Patent Publication Number: US-6659057-B2

Title: Small four-cycle engine having compression relief to facilitate cranking

Description:
This is a divisional of application Ser. No. 09/499,973 filed on Feb. 08, 2000, now U.S. Pat. No. 6,401,678 B1. 
    
    
     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. Pat. No. 4,651,687; Holsehub in U.S. Pat. No. 4,977,868; Teral, et al in U.S. Pat. No. 4,991,551; and Kojima, et al in U.S. Pat. No. 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 
     FIG. 1 is a cross-sectional side elevation of a single piston four-cycle gasoline engine; 
     FIG. 2 is a side cross-sectional view of the engine shown in FIG. 1; 
     FIG. 3 is an enlarged schematic illustrating the cam lobe and cam follower mechanisms; 
     FIG. 4 is a perspective of the cam; 
     FIG. 5 is a perspective of the intake valve cam follower; 
     FIG. 6 is a schematic showing the primary cam engagement surfaces of the intake cam follower in contact with the cam surface; 
     FIG. 7 is a schematic having the secondary cam engagement surface of the intake cam follower in contact with the second cam surface; 
     FIGS. 8 a  and  8   b  are graphs showing the displacement of the exhaust and intake valves during the four-cycles of the engine; 
     FIG. 9 is a front view of an alternate configuration of the cam; 
     FIG. 10 is a cross-sectional side view of the cam shown on FIG. 8; 
     FIG. 11 is a perspective of a cam follower; 
     FIG. 12 is a front view of an alternate embodiment of the invention; and 
     FIG. 13 is a cross-sectional view of the alternate embodiment shown on FIG.  12 . 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     FIG. 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. Pat. No. 5,738,062 issued to Everts on Apr. 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. 
     FIG. 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 FIG. 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 FIGS. 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 FIG. 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&#39;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 FIGS. 4 and 5. FIG. 4 shows a cam  106  corresponding to cam  84  shown in FIG.  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 FIG. 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 FIG. 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 FIG. 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. FIG. 8 a  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 cycle  136  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 four-cycle 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 FIG. 8 b . 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 FIGS. 9 through 11. Referring first to FIGS. 9 and 10, the cam  150  corresponds to cam  86  shown on FIG.  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 FIG. 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 FIGS. 4 through 8. 
     FIGS. 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 FIG.  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.