Method and apparatus for automatically setting clearances in an internal combustion engine

Abstract: A method for automatically setting valve clearances in internal combustion engines (also known as "tappet setting" or "valve lash setting") comprises a series of steps in which a rocker arm (10) is set to a zero position which is recorded as a reference datum and an adjustment screw (18) is then operated to set the rocker arm to a first reference position. The adjustment screw is then rotated through a predetermined angle so that the rocker arm is moved to a second reference position. The difference between the first and second reference positions and the predetermined angle are used to determine a coefficient relating the angular movement of the adjustment screw to linear movement of the rocker arm. The coefficient is then used to calculate the angular rotation of the adjustment screw required to set a predetermined valve clearance relative to the zero position. The initial adjustment of the rocker arm position serves to neutralise backlash in the valve drive train prior to setting the valve clearance.

Description: Technical Field The present invention relates to an automated method for setting valve clearances for inlet and exhaust valves in the cylinder(s) of internal combustion engines. Background Art As is well known in the art, the operation of inlet and exhaust valves in internal combustion engines is often controlled by a rocker arm which reciprocates about a rocker shaft. A first end of the rocker arm, located on a first side of the rocker shaft, is reciprocated by a push rod connected to a cam follower, which in turn is driven by a cam mounted on a camshaft. The second end of the rocker arm, located on the second side of the rocker shaft, drives the valve stem of an inlet or exhaust valve which is spring-biased into a normally closed position. Each inlet valve and each exhaust valve has an associated rocker arm. When the valves of a particular piston are fully closed (i.e. when the piston is in its top dead centre (TDC) position on the compression stroke of a four stroke engine), a certain predetermined clearance is required between the second end of the rocker arm and the end of the valve stem which is contacted by the rocker arm in operation of the engine. This clearance must be set within fine tolerances, typically of the order of +/-2/1000 inch (0.051 mm). The process of setting this clearance is referred to herein as "valve clearance setting" and is commonly referred to in the art as "tappet setting" in the United Kingdom or "valve lash setting" in the USA. The valve clearance is adjusted by means of a threaded adjustment screw which extends through the first end of the rocker arm and is seated in a cup formed in the end of the push rod. The adjustment screw may be locked in the required position by a lock nut, or may be a friction screw or the like which does not require a lock nut. The combination of the cam, cam follower, push rod, adjustment screw, rocker arm and rocker shaft is referred to herein as the "valve drive train". Conventionally, such as in accordance with GB-A-448530, valve clearances are adjusted manually, by use of a feeler gauge which is inserted between the second end of the rocker arm and the end of the valve stem whilst manually adjusting the adjustment screw at the first end of the rocker arm. This process is labour intensive, time consuming and relatively inaccurate/inconsistent. It would clearly be desirable to automate the process of valve clearance setting. To date, however, attempts at automation have failed to deliver satisfactory results. One previously proposed method of performing automatic valve clearance setting utilises an automatic machine tool for adjusting the adjustment screw, a linear position sensor which senses the position of the second end of the rocker arm and a linear actuator having a clip member which engages the rocker arm on the second side of the rocker shaft and which is capable of pushing the rocker arm in its valve-actuating direction and pulling the rocker arm in the opposite direction. This method comprises the steps of pushing the second end of the rocker arm in its valve-actuating direction to a predetermined zero position (reference datum) in which the second end of the rocker arm contacts the end of the valve stem but does not displace it from its normally closed position, pulling the rocker arm in the opposite direction by an amount sufficient to remove all backlash from the valve drive train, and adjusting the adjustment screw against the pulling force until the position sensor indicates that the second end of the rocker arm is at a predetermined distance (the required valve clearance) from the zero position. This method has been found to be unsatisfactory in practice, failing to provide consistently accurate setting of valve clearances. The present inventors have determined that this prior method does not take sufficient account of variations in the relative positions of the various elements of the valve drive train caused by backlash in the valve drive train and movement of the rocker arm during the setting process, and does not take sufficient account of variations in the dimensions of the valve drive train elements between individual valves of an engine and between different engines. As used herein, "backlash" refers generally to clearances between adjacent, mutually coupled components and is not restricted to clearances between relatively rotatable components. The backlash in the valve drive train additionally includes backlash between the rocker shaft and its mounting pedestals. Disclosure of the Invention In accordance with the present invention, there is provided a method of setting a predetermined valve clearance in an internal combustion engine. The engine includes a rocker arm rotatably mounted on a rocker shaft for reciprocating movement relative thereto. The rocker arm has a first end located on a first side of the rocker shaft and a second end located on a second side of the rocker shaft. The first end of the rocker arm has an adjustment screw extending therethrough to act on an end of a push rod. The second end of the rocker arm is movable in a first, valve-actuating, direction and in a second direction opposite to said first direction, and has a valve engaging surface which co-operates with an end of a valve stem. The valve stem is resiliently biased in said second direction towards a first position and is movable against the bias force in said first direction towards a second position. A first step of the method comprises setting the rocker arm to a zero position in which said valve engaging surface thereof contacts the end of the valve stem without displacing the valve stem from its first position, and recording said zero position as a reference datum. A second step of the method comprises using the adjustment screw to adjust the position of said rocker arm to a first reference position in which backlash affecting the position of the rocker arm is substantially neutralised. A third step of the method comprises rotating the adjustment screw through a reference angle and recording a corresponding second reference position thereof. A fourth step of the method comprises calculating a coefficient from the difference between said first and second reference positions and said reference angle. A fifth step of the method comprises using said coefficient to calculate an angular rotation of the adjustment screw corresponding to the predetermined valve clearance. A sixth step of the method comprises rotating the adjustment screw on the basis of said calculated angular rotation to set the predetermined valve clearance relative to said reference datum. Preferably, said first step includes moving said rocker arm in said first direction to said zero position. Preferably also, the adjustment screw is rotatable in a first angular direction for movement towards the push rod and in a second angular direction for movement away from the push rod. Said second step preferably includes rotating said adjustment screw in said first angular direction such that said valve stem is displaced from its first position to a third position intermediate said first and second positions. Preferably also, said second step further includes rotating the adjustment screw in said second angular direction through a predetermined angle such that said valve stem moves from its third position to a fourth position intermediate said third and first positions. Preferably also, said third step includes rotating the adjustment screw through said reference angle in said second angular direction such that the valve stem moves from said fourth position towards said first position. Where the adjustment screw has a lock nut associated therewith, said first step preferably includes loosening said lock nut prior to setting the rocker arm to said zero position. The lock nut is preferably tightened slightly following said third step and prior to said sixth step. The lock nut is preferably tightened fully following said sixth step. In this case, the angular rotation calculated in said fifth step preferably corresponds to said predetermined valve clearance plus a correction distance representing a displacement of the second end of the rocker arm caused by said slight tightening of the lock nut. The sixth step then preferably comprises rotating the adjustment screw such that the second end of the rocker arm is displaced from the zero position in said first direction by said correction distance and the predetermined valve clearance is then set by rotating the adjustment screw through the angular rotation calculated in the fifth step. Brief Description of the Drawings Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a graph showing variations in the position of the second end of a rocker arm against the angle of rotation of a valve adjustment screw while performing a valve clearance setting operation in accordance with a preferred embodiment of the present invention; Fig. 2 is a schematic end view of part of a rocker arm assembly and an associated valve stem and of components of an automated system for setting the valve clearance in accordance with the preferred embodiment of the present invention; and Figs. 3A to 3L are a series of views similar to that of Fig. 2, illustrating the sequence of operations represented by the graph of Fig. 1. Best Mode for Carrying Out the Invention Referring firstly to Fig. 2, there is shown a rocker arm 10 rotatably mounted on a rocker shaft 12 for reciprocating movement relative thereto in'a first, valve-actuating, direction A and in a second opposite direction B. The rocker arm 10 has a first end 14 located on a first side of the rocker shaft 12 and a second end 16 located on a second side of the rocker shaft 12. The first end 14 of the rocker arm 10 has an adjustment screw 18 extending therethrough and engaging a cup 19 formed in an end of a push rod 20. In this embodiment, the adjustment screw 18 has an associated lock nut 21. It will be understood that if the adjustment screw 18 was a friction screw or the like then the lock nut 21 would not be required. The adjustment screw 18 is rotatable in a first angular direction (clockwise, in this embodiment, for a right hand thread) for downwards movement towards the push rod 20 and in a second angular direction (anti-clockwise, in this embodiment) for upwards movement away from the push rod 20. The second end 16 of the rocker arm 10 has a valve engaging surface 22 co-operating with an end 24 of a valve stem 26 which is resiliently biased in the direction B towards a first position (normally closed) and which is movable towards a second (open) position by rotation of the rocker arm 10 in the first direction A. For the purposes of performing the method of the present invention, there is provided a rocker arm actuating means, suitably a linear actuator 27 such as a pneumatic cylinder device, adapted to selectively engage the rocker arm 10 on the second side thereof so as to rotate the rocker arm 10 in the first direction A. The linear actuator 27 can be moved in and out of engagement with the rocker arm 10 and is preferably adapted to apply a predetermined force to the rocker arm 10. The linear actuator 27 may be any of a variety of known types and will not be described in detail herein. Also provided is a position sensing means, suitably a linear position sensor 28, for monitoring the position of the second end 16 of the rocker arm 10. The linear position sensor 28 may be any of a variety of known types and will not be described in detail herein. The sensor 28 should have an accuracy better than the required tolerance of the valve clearance setting, suitably of the order of +/- 0.01mm. The small range of movement of the rocker arm 10 during the valve clearance setting process is such that the arcuate movement of the rocker arm 10 about the rocker shaft 12 may be treated as linear. Also provided is an adjustment screw actuator means, suitably a machine tool 30, for rotating the adjustment screw 18 in its first and second angular directions. In this embodiment the machine tool 30 has a first, inner rotary actuating element 32 for engaging and rotating the adjustment screw 18 and a second, outer rotary actuating element 34, co-axial with the first, for engaging and rotating the lock nut 21. The first rotary actuating element 32 has associated therewith an angle sensor 36, for measuring the angular rotation of the element 32. The second rotary actuating element 34 has associated therewith a load sensor 38 for measuring the force applied to the lock nut 21 and an angle sensor 40, for measuring the angular rotation of the element 34. The machine tool 30 and its associated sensors may be any of a variety of known types and will not be described in detail herein. The machine tool 30, linear actuator 27, linear position sensor 28, and the sensors 36, 38 and 40 of the machine tool 30, are connected to a control system 42, such as a digital computer, which provides automatic control of the valve clearance setting process. Control systems of this type are well known in the art and will not be described in detail herein. The adjustment screw 18 and associated rotary actuator 32 are preferably of the Torx-head (Trade Mark) type. Industrial Applicability Figs. 1 and 3A to 3L illustrate the valve clearance setting process, which will now be described in detail. At the beginning of the process, the relevant piston of the engine is in its top dead centre (TDC) position so that the relevant valve is fully closed and the rocker arm 10 is in the correct orientation for the valve clearance setting process. The lock nut 21 is also at a pre-set position on the adjustment screw 18. As shown in Fig. 3A, the linear actuator 27 is engaged on the second side of the rocker arm 10 and operated to apply a predetermined force, less than the resilient bias force urging the valve stem 26 into its first position, to the rocker arm 10 so as to move the rocker arm 10 in the first direction A to a zero position in which the valve engaging surface 22 contacts the end 24 of said valve stem 26 without displacing the valve stem 26 from its first position. This zero position is recorded as a reference datum, using the linear position sensor 28. This is illustrated at point 50 in Fig. 1. At this point the adjustment screw 18 is also shown as having zero degrees of angular rotation. Referring to Fig. 3B, the linear actuator 27 is moved away out of engagement with the rocker arm 10. The machine tool 30 is applied to the adjustment screw 18 and lock nut 21, pushing the adjustment screw 18 into engagement with the cup 19 of the push rod 20 and at the same time displacing the rocker arm 10 and the linear position sensor 28 in the direction B, and eliminating backlash through the push rod 20 and cam follower. At this stage a check may be performed to ensure that the linear position sensor 28 has been displaced in the direction B by a pre-determined minimum value (typically of the order of 0.05 mm); i.e. that there has been a movement of the rocker arm 10. This ensures that the lock nut pre-set was correct. As shown in Fig. 3C, the outer rotary actuator 34 of the machine tool 30 is operated to unfasten the lock nut 21 by one turn, whilst the adjustment screw 18 is held at zero degrees rotation by the inner rotary actuator 32, in order to allow subsequent adjustment of the adjustment screw 18. As shown in Fig. 3D, the lock nut 21 is held while the adjustment screw 18 is rotated in its first direction until the linear position sensor 28 indicates a predetermined displacement of the second end 16 of the rocker arm 10 in the direction A, moving the valve stem 26 in the first direction to a third position intermediate its first and second positions (point 52 in Fig. 1.). The predetermined displacement is typically of the order of 2 mm, selected to be greater than or equal to a minimum value sufficient to place the valve drive train in tension with the backlash between the various drive train components biased in one direction. The value is sufficiently small that the arcuate movement of the second end 16 of the rocker arm 10 can be regarded as linear. As shown in Fig. 3E, the adjustment screw is then rotated in its second direction through a first predetermined angle, displacing the rocker arm 10 by a small amount in the second direction B (54 in Fig. 1). This predetermined angle, typically of the order of 90 degrees, is selected to be sufficient to neutralise the backlash at least between the rocker arm 10 and rocker shaft 12 and, preferably, between the adjustment screw 18 and the rocker arm 10. Generally speaking, this means that the backlash between the rocker arm 10 and the rocker shaft 12 is shifted in the opposite direction from that caused by the previous displacement of the rocker arm 10 in the direction A, moving the clearance between the rocker arm and rocker shaft from one side of the rocker shaft to the other. This takes the process to point 56 in Fig. 1. The process described thus far comprises setting a zero position (reference datum) for subsequent measurements of the linear position of the second end 16 of the rocker arm 10 and then adjusting the rocker arm position in such a way as to neutralise backlash affecting the position of the rocker arm which might compromise the accuracy of the subsequent process steps. At point 56 in Fig. 1, the linear position of the second end 16 of the rocker arm 10 relative to the zero position is recorded as a first reference position A1 (Fig. 3F). Next (Fig. 3G), the adjustment screw 18 is rotated further in its second direction through a predetermined reference angle  (suitably 360 degrees) and the corresponding rocker arm position is recorded as a second reference position A2 (point 58 in Fig. 1). Next (Fig. 3H, step 60 in Fig. 1), a coefficient X is calculated as follows: X = (A2 - A1)/ mm/degree i.e. X represents mm of linear movement of the second end 16 of the rocker arm 10 per degree of rotation of the adjustment screw 18, under the neutral backlash conditions established by the preceding adjustments of the rocker arm position. This has the effect of compensating for variables present in the valve drive train, including rocker shaft tolerances etc., and the coefficient X is specific to the particular combination of rocker arm and adjustment screw. This would not be achieved by calculating the value of X from position measurements made without previously adjusting the rocker arm position to neutralise backlash as described or by calculating X directly. from the nominal pitch of the adjustment screw 18 or the like. Next (Fig. 3I, step 62 in Fig. 1); the lock nut 21 is tightened slightly ("snugged") by a predetermined force applied by the machine tool 30. This induces a slight additional movement of the rocker arm 10 in the second direction B. To compensate for this, the adjustment screw 18 is rotated in its second angular direction until the second end 16 of the rocker arm 10 is displaced by a small predetermined correction distance d in the direction A relative to the zero position. The distance d is an arbitrary small value which is just large enough to be measured accurately by the position sensor 28, typically of the order of 0.03 mm (point 63 in Fig. 1). This step is not required if the adjustment screw does not have a lock nut. Next (Fig. 3J, step 64 in Fig. 1), the angular rotation R of the adjustment screw 18 corresponding to the linear displacement required to set the desired clearance gap C relative to the zero position is calculated as follows: R = (C + d)/X. Typical values of C might be 0.203 mm (0.008 inch) for an inlet valve and 0.457 mm (0.018 inch) for an exhaust valve. The adjustment screw 18 is then rotated in its second angular direction through the angle R to achieve the desired clearance C between the rocker face 22 and the end 24 of the valve stem 26, thus setting the required valve clearance gap (Fig. 3K), point 66 in Fig. 1). The lock nut 21 is then tightened fully by applying a predetermined force thereto. Finally, the clearance is checked using the linear position sensor 28 to ensure that the clearance is within the required tolerance relative to the zero position. (Fig. 3L, point 68 in Fig. 1). The invention thus provides a method of reliably and accurately setting a valve clearance gap in an automatic process.

Claims: A method of setting a predetermined clearance (C) in an internal combustion engine between a rocker arm (10) and a rocker arm actuated component (26), the engine having said rocker arm (10) rotatably mounted on a rocker shaft (12) for reciprocating movement relative thereto, the rocker arm having a first end (14) located on a first side of the rocker shaft (12) and a second end (16) located on a second side of the rocker shaft (12), the first end (14) of the rocker arm (10) having an adjustment screw (18) extending therethrough to act on an end of a push rod (20), the second end of the rocker arm being movable in a first, component-actuating, direction (A) and in a second direction (B) opposite to said first direction and having a component engaging surface (22) co-operating with a portion (24) of the rocker actuated engine component (26), at least a portion of the rocker arm actuated engine component (26) being biased in said second direction (B) towards a first position and being movable against the bias in said first direction (A) towards a second position; the method comprising the steps of: (a) setting the rocker arm (10) to a zero position and recording said zero position as a reference datum; (b) rotating the adjustment screw (18) to adjust the position of said rocker arm (10) to a first reference position (A1); (c) rotating the adjustment screw (18) through a reference angle (0) and recording a corresponding second reference position (A2) thereof; (d) calculating a coefficient (X) from the difference between said first (A1) and second (A2) reference positions and said reference angle (); (e) using said coefficient (X) to calculate an angular rotation (R) of the adjustment screw (18) corresponding to said predetermined clearance (C); and (f) rotating the adjustment screw (18) on the basis of said calculated angular rotation (R) to set the predetermined clearance (C) relative to said reference datum. A method as claimed in claim 1, wherein said rocker arm actuated engine component includes an engine valve having a valve stem (26), and wherein said rocker arm (10) includes a valve-engaging surface (22) cooperating with an end (24) of the valve stem(26). A method according to claim 1, in which said rocker arm actuated engine component includes a bridge connecting plural engine valves. A method according to claim 1, wherein said rocker arm actuated engine component includes a tappet of a mechanically actuated fuel injector. A method as claimed in Claim 2, wherein the step of setting the rocker arm (10) to said zero position comprises setting the rocker arm (10) to a position in which said valve engaging surface (22) of the rocker arm (10) contacts the end (24) of the valve stem (26) without displacing the valve stem (26) from its first position. A method as claimed in any preceding Claim, wherein said first reference position (A1) is a position in which backlash affecting the position of the rocker arm (10) is substantially neutralised. A method as claimed in any preceding Claim, wherein step (a) includes moving said rocker arm (10) in said first direction (A) to said zero position. A method as claimed in Claim 7, wherein the adjustment screw (18) is rotatable in a first angular direction for movement towards the push rod (20) and in a second angular direction for movement away from the push rod (20); and wherein step (b) includes rotating said adjustment screw (18) in said first angular direction such that said rocker arm (10) is displaced from its first position (A1) to a third position intermediate said first (A1) and second (A2) positions. A method as claimed in Claim 8, wherein step (b) further includes rotating the adjustment screw (18) in said second angular direction through a predetermined angle (R) such that said rocker arm (10) moves from its third position to a fourth position intermediate said third and first (A1) positions. A method as claimed in Claim 9, wherein step (c) includes rotating the adjustment screw (18) through said reference angle (R) in said second angular direction such that the rocker arm (10) moves from said fourth position towards said first (A1) position. A method as claimed in any preceding Claim, wherein the adjustment screw (18) has a lock nut (21) associated therewith, and wherein step (a) includes loosening said lock nut (21) prior to setting the rocker arm (10) to said zero position. A method as claimed in Claim 11, wherein the lock nut (21) is tightened slightly following step (c) and prior to step (f). A method as claimed in Claim 12, wherein the lock nut (21) is tightened fully following step (f). A method as claimed in Claim 12 or Claim 13, wherein said angular rotation (R) calculated in step (e) corresponds to said predetermined clearance (C) plus a correction distance (d) representing a displacement of the second end (16) of the rocker arm (10) caused by said slight tightening of the lock nut (21). A method as claimed in Claim 14, wherein step (f) comprises rotating the adjustment screw such that the second end (16) of the rocker arm (10) is displaced from the zero position in said first direction (A) by said correction distance (d) and the predetermined clearance (C) is then set by rotating the adjustment screw through the angular rotation (R) calculated in step (e). Apparatus for setting a predetermined clearance (C) in an internal combustion engine between a rocker arm (10) and a rocker arm actuated component (26), the engine having said rocker arm (10) rotatably mounted on a rocker shaft (12) for reciprocating movement relative thereto, the rocker arm having a first end (14) located on a first side of the rocker shaft (12) and a second end (16) located on a second side of the rocker shaft (12), the first end (14) of the rocker arm (10) having an adjustment screw (18) extending therethrough to act on an end of a push rod (20), the second end of the rocker arm being movable in a first, component-actuating, direction (A) and in a second direction (B) opposite to said first direction and having a component engaging surface (22) co-operating with a portion (24) of the rocker actuated engine component (26), at least a portion (24) of the rocker arm actuated engine component (26) being biased in said second direction (B) towards a first position and being movable against the bias in said first direction (A) towards a second position; the apparatus comprising: an electronic controller (42); a rocker arm actuator (27) responsive to the electronic controller (42) to selectively rotate the rocker arm (10) relative to the rocker shaft (12); a rocker arm position sensor (28) operably connected with said electronic controller (42) to record with said electronic controller (42) the position of the second end (16) of said rocker arm (10); and an adjustment screw rotator (32,34) responsive to said electronic controller (42) to selectively rotate the rocker arm adjustment screw (18); wherein said electronic controller (42) is programmed to (a) cause said rocker arm actuator (27) to set the rocker arm (10) to a zero position and record said zero position as a datum, (b) cause the adjustment screw rotator (32,34) to rotate the adjustment screw (18) to adjust the position of said rocker arm (10) to a first reference position (A1) and then rotate the adjustment screw (18) through a reference angle (), (c) record a corresponding second reference position (A2) of the rocker arm (10), (d) calculate a coefficient (X) from the difference between said first (A1) and second (A2) reference positions and said reference angle (), (e) use said coefficient (X) to calculate an angular rotation (R) of the adjustment screw (18)corresponding to said predetermined clearance (C), and (f) cause said adjustment screw rotator (32,34) to rotate the adjustment screw (18) on the basis of said calculated angular rotation (R) to set the predetermined clearance (C) relative to said reference datum.

Inventors: BATCHELOR GRAHAM P, CROZIER WILLIAM MATTHEW, BATCHELOR, GRAHAM P., CROZIER, WILLIAM MATTHEW
Applicants: PERKINS ENGINES CO LTD, PERKINS ENGINES COMPANY LIMITED
Application year: 2001
Main classification: F01L1
IPC codes: F01L1