External drive double shaft overhead cam engine (dschc)

Abstract: An overhead cam engine having a drive train including a crankshaft (32) rotatably disposed within a crankcase (8) and driven by a piston-connecting rod assembly (25, 37). The crankshaft drives a timing shaft (42) at half the speed of the crankshaft through a gear set (38, 40) within in the crankcase. The timing shaft is parallel to the crankshaft, and has an end extending externally of the crankcase (8) with a timing pulley (44) mounted thereon. The timing pulley is drivingly attached through an external timing belt (46) to a camshaft pulley (48) mounted on an end of a camshaft (50) which extends externally of the cylinder head (18). The camshaft has a pair of cam lobes mounted thereon inside the cylinder head for actuating intake and exhaust valves (62, 68) within the cylinder head.

Description: The field of the present invention relates to overhead cam engines, for use in a variety of applications, such as lawnmowers, generators, pumps, tillers, pressure washers and other lawn and garden implements, or in small utility vehicles such as riding lawnmowers, lawn tractors, and the like. Generally, the intake and exhaust valves of prior known engines are actuated directly by a camshaft located in the cylinder head, or indirectly through the use of rocker arms, tappets, or other similar means. For example, in many existing L-head and overhead valve engines, the crankshaft drives a camshaft which is parallel to the crankshaft and located in the crankcase, and lobes on the camshaft actuate push rods and rocker arms to open and close the valves. Another arrangement is illustrated in U.S. Patent No. 5,090,375, assigned to the assignee of the present application, in which the crankshaft extends externally of the crankcase and the camshaft extends externally of the cylinder head. The camshaft is driven by the crankshaft by means of a belt and pulley, chain and sprocket or the like at one half the rotational speed of the crankshaft. Such speed reduction is typically achieved by mounting a pulley or sprocket on the camshaft that is twice as large as the pulley or sprocket on the crankshaft. DE 3 641 201 A discloses an overhead cam engine having a crankcase, a cylinder head, a crankshaft and a timing shaft. An end of the timing shaft extends externally of the crankcase and has a pulley mounted thereon for driving a camshaft pulley mounted on an end of a camshaft that extends externally of the cylinder head. The diameter of the camshaft pulley is twice the diameter of the timing shaft pulley. The said cam engine has various disadvantages. First, the height of the engine is relatively great. Second, the timing shaft is relatively short and supported in bearings that are close to each other on one side of the crankcase. A problem with this and similar designs is that the pulley or sprocket is mounted directly on the portion of the crankshaft which extends from the crankcase, which produces a high force moment on the end of the crankshaft and stress on the crankshaft bearing. To compensate for the stress, the crankshaft bearing must be enlarged and/or the crankcase wall reinforced around the bearing, which increases the cost and size of the crankcase. An additional problem is that speed reduction is accomplished outside of the crankcase by means of the pulley or sprocket mounted on the camshaft which is twice as large as the pulley or sprocket mounted on the crankshaft. The large pulley or sprocket on the camshaft increases the overall size of the cylinder head and length of the engine. What is needed is an engine in which the camshaft is not driven directly from the crankshaft, to reduce the stress on the crankshaft bearing and obviate the need for a larger bearing and/or reinforcement of the crankcase wall around the bearing. A further need is for an engine in which the speed reduction is carried out within the crankcase by the same reduction gearing as in overhead valve and L-head engines, such that a smaller pulley may be to be mounted on the camshaft to reduce the size of the cylinder head and length of the engine and to utilize existing bearings in the crankcase. A still further need is for an engine in which the above needs are accomplished, yet may be manufactured using existing components and tooling. The present invention provides an overhead cam engine according to claim 1. The present invention provides a drive train for an overhead cam engine, including a timing shaft parallel to the crankshaft which is driven by the crankshaft at one half crankshaft speed through a gear set within the crankcase. An end of the timing shaft extends externally of the crankcase, and has a pulley mounted thereon for driving a camshaft pulley mounted on an end of a camshaft that extends outside of the cylinder head through a belt connecting the timing shaft pulley to the camshaft pulley. The bearing and reduction gear set are disposed on the opposite end of the timing shaft and located on the opposite side of the crankcase with respect to the timing shaft pulley. An advantage of this arrangement is that carrying out the speed reduction through a gear set within the crankcase reduces the end force on the crankshaft and the stress on the crankshaft bearings, which in turn reduces cost by obviating the need for a larger crankcase bearing and/or reinforcement of the crankcase wall around the bearing, therefore allowing a crankshaft of smaller diameter and a thinner crankcase wall. Another advantage is that the speed reduction within the crankcase, provided by the timing shaft and gear set, eliminates the need for a pulley on the camshaft that is twice as large as the pulley on the crankshaft, which reduces the size of the cylinder head and the length of the engine. In addition, because many existing L-head and overhead valve engines already include an internal camshaft driven by the crankshaft through a gear set, such engines can conveniently be modified for overhead cam operation by replacing the camshaft with a timing shaft and extending the timing shaft through the crankcase wall so that a timing belt or chain can extend from the timing shaft to the camshaft externally of the crankcase. This enables the utilization of existing tooling and engine components, thereby significantly reducing the cost of the engine. The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: Fig. 1 is a perspective cutaway view of an overhead cam engine, showing the crankshaft, timing shaft with timing gear and timing pulley, timing belt, camshaft and camshaft pulley, with the crankcase, cylinder block and cylinder head partially shown; Fig. 2 is a perspective fragmentary sectional view of the crankcase, showing the lower crankshaft bearing, oil sump, lower timing shaft bearing, and gerotor oil pump; Fig. 3 is a perspective view of the timing shaft, showing the timing gear, gerotor oil pump inner rotor, and oil pump cover; Fig. 4 is a perspective view of the engine; Fig. 5 is a further perspective view of the engine taken from the opposite side; and Fig. 6 is a longitudinal sectional view of the engine. Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. Referring to Figs. 1, 4, 5 and 6, part of the drive train 10 of an overhead cam engine, which can be either a vertical crankshaft or horizontal crankshaft type engine, is shown. Drive train 10 is generally enclosed within crankcase 8, mounting flange or crankcase base 14, cylinder block 16, cylinder head 18, and valve head cover 19. Crankcase 8 generally comprises crankcase base 14 and crankcase upper casing 12, where the upper portion of casing 12 defines a first side 12a of crankcase 8, and the lower portion of casing 12 and crankcase base 14 defines a second side 12b of crankcase 8 opposite the first side of crankcase 8. Crankcase casing 12 includes upper crankshaft bearing 20 and upper timing shaft bearing 24. Crankcase base 14 includes lower crankshaft bearing 22, lower timing shaft bearing 26, mounting flange 23, and oil sump 28. Crankcase casing 12 and crankcase base 14 are attached in a conventional manner. Journals 32a and 32b of crankshaft 32 are rotatably carried in upper and lower crankshaft bearings 20 and 22, respectively, and crankshaft 32 is disposed along an axis L 1 -L 1 . Piston 25 (Fig. 6) is slidably received in cylinder bore 15 within cylinder block 16 along an axis L 2 -L 2 , perpendicular to crankshaft axis L 1 -L 1 . Cylinder block 16 has integral supports 43 for mounting an electric ignition module (not shown) thereon, and integral cooling fins 34 for dissipating heat. As shown in Fig. 6, connecting rod 37 is rotatably connected to piston 25 by wrist pin 39, and is also rotatably connected to crankshaft 32 between throws 36 in a conventional manner. A flywheel, recoil starter, and blower (not shown) are secured to crankshaft 32. A conventional muffler (not shown) is connected to exhaust port 31 (Fig. 5) and carburetor 33 is connected to intake port 35 (Fig. 4). Carburetor 33 includes breather 47 and throttle lever 49 connected to governor linkage (not shown). As may be seen in Figs. 4 and 5, intake port 35 and exhaust port 31 extend inwardly into cylinder head 18 on opposite sides of cylinder head 18 in a cross flow orientation. Spark plug 41 is received within cylinder head 18. Drive gear 38, secured to crankshaft 32 between upper and lower crankshaft bearings 20 and 22 and disposed in the second or lower side 12b of crankcase 8, is driven by crankshaft 32, and drive gear 38 drives timing gear 40, which is twice the diameter of drive gear 38. Timing gear 40 is secured to timing shaft 42, which is rotatably carried in upper and lower timing shaft bearings 24 and 26, respectively, and extends substantially completely across crankcase 8. Timing gear 40 is disposed in the lower side of crankcase 8. Therefore, crankshaft 32 is directly rotatably coupled to timing shaft 42, via drive gear 38 and timing gear 40, at a location in crankcase base 14, and crankshaft 32 drives timing shaft 42 at half the speed of crankshaft 32 through a gear set including drive gear 38 and timing gear 40. Timing shaft 42 is disposed parallel to crankshaft axis L 1 -L 1 , and extends externally out of crankcase first or upper side 12a at one end, on which is secured a drive member in the form of toothed timing shaft pulley 44 held in place by snap ring 45 (Figs. 5 and 6). Timing shaft pulley 44 drives toothed timing belt 46 and toothed camshaft pulley 48 (Figs. 4 and 6) which is secured to an end of camshaft 50 which extends externally of cylinder head 18. Alternatively, other endless loop drives can be employed, such as a chain and sprocket mechanism (not shown). Belt guard 52 substantially covers timing belt 46, and is fixed to cylinder head 18 and crankcase casing 12. As shown in Figs. 1 and 4-6, a portion of timing belt 46 around timing shaft pulley 44 is not covered by belt guard 52 but rather is exposed. Camshaft 50 is carried in upper and lower camshaft bearings 54 and 56, respectively, within cylinder head 18, and is disposed along an axis substantially parallel to crankshaft axis L 1 -L 1 . Camshaft 50 has spaced cam lobes 58 which periodically actuate tappets 60 as camshaft 50 rotates. Tappets 60 are connected to valves 62 extending through valve guides 64 within the cylinder head 18. Valves 62 seat against valve seats 66 which are press-fitted into cylinder head 18. As piston 25 reciprocates, crankshaft 32 drives timing gear 40 at half crankshaft speed, which in turn drives timing shaft 42, timing pulley 44, timing belt 46, camshaft pulley 48, and camshaft 50 at a rotational speed equal to timing gear 40. Referring to Fig. 2, crankcase base 14 includes integral lower crankshaft bearing 22, and lower timing shaft bearing 26 located within oil pump housing 68. Oil pump housing 68 encloses a conventional gerotor oil pump (or similar pump), including external rotor 70 within oil pump housing 68 which is disposed above lower timing shaft bearing 26. Timing shaft 42 is shown in Fig. 3, on which is secured timing gear 40, internal rotor 74 and cover 76, which closes oil pump housing 68. As timing gear 40 and timing shaft 42 are driven by crankshaft 32, internal rotor 74 operatively engages external rotor 70, thereby drawing oil from oil sump 28 through oil inlet 72, and pumping oil throughout the engine via oil passageways (not shown). Because timing gear 40 is driven by crankshaft 32 at a position adjacent lower crankshaft bearing 22, existing, standard size crankshaft bearings are sufficient to handle the torque involved. This is in contrast with the conventional technique for driving a timing belt wherein a pulley is disposed on the end of the crankshaft external of the crankcase. In such an arrangement, the force moment on the end of the crankshaft is sufficiently large to require a larger bearing and a thicker or reinforced crankcase wall. As described above, speed reduction between crankshaft 32 and camshaft 50 occurs within crankcase 8, thereby enabling camshaft pulley 48 to be the same diameter as timing shaft pulley 44. This reduces significantly the size of cylinder head 18 and valve head cover 19, and also the length of the engine as measured from crankshaft 32 to the top of the valve head cover 19. This reduced engine silhouette is important in applications where space is at a premium, such as in walk behind lawnmowers. While the present invention has been described as having a preferred design, the present invention can be further modified within the scope of claim 1.

Claims: An overhead cam engine, comprising: a crankcase (8) having a first side (12a) and an opposite second side (12b); a cylinder head (18) attached to said crankcase; a crankshaft (32) rotatably disposed in said crankcase, said crankshaft driven by a piston (25) and connecting rod (37) assembly; a timing shaft (42) rotatably disposed in a first bearing (24) in said first side (12a) of said crankcase and in a second bearing (26) in said second side of said crankcase, said timing shaft directly rotatably coupled to said crankshaft at a location in said second side (12b), said timing shaft having an end extending externally out of said first side of said crankcase; a camshaft (50) rotatably disposed in said cylinder head (18) and actuating a pair of valves (62), said camshaft having an end extending externally out of said cylinder head; and drive linkage (44, 46, 48) disposed externally of said crankcase and connecting said timing shaft (42) and said camshaft (50), said timing shaft (42) and said crankshaft (32) being directly rotatably coupled through a gear set (38, 40) disposed in said second side of said crankcase (8). whereby the timing gear (40) on said timing shaft (42) is twice the diameter of the drive gear (38) on said crankshaft, such that said timing shaft is driven at half the speed of said crankshaft. The engine of Claim 1, characterized in that said drive linkage comprises: a timing pulley (44) mounted on said end of said timing shaft extending externally of said crankcase (8); a camshaft pulley (48) mounted on said end of said camshaft (50) extending externally of said cylinder head (18); and a timing belt (46) connecting said timing pulley and said camshaft pulley. The engine of Claim 2, characterized in that said timing pulley (44) and said camshaft pulley (48) have the same diameter. The engine of any of the preceding claims, characterized in that said crankcase (8) includes a base (14) with an oil pump (68, 70) disposed therein, said oil pump driven by said timing shaft (42). The engine of any of the preceding claims wherein said timing shaft (42) extends substantially completely across said crankcase (8).

Inventors: GHELFI GIUSEPPE, GOLZIO LUCIANO, MION PAOLO, MOLINA ROBERTO, GHELFI, GIUSEPPE, GOLZIO, LUCIANO, MION, PAOLO, MOLINA, ROBERTO
Applicants: TECUMSEH PRODUCTS CO, TECUMSEH PRODUCTS COMPANY
Application year: 2000
Main classification: F01L1
IPC codes: F01L1, F02B63, F02B75