A SMALL FOUR-CYCLE ENGINE HAVING COMPRESSION RELIEF TO FACILITATE STARTING

Abstract: The engine has a single cam (106) actuating both the intake and exhaust valves (50, 56). The cam has a primary cam surface (110) and a boss (112) extending from its side. An exhaust valve cam follower (90) engages only the primary cam surface. The intake valve cam follower (86, 114) has a first cam follower surface (124) engaging only the primary cam surface and a secondary cam engagement surface (128) engaging only the boss to open the intake valve during a predetermined portion of the engine's compression cycle to provide compression relief facilitating cranking. The secondary cam follower surface may be provided on the exhaust cam follower instead. In an alternate embodiment, the secondary cam surface (212) is displaced by centrifugal force to a location inhibiting the secondary cam engagement surface from engaging the secondary cam surface at normal engine operating speeds.

Description: A SMALL FOUR-CYCLE ENGINE HAVING COMPRESSION RELIEF TO FACILITATE STARTING TECHNICAL FIELD This invention is related to small four-cycle internal combustion engines and in particular to a compression relief mechanism to facilitate engine cranking. BACKGROUND ART Small internal combustion engines have found wide acceptance in garden implements such as line trimmers and leaf blowers and power tools such as chain saws. Initially, small two-cycle engines were used for these applications. However, two-cycle engines have well recognized exhaust emission problems that often make them unacceptable for their use in engines that must comply with exhaust emission regulations such as the California Air Resource Board and the Federal Environmental Protection Agency ("EPA") regulations. Limitations on exhaust emissions of carbon monoxide, hydrocarbons and nitrogen oxide that will be required in the near future cannot feasibly be met by outdoor power tools powered by two-cycle internal combustion engines. Four-cycle internal combustion engines in contrast provide a distinct advantage in that they are capable of meeting the new exhaust regulations and are quieter compared to a comparable two-cycle engines. A problem currently being faced with the small four-cycle engine is the force required to crank them to start. Since there is no substantial overlap between the exhaust and fuel intake cycles of a four-cycle engine, the force required to overcome the compression cycle of the four-cycle engines becomes much higher. This problem was recognized by the prior art and various mechanisms have been disclosed to reduce the manual force required to overcome the compression stroke. For example, Yamashita, et al in U.S. Patent 4,651,687; Holsehub in U.S. Patent 4,977,868; Teral, et al in U.S. patent 4,991,551 ; and Kojima, et al in U.S. Patent 5,948,992 all teach pressure release mechanisms deactivated by centrifugal force when the engine reaches operating speed. These mechanisms require moving parts and are equally actuated during the exhaust as well as the compression cycles keeping the exhaust valve partially open during the intake stroke as well. DISCLOSURE OF INVENTION The invention is an improved compression relief mechanism for small four-cycle engines of the type having a single cam actuating the exhaust and intake valves. The invention comprises a second cam surface provided on the single cam and either the intake valve cam follower or the exhaust valve follower has a second cam engagement surface which engages the second cam surface to partially open either the intake or the exhaust valve during the compression cycle to effect a compression relief reducing the force required to crank the engine. A first object of the invention is to provide a compression relief mechanism having no moving parts. Another object of the invention is to provide a compression relief mechanism for a four-cycle engine which is actuated only during the compression cycle. Another object of the invention is to provide a second cam surface provided on single cam engageable with a second cam engagement surface on either the intake valve cam follower or the exhaust valve cam follower. Still another object of the invention is to provide a boss extending from the side of the single cam lobe which provides the second cam surface and the cam follower has a second cam engagement surface which engages the boss to partially open either the intake or exhaust valve during a predetermined period during the compression cycle. Yet another object of the invention is a mechanism for disabling the engagement of secondary cam engagement surface with the secondary cam surface at normal engine operating speeds. These and other objects of the invention will become more apparent from a reading the detailed description of the preferred embodiment in conjunction with the appended drawings. BRIEF DESCRIPTION OF DRAWINGS FIGURE 1 is a cross-sectional side elevation of a single piston fourcycle gasoline engine; FIGURE 2 is a side cross-sectional view of the engine shown in Figure 1; FIGURE 3 is an enlarged schematic illustrating the cam lobe and cam follower mechanisms; FIGURE 4 is a perspective of the cam; FIGURE 5 is a perspective of the intake valve cam follower; FIGURE 6 is a schematic showing the primary cam engagement surfaces of the intake cam follower in contact with the cam surface; FIGURE 7 is a schematic having the secondary cam engagement surface of the intake cam follower in contact with the second cam surface; FIGURES 8a and 8b are graphs showing the displacement of the exhaust and intake valves during the four-cycles of the engine; FIGURE 9 is a front view of an alternate configuration of the cam; FIGURE 10 is a cross-sectional side view of the cam shown on Figure 8 ; FIGURE 11 is a perspective of a cam follower; FIGURE 12 is a front view of an alternate embodiment of the invention; and FIGURE 13 is a cross-sectional view of the alternate embodiment shown on Figure 12. BEST MODE FOR CARRYING OUT THE INVENTION Figure 1 illustrates a lightweight, single piston four-cycle internal combustion engine incorporating the compression relief mechanism. This internal combustion engine is of the type disclosed in U.S. Patent 5,738,062 issued to Everts on April 14, 1998, which is incorporated herein by reference. These engines are relatively lightweight and may be incorporated on various types of hand-held devices such as known in the art. Figure 2 is a side cross-sectional view of the four-cycle internal combustion engine 30. The engine 30 has a lightweight aluminum housing which has an engine block 32. The engine block 32 has a cylindrical piston bore 34 receiving a reciprocating piston 38. A crankshaft 36 is rotatably mounted within the engine block in a conventional manner. The piston 38 reciprocates within the piston bore 34 and is connected to the crankshaft by connecting rod 48. A cylinder head 42 is attached to the engine block 32 and defines in conjunction with the piston bore 34 and the piston 38 a combustion chamber 44. Cylinder head 42 is provided with an intake port 46 coupled to a carburetor 48 which provides a combustible air/fuel mixture. The carburetor 48 is intermittently connected to the combustion chamber 44 via an intake valve 50. Cylinder head 42 also has an exhaust port 52 connected to the combustion chamber 44 via an exhaust valve 56. Engine block 32 is part of the housing that provides an enclosed oil reservoir 58. The oil reservoir 58 is relatively deep so that ample clearance between the crankshaft and the level of the oil during normal use in which the engine may be tilted from the vertical by 20° or more. As illustrated in Figure 2, the crankshaft 36 is cantilevered and is provided with an axial shaft 62 having an output end 64 adopted to be coupled to a tool or implement. The opposite end of the shaft 62 is coupled to a crank 70 having an appropriate counterweight 68. Crank 70 cooperates with a series of roller bearing 72 mounted in the connecting rod 48 to rotate the crankshaft 36 with the reciprocation of the piston 38. The axial shaft 62 of the crankshaft 36 is rotatably attached to the engine block 32 by conventional bearings 74 and 76. A cam shaft drive gear 78 is attached to the crankshaft 36 intermediate bearings 74 and 76. The camshaft device and valve lifter mechanism of the four-cycle engine shall be discussed with reference to Figures 2 and 3. Drive gear 78 attached to the crankshaft engages a cam gear 80 journalled to the engine block 32 by a journal 33. Cam gear 80 rotates the camshaft assembly 82 having a single cam 84 at one-half the rotational speed of the crankshaft as is known in the art. As shown in Figure 3, the cam 84 is engaged by an intake valve cam follower 86 and an exhaust valve cam follower 90. Intake valve cam follower 86 actuates the intake valve 80 by means of push rod 88 and rocker arm 96 while exhaust valve cam follower 90 actuates the exhaust valve 56 by means of push rod 92 and rocker arm 94. The cam followers 86 and 90 are pivotably connected to the engine block 32 by means of pivot pin 93. The intake valve cam follower 86 and the exhaust valve cam follower are oriented to open the intake valve 50 during the intake engine cycle and to open the exhaust valve 56 during the engine's exhaust cycle in a conventional manner. A valve cover 98 is attached to the cylinder head 42 and the pair of push rod tubes surround the intake and exhaust push rods 88 and 92, respectively, in order to prevent the entry of dirt and other contaminants from entering into the engine block 32. A spark plug 104 is mounted in a threaded spark plug mounting bore provided in the cylinder head. The spark plug is periodically energized to ignite the air fluid mixture in the combustion chamber 44 during the combustion cycle of the engine. The engine 30 operates in a conventional four-cycle mode. The details of the cam 84 and the intake valve cam follower 86 which provide a desired compression relief to make the engine easier to manually crank, such as by a recoil starter, is shown in Figures 4 and 5. Figure 4 shows a cam 106 corresponding to cam 84 shown in Figure 3. Cam 106 has a mounting slot 108 which locks the rotation of the cam to the rotation of the cam gear 80, a primary cam surface 110 and a boss 112 which protrudes from the side of the cam and which provides a secondary cam surface. The cam follower 114 shown in Figure 5 which corresponds to the intake valve cam follower 86 has a pivot boss 116. The pivot boss 116 has a pivot bore 118 by means of which it is journalled to the housing 32 by journal 93 and an arm 120 which is engaged by push rod 88 at an end thereof. The cam follower also has a follower arm 122 having a primary cam engagement surface 124 which engages only the primary cam surface 110 of the cam 106. The cam follower 114 has an extension leg 126 which extends from the side of the cam follower arm 124 and has a secondary cam engagement surface 128 which is engageable with the boss 112 to disengage the primary cam engagement surface from engagement with the cam surface 110 during a predetermined rotational interval of the cam 106. The engagement of the secondary cam engagement surface with the boss 112 opens the intake valve for a predetermined portion of the compression cycle providing a compression relief reducing the cranking force on the cam shaft during cranking. As shown in Figure 6, when the boss 112 of the cam 106 is in a region displaced from the secondary cam engagement surface 128, the primary cam engagement surface is in intimate contact with the primary cam surface 110 and the position of the input cam follower is determined by the profile of the cam 106 as in a conventional prior art engine. In this position, the extension leg 126 extends along the side of the cam 106. However, when the position of the cam 106 is such that the boss 112 is engaged by the secondary cam engagement surface 128 as shown in Figure 7, the primary cam engagement surface 124 is displaced from the primary cam surface 110. This causes the intake valve cam follower to be rotated through a small angle activating the intake valve to remain slightly opened decreasing the pressure in the combustion chamber 44 as desired. The extended open period of the intake valve 50 during cranking results in only minimal degradation of engine performance when operating at higher engine speeds. Since the exhaust cam follower does not have an extension leg comparable to extension leg 126, the exhaust cam follower is unaffected by the presence of the boss 112 and it operates in a normal manner. Figure 8a is a graph showing the displacement of the exhaust valve, during the exhaust cycle 132 of the engine, curve 130, and the displacement of the intake valve 50, during the intake cyclel36 of the engine, curve 134. The portion of the curve 140 which is an extension of the curve 134 shows the continued opening of the intake valve during the compression cycle of the engine. The position of the intake valve 50 and the exhaust valve 56 during the combustion cycle remains the same as in prior art fourcycle internal combustion engines. Although the invention has been described and illustrated showing the intake valve cam follower being actuated by the secondary cam surface, it would be obvious to one skilled in the art that the exhaust valve cam follower rather than the intake valve cam follower could have an extension leg comparable to extension leg 126 and a secondary cam engagement surface corresponding to secondary cam engagement surface 128 and the boss 112 being located such that the exhaust valve rather than the intake valve is opened for a predetermined period of the compression cycle as shown in Figure 8b. This set of curves shows the temporary opening of the exhaust valve 56, curve 144, during the compression cycle 142. The invention contemplates opening either the intake valve or the exhaust valve for a short period of time during the compression cycle to provide the desired compression relief during cranking of the engine. An alternate embodiment of the cam and the cam follower is shown on Figures 9 through 11. Referring first to Figures 9 and 10, the cam 150 corresponds to cam 86 shown on Figure 3 and has a primary cam surface 152 and a mounting slot 154 which locks the rotation of the cam 150 to the rotation of the cam gear 82. The cam 150 further has an enlarged portion 16 which protrudes from one side of the cam 150. The peripheral surface of the enlarged portion 156 is a lateral extension of the cam surface 152. The enlarged portion 156 further includes radial protrusion or bump 158 which provides a secondary cam surface laterally displaced from the primary cam surface 152. The radial protrusion or bump 158 provides the secondary cam surface corresponding to the secondary cam surface provided by boss 112. The cam valve follower 160 shown on Figure 11 has a mounting bore 162 by means of which it is pivotably attached to the housing. The cam valve follower 150 has an arm 164 which is engaged by the exhaust or intake valve push rods 88 or 92 to open and close the exhaust and intake valves respectively. The cam follower 150 also has a follower arm 168 which engages the primary cam surface 152 of the cam 150. The width of the follower arm 168 at the end which engages the cam 150 is enlarged having a secondary cam engagement portion 170 which is capable of engaging the secondary cam surface of the radial protrusion 158. The cam 150 and cam follower 160 may be arranged to partially open the intake or exhaust valves during a predetermined portion of the compression cycle. The cam follower controlling the opening and closing of the valve not associated with cam follower 160 will not have a secondary cam engagement portion 170 and therefore will only follow the profile of the primary cam surface 152 and be unaffected by the secondary cam surface. The operation of this alternate embodiment is substantially the same as the embodiment shown on and discussed relative to Figures 4 through 8. Figures 12 and 13 illustrate still another embodiment of the cam activating the intake and exhaust valves of the engine. In this embodiment, the cam 206 has a primary cam surface 208 which is engaged by both the intake valve cam follower 86 and the exhaust valve cam follower 90 to actuate the intake vale and exhaust valve respectively. The secondary cam surface 212 is provided on a secondary cam 210 slidably attached to cam 206. The secondary cam 210 has a cam shaft slot 216 through which the cam shaft 82 is received. The cam shaft slot 216 is arranged to permit radial displacement of the secondary cam 210 but the sides of the cam shaft slot 216 prohibits transverse displacement of the secondary cam 210. The end 214 of the secondary cam 210 opposite the secondary cam surface 212 functions as a weight which produces a force biasing the secondary cam surface 212 away from the primary cam surface 208 at normal engine operating speeds. A guide pin 222 attached to the primary cam 206 is received in a guide pin slot 220 and controls the orientation of the secondary cam relative to cam 206. The guide pin slot 220 is dimensioned such that when the secondary cam 210 is displaced as far as it can go radially away from the cam shaft 82, the secondary cam surface 212 is engageable by the secondary cam engagement surface 124 of the intake valve cam follower 114 to produce the desired compression relief. However, when the engine is running, the radial force generated by the weight at the opposite end 214 of the secondary cam 210 will radially displace the secondary cam 210 and the secondary cam surface 212 towards the cam shaft 82 a distance sufficient to prevent engagement of the secondary cam surface 112 by the secondary cam engagement surface of the intake cam follower 114. The secondary cam is biased away from the cam shaft 82 to its operative position by a spring 224. One end of the spring 224 is received in a spring bore 218 provided in the secondary cam and the other end of the spring 224 engages camshaft 82. The spring 224 is selected to have a force sufficient to maintain the secondary cam 210 in the extended position at cranking speeds of the cam shaft 82, but will permit the secondary cam 210 to be radially retracted at nominal engine speeds to prevent the engagement of the secondary cam surface 212 by the secondary cam engagement surface 128 of the intake cam follower 114. The radial length of the secondary cam is selected so that neither end is engageable by the secondary cam engagement surface 128 at normal operating rotational speeds of the engine. The secondary cam 210 is slidably held against cam 206 by a conventional "C" washer received in an annular groove 228 provided in the cam shaft 82 as shown in Figure 13. As discussed above, the engagement of the secondary cam surface 212 by the secondary cam engagement surface 128 of the intake valve cam follower only produces compression relief during cranking of the engine. This mechanism is disabled by the withdrawal of the secondary cam by centrifugal force once the engine reaches a normal operating speed. Therefore, the compression relief is only obtained during cranking of the engine. As in previous embodiments, the secondary cam surface and secondary cam engagement surfaces may be arranged to open either the intake valve or exhaust valve during the compression cycle. While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.

Claims: WHAT IS CLAIMED IS: 1. A four-cycle engine having reduced cranking force, the engine having an intake cycle, a compression cycle, a combustion cycle and an exhaust cycle, the engine comprising: an engine block having a piston bore; a crankshaft rotatably mounted in the engine block; a piston disposed in the piston bore and operative to reciprocate therein; a connecting rod connecting the piston to the crankshaft, the reciprocation of the piston rotating the crankshaft; a cylinder head attached to the engine block closing the piston bore at the end opposite the crankshaft, the cylinder head cooperating with the piston and the engine block to form a combustion chamber; means for supplying a combustibly air/fuel mixture to the combustion chamber; an intake valve disposed between the means for supplying an air/fuel mixture and the combustion chamber to control the quantity of the air/fuel mixture being supplied to the combustion chamber; an exhaust valve connected to the combustion chamber to control the exhaustion of the combusted air/fuel mixture from the combustion chamber during the exhaust cycle; a cam rotatably connected to the engine block, the cam having a primary cam surface and a secondary cam surface provided at a predetermined rotational orientation relative to the primary cam surface; a gear train disposed between the cam and the crankshaft to rotate the cam at one-half the rotational speed of the crankshaft; a first valve cam follower disposed between the cam and one of the intake valve and the exhaust valve, the first cam follower having a primary cam engagement surface engaging the primary cam surface, the cam follower oriented to open a selected one of the intake valve and the exhaust valve, the first cam follower further having a secondary cam engagement surface engageable with the secondary cam surface to partially open selected one of the intake valve and the exhaust valve during the compression cycle to partially relieve the compression pressure in the combustion chamber during cranking; a second valve cam follower connected between the cam and the other of the intake valve and the exhaust valve, the second cam follower having a cam engagement surface engageable with only the primary cam surface, the second cam follower oriented relative to the cam to open the other valve; and a spark plug for igniting the air/fuel mixture in the combustion chamber to burn during the combustion cycle. 2. The engine of claim 1 wherein the secondary cam surface is displaced laterally from the primary cam surface. 3. The engine of claim 2 wherein the secondary cam surface is a boss extending from a side of the cam. 4. The engine of claim 3 wherein the boss is formed integral with the cam. 5. The engine of claim 3 wherein the secondary cam surface is a radial protrusion on a lateral extension of the primary cam surface. 6. The engine of claim 1 wherein the secondary cam surface is displaced at normal engine speeds to a position preventing the engagement of the secondary cam engagement surface with the secondary cam surface by centrifugal force. 7. A mechanism for partially opening a selected one of the intake valve and the exhaust valve of a four cycle engine during the compression cycle, the engine having at least a crankshaft, an intake valve and an exhaust valve, the mechanism comprising: a cam having a primary cam surface and a secondary cam surface; an exhaust valve cam follower disposed between the exhaust valve and the cam, the exhaust valve cam follower having a cam valve engagement surface engageable only with the primary cam surface, the exhaust valve cam follower oriented relative to the cam to open the exhaust valve during the exhaust cycle of the four-cycle engine; and an intake valve cam follower disposed between the intake valve and the cam, the intake valve cam follower having a primary cam engagement surface engageable only with the primary cam surface, the intake cam follower being oriented relative to the cam to open the intake valve during the intake cycle of the engine in response to the primary cam engaging surface engaging the primary cam surface, one of the intake valve followers and the exhaust cam follower having a secondary cam engagement surface engageable with the secondary cam surface of the cam to partially open the associated valve during the compression cycle to provide compression relief. 8. The mechanism of claim 7 wherein the primary and secondary cam surfaces on the cam are laterally displaced from each other. 9. The mechanism of claim 7 wherein the secondary cam surface is displaced radially inwardly from the primary cam surface and the secondary cam engagement surface extends radially from the primary cam engagement surface. 10. The mechanism of claim 9 wherein said secondary cam surface is the surface of a boss extending from the side of the cam. 11. The mechanism of claim 8 wherein the secondary cam surface is a radial protrusion on a lateral extension of the primary cam surface. 12. The mechanism of claim 10 wherein the boss is formed integral with the cam. 13. The mechanism of claim 7 wherein the secondary cam surface is displaceable radially by centrifugal force to a location prohibiting the engagement of the secondary cam surface by the secondary cam engagement surface of provided on one of the intake valve cam follower and the exhaust cam follower. 14. A compression relief mechanism to facilitate the cranking of a four-cycle internal combustion engine having intake and exhaust valves actuated by a single cam comprising: the cam having a primary cam surface and a secondary cam surface; an exhaust valve cam follower engaging only the primary cam surface, the exhaust cam follower oriented relative to the cam to open the exhaust valve during the engine's exhaust cycle; and an intake valve cam follower having a first cam engagement surface engageable with only the primary cam surface oriented relative to the cam to open the intake valve during the engine's intake cycle, one of the exhaust cam follower and the intake cam follower having a secondary cam engagement surface operative to engage the secondary cam surface during the compression cycle of the engine to provide compression relief. 15. The compression relief mechanism of claim 14 wherein the secondary cam engagement surface is displaced radially from the primary cam engagement surface. 16. The compression relief mechanism of claim 14 wherein the secondary cam surface is displaced radially inwardly from the primary cam surface and the second cam engagement surface extends from the first cam engagement surface along the side of the cam. 17. The compression relief mechanism of claim 16 wherein the secondary cam surface is provided by a boss extending from the side of the cam. 18. The compression relief mechanism of claim 14 wherein the secondary cam surface is provided on a second cam slidably attached to the cam, the second cam radially displaceable to a location prohibiting the engagement of the secondary cam surface by the secondary cam engagement surface.

Inventors: BILLETDEAUX MICHAEL, BILLETDEAUX, MICHAEL
Applicants: MTD SOUTHWEST INC, MTD SOUTHWEST INC.
Application year: 2001
Main classification: F01L13
IPC codes: F01L13