Abstract:
An internal combustion engine includes at least one cylinder block having at least one combustion cylinder. A number of cylinder liners are respectively associated with each combustion cylinder. Each cylinder liner defines a corresponding cylinder inside surface. Each cylinder liner includes a plurality of discrete oil retaining indentations in a predefined pattern on the cylinder inside surface. The plurality of oil retaining indentations are bounded in both peripheral and longitudinal directions of the corresponding cylinder liner.

Description:
FIELD OF THE INVENTION 
   The present invention relates to internal combustion engines, and, more particularly, to lubrication of combustion cylinders in such engines. 
   BACKGROUND OF THE INVENTION 
   In an internal combustion (IC) engine, when a piston reciprocally moves in sliding contact within the cylinder, friction and wear are most severe at top dead center (TDC) and bottom dead center (BDC) of the stroke positions of the piston. At these exact points where the piston changes direction, a condition of zero velocity occurs causing a reduction in hydrodynamic oil film thickness which can lead to metal-to-metal contact. This condition is most severe at TDC because lubricating oil on the surface is exposed to combustion temperatures which may cause unfavorable changes in its viscosity. This condition renders it more difficult to retain oil in the pores of the metal surfaces, accelerating the oil film diminishment rate subsequent to when the liner is wiped clean by the oil wipe rings situated beneath the combustion rings of the piston. 
   It is known to hone the inside surface of a combustion cylinder to produce scratches that retain lubricant oil. For example, a commonly used plateau honing operation provides deep scratches extending entirely around the inside surface of the combustion cylinder that retain lubricant oil. A second honing operation provides a smooth finish for the piston ring and piston to ride on. The deep scratches are not well controlled and are not conducive to the build up of a good squeeze film or hydrodynamic oil film. 
   What is needed in the art is an internal combustion engine providing improved oil lubrication of the combustion cylinders and reduced oil consumption. 
   SUMMARY OF THE INVENTION 
   The present invention provides an internal combustion engine including a combustion cylinder having an inside surfaces which is ablated to have discrete pock marks which vary in density distribution along the length of the combustion cylinder. 
   The invention comprises, in one form thereof, an internal combustion engine including at least one cylinder block having at least one combustion cylinder. A number of cylinder liners are respectively associated with each combustion cylinder. Each cylinder liner defines a corresponding cylinder inside surface. Each cylinder liner includes a plurality of discrete oil retaining indentations in a predefined pattern on the cylinder inside surface. The plurality of oil retaining indentations are bounded in both peripheral and longitudinal directions of the corresponding cylinder liner. 
   An advantage of the present invention is that the discrete indentations formed as pock marks better hold oil than conventional scratches formed in the inside surface of a combustion cylinder. 
   Another advantage is that the discrete indentations decrease radiation and convection heat transfer, thereby reducing volotization and pyrolysis. 
   Yet another advantage is that the discrete indentations provide lower oil consumption, longer particulate trap life and better performance. 
   A still further advantage is that the discrete indentations provide lower friction and wear resulting in longer engine life and better fuel economy. 
   A still further advantage is that multiple ablation manufacturing methods may be used to form the discrete indentations in the inside surface of the cylinder liner. 
   A further advantage is that the discrete indentations may be formed with precise uniformity and spacing. 
   Another advantage is that the improved oil lubrication using discrete indentations provides very low emissions levels and reduces contamination of after treatment devices. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a schematic, sectional view of a portion of a cylinder block of an internal combustion engine, illustrating an embodiment of discrete indentations of the present invention; 
       FIG. 2  is a graphical illustration of a relationship between piston position, speed and density distribution of the discrete indentations of the present invention; 
       FIGS. 3A and 3B  are top and side representations of one embodiment of a discrete indentation of the present invention; 
       FIGS. 4A and 4B  are top and side representations of another embodiment of a discrete indentation of the present invention; and 
       FIGS. 5A and 5B  are top and side representations of yet another embodiment of a discrete indentation of the present invention. 
   

   Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, and more particularly to  FIG. 1 , there is shown a portion of an IC engine  10  of the present invention. IC engine  10  generally includes a cylinder block  12 , cylinder liner  14 , piston  16  carrying a pair of piston rings  18 , and connecting rod  20  interconnecting piston  16  with a crankshaft (not shown). It will be appreciated that although IC engine  10  is shown with a single cylinder block  12  carrying a single cylinder liner  14 , IC engine  10  typically includes multiple cylinder blocks  12 , with each cylinder block carrying multiple cylinder liners defining multiple cylinders. 
   Piston  16  is reciprocally movable within cylinder liner  14  between a TDC position and a BDC position, indicated generally in  FIG. 1 . Connecting rod  20  in known manner is reciprocally connected to the crank shaft and pivotally connected to piston  16  via a pin (not shown), such that connecting rod  20  moves through an angular arc upon reciprocating movement of piston  16  within cylinder liner  14 . 
   At the TDC position and the BDC position, piston  16  reverses reciprocating movement within cylinder liner  14 , and thus reaches a piston speed of zero at the TDC and BDC positions. When moving from the TDC position to the BDC position, or vice versa, piston  16  is accelerated and reaches a maximum piston speed approximately at the center of the piston stroke. 
   From an oil lubrication perspective, the worst case position of piston  16  during a piston stroke is at the TDC position, whereat piston  16  is at a zero traveling speed and the operating temperate is the highest as a result of combustion at or near the TDC position. High combustion gas pressures at TDC apply high loads to the piston rings, decreasing the oil film thickness. The traveling speed of piston  16  as a result of the piston position within cylinder liner  14  is graphically illustrated in  FIG. 2 . 
   Cylinder liner  14  includes an inside surface  22  against which piston rings  18  slide. As described above, inside surface  22  is conventionally formed with a plurality of generally annularly extending deep scratches which retain oil for lubrication of piston  16  and rings  18 . Since the scratches are typically formed with a specified honing operation, the scratches extend around the entire periphery of inside surface  22 . Control of the exact position of the scratches is not easily accomplished, since the exact positioning of the scratches depends upon the rotational speed, axial feed rate and characteristics of the honing tool. 
   In contrast, the present invention forms a plurality of discrete oil retaining indentations at inside surface  22  of cylinder liner  14 , which are preferably in the form of pock marks. Rather than extending around the entire periphery of inside surface  22  as is the case with annularly extending scratches, pock marks  24  are bounded in both peripheral and longitudinal directions of cylinder liner  14 . 
   According to another aspect of the present invention, the distribution density of pock marks  24  is dependent upon a longitudinal position on cylinder liner  14 . Since the oil lubrication needs are greater at the TDC and BDC positions, pock marks  24  have a distribution density which is greater at the longitudinal ends of cylinder liner  14  and less at the longitudinal middle of cylinder liner  14 . In other words, the distribution density of pock marks  24  is greater at the TDC and BDC positions. A distribution density which is greater at the TDC and BDC positions and less at the middle position of cylinder liner  14  is shown in  FIG. 1 , and illustrated graphically in  FIG. 2 . 
   In the embodiment shown in  FIG. 1 , pock marks  24  are formed with a generally spiral pattern on inside surface  22  of cylinder liner  14 . The spiral pattern has a lesser pitch at the longitudinal ends of cylinder liner  14  (corresponding to the greater distribution density), and a greater pitch at the longitudinal middle of cylinder liner  14  (corresponding to the lesser distribution density). The exact angular pitch of course depends upon the desired distribution density, and varies from one application to another. 
   Pock marks  24  have a generally dot shape as shown in  FIG. 1 , and illustrated in more detail in  FIGS. 3A and 3B . Each dot shaped pock mark has a depth of between approximately 5 to 20 microns, preferably approximately 10 microns. Additionally, each dot shaped pock mark has a diameter of between approximately 50 to 100 microns. Dot shaped pock marks with these dimensions have been shown to be effective in retaining oil for lubrication of piston  16  and rings  18 . 
   Cylinder liner  14  may be formed from any suitable liner material allowing formation of pock marks  24 , such as iron, steel, etc. Liners formed from iron are typically much more common than liners formed from steel, since iron includes graphite pockets which retain oil for lubrication. With the present invention, steel liners can also be used since pock marks  24  likewise retain oil for lubrication. Steel liners have the advantage of being much stronger than iron liners. 
   In another embodiment illustrated in  FIGS. 4A and 4B , indentations or pock marks  24  have an elliptical shape with a bottom surface which tapers in the running direction of piston  16 . For example, pock marks near the TDC position may have an elliptical shape with a major axis extending parallel to the longitudinal axis of cylinder liner  14 , and a bottom surface which tapers toward the upper end of cylinder liner  14  so that a squeeze film of oil is created near the TDC position for maximum lubrication of rings  18 . 
     FIGS. 5A and 5B  illustrate another example of a pock mark  24   b  which is configured to create a squeeze film of lubricating oil in both directions, such as may be desirable near the longitudinal middle of cylinder liner  14  during reciprocating movement of piston  16 . 
   During manufacture, cylinder liner  14  is formed with pock marks  24  as described above, prior to being pressed within cylinder block  12 . In one embodiment, pock marks  24  are formed on inside surface  22  using a photolithography ablation process, similar to a photolithography ablation process used on ceramics in the micro-electronics industry. In general, a photosensitive layer is placed on inside surface  22  and exposed to light to remove portions of the photosensitive layer. The light preferably is produced by a laser which is targeted at selected locations on inside surface  22  where the pock marks are desired to be formed. The laser can be controllably movable to aim the laser at the selected pock mark locations. Alternatively, cylinder liner  14  can be rotated and moved in a longitudinal direction relative to a stationary laser which is then actuated at selected locations to remove portions of the photosensitive layer. The cylinder liner is then exposed to an etching agent, such as an acid, to remove material from inside surface  22  at selected pock mark locations. The dot shaped pock mark shown in  FIGS. 3A and 3B  may be formed using a photolithography or other suitable chemical etching manufacturing process. 
   In another embodiment, pock marks  24  may be formed using a laser ablation process in which the laser is configured to actually remove material from inside surface  22  of cylinder liner  14 . This type of ablation process may be more suitable for forming the custom shaped pock marks as shown in  FIGS. 4A and 4B , and  5 A and  5 B. 
   In the embodiment shown and described above, oil retaining indentations are formed in the inside surface of a cylinder in an IC engine. However, it should be understood that such oil retaining indentations may be formed in other reciprocating piston and cylinder arrangements, such as a fluid compressor (e.g., air compressor). 
   Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.