Abstract:
A hydraulic system ( 1 ) for a switchable valve drive element ( 10 ) of an internal combustion engine ( 33 ) is provided and includes a throttled connection ( 13 ) created from a second channel ( 9 ), used to actuate a hydraulic play-compensation element ( 11 ) using hydraulic fluid, directly to an external radial side ( 12 ) of a coupling element ( 8 ). The coupling element ( 8 ) is supplied by a first channel ( 7 ) with the switching hydraulic pressure. These measures make it possible to keep the first channel ( 7 ) as free as possible of undesired air.

Description:
BACKGROUND 
     This invention relates to a hydraulic system for an internal combustion engine with at least one hydraulically actuated coupling element such as a sliding coupling, preferably in a switchable valve drive element such as a cam follower or support element for it. The valve drive element is preferably of the type having at least two parts that move with respect to one another to attain different cam strokes, wherein on at least one side of the coupling element, a path runs in or on the valve drive element to feed switching hydraulic fluid pressure. The valve drive element moves within a bore in the internal combustion engine into which a first channel opens to feed the switching hydraulic fluid pressure, one end of the channel being supplied by a hydraulic fluid pump followed by a directional valve to turn on and turn off the switching pressure, and the other end of the channel being hydraulically connected to the path. 
     Hydraulic systems of this type in an internal combustion engine, for example to actuate a coupling element of a switchable valve drive element such as a flat tappet, roller tappet, support element or finger lever or rocker arm or the like, exhibit a list of system-dependent disadvantages (see also DE 196 04 866 or U.S. Pat. No. 5,351,662). When a switch command is issued, delays or fluctuations in the switching time occur that are dependent on RPM, temperature, wear, tolerances or oil viscosity. An important factor influencing the delay in the switching time is the undesirable high compressibility of the hydraulic fluid used caused by entrained air bubbles or oil foaming that occurs on top of the inherent compressibility of the hydraulic fluid that always exists but is relatively minimal. These air bubbles can make their way bit by bit, for example, into a hydraulic fluid feed channel ahead of its respective coupling element when the internal combustion engine is shut off and the channel is idle, even if the corresponding switchable valve, or a check valve, prevents backflow out of the channel. After starting the internal combustion engine, this channel must be bled long enough prior to the first switch command until any amounts of air are removed from it, or at least most of it is. However, there are always areas in this channel that are at geodetically high relative points or at the end of the channel, for example directly in front of the corresponding coupling element, that despite everything are not affected by the bleed stream produced in the channel by the pump-channel connection effected by the switchable valve. A person trained in the art could of course install bypasses around the switchable valve, for example, in order to produce a permanent bleed stream, or to define blow down or leakage points, but this unnecessarily increases the design effort and the costs associated with the hydraulic system and with the overall internal combustion engine. 
     Also, the hydraulic fluid used can foam up, for example when the internal combustion engine is at hot idle. This foaming can also lead to the undesired hydraulic fluid compressibility mentioned. In worst case, after issuing the switch command, no switching occurs at all at the coupling element since all that occurs is the compression of air or oil foam. Here, as well, a permanent bleed stream could be used to remove the undesired air as much as possible from the corresponding coupling channel when the hydraulic fluid is disconnected. However, as mentioned, this bleed stream does not reach the entire range of the channel up to directly in front of the coupling element, resulting in this air cushion merely being pushed back and forth in the channel between coupling cycles. 
     SUMMARY 
     The object of this invention is thus to create a hydraulic system of the above type in which the disadvantages cited are remedied using simple means. 
     According to the invention, this object is met by providing the path with a connection to a second hydraulic fluid channel at least near the coupling element. This second channel feeds high-pressure hydraulic fluid when the switching pressure is shut off in the first channel, the pressure in the second channel being less than the necessary switching pressure. 
     Useful embodiments of the invention are discussed below, which can also include independently protectable features. 
     At this point, it is stressed that the area of protection of the invention refers in particular to a hydraulic system of an internal combustion engine and here especially to a hydraulic system to actuate a coupling element for a slide coupling of a switchable valve driver. However, the concept of the invention goes so far as to include a multitude of hydraulic systems in the design of engines, as well as in other technologies where a slide valve or similar element is to be hydraulically shifted. For example, the invention can also be used for block pistons or slide valves in hydraulic camshaft positioning devices. Also, the area of protection does not extend only to valve drive elements that are installed in slots or bore holes in internal combustion engines, but for example can also extend to finger levers, valve rockers or rocker arms next to one another that can be coupled together selectively using at least one hydraulically moving slide coupling. 
     According to the invention, by producing a connection to a second high-pressure channel directly in front of the coupling element or its path in the valve drive element, the first channel can be completely or almost completely bled free of air bubbles during the critical times described above, at least while the coupling pressure in the first channel is disconnected. It is helpful in the process to keep this “bleed pressure” to a minimum so as to prevent it from moving the coupling elements in their movement direction. This measure can be implemented extremely inexpensively, for example by means of a simple notch in the valve drive element, as illustrated below in more detail. 
     Although it is not required, it is expedient to make use of a second channel that supplies a hydraulic play-equalization element in the valve drive element. It is however possible to use separate controls as well. 
     In this manner, the connection from the second channel to the path directly in front of the side of the coupling element or directly near the side of the coupling element is created by means of a pressure-reducing design such as anozzle or a throttle. This allows the full hydraulic fluid pressure, which is used to actuate the hydraulic play-equalization element to be turned on since only a single hydraulic fluid pump is used, which is an advantage. This is because directly in front of the path or coupling element, the pressure is reduced. 
     If a person skilled in the art is able to place the connection between the second and the first channel directly behind the side of the coupling element, the best success can be expected according to the invention. 
     Instead of the suggested nozzle or throttling device, there are other pressure-reducing measures that are already available to the person trained in the art. 
     According to an additional embodiments of the invention, the hydraulic system can be applied to a hydraulic flat tappet. Here, the “bleed pressure” should pass from a supply chamber in the flat tappets fed from the second channel to a supply chamber in the flat tappets fed from the first channel. In this manner, a transfer line can be implemented at an edge region at the bottom of a slot for the coupling means in the flat tappet, it being useful to locate said slot near the base (but not necessarily). 
     Another preferred embodiment of the invention relates to a hydraulic system of a roller tappet or similar device. Here, the connection can be produced as an axial path that leads from the second channel at the bore hole in the internal combustion engine to the path in front of the coupling element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of invention will be explained in detail below with reference to the drawings. In the drawings: 
     FIG. 1 is a schematic view of a hydraulic system in accordance with the invention; and 
     FIG. 2A is a cross-sectional view of a flat tappet in accordance with the invention. 
     FIG. 2B is a bottom view of the tappet of FIG. 2A for use in the hydraulic system according to FIG.  1 . 
     FIG. 3 is a cross-sectional view of a cam following tappet in accordance with the invention for use in the hydraulic system according to FIG.  1 . 
     FIG. 4 is a cross-sectional view of another embodiment of a cam following tappet in accordance with the invention for use in the hydraulic system according to FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 discloses in a schematic view a hydraulic system  1  that in this case is used in the supply of switchable support elements  2 . A hydraulic fluid pump  3  is shown, downstream of which are essentially an oil filter  4 , an oil cooler  5  and a directional valve  6  (here a 3/2 design). A first channel  7  to feed switchable hydraulic fluid pressure to coupling elements  8  (see FIGS. 2-4) connects to a working connection of the directional valve  6  identified by A. The directional valve  6  also has a tank connection T and a pump connection P. A second hydraulic fluid channel  9  branches off just in front of the pump connection P, and supplies hydraulic fluid pressure to a hydraulic play-compensation element  11  located in each of the valve drive elements  10 , regardless of the switch position of the directional valve  6 . 
     The directional valve  6  according to FIG.  1 is shown in a switch position in which the pressurized hydraulic fluid is disconnected from the first channel  7 , which is then connected to the tank connection T. In the second channel  9 , there is permanent hydraulic fluid pressure supplying the play-compensation elements  11 . 
     As illustrated in the introductory description, air bubbles can accumulate or oil foaming can occur in the first channel  7 . It is clear that the air bubbles tend to collect at high points geodetically. According to this invention, the air bubbles are completely, or almost completely, eliminated from the entire first channel  7  by means of a technically simple measure that is inexpensive to implement. This channel  7  runs from one side  12  (see FIGS. 2-4) of the coupling elements  8  to the pump connection A. This does a good job of eliminating the delays in switching time that are caused by the air bubbles or at least reduces them to a large degree, as is described in more detail below. 
     According to the invention, this result is accomplished by the entire first channel  7  being bled, when it is disconnected, by the second channel  9  directly at the coupling element  8  itself, namely beginning on its side  12 . This is accomplished by means of a connection  13  directly “on the spot”, according to the invention, that throttles the pressure in the second channel  9 . Of course, it is conceivable to instead run a separate hydraulic fluid line directly to the side  12  of the coupling element  8  and to not directly use the fluid that is used to supply the hydraulic play-compensation element  11 . 
     FIGS. 2 through 4 disclose valve drive elements  10  for which the above-mentioned connection  13  has been implemented. FIGS. 2A and 2B show a known switchable flat tappet  14 . One part  15  is made in the form of an annular section and another part  16  is made as a round section. The other part  16  is held within the first part  15  telescopically and moves axially with respect to it. The coupling element  8  here moves in a radial slot  17  near a base  18 . At a bottom  19  of the slot  17 , opposite the base  18 , is an annular section  20 . This sets off, together with the base  18 , one supply chamber  21 ,  22  on each side of the slot  17  for hydraulic fluid. Supply chamber  21  is fed from the first channel  7 . The other supply chamber  22  is connected to the hydraulic fluid from the second channel  9  and is used to supply the hydraulic play-compensation element  11  installed in the other part  16 . One supply chamber  21  is provided to actuate the coupling element  8 . To this end, the slot  17  has an orthogonal transfer line  23 . This is a component of a path  24  within the flat tappet  14  to feed the switching hydraulic fluid pressure from the first channel  7  and the first supply chamber  21 . 
     The connection  13  according to the invention is made here at the bottom  19  of the slot  17 . It runs between the annular section  20  and the slot  17  directly in the outer radial edge. In this way, the hydraulic fluid pressure present in the other supply chamber  22  through the second channel  9 , the purpose of which is to supply the play-equalization element  11 , can be fed to the supply chamber  21  through the connection  13  when the hydraulic fluid pressure is disconnected in the first channel  7 . The connection  13  is designed as a throttle so that the full hydraulic fluid pressure is not applied, thus shifting the coupling means  8  in worst case. Air bubbles etc, are thus bled as much as possible from the first supply chamber  21  near side  12  into the first channel  7 , which also can contain air bubbles, and go from there out into the open. 
     FIGS. 3 and 4 disclose a valve drive element  10  that is shown as a cam-following tappet  25 . Its one part  26  is also made in the form of an annular section that holds the other part  27 . The other part  27  is made to move relative to the first part  26  axially. The first part  26  has a cam contact surface  28  that is designed here as a roller. The other part  27  in turn has a seat  29  opposite the cam contact surface  28  for one end of a push rod (not illustrated). 
     In tappet  25 , there is a radial slot  30  that penetrates both parts  26 ,  27 . The coupling element  8  is held in slot  30  of the other part  27  in its decoupled state. The path  24  is thus formed here by the slot  30  in the outer part  26 . 
     The outer surface  31  of the tappet  25  moves inside a bore hole  32  in an internal combustion engine  33 . The first channel  7  for the switchable pressurized hydraulic fluid is placed radially outside in front of path  24 , which is directly adjacent to the side  12  of the coupling means  8 . The second channel  9  leads to a bore hole  32  at an axial distance away from the first channel  7 . This communicates with a radial transfer line  35  on or in part  26 . The hydraulic play-equalization element  11  is fed through this radial transfer line  35 . 
     According to FIG. 3, the connection  13  according to the invention is implemented at the outer surface  3   1  of the first part  26 . As seen to the left of the symmetrical line in FIG. 3, the connection runs from the radial transfer line  35  directly to the path  24  in front of the coupling element  8 . According to FIG. 4, on the other hand, the connection can also be located at an inner surface  36  of the first part  26 , starting at the radial transfer line  35 , and can run behind the side  12  of the coupling element  8 . 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 List of Reference Numbers and Letters 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Hydraulic System 
               
               
                 2 
                 Support Element 
               
               
                 3 
                 Hydraulic Fluid Pump 
               
               
                 4 
                 Oil Filter 
               
               
                 5 
                 Oil Cooler 
               
               
                 6 
                 Directional Valve 
               
               
                 7 
                 First Channel 
               
               
                 8 
                 Coupling Element 
               
               
                 9 
                 Second Channel 
               
               
                 10 
                 Valve Drive Element 
               
               
                 11 
                 Hydraulic Play-Compensation Element 
               
               
                 12 
                 Side 
               
               
                 13 
                 Connection 
               
               
                 14 
                 Flat tappet 
               
               
                 15 
                 Part 
               
               
                 16 
                 Part 
               
               
                 17 
                 Slot 
               
               
                 18 
                 Base 
               
               
                 19 
                 Bottom 
               
               
                 20 
                 Annular section 
               
               
                 21 
                 Supply chamber 
               
               
                 22 
                 Supply chamber 
               
               
                 23 
                 Transfer line 
               
               
                 24 
                 Path 
               
               
                 25 
                 Tappet 
               
               
                 26 
                 Part 
               
               
                 27 
                 Part 
               
               
                 28 
                 Cam Contact Surface 
               
               
                 29 
                 Seat 
               
               
                 30 
                 Slot 
               
               
                 31 
                 Outer surface 
               
               
                 32 
                 Bore Hole 
               
               
                 33 
                 Internal Combustion engine 
               
               
                 35 
                 Radial Transfer line 
               
               
                 36 
                 Inner Surface 
               
               
                 A 
                 Working connection 
               
               
                 P 
                 Pump Connection 
               
               
                 T 
                 Tank Connection