Hydraulic lash adjuster with an open ended top plunger surface

A hydraulic lash adjuster mechanism for an internal combustion engine having a body portion with a bore formed in the body portion having a bottom surface. A plunger having a top surface is slidingly received within the bore of the body portion. The plunger has an internal channel with a bleed hole formed therein connecting the top surface of the plunger to a high pressure chamber formed between the bottom surface of the bore and the bottom of the plunger. The body portion has a valve opening formed therein that is in fluid communication with an engine fluid supply. A check valve mechanism selectively opens and closing the valve opening in response to pressure differences between the engine fluid reservoir and the high pressure chamber. The diameter and length of the bleed hole and the force applied to said top surface of said plunger control the leak down rate of the hydraulic lash adjuster to eliminate lash in the engine valve train components.

TECHNICAL FIELD 
The present invention relates generally to hydraulic lash adjusters. More 
specifically, the present invention relates to hydraulic lash adjuster 
mechanism for internal combustion engines having a bleed hole formed 
through the top surface of the plunger allowing the leak down rate to vary 
under a full range of operating conditions. 
BACKGROUND 
Hydraulic lash adjusters are well known for use in internal combustion 
engines. Lash adjusters are typically used to eliminate clearance or lash 
between engine valve train components that can result from varying 
operating conditions. Hydraulic lash adjusters are used to maintain engine 
efficiency, to reduce engine noise, and minimize wear on the valve train. 
Hydraulic lash adjusters operate by transmitting the energy of a valve 
actuating cam through hydraulic fluid trapped in a pressure chamber 
beneath a plunger in the lash adjuster body. During each operation of the 
cam, as the length of the valve actuating components vary (due to 
temperature changes, for example), small quantities of hydraulic fluid are 
permitted to enter or escape from the pressure chamber. As the hydraulic 
fluid enters or escapes the pressure chamber (leak down), the position of 
the plunger is adjusted and consequently the effective total length of the 
valve train is adjusted which minimizes or eliminates the lash. 
Conventional hydraulic lash adjusters have a leak down rate controlled by 
precise clearance between two concentric tubes, namely, the plunger and 
the outer cylinder, such as disclosed in U.S. Pat. No. 5,622,147. The leak 
down rate is controlled by a leak path located between the outer periphery 
of the plunger and the inner wall of the lash adjuster body. Since the 
leak down rate of these prior lash adjusters depends on the magnitude of 
the gap between the two concentric tubes raised to the third power, slight 
changes in dimensions can have a large effect on the leak down rate. As a 
result, these tubes typically are provided with a lapped or polished 
finish and are matched to provide a leak path of the appropriate 
dimensions to ensure the required accuracy in leak down rate. The process 
used to provide tubes with these precise dimensions in order to achieve 
the desired accuracy is an expensive process. 
To properly minimize lash, the leak down rate must be sufficiently fast so 
that as the engine valve heats and expands, the lash adjuster can relax 
and accommodate the expansion. If the lash adjuster does not accommodate 
the engine valve expansion, the engine valve may not seat completely. The 
inability of a lash adjuster to accommodate engine valve expansion could 
potentially cause engine problems such as loss of power output and deposit 
buildup on the engine valve stem. These problems can be exacerbated with 
new engine designs that heat the catalyst more rapidly causing the engine 
exhaust valves to also quickly heat and expand. 
While hydraulic lash adjusters typically can increase their length quickly, 
they require more time to shrink, which is a function of the oil viscosity 
and temperature. For example, as the engine's oil gets cooler and more 
viscous, the leak down rate decreases. However, the engine valve train 
growth rate is at its maximum during the initial warm up from a cold 
start. Thus, only the minimum leak down rate is available at the time the 
maximum leak down rate is required. Current lash adjusters are unable to 
provide the required leak down rate during the initial warm up from a cold 
start. 
Similarly, a leak down rate that is too fast can cause a hydraulic lash 
adjuster to relax sufficiently during a single cycle causing the cam 
follower to lose contact with the cam. Under this circumstance, the engine 
valve could potentially slam shut, causing noise which is most evident 
under hot idle conditions. Since the leak down rate varies with engine 
fluid viscosity, both the grade of engine fluid used and the temperature 
will affect the leak down rate, with the result that there may not be a 
single leak down rate setting that is satisfactory under all conditions. 
For example, as the engine's oil gets hotter and less viscous, the leak 
down rate increases. However, the engine valve train growth rate is at its 
minimum during hot running conditions. Thus, the maximum leak down rate is 
available at the time the minimum leak down rate is required. Therefore, 
the current lash adjuster mechanisms do not adequately compensate for lash 
under all engine parameters and conditions. 
SUMMARY OF THE INVENTION 
The present invention is directed to overcoming one or more of the problems 
as set forth above. It is an object of the present invention to provide a 
lash adjuster with variable leak down rates to accommodate rapid engine 
valve stem growth, such as experienced during a fast warm up from a cold 
start. 
It is yet another object of the present invention to provide a lash 
adjuster mechanism for minimizing engine valve lash during virtually all 
engine conditions. 
According to the present invention, the foregoing and other objects are 
attained by providing an improved hydraulic lash adjuster for an internal 
combustion engine. The lash adjuster has a body portion having a bore 
formed therein that terminates at a bottom surface. A plunger having a top 
surface is slidingly received within the bore. The plunger top surface has 
a bleed hole formed through it. A high pressure chamber is formed in the 
body portion between the bottom surface of the bore and the bottom of the 
plunger. The body portion has a valve opening formed therein that is in 
communication with an engine fluid supply apart from the lash adjuster. 
The valve opening is in communication with a check valve mechanism that 
selectively opens and closes the valve opening in response to pressure 
differences between the engine fluid supply and the high pressure chamber. 
The top surface of the plunger is in communication with a cam follower. The 
cam follower has a cup formed therein that communicates with the bleed 
hole. As more force is applied to the cam follower, the engine fluid film 
thickness between the cup and the plunger ball end reduces and slows down 
the leak rate. The leak rate is thus controlled by the diameter of the 
bleed hole and the force applied to the top surface of the plunger. 
Additional objects and features of the present invention will become 
apparent upon review of the drawings and accompanying detailed description 
of the preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 1, 2, and 3 illustrate a preferred embodiment of a lash adjuster in 
accordance with the present invention. The lash adjuster 10 includes a 
body member 12 in which a bore 14 is formed therein. The bottom of the 
bore 14 is defined by a bottom bore surface 16. A plunger 18 is 
telescopically positioned within the bore 14, such that the plunger 18 can 
move with respect to the body member 12. The plunger 18 is preferably in 
communication with a engine valve actuated cam 20 through a cam follower 
22 which limits the motion of the plunger 18 away from the bottom bore 
surface 16. 
The plunger 18 has a high pressure chamber 24 formed between its bottom 
surface 26 and the bottom bore surface 16. The high pressure chamber 24 
has a valve opening 28 preferably formed through the bore bottom surface 
16. The valve opening 28 is in communication with a check valve 30, 
preferably a ball valve for selectively opening and closing the valve 
opening 28. The high pressure chamber 24 is defined by the area between 
the bottom surface 26 of the plunger 18, the bottom bore surface 16, the 
inner periphery of the lash adjuster body 12, the periphery of the 
internal channel 54 in the plunger 18, and the bleed hole 56 in the 
plunger 18. It should be understood that the high pressure chamber 24 can 
be positioned in other places and is not limited to the preferred area. 
For example, the bleed hole 56 may be located at the bottom end of the 
internal channel 54 in the plunger in which case the internal channel 54 
would not form part of the high pressure chamber 24. 
The high pressure chamber 24 is preferably sealed at the interface between 
the lash adjuster body 12 and the plunger 18 by the seal 32. The seal 32 
prevents any leakage of engine fluid from the high pressure chamber 24 
between the plunger 18 outer surface and the inner periphery of the valve 
body 12. The seal 32 can be any commercially available seal, including one 
made out of Teflon or other suitable material. The use of the seal 32 
eliminates the need for a precision fit between the plunger and the 
cylinder, thus reducing manufacturing costs. The use of a seal 32, 
although preferred, is not necessary to achieve the objects of the present 
invention. 
The high pressure chamber 24 receives an engine fluid, such as oil or the 
like, which enters the lash adjuster 10 through a fluid passageway 34 and 
valve opening 28. The fluid passageway 34 receives fluid from the engine's 
oil galleries. It should be understood that a low pressure reservoir is 
not required in the disclosed lash adjuster, as the configuration of the 
disclosed lash adjuster can quickly clear any air which enters the high 
pressure chamber. This is because of the disclosed flow through design. 
However, a low pressure reservoir or chamber can be machined into the 
lower portion of the body member 12 or into the engine cylinder head 
itself, if a low pressure reservoir is deemed necessary or is desired. 
After entering the lash adjuster, the engine fluid passes from the fluid 
passageway 34 to the valve opening 28. When the check valve 30 is in the 
position shown in FIG. 2, fluid is prevented from flowing into the high 
pressure chamber 24. Conversely, when the check valve 30 is in the 
position shown in FIG. 1, engine fluid flows from the fluid passageway 34 
through the valve opening 28 and into the high pressure chamber 24. 
The check valve 30 preferably includes a ball valve member 40 that is of a 
diameter large enough to seal off the valve opening 28. The ball valve 
member 40 is preferably biased by a first spring member 42 into a closed 
position wherein the check valve 30 is normally closed and engine fluid is 
prevented from flowing from the high pressure chamber 24 back through the 
valve opening 28. The first spring member 42 is maintained in contact with 
the ball valve member 40 by a platform member 44. The platform member 44 
is generally M-shaped in cross-section with a peripheral foot portion 46 
and an upper surface 48. 
The peripheral foot portion 46 rests on the bottom bore surface 16 while 
the upper surface 48 lies generally parallel to the bottom bore surface 16 
and has an downwardly extending protrusion 50 which limits the travel of 
the ball valve member 40 with respect to the valve opening 28. A second 
spring member 52 is interposed between the bottom surface 26 of the 
plunger and the peripheral foot portion 46. The second spring member 52 
maintains the peripheral foot portion 46 and thus the platform member 44 
in contact with the bottom bore surface 16. 
The high pressure chamber 24 is in fluid communication with an internal 
channel 54. The internal channel 54 extends between the high pressure 
chamber 24 and a bleed hole 56. It should be understood that the bleed 
hole 56 may be located at the top of the internal channel 54, or at the 
bottom of the internal channel 54 or at any location along the length of 
the internal channel 54. Alternatively, the bleed hole 56 may extend along 
the entire length of the internal channel 54. In operation, engine fluid 
passes from the high pressure chamber 24 through the internal channel 54 
and out the bleed hole 56. The rate at which engine fluid passes through 
the bleed hole is dependent upon a number of factors, including engine 
fluid viscosity, and the diameter and length of the bleed hole 56 and the 
oil film thickness between the plunger ball end 61 and the cam follower 
cup 62. 
The cam follower 22 is in communication with an engine valve 70. The cam 
follower 22 has a cup portion 62 that is separated from the plunger ball 
end 61 by an engine fluid film 63. The thickness of the engine fluid film 
63 regulates the flow of engine fluid from the lash adjuster 10. 
In operation, the plunger 18 is moved within the lash adjuster body 12 by 
the second spring member 52 to extend the plunger 18 and by the engine 
valve spring 91 to retract the plunger 18. During the engine valve 
actuation or lift event, the check ball 40 is seated in the valve opening 
28 preventing engine fluid from flowing from the fluid passageway 34 to 
the high pressure chamber 24. At this time, the force from the cam 
follower 22 is applied to the top of the plunger 18 via the engine fluid 
film 63 which in turn reduces the fluid film thickness and hence the leak 
down rate. The higher the force applied by the cam follower 22, the slower 
the leak down rate. 
When the cam is on base circle, the check ball 40 will be unseated from the 
valve opening 28 when the pressure in the fluid passageway 34 exceeds the 
pressure in the high pressure chamber 24 by an amount sufficient to 
overcome the force applied by the first spring member 42. The engine oil 
gallery pressure and flow through the valve opening 28 combined with the 
force from the spring 52 will lift the plunger 18 upwardly toward the cam 
follower 22. Engine fluid will also flow from the high pressure chamber 
24, through the internal channel 54, and out the bleed hole 56. The rate 
of flow is controlled by the pressure in the high pressure chamber, the 
diameter and length of the bleed hole 56, and by the engine fluid film 63 
thickness between the ball end 61 of the plunger 18 and the cup 62 in the 
cam follower 22. 
When a light load is applied to plunger 18 by the cam follower 22, for 
example, when the first spring member 42 is causing the check valve 30 to 
almost close off the valve opening 28, the leak down through the bleed 
hole (FIG. 1) is at its fastest rate. At this point, the engine fluid film 
63 thickness between the cup 62 and the plunger ball end 61 is relatively 
large and engine fluid can leak down as shown by the arrows 80 in FIGS. 1 
and 2. The engine fluid that exits the bleed hole 56 falls back into the 
cylinder head and eventually to the engine sump (not shown). Conversely, 
when a high force is applied to the plunger 18, for example when the force 
is at a point to start opening the engine valve 70, the leak down rate 
will be at its slowest rate. This is because the cup 62 in the cam 
follower 22 and the ball end 61 of the plunger 18 will be squeezing the 
engine fluid film 63, thus reducing its thickness and hence increasing the 
resistance to engine fluid flow through the bleed hole 56. 
The diameter and length of the bleed hole 56 control the maximum leak rate, 
but not the operating leak rate. The operating leak rate is controlled by 
the engine fluid film thickness between the top surface of the ball end 61 
of the plunger 18 and the cup 62 into which it fits. The maximum leak rate 
controls how fast the lash adjuster will grow. The lower the maximum leak 
rate, the faster the lash adjuster can grow. The operating leak rate 
controls how fast the lash adjuster will contract. The faster the leak 
rate, the faster the lash adjuster will contract. By choosing suitable 
dimensions, this lash adjuster can be optimized for the desired growth and 
contraction characteristics. An added benefit of the configuration of the 
present invention is that any air that is introduced into the high 
pressure chamber 24 by the engine fluid supply system will bleed out of 
the high pressure chamber 24 quickly due to the flow through nature of the 
design. 
It should be understood that the leak down rate can be varied. The leak 
down time of a lash adjuster is proportional to the engine fluid's 
viscosity, the diameter of the plunger 18, the leak path length, and is 
inversely proportional to the pressure change across the leak path and the 
cube of the leak path clearance. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof; therefore, the 
illustrated embodiments should be considered in all respects as 
illustrative and not restrictive, reference being made to the appended 
claims rather than to the foregoing description to indicate the scope of 
the invention.