensors and/or knock sensors or the like, that are not shown in  FIG. 1 , can also be evaluated. 
         [0037]    The method according to the present invention provides a preferably iterative reduction in the residual gas proportion present in combustion chamber  12 , in order to set as lean as possible an operation and with that a fuel saving operation of internal combustion engine  10 . Within the scope of this iterative reduction in the residual gas proportion, and starting from the usual HCCI operation of internal combustion engine  10 , the residual gas proportion is reduced stepwise until the characteristic quantity, in this example ascertained in step  100  ( FIG. 2 ), has reached a specifiable stability boundary. 
         [0038]    After the characteristic quantity has been ascertained first of all in step  100  of the method according to the present invention, an evaluation of the characteristic quantity is made in following step  110 , of the method according to the present invention, particularly with respect to the reaching or the falling below the specifiable stability boundary. Insofar as one may conclude, from the characteristic quantity ascertained in step  100 , that a further reduction in the residual gas proportion is possible without impairing the stability in the operation of internal combustion engine  10 , a corresponding reduction in the residual gas proportion by a specifiable increment is subsequently undertaken in method step  120 . The reduction in the residual gas proportion preferably takes place for a subsequent power cycle of the cylinder of internal combustion engine  10  shown in  FIG. 1 , the increment for reducing the residual gas proportion being preferably selected as a function of the characteristic quantity and/or as a function of further operating variables of internal combustion engine  10 . This advantageously makes possible a particularly precise approximation of the actually set residual gas proportion to a residual gas proportion required at a minimum for a stable operation of internal combustion engine  10 , whereby, among other things, the robustness of the method according to the present invention is advantageously increased. 
         [0039]    To the extent that the evaluation of the characteristic quantity in step  110  indicates that the characteristic quantity has already reached or has even fallen below the specifiable stability boundary, no further reduction in the residual gas proportion is undertaken, according to the present invention. Rather, a residual gas proportion can be advantageously increased for subsequent power cycles of the cylinder, so as to produce again and ensure stable operation of internal combustion engine  10 . 
         [0040]    The setting of the residual gas proportion takes place, in a manner known to one skilled in the art, by appropriate control of intake camshaft  32  and exhaust camshaft  34 , or an alternatively usable fully variable valve control system by control unit  38  of internal combustion engine  10 . 
         [0041]    In a valve control strategy having, for instance, negative valve overlap, the residual gas proportion can be reduced particularly by the following interventions:
       The closing instant of exhaust valve  28  and the opening time of intake valve  18  are shifted symmetrically in the direction of upper dead center in the gas exchange cycle (GWOT), the position in time, but not the duration, of the opening of exhaust valve  28  is shifted in direction GWOT, while the control times for intake valve  18  are maintained.   The setting of a higher residual gas proportion can accordingly be effected by an inverse procedure or by further measures known to one skilled in the art.       
 
         [0044]    The increments used for the reduction or the increase in the residual gas proportion may also particularly advantageously be selected as a function of a deviation of the characteristic quantity from the specifiable stability boundary, which yields an even more precise setting of a fuel-optimized and yet stable operation of internal combustion engine  10 . 
         [0045]    Since the operating method according to the present invention, and especially the modification of the residual gas proportion undertaken in it, in general also change other features of the combustion, such as the combustion position and the efficiency, the method according to the present invention can advantageously be combined with a corresponding control method or regulating method, for the regulation of the combustion position and/or the energy delivered by internal combustion engine  10 . 
         [0046]    In general, while using the characteristic quantity according to the present invention, all the methods known to one skilled in the art for setting the residual gas proportion in the cylinder charge, can be used in order to ensure a fuel-optimizing and yet stable operation of internal combustion engine  10 . 
         [0047]    Besides the setting of the residual gas proportion described above, it is also conceivable to set a fuel quantity to be injected into combustion chamber  12  as a function of the characteristic quantity, or, in general, to set further parameters of internal combustion engine  10  known to one skilled in the art, which influence the air/fuel ratio. When it comes to minimizing the fuel usage, the parameters are changed, analogously to the setting of the residual gas proportion, until the characteristic quantity according to the present invention reaches an appropriate stability boundary. 
         [0048]    In order to carry out the method according to the present invention described above, control unit  38  can have an appropriate processing unit, such as a microcontroller or a digital signal processor that has an electronic storage medium assigned to it, which includes a computer program for implementing the method according to the present invention.