Abstract:
To achieve a reliable and exhaust-optimized combustion starting already from the beginning of a combustion phase in the startup of an internal combustion engine, the internal combustion engine is brought to a target speed by an electric motor in a drag phase, the target speed being higher than the previously known starting speed. The target speed corresponds approximately to an idling speed assigned to the internal combustion engine, for example. After exceeding the starting speed, first a mixture enrichment is determined, taking into account a prevailing state of a wall film here in particular. Only after reaching the target speed is the fuel metered and does the combustion take place in the combustion phase. The fuel is metered here in particular on the basis of the mixture enrichment determined after exceeding the starting speed.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for starting an internal combustion engine, in which an electric motor is activated after a shut-down phase and the internal combustion engine is brought to a starting speed by the electric motor. The present invention also relates to an internal combustion engine as well as a control unit for controlling and regulating an internal combustion engine; an electric motor provided for the internal combustion engine is activatable after a shut-down phase and is capable of inducing a starting speed in the internal combustion engine. The present invention also relates to a computer program capable of running on a computer, in particular a control unit for controlling and regulating an internal combustion engine. 
       BACKGROUND INFORMATION 
       [0002]    Internal combustion engines may be started by using an electric motor known as a starter to bring the internal combustion engine to a starting speed. The power to be delivered by the electric motor for reliable starting of the internal combustion engine depends on the design of the electric motor, the efficiency of the automotive battery available and in particular the starting speed. To minimize the electric power required for startup and to also minimize the weight of the starter system, which is determined decisively by the efficiency of the automotive battery and the electric motor, the starting speed selected is significantly below the idling speed above which controlled and regulated automatic operation of the internal combustion engine is reliably possible and a torque suitable as the basis for implementation of additional torque demands is generated. 
         [0003]    The starting speed is usually just so selected that reliable starting of the internal combustion engine—i.e., operation of the internal combustion engine on the basis of its own combustion power—is possible. In a gasoline engine, the starting speed is often on the order of magnitude of 100 revolutions per minute and in diesel engines it is on the order of 200 revolutions per minute, whereas the idling speed is between 600 and 800 revolutions per minute, for example. 
         [0004]    For reliable operation of an internal combustion engine, a plurality of input parameters is detected, analyzed by a control unit and taken into account in controlling and regulating the internal combustion engine; this includes in particular the functions of mixture formation and ignition. During the starting phase, the internal combustion engine is usually operated according to a pre-control program on the basis of the particularly high rotational speed dynamics in the starting phase and the poorly detectable operating conditions until reaching the idling speed. For example, the position of the throttle valve, the fuel quantity and the ignition points in time are preselected here using engine characteristics maps. A torque structure which takes into account the determination and analysis of a torque demand is not active in the starting phase. Only after the end of the starting phase is it possible for the torque of the internal combustion engine to be set as a function of the driver&#39;s intent, for example. 
         [0005]    During operation of the internal combustion engine, a certain quantity of fuel, the so-called wall film, is deposited in the combustion chambers of the internal combustion engine. If an intake manifold is allocated to the internal combustion engine and if the fuel is metered via intake manifold injection, then a wall film is also deposited on the inside walls of the intake manifold. The wall film behaves like a fuel reservoir, first withdrawing fuel from a metered air-fuel mixture and then releasing it again. 
         [0006]    The wall film evaporates during a shut-down phase of the internal combustion engine. To prevent the air-fuel mixture from becoming lean during the first operating cycles in the starting phase and to ensure reliable combustion, the proportion [of fuel] in the air-fuel mixture is increased at first at least briefly for this reason. This is referred to as enrichment of the mixture. This achieves the result that fuel may be deposited on the walls of the intake manifold or the combustion chambers while nevertheless a portion of the fuel in the air-fuel mixture remains available for combustion. 
         [0007]    For the reasons given above, the enrichment of the mixture is also pre-controlled during the starting phase. However, this pre-control has the disadvantage that the air-fuel mixture is frequently suboptimal, prolonging the starting phase on the one hand while also increasing fuel consumption on the other. In particular, exhaust-optimized combustion is not usually possible in the starting phase, resulting in undesirably high emissions in the exhaust. 
       SUMMARY OF THE INVENTION 
       [0008]    An object of the exemplary embodiments and/or exemplary methods of the present invention is to create a possibility for reliable exhaust-optimized combustion from the very start of the combustion phase. 
         [0009]    This object is achieved by a method of the type defined in the introduction by bringing the internal combustion engine to a target rotational speed using the electric motor in a drag phase wherein the target speed is above the starting speed. 
         [0010]    The target speed here corresponds approximately to the idling speed, for example. If the internal combustion engine is started by an electric motor, there is initially no injection or combustion on reaching the starting speed known in the related art. Instead, an enrichment of the mixture is determined first after exceeding the starting speed, the prevailing status of the wall film being taken into account here in particular. Only on reaching the target speed do fuel metering and combustion take place in a combustion phase. Fuel is metered here in particular taking into account the enrichment of the mixture determined after exceeding the starting speed. 
         [0011]    With the method according to the present invention, the internal combustion engine is consequently brought to a much higher rotational speed—the target speed—during a start than was possible in the past, and combustion is started only on reaching this significantly higher speed. With an appropriate design of the electric motor, the target speed may be reached rapidly in particular, so that a faster start is feasible on the whole. Furthermore, the steady-state target speed allows operating conditions to be detectable and analyzable so that an improved determination of the enrichment of the mixture is possible. 
         [0012]    Enrichment of the mixture may be determined as a function of the measured duration of the shut-down phase. Since the wall film evaporates during the shut-down phase of the internal combustion engine, it is possible to determine any wall film that might still be present for a known duration of the shut-down phase and to take it into account in enriching the mixture. 
         [0013]    Alternatively or additionally, the duration of the drag phase, the target speed and/or a measured temperature may also be taken into account in determining the enrichment of the mixture. These operating parameters have an influence on the reduction of the wall film and thus permit a particularly precise determination of the enrichment of the mixture which is necessary to compensate for the wall film effect. 
         [0014]    During the drag phase, air is passed through the intake manifold and the combustion chambers without any fuel being injected. The wall film is consequently reduced as a function of the duration of the drag phase. Taking into account the target speed, it is possible to ascertain the quantity of air flowing through the intake manifold and the combustion chambers during the drag phase and thus accelerating the reduction of the wall film. In this way, the reduction of the wall film may be determined again with even greater accuracy. 
         [0015]    The reduction of the wall film also depends on the temperature of the internal combustion engine because evaporation of fuel is greater at higher temperatures. This may be taken into account in determining the prevailing temperature of the internal combustion engine. For example, the temperature of the motor oil or the water coolant may be used as the basis here. 
         [0016]    According to an embodiment of the method according to the present invention that is particularly easy to implement, the enrichment of the mixture is determined as the maximum possible value. The combustion phase is thus begun initially with the maximum possible enrichment of the mixture. This ensures that the wall film is built up particularly rapidly. 
         [0017]    In determining the enrichment of the mixture, a prevailing torque demand may be taken into account. Since the combustion phase begins only on reaching the target speed, at which point in time the operating conditions are detectable, a prevailing torque demand may be detected, e.g., by analysis of a signal supplied by a pedal value sensor. If there is a high torque demand, an even greater enrichment of the mixture may be determined, this in turn ensuring a rapid buildup of the wall film, while on the other hand supplying the required high fuel ratio in the air-fuel mixture for implementing the high torque demand. 
         [0018]    According to an exemplary embodiment of the method according to the present invention, the enrichment of the mixture in the combustion phase is decreased continuously or in increments. Operating parameters that influence the buildup of the wall film may be detected and taken into account. These include in particular the prevailing value of the enrichment of the mixture, the target speed, the prevailing speed detected, the duration of the combustion phase, the prevailing torque, and/or the prevailing temperature of the internal combustion engine. This makes it possible to ensure that the enrichment of the mixture on the one hand allows the fastest possible buildup of the wall film while on the other hand the enrichment of the fuel is decreased as a function of the thickness of the wall film already built up, so that there is no unnecessary metering of fuel which is not needed for further buildup of the wall film or for combustion. 
         [0019]    According to another advantageous embodiment of the method according to the present invention, the target speed is not fixedly predetermined but instead is ascertained as a function of a prevailing vehicle electrical system voltage, a prevailing torque demand, a temperature detected in the internal combustion engine and/or the measured duration of the shut-down phase. This allows a further improvement in the starting performance of the internal combustion engine, taking into account differences in operating parameters. For example, if the prevailing vehicle electrical system voltage is particularly low, it is possible that the power required to achieve a predetermined target speed may not be available from the automotive battery. To nevertheless ensure reliable starting of the internal combustion engine, the target speed may be lowered, so that it is achievable with the available vehicle battery power. It is possible in particular to then provide for the engine to be started in the traditional manner, with the internal combustion engine being accelerated by the electric motor only until reaching the starting speed. 
         [0020]    The target speed may be selected to be higher than the idling speed. This is advantageous in particular if the internal combustion engine is operable in a so-called coasting mode. In this case, for startup of the internal combustion engine and in particular for metering of fuel and enrichment of the mixture, it is possible to rely on individual parts of methods that are already used in the control unit for implementing reinstatement after the end of the coasting mode. The method according to the present invention may be used to advantage in particular when the internal combustion engine and the electric motor are intended to work together in a hybrid drive because in a hybrid drive, strategies for a reinstatement after a coasting phase are already implemented in a hybrid drive on the one hand, while on the other hand the electric motor is designed so that the target speed required for implementing the method according to the present invention is achievable with no problem. 
         [0021]    This object is also achieved by a control unit and by an internal combustion engine of the type defined previously, so that the control unit or the internal combustion engine is equipped to implement the method according to the present invention. 
         [0022]    This object is also achieved by a computer program of the type defined previously, so that the computer program is programmed for implementing the method according to the present invention when the computer program is running on the computer. The computer program thus represents the exemplary embodiments and/or exemplary methods of the present invention as does the method for whose execution the computer program is programmed. 
         [0023]    Additional features, possible applications and advantages of the exemplary embodiments and/or exemplary methods of the present invention are derived from the following description of exemplary embodiments of the present invention which are illustrated in the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  shows a schematic diagram of a vehicle having an internal combustion engine and a control unit set up for implementing the method according to the present invention. 
           [0025]      FIG. 2  shows a schematic diagram of the chronological sequence of the method according to one possible embodiment of the present invention. 
           [0026]      FIG. 3  shows a schematic flow chart of an exemplary embodiment of the method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]      FIG. 1  shows in a highly schematized form a vehicle  1  which includes an internal combustion engine  2  and a control unit  3 . Internal combustion engine  2  has cylinders  4 , which are connected to an intake manifold  5  and an exhaust system  9 . A fuel injector  6  is provided in intake manifold  5  and is connected via a fuel line  7  to a fuel tank  8 . Fuel injector  6  is connected to control unit  3  via a signal line  18 . 
         [0028]    Exhaust system  9  includes an exhaust purification system, e.g., a catalytic converter  10 . A sensor for measuring the quality of the exhaust, e.g., a lambda sensor (not shown), is provided in exhaust system  9  and connected via a signal line  11  to a control unit  3 . 
         [0029]    Internal combustion engine  2  is provided with an electric motor  12 . Electric motor  12  may also be part of a hybrid drive in particular. Electric motor  12  is connected to control unit  3  by a signal line  13 . 
         [0030]    Vehicle  1  also includes a pedal value sensor  14 , which is also connected to control unit  3  by a signal line  15 . 
         [0031]    Control unit  3  includes a processor  16  and a memory element  17 . Memory element  17  may be designed as a RAM or a ROM, for example. Memory element  17  may also be designed as a flash memory or as an optical and/or magnetic memory medium. For example, a computer program programmed for performing the method according to the present invention is stored in memory element  17 . 
         [0032]    Control unit  3  is suitable for controlling and regulating the operation of internal combustion engine  2 . Control unit  3  is programmed for implementing the method according to the present invention in particular. 
         [0033]    Cooperation of the individual components of vehicle  1  shown in  FIG. 1  is described on the basis of the embodiments of the method according to the present invention illustrated in  FIGS. 2 and 3 . 
         [0034]      FIG. 2  shows first a possible chronological chart for implementation of the method according to the present invention. The embodiment shown here as an example includes phases of the method, i.e., shut-down phase  20 , drag phase  21  and combustion phase  22 . 
         [0035]    After the end of shut-down phase  20  and at the start of drag phase  21 , rotational speed  25  of internal combustion engine  2  is increased by electric motor  12  until reaching target speed  23 . During the drag phase, the engine control is brought to and/or operated in a state resembling coasting operation of internal combustion engine  2 . If starting speed  24  is reached or exceeded when speed  25  is increased, then first there is no injection—as would occur in the methods known from the related art and as represented by starting condition  27  in  FIG. 2 . Instead, an extended starting condition is activated, describing the functionality of a start while driving and thus a reinstatement functionality. Extended starting condition  29  thus describes the state of affairs in which starting condition  27  has already occurred, i.e., minimal starting speed  24  has already been reached, but fuel metering, i.e., injection, has not yet been enabled. 
         [0036]    After activation of extended start condition  29 , a mixture enrichment  26  is determined. This may be set at the maximum possible value, for example. However, it is advantageous here to detect prevailing operating parameters and allow a highly exhaust-optimized mixture enrichment  26  on the one hand and the quickest possible buildup of a wall film on the other hand on the basis of the operating parameters thereby ascertained. 
         [0037]    However, if a complete wall film reduction is to be expected on the basis of drag phase  21 , e.g., because drag phase  21  lasts a particularly long period of time or target speed  23  is selected to be particularly high, then it is possible to provide for mixture enrichment  26  to always be set at the maximum level. However, it is advantageous to ascertain mixture enrichment  26  as a function of the duration of shut-down phase  20  to thereby take into account the cooling of the combustion chamber and the resulting reduction in combustion efficiency. 
         [0038]    Finally, injection enabling  28  during combustion phase  22  occurs through suitable triggering of fuel injector  6  by control unit  3  via signal line  18 . In the next operating cycles, mixture enrichment  26  is reduced by a fixedly predetermined amount per operating cycle or by a dynamically ascertained amount, either immediately or with a time lag, until the wall film is built up completely. 
         [0039]      FIG. 3  shows a schematic flow chart of an exemplary embodiment of the method according to the present invention. The method starts in a step  100  in which a start demand is detected. A start demand may be initiated by a driver or generated automatically in a transition from a stop phase to a start phase during start-stop operation of internal combustion engine  2 . 
         [0040]    The start demand detected causes the transition from shut-down phase  20  to drag phase  21  in one step  101  in which electric motor  12  is activated first. 
         [0041]    A check is performed in a step  102  to ascertain whether speed  25  has reached or exceeded starting speed  24 . If this is the case, then starting condition  27 , which describes the starting method known from the related art, is concluded. 
         [0042]    In a step  103 , prevailing operating parameters are then detected. The prevailing operating parameters describe, for example, a temperature of internal combustion engine  2  or a prevailing torque demand transmitted from pedal value sensor  14  via signal line  15  to control unit  3 . 
         [0043]    In a step  104 , mixture enrichment  26  is determined within extended starting condition  29  as a function of the prevailing measured operating parameters. Mixture enrichment  26  is designed here so that the wall film is built up as rapidly as possible and nevertheless exhaust-optimized combustion is possible with the start of combustion at the beginning of combustion phase  22 . 
         [0044]    A check is performed in a step  105  to ascertain whether target speed  23  has been reached. If this is not the case, the check is repeated. It is also conceivable for the program to branch back to step  103  and detect the prevailing operating parameters again and/or to determine a prevailing mixture enrichment  26  in step  104 . 
         [0045]    If target speed  23  has been reached, injection enabling  28  takes place in step  106 , representing the transition from drag phase  21  to combustion phase  22 . 
         [0046]    In a step  107 , mixture enrichment  26  is reduced as a function of the prevailing measured operating parameters or of a predetermined engine characteristics map, so that the wall film is still built up completely and exhaust-optimized combustion is possible. The effect of the wall film that has already been built up is taken into account here with regard to the prevailing combustion. In particular, a prevailing torque demand may also be taken into account here. 
         [0047]    This method ends in a step  108 , in which the wall film is built up completely and mixture enrichment  26 , which is provided for buildup of the wall film, has been reduced completely. 
         [0048]    If internal combustion engine  2  is operable in a coasting phase, e.g., during a coasting shutdown, then the method according to the present invention may be implemented in a particularly efficient manner if a basic reinstatement functionality that is already present is used here. The method according to the present invention is then based on mixture enrichment  26 , which is provided for a successful reinstatement after a coasting phase. Therefore, the method according to the present invention is particularly simple to implement on the one hand, while on the other hand it is implementable in a particularly exhaust-optimized manner because many of the parameters needed in implementation of the method according to the present invention have already been detected and analyzed for the basic reinstatement functionality. 
         [0049]    In drag phase  21 , internal combustion engine  2  is put in a state resembling that of coasting mode by electric motor  12  which is particularly strong, such as the motor used with a hybrid drive, for example, and the altered engine control, i.e., drive control, associated with this. If starting speed  24  is exceeded, the starting method characterized by starting condition  27  and known from the related art is reset after reaching starting speed  24  without triggering actuators of internal combustion engine  2 , in particular without enabling the injection, and the reinstatement functionality which is provided for control of the hybrid drive anyway is started by an extended starting condition  29 . Now the method may begin with the basic reinstatement functionality and injection enabling  28  may be possible as soon as target speed  23  has been reached. 
         [0050]    Modifications of the method are of course conceivable. For example, the method may be implemented completely without any prior determination of starting speed  24  and/or starting condition  27 . Instead, a speed above which mixture enrichment  26  is determined may be preselected. In this case, the rotational speed may be predetermined in such a way that enough time is available to reliably determine mixture enrichment  26 .