Abstract:
A method for operating an internal combustion engine. The latter has at least one combustion, one induction pipe, and one throttle valve. From the actual position of the throttle valve, a gas charge of the combustion chamber is defined. To improve the emissions and fuel consumption characteristics of the internal combustion engine, the minimum gas pressure is measured that exists in the induction pipe assigned to the combustion chamber at the end of the intake stroke. From this gas pressure, a value is determined which more closely approximates the actual gas charge of the combustion chamber.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for operating an internal combustion engine having at least one combustion chamber, one induction pipe, and one throttle valve. 
     BACKGROUND INFORMATION 
     In modern internal combustion engines having direct fuel injection and/or having an electronic gas pedal, the quantity of gas to be introduced into the combustion chamber is determined, inter alia, according to the quantity of fuel to be injected into the combustion chamber. This is necessary, inter alia, in order to generate a mixture in the combustion chamber in which combustion-generated emissions and fuel consumption are minimized. In this context, the quantity of gas, or the “gas charge,” is defined as a function of the actual position of the throttle valve, because the assumption is made that at a defined throttle position only a certain quantity of gas can reach the combustion chamber. 
     However, the problem in defining the gas charge of the combustion chamber in this manner is that the throttle valves themselves are manufactured within certain tolerances, so that when different throttle valves are set at the same angle, the result can be differing gas charges in corresponding combustion chambers. The gas charge actually present in the combustion chamber can therefore differ from the gas charge defined by the throttle valve position in ways that are not immediately predictable, which makes the creation of an optimal mixture dependent on the accidental presence of a “standard throttle valve.” 
     SUMMARY OF THE INVENTION 
     The present invention therefore has the objective of refining a method of the type cited above so that the mixture can always be adjusted with great precision. 
     This objective is achieved as a result of the fact that the minimum gas pressure is measured that is present in the induction pipe assigned to the combustion chamber at the end of the intake stroke, and that from this a value is determined which better approximates the actual gas charge of the combustion chamber. 
     The method according to the present invention is based on the following consideration: In an internal combustion engine having one intake valve, the piston at the beginning of the intake stroke is located in the upper dead center and then travels to the lower dead center. In this context, the quantity of gas behind the throttle valve continually expands in volume. This has the consequence that the pressure falls. In the lower dead center, the maximum volume and therefore the minimum pressure is achieved. Shortly thereafter, the intake valve closes. At this point in time, roughly the same pressure exists in the combustion chamber as in the induction pipe. By taking account of the characteristic data of the internal combustion engine, the gas charge in the combustion chamber can be calculated from this minimum pressure. Since this gas charge is calculated from the gas pressure that actually exists in the induction pipe, the leakage through the throttle valve and behind the throttle valve due to manufacturing tolerances is taken into account in the gas charge. This value is therefore more precise and can be used to produce mixtures more precisely. The pressure in the induction pipe is preferably measured by a sensor provided in the induction pipe. 
     In one very advantageous refinement, a gas charge of the combustion chamber is defined from the actual position of the throttle valve, the defined value is compared with the measured value, and then, if the comparison shows that the difference between the measured and the defined gas charge lies outside of a permissible range, the position of the throttle valve is corrected. Therefore, in this refinement, the gas charge measured from the minimum gas pressure and the gas charge defined by the actual position of the throttle valve are combined with each other. In this context, by determining a permissible range, a range of tolerance created, which makes it unnecessary to resort to control interventions too frequently. 
     In this context, the correction takes place preferably so that the difference between the measured and the defined gas charge is equal to zero. This means that the gas charge is optimized. 
     The above-mentioned method is particularly well suited for internal combustion engines that have a plurality of combustion chambers and also is particularly well suited for internal combustion engines in which each combustion chamber or a group of combustion chambers (e.g., a cylinder bank) has assigned to it its own induction pipe and its own throttle valve. For this reason, it is proposed in one refinement that the above-mentioned method be carried out independently for a plurality of combustion chambers having their own induction pipe and especially their own throttle valve. In this way, for each individual combustion chamber or for each group of combustion chambers, a value can be calculated which more closely approximates the actual gas charge of this individual combustion chamber, and the position of the throttle valve. assigned to this combustion chamber can be corrected. In this manner, the emissions and fuel consumption characteristics of the individual cylinders of the internal combustion engine are optimized. 
     Correcting the position of the throttle valve can take place in varying ways. One possibility is to correct an offset, usually taken into account in calculating the gas charge, and a slope. This offset is a value which makes it possible to take into account the air leakage streams through gaps between the throttle valve and the wall of the induction pipe and through other leakage points between the throttle valve and the combustion chamber. The slope takes into account the multiplicative errors in the throttle valve system. In response to a difference between the value for the gas charge defined by the actual position of the throttle valve and a value calculated on the basis of the minimal gas pressure, it can be assumed that the offset and the slope do not optimally reflect the actual reality. This can be partially compensated for by correcting the offset as proposed in accordance with the present invention. The same applies to the slope. In this context, the correction can take place in a multiplicative manner, meaning a change in the slope, or in an additive manner, meaning a change in the “offset.” A correction of the offset and/or the slope is recommended especially in response to generally low pressure levels in the induction pipe, i.e., in an operating state in which the throttle valve is closed relatively far, because in an operating state of this type the aforementioned leakage streams play a relatively large role. 
     Intervening in the control of the throttle valve position has the advantage over regulating the throttle valve position directly on the basis of the pressure-based gas charge that the optimal gas charge can be achieved more rapidly and without transient effects. Calculating the gas charge on the basis of the throttle valve position makes it possible to react immediately to a change in the driver&#39;s requests. On the other hand, the minimum pressure in one cylinder can only be measured anew after an entire rotation of the camshaft. 
     At an overall high induction pipe pressure, as is also mentioned in one refinement of the present invention, the regulation of the throttle valve position is influenced. In addition, it is also possible to influence the calculation of a setpoint value of the throttle valve. Both measures make it possible to react quickly to differences between the defined and the measured values. 
     The precision of the above-mentioned method is even more improved in one refinement which, in calculating the gas charge, takes into account the partial pressure of the internal residual gas. 
     The present invention also relates to a computer program, which is suited to carrying out the above-mentioned method, if it is executed on a computer. In one preferred refinement of this computer program, it is stored in a storage device, in particular in a flash memory. 
     Finally, the present invention relates to a control and regulating unit for an internal combustion engine, especially of a motor vehicle, having at least one combustion chamber, one induction pipe, and one throttle valve, the unit defining a gas charge of the combustion chamber from the actual position of the throttle valve. To improve the emissions and fuel consumption behavior of the internal combustion engine, the control and regulating unit is connected to a pressure sensor arranged in the induction pipe, and it determines a value that more closely approximates the actual gas charge from the minimal gas pressure existing in the induction pipe assigned to the combustion chamber at the end of the intake stroke. 
     Especially preferred is the refinement of the control and regulating unit, in which, if the comparison shows that the calculated gas charge of the combustion chamber does not roughly correspond to the defined gas charge, then a correction signal for the position of the throttle valve is generated. 
     Finally, a control and regulating unit of this type is particularly advantageous if it is suited for internal combustion engines having a plurality of combustion chambers and a plurality of pressure sensors and throttle valves each assigned to one combustion chamber. This is the case in the refinement in which the control unit is connected to a multiplicity of pressure sensors, each assigned to one combustion chamber, and, in particular, generates a plurality of independent correction signals for the corresponding throttle valves. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a schematic representation of components of an internal combustion engine having two combustion chambers. 
     FIG. 2 depicts a simplified representation of a method for operating the internal combustion engine from FIG.  1 . 
     FIG. 3 depicts a detailed flowchart of the method represented in FIG.  2 . 
     FIG. 4 depicts a diagram of the pressure curves in the induction pipe of the internal combustion engine represented in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, an internal combustion engine is designated overall by reference numeral  10 . It has two combustion chambers  12  and  14 , each of which is supplied with air via its own induction pipe  16  and  18 , respectively. The internal combustion engine can be, e.g., an SI engine having an electronic gas pedal (“e-gas”). The corresponding pistons, etc., are not depicted. In the area of the intake of induction pipe  16  and  18  feeding into combustion chambers  12  and  14 , respectively, intake valves  20  and  22  are schematically depicted in FIG.  1 . The exhaust gases can escape via exhaust pipes  24  and  26 , which are connected via exhaust valves  28  and  30  to combustion chambers  12  and  14 , respectively. 
     Arranged in induction pipes  16  and  18  are throttle valves  32  and  34 , whose positions are set by servomotors  36  and  38 , respectively. The actual position of throttle valves  32  and  34  is transmitted in each case by position sensors  40  and  42  to a control and regulating unit  44 , which in turn drives servomotors  36  and  38 , respectively. Between throttle valves  32  and  34  and intake valve  20  and  22 , respectively, there are in each case pressure sensors  46  and  48 , which measure the pressure in induction pipes  16  and  18  between throttle valves  32  and  34  and intake valves  20  and  22 , respectively, during the operation of the internal combustion engine  10 . Pressure sensors  46  and  48  also convey corresponding signals to control and regulating unit  44 . The latter is also connected to a remote-position indicator  50  of a gas pedal  52 . 
     The quantity of air flowing through induction pipes  16  and  18  into combustion chambers  12  and  14  (arrows  54  and  56 ), respectively, is fundamentally influenced by the piston of throttle valves  32  and  34 . The control and regulation of the latter is now described with reference to FIGS. 2-4. 
     First, an air charge setpoint value rlsol is calculated by control and regulating unit  44  as a function of a signal which control and regulating unit  44  receives from remote-position indicator  50  of gas pedal  52 . This air charge setpoint value rlsol is identical for both combustion chambers  12  and  14  and represents the gas charge that is optimal for the fuel quantity to be injected. Calculating this value takes place in a block  58 . 
     As a function of air charge setpoint value rlsol, for each throttle valve  32  and  34  in a charge control process  60  and  62 , a setpoint wdksl and wdks 2  is established for the position of throttle valves  32  and  34 , respectively. These setpoint values wdksl and wdks 2  are in turn supplied to a position controller  68  and  70 , which drives servomotors  36  and  38  of throttle valves  32  and  34 , respectively. Actual position wdk 1  (block  72 ) of throttle valve  32  and actual position wdk 2  (block  74 ) of throttle valve  34  are measured by remote-position indicator  40  and  42 , respectively, and are conveyed to position controllers  68  and  70 , respectively. Position controller  68 , servomotor  36 , and remote-position indicator  40  as well as position controller  70 , servomotor  38 , and remote-position indicator  42  therefore constitute a closed control loop. 
     Actual positions wdk 1  and wdk 2  of throttle valves  32  and  34 ,. respectively, are also supplied to a throttle-valve-based charge measurement system  76  and  78 , which from supplied values wdk 1  and wdk 2  in blocks  80  and  82  define a “theoretical” actual position rldk 1  for combustion chamber  12  and rldk 2  for combustion chamber  14 . This gas charge is theoretical because it does not take into account the individual tolerances of throttle valves  32  and  34  and can therefore distinguish these gas charges from the actual gas charges. 
     From pressure sensors  46  and  48  arranged in induction pipes  16  and  18 , respectively, pressures P 1  and P 2  are continuously determined which exist in induction pipes  16  and  18 , respectively, corresponding to the curves in FIG.  4 . Using a minimum value characterizer that is not depicted in the drawing, for each curve P 1  and P 2 , minimum value P 1 min and P 2 min is determined in blocks  84  and  86 , respectively. 
     Both of these pressures P 1 min and P 2 min are the pressures at the end of the intake stroke and this is so for the following reason: when intake valves  20  and  22  open during the charge changing phase, the (undepicted) piston is situated at the upper dead center and then travels to the lower dead center. In this context, the gas quantity behind throttle valve  32  and  34  expands to an ever greater volume, and the pressure therefore falls. At the lower dead center, the maximum volume; and therefore minimum pressures P 1 min and P 2 min are reached. 
     Shortly thereafter, intake valves  20  and  22  close. Pressures P 1 min and P 2 min, measured in induction pipes  16  and  18 , closely approximate the pressure in combustion chambers  12  and  14 , respectively, from which the charge can be calculated. 
     In a computing loop that is also not depicted in the drawing, from minimum pressure values P 1 min and P 2 min, gas charge rldss 1  (block  88 ) and rldss 2  (block  90 ) are calculated, which more closely correspond to the actual gas charge. 
     Gas charge rldk 1  (block  80 ) in combustion chamber  12 , defined by the position of throttle valve  32 , is now compared in a comparator  92  to gas charge value rldss 1  (block  88 ), calculated from minimum pressure P 1 min in induction pipe  16 . In response to a difference between two gas charges rldk 1  and rldk 2 , a query is raised in comparator  92  in block  94  as to whether pressure level P 1  in induction pipe  16  is relatively low overall. This is the case, e.g., when throttle valve  32  is closed relatively far. If the answer in block  94  is yes, then learned additive quantity msndko 1 , representing a measure for the air leakage streams through throttle valve  32  and behind this throttle valve  32 , is changed. Quantity msndko 1  is used for correcting the charge measurement in block  76  and for correcting the charge control in block  60 . 
     If the answer in block  94  is no, then a multiplicative correction of the charge measurement in block  76  and of the charge control in block  60  is carried out by a factor fkmsdk 1 . (block  98 ). 
     Analogously, for other combustion chamber  14 , a comparator  100 , a decision block  102 , and correction quantities msndko 2  (block  104 ) and fkmsdk 2  (block  106 ) are provided. In this manner, despite identical setpoint value rlso 1  (block  58 ) for both combustion chambers  12  and  14 , different setpoint angles wdks 1  and wdks 2  result in blocks  64  and  66 , which compensate for the tolerance differences between two throttle valves  32  and  34 , respectively. 
     Monitoring the normal operation of the method can be carried out in a simple manner: The pressure signal made available by pressure sensors  46  and  48  is superposed on a minimum, which is smaller than p 1 min and p 2 min in blocks  84  and  86  and which does not coincide temporally with the closing of intake valves  20  and  22 , respectively. The charge measurements in blocks  76  and  78  in blocks  80  and  82  now supply a value rldk 1  and rldk 2  that is too small. The system (controller  68  and  70 ) must now react to a leak behind a throttle valve  32  and  34 , simulated in this manner, by closing upstream throttle valve  32  and  34 .