Abstract:
In an hydraulic actuator, a movement of an actuating element of the actuator is effected in that a working chamber of the actuator, with the aid of a valve device, is able to be selectively connected to, and disconnected from, a fluid reservoir in which pressurized hydraulic fluid is stored. The lift of the actuating element of the actuator is a function of a fluid volume present in the working chamber. It is provided that, to ascertain an instantaneous operating performance of the actuator, the working chamber is briefly connected to the fluid reservoir, the corresponding pressure drop in the fluid reservoir is recorded, and the corresponding lift is determined from the pressure drop with the aid of known geometrical variables of the actuator.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method for operating an hydraulic actuator, in particular for a gas-exchange valve of an internal combustion engine, in which a movement of an actuating element of the actuator is effected in that a working chamber of the actuator, by means of a valve device, is able to be connected to, and disconnected from, a fluid reservoir in which pressurized hydraulic fluid is stored, the lift of the actuating element of the actuator being a function of a fluid volume present in the working chamber.  
       BACKGROUND INFORMATION  
       [0002]     Published German patent document DE 198 26,047 describes a device for controlling a gas-exchange valve of an internal combustion engine and the corresponding operating method. A high-pressure pump pumps hydraulic fluid into a piping system in which the hydraulic fluid is stored under very high pressure. A working chamber of an hydraulic cylinder whose piston is connected to a valve element of a gas-exchange valve of an internal combustion engine is connected to the fluid reservoir via a two-way valve. An outlet of the working chamber is also connected to a low-pressure region via a two-way valve. Depending on the valve setting, high or low pressure prevails in the working chamber of the hydraulic actuator and a corresponding fluid volume is present in the working chamber, which affects the piston position.  
         [0003]     The advantage of such a gas-exchange valve is that it may be triggered independently of a setting of a camshaft of the internal combustion engine. For cost reasons, no detection of the instantaneous piston position takes place. As a result, the positioning of the piston of the hydraulic actuator is not able to be precisely regulated.  
         [0004]     An object of the present invention is to provide a method such that the actuating element of the actuator is able to be positioned as precisely as possible.  
       SUMMARY  
       [0005]     In a method according to the present invention, this objective is achieved in that, to ascertain an instantaneous operating performance of the actuator, the working chamber is briefly connected to the fluid reservoir, the corresponding pressure drop in the fluid reservoir is recorded and the corresponding lift is ascertained from the pressure drop with the aid of known geometric variables of the actuator, and at least one value pair is formed, which is made up of an opening duration and the lift.  
         [0006]     The ascertained value pair may be compared with a value pair determined on a test stand, for example, or during a previous method run. In this manner age manifestations, changed ambient conditions etc. may be detected and taken into account in the triggering of the valve devices. The outputting of an information when the instantaneous operating performance of the actuator has changed in an impermissible manner is possible as well. This increases the reliability of the actuator operation since it allows countermeasures to be taken even before the operation of the actuator possibly results in damage.  
         [0007]     In a first example embodiment of the invention, it is provided that the pressure drop in the fluid reservoir be recorded for different durations during which the working chamber of the actuator is connected to the fluid reservoir and that an instantaneous characteristic curve be formed from the ascertained value pairs. In this case the actuating element of the hydraulic actuator may be positioned very precisely in normal operation without the need for complex regulation and the cost-intensive installation of a sensor that detects the lift of the actuating element of the hydraulic actuator. Therefore, the precise positioning of the actuating element is basically possible without additional hardware and, consequently, at low cost.  
         [0008]     According to an example embodiment of the method according to the present invention, the actuating element is brought from a known initial position to a known limit position, the corresponding pressure drop in the fluid reservoir is recorded, and the at least one ascertained value pair is standardized with the aid of the recorded pressure drop and the lift between initial position and limit position. Measuring inaccuracies are able to be eliminated by this method and the precision of the characteristic curve of the hydraulic actuator may be improved even further. Due to the additional method step provided in this example embodiment, the actual method by which at least one value pair is determined is calibrated, so to speak.  
         [0009]     The actuating element may be brought into the initial or the limit position simply in that the valve device is in one or the other position for a particular length of time. Alternatively or additionally, however, the reaching of the initial and/or the limit position of the actuating element may also be detected with the aid of a knock sensor. This improves the precision of the aforesaid standardization or calibration.  
         [0010]     It is also provided that the at least one value pair be formed taking the elasticity module of the hydraulic fluid and/or the elasticity of the fluid reservoir into account. This, too, results in even greater precision of the instantaneous characteristic curve of the hydraulic actuator. In addition, it may also be taken into account that the elasticity module of the hydraulic fluid is temperature and pressure dependent. The elasticity of the fluid reservoir, too, i.e., the elasticity of its walls, may change, primarily as a function of the temperature.  
         [0011]     In a further example embodiment of the method according to the present invention, it is also indicated that the temperature and/or the viscosity of the hydraulic fluid are/is recorded during the detection of the instantaneous operating performance of the actuator and the at least one value pair is formed for a particular viscosity and/or a particular temperature of the hydraulic fluid. Therefore, it is possible in this way to generate a whole set of value pairs or characteristic curves, one value pair or one characteristic curve in each case being valid only for quite specific operating or ambient conditions. This, too, ultimately results in an even further improvement of the precision of the positioning of the actuating element of the hydraulic actuator.  
         [0012]     It is also advantageous if the response time of the valve device is ascertained from the onset of the pressure drop in the fluid reservoir. For the accuracy of the positioning of the actuating element of the hydraulic actuator, in particular with respect to the temporal accuracy, the response time—i.e., the time between the generation of the trigger signal and the onset of the pressure drop caused by the movement of the actuating element—is particularly important. In the method according to the present invention, this response time may be determined “as an aside”, so to speak, and be taken into account in the triggering of the valve device during normal operation of the hydraulic actuator.  
         [0013]     To determine the instantaneous operating performance of the hydraulic actuator, it is advantageous if the fluid reservoir is fluidly separated from a pressure reservoir, and/or a high-pressure pump for the supply of the fluid reservoir is de-energized. While the method according to the present invention may also be carried out when a pressure reservoir is connected to the fluid reservoir or when a high-pressure pump delivers into the fluid reservoir, these cases require fairly complex consideration of the form change of the pressure reservoir (for example by means of a position detection at the pressure reservoir) or the conveying capacity of the high-pressure pump. This will not be required if, as provided in the example embodiment, the fluid reservoir is simply separated from the pressure reservoir or from the high-pressure pump. Furthermore, this improves the precision of the method according to the present invention, since the volume of the fluid reservoir is reduced by this measure, which leads to a greater pressure drop in a corresponding triggering of the valve device at the same lift of the actuating element of the hydraulic actuator, the pressure drop being able to be measured with greater accuracy.  
         [0014]     If the hydraulic actuator is used to activate a gas-exchange valve of an internal combustion engine, it is advantageous if the instantaneous operating performance is determined after the internal combustion engine has been shut off or during an overrun operation of the internal combustion engine. In this case the method according to the present invention may be carried out without adverse effect on the normal operation of the internal combustion engine.  
         [0015]     However, it should be ensured that the triggering of the hydraulic actuator for the purpose of determining the instantaneous characteristic curve is implemented in such a way that the particular gas-exchange valve neither collides with a piston of the internal combustion engine nor with another gas-exchange valve. In overrun operation, for example, a triggering of the hydraulic actuator is consequently conceivable only in a partial lift range. Given a multi-cylinder internal combustion engine, it is thus possible that a plurality of de-energize phases are required to ascertain the instantaneous operating performance of the actuators of all gas-exchange valves.  
         [0016]     Furthermore, it may be provided according to the present invention that the pressure in the fluid reservoir be recorded when the hydraulic actuator is at rest and a report be output in the case of an impermissible pressure drop. This allows the tightness or the leakage of the hydraulic system of the fluid reservoir supplying the actuator to be checked. In this way, the user may detect the availability of the correct operating mode of the hydraulic actuator and thus ultimately of the gas-exchange valve; if warranted, the operation of the internal combustion engine may be terminated automatically or be restricted to a safety zone so as to avoid damage to the internal combustion engine due to an incorrectly working gas-exchange valve. It is understood that the monitoring of the pressure drop is facilitated if a high-pressure pump, which supplies the fluid reservoir with hydraulic fluid, is switched off or is completely disconnected from the fluid reservoir. The same also holds for a pressure reservoir.  
         [0017]     The present invention also provides a computer program, which is programmed to carry out the afore-described method and is stored on a computer-readable storage medium.  
         [0018]     The present invention also provides a control and/or regulating device for an internal combustion engine, which is programmed to be used in the above-described method.  
         [0019]     The present invention also provides an internal combustion engine, in particular for a motor vehicle, having a control and/or regulating device, which is programmed to be used in the above-described method. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0020]      FIG. 1  shows a schematic illustration of an internal combustion engine of a motor vehicle having gas-exchange valves, which are activated by an hydraulic actuator connected to an hydraulic system.  
         [0021]      FIG. 2  shows a more detailed representation of the hydraulic system of  FIG. 1 .  
         [0022]      FIG. 3  shows a flow chart illustrating a method for operating the hydraulic actuator of  FIG. 1 .  
         [0023]      FIG. 4  shows a schematic illustration of an alternative exemplary embodiment of an hydraulic system.  
         [0024]      FIG. 5  shows a flow chart of a method for operating the hydraulic actuator of  FIG. 1  using the hydraulic system of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION  
       [0025]     In  FIG. 1 , an internal combustion engine is generally denoted by reference numeral  10 . It is used to drive a motor vehicle  12 , which is shown only symbolically in  FIG. 1 , using a dashed line. Internal combustion engine  10  is a multi-cylinder internal combustion engine having reciprocating pistons. However, for reasons of clarity, only the essential elements of a single cylinder are shown in  FIG. 1 .  
         [0026]     The cylinder shown in  FIG. 1  includes a combustion chamber  14 , which is delimited by a piston  16  among others. Air is supplied to combustion chamber  14  via an inflow pipe  18  and a first gas-exchange valve  20 . First gas-exchange valve  20  thus is the intake valve of combustion chamber  14 . The combustion waste gases are conducted from combustion chamber  14  to an exhaust-gas pipe  24  via a second gas-exchange valve  22 . The second gas-exchange valve thus is a discharge valve of combustion chamber  14 .  
         [0027]     In internal combustion engine  10  shown in  FIG. 1 , intake valve  20  and discharge valve  22  are not activated by a camshaft but by an hydraulic actuator  26  or  28 , respectively. Hydraulic actuator  26  is triggered by an hydraulic system  30 , and actuator  28  is triggered by an hydraulic system  31  whose exact configuration is discussed in greater detail at a later point. Hydraulic systems  30  and  31  are in turn controlled by a control device  32 .  
         [0028]     The fuel arrives in combustion chamber  14  of internal combustion engine  10  via an injector  34 , which injects the fuel directly into combustion chamber  14 . Injector  34  is connected to a fuel system  36 . The fuel-air mixture present in combustion chamber  14  is ignited by a spark plug  38 , which is controlled by an ignition system  40 . Elements  38  and  40  may be dispensed with in a diesel gasoline engine.  
         [0029]     Hydraulic systems  30  and  31  are identically configured. They will now be discussed on the basis of hydraulic system  30  according to  FIG. 2 :  
         [0030]     Hydraulic fluid (not shown) is stored in a storage reservoir  42 . An adjustable high-pressure pump  44 , which is driven by an electric motor  46 , supplies the hydraulic fluid out of storage reservoir  42  into a high-pressure line  50 , via a one-way valve  48 . Connected to high-pressure line  50  is a pressure reservoir  52 , which may be, for instance, a pressure reservoir having a spring-loaded piston. A pressure sensor  54  detects the pressure in high-pressure line  50  and transmits corresponding signals to control device  32 .  
         [0031]     Hydraulic actuator  26  is a two-way hydraulic cylinder. A piston  58  is arranged in a housing  56  in a movable manner. A fluid chamber between the upper face of piston  58  (here and hereinafter, “upper” and “lower” refer only to the representation in the figures) and housing  56  forms a first working chamber  60 . A fluid chamber between the bottom side of piston  58 , a piston rod  62  being connected thereto, and housing  56  form a second working chamber  64 . Braced between the bottom side of piston  58  and housing  56  is a compression spring  66 . Piston rod  62  is connected to intake valve  20 .  
         [0032]     Between hydraulic actuator  26  and pressure sensor  54  is a storage chamber  68  in high-pressure line  50 , which forms a collection line in the sense of a “high-pressure rail”. Via a branch line  70 , second working chamber  64  is permanently connected to high-pressure line  50  or storage chamber  68 . Arranged between storage chamber  68  and first working chamber  60  is a two-way valve  72 , which is closed in its spring-loaded rest position  74  and open in its activated position  76  (two-way valve  72  is activated by an electromagnet  78 ). High-pressure line  50 , pressure reservoir  52 , storage chamber  68 , branch line  70  and second working chamber  64  together form a fluid reservoir  80 , which is sealed in the direction of high-pressure pump  44  by one-way valve  48  and may be sealed with respect to first working chamber  60  by valve  72 .  
         [0033]     First working chamber  60  is connected to storage reservoir  42  by a return line  82 . A two-way valve  84  and a one-way valve  86  are arranged in return line  82 . Two-way valve  84  is open in its spring-loaded rest position  88  and closed in activated position  90 . It is brought into closed position  90  by an electromagnet  92 .  
         [0034]     In normal operation of the internal combustion engine, a back-and-forth movement of intake valve  20  is effected by an alternating activation of the two solenoid valves  72  and  84 . When solenoid valve  84  is closed, the opening duration of solenoid valve  72  determines how much hydraulic fluid reaches working chamber  60  of hydraulic actuator  26 . The quantity of hydraulic fluid present in first working chamber  60  in turn determines the position or the lift of piston  58  and thus ultimately the lift of intake valve  20  as well. Intake valve  20  is closed by opening solenoid valve  84  when solenoid valve  72  is closed.  
         [0035]     To ascertain the instantaneous operating performance of hydraulic actuator  26 , a method is used that is stored as computer program in a memory  94  of control device  32 . The method will now be explained with reference to  FIG. 3 :  
         [0036]     Following a start block  96 , high-pressure pump  44  is switched off in a block  98 . Magnets  78  and  92  of both solenoid valves  72  and  84  are de-energized in same block  98 . Solenoid valve  72  is thus closed whereas solenoid valve  84  is open. This pushes piston  58  into its upper limit position in  FIG. 2 . In block  100 , solenoid valve  84  is then brought into its closed position  90 . In a block  102 , solenoid valve  72  is opened during a defined time period dt and then closed again. Pressure sensor  54  detects pressure drop dp in fluid reservoir  80  (block  104 ). This pressure drop, together with corresponding time period dt, is stored as value pair dp,dt.  
         [0037]     In a block  106  it is queried whether piston  58  has moved to its lower limit position shown in  FIG. 2 . This is detected by a knock sensor, which is not shown in  FIGS. 1 and 2 . If the answer in block  106  is “no”, solenoid valve  84  is opened in block  108  and then closed again. This relieves first working chamber  60 , and piston  58  reaches its upper initial position in  FIG. 2  again. In a time block  110 , time period dt is increased by a fixed differential value dt 1 . A return to block  102  then takes place.  
         [0038]     Using the method shown in  FIG. 3 , solenoid valve  72  is thus opened successively during an increasingly longer period of time, so that a correspondingly larger quantity of hydraulic fluid flows out of fluid reservoir  80  into first working chamber  60  and a correspondingly different pressure drop is recorded by pressure sensor  54 . It is to be understood in this context that a pressure drop at pressure sensor  54  is detected only when pressure reservoir  52  is blocked, for instance. If this is impossible, the state change of pressure reservoir  52  would also have to be detected as an alternative.  
         [0039]     The method loop is run through repeatedly until piston  58  has reached its lower limit stop shown in  FIG. 2 . In this case, a switch is made from block  106  to block  112  in which the quotient is formed from pressure drop dpa and the corresponding maximum lift dha between the upper limit stop and the lower limit stop of piston  58 .  
         [0040]     The corresponding lifts of piston  58  are calculated in block  114  of  FIG. 3  from the stored pressure differentials dp. The following formula is used  
       dh   =         VO   *     ⅆ   p         E   OIL         ⅆ   A           
 
         [0041]     In the above formula, dh is the lift of piston  58 ; VO the original volume in fluid reservoir  80  prior to the opening of solenoid valve  72 ; dp the pressure drop detected by pressure sensor  54 ; E OIL  the elasticity of the hydraulic fluid; and dA the difference between the upper and the lower boundary surfaces of piston  58 . In this manner, value pairs dp,dh are formed from which, furthermore, a characteristic curve dh=f(dt) is formed in block  114  of  FIG. 3 . This characteristic curve links lift dh of piston  58  to corresponding opening duration dt of solenoid valve  72 . This characteristic curve is then utilized in normal operation to trigger solenoid valve  72  so as to achieve a certain desired lift. Value pairs dp,dh are standardized or calibrated on the basis of quotient dpa/dha formed in block  112  of  FIG. 3 .  
         [0042]     With reference to  FIGS. 4 and 5 , a second exemplary embodiment of an hydraulic system  30  will now be discussed. Those elements and function blocks that have functions which are equivalent to those of elements and function blocks of the exemplary embodiment described in connection with  FIGS. 2 and 3  have the same reference numerals and are not discussed again in detail.  
         [0043]     First of all, hydraulic system  30  shown in  FIG. 4  differs from that in  FIG. 2  by an additional solenoid valve  118 , which is arranged between one-way valve  48  and pressure reservoir  52  on one side, and pressure sensor  54  on the other side. With the aid of additional solenoid valve  118 , it is thus possible to separate fluid reservoir  80  from pressure reservoir  52 , which facilitates the detection of pressure drop dp. Also provided in hydraulic system  30  shown in  FIG. 4  are a temperature sensor  120  and a viscosity sensor  122 , which record the temperature and the viscosity, respectively, of the hydraulic fluid present in fluid reservoir  80  and transmit corresponding signals to control device  32 .  
         [0044]     The instantaneous operating performance of hydraulic actuator  26  of  FIG. 4  is determined by means of a method which will now be discussed with reference to  FIG. 5 :  
         [0045]     In contrast to the method of  FIG. 3 , valve  118  is also de-energized in block  100  in the method illustrated in  FIG. 5 . This, as already mentioned earlier, separates pressure reservoir  52  from fluid reservoir  80 , and high-pressure pump  44 , too, is separated from fluid reservoir  80 . If appropriate, it may also continue running while the method illustrated in  FIG. 5  is executed.  
         [0046]     In block  102 , valve  72  is opened during a plurality of method loops during a same time period dt 1 . That is to say, it is opened further and further in a step-wise manner. In block  110 , a counter n is incremented by 1 in each case, and in block  124  it is queried whether counter n is greater than a limit value G. Limit value G thus restricts the number of measuring procedures to a fixed value. In block  106 , valve  72  is opened during a time period dt 2 , which is long enough for piston  58  to attain its lower limit position in  FIG. 4  under all circumstances. As a result, this procedure will not have to be detected by a knock sensor. In block  114 , characteristic curve dh=f(dt) is determined and stored for temperature temp 1  recorded by temperature sensor  120  and viscosity visc 1  of the hydraulic fluid recorded by viscosity sensor  122 . If the method of  FIG. 5  is run through under different ambient conditions, a set of characteristic curves is produced, each of which is suited to particular ambient conditions.  
         [0047]     The methods illustrated in  FIGS. 3 and 5  may be initiated by control device  32  immediately after internal combustion engine  10  has been shut off. Control device  32  is aware of the position of pistons  16  of the individual cylinders of internal combustion engine  10 , and the methods illustrated in  FIGS. 3 and 5  will be implemented only for those cylinders for which it is ensured that no collision will occur between intake valve  22  and corresponding piston  16  or with other valves. If the methods are implemented with a certain regularity after the internal combustion engine has been shut off, it is nevertheless ensured that the instantaneous operating performance of hydraulic actuators  26  of intake valves  20  of all cylinders is known. However, it is also possible to implement the methods during an overrun operation of the motor vehicle as long as it is ensured that no collisions will occur between the piston and the corresponding gas-exchange valve.  
         [0048]     In an analogous manner, the instantaneous operating performance of hydraulic actuators  28  of discharge valves  22  is determined as well. It may also be considered here that collisions may occur between intake valve  20  and discharge valve  22  of a cylinder. In a repeated implementation of the methods shown in  FIGS. 3 and 5 , it is also possible to form averaged values, for example across the three last method sequences, so as to improve the accuracy of the method result. Furthermore, the response time of solenoid valve  72  may be determined from the onset of pressure drop dp in fluid reservoir  80 .  
         [0049]     In exemplary embodiments not shown here, the afore-described method is used with internal combustion engines having manifold injection and with diesel gasoline engines.  
         [0050]     In an exemplary embodiment also not shown, in an operating phase in which discharge valve  20  is at rest, valve  118  is closed and the pressure development in fluid reservoir  80  is monitored. A message is output if the pressure drops too much during a particular time period. This may be an entry in a fault memory, or a warning display may light up for the user of internal combustion engine  10 . It is also conceivable in such a case to shut down internal combustion engine  10  completely or to allow only a restricted operational safety operation so as to avoid further damage to internal combustion engine  10 .