Abstract:
A chain drive assembly for an internal combustion engine comprises a chain tensioner ( 6 ) that acts upon a chain ( 2 ) that is used to drive the cam shafts ( 4 ) of the engine. The tensioner has a hydraulically actuated plunger ( 30 ) which is moveable within a housing and biasing means in the form of a solenoid ( 34 ), compression spring or motor that urges the plunger outwards of the housing at least when there is no hydraulic pressure. The tensioner is controlled by means of a control unit ( 8 ) that receives signals from various transducers that transmit signals providing information in relation to the performance of the engine. The control unit receives the measured information, calculates the desired tension to be applied to the chain and transmits the control signal to the chain tensioner so as to operate the biasing means or control the hydraulic pressure so that the desired chain tension is applied.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to PCT Application No. PCT/GB99/02043 filed on Jun. 29, 1999, and Great Britain Application No. 9813961.1 filed on Jun. 30, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method for tensioning the chain of an internal combustion engine and to the related chain tensioner apparatus for performing the method. 
     In an internal combustion engine a continuous loop of chain is used for driving the camshafts of an engine from the output of its crankshaft. In order to compensate for any slackness in the chain that often occurs as a result of wear, a chain tensioner is used. Generally a chain tensioner comprises a hydraulic piston that is guided longitudinally in a bore of a housing and is biased into contact with the chain by meanes of the hydraulic pressure within the bore and/or a compression spring that acts between the piston and the housing. An end of the piston that projects from the housing is fitted with a shoe that rests on the chain to apply the appropriate tensioning force. When the piston is acted upon by the hydraulic pressure and/or spring force the chain is tensioned by the shoe. The hydraulic fluid also provides damping of the piston movement during use. 
     The hydraulic fluid in the chain tensioner is generally derived from the engine supply. The compression spring is designed to apply tension to the chain when the engine is off or during engine start up as at these times there is no or little hydraulic pressure. During normal operation of the engine the chain tensioner is tensioned by both the hydraulic pressure and the spring force. In most existing systems the chain tensioner is designed to ensure that sufficient tension is applied to accommodate all operating conditions and this results in the chain being over-tensioned for significant periods of it use. Maximum tension is required at engine start-up or at periods of rapid acceleration or deceleration when significant wear and noise can occur in taking up the slack of the chain, thereafter during normal use the chain is generally over-tensioned which itself contributes to noise and wear. 
     Mechanical tensioner that use only a spring force to bias the tensioning shoe are often used on belt systems offer constant tensioning loads that may be adequate for the start-up of the engine but which are over-compensatory when the engine is idling or operating at a constant speed. 
     In chain tensioner designs where hydraulic fluid is taken from the engine supply the pressure of the hydraulic fluid may increase with engine speed thereby increasing the tension of the chain unnecessarily. This increases the wear of the chain. 
     It will be appreciated that the problem with existing chain tensioners is that they are not responsive to the condition of the engine and are unable to apply a tension force that is variable in accordance with the requirements of the chain. 
     U.S. Pat. No. 5,159,904 describes a chain tightener system in which an electric control unit records one or more parameters of the operation of the engine and, in response effects a progressive and constant variation of the angular phase of rotation of the crankshaft and camshafts of an engine. This is achieved by using a pair of chain tighteners; one on the slack side of the chain and one on the tight side, However this arrangement cannot provide control over the chain noise, wear, vibration or harshness. 
     It is an object of the present invention to obviate or mitigate the aforesaid 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a chain drive assembly for reducing noise, vibration or wear of a chain in an internal combustion engine, the assembly comprising a chain tensioner having a hydraulically actuated plunger for engaging a chain, the plunger being moveable within a housing and biasing means to bias the plunger outwards of its housing at least when there is no hydraulic pressure, means for controlling hydraulic pressure in the housing, means for measuring the projection of the plunger from the housing, means for measuring an operational parameter of the engine or drive assembly, means for calculating the desired chain tension in response to the measured operational parameter and means for transmitting a control signal to the chain tensioner so as to operate the biasing means or thee means for controlling the hydraulic pressure in the housing so as to control the projection of the plunger to apply the desired chain tension. 
     The achieve control of the chain tension not only reduces the noise etc. but provides for extended chain life. 
     The biasing means for controlling the projection of the plunger from the housing is preferably a solenoid, motor or a compression spring. 
     In one preferred embodiment the plunger is biased against a sliding member by actuation of the solenoid or motor. Hydraulic fluid may be supplied to the housing so as to act separately on the sliding member. 
     Preferably the desired chain tension is applied during operation of the chain. 
     The control signal may operate a control valve to control the flow of hydraulic fluid to the housing. The housing may have a plurality of chambers each receiving a different sized plunger member. 
     The operational parameter may be one or more of any of the following: the speed of r revolution of a crankshaft of the engine; the speed of revolution of at least one camshaft of the engine; the torque of the crankshaft; the torque of at least one of the camshafts; the relative phase of rotation of the crankshaft and at least one camshaft; the pressure of the hydraulic fluid in the chain tensioner; or the strain in the chain. 
     According to a second aspect of the present invention there is provided a method for tensioning the chain of a chain drive assembly so as to reducing noise, vibration or wear of the chain in an internal combustion engine, the drive assembly including a chain tensioner having a hydraulically, operated plunger that is moveable within a housing and biasing means to bias the plunger outwards of its housing at least when there is no hydraulic pressure, the method comprising the steps of measuring an operational parameter of the engine or drive assembly, measuring the projection of the plunger from the housing, calculating the desired chain tension in response to the measured operational parameter, transmitting a control signal to the chain tensioner so as to operate the biasing means or to change the hydraulic pressure in the housing so as to move the plunger to apply the desired chain tension. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 is a diagrammatic end view of an internal combustion engine shown with a drive chain and chain tensioner of the present invention; 
     FIG. 2 is a schematic view of a first embodiment of a chain tensioner of the present invention; 
     FIG. 3 is a schematic view of a second embodiment of a chain tensioner of the present invention; and 
     FIG. 4 is a sectioned schematic representation of a third embodiment of a chain tensioner of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1 of the drawings, there is shown in a diagramatic representation an internal combustion engine  1  of an automotive vehicle with a drive chain  2  for transmitting power from the output of a crankshaft  3  of the engine  1  to overhead camshaft  4 . 
     The drive chain  2  is in die form of an endless loop that passes around sprockets  5  attached to the ends of the crankshaft  3  and the camshafts  4 . A chain tensioner  6 , examples of which are described in detail below, maintains tension in the chain  2  via a tensioning shoe  7  throughout its use. 
     The operation of the chain tensioner  6  is controlled by a control unit  8  which includes a processor that receives information signals from various parts of the engine  1  and activates the chain tensioner  6  in response to those signals. The control unit  8  may form part of an engine management system. In the example shown in FIG. 1, signals are received from appropriate transducers associated with the crankshaft  3 , the camshafts  4 , the chain  2  itself and tie chain tensioner  6  and this information is processed to determine the appropriate tension that should be applied to the chain  2  via the chain tensioner  6 . The signals received from the crankshaft  3  and camshafts  4  may convey information relating to the torque and/or the relative angular rotational phase of the shafts. Transducers such as optical encoders, accelerometers may be used for this purpose. The chain may be fitted with strain gauges so that information relating to thy strain or load of the chain can be processed by the control unit. In addition, transducers such as proximity sensors or infra-red or laser sensors may be used to transmit signals relating to the position of a plunger of the chain tensioner or tensioning shoe to the control unit. 
     The purpose of the above arrangement is to control the chain tensioner such that an appropriate tensioning force is applied so as to that take into account the operating conditions of the drive chain and engine and to ensure that over-compensatory tension is not applied to the chain. The operation of this reactive chain tensioner ensures that undue chain wear, noise vibration or harshness is not incurred. 
     Example embodiments of the chain tensioner  6  are shown in FIGS. 2,  3  and  4 . In the embodiment shown in FIG. 2, the tensioner  6  is principally hydraulic and comprises a plunger  10  that is axially guided within a housing  11  and has an end  12  that projects from the housing  11  to make contact with the chain  2  via a tensioning shoe (not shown in FIG.  2 ). The plunger  10  has two spaced cylindrical heads  13 ,  14 , one ( 13 ) relatively large and one ( 14 ) relatively small, both being moveable with the plunger  10  relative to first and second chambers  15 ,  16  in the housing  11 . Each chamber  15 ,  16  is fed with a supply of hydraulic fluid from the engine supply via separate chamber inlet conduits  17 ,  18  in the housing  11 . Upstream of the inlets  17 ,  18  there is a control valve  19  that operates to switch the hydraulic fluid from the supply  20  to one or the other of the chamber inlets  17 ,  18 . In use, high tensioning forces are applied by the hydraulic fluid being directed by the control valve  19  into the first chamber  15  For a given hydraulic pressure a large force is applied by virtue of the large surface area of the plunger head  13  Low tensioning forces are applied by directing the hydraulic fluid into the second chamber  16  where a smaller surface area of the plunger head  14  means that a smaller force is applied to the plunger for a given hydraulic fluid pressure. This design of chain tensioner  6  enables a relatively high tension to be applied to the chain when required at low engine speeds and a relatively low tension to be applied at relatively low engine speeds. Signals from a transducer associated with the crankshaft provide information relating to the revolutions per minute of the engine and are used to determine the tension to be applied to the chain tensioner. Once the appropriate tensioning force has been calculated by the control unit a signal is sent to switch the control valve  19  to the appropriate position: It will be appreciated that whilst the tensioner  6  is shown with only two chambers it may be designed to have more so that the tension of the chain can be adjusted in finer increments according to the speed of the engine. The tensioner is also provided with means, such as a compression spring, for biasing the plunger into contact with the chain during start-up when there is little or no hydraulic pressure in the chain tensioner housing. 
     The hydraulic fluid present in the housing of the chain tensioner also serves to damp movement of the plunger, as is known. 
     In FIG. 3 there is shown a chain tensioner that is particularly suitable for controlling the tension at engine start-up of the engine. The tensioner alas a plunger  30  that is moveable axially within a housing  31 . One end  32  of the plunger  30  is received in the coil  33  of an electric solenoid  34  attached to the rear of the housing  31 , whilst the other end  35  extends into a bore  36  in a cylinder  37  that is slidably received in a chamber  38  in the housing. The chamber  38  has an inlet conduit  39  that is supplied with hydraulic fluid from the engine oil supply. In use activation of the solenoid  34  by a control signal biases the plunger  30  into the sliding cylinder  37  so as to force the cylinder  37  to project out of the housing  31  and bear against the drive chain via a tensioning shoe if necessary. The supply of fluid to the chamber  38  applies pressure to the ends of the cylinder  37  to bias it out of die housing  31 . If the engine is started after a prolonged stoppage the hydraulic pressure of the engine supply is generally not sufficient to supply the chamber  38  of the chain tensioner housing  31 . Accordingly the solenoid  34  is activated by a control signal so as to apply a tensioning force to the chain via the plunger  30  and cylinder  37 . When normal operating conditions of the engine have resumed the hydraulic pressure has risen to an extent that tensioning force can be applied to the chain by virtue of the hydraulic pressure acting on the cylinder  37  and the solenoid  34  may be switched off. This design is beneficial not only at start up but at tires of rapid acceleration or deceleration where the increase or decrease in hydraulic pressure lags the increase or decrease in speed of the engine. In such circumstances the solenoid may be operated so as to apply sufficient tensioning force during acceleration or deactivated to reduce the applied tension during deceleration. 
     A similar but more detailed design is shown in FIG.  4 . The tensioner, as before, has a plunger  40  that is axially moveable within a housing  41 . The end of the plunger  40  received in the housing  41  carries a magnet  42  which is moveable with the plunger  40  in a first chamber  43  of the housing  41  under the influence of a solenoid coil  44  that surrounds the first chamber  43 . 
     A second chamber  45  is disposed in the housing  41  between the first chamber  43  and the exposed end of the plunger  40 . The second chamber  45  is connected to a supply of hydraulic fluid by a two limbed conduit  46  that is disconnectable from the supply by a electrically operated needle valve  47 . The plunger  40  has a head  48  that resides in the second chamber  45  and serves as a hydraulic damper. 
     The tensioner is typically powered from a  12 V battery of the vehicle and the solenoid coil  44  is supplied with a variable electric current to modulate the tensioner load as required. The variable current may be either constant at a constant engine speed or alternatively may be variable to stimulate the hydraulic damper if the control is a closed loop system. The hydraulic damper can be switched on or off as desired by the electrically operated needle valve  47 . 
     The solenoid of the above embodiments may be substituted by an alternative actuator such as a stepper motor with leadscrew. In all these designs the tensioning force applies is variable in discrete steps by applying an appropriate control signal that governs the power supplied to the motor or solenoid to control length of stroke of the plunger. 
     In all designs the information signals received by the control unit are processed by a feedback control loop that may include an integrator or differentiator. Alternatively the information may be processed by using a simple look-up table to calculate the required tension to be applied. 
     It is to be appreciated that in all embodiments the tension applied to the chain can be varied during operation of the chain so that rapid responses are made to changes in the engine speed etc. 
     The provision of an active chain tensioner that reacts to the precise loading requirements as described above enables chain noise and vibration to be efficiently and effectively controlled whilst keeping the load applied to a chain to a minimum thereby reducing chain wear. This is particularly desirable during rapid changes in the engine speed or when the engine is started from cold. The chain drive thus has increased durability and by monitoring the tension applied the life of the chain can be accurately predicted. 
     It will be appreciated that numerous modifications to the above described design may be made without departing from the scope of the invention as defined in the appended claims. For example, signals can be derived from appropriate transducers do obtain other information from the engine and/or drive assembly such as the strain in or position of chain guide shoes, or the pressure of the hydraulic fluid as supplied to the chain tensioner. In the latter case the chain tensioner can be controlled to ensure that unnecessary tension is not applied when the hydraulic pressure is abnormally high. 
     Whilst the chain tensioner is described above in relation to a camshaft drive it is to be appreciated that it has application to other engine chain drives such as those used to drive an oil pump, balance shaft drives or a water pump. In these cases signals relating to torque or speed of rotation arc derived from the pump or shaft drives in order to ascertain the tension to be applied to the chain.