Patent Publication Number: US-10787977-B2

Title: Method for operating a combustion machine, combustion machine and motor vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from German Patent Application No. 10 2017 129 769.3, filed Dec. 13, 2017, which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a method for operating a combustion machine having a fuel tank system that comprises a fuel tank vent valve. The invention also relates to a combustion machine that is suitable for carrying out such a method as well as to a motor vehicle having such a combustion machine. 
     BACKGROUND OF THE INVENTION 
     As a rule, a fuel tank system for a combustion machine of a motor vehicle has a vent line so that a rising pressure in the fuel tank of the tank system caused, for instance, by fuel that is evaporating at high ambient temperatures can be relieved to the atmosphere. In this process, also in view of the emissions standards, only the smallest possible amounts of fuel vapors are allowed to escape into the environment. This is prevented in that a fuel vapor filter, which is usually configured in the form of an activated carbon filter that absorbs the fuel vapors, is integrated into the vent line. 
     In order to regenerate such a fuel vapor filter, such a tank system is additionally provided with a purge gas line which is connected, on the one hand, to the fuel vapor filter and, on the other hand, to the fresh gas line of the combustion machine. During operation of the combustion machine, ambient air can at times be drawn in via an atmospheric port of the fuel vapor filter by means of the negative pressure that prevails in the area of the opening of the purge gas line, and this ambient air flows through the fuel vapor filter in the direction opposite to the direction in which the fuel vapors are flowing out of the fuel tank and into the fuel vapor filter, thus purging the filter. In this manner, the fuel vapors from the fuel vapor filter are fed via the fresh gas line to the combustion chambers of the internal combustion engine of the combustion machine. 
     U.S. Pat. Appln. No. 2003/0047161 A1 discloses a fuel tank system of a combustion machine having a direct-injection gas engine in which the type of combustion process and the amount of fuel injected during the purging of the fuel tank system are set as a function of a target value deviation for the fuel-to-air ratio, said deviation being determined on the basis of the measured value of a lambda sensor. 
     U.S. Pat. Appln. No. 2002/0162457 A1 describes a method for venting a fuel tank system of a combustion machine, whereby, on the basis of the hydrocarbon (HC) content in the purge gas, a decision is made as to when the fuel vapor filter has to be purged and when this purging can be ended again. In this context, the hydrocarbon content in the purge gas is determined by means of an appropriate HC sensor. 
     SUMMARY OF THE INVENTION 
     The invention was based on the objective of putting forward an advantageous method for determining the hydrocarbon content in the purge gas of a fuel tank system of a combustion machine. 
     This objective is achieved by means of a method according to patent claims. A combustion machine that is suitable for carrying out such a method is the subject matter of patent claims. Advantageous embodiments of the method according to the invention and preferred embodiments of the combustion machine according to the invention are the subject matters of the other patent claims and/or they ensue from the description of the invention below. 
     According to the invention, a method to determine the hydrocarbon content in the purge gas of a fuel tank system of a combustion machine is being provided, whereby the fuel tank system comprises at least
         a fuel tank,   a fuel vapor filter that is fluidically connected to an atmospheric port,   a vent line leading from the fuel tank to the fuel vapor filter,   a purge gas line leading from the fuel vapor filter to the fresh gas line of the combustion machine,   a control valve (fuel tank vent valve) that is integrated into the purge gas line,   a pressure sensor that is integrated into the purge gas line and that can be configured as a relative pressure sensor or as an absolute pressure sensor or as a differential pressure sensor, and   a compressor, that is to say, a so-called purge air pump, that is integrated into the purge gas line.
 
It is provided according to the invention that, during operation of the compressor, at least three pressure measurements are carried out by means of the pressure sensor in the purge gas line over a defined period of time and, only at the end of this period of time, on the basis of the determined pressure values, the loading of the purge gas with hydrocarbons is qualitatively determined in the purge gas that had flowed through the purge gas line during the period of time.
       

     In this context, the expression determining the “qualitative” loading refers to the fact that at least one statement is made about the concrete composition of some of the hydrocarbons in the mixture of hydrocarbons contained in the purge gas. 
     The term “fuel vapor filter” does not mean according to the invention that it has to filter the volatile fuel in gaseous form. Rather, the fuel can already have (partially) condensed out again during the filtering procedure. 
     Aside from the determination of the quantitative loading, which is preferably carried out parallel to the determination of the qualitative loading, the determination according to the invention of the qualitative loading allows a very precise assessment of the influence that the purge gas—which is fed to the fresh gas line of the combustion machine and from there to the internal combustion engine—exerts on the combustion processes during operation of the internal combustion engine. As a result, countermeasures can thus be carried out very precisely such as, for instance, adjusting the amount of fuel that is injected into the combustion chambers of the internal combustion engine for the individual work cycles, and this adjustment serves to compensate for the hydrocarbons that are introduced into the combustion chambers along with the purge gas. In this manner, the fuel-fresh gas mixtures that are thermally converted in the combustion chambers during the individual work cycles can be kept very exactly within the range of a prescribed target value or within a target value range, and this has a positive impact on the operating behavior of the internal combustion engine or of the entire combustion machine, and especially on the volumetric efficiency and/or on the emission behavior. 
     Accordingly, the invention also relates to a method for operating a combustion machine, said method encompassing the determination according to the invention of the hydrocarbon content in the purge gas of a fuel tank system of the combustion machine and, based on the result of this determination, encompassing the adjustment of parameters that influence combustion processes that take place in an internal combustion engine during operation of the combustion machine. 
     The preferably quantitative determination of the loading of the purge gas with hydrocarbons—a procedure in which preferably the amount of the hydrocarbons in relation to the total amount of purge gas, that is to say, the relative hydrocarbon content (in percent by volume or in percent by weight) is determined—preferably takes place only or one time (for the period of time) at the end of the period of time, as a result of which, based on pressure measurements, very precise results can be obtained pertaining to the quantitative loading of the purge gas with hydrocarbons. 
     It can preferably be provided that the defined period of time begins with the start-up of the compressor, and thus with the beginning of a purging procedure, and/or that it ends with the determination of several pressure values that are within the same value range. In this context, this value range is relatively small in comparison to the maximum difference among the measured pressure values, so that it is thus provided that the defined period of time ends when pressure values that are the same or approximately the same occur multiple times. As an alternative or in addition, however, it can also be provided that the defined period of time extends precisely to, or at least over, the defined number of pressure measurements. 
     In accordance with a preferred embodiment of a method according to the invention, it can be provided that, for the pressure measurements, in each case, a pressure gradient, a pressure differential relative to one of the other pressure measurements and/or a pressure gradient differential is determined, which are then analyzed with respect to at least one of these descriptive values that is characteristic of certain types of hydrocarbons in order to qualitatively determine the loading of the purge gas with hydrocarbons. In this manner, it can be provided, for example, that, in the case of a relatively large pressure gradient, the presence of a relatively high content of ethanol in the purge gas can be assumed. 
     It can likewise be preferably provided that the positions of at least some of the pressure measurements taken within the defined period of time and/or the time interval of at least some of the pressure measurements taken during a preceding start-up of the compressor are used in order to determine the qualitative loading of the purge gas with the hydrocarbons. This yields a very precise determination of the qualitative loading. For example, it can be provided that a relatively high content of ethanol in the purge gas is only assumed if a relatively large pressure gradient has been determined within a relatively early segment of the defined period of time and especially after the compressor has been started up. 
     In order to enhance the precision of the determination of the quantitative and/or qualitative loading of the purge gas with hydrocarbons, it can preferably be provided that the time interval between the pressure measurements is varied, at least at times. In particular, it can be provided that, within defined limits and relative to reference values, the larger a pressure gradient determined on the basis of the first pressure measurement is, the shorter the time interval selected between a first pressure measurement and a second pressure measurement. Consequently, it can preferably be provided that a first pressure gradient is determined on the basis of a first pressure measurement and a second pressure gradient is determined on the basis of a subsequent second pressure measurement, and, on the basis of the magnitude of the difference between these pressure gradients, the time interval between the second and a subsequent third pressure measurement is varied in such a way that a relatively small time interval is selected in the case of a relatively large difference between the pressure gradients. 
     It can preferably be provided that a method according to the invention is carried out at least once for each start-up of the compressor, so that, for each purging procedure, a determination according to the invention is made of the hydrocarbon content in the purge gas that is then being conveyed, so that, for each purging procedure, it is accordingly possible to carry out the most exact compensation possible of the influence exerted by the hydrocarbons that have been introduced via the purge gas into the combustion chambers of the internal combustion engine. 
     A combustion machine according to the invention comprises, on the one hand, at least
         an internal combustion engine,   a fresh gas line for feeding fresh gas to the internal combustion engine,   an exhaust gas line for discharging exhaust gas from the internal combustion engine, and   a fuel tank system that comprises
           a fuel tank,   a fuel vapor filter that is fluidically connected to an atmospheric port,   a vent line leading from the fuel tank to the fuel vapor filter,   a purge gas line leading from the fuel vapor filter to the fresh gas line of the combustion machine,   a control valve (fuel tank vent valve) that is integrated into the purge gas line,   a pressure sensor that is integrated into the purge gas line, and   a compressor, that is to say, a so-called purge air pump, that is integrated into the purge gas line.
 
Moreover, a combustion machine according to the invention has a control device that is configured to automatically carry out a method according to the invention.
   
               

     The method according to the invention can especially be employed in a fuel tank system of a combustion machine (according to the invention) whose internal combustion engine can be operated in an externally ignited manner, especially in accordance with the Otto principle, since the fuel used to operate such an internal combustion engine is usually relatively highly volatile (especially in comparison to diesel fuel), as a result of which the special need for a fuel tank vent system can be warranted. 
     A combustion machine according to the invention can especially be part of a motor vehicle. In this context, the internal combustion engine of the combustion machine can especially be designed to directly or indirectly provide the drive power for the motor vehicle. Therefore, the invention also relates to a motor vehicle, particularly to a motor vehicle on wheels (preferably a passenger car or a truck) having a combustion machine according to the invention. 
     The indefinite articles (“a”, “an”), especially in the patent claims and in the description that generally explains the patent claims, are to be understood as such and not as numbers. Therefore, components described in a concrete manner should be understood in such a way that they are present at least once and can also be present several times. 
    
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in greater detail below, making reference to an embodiment shown in the drawings. The drawings show the following: 
         FIG. 1 : a combustion machine according to the invention, having a fuel tank system, in a schematic depiction, and 
         FIG. 2 : a diagram illustrating the execution of a method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a combustion machine according to the invention for a motor vehicle having a fuel tank system. This system comprises a fuel tank  10  that is connected via a vent line  12  to a fuel vapor filter  14  which especially can be configured in the form of an activated carbon filter or which at least comprises such a filter. The fuel vapor filter  14  is also connected via a purge gas line  16  to a fresh gas line  18  of the combustion machine, whereby the purge gas line  16  opens up into the fresh gas line  18  upstream (relative to the flow direction of fresh gas in the fresh gas line  18  in the direction of an internal combustion engine  20  of the combustion machine) of a charge-air compressor  22  that is integrated into the fresh gas line  18 . The charge-air compressor  22  is part of an exhaust-gas turbocharger that also comprises an exhaust-gas turbine  24  that is integrated into an exhaust gas line  26  of the combustion machine. In the charge-air segment of the fresh gas line  18  located between the charge-air compressor  22  and the internal combustion engine  20 , there is also a throttle valve  36  that divides the charge-air segment into an upstream section, which is often referred to as a pressure pipe, and a downstream section, which is often referred to as an intake pipe. 
     During operation of the combustion machine, in a known manner, gas mixtures consisting of fresh gas that is made up completely or primarily of ambient air and of fuel that is injected, for example, directly into the combustion chambers  28  by means of injection valves (not shown here) are burned in a defined sequence in the combustion chambers  28  of the internal combustion engine  20 , said chambers being partially delimited by cylinders  30  of the internal combustion engine  20 , whereby the pressure rises thus generated in the combustion chambers  28  are used to move pistons  32  that can be moved along the longitudinal axis in the cylinders  30 . Through the interaction of connecting rods (not shown here), these movements of the pistons  32  can be converted into a rotational motion of a crankshaft (not shown here), whereby the movement of the pistons  32  via the connecting rods by means of the crankshaft simultaneously brings about a cyclical back-and-forth movement of the pistons  32 . The exhaust gas generated in the combustion chambers  28  during the combustion of the fuel-fresh gas mixtures is discharged via the exhaust gas line  26 , thereby flowing through the exhaust gas turbine  24 , which causes a turbine impeller (not shown here) to rotate. This rotation of the turbine impeller is transferred by means of a shaft  34  to a compressor impeller (not shown here) of the charge air compressor  22 , as a result of which the charge air compressor  22  compresses the fresh gas that is fed to the internal combustion engine  20  via the fresh gas line  18 . 
     The side of the fuel vapor filter  14  of the fuel tank system facing away from the vent line  12  and the purge gas line  16  (with respect to its filtering effect for fuel vapors) is fluidically connected to the atmosphere via an ambient air line  38 , for which purpose the ambient air line  38  forms an atmospheric port  44 . 
     The fuel tank  10  is partially filled with fuel, whereby a part of this fuel, which is actually liquid, has usually evaporated, so that fuel in a gaseous state of aggregation is also present in the fuel tank  10 . Such an evaporation of fuel in the fuel tank  10  is intensified by a relatively high temperature of the fuel, and this can occur especially at relatively high ambient temperatures as well as in the case of a change in the ambient pressure, for instance, if a motor vehicle comprising the combustion machine is being driven up a mountain. In order to prevent an impermissibly high excess pressure in the fuel tank  10  due to such evaporation, the possibility exists to equalize the pressure with the ambient pressure via the vent line  12  and the fuel vapor filter  14  as well as via the ambient air line  38 , whereby the fuel vapor filter  14  prevents such a pressure equalization from allowing fuel vapors to escape into the environment. 
     Such a venting of the fuel tank  10  leads to an increasing saturation of the fuel vapor filter  14  which, in turn, requires the filter to be regenerated at regular intervals. For this purpose, it is provided for the fuel vapor filter  14  to be purged in that ambient air is drawn in via the atmospheric port  44  and via the ambient air line  38 . This ambient air flows through the fuel vapor filter  14  in the opposite direction from that of the flow during the venting of the fuel tank  10 , as a result of which fuel molecules absorbed in the fuel vapor filter  14  are carried along by the ambient air and introduced into the fresh gas line  18  via the purge gas line  16 . Consequently, this fuel, which usually contains a mixture of different hydrocarbons, is conveyed to the combustion chambers  28  of the internal combustion engine  20  in order to be burned. 
     Such a purging of the fuel vapor filter  14  is only provided for at times and always during operation of the internal combustion engine  20  because only then can the fuel that has been introduced into the fresh gas line  18  by the purging of the fuel vapor filter  14  also be reliably fed into the combustion chambers  28  in order to be burned. In contrast, if fuel is introduced into the fresh gas line  18  when the internal combustion engine  20  is not being operated, this could cause the gaseous fuel to escape into the environment via leaks in the fresh gas line  18  and especially via an intake opening of the fresh gas line  18 . 
     A fuel tank vent valve  42  that is integrated into the purge gas line  16  is arranged as close as possible to the purge gas line opening  40  leading into the fresh gas line  18 , or else it is integrated into it. 
     In order to purge the fuel vapor filter  14 , there is a need for a sufficient pressure gradient between, on the one hand, the ambient pressure and, on the other hand, the pressure in the fresh gas line  18  in the vicinity of the opening  40  of the purge gas line  16 , whereby this pressure gradient is not always present due to greatly fluctuating pressures in the fresh gas line  18  during operation of the internal combustion engine  20 . During operation of the internal combustion engine  20  and thus of the charge air compressor  22 , the pressure of the fresh gas in the section of the fresh gas line  18  in the area of the opening  40  of the purge gas  16  is usually so low that there is a sufficient pressure gradient relative to the ambient pressure that is present at the atmospheric port  44 . However, this is not always the case. 
     In order to allow purging of the fuel vapor filter  14  at any time so that its complete saturation can be reliably prevented, the fuel tank system of the combustion machine also comprises a compressor  46  that is integrated into the purge gas line  16 , whereby said compressor  46  is also normally referred to as a “purge air pump” and it can be configured in the form of a piston compressor, especially as a rotary vane compressor or else as a radial fan. When this compressor  46  is being operated, ambient air can be actively drawn in via the atmospheric port  44 , and this air then flows through the fuel vapor filter  14  in order to purge it, and is then conveyed via the compressor  46  all the way to the opening  40  of the purge gas line  16 . 
     At least the compressor  46 , the fuel tank vent valve  42 , the throttle valve  36  and a pressure sensor  50  that is integrated into the purge gas line  16  can be actuated by means of a control device  48  (for example, the engine control unit of the combustion machine). 
       FIG. 2  is a diagram illustrating the procedure when the method according to the invention is being carried out. In this figure, for two purging procedures of a fuel tank system of a combustion machine according to the invention as shown, for example, in  FIG. 1 , the pressure curves in a section of the purge gas line  16 , which can especially be arranged downstream from the compressor  46 , are plotted over time for a defined period of time that begins with the start-up of the compressor (initial point in time t A ) and that extends over a time span until the end point in time t E  depicted in  FIG. 2 . During this defined period of time, multiple pressure measurements are carried out for each one of the purging procedures, and these measurements are depicted in  FIG. 2  by appropriate measuring points along the pressure curves, and the associated pressure curves are interpolated on the basis of the measured pressure values. The end point in time t E , which can also be different for the various purging procedures, can be a defined time span after the appertaining initial point in time t A . As an alternative, it can be provided for the end point in time t E  to coincide with the execution of the last of a defined number of pressure measurements. And, in turn, it can be alternatively provided that the end point in time t E  is set when the specific pressure curve under consideration no longer changes—or else only by a value that is below a defined limit value—over a defined period of time and/or over a defined number of pressure measurements. 
     At the same time as the end of the defined period of time, an evaluation is made of the pressure curve or of each pressure curve, on the basis of which it is possible to derive, on the one hand, the quantitative and, on the one hand, the qualitative, loading of the purge gas that is being conveyed by the compressor during the appertaining period of time and that is subsequently flowing or already has flowed into the fresh gas line, together with hydrocarbons. This evaluation is especially based on the fact that the various hydrocarbons contained in the purge gas exhibit different densities, on the one hand, in comparison to each other and, on the other hand, in comparison to ambient air that is likewise contained in the purge gas. In this context, on the basis of the averaged rise in the pressure over the appertaining period of time, especially the quantitative loading of the purge gas with hydrocarbons can be determined, whereas on the basis of the characteristic curve of the pressure during the appertaining period of time, conclusions can be drawn about the qualitative loading and thus about the composition of the mixture containing various hydrocarbons in the purge gas. For instance, the upper pressure curve of the two curves depicted in  FIG. 2  exhibits a clearly steeper rise in a relatively early section of the defined period of time than the lower pressure curve in a corresponding section of the associated period of time, as a result of which a relatively high content of ethanol in the purge gas being conveyed in the corresponding purging procedure can be derived. 
       FIG. 2  also shows that the time intervals between the pressure measurements for the individual pressure curves are varied, whereby as a tendency, it is provided that the steeper the rise or the larger the pressure gradient at the point in time of each preceding pressure measurement was, the shorter the time interval that is selected after which a pressure measurement that still has to be carried out is actually carried out. This enhances the resolution during the determination of the pressure curves and thus improves the evaluation result. 
     LIST OF REFERENCE NUMERALS 
     
         
           10  fuel tank 
           12  vent line 
           14  fuel vapor filter 
           16  purge gas line 
           18  fresh gas line 
           20  internal combustion engine 
           22  charge air compressor 
           24  exhaust gas turbine 
           26  exhaust gas line 
           28  combustion chamber of the internal combustion engine 
           30  cylinder of the internal combustion engine 
           32  piston of the internal combustion engine 
           34  shaft 
           36  throttle valve 
           38  ambient air line 
           40  opening of the purge gas line 
           42  fuel tank vent valve 
           44  atmospheric port 
           46  compressor 
           48  control device 
           50  pressure sensor