Abstract:
In an internal combustion engine with variable actuation valves, each variable actuation valve is actuated by an actuator assembly including an actuating piston slidably mounted in a guide bushing. Between the actuating piston and the stem of the respective valve is interposed an auxiliary hydraulic tappet comprising a first bushing and a second bushing positioned inside the first bushing in such a way as to define a first chamber between the second bushing and the actuating piston, and a second chamber between the two bushings of the hydraulic tappet. The first chamber is fed a pressurised chamber of the engine lubrication loop. A check valve controls a communication between the two chambers of the tappet, to allow the passage of fluid in the direction of the second chamber. The first bushing of the auxiliary hydraulic tappet is positioned outside the guide bushing of the actuating piston, so that said bushing can be dimensioned with a relatively small diameter, regardless of the outer diameter of the auxiliary hydraulic tappet.

Description:
SUMMARY OF THE INVENTION  
       [0001]     The present invention relates to internal combustion engines with multiple cylinders, of the type comprising:  
         [0002]     at least an intake valve and at least an exhaust valve for each cylinder, each provided with respective elastic return means which bias the valve towards a closed position, to control respective intake and exhaust conduits,  
         [0003]     at least a camshaft, to actuate the intake and exhaust valves of the engine cylinders by means of respective tappets,  
         [0004]     in which at least each intake valve has variable actuation, being actuated by the respective tappet, against the action of the aforesaid elastic return means, by the interposition of hydraulic means including a pressurised fluid chamber, into which projects a pumping piston connected to the tappet of the intake valve,  
         [0005]     said pressurised fluid chamber being able to be connected by means of a solenoid valve with an exhaust channel, in order to uncouple the variable actuation valve from the respective tappet and cause the rapid closure of the valve by effect of the respective elastic return means,  
         [0006]     electronic control means for controlling each solenoid valve in such a way as to vary the time and travel of opening of the variable actuation valve as a function of one or more operative parameters of the engine,  
         [0007]     in which the aforesaid hydraulic means further comprise an actuation assembly for each variable actuation valve, including an actuating piston slidably mounted in a guide bushing,  
         [0008]     said actuating piston facing a variable volume chamber communicating with the pressurised fluid chamber both through first communication means controlled by a check valve which allows only the passage of the fluid from the pressurised fluid chamber to the variable volume chamber, and through second communication means which allow the passage between the two chambers in both directions;  
         [0009]     in which said hydraulic means further comprise hydraulic braking means able to cause a narrowing of said second communication means in the final phase of closure of the engine valve,  
         [0010]     in which between the actuating piston of each variable actuation valve and the stem of the intake valve is interposed an auxiliary hydraulic tappet, in which said auxiliary hydraulic tappet comprises:  
         [0011]     a first bushing having an end wall in contact with one end of the stem of the variable actuation valve,  
         [0012]     a second bushing slidably mounted within said first outer bushing and having an end in contact with a corresponding end of said actuating piston,  
         [0013]     a first chamber defined between said second bushing and said actuating piston, which is in communication with a passage for feeding the pressurised fluid to said first chamber,  
         [0014]     a second chamber defined between said first bushing and said second bushing, and  
         [0015]     a check valve which controls a passage in a wall of said second bushing to allow the passage of fluid only from said first chamber to said second chamber of said auxiliary hydraulic tappet.  
         [0016]     An engine of the type specified above is described and illustrated for example in European patent application 1 344 900 A2 by the same Applicant.  
         [0017]     In engines of this type, it is important that the closing movement of each valve, determined by the elastic means associated with the valve when the pressurised chamber of the actuation system is discharged, be as fast as possible, and then to be braked in the final phase of the valve travel by the aforesaid hydraulic braking means. This requirement is particular important when starting the engine at low temperature. However, there are limits to the possibility of making the closing phase of the valve substantially instantaneous, which derive in particular from the mass of the moving members, from the load of the elastic means which return the valve to the closed position and from the viscosity of the fluid (the engine lubricating oil) used in the hydraulic system. To increase the closing speed of the valve, it would in particular be advantageous to minimise the diameter of the aforesaid variable volume chamber which is defined by the actuating piston of the valve within the related guide bushing, since said chamber must be, emptied of oil during the return movement of the actuating piston caused by the closing of the valve. However, in known solutions, here too there is a limit to the possibility of reducing said diameter, since the inner diameter of the guide bushing of the actuating piston must be sufficient to house the aforesaid auxiliary hydraulic tappet which is interposed between the actuating piston and the stem of the valve. If a tappet of any conventional type available on the market is to be used, the diameter of said tappet cannot be reduced beyond a certain limit.  
         [0018]     To eliminate or at least reduce said drawbacks, the present invention relates to an engine of the type indicated at the start of the present description, characterised in that said first bushing of the auxiliary hydraulic tappet is mounted outside the guide bushing of the actuating piston.  
         [0019]     Thanks to said characteristic, in the engine according to the invention the dimensioning of the inner diameter of the guide bushing of the actuating piston of the valve becomes completely independent from the outer dimension of the aforesaid auxiliary hydraulic tappet. It is thus possible, in particular, to adopt a guide bushing of the actuating piston with a smaller inner diameter than the outer diameter of said auxiliary hydraulic tappet. Therefore, it is possible considerably to reduce the diameter of said variable volume chamber with respect to known solutions, with consequent possibility of greatly accelerating the valve closing motion. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The invention shall now be described with reference to the accompanying drawings, provided purely by way of non limiting example, in which:  
         [0021]      FIG. 1  is a section view of a prior art engine, of the type described for example in European Patent EP 0 803 642 B1 by the same Applicant, which is shown herein to illustrate the fundamental principles of a variable actuation system of the valves,  
         [0022]      FIG. 2  is a section view in enlarged scale of an auxiliary hydraulic tappet associated with an intake valve of an engine of a type similar to that of  FIG. 1 , as previously proposed in the European Patent application EP 1 344 900 by the Applicant,  
         [0023]      FIG. 3  is a schematic section view of an auxiliary hydraulic tappet in an engine according to the present invention,  
         [0024]      FIG. 4  is a similar view to  FIG. 3 , showing an embodiment example, and  
         [0025]      FIG. 5  shows a diagram that shows the advantages of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     With reference to  FIG. 1 , the internal combustion engine described in the prior European patent application EP A 0 803 642 by the same Applicant is a multi-cylinder engine, for instance an engine with four cylinders in line, comprising a cylinder head  1 . The head  1  comprises, for each cylinder, a cavity  2  formed in the base surface  3  of the head  1 , defining the combustion chamber, into which end two intake conduits  4 ,  5  and two exhaust conduits  6 . The communication of the two intake conduits  4 ,  5  with the combustion chamber  2  is controlled by two intake valves  7 , of the traditional mushroom type, each comprising a stem  8  slidably mounted in the body of the head  1 . Each valve  7  is returned towards the closed position by springs  9  interposed between an inner surface of the head  1  and an end cup  10  of the valve. The opening of the intake valves  7  is controlled, in the manner described below, by a camshaft  11  rotatably mounted around an axis  12  within supports of the head  1 , and comprising a plurality of cams  14  for actuating the valves  7 .  
         [0027]     Each cam  14  which controls an intake valve  7  cooperates with the washer  15  of a tappet  16  slidably mounted along an axis  17  which, in case of the example illustrated in the aforementioned prior document, was directed substantially at  900  relative to the axis of the valve  7 . The tappet  16  is slidably mounted within a bushing  18  borne by a body  19  of a pre-assembled assembly  20  incorporating all the electrical and hydraulic devices associated with the operation of the intake valve, as described in detail below. The tappet valve  16  is able to transmit a bias to the stem  8  of the valve  7 , in such a way as to cause the opening thereof against the action of the elastic means  9 , by means of pressurised fluid (typically oil from the engine lubrication loop) present in a pressure chamber C, and a piston  21  mounted slidably in a cylindrical body constituted by a bushing  22  which is also borne by the body  19  of the subgroup  20 . In the known solution shown in  FIG. 1 , the pressurised fluid chamber C associated to each intake valve  7  can be placed in communication with the exhaust channel  23  by means of a solenoid valve  24 . The solenoid valve  24 , which can be of any known type, suited to the function illustrated herein, is controlled by electronic control means, schematically designated by the number  25  according to signals S indicative of engine operating parameters, such as the position of the accelerator pedal and the number of engine revolutions per minute. When the solenoid valve  24  is opened, the chamber C comes in communication with the channel  23 , so the pressurised fluid present in the chamber C flows into said channel and an uncoupling is obtained of the cam  14  and of the respective tappet  16  from the intake valve  7 , which then rapidly returns to its closed position under the action of the return spring  9 . By controlling communication between the chamber C and the outlet channel  23 , it is therefore possible to vary at will the time and opening stroke of each intake valve  7 .  
         [0028]     The outlet channels  23  of the various solenoid valves  24  all end in a same longitudinal channel  26  communicating with pressure accumulators  27 , only one whereof is visible in  FIG. 1 . All the tappets  16  with the associated bushings  18 , the pistons  21  with the associated bushings  22 , the solenoid valves  24  and the related channels  23 ,  26  are borne and formed in the aforesaid body  19  of the pre-assembled set  20 , to the advantage of the rapidity and ease of assembly of the engine.  
         [0029]     The exhaust valves  70  associated to each cylinder are controlled, in the embodiment illustrated in  FIG. 1 , in traditional fashion, by a respective cam shaft  28 , by means of respective tappets  29 , although in principle, both in the case of the prior document mentioned above, and in the case of the present invention, an application of the variable actuation system to command the exhaust valves is not excluded.  
         [0030]     Also with reference to  FIG. 1 , the variable volume chamber defined inside the bushing  22  by the piston  21  (which in  FIG. 1  is shown in its minimum volume condition, the piston  21  being in its upper top stroke end position) communicates with the pressurised fluid chamber C through an opening  30  obtained in an end wall of the bushing  22 . Said opening  30  is engaged by an end nose  31  of the piston  21  in such a way as to obtain a hydraulic braking of the motion of the valve  7  in the closing phase, when the valve is near the closed position, since the oil present in the variable volume chamber is forced to flow into the pressurised fluid chamber C passing through the play existing between the end nose  31  and the wall of the opening  30  engaged thereby. In addition to the communication constituted by the opening  30 , the pressurised fluid chamber C and the variable volume chamber of the piston  21  communicate with each other by means of internal passages formed in the body of the piston  21  and controlled by a check valve  32  which allows the passage of fluid only from the pressurised chamber C to the variable volume chamber of the piston  21 .  
         [0031]     During the normal operation of the prior art engine illustrated in  FIG. 1 , when the solenoid valve  24  excludes the communication of the pressurised fluid chamber C with the exhaust channel  23 , the oil present in this chamber transmits the motion of the tappet  16  imparted by the cam  14  to the piston  21  that commands the opening of the valve  7 . In the initial phase of the opening movement of the valve, the fluid coming from the chamber C reaches the variable volume chamber of the piston  21  passing through an axial hole  30  drilled in the nose, the check valve  32  and additional passages which place in communication the inner cavity of the piston  21 , which has tubular shape, with the variable volume chamber. After a first displacement of the piston  21 , the nose  31  comes out of the opening  30 , so the fluid coming from the chamber C can pass directly into the variable volume chamber through the opening  30 , which is now free. In the inverse movement of closure of the valve, as stated, during the final phase the nose  31  enters into the opening  30  causing the hydraulic-braking of the valve, to prevent any impacts of the body of the valve against its seat.  
         [0032]      FIG. 2  shows the device described above in the modified form which was proposed in the previous European Patent application EP 0 1 344 900 by the same Applicant.  
         [0033]     In  FIG. 2 , the parts in common with  FIG. 1  are designated by the same reference number.  
         [0034]     A first evident difference of the device of  FIG. 2  with respect to that of  FIG. 1  is that in the case of  FIG. 2 , the tappet  16 , the piston  21  and the stem  8  of the valve are mutually aligned along an axis  40 . This difference does not fall within the scope the invention, as it is already contemplated in the prior art. Similarly, the invention would also apply to the case in which the axes of the tappet  16  and of the stem  8  were to form an angle between them.  
         [0035]     Similarly to the solution of  FIG. 1 , the tappet  16 , with the related washer  15  which co-operates with the cam of the camshaft  11  is slidably mounted in a bushing  18 . In the case of  FIG. 2 , the bushing  18  is screwed into a threaded cylindrical seat  18   a  obtained in the metal body  19  of the pre-assembled set  20 . A sealing gasket  18   b  is interposed between the bottom wall of the bushing  18  and the bottom wall of the seat  18   a.  A spring  18   c  returns the washer  15  in contact with the cam of the camshaft  11 .  
         [0036]     In the case of  FIG. 2  also, as in  FIG. 1 , the piston  21  is slidably in a bushing  22  which is received in a cylindrical cavity  32  obtained in the metallic body  19 , with the interposition of sealing gaskets. The bushing  22  is held in the condition mounted by an end threaded ring nut of the cavity  32  and which presses the body of the bushing  22  against an abutment surface  35  of the cavity  32 . Between the locking ring nut  33  and the flange  34  is interposed a Belleville washer  36  to assure a controlled axial load to compensate for the differential thermal expansions between the different materials constituting the body  19  and the bushing  22 .  
         [0037]     The main difference of the prior art solution shown in  FIG. 2  and the one, also known, of  FIG. 1  is that in this case the check valve  32  which allows the passage of pressurised fluid from the chamber C to the chamber of the piston  21  is not borne by the piston  21  but rather by a separate element  37  which is fixed relative to the body  19  and it superiorly closes the cavity of the bushing  22  within which is slidably mounted the piston  21 . Moreover, the piston  21  does not have the complicated conformation of  FIG. 1 , with the end nose  31 , but it is shaped as a simple cup-like cylindrical element, with a bottom wall facing the variable volume chamber which receives pressurised fluid from the chamber C through the check valve  32 .  
         [0038]     The element  37  is constituted by an annular plate which is locked in position between the abutment surface  35  and the end surface of the bushing  22 , as a result of the tightening of the locking ring nut  33 . The annular plate has a central cylindrical projection which serves as a container for the check valve  32  and which has an upper central hole for the passage of the fluid. In the case of  FIG. 2  as well, the chamber C and the variable volume chamber delimited by the piston  21  communicate with each other, as well as through the check valve  32 , through an additional passage, constituted by a lateral cavity  38  obtained in the body  19 , a peripheral cavity  39  defined by a flattening of the outer surface of the bushing  22 , and by an opening (not showing in  FIG. 2 ) of greater size and a hole  42  of smaller size obtained radially in the wall of the bushing  22 . These openings are shaped and mutually arranged in such a way as to achieve operation with hydraulic brake in the final closing phase of the valve, for when the piston  21  has obstructed the opening of greater size, the hole  42  remains free, which intercepts a peripheral end throat  43  defined by a circumferential end groove of the piston  21 . To assure that the aforesaid two openings correctly intercept the fixed passage  38 , the bushing  34  must be mounted in a precise angular position, which is assured by an axial pin  44 . This solution is preferred with respect to the arrangement of a circumferential throat on the outer surface of the bushing  22 , for this would entail an increase in the oil volumes in play, with consequent drawbacks in operation. A calibrated hole  320  is also provided in the element  37 , which directly places the annular chamber defined by the throat  43  in communication with the chamber C. Said hole  320  assures correct operation at low temperature, when the fluid (engine lubrication oil) is very viscous.  
         [0039]     In operation, when the valve needs to be opened, pressurised oil, bias by the tappet  16 , flows from the chamber C to the chamber of the piston  21  through the check valve  32 . As soon as the piston  21  has moved away from its upper end stop position, the oil can then flow directly into the variable volume chamber through the passage  38  and the two aforesaid openings (the larger one and the smaller one  42 ), bypassing the check valve  32 . In the return movement, when the valve is near its closed position, the piston  21  intercepts first the large opening and then the opening  42  determining the hydraulic braking. A calibrated hole can also be provided in the wall of the element  37  to reduce the braking effect at low temperatures, when the viscosity of the wall would cause excessive slowing in the movement of the valve.  
         [0040]     As is readily apparent, the main different with respect to the solution shown in  FIG. 1  is that the operations for fabricating the piston  21  are much simpler, since said piston has a far less complicated conformation than the one contemplated in the prior art. The solution according to the invention also allows to reduce the oil volume in the chamber associated with the piston  21 , which allows to obtain a regular closing movement of the valve, without hydraulic bounces, a reduction in the time required for closing, a regular operation of the hydraulic tappet, without pumping, a reduction in impulsive force in the springs of the engine valves and reduction in hydraulic noise.  
         [0041]     An additional characteristic of the prior art solution shown in  FIG. 2  is the provision of a hydraulic tappet between the piston  21  and the stem  8  of the valve. The tappet  400  comprises two concentric slidable bushings  401 ,  402 . The inner bushing  402  defines with the inner cavity of the piston  21  a chamber  403  which is fed a pressurised fluid through passages  405 ,  406  in the body  19 , a hole  407  in the bushing  22  and passages  408 ,  409  in the bushing  403  and in the piston  21 .  
         [0042]     A check valve  410  controls a central hole in a frontal wall borne by the bushing  402 .  
         [0043]     In regard to the present invention,  FIG. 3  shows a schematic section view of the end wall of the actuating piston  21  of a variable actuation valve and the related guide bushing  22 , as well as the auxiliary hydraulic tappet  400  associated with the actuator assembly constituted by the piston  21  and by the bushing  22 . As  FIG. 3  clearly shows, the main different with respect to the prior art solution illustrated in  FIG. 2  is that in this case the auxiliary hydraulic tappet  400  is completely positioned outside the actuator assembly of the variable actuation valve. More specifically, the first bushing  401  of the auxiliary hydraulic tappet  400  is not positioned inside the guide bushing  22 . Thanks to this characteristic, the dimensioning of the guide bushing  22  is completely independent of the dimensions of the auxiliary hydraulic tappet  400 . This is an advantage, since, if a hydraulic tappet of any conventional type available on the market is to be used, the outer diameter of said tappet cannot be reduced beyond a certain limit. On the other hand, there is an advantage, as discussed at the start of the present description, in reducing the diameter of the guide bushing  22 , since said reduction in diameter entails a reduction in the quantity of oil which must flow out of the variable volume chamber defined inside the guide bushing  22  from the upper end of the piston  21  when the engine valve has to close. It is thereby possible to obtain a substantial reduction in the closing time of the valve, with consequent advantages in terms of the efficient operation of the engine, with respect to the prior art solution illustrated in  FIG. 2 .  
         [0044]     With reference again to  FIG. 3 , the inner chamber  403  of the hydraulic tappet is fed with oil from the engine lubrication oil in similar fashion to the one illustrated in  FIG. 2 . The oil coming from a feeding channel  405  ( 2 ) reaches a circumferential chamber  406  ( 3 ) defined by an outer peripheral throat of the guide bushing  22 . From said circumferential chamber  406 , the oil flows, through a radial hole  407  obtained in the wall of the guide bushing  22  into a, peripheral chamber  408  defined by a circumferential throat of the outer surface of the piston  21 . Thence the oil passes into the chamber  403  through a radial hole  409  obtained in the wall of the piston  21 . The communication between the chamber  403  defined between the piston  21  and the bushing  402 , and the chamber  411  defined between the two bushings  401 ,  402 , is controlled by the check valve  410 , subjected to the action of the return spring  412 . The operation of the actuator assembly  21 ,  211  and of the auxiliary hydraulic tappet  400  is wholly similar to the one described above with reference to prior art solutions.  
         [0045]     In the case of the solution illustrated in  FIG. 3 , both bushings  401 ,  402  constituting the auxiliary hydraulic tappet  400  are positioned outside the guide bushing  22  of the actuator piston  21 .  
         [0046]      FIG. 4  shows a variant, wholly similar, in principle, to the solution of  FIG. 3 , which differs therefrom in that only the bushing  401  of the auxiliary hydraulic tappet  400  is positioned outside the guide bushing  22 , whilst the bushing  402  is mounted within it. Otherwise, the solution shown in  FIG. 4  differs from the solution shown only schematically in  FIG. 3  solely in some constructive details.  FIG. 4  also partially shows the upper end of the stem  8  of the valve with the respective return valve  9  and the respective end element  10  for bearing the spring  9 .  
         [0047]      FIG. 5  is a diagram that shows the advantages of the invention. It illustrates the displacement X of the engine valve in the closing phase, as the angle of the drive shaft changes in three different situations. Diagrams A and B refer to the case in which, all other dimensions being equal, the inner diameter of the guide bushing  22  of the piston is respectively 11 mm (diagram A) and 9 mm (diagram B). The solution A substantially corresponds to the one illustrate in  FIG. 2 , while the solution B becomes possible thanks to the present invention, because of the positioning of the auxiliary hydraulic tappet  14  outside the valve actuator assembly. As is readily apparent, the angle of rotation of the drive shaft required to obtain the complete closing of the valve is substantially reduced in the case of the present invention.  
         [0048]     Naturally, a determining factor influencing the closing speed of the valve is the ratio between the narrow passage area of the solenoid valve ( 24 ,  FIG. 1 ) through which the oil present in the chamber of the actuator assembly returns into the low pressure area ( 23 ,  FIG. 1 ) and the area of the chamber of the actuator assembly, defined by the upper end of the piston  21  inside the guide bushing  22 . The diagram C shows the situation of an ideal actuator, in which the ratio between said areas is equal to 1. Obviously, this solution cannot be achieved in practice, but it is interesting to note that, thanks to the invention, a closing speed of the valve is obtained (diagram B) that is not much lower than the ideal solution represented by diagram C.  
         [0049]     Naturally, without altering the principle of the invention, the construction details and the embodiments may be widely varied relative to what is described and illustrated purely by way of example herein, without thereby departing from the scope of the present invention.