Engine piston

Abstract: A piston, which reciprocates in a cylinder bore (23) of an engine (10), is supplied with lubricating oil. The piston includes pin bosses (13) provided under a piston head (12), a symmetrical pair of skirts (17), and side walls (18) connecting the pin bosses (13) and the skirts (17). The side walls (18) have openings (19), which communicate the outer side of the walls (18) with the inner side of the walls (18) under the middle section of the head (12). A recess (26) is defined in the lower surface (12a) of the head (12) to receive a spray of lubricating oil. The width (W1) of the recess (26) is wider than the width (D2) of each skirt (17) at its basal portion. This causes the oil supplied to the lower surface (12a) of the head (12) during movement of the piston to be injected against the recess (26) and thus be diffused. As a result, oil passes by the basal section of the skirts (17) and permeates into the space defined by the cylinder bore (23) and the skirts (17).

Description: TECHNICAL FIELD The present invention relates to an engine piston, and more particularly, to a piston that receives a spray of oil on its lower surface and deflects the spray to a desired location. RELATED BACKGROUND ART In a conventional engine, pistons that reciprocate in cylinder bores expand due to the high temperature of the heat produced in combustion chambers. Overheating of the pistons lowers the knock limit value with respect to the ignition timing of the air-fuel mixture in the combustion chamber. In addition, excessive heat expansion of each piston increases friction produced between the piston and the cylinder bore. To solve these problems, pistons are cooled by injecting lubricating oil toward the lower surface of the piston's head with an oil supplying device such as an oil jet provided in the engine. During recent years, modifications made in the shape of pistons has contributed to reducing the weight of engines. Japanese Unexamined Patent Publication 5-172001 discloses such a uniquely shaped piston together with its improved cooling structure. As shown in Fig. 7, the publication describes a piston 31 that includes a pin boss 33 and skirts 34, which are provided below a piston head 32 and connected to one another by side walls 35. The piston 31 has a hollowed section 36 below the head 32 that contributes to a light weight structure. The section 36 includes first and second passages 37, 38. The first passage 37 is defined about the boss 33 to allow passage of lubricating oil and is connected with the section 36. The second passage 38 allows passage of oil directed downward of the piston 37. When lubricating oil is supplied to the lower side of the head 32 through an oil supply bore in a lower wall of the first passage 37 in Fig. 7, the oil cools the head 32 and then flows through the first passage 37 to cool the boss 33, side walls 35, and cylinder bore (not shown). The oil then passes through the second passage 38 and flows downward of the piston 31. However, although the above structure of the piston 31 enables some of the oil supplied to the lower side of the head 32 to be conveyed to the cylinder bore through the hollow section 36, the oil is not positively provided to the space between the cylinder bore and the skirts 34 due to the structure of the first passage 37. As a result, the oil supplied between the skirts 34 and the bore is insufficient. The unexamined utility model application JP1-88084 discloses a piston for an internal combustion engine in which some of an oil enters a recess and falls as it cools the piston. Also in this prior art document, oil is not positively provided to a space between a cylinder bore and a skirt. DISCLOSURE OF THE INVENTION Accordingly, it is a primary objective of the present invention to provide an engine piston that improves the sliding performance of a skirt by positively providing lubricating oil, supplied to the lower side of the head, to a space between the skirt and the cylinder bore. This object is solved by an engine piston according to claim 1. More specifically, to achieve the foregoing and other objects and in accordance with the purpose of the present invention, an engine piston adapted to be reciprocally disposed in a cylinder bore of an engine is provided. The piston is arranged to receive lubricant oil from oil supplying device. A pair of pin bosses are coaxially arranged with each other under a head. A pair of skirts extend downward from a lower surface of the head in a symmetrized manner with respect to an axis of the bosses. The skirts are arranged to move along the cylinder bore. A pair of side walls provide an connection with the associated boss and the associated skirt. Each of the side walls have an outer side and an inner side connecting to each other through an opening. The lower surface receives the lubricant oil, and the lower surface has a recess to receive the oil therein. The piston shows a width of the recess in the axial direction with respect to the bosses which is larger than a width of the skirt at a boundary to the head in the direction of the axis. Further embodiments are disclosed in the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: Fig. 1 is a front view showing a piston; Fig. 2 is a cross-sectional view taken along line 2-2 of Fig. 1; Fig. 3 is a cross-sectional view taken along line 3-3 of Fig. 2; Fig. 4 is an exaggerated view of the barrel profile of the skirt; Fig. 5 is an assembling drawing showing the piston in a cylinder bore; Fig. 6 is a cross-sectional view based on Fig. 3; and Fig. 7 is a front view of a prior art piston. DESCRIPTION OF SPECIAL EMBODIMENT Fig. 1 shows a front view of a piston 11. Fig. 2 shows a cross-sectional view taken along line 2-2 of Fig. 1, and Fig. 3 shows a cross-sectional view taken along line 3-3 of Fig. 2. The piston 11 includes a substantially disk-shaped head 12 and a pair of pin bosses 13 provided under the head 12. Three ring grooves 14, 15, 16 extending parallel to one another are provided in the peripheral surface of the head 12. The pair of bosses 13 are arranged along the same axis L1. A piston ring is arranged in each of the grooves 14, 15 and an oil ring is arranged in the groove 16. The bosses 13 are opened toward the front and rear sides of the piston 11. A piston pin 22 (shown in Fig. 5) is arranged in the two bosses 13. A pair of skirts 17 are arranged symmetrically about the axis L1 of the two bosses 13 and extend downward along the outer periphery of the head 12. The two skirts 17 have identical shapes. Under the head, side walls 18 are arranged around the bosses 13 and connect the bosses 13 to the skirts 17. The side walls 18 have openings 19, which communicate the outer side of the walls 18 with the inner side of the walls 18 under the middle section of the head 12. The openings 19, which are opposed to each other in the direction of the axis L1, are also communicated with each other. Fig. 4 shows an exaggerated barrel profile of the skirt 17. The skirt 17 has an upper end 17a located at the boundary, a lower end 17b opposed to said upper end. The skirt 17 has a barrel profile, which is projected most outwardly at its vertically middle section. At peak P1, where the skirt 17 projects most outwardly, the clearance between the skirt 17 and the cylinder bore is smallest. Thus, the pressure acting on the surface of the skirt 17, or the planar pressure acting on the skirt 17, is highest at peak P1. The peak P1 extends around the circumference of the skirt 17. Distance D1, which is the length from the upper end 17a of the skirt 17 to the bottom 19a of the opening 19 in the vertical direction, is equal to or longer than distance D2, which is the length from the upper end 17a to the peak P1 in the vertical direction. That is, the bottom 19a is arranged at a height equal to or lower than the peak P1. The piston 11 is employed in an engine. As shown in Fig. 5, the piston 11 is connected to a connecting rod 21, which is connected to a crankshaft 20, by the piston pin 22. The piston 11 is installed in a cylinder bore 23, which is formed in the engine 10. The piston 11 reciprocates along the walls of the bore 23 when the engine 10 is operated. Lubricating oil under a predetermined pressure is supplied to an oil passage 25 of the crankshaft 20 and injected from an oil nozzle 24, provided in the connecting rod 21, toward the piston 11 and bore 23 during operation of the engine 10. The oil directed toward the piston 11 is injected against a lower surface 12a of the head 12 between the two skirts 17. As shown in Fig. 2, the lower surface 12a has a recess 26 where the oil is received. The shape of the recess 26 is symmetrical with respect to the axis L1. The recess 26 receives and diffuses the injected oil. As shown in Fig. 3, the recess 26 is obtained by hollowing out a portion of the lower surface 12a of the head 12. The peripheral wall of the recess 26 has a smoothly curved cross section which defines a curved surface 26a. A predetermined angle  is defined between a line extending outward from the curved surface 26a and a axis of the head 12. In this embodiment, the value of the angle  is equal to or larger than 10 degrees. As shown in Fig. 2, width W1, which corresponds to the width of the recess 26 in the direction of the axis L1, is larger than the width W2, which corresponds to the width of the skirt 17 at the vicinity of its basal portion in the direction of the axis L1. That is, the distance between the two outer ends of the recess 26 is longer than the distance between the two basal outer ends of the skirt 17 on each side of the axis L1. Accordingly, the above structure allows lubricating oil to be injected from the oil nozzle 24 toward the piston 11 when the piston 11 reciprocally moves along the walls of the cylinder bore 23 during the operation of the engine 10. The oil is diffused when it is injected against the recess 26 defined in the lower surface 12a of the head 12. During the diffusion, the curved surface 26a of the recess 26 and its angle  enables the oil to be efficiently diffused outward. The diffused oil is applied to the pin bosses 13 and the inner side of the skirts 17. The oil is also applied to the cylinder bore 23 when it passes through the openings 19 in a direction parallel to the axis L1 of the bosses 13. Therefore, the oil diffused by the recess 26 is efficiently applied to the various components of the piston 11 under the head 12 and thus efficiently cools the piston 11. In addition, some of the oil diffused by the recess 26 passes by the basal portion of the skirts 17 and is applied to the cylinder bore 23 thus permeating into the space defined between the bore 23 and the skirts 17. Since oil is positively supplied to the space defined between the bore 23 and the skirts 17, a sufficient amount of oil is applied between the bore 23 and skirts 17 in an efficient manner. This further improves the lubrication and sliding performance of the skirts 17. The improvement in the sliding ability enables a further reduction in the area of the skirts 17. The increase in the amount of oil supplied between the skirts 17 and the bore 23 results in an improvement in the prevention of scuffing of the piston 11. Furthermore, since the oil film formed between the skirts 17 and the bore 23 is relatively thick, the film serves as a damper and suppresses slapping between the skirt 17 and the bore 23. The recess 26 defined in the lower surface 12a of the head 12 and the openings 19 in the side walls 18 contribute to a further reduction in the weight of the piston 11. Additionally, the bottom of the opening 19 is arranged at a height equal to or lower than the peak P1 on the barrel profile of the skirts 17. The clearance between the skirts 17 and the bore 23 is minimum at the height corresponding to the peak P1. The pressure on the outer peripheral surface of the skirts 17 where the skirts 17 are connected to the side walls 18 is greater than that at other parts of the skirts 17 at the same height. The region where the pressure is the highest, or the peak P1, and the regions where the side walls 18 causes the pressure to be high do not overlap each other. This enables the value of the maximum pressure at the region corresponding to the peak P1 to be uniformly maintained around the circumference of the skirts 17 despite the existence of the side walls 18. It is required to uniformly maintain the planar pressure at peak P1 about the circumference of the skirts 17. In other words, the side walls 18 do not cause the value of the maximum pressure at the region corresponding to the peak Pi to vary. As a result, the distance between the opposed side walls 18 is shortened in the direction of the axis L1. This, in turn, allows the width W2 of each skirt 17 to be narrowed. The narrowing of the width W2 of each skirt 17 increases the difference between the width W1 of the recess 26 and width W2. This allows a larger amount of lubricating oil, injected toward the lower side of the piston 11, to be supplied between the skirts 17 and the cylinder bore 23. Although only one embodiment of the present invention has been described so far, it should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the scope of the claims. Particularly, it should be understood that the only embodiment of the present invention may also be modified as described below. Such modifications achieve the same operation and effects of the above embodiment. In the above embodiment, the peripheral area of the recess 26 has a smoothly curved surface 26a which extends outwardly at a predetermined angle  with respect to the axis of the head 12, as shown in Fig. 3. However, as shown in Fig. 6, the peripheral area of the recess 26 may have an inclined conical surface 26b that extends outwardly at a predetermined angle 6 with respect to the axis of the head 12. In the first embodiment, the oil nozzle 24, which supplies lubricating oil to the lower side of the piston 11, was provided in the connecting rod 21. However, an oil jet that supplies oil to the lower side of the piston 11 may be provided separately from the connecting rod 24. The piston 11 employed in the first embodiment has three grooves 14-16. However, the present invention may be embodied in a piston having more or less than three grooves. Although the pair of skirts 17 have an identical shape in the first embodiment, skirts having different shapes may be used. In the first embodiment, the shape of the recess 26 is symmetrical about the axis L1 of the pin bosses 13. However, a recess having a shape which is not symmetrical may be used instead. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.

Claims: An engine piston reciprocally disposed in a cylinder bore (23) of an engine (10), the piston comprising : a pair of pin bosses (13) coaxially arranged with each other under a head (12), a pair of skirts (17) extending downward from a lower surface (12a) of the head (12) in a symmetrized manner with respect to an axis (L1) of the bosses (13), said skirts (17) being arranged to move along the cylinder bore (23), a pair of side walls (18) providing a connection with each boss (13) and the skirts (17), each of said side walls (18) having an opening (19) provided between an outer side and an inner side of said side walls, said lower surface (12a) having a recess (26) for receiving oil injected by an oil supplying device (24, 25) therein, wherein a width (W1) of the recess (26) in the axial direction with respect to the bosses (13), is larger than a width (W2) of the skirt (17) at a boundary to the head (12) in the direction of the axis (L1), and wherein a peripheral wall of the recess (26) includes a smoothly curved or an inclined conical surface (26a; 26b) which extends outwardly at a predetermined angle () which is defined between a line extending outward from the surface (26a; 26b) and an axis of the head (12), for enabling the oil to be efficiently diffused outward. The piston as set forth in Claim 1, characterized in that said recess (26) has a shape symmetrical with respect to the axis (L1) of the pin bosses (13). The piston as set forth in any one of the preceding claims, characterized in that said_angle () is at least 10 degrees. The piston as set forth in any one of the preceding claims, characterized in that each of said skirts (17) has an upper end (17a) located at the boundary to the head (12), a lower end (17b) opposed to said upper end (17a) and a barrel profile provided between the upper end (17a) and the lower end (17b) in longitudinal direction of the skirt which is projected most outwardly at a_middle section of the skirt (17) in longitudinal direction, wherein said opening (19) of the side wall (18) has a bottom (19a), said upper end (17a) of the skirt (17) and the bottom (19a) of the opening (19) define a first distance (D1) in the longitudinal direction of the piston, said upper end (17a) and the middle section (P1) of the skirt (17) define a second distance (D2) in the longitudinal direction of the piston which is shorter than the first distance (D1). The piston as set forth in any one of the preceding claims, characterized in that said skirts (17) have shapes identical to each other. Engine having a piston according to anyone of the preceding claims, including a crankshaft (20) which is rotatable and a connecting rod (21) for connecting the pin bosses (13) to the crankshaft (20), wherein said crankshaft (20) includes an oil passage (25) therein to receive the oil under a predetermined pressure, wherein said oil supplying device includes an oil nozzle (24) provided with the connecting rod (21) to inject the oil, supplied to the oil passage (25), toward the lower surface (12a) of the piston.

Inventors: MASUDA YOSHIHIKO, SUGIYAMA MASANORI, MASUDA, YOSHIHIKO, SUGIYAMA, MASANORI
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1996
Main classification: F02F3
IPC codes: F02F3, F01P3, F02F1