Oil pan for an internal combustion engine
TECHNICAL FIELD 
     The present invention relates to an oil pan for an internal combustion engine, and in particular to an oil pan which is formed by a casting process. 
     BACKGROUND OF THE INVENTION 
     It is conventional to connect a cylinder block, a transmission case and a cast oil pan with each other so as to improve the overall rigidity of the engine power plant and to reduce the vibration of the engine as disclosed in Japanese patent laid open publication (kokai) No. 63-170558. 
     According to this conventional structure, an annular flange is provided along the upper edge of the oil pan for attachment to the cylinder block, and another annular flange is provided on a surface of the oil pan perpendicular to the axial center line of the crankshaft for attachment to an opposing surface of the transmission case. Mounting bolts are provided for the mounting holes of the flange for attachment to the cylinder block near the intersection between these two annular flanges, and these mounting bolts are a pair of long threaded bolts which pass through the annular flange for attachment to the transmission case. The heads of these long threaded bolts are supported by the lower end of the flange for attachment to the transmission case. Interference is thereby avoided between the mounting bolts and the two annular flanges. 
     However, according to this conventional arrangement, it is not possible to directly clamp the flange during attachment to the mounting surface of the cylinder block, and it is therefore difficult to achieve a contact pressure sufficient for sealing. As a result, the structure of the power plant is undesirably complex. The long threaded bolts contribute to rigidity of the flange and it is difficult to modify the flange so as to directly clamp it to the mounting surface of the cylinder block. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of such problems in the prior art, a primary object of the present invention is to provide an oil pan for an internal combustion engine which can be securely attached to the mounting surfaces of the cylinder block and the transmission case without requiring a complex overall structure. 
     A second object of the present invention is to provide an oil pan for an internal combustion engine which can be securely attached to the mounting surfaces of the cylinder block and the transmission case while providing a sufficient overall rigidity to the power plant. 
     A third object of the present invention is to provide an oil pan for an internal combustion engine which is suitable for fabrication by a casting process. 
     These and other objects of the present invention can be accomplished by providing an oil pan for an internal combustion engine which includes a main body including a pan-shaped bottom wall, a first annular flange for attachment to a cylinder block, and a second annular flange for attachment to a transmission case. The first and second annular flanges have substantially perpendicular flange surfaces intersecting at a longitudinal end of the main body, a plurality of first mounting holes provided circumferentially along the first flange, a plurality of second mounting holes provided circumferentially along the second flange, and access holes provided in parts of the bottom wall adjacent to a bottom end of the second flange. The access holes allow access to heads of mounting bolts in the first mounting holes. An annular rib is formed around each of the access holes, and a connecting rib is formed in a part of the bottom wall and has a certain depth in a direction perpendicular to the flange surface of the first flange and connects the annular ribs with each other. 
     Thus, the first flange can be securely clamped onto the opposing mounting of the cylinder block because all of the mounting bolts can directly clamp the first flange onto the mounting surface. Furthermore, the annular rib and the connecting rib ensure rigidity of the second flange, and the resulting power unit is provided with a high rigidity. Thus, the sealing performance can be ensured, and a highly vibration free power structure is achieved. These effects are enhanced when the oil pan is made of cast metal or alloy. 
     To further increase the rigidity of the oil pan, the oil pan may further comprise annular bosses surrounding the second mounting holes, in which case the connecting rib is additionally connected to the annular bosses. Also, a pair of reinforcement ribs can be provided in the bottom wall extending from the first flange and to a part of the second flange adjacent to a corresponding one of the annular bosses. 
     To allow servicing of the transmission or the torque converter without removing the oil pan from the engine, a maintenance hole is provided in the bottom wall so as to allow access to an opposing surface of the transmission case. In this case, a part of the connecting rib can be formed by sections of a semi-cylindrical part of the bottom wall surrounding the maintenance hole. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is described in the following with reference to the appended drawings, in which: 
     FIG. 1 is a bottom view of an embodiment of the oil pan according to the present invention; 
     FIG. 2 is a sectional view taken along line II--II of FIG. 
     FIG. 3 is a front view of the oil pan; 
     FIG. 4 is a sectional view taken along line IV--IV of FIG. 1; 
     FIG. 5 is a sectional view taken along line V--V of FIG. 1; 
     FIG. 6 is a sectional view taken along line VI--VI of FIG. 1; and 
     FIG. 7 is a side view of the oil pan. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 to 7 show a preferred embodiment of the oil pan for an internal combustion engine according to the present invention. The oil pan 1 is made of cast aluminum alloy, and has a generally pan-shaped bottom wall 1a. The upper edge of the bottom wall 1a of the oil pan 1 has an annular flange 2. The rear portion of the bottom wall 1a has an extension 1b, and the rear edge of the extension 1b defines an annular flange 3 for attachment to a transmission case in cooperation with the bottom wall 1a and the flange 2. The transmission case may accommodate an automatic transmission system including a fluid torque converter among other possibilities. Thus, the flange 2 has a substantially horizontal flange surface while the flange 3 has a vertical flange surface perpendicular to a central axial line of a crankshaft (not shown in the drawing). The flange 2 is used for attachment to a cylinder block, and the flange 3 is used for attachment to a transmission case. 
     A plurality of mounting holes 5 are provided in the annular flange 2. Two mounting holes 5a and 5b are located adjacent to the flange 3, and have respective seat surfaces 6a and 6b for supporting heads of mounting bolts. The bottom wall extension 1b adjacent to the lower end of the flange 3 has a pair of access holes 7a and 7b allowing access to the heads of the threaded bolts disposed in the mounting holes 5a and 5b. These access holes 7a and 7b are large enough for entry of a bolt fastening tool and are normally closed by plugs of rubber or other resilient synthetic resin material. 
     Referring to FIGS. 3 and 4, a bottom wall extension 1b and the annular flange 3 border a central recess 9. The bottom wall extension 1b has a maintenance hole 10 and access holes 7a and 7b, for allowing access to a drive plate of a torque converter of the transmission system (not shown in the drawings) from the exterior of the oil pan. Mounting holes 5a and 5b are shown in FIG. 3. Thus, by removing the drive plate from this maintenance hole 10 and unfastening the mounting bolts from the flange 3, it is possible to remove the transmission case from the engine without removing the oil pan 1 from the engine. Therefore, the transmission case can be removed from the power unit without risking spilling lubricating oil from the engine or risking oil leakage. This maintenance hole 10 is normally closed by a plug 11. The plug 11 can be made of rubber or other resilient synthetic resin material, and is removed when the transmission case is serviced. 
     As illustrated in FIGS. 1, 3 and 5, a pair of annular transmission mounting bosses 12a and 12b are adjacent to the access holes 7a and 7b. As best illustrated in FIG. 3, a pair of reinforcement ribs 17a and 17b are integrally provided in the bottom wall 1a and in the bottom wall extension 1b of the oil pan 1. The ribs 17a and 17b are respectively connected between the two annular flanges 2 and 3. In this embodiment, each of the reinforcement ribs 17a and 17b connects the slightly thickened seat region 6a surrounding one of the mounting holes 5a and 5b with a corresponding one of the transmission mounting bosses 12a and 12b. 
     Each of the access holes 7a and 7b are respectively surrounded by reinforcing annular ribs 13a or 13b which are joined by a U-shaped connecting rib 14 extending along the upper periphery of the maintenance hole 10. The connecting rib 14 has depth in a direction perpendicular to the flange surface of the flange 3. The connecting rib 14 is a semi-cylindrical wall in the bottom wall 1a. This semi-cylindrical wall defines a concave surface on the exterior of the bottom wall of the oil pan, and has a sufficient axial length to allow insertion of a special tool for the removal of a drive plate. 
     The annular ribs 13a and 13b are also respectively connected to the bosses 12a and 12b of the mounting holes for the flange 3. Thus, the connecting rib 14, the reinforcement ribs 17a and 17b, the transmission mounting bosses 12a and 12b and the annular ribs 13a and 13b merge with each other at two regions adjacent to the transmission mounting bosses 12a and 12b. In FIGS. 1 and 7, numeral 15 denotes a seat surface of an oil filter canister, and numeral 16 denotes a seat surface for an engine mount. 
     When installing the oil pan 1 on an engine, the oil pan 1 is fitted on the open bottom end of the cylinder block, and threaded bolts are inserted in the mounting holes 5. The seat surfaces 6a and 6b for the heads of these mounting bolts provided in the flange 2 allow heads of the bolts to be directly applied to the flange 2 without requiring long bolt stems or long bosses in the oil pan. As a result, a sufficient sealing performance can readily be achieved. In particular, because no bolt stems pass across the recess 9, there is greater freedom of design. For instance, it is possible to increase the clearance between the flywheel of the engine and the recess 9, and the tuning of the mass of the flywheel can be facilitated. 
     As discussed above, according to the oil pan of the present invention, access holes are provided in the oil pan, and mounting bolts used in the mounting holes of the flange are used for attachment of the flange to the cylinder block such that heads of the bolts are directly applied to the flange. Thus, a sufficient sealing performance can be achieved without any difficulty. Furthermore, annular ribs surround the access holes, and are connected with each other by a rib structure, thereby imparting a sufficient rigidity to the flange. The rib structure may include a part of the bottom wall of the oil pan. 
     Although the present invention has been described in terms of a preferred embodiment thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims.