1. Field of the Invention
The invention is related generally to pistons for internal combustion engines, and more particularly to pistons having internal cooling chambers for cooling the pistons during operation.
2. Description of the Related Art
It is desirable for engine manufacturers to increase the temperature of the combustion of the air/fuel mixture in the combustion chamber of the cylinder in order to increase the fuel efficiency of the engine and decrease emissions. Therefore, there remains a significant and continuing need for improved pistons with cooling systems capable of allowing the piston to withstand increased temperatures.
However, with increased heat of combustion, there is a corresponding need to design pistons that can operate in such an environment. One approach has been to cool the upper region of the piston that is exposed to the heat of combustion by forming an open or a closed cooling chamber in the piston and directing a stream of cooling oil from below into the chamber to help extract some of the heat of combustion from the affected regions of the piston. Such a cooling approach, however, has its limits and may not be adequate or most efficient under all conditions. Another alternative approach has been to encapsulate a cooling medium, such as sodium, within a sealed chamber in the upper portion of the piston as the principle means to extract the heat of combustion from the piston. This too has its limits and cannot always adequately cool the piston with ever-increasing combustion temperatures.
According to an aspect of the present invention, a piston for an internal combustion engine comprises a piston body having first and second cooling chambers. The first chamber is sealed closed and contains a first cooling medium other than air for extracting heat from the surrounding regions of the piston body during operation of the piston. The second cooling chamber is disposed adjacent to the first cooling chamber and is at least partially open to receive a flow of cooling oil during operation of the piston to extract additional heat from the piston body.
One advantage of such a piston is the ability to operate at increased combustion temperatures. In operation, both the first cooling medium in the first cooling chamber and the cooling oil in the second cooling chamber extract heat from the piston body to allow the piston to operate in the high temperature environment. Further, at least some of the cooling oil in the second cooling chamber is directed to the portion of the piston body separating the first and second cooling chambers to extract heat from the first cooling medium. In other words, the cooling oil extracts heat from both the piston body and the first cooling medium. Thus, the effectiveness of the first cooling medium is increased by the cooling oil.
Another advantage of such a piston is the substantially uniform cooling of the top of the piston body, which is directly exposed to the high temperatures from the combustion of the air/fuel mixture in the combustion chamber. The uniform cooling reduces thermal stresses within the piston body, thus increasing the operating life of the piston body. The uniform cooling is additionally advantageous when the piston is employed in a diesel engine because hot spots in diesel engine pistons can lead to undesirable mono-nitrogen oxides (NOx) generation.
According to another aspect of the invention, the first cooling medium in the first cooling chamber is of a material which is solid at ambient temperatures and melts at a predetermined temperature corresponding to an improved operating temperature of the engine. Such a fluid is advantageous because it allows the piston body to quickly warm to a more efficient operating temperature before the first operating medium melts and begins to cool the crown of the piston body in what is commonly referred to as the “cocktail shaker” effect. In other words, the time for the piston body to reach an increased operating temperature is reduced without compromising the cooling ability of the first cooling medium.
According to yet another aspect of the invention, the first cooling chamber is sealed closed, thus protecting the first cooling medium from contaminants and from oxidation, both of which could reduce the rate of cooling the piston.
These and other features and advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, a piston including a piston body 20 is generally shown in
As shown in
As shown in
In the exemplary embodiment, the piston body 20 is a monobloc piston body 20, i.e. the piston body 20 includes a skirt 41 integrally formed with or secured to at least the bottom of the crown 24. The skirt 41 could be secured to the crown 24 through friction welding, for example. Alternately, the skirt 41 could be hingedly connected to the crown 24 with a pin (not shown). In the exemplary embodiment, the skirt 41 has a generally cylindrical shape and extends axially downward from the bowl rim 28 of the top surface 26. The skirt 41 presents a pair of diametrically opposed bosses 42 spaced axially from the piston rings 40, and the bosses 42 are sized to receive a pin (not shown) to connect the skirt 41 to a piston rod (not shown), as will be appreciated by those skilled in the art.
As best shown in
In the exemplary embodiment, the first cooling chamber 46 is hermetically sealed closed, and therefore, the first cooling medium 48 is protected from contamination and from oxidation. It should be appreciated that the first cooling chamber 46 could take many different sizes and shapes other than those shown in the Figures. For example, the first cooling chamber 46 could be substantially disposed only adjacent to the bowl rim 28 of the crown 24, or the first cooling chamber 46 could extend downwardly past each of the piston rings 40 for cooling all of the piston rings 40.
Many engines, but particularly diesel engines, operate most efficiently at higher temperatures and pressures. In the exemplary embodiment, the first cooling medium 48 has a melting point temperature of between fifty and two hundred degrees Celsius (50-200° C.) to increase the efficiency of the engine. Most preferably, the first cooling medium 48 is of sodium or a sodium-based compound having a melting point temperature of approximately one hundred degrees Celsius (100° C.). In operation, the sodium 48 remains solid until it reaches the 100° C. melting point, i.e. after the piston body 20 has reached a more efficient operating temperature. Upon reaching the melting point, the sodium 48 melts and the “cocktail shaker” effect described above begins to increase the cooling of the top surface 26 of the crown 24 and the piston rings 40. Because this increased cooling only starts after the sodium 48 melts, the time that the piston 20 spends at the less efficient low temperatures is reduced. It should be appreciated that the first cooling medium could alternately be lithium, a lithium-based compound, silicone oil or any other cooling medium. It may be desirable to select the first cooling medium 48 as a function of the most efficient operating temperature of the piston 20.
The piston body 20 further includes a second cooling chamber 50 adjacent to the first cooling chamber 46. The second cooling chamber 50 is at least partially open to receive a cooling oil 51 into the second cooling chamber 50 during operation of the piston to extract additional heat from the piston body 20. In the exemplary embodiment, the second cooling chamber 50 is defined by a flange 52 spaced axially from the wall 44 of the first cooling chamber 46. In other words, the first and second cooling chambers 46, 50 are separated by the wall 44. The flange 52 curves axially upward toward the top surface 26 of the crown 24 to an end that is axially below the combustion bowl 30 of the crown 24 to define a dish-tray shape, as best shown in
As shown in
Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while within the scope of the appended claims. In addition, the reference numerals in the claims are merely for convenience and are not to be read in any way as limiting.