Patent Application
20120174887
The invention relates to engine braking incorporating cam operated engine braking exhaust valves open into the engine cylinders.
In a conventional “lost motion” cam operated engine compression braking system, a plurality of engine-braking cam lobes are provided on an engine cam that normally operates the engine exhaust valves. For overhead cam engines, a plurality of engine-braking followers or tappets respectively ride on the cam lobes and are at least partially contained in brake housings containing oil. Mechanical force from the camshaft lobes is converted to hydraulic pressure inside the brake housings by providing lift to the followers. The followers act as master pistons that are driven into the brake housings. Hydraulic controls that are fluidly connected to the oil in the brake housing determine whether the movement of the followers into the oil has a hydraulic pressure effect on the oil contained in the brake housings or no effect. When the hydraulic controls are activated by an engine braking signal, the movement of the followers into the brake housings increases hydraulic pressure which causes movement of slave pistons moving out of their respective brake housings. The slave pistons are operatively engaged to a engine-braking exhaust valves to open a the engine cylinders to the exhaust manifold.
A more complete description of such engine braking systems can be found for example in U.S. Pat. Nos. 5,680,841; 6,415,752 and 6,321,701 and published application US20030164153, all herein incorporated by reference to the extent the references are not inconsistent with the present disclosure.
The present inventors have recognized that the cam lobe and follower contact area is difficult to lubricate. When the loads and sliding speed at the contact area is high, significant contact temperature can be developed which reduces lubrication effectiveness. Because the contact geometry between the cam lobe and the flat face of the follower is counter-formal (i.e., unlike the conformal contact area in engine bearings), the contact pressure is much higher in the cam lobe/follower contact area and the oil film thickness is much lower than that in engine bearings.
In many early overhead cam (OHC) designs, the cam lobe and follower contact is lubricated by the oil splash from the camshaft bearings to provide adequate amount of lubricant. Many manufacturers solved the pivoted OHC cam's excessive wear problem by adopting high specification materials and the use of spray bars and even holes in the cam lobes to supply sufficient lubricant to the contact area. The additional supply not only serves to lubricate the contact area but also acts as a coolant to reduce the cam lobe/follower contact temperature.
The present inventors have recognized that Hertzian contact stress, oil entraining velocity, oil film thickness and flash temperature are all important for the cam lobe/follower friction and wear. Lubricant supply can be very important for the cam lobe/follower contact area and may change positively the behavior of the wear characteristic and affect the lubrication mechanism of the contact area.
The paper “Improvement of Lubrication for Cam and Follower” in the journal of “Tribology Transaction” Vol 42, 1999, No. 4, pp. 755-762, shows that using oil supply from a cam-lobe oil hole can improve lubrication.
The present inventors have recognized that it is difficult to provide a camshaft and lobe configuration that will provide adequate lift for engine braking and also have adequate life under the contact stress developed at the lobe to follower interface. Solutions which reduce the contact stress can result in lower hydraulic pressure being developed in the brake manifold which can limit overall engine brake performance.
One exemplary embodiment of the invention is incorporated into an engine braking system of the type wherein a cam lobe lifts a follower by direct contact, wherein the follower is at least partially contained in a brake housing containing oil. A slave piston is also at least partially contained in the brake housing, and during engine braking, movement of the follower causes the oil to move the slave piston outward of the brake housing. Movement of the slave piston opens an engine braking exhaust valve to open a flow path between an engine cylinder and an exhaust conduit. The exemplary embodiment of the invention provides an oil passage through the follower to lubricate an interface between a flat face of the follower and the cam lobe with oil from the brake housing.
As a further enhancement, the oil passage has an outlet hole that is open on the flat face of the follower outside of the contact area between the flat face and the cam lobe.
According to another enhancement, the oil passage extends axially through the follower with a first portion and with a second portion extending obliquely from the flat face to intersect with the first portion. As one example, an angle between the first portion and the second portion is about 145 degrees.
According to another enhancement, the oil passage includes a first linear portion that extends from an end of the follower along a first direction to an intersecting position within the follower and a second linear portion that extends from the flat face to intersect the first linear portion at the intersecting position, the first and second portions being non-collinear.
Advantageously, the first linear portion has a greater diameter than the second linear portion.
According to an exemplary embodiment of the present invention, the follower is arranged to ride directly on a camshaft brake lobe. High pressure lube oil in the brake housing will lubricate the follower to cam lobe interface. The high pressure lube oil should lubricate the follower to cam lobe interface and reduce lube oil flash temperature scuffing.
The oil passage can be advantageously drilled through the follower at a location that avoids the highly loaded area or contact area between the follower flat face and the cam lobe. For example, the oil passage through the follower can be offset from the axis of the follower to avoid this contact area. It is anticipated that by avoiding the contact area the outlet hole will not adversely affect the lubricating condition in the highly loaded area.
Thus, a properly placed outlet hole position can promote the lubricating oil flow for the cam lobe surface. The combination of cam stress, oil flow entrainment, flash temperature, etc. determine the life of the cam lobe-follower surface.
The optimum position of the oil passage outlet hole is related to cam rotation direction. Also, if the flat follower axially rotates, the offset hole position will rotate relative to camshaft rotating direction. Accordingly, a guide can be provided to prevent axial rotation of the follower so that the offset outlet hole will be maintained in the optimal position.
The lubricated brake cam lobe should allow higher brake pressure to be run for greater brake capability as well as longer follower/cam lobe service life.
Another aspect of the invention is not limited to cam/follower activation of engine brake valves but may be useful to any follower/cam lobe mechanism. The embodiment can be incorporated into an engine having a cam with cam lobes and a plurality of cam followers riding on respective cam lobes. The embodiment provides an enhanced lubrication arrangement for the followers and cam lobes. This aspect provides an oil passage through each of the followers to lubricate an interface between a flat face of each follower and a respective cam lobe. Advantageously, the oil passage can comprise an outlet hole on the flat face that is outside the contact area between the flat face and the cam lobe. The oil passage can include an axial portion extending from an end of each follower opposite the flat face and an oblique portion extending from the outlet hole to the axial portion.
The angle between the axial portion and the oblique portion can be about 145 degrees. The axial portion can have a diameter that is greater than the oblique portion.
Numerous other advantages and features of the present invention will be become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.
While this invention is susceptible of embodiment in many different forms, there are shown in the drawings, and will be described herein in detail, specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated.
A hydraulic control circuit 66 is in fluid communication with the oil manifold 55. Depending on whether engine braking is demanded by the vehicle operator, the hydraulic control circuit either contains the oil within the oil manifold 55, or relieves the oil manifold 55 of oil pressure to allow movement of the follower 44 into the brake housing 52 to have no hydraulic effect on the slave piston 58. Oil is supplied to the oil manifold 55 by a pump circuit 67.
Each slave piston 58 is pressed to a braking exhaust valve 70 that opens each engine cylinder 72 to an exhaust manifold 73. By opening the exhaust valves 70 to exhaust during engine braking, the engine can be effectively tasked as an air compressor to absorb vehicle kinetic energy to compress air. A more complete description of such engine brake systems can be found for example in U.S. Pat. Nos. 5,680,841; 6,415,752 and 6,321,701 and published application US20030164153, all herein incorporated by reference to the extent the references are not inconsistent with the present disclosure.
The first portion 84 defines an inlet hole 84a and the second portion defines an outlet hole 94a.
The first portion 84 can have a greater diameter than the second portion 94. The first portion 84 may be drilled into each follower 44a and the second portion 94 may be laser drilled into each follower 44a to intersect the first portion 84. Based on the reduced diameter, the flow resistance of the second portion 94 would be far greater than the first portion 84 so a precise tuning of the oil flow can be achieved by only the laser drilling of the second portion 94.
By obliquely arranging the second portion 94, the outlet hole 94a will be outside an area of contact 100 between the flat face 96 and the lobe 36 so that the presence of the hole in a high contact stress area is not detrimental to the lubrication of the interface or service life of the follower or cam lobe.
If needed, guides can be provided to prevent the followers from axially rotating so that a predetermined, constant location of the hole 94a can be ensured.
From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred.
This application claims the benefit of U.S. provisional application Ser. No. 61/246580, filed Sep. 29, 2009.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2010/036121 | 5/26/2010 | WO | 00 | 3/24/2012 |
| Number | Date | Country | |
|---|---|---|---|
| 61246580 | Sep 2009 | US |
