Patent Publication Number: US-2015075469-A1

Title: Ohv engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to Over Head Valve or OHV engines, and more specifically to an OHV engine preferably for use as a utility engine. 
     2. Description of the Related Art 
     A utility engine typically has its fuel tank located on an upper region of the engine so as to supply fuel without employing an expensive fuel pump. The higher the location of the fuel tank, the higher the center of gravity of the engine including the fuel tank with the fuel inside it, leading to increased vibration of the engine, limited maneuverability of the equipment mounted with the engine, etc. So, there is a requirement to lower a height of the engine. One idea for achieving this is to tilt cylinders of the engine, but tilting the cylinders will limit mounting dimensions of the cylinders for installation onto the equipment. Therefore, it is also required that engine&#39;s horizontal dimensions (width and depth) are also reduced. 
     In this case, an OHV engine such as disclosed in Japanese Unexamined Utility Model Application Publication No. H3-5909 is advantageous, i.e., an engine in which a cam shaft is disposed below a tilted cylinder helps reducing engine dimensions (height, width and depth). 
     Recently, however, there is another requirement for increased length of piston stroke for better fuel economy. A longer piston stroke results in a longer cylinder and an increased height of the engine, but it is still desirable to have a short engine height even if the piston stroke is increased. 
     SUMMARY OF THE INVENTION 
     Therefore, preferred embodiments of the present invention provide an OHV engine having a reduced height. 
     According to an aspect of various preferred embodiments of the present invention, an OHV engine includes a cylinder; a piston inside the cylinder; a crank shaft configured to convert reciprocating movement of the piston into rotating movement; a connecting rod configured to connect the piston and the crank shaft to each other; and a cam shaft configured to move in association with the crank shaft. In this engine, the cylinder is inclined in an obliquely upward direction so that an angle θ 1  defined by a cylinder center axis and a horizontal plane preferably is greater than 0 degree and smaller than about 45 degrees, for example, when the cylinder is viewed from the crank shaft; the camshaft is at a lower position than the cylinder center axis; and the cylinder center axis is offset with respect to a line which passes through a rotation axis of the crank shaft and is parallel or substantially parallel to the cylinder center axis, to a side that is spaced away from the cam shaft. 
     According to various preferred embodiments of the present invention, it is possible to increase a distance between the cylinder and the cam shaft by offsetting the cylinder center axis to the side away from the cam shaft. This means that it is possible to achieve more inclination in the cylinder without changing a distance between the cam shaft and the crank shaft (distance between the cam shaft&#39;s rotation axis and the crank shaft&#39;s rotation axis) within a range where there is no interference between the cylinder and the camshaft. Consequently, it is possible to lower the height of the engine. Further, the cylinder inclines such that the angle θ 1  which is defined by the cylinder center axis and the horizontal plane is greater than 0 degree and smaller than about 45 degrees, for example. Therefore, even when the cylinder is given a longer dimension in the direction of the cylinder center axis, an increase in the height of the engine is smaller than an increase in horizontal dimensions of the engine. As a result, it becomes possible to reduce the height of the engine even if the length of the cylinder is increased. 
     Preferably, the OHV engine further includes a crank case attached to the cylinder and accommodating the crank shaft and the cam shaft; a drive gear provided around the crank shaft; and a driven gear provided around the cam shaft and rotating in association with rotation of the drive gear. With this arrangement, the cam shaft has its rotation axis at a lower position than a horizontal plane which passes through the rotation axis of the crank shaft; and the horizontal plane and a straight line which connects the rotation axis of the cam shaft and the rotation axis of the crank shaft define an angle θ 2  which is greater than 0 degree and smaller than about 45 degrees, for example. When increasing the piston stroke, the crank webs have to have an increased outward form, and therefore the cam shaft must be moved away from the crank shaft in order to prevent interference with the crank webs. This results in an increase in the distance between the rotation axis of the cam shaft and the rotation axis of the crank shaft, and an increase in a diameter (radius) of the cam shaft driven gear, so the camshaft driven gear is dipped in oil inside the crank case to an increased height (depth). If the driven gear is dipped into the oil to an increased height (depth) when splashing the oil, more horse power is lost. 
     However, if the angle θ 2  which is defined by the horizontal plane and the straight line that connects the rotation axis of the cam shaft and the rotation axis of the crank shaft with each other is greater than 0 degree and smaller than about 45 degrees, for example an increase in the vertical distance is smaller than an increase in the horizontal distance even when the distance between the rotation axis of the cam shaft and the rotation axis of the crank shaft is increased. Because of this, and due to the fact that the horizontal plane is parallel or substantially parallel to a surface of the oil, the arrangement makes it possible to significantly reduce or prevent an increase in a surface area of the portion of the driven gear in the cam shaft that is dipped into the oil, and therefore makes it possible to reduce an increase in loss of horse power. In addition, it is also possible to reduce an increase in the height of the engine. 
     Further preferably, the angle θ 1  is greater than the angle θ 2 . In this case, location of the cam shaft is limited so that the angle θ 2  will be smaller than the angle θ 1 , whereas the rotation axis of the camshaft is disposed at a higher position, i.e., more closely, to the horizontal plane which passes through the rotation axis of the crank shaft. The cam shaft disposed at the higher position makes it possible to increase a distance between the cam shaft and the surface of the oil inside the crank case, which then makes it possible to raise the surface of the oil (more closely to the horizontal plane which passes the rotation axis of the crank shaft). Consequently, it becomes possible to reduce dimensions of the crank case in the up-down direction. As a result, it becomes possible to reduce the dimensions of the engine in the up-down direction, i.e., the height of the engine. 
     Further, preferably, a rotation direction of the crank shaft is counterclockwise when the OHV engine is viewed from a position where the crank shaft is located on the left side and the cam shaft is located on the right side. Generally in an OHV engine, a maximum combustion pressure is reached slightly after the piston has passed the top dead center. At this point, the combustion pressure in a space surrounded by the cylinder and the piston peaks out, with the piston under an increased thrust force. However, as described above, the cylinder center axis is preferably offset to the side spaced away from the cam shaft and the crank shaft is rotated counterclockwise. This makes it possible to bring the connecting rod (especially a portion thereof which is closer to the crank shaft) closely to the cylinder center axis slightly after the piston has passed the top dead center. Specifically, during the time when the combustion pressure is high, the connecting rod and the cylinder are in a positional relationship which advantageously reduces a thrust force that acts on the piston. This reduces friction which acts on the piston, and thus improves fuel economy. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view from a front left view point, of an engine generator which includes an OHV engine according to a preferred embodiment of the present invention. 
         FIG. 2  is a perspective view from a rear right view point, of the engine generator which includes the OHV engine according to a preferred embodiment of the present invention. 
         FIG. 3  is a diagram of the engine, showing a longitudinal section thereof. 
         FIG. 4  is a graph which shows a relationship between a crank angle and a pressure inside a cylinder of the engine. 
         FIG. 5A  is a diagram for describing a force which acts on a piston if the cylinder is not offset, whereas  FIG. 5B  is a diagram for describing a force which acts on the piston if the cylinder is offset. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. 
       FIG. 1  and  FIG. 2  show an engine generator  10  which includes an OHV engine (Over Head Valve Engine, hereinafter called “engine”)  24  (which will be described later) according to a preferred embodiment of the present invention. In the present specification, a “fore-aft direction” and a “left-right direction” in the engine generator  10  are defined as shown in  FIG. 1  and  FIG. 2  for the sake of descriptive convenience. Thus, a side on which the engine  24  is provided is a “front side”, a side on which a generator  26  (to be described later) is provided is a “rear side”, and a side on which an operation panel  48  (to be described later) is provided is a “left side”. 
     The engine generator  10  preferably is a portable generator, including a generator frame  12 . The generator frame  12  includes a front frame  14 , a rear frame  16 , an upper frame  18 , and a pair of lower frames  20 ,  22 . The front frame  14  is provided by a pipe-shaped member which is preferably formed into a general shape of inverted letter of U in a front view, whereas the rear frame  16  is provided by a pipe-shaped member which is preferably formed into a general shape of inverted letter of U in a rear view. The front frame  14  and the rear frame  16  are connected with each other at both of their end portions. The upper frame  18  is provided by a pipe-shaped member and extends in the fore-aft direction, connecting upper left end portions of the front frame  14  and the rear frame  16 , respectively. The upper frame  18  defines and serves as a grip. The lower frame  20  is a platy member extending in the left-right direction, connecting left and right lower portions of the front frame  14  with each other. The lower frame  22  is a platy member extending in the left-right direction, connecting left and right lower portions of the rear frame  16  with each other. 
     The engine  24  is installed on the lower frame  20 , whereas the generator  26  is installed on the lower frame  22 . The engine  24  and the generator  26  are arranged in the fore-aft direction, with the engine  24  being on the front side and the generator  26  being on the rear side. The engine  24  includes a crank shaft  66  (to be described later), which is connected with a rotating shaft (not illustrated) of the generator  26 . 
     The engine  24  includes, on its front side, an air intake section  28  to introduce outside air. The air intake section  28  includes a cooling fan (not illustrated). An air cleaner  30  is provided on the right side of the air intake section  28 . As the cooling fan is driven, outside air introduced from the air intake section  28  cools the engine  24 . Near the air intake section  28 , a recoil starter  32  is provided. 
     A muffler  34  is provided behind the engine  24 , on the right side of the generator  26 . Exhaust gas from the engine  24  is discharged to outside via the muffler  34 . A canister  36  is provided below the engine  24 . A fuel tank  38  is connected to the air cleaner  30  via the canister  36 . Gasoline vapor from the fuel tank  38  is adsorbed in the canister  36 . 
     The fuel tank  38  is arranged to cover the engine  24  and the generator  26  from above. The fuel tank  38  stores fuel (for example, gasoline, in the present preferred embodiment) which is to be supplied to the engine  24 . The fuel tank  38  has its right side portion attached to a support frame  40  which connects an upper right end portion of the front frame  14  and an upper right end portion of the rear frame  16  to each other. The fuel tank  38  has its left front portion and left rear portion connected to the front frame  14  and the rear frame  16  respectively via brackets  42  and  44 . 
     An operation box  46  is provided on the left side of the fuel tank  38 . The operation box  46  includes the operation panel  48 , and a case  50  which is provided on the right side of the operation panel  48  and incorporates an operation section (not illustrated), etc. A battery  52  is provided below the case  50 . 
     In the engine generator  10  described as above, the recoil starter  32  is pulled to rotate the crank shaft  66  and start the engine  24 . As the engine  24  starts, the generator  26  starts its power generating operation. The electric power from the generator  26  can be taken out of the operation panel  48  or stored in the battery  52 . 
     Reference will now be made to  FIG. 3  to describe the engine  24 . 
     The engine  24  preferably is an air-cooled, single-cylinder, four-cycle engine for example, of a slanted type in which a cylinder center axis A is slanted obliquely. The engine  24  includes a cylinder  54 . The cylinder  54  includes a cylinder body  56  and a cylinder head  58  which is attached to an upper end portion of the cylinder body  56 . A cylinder head cover  60  is attached to an upper end portion of the cylinder head  58 . A crank case  62  is provided in a lower portion of the cylinder body  56 . 
     The cylinder body  56  has an inner circumferential surface provided with a cylinder liner  56   a.  Inside the cylinder body  56 , a piston  64  is provided slidably with respect to the cylinder liner  56   a.  The crank case  62  accommodates the crank shaft  66  and a camshaft  68  which moves in association with the crank shaft  66 . The crank shaft  66  is disposed horizontally. The crank shaft  66  and the cam shaft  68  are parallel or substantially parallel to each other. The camshaft  68  is disposed not to interfere (contact) with crank webs  70  of the crank shaft  66 . The piston  64  and the crank shaft  66  are connected to each other by a connecting rod  72 , such that reciprocating movement of the piston  64  is converted into rotating movement by the crank shaft  66 . The crank shaft  66  is provided with a drive gear  74 , whereas the cam shaft  68  is provided with a driven gear  76  which rotates in association with rotation of the drive gear  74 . The crank case  62  also accommodates a balancer  78 . The balancer  78  is in engagement with a gear  80  provided in the crank shaft  66 , to reduce vibration. 
     From the cylinder body  56  to the cylinder head  58 , there is provided a communication path  84  which provides communication between inside of the crank case  62  and inside of a rocker arm chamber  82  in the cylinder head cover  60 . A pushrod  86 , and a tappet  88  provided on an end portion of the pushrod  86  are inserted through the communication path  84 . Inside the crank case  62 , the tappet  88  has its tip portion contacted to a cam  90  of the cam shaft  68 . The pushrod  86  has another end portion contacted to a rocker arm  92  which is provided inside the rocker arm chamber  82 . The rocker arm  92  drives an exhaust valve  94 . Additionally, though not illustrated in  FIG. 3 , the engine  24  accommodates a pushrod, a tappet and a rocker arm for driving an inlet valve, in parallel or substantially in parallel to the pushrod  86 , the tappet  88  and the rocker arm  92  to drive the exhaust valve  94 . 
     An oil dipper  96  is attached to a big end portion  72   a  of the connecting rod  72 , and oil  97  is stored inside the crank case  62 . The oil  97  is splashed by the oil dipper  96  to the cylinder body  56 , the cylinder head  58 , the cylinder head cover  60  and so on, directly or indirectly after spattering on the crank shaft  66 , the cam shaft  68 , etc., such that lubrication of the crank shaft  66 , the cam shaft  68 , the cylinder body  56 , the rocker arm  92 , etc. is achieved. 
     With the above-described arrangement, the cylinder body  56  is inclined in an obliquely upward direction so that the cylinder center axis A and a horizontal plane H define an angle θ 1  which is greater than 0 degree and smaller than about 45 degrees, for example, when the cylinder body  56  is viewed from the crank shaft  66 . The cam shaft  68  is at a lower position than the cylinder center axis A. The cylinder center axis A is offset with respect to a line C which passes through a rotation axis B of the crank shaft  66  and is parallel or substantially parallel to the cylinder center axis A, by a distance X to a side away from the cam shaft  68 . The horizontal plane H is a plane that is parallel or substantially parallel to a liquid surface  97   a  of the oil  97  stored in the crank case  62 .  FIG. 3  shows a horizontal plane H which passes through the rotation axis B of the crank shaft  66 . 
     The cam shaft  68  has its rotation axis D at a lower height than the horizontal plane H which passes through the rotation axis B of the crank shaft  66 . A straight line E which connects the rotation axis D of the cam shaft  68  and the rotation axis B of the crank shaft  66  with each other defines an angle θ 2  with the horizontal plane H, and the angle θ 2  is greater than 0 degree and smaller than about 45 degrees, for example. It is preferable that the angle θ 1  is greater than the angle θ 2 . 
     Further, as shown in  FIG. 3 , when the engine  24  is viewed from a position where the crank shaft  66  is located on the left side and the camshaft  68  is located on the right side, a rotation direction of the crank shaft  66  is counterclockwise as indicated by Arrow F. 
     According to the engine  24  as has been described thus far, it is possible to increase a distance between the cylinder body  56  and the cam shaft  68  by offsetting the cylinder center axis A to the side away from the cam shaft  68 . Thus, it is possible to achieve more inclination in the cylinder body  56  without changing a distance between the cam shaft  68  and the crank shaft  66  (distance between the rotation axis D of the cam shaft  68  and the rotation axis B of the crank shaft  66 ) within a range where there is no interference between the cylinder body  56  and the cam shaft  68 . Consequently, it is possible to lower the height of the engine  24  within this range. Further, the cylinder body  56  inclines within the condition that the angle θ 1  which is defined by the cylinder center axis A and the horizontal plane H is greater than 0 degree and smaller than about 45 degrees, for example. Therefore, even when the cylinder body  56  is given a longer dimension in the direction of the cylinder center axis A, an increase in the height of the engine  24  is smaller than an increase in the horizontal dimensions of the engine  24 . As a result, it becomes possible to reduce the increase in the height of the engine  24  even if the length of the cylinder body  56  is increased. 
     When increasing the piston stroke generally in an OHV engine, the crank webs have to have an increased outward form, and therefore the cam shaft must be moved away from the crank shaft in order to prevent interference with the crank webs. This results in an increase in the distance between the rotation axis of the cam shaft and the rotation axis of the crank shaft, and an increase in a diameter (radius) of the cam shaft driven gear, so the cam shaft driven gear is dipped in oil inside the crank case to an increased height (depth). If the driven gear is dipped into the oil to an increased height (depth) when splashing the oil, more horse power is lost. According to the engine  24 , however, the angle θ 2  which is defined by the horizontal plane H and the straight line E that connects the rotation axis D of the cam shaft  68  and the rotation axis B of the crank shaft  66  with each other preferably is greater than 0 degree and smaller than about 45 degrees, for example. Due to this arrangement, even when the distance between the rotation axis D of the cam shaft  68  and the rotation axis B of the crank shaft  66  is increased, an increase in the vertical distance is smaller than an increase in the horizontal distance. Because of this, and due to the fact that the horizontal plane H is in parallel or substantially parallel to the liquid surface  97   a  of the oil  97 , the arrangement makes it possible to significantly reduce or prevent an increase in a surface area of the portion of the driven gear  76  in the cam shaft  68  that is dipped into the oil  97 , and therefore makes it possible to significantly reduce or prevent an increase in loss of horse power. In addition, the arrangement also significantly reduces or prevents an increase in the height of the engine  24 . Further, if the distance X from the line C which is parallel or substantially parallel to the cylinder center axis A is increased without moving the rotation axis B of the crank shaft  66  to a higher position, it becomes possible to decrease the distance between the rotation axis D of the cam shaft  68  and the rotation axis B of the crank shaft  66  while significantly reducing or preventing an increase in the height of the engine  24 , and therefore it becomes possible to also decrease the diameter (radius) of the driven gear  76  of the cam shaft  68  to an extent that the driven gear  76  of the cam shaft  68  does not have its lower end contacting the liquid surface  97   a  of the oil  97 . The arrangement described above significantly reduces or prevents loss of horse power. 
     In the engine  24 , location of the cam shaft  68  is limited so that the angle θ 2  is smaller than the angle θ 1 , whereas the rotation axis D of the cam shaft  68  is disposed at a higher position, i.e., more closely to the horizontal plane H which passes through the rotation axis B of the crank shaft  66 . The cam shaft  68  disposed at the higher position allows to increase the distance between the cam shaft  68  and the liquid surface  97   a  of the oil  97  inside the crank case  62 , which then makes it possible to raise the liquid surface of the oil  97  (closer to the horizontal plane H which passes through the rotation axis B of the crank shaft  66 ). Consequently, it becomes possible to reduce a dimension of the crank case  62  in the up-down direction. As a result, it becomes possible to reduce the dimension of the engine  24  in the up-down direction, i.e., the height of the engine  24 . 
     Generally in an OHV engine, a maximum combustion pressure is reached slightly after the piston has passed the top dead center. At this point, the combustion pressure in a space surrounded by the cylinder body and the piston peaks out, with the piston under an increased thrust force. According to the engine  24 , as described earlier, the cylinder center axis A preferably is offset to the side spaced away from the cam shaft  68  and the crank shaft  66  is rotated counterclockwise (as indicated by Arrow F in  FIG. 3 ). This arrangement makes it possible to bring the connecting rod  72  (especially the center of the big end portion  72   a ) closely to the cylinder center axis A slightly after the piston  64  has passed the top dead center. Specifically, during the time when the combustion pressure is high, the connecting rod  72  and the cylinder body  56  are in a positional relationship which advantageously reduces a thrust force that acts on the piston  64 . This reduces friction which acts on the piston  64 , and thus improves fuel economy. 
     This will be elaborated with reference to  FIG. 4 ,  FIG. 5A  and  FIG. 5B . 
     In the engine  24 , a combustion pressure (pressure inside the cylinder) which acts on the piston  64  rotates the crank shaft  66  via the connecting rod  72 . Although an ignition occurs before the piston  64  reaches the top dead center, combustion takes a certain amount of time, and as shown in  FIG. 4 , the pressure inside the cylinder is reached to its peak after the top dead center is reached. Here, an angle difference a will be defined as a difference between a crank angle when the piston  64  is at the top dead center and a crank angle when the pressure inside the cylinder is at its peak. 
     With reference to  FIG. 5A , if there is no cylinder offset, a connecting rod reaction force has a thrust component as a function of the angle difference a, and this thrust force acts on the piston  64 . In this case, there is a large friction force generated between the piston  64  and the cylinder  54 , which causes a friction loss. 
     In  FIG. 5B , on the other hand, when the pressure inside the cylinder is around its peak, the connecting rod  72  has its longitudinal axis (a straight line which passes through a connecting section between the connecting rod  72  and the piston  64 , and a connecting section between the connecting rod  72  and the crank shaft  66 ), in or substantially in alignment with the cylinder center axis A or in parallel or substantially in parallel thereto, so a connecting rod reaction force has zero or a small thrust component due to the amount of offset of the cylinder  54 . This decreases the friction force and the friction loss between the piston  64  and the cylinder  54 . 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.