Abstract:
The volume of lubricating oil stored in the sump of an internal combustion engine for a vehicle is in significant excess of the volume of oil circulating through the engine at any one time. The circulating oil, drawn from the sump, may be rapidly heated during its passage through the engine, but the excess volume remaining in the sump dilutes and cools the circulating oil as it returns to the sump. By separating the oil volume into a portion which is circulated through the engine and a second portion which has only limited opportunity to mix with and cool the circulating oil the circulating oil may attain its operating temperature more rapidly. Once the stored volume of oil in the engine has also reached its operating temperature the circulating oil and stored oil may be recombined.

Description:
TECHNICAL FIELD 
     This invention pertains to enhancing the efficiency of an internal combustion engine by rapid heating of circulating engine oil though preferential circulation of previously-heated oil. Mixing of the previously-heated oil and cold oil in the engine sump is discouraged through the use of a selectively-permeable screen which only promotes mixing when the sump oil attains a preselected viscosity. 
     BACKGROUND OF THE INVENTION 
     The text of this background section is to prepare the reader for understanding practices described in this disclosure. The text is not presented with a consideration of whether it discloses prior art. 
     Multi-cylinder, reciprocating piston, internal combustion engines for automotive vehicles typically contain an oil circulation system for lubrication of valves, cylinder walls, pistons, connecting rods, cranking mechanisms, and the like. Generally, a predetermined quantity of lubricating oil (e.g., four to six quarts) is stored in bucket-like sump container attached to the engine below the cranking mechanism. When the engine is operating, an oil pumping mechanism, often driven off the engine, draws lubrication oil from the sump container and pumps it upwardly over all moving engine parts. The oil is drawn through an oil pick-up or inlet tube positioned below the surface of the sump oil. The oil flows in an oil circulation path, as intended and provided, over engine parts requiring lubrication. As it completes its flow, the oil drains downwardly back into the sump container. Typically, less than half of the stored volume of oil is in circulation at any moment of engine operation. In this way an adequate supply of oil is assured despite irregular motion of the vehicle, or leakage of the oil or burning of some of the oil as it is exposed on cylinder walls. 
     The oil is heated during engine operation, often to temperatures of about 90° C. to about 110° C. and at this temperature the oil has a viscosity and flow properties well suited for lubrication of engine surfaces. But when engine operation has ceased, the stored and now quiescent oil is cooled to the ambient temperature in which the vehicle is situated. Since this temperature may be well less than about 30° C., temperature-dependent properties of the oil are often less than desired for engine operation. So the oil may be relatively cold and viscous as its circulation is commenced immediately following an engine cold start. Sometimes vehicles intended for cold climates have special oil heaters located in the sump container for keeping the oil at a desired temperature between intermittent usages of the engine. Most vehicles do not have such an oil heater. But there is a need to reheat the circulating oil for better engine operation and less engine wear. A difficulty is that the total volume of oil is considerably larger than the amount being circulated and heated by the engine at any operating moment. 
     SUMMARY OF THE INVENTION 
     In accordance with practices of this invention, the oil storage volume in the sump container is divided into two volumes using a separator which may be a thin metal sheet with many small holes or small mesh metal screen member. The size of the small holes in the sheet or the mesh openings in the screen are determined to impede flow-through by a cold viscous oil but to permit passage of the same oil heated for engine operation. 
     The sheet or screen separator member is shaped, located, and fixed in the sump container to catch and contain circulated, returning engine oil from a started and operating engine and direct it to an oil pick-up in the sump for continued circulation. The cold oil-retaining separator member is also shaped and located to enclose a volume of oil from the total stored oil volume, the enclosed volume lying between the separator and the sump walls and bottom. The circulating oil is drawn from and returned to the free volume defined by the separator. The remaining portion of the oil in the sump container volume is outside the circulated oil volume and contained within the screen member enclosed volume. For example, in a five or six quart oil capacity engine, the circulating volume of oil within the separator defined space may be about one and one-half quarts, or about 25 to 30% of the total oil volume, with the remaining cold oil contained within the enclosed volume. 
     Thus, immediately following an engine cold-start, a selected portion of the oil from the overall sump container volume is pumped upwardly into the oil circulation paths through the engine, and this volume of circulating oil is drained back into the free storage volume defined within the screen or sheet member. This smaller portion of oil is determined for adequate lubrication of the parts of the engine. But this smaller portion is also more rapidly heated by engine operation from the stored oil&#39;s ambient temperature to its preferred operation temperature, somewhat above 90° C. 
     So, during a period of a few minutes following an engine cold start, the total oil volume within the sump container has been divided by the screen or sheet member into two portions. The smaller free portion contained within an upper and central volume (with respect to the return drain path of the circulated oil) is being heated as it is circulated through the engine. The larger oil volume contained within the sump vessel, but temporarily and partially excluded from circulation by the separator member, is cooler. But the separation of the warming circulating oil from the excluded outer oil volume in the sump container is temporary. 
     The screen or shell member is formed of a metal or other suitable thermally conductive material so that heat is transferred through the member from the engine heated oil to the temporarily non-circulated oil. Further, the small screen or sheet openings of the separator become less resistant to oil flow as the oil is heated. The screen mesh opening or sheet perforations are sized to permit easy passage of heated oil (e.g., at 60° C. or higher) while slowing and impeding passage of colder oil through the perforations. It is in this way that the perforated sheet or screen member temporarily excludes much of the cold oil from the enclosed volume defined by the shell member. But some circulated and warming oil can enter the enclosed volume as it is returned to the circulating oil volume. As engine operation continues, oil flow through the perforations in the sheet member permits heating of the total oil volume, and the temporarily separated oil volumes are, in effect, recombined by easy flow of heated oil through the perforations in the separator shell member. 
     Thus, the openings in the screen or sheet are sized to permit a slow flow of relatively cold oil and to permit easy flow of hot oil. As described the function of the screen or perforated sheet is simply to permit the recirculation of a loosely confined portion of the total oil volume to hasten heating of the oil following an engine cold start. But the goal is to continually heat and circulate all of the stored oil during continued engine operation so that the screen or sheet presents only a modest resistance to flow of heated oil. The sheet serves its task mainly following an engine cold-start and reduces the time required to heat some oil to its effective lubricating temperature. Thereafter, during continued engine operation, the rest of the stored oil is heated. But the duration of the cold start period with less effective lubrication is reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows, in cross-section, an internal combustion engine schematically illustrating the circulating path followed by the lubricating oil. 
         FIG. 2  shows, in sectional view, a schematic representation of oil flow in a portion of a schematic, but representative engine oil sump with an associated oil intake. The oil in the sump is partitioned into two volumes by a temperature-sensitive separator, and the oil flow shown is representative of the initial oil flow expected during practice of the invention immediately after a cold engine start when the oil is at a temperature of less than about 60° C. 
         FIG. 3  shows a representation of the oil flow in the engine oil sump portion of  FIG. 2  after some period of engine operation during which one of the oil volumes has attained a temperature of greater than 60° C. while the bulk of the second oil volume remains at a temperature of below about 60° C. 
         FIG. 4  shows a representation of the oil flow in the engine oil sump portion of  FIGS. 2 and 3  after both oil volumes have attained a temperature of greater than about 60° C. 
         FIGS. 5A and 5B  show two exemplary separators.  FIG. 5A  shows a perspective view of a portion of a sheet separator incorporating a plurality of openings;  FIG. 5B  shows a woven mesh separator. Both separators are suited to prevent or restricting passage of lubricating oil at a temperature of less than about 60 20   C. while allowing passage of lubricating oil at temperatures of greater than 60° C. 
         FIGS. 6A-C  show, in cross-section, several orifice configurations and indicate the difference in flow capacity of these orifice configurations. 
         FIG. 7  shows a representative curve showing the difference between the circulating oil temperature in an engine adapted for practice of the invention and a conventional engine with a conventional sump after a cold start. The result illustrates the increase in oil temperature and hence the reduction in viscosity and associated fuel economy enhancement obtainable through practice of the invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Lubricating oils in internal combustion engines, in common with most liquids, become less viscous as their temperature increases. Although such oils commonly include, as part of a more extensive additive package, a viscosity modifier, this will only reduce, not eliminate, the extent of the viscosity reduction. Hence an oil formulated to develop an appropriate viscosity for effective lubrication at normal engine operating temperatures of 90-110° C. or so will exhibit a higher viscosity as the engine, and its lubricating oil, is warming to its steady-state operating temperature after a cold start. This higher viscosity results in increased friction and reduced vehicle fuel economy during the 800-1200 seconds or so required for the engine to reach its operating temperature. It is an object of this invention to mitigate the negative impact of cold starts on vehicle fuel economy. 
       FIG. 1  shows, in schematic cross-section, an engine  5  suitable for use in a motor vehicle. Oil  11  is stored in sump  10  for delivery to the engine. Oil is withdrawn from sump  10  at oil pick-up  14  under the urging of oil pump  15  and flows as flow  22  through tube  18  to filter  17 . After passing through filter  17  the oil is distributed under pressure by an appropriate arrangement of channels and orifices to all regions of the engine requiring lubrication. These lubricated elements include rocker arm  13  and the valve system located at the topmost location of the engine as well as bearings  19 ,  21  and  23 . After performing its lubrication function the oil returns to sump  10  as droplets  24  under the influence of gravity. 
     An exemplary embodiment of the invention is shown in  FIG. 2  which is a schematic sectional view of a portion of a sump  10  of an internal combustion engine like that shown in  FIG. 1 , showing the oil pick-up  14  contained within oil pan  12 . Oil fills the oil pan  12  to a predetermined level  16  and oil entering oil pick-up  14  is conveyed to the engine through tube  18  by an oil pump (not shown). Oil flow within the sump is shown by arrows  20  and the aggregated oil flow within tube  18  by arrows  22 . Heated oil which has previously passed through the engine and is returning, under the action of gravity to sump  10  is shown as droplets  24 . Individual droplets may also deposit on some of the unseen sump surfaces and consolidate into flow  26  directed into the oil pan. 
     In an embodiment the oil in oil pan  12  is sequestered into two layers  28  and  30  by separator  32 . Separator  32  is a generally planar and horizontally mounted below the oil surface indicated by oil level  16 . Separator  32  has an opening surrounding oil pick-up  14  allowing upper oil layer  30  free access to pick-up  14 . The opening is bounded by a downwardly extending flange  40  extending to inner bottom surface  36  of oil pan  12 . It is intended that separator  32  seal against the surfaces of oil pan  12  wherever the perimeter edges or flange edges of the separator contact the oil pan to prevent passage of oil from one volume to the other at the oil pan interior surfaces. Lower oil layer  28  is contained between the inner surface  34  of separator  32  and the inner bottom surface  36  and the sidewalls  38  of oil pan  12 . It will be appreciated that the respective volumes of upper oil layer  30  and lower oil layer  28  may not be readily estimated from this figure since the lateral extent of the oil pan, shown in the section, is much less than its longitudinal extent. Thus the volume of oil accessible to oil pick-up  14  is disproportionately emphasized in lateral section. 
     Separator  32  comprises a plurality of openings in a thin sheet or a fine mesh screen. Commonly such a sheet would be metal, but any material which may be fabricated as a thin sheet and not react with hot oil or any of the fuel or water-based or other impurities in the oil pan would be suitable. However it is preferred that the separator possess good thermal conductivity to promote heat flow from heated oil on one side of the separator to colder oil on the other side. Thus metallic separators may be commonly used. Such separators may be fabricated of those metals and alloys, optionally coated, currently in use for oil pans since these have clearly demonstrated durability in an engine oil environment. 
     An exemplary arrangement of orifices in a sheet is shown in  FIG. 5A . Commonly such orifices may be circular in plan view as shown, but alternate geometries, such as ovals, slits or regular or irregular polygons may be employed provided at least one dimension of the orifice does not exceed a characteristic dimension. The characteristic dimension is selected so that the orifices severely impede the flow of higher viscosity oil, that is oil at a temperature of about 60° C. or less, but enable flow of the same oil at a temperature of greater than about 60° C. or so when it is in a lower viscosity state. The particular characteristic dimension will vary with the particular lubricating oil but will generally range from about 100-300 micrometers. An exemplary polygonal opening will generally obtain in woven wire mesh separators such as that shown in  FIG. 5B  in which openings  76  of dimension ‘d’ are defined by the spacings between arrays of interwoven arrays of orthogonal wires  72 ,  74 . Opening shapes other than the generally square openings shown in  FIG. 5B  may be developed under more complex weaves. 
     Referring to  FIG. 5A  it will be noted that orifices  60 , only some of which are shown extending through sheet  62  for clarity, are arranged in a hexagonal arrangement highlighted at  64 . This particular arrangement enables close packing but it is intended to be illustrative rather than limiting and other arrangements of the orifices may be used without limitation. The orifices are shown as spaced apart to avoid unduly weakening supporting sheet  62 . As shown the orifices may be spaced apart by a distance ‘d’ substantially equal to the diameter ‘d’ of the orifices but other suitable spacings may also be used. 
     The area density of orifices should be sufficient to enable an oil flow rate substantially equal to or greater than the oil flow rate through the engine. As an example, an array of orifices 200 micrometers in diameter arranged as shown in  FIG. 5A  on a separator with an area of 100 square inches or so, may pass up to about 30 gallons per minute under a 1 inch head. This flow rate is sufficient for a high performance V8 engine for a sports car. The more open weave mesh of  FIG. 5B  enables yet greater flow. 
     The flow characteristics of the interface may be enhanced by shaping the exit geometry of the orifice. The calculated results referred to above were representative of the orifice of  FIG. 6A , that is an orifice in a very thin sheet of thickness (indicated as ‘t’ in  FIG. 5A ) of less than one quarter of the orifice diameter. For the 200 micrometer orifice discussed above this would imply that the sheet be a foil of 50 micrometers or so. Such a foil may require that it be mounted on a frame or similar support structure to support the loads which might be applied to it, for example by sloshing oil on cornering or abruptly stopping the vehicle. It may be noted that use of this thin foil exacts a flow performance penalty of about 25% over the use of the thicker sheet shown in  FIG. 6B . 
     Increasing the sheet thickness to between two and three times the orifice dimension as shown in  FIG. 6B  enables, for a two hundred micron orifice, a sheet thickness of between 0.2 and 0.3 millimeters enabling the sheet to be self-supporting eliminating and eliminating any need for a frame or support structure. As noted, the thicker sheet enables greater fluid flow than the thinner sheet. While such theory is not relied on it appears that the extended channel length may result in a more organized flow pattern and induce less backpressure. 
     Yet further modification of the orifice, while maintaining the same exit diameter, is shown in  FIG. 6C . Again the orifice extends to between two and three times the orifice dimension, but, in addition, the orifice inlet is tapered, resulting in a smoother flow transition and a further increase in flow by about 18% over the straight-sided orifice of  FIG. 6B . For ease of manufacture, preferably the tapered geometry of  FIG. 6C  is developed on a foil, as in  FIG. 6A , again raising the issues of the mechanical stability of the foil under applied loads. Also, as will become apparent in consideration of the oil flow paths the direction in which the flared section extends from the sheet may need to be modified consistent with the anticipated oil flow paths. 
     The straight-sided orifices of  FIGS. 6A and 6B  may be made by drilling using conventional microdrills or by spark machining or laser drilling. The orifice geometry of  FIG. 6C  may be developed by piercing and flaring using a tapered point cylindrical punch which will, when the point penetrates the sheet, flare the surrounding material provided the sheet&#39;s ductility is sufficient to resist flange cracking 
     The influence of separator  32  on the oil flow paths in the oil pan  12  may be appreciated by consideration of  FIGS. 2, 3 and 4  which are illustrative of the evolution in oil flow path as the engine, after a cold start, progressively heats up to its operating temperature. 
     As illustrated in  FIG. 2 , immediately after cold start up, the oil of lower oil layer  28 , at a temperature of less that 60° C. is initially prevented from accessing oil pick-up  14  by separator  32 . Oil pick-up  14  therefore draws oil substantially exclusively from upper layer oil  30  conveying it to the engine (not shown) as aggregated flow  22  under the urging of an oil pump (not shown). The oil, now heated after its passage though the engine, returns to the sump as droplets  24  and consolidated flow  26 . The oil of upper oil layer  30 , though warmed by the engine-heated returning oil remains below 60° C. and so substantially cannot pass through separator  32 . Oil in upper oil layer  30  therefore flows parallel to the surface of separator  32  as indicated by arrows  20  and returns to oil pick-up  14  without significantly mixing with the oil of lower oil layer  28 . The individual oil flows  20 , on converging at the oil pick-up  14  are aggregated into oil flow  22  and conveyed into engine. 
       FIG. 3  is illustrative of the oil flow at a later stage in the engine warm-up. The oil of the upper oil layer  30  upper layer continually heated by returning heated returning oil droplets  24  and returning consolidated oil flow  26  achieves a temperature of about 60° C. or so at which it may pass through separator  32 . However, because of its lower density than the cooler oil of lower oil layer  28 , the preponderance of flow is still parallel to separator  32  as indicated by arrows  20 . But passage of heated oil flow  20  serves to warm separator  32  and elevate the temperature of some volume of the oil of lower oil layer  30  in contact with inner surface  34  of the separator. When the temperature of the volume of oil in contact with inner surface  34  is sufficient to enable flow through separator  32  some volume of oil from lower oil layer  28  may pass though separator  32  as flow  120  to merge and mingle with flow  20  as it merges into aggregated flow  22 . The volume of oil corresponding to flow  120  may be replaced by leakage of some oil from the upper oil layer into lower oil volume  28  via flow  20 ′. Continued engine operation will further elevate the temperature of the oil of upper oil layer  30  and promote further heating of, and flow into and out of lower oil volume  28 . 
     When all oil, in both the upper and lower oil layers, achieves a temperature above about 60° C. or so, rendering separator  32  fully permeable to all of the oil, the flow will be as shown in  FIG. 4 . Flow  20  in upper oil layer  30  will continue but the volume of flow  20 ′ from the upper oil layer  30  to lower oil layer  28  and the volume of flow  120  from the lower oil layer to the upper oil layer will both increase, promoting full circulation and engaging all the oil in the sump. 
     The effectiveness of this approach is shown in  FIG. 7 , a representative curve illustrating the difference in oil temperature with time after cold start resulting from practice of this invention. The curve shows the difference in circulating oil temperature recorded for an engine with an oil pan containing a separator as described and an engine with a conventional oil pan without a separator. In both cases the oil attains its normal operating temperature about 800-1000 seconds after cold start, leading to a temperature differential of substantially zero. But the engine with the separator enables a rise in circulating oil temperature during the warmup period. The temperature difference rise develops immediately after start-up and increases rapidly to a maximum value of about 10-12° C. at about 200 seconds or so after engine start, before starting to decline as the circulating oil in both engines progresses to its steady-state normal operating temperature after about 800-1000 seconds or so. 
     The relative partitioning of the total oil volume may depend on the specifics of a particular engine but the volume should be informed by the need to not starve the engine of oil during warm-up, particularly during the first 10-20 seconds after start-up. During this initial period the gravitational return flow of the still-cool, viscous oil to the sump may be delayed resulting in an initial circulating oil volume which is greater than would occur at steady-state. 
     The volume of oil participating in engine lubrication should also be informed by its ability to temporarily accept and hold contaminants, such as water and unburned fuel, from the combustion chamber, which blow by the piston rings. Such contaminants may exist as vapors in a hot engine and be eliminated by the positive crankcase ventilation system of the engine. In cold engine and during warm-up they will condense and temporarily dissolve and be dispersed in the cold oil. Thus another constraint on the oil volume partition effected by the separator is that the circulating oil volume be sufficient to accommodate the oil contaminants produced on cold start without prejudicing its lubricating properties. All of these requirements may be met if the sump is so partitioned as to enable an initial circulating oil flow of at least one and one-half quarts. This will correspond to about 25 to 30% of the total oil volume in a conventional engine whose normal oil requirement is for five or six quarts. 
     While preferred embodiments of the invention have been described as illustrations, these illustrations are not intended to limit the scope of the invention.