Abstract:
An apparatus for tensioning a chain is provided that compensates for the affect of temperature on oil viscosity by providing, in addition to a continuously open oil leakage path, an additional leakage path open only at lower temperatures when oil is more viscous and leaks at a slower rate. Thus, overall oil leakage at lower temperatures is similar to that at higher temperatures, when the additional leakage path is closed, and the less viscous higher temperature oil leaks at a faster rate from the continuously open oil leakage path. Accordingly, the affect of temperature on hydraulic stiffness of the tensioner assembly is minimized.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to an apparatus for tensioning a chain, such as on an automotive engine. 
       BACKGROUND OF THE INVENTION 
       [0002]    Chain drive tensioner assemblies are used to operate ancillary components associated with automotive engines. For example, chain drive tensioner assemblies are used to drive complex valve trains, balance shafts, oil pumps, high pressure fuel injection pumps and water pumps. 
         [0003]    Known chain drive tensioner assemblies include a chain and a chain drive tensioner system that is operable to create an initial required tension on the chain. Known chain drive tensioner assemblies generally include a tensioner body and a tensioner piston attached to a shoe. Oil pressure within a hydraulic chamber of the tensioner body exerts pressure against the tensioner piston and shoe assembly, which engages the chain to create the required tension. Under impact forces from the chain, some oil leaks from the hydraulic chamber along an oil leakdown path with an orifice of fixed size. The stiffness of the chain drive tensioner assembly is controlled by the amount of oil leakage allowed from the hydraulic chamber. Increased tensioner stiffness leads to improved chain drive control. However, as oil temperature is increased, the viscosity of oil in the hydraulic chamber is decreased, and thus more oil leaks out of the leakdown path, decreasing stiffness of the assembly. When the oil temperature is relatively low, viscosity increases and therefore less oil can leak out of the fixed orifice leakdown path, increasing stiffness of the tensioner assembly and potentially causing the tensioner to experience “pump up” (i.e., an over-extension of the piston and shoe) which decreases chain drive control. 
       SUMMARY OF THE INVENTION 
       [0004]    An apparatus for tensioning a chain is provided that compensates for the affect of temperature on oil viscosity by providing, in addition to a continuously open oil leakage path, an additional leakage path open only at lower temperatures when oil is more viscous and leaks at a slower rate. The additional leakage path is also referred to as a variable flow passage or second oil leakdown path. Thus, overall oil leakage at lower temperatures is similar to that at higher temperatures, when the additional leakage path is closed, and the less viscous higher temperature oil leaks at a faster rate from the continuously open oil leakage path. Accordingly, the affect of temperature on hydraulic stiffness of the tensioner assembly is minimized. 
         [0005]    The apparatus includes a chain drive tensioner assembly operatively connected with the chain. The chain drive tensioner has a hydraulic chamber subjected to force from the chain. A first oil leakdown path provides continuous oil leakage from the hydraulic chamber when under force from the chain and a second oil leakdown path selectively provides additional oil leakage from the hydraulic chamber under predetermined temperature conditions when under force from the chain. 
         [0006]    The chain drive tensioner assembly may include a tensioner body with a bore therein. The tensioner body defines first and second passages (i.e., the first and second leakdown paths) operatively connected with the tensioner bore. An actuator is provided that is movable in response to a predetermined temperature to block the second passage such that the first passage is operable to leak oil at both the first and second temperatures and the second passage is operable to leak oil at the first temperature but is blocked by movement of the actuator at the second temperature. 
         [0007]    In one embodiment, the actuator is a thermal wax actuator, containing thermal wax that melts at a predetermined temperature, thereby expanding to move a piston that blocks the second passage. In another embodiment, the actuator is a pulse width modulated actuator that responds to a pulse width modulated current or voltage triggered by a control signal that corresponds with sensed oil temperature. Movement of the actuator in response to the pulse width modulated current or voltage may provide intermediate actuator positions in which the second passage is only partially opened, providing more fine tuning of the amount of oil leakage from the second passage as a function of temperature. 
         [0008]    The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic perspective illustration of an internal combustion engine with a chain driven balance shaft including a first embodiment of a chain drive tensioner assembly; 
           [0010]      FIG. 2  is a schematic cross-sectional illustration of the chain drive tensioner assembly of  FIG. 1  taken at the lines  2 - 2  in  FIG. 1 , with an actuator in a first position to open an additional leakdown path under a relatively cold oil temperature; 
           [0011]      FIG. 3  is a schematic cross-sectional illustration of the chain drive tensioner assembly of  FIG. 2 , with the actuator in a second position to close the additional leakdown path under a relatively hot oil temperature; 
           [0012]      FIG. 4  is a schematic cross-sectional illustration of a second embodiment of a chain drive tensioner assembly for use with the engine of  FIG. 1 , with an electronically controlled actuator in the first position to open the additional leakdown path under the relatively cold oil temperature; and 
           [0013]      FIG. 5  is a schematic cross-sectional illustration of the chain drive tensioner assembly of  FIG. 4  with the actuator in a second position to close the additional leakdown path under the relatively hot oil temperature. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    Referring to the drawings, wherein like reference numbers refer to like components,  FIG. 1  is a schematic perspective illustration of an engine assembly  10  that includes an internal combustion engine  11  and a chain driven component, i.e., a balance shaft drive, indicated generally at  12 . 
         [0015]    An apparatus referred to as a chain drive tensioner assembly  14  tensions a drive chain  16  of the chain driven balance shaft drive  12 . The chain drive tensioner assembly  14  includes a tensioner body  18  and tensioner piston assembly  20  in contact with the drive chain  16 . 
         [0016]    It should be appreciated that, while the variable leakdown chain drive tensioner assembly  14  is applied to the chain driven balance shaft drive  12 , the variable leakdown chain drive tensioner assembly  14  may alternatively be applied to other chain driven components, which may include but are not limited to, chain drive components used to drive complex valve trains, oil pumps, high pressure fuel injection pumps and water pumps. 
         [0017]      FIG. 2  is a schematic illustration of the chain drive tensioner assembly  14  of  FIG. 1  and tensioning chain  16 . The tensioner body  18  defines a bore  22 . The tensioner piston assembly  20  has a tensioner piston portion  24  and a tensioner shoe portion  26 . 
         [0018]    The bore  22  and a tensioner piston cavity  23  of the tensioner piston assembly  20  cooperate to form a hydraulic chamber  28 , which, as discussed further below, is substantially fluid-tight and, when filled with fluid (also referred to herein as oil), is characterized by a hydraulic stiffness that substantially prevents inward movement of the tensioner piston assembly  20  when under loading by the chain  16 . 
         [0019]    The tensioner piston portion  24  of the tensioner piston assembly  20  is installed within the bore  22  of the tensioner body  18 . A spring  30  connects the tensioner body  18  and the tensioner piston assembly  20 . 
         [0020]    Hydraulic fluid is received within the chamber  28  of the tensioner body  18  from an oil supply  32 , overcoming a ball check valve  34 . When filled with hydraulic fluid, a hydraulic stiffness or tensioner reaction stiffness is created within the chamber  28 , which substantially prevents inward movement of the tensioner piston assembly  20  when under loading by the chain  16 . The oil supply  32  provides additional oil to the chamber  28  when chain loading lessens in order to replace oil that leaks from chamber  28  when under chain loading. 
         [0021]    The tensioner body  18  includes a first leakage passage  38 , also referred to as a first leakage path, in fluid communication with the chamber  28 . A leakdown disk  40  is disposed within the first leakage path  38 . The leakdown disk  40  is operable to regulate flow through the first leakage path  38  at a fixed flow rate based on pressure within the chamber  28  (i.e., dependent on the loading of the chain). However, because viscosity of oil within the hydraulic chamber  28  decreases as oil temperature increases, a greater volume of oil will flow out of the first leakdown path  38  at a higher temperature (and lower viscosity) than at a lower temperature (and higher viscosity). In typical chain drive temperature assemblies having only one leakdown path, as the leakage increases, the tensioner reaction stiffness decreases and, conversely, as the leakage decreases the tensioner reaction stiffness increases. 
         [0022]    However, to compensate for the affect of increased leakage through the first leakage passage  38  as temperature increases, the valve body  18  defines a second passage  42 , also referred to as a variable flow passage or a selective leakdown path, in fluid communication with the chamber  28 . The second passage  42  is a selective leakdown path because an actuator  44  selectively blocks flow or permits flow through the second passage  42  as a function of temperature. Specifically, in the chain drive tensioner assembly  14 , the actuator  44  is a thermal wax actuator that has thermal wax  46  confined within a piston cavity  48 . Thermal wax motors or actuators are commercially available. The thermal wax  46  is selected based on its melting temperature in relation to the desired hydraulic stiffness of the chain drive tensioner assembly  14 . At relatively low temperatures below the thermal wax melting temperature, the thermal wax is in solid form, such as is illustrated in  FIG. 2 . Thus, at a first oil temperature below the wax melting temperature, the thermal wax  46  occupies a relatively small space in the piston cavity  48 , and a movable piston  50  in contact with the wax is biased by spring  52  to a first position as shown in  FIG. 2 . In the first position of  FIG. 2 , a narrowed portion  54  of the piston  50  aligns with the second passage  42  to permit oil flow out of the chamber  28  past the piston  50 . Thus, at the first oil temperature (and at all oil temperatures below the thermal wax melting temperature), oil leakage is permitted through both the first and second passages  38 ,  42 . 
         [0023]    However, at relatively high temperatures at or above the thermal wax melting temperature, the thermal wax melts, indicated as  46 A in  FIG. 3 , expanding to occupy greater space within the piston cavity  48  and overcoming the bias of spring  52  to move the piston  50  such that the narrowed portion  54  is out of alignment with the second passage  42  and the piston  50  blocks oil flow out of the second passage  42 . Thus, at a second oil temperature at or above the wax melting temperature, as indicated in  FIG. 3 , leakage is permitted only from the first passage  38 . 
         [0024]    Referring to  FIGS. 4 and 5 , a second embodiment of a chain drive tensioner assembly  14 A is illustrated. The chain drive tensioner assembly  14 A may be used in place of chain drive tensioner assembly  14  in the engine assembly  10  of  FIG. 1 . Components of the chain drive tensioner assembly  14 A substantially identical to those of the chain drive tensioner assembly  14  are represented with identical reference numbers. In this embodiment, the actuator  44 A operable to move the piston  50  from the first position of  FIG. 4  to the second position of  FIG. 5  is a pulse width modulated actuator. The actuator  44 A is an electromechanical actuator, such as a solenoid, moving in response to an electronic control signal  56  sent along conductor wire  58  from an electronic controller  60 . The electronic controller  60  receives a sensor signal  62  from a temperature sensor  64  in communication with oil in the fluid supply  32 . The controller  60  processes the sensor signal  62  according to a stored algorithm and generates a control signal  56  corresponding to the sensed oil temperature. The sensor  64  is shown mounted in the oil supply  32 , but may be anywhere in the engine assembly where a temperature corresponding with oil temperature may be sensed, including within the chamber  28 . 
         [0025]    A power converter  68  utilizes power electronics to direct a pulse width modulated current or voltage corresponding to the control signal to a solenoid  66  within the actuator  44 A. The power electronics are connected with a power source, such as a battery (not shown), that supplies power converted to a pulse width modulated voltage or current by the power electronics. The solenoid  66  responds to the pulse width modulated voltage or current to cause linear movement of the piston  50  corresponding to the pulse width modulated current or voltage. The pulse width modulated actuator  44 A may be finely tuned to control movement of the piston  50  to multiple intermediate positions between the first position of  FIG. 4  and the second position of  FIG. 5 , where the thinned portion only partially aligns with the second passage  42  to control the passage  42  to partially open (i.e., partially blocked) states between the open state of  FIG. 4  and the blocked (i.e., closed) state of  FIG. 5 . Thus, the leakage from the second passage  42  may be finely tuned in correlation with oil temperature over a range of temperatures. 
         [0026]    While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.