Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority pursuant to 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 60/583,001 filed Jun. 25, 2004, which is hereby incorporated by reference for all purposes. 

   FIELD OF THE INVENTION 
   The present invention generally relates to a piston engine cooling system. More specifically, the present invention relates to a system for cooling a piston engine by passing lubrication oil through one or more piston heads so as to efficiently transfer heat away from the engine. 
   BACKGROUND 
   Piston engines, and in particular internal combustion engines, are often cooled using lubrication oil. This is conventionally achieved by spraying lubrication oil onto the piston to facilitate heat transfer between the piston head and the sprayed lubricant. The heated oil then flows down to a sump from where it is recycled by a pressurized lubrication system. In a dry-sump lubrication system, the sump flow is first scavenged to a storage tank which is usually located remotely from the sump itself. 
   Such heat transfer, however, is inefficient, as the contact time between the piston and the oil spray is short. Moreover, the small contact area at the rear face of the piston also hampers efficient heat transfer. Due to these inefficiencies, a relatively large volume of oil spray having a high flow rate is required to cool the piston. This large volume of oil having a high flow rate requires additional components such as larger-than-necessary oil storage tanks, thereby reducing the engine&#39;s power-to-weight ratio and increasing the manufacturing and operational costs of the engine. 
   Some systems, however, teach a closed-loop oil system in which lubrication oil flows through the crankshaft, the connecting rod, and the piston. There are a number of drawbacks associated with such systems. First, lubrication oil does not make sufficient contact with the piston for a sufficient length of time to efficiently remove heat from the piston. Second, flow channels within different pistons are typically serially connected such that lubrication oil heated by a preceding piston is used for cooling a subsequent piston. Therefore, the lubrication oil cooling different pistons has different temperatures. Accordingly, heat transfer between a piston and the lubrication oil is not uniform across the engine. This causes thermal gradients and strains within the engine potentially leading to the formation of cracks, etc. 
   In light of the above, it would be highly desirable to provide an efficient cooling system for a piston engine while maintaining a high power-to-weight ratio and reducing costs. 
   SUMMARY 
   The present invention provides a piston cooling system that injects lubrication oil into a cooling chamber in the piston head of a piston engine. The cooling chamber includes a tortuous flow channel that is configured to increase the contact surface between the lubrication oil flowing through the cooling chamber and the piston head and prolong the contact time period during which the lubrication oil contacts the piston head. Lubrication oil is injected into the cooling chamber through a series of fluidly coupled channels embedded in a crankshaft and a rod connecting the piston head to the crankshaft. 
   After heat exchange with the piston head in the cooling chamber, the lubrication oil is either returned to a lubrication pressure pump inlet for reuse or flows into an oil reservoir without being mixed with air in a crankcase associated with the piston engine. 
   The crankshaft has two embedded oil flow channels, a crankshaft inlet channel allowing cooling oil entering different pistons to have substantially similar parameters, such as temperature, and a crankshaft outlet channel allowing heated lubrication oil exiting each individual piston to be recycled. As a result, heat transfer is conducted uniformly from one piston head to another. This can significantly reduce the chance of engine failures caused by thermal gradients and strains within the engine. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects and advantages of the present invention will be better understood from the following detailed description when read in conjunction with the drawings, in which: 
       FIG. 1  is a schematic flow diagram of an embedded cooling system used by a piston engine, according to an embodiment of the present invention; 
       FIG. 2  is a schematic flow diagram of an embedded cooling system used by a piston engine, according to another embodiment of the present invention; 
       FIG. 3  is a cross-sectional view of a piston engine that uses an embedded cooling system, according to some embodiments of the present invention; and 
       FIG. 4  is a cross-sectional view of a piston head of a piston engine taken along line A–A′ of  FIG. 3 . 
   

   Like numerals refer to similar elements throughout the drawings. 
   DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1  is a schematic flow diagram of an embedded cooling system  100  used by a piston engine  300 , according to a first embodiment of the present invention. Different types of cooling fluid can be used by the cooling system  100 . For illustrative purposes, lubrication oil is chosen to describe various embodiments of the present invention. In the first embodiment, lubrication oil flows through the piston engine  300  to reduce its temperature. After exiting the piston engine  300 , the heated lubrication oil flows back to the pressure lubrication system  100 . Before being re-used by the pressure lubrication system  100 , the lubrication oil flows through an oil filter  118  along an oil flow path  115  and is then pressurized by an oil pump  117  or other pressure control device to maintain a high fluid pressure within the embedded cooling system. 
   The lubrication oil is cooled-down by passing it through a heat exchanger  119  to remove at least some heat transferred from the piston engine  300 . The cooled lubrication oil then passes into the piston engine  300  to remove more heat generated by the piston engine. A more detailed discussion about the oil flow inside the piston engine  300  is provided below in connection with  FIGS. 3 and 4 . 
   In this embodiment, the lubrication oil flow from the piston engine  300  flows directly to the inlet of the oil pump  117 , thereby significantly reducing the amount of lubrication oil that must be collected from the crankcase for a dry sump system. This configuration significantly reduces the dimensions of the scavenge pump and the oil reservoir and therefore increases the engine&#39;s power-to-weight ratio. The reduced cooling flow also reduces the power consumption of the lubrication pressure pump. 
     FIG. 2  is a schematic flow diagram of an embedded cooling system used by a piston engine, according to another embodiment of the present invention. The returned lubrication oil is first collected by a dry sump  212  and then directed to a lubrication system storage reservoir  216  through a scavenge pump  214 . Since the lubrication oil flow returned to the reservoir  216  does not mix with any crankcase ambient air, it does not require any further conditioning processes such as air/oil separation. Before being re-injected into the piston engine  300  by the pressure lubrication system  200 , the oil flows through an oil filter  218 , an oil pump  117 , and a heat exchanger  219  to be cooled down. This cooling process effectively removes at least some of the heat which was transferred to the lubrication oil from the piston engine. The removed heat can then dissipate to atmosphere or be used, such as to heat the interior of a vehicle. 
   In both embodiments, the cooling system allows the cooling lubrication oil to directly contact a large surface area within the piston head for a predetermined length of time. This, when combined with a predetermined flow rate, optimizes the heat transfer process and minimizes the amount of cooling lubricant required to maintain the piston engine at the desired temperature. 
     FIG. 3  is a cross-sectional view of the piston engine  300  that uses either embedded cooling system  100  or  200 , according to some embodiments of the present invention. The piston engine  300  includes one or more pistons  301 . Each piston  301  includes a piston head  321  coupled to a piston connecting rod  311 . Each piston connecting rod  311  is rotatably coupled to a crankshaft  307 . 
   Each piston head  321  contains one or more flow channels  302 ,  303  at its rear (crankcase side) face, i.e., disposed behind the front face  305  of each piston. These channels allow pressurized lubrication oil to flow from a pressure lubrication system as shown in  FIGS. 1 and 2  to a cooling chamber behind the piston&#39;s front face  305 . 
   In some embodiments, each piston head  321  includes a cooling chamber  304  behind its corresponding front face  305 . A piston head inlet channel  302  introduces cooled lubrication oil into the cooling chamber  304 , while a piston head outlet channel  303  allows heated lubrication oil to be expelled from the cooling chamber  304 . The cooling chamber  304  is configured to include appropriate flow channels and/or interleaved cooling fins  313 ,  314  to maximize heat transfer from the piston head  321  to the lubrication oil, e.g., by increasing the contact area between the piston head and the lubrication oil. Sometimes, the space or compartment defined in the cooling chamber  304  is reduced to a tortuous flow path from the piston head inlet channel  302  to the piston head outlet channel  303 . A more detailed description of the cooling chamber  304  is provided below in connection with  FIG. 4 . 
   As shown in  FIG. 3 , the piston head inlet channel  302  is fluidly coupled to a connecting rod inlet channel  310  passing through the length of the connecting rod  311 . Similarly, the piston head outlet channel  303  is fluidly coupled to a connecting rod outlet channel  312  that also passes through the length of the connecting rod  311 . The connecting rod inlet channel  310  and connecting rod outlet channel  312  are fluidly coupled to a respective crankshaft inlet channel  306  and crankshaft outlet channel  308  via rotatable seals or oil journals  309 . 
   During operation of the piston engine  300 , pressurized lubrication oil flows under pressure from the crankshaft inlet channel  306 , through an inlet oil journal  309 , through the connecting rod inlet channel  310  and the piston head inlet channel  302  and into the cooling chamber  304 . As the pressurized lubrication oil flows through the cooling chamber  304 , heat is transferred to the lubrication oil from the piston head  321 . The lubrication oil exiting the cooling chamber  304  flows through the piston head outlet channel  303 , through the connecting rod outlet channel  312  and an outlet oil journal  309  and into the crankshaft outlet channel  308 . In some embodiments shown in  FIG. 1 , the lubrication oil exiting the crankshaft outlet channel  308  directly flows into an oil filter  117 , while in some other embodiments shown in  FIG. 2 , the lubrication oil exiting the crankshaft outlet channel  308  directly flows into a dry sump  212  from where it is recycled by the pressure lubrication system. 
   Note that the inlet flow paths of cooling lubrication oil within different pistons  301  of  FIG. 3  are fluidly coupled in parallel, not in series. In other words, the lubrication oil entering the cooling chambers  304  within different piston heads  321  shares a similar set of parameters including pressure, temperature, flow rate, etc., thereby rendering a substantially uniform heat exchange rate within different piston heads  321 . This configuration allows each piston head  321  to be cooled to substantially the same temperature, thereby increasing performance uniformity across all of the pistons and reducing thermal warping and system failures caused by temperature differentials. 
   As mentioned above in connection with  FIG. 3 , the cross-sectional view of the cooling chamber  304  includes a tortuous path to increase the surface contact area between the lubrication oil and the piston head.  FIG. 4  shows such a cross-sectional view of a piston head  321  of the piston engine  300  taken along line A–A′ of  FIG. 3 . Lubrication oil flows into cooling chamber  304  from the piston head inlet channel  302 . In some embodiments, there are two sets of interleaved cooling fins, one set of cooling fins  313  attached to the ceiling of the cooling chamber  304  and the other set of cooling fins  314  attached to the floor of the cooling chamber  304 . In some other embodiments, the two sets of interleaved cooling fins are alternatively attached to two opposing walls of the cooling chamber. The dots and crosses in  FIG. 4  depicts that lubrication oil flows up and down in the cooling chamber to navigate through the two sets of interleaved cooling fins before reaching piston head outlet channel  303 . Heat generated by the piston engine is therefore conducted from the fins to the lubrication oil, which transfers the heat out of the cooling chamber  304 . This type of chamber profile or cross-sectional area prolongs the contact period during which the lubrication oil is exposed to the hot piston head  321 . The longer the exposure period, the more heat is removed from the piston head through the lubrication oil. For simplicity, the piston head shown in  FIG. 4  has a square contour, but it will be apparent to one skilled in the art that this approach is applicable to any shape of piston head. 
   The foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations are possible in view of the above teachings. For example, the pressure lubrication system  100  or  200  may include more or less components depending on the overall working environment. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.

Technology Category: 2