Abstract:
An exhaust control valve assembly for a two-cycle engine includes a governor for moving an exhaust control valve in opening and closing directions to advance or retard a timing of opening of an exhuast port. The governor includes a governor spring for urging the exahust control valve in the closing direction. The governor spring exhibits a changing spring constant which increases as the exhaust control valve moves in the opening direction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an exhaust control valve assembly for an engine, such as a two-cycle engine. 
     2. Description of the Relevant Art 
     In accordance with the background art, an exhaust control valve assembly includes an exhaust control valve. The exhaust control valve is opened or closed in order to advance or retard the opening of an exhaust port. A drive system causes opening and closing of the exhaust control valve. Such an arrangement in disclosed, for example, in Japanese Patent Laid-Open Publication No. Sho. 63-306227. 
     The background art&#39;s exhaust control valve assembly suffers drawbacks. When the engine is running at high speed, the actual timing or degree of opening of the exhaust control valve gets ahead of the desired timing or degree of opening. When the actual timing or degree of opening is ahead of the desired timing or degree of opening, the engine&#39;s output performance is reduced. 
     SUMMARY OF THE INVENTION 
     The present inventors have discovered the source of the drawbacks, and invented a solution to the drawbacks. The present inventors observed that pressure within the exhaust port increases with increases in engine speed. The increased pressure, within the exhaust port, acts on the exhaust control valve, in the opening direction of the exhaust control valve, causing the exhaust control valve to open sooner or more than required. 
     It is therefore the object of the present invention to provide an exhaust control valve assembly, which increases the output performance of an engine. 
     It is another object of the present invention to provide an exhaust control valve assembly, which opens the exhaust control valve to a desired timing and degree. 
     These and other objects of the invention are accomplished by an engine comprising: a cylinder block having a cylinder bore; an exhaust opening at an inner wall of said cylinder bore; an exhaust control valve moveable in an opening direction and a closing direction to at least partially block said exhaust opening, and a governor attached to said exhaust control valve for moving said exhaust control valve in the opening direction in response to an increased engine speed, said governor including a governor spring urging said exhaust control valve in the closing direction, wherein said governor spring has a spring force which changes as said exhaust control valve moves. 
     Further, these and other objects of the invention are accomplished by an engine comprising: a cylinder block having a cylinder bore; an exhaust opening at an inner wall of said cylinder bore; an exhaust control valve moveable in an opening direction and a closing direction to at least partially block said exhaust opening; and a governor attached to said exhaust control valve for moving said exhaust control valve in the opening direction in response to an increased engine speed, said governor including a first spring and a second spring urging said exhaust control valve in the closing direction, wherein said first and second springs present an overall spring force which changes as said exhaust control valve moves. 
     Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein: 
     FIG. 1 is a side view of a two-cycle engine, equipped with the exhaust control valve of the present invention; 
     FIG. 2 is a cross-sectional view of the engine of FIG. 1; 
     FIG. 3 is a cross-sectional view taken along line  3 — 3  of FIG. 2; 
     FIG. 4 is a cross-sectional view taken along line  4 — 4  of FIG. 1; 
     FIG. 5 is an enlarged view of a portion of the engine within a dashed circle  5  of FIG. 1, 
     FIG. 6 is a graph of a spring characteristic of a governor spring, and 
     FIG. 7 is a cross-sectional view, similar to FIG. 4, illustrating a modified embodiment of the governor spring. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-3 illustrate a two-cycle motorcycle engine E, with an engine main body  1 . The engine main body  1  includes a crankcase  2 , a cylinder block  3  connected to an upper end of the crankcase  2 , and a cylinder head  4  connected to an upper end of the cylinder block  3 . A piston  5 , rising and falling in a cylinder bore  3   a  of the cylinder block  3 , is connected to a crankshaft  6  via a connecting rod  7 . The crankshaft  6  is housed in and supported by the crankcase  2 . One or more spark plugs  8  are then screwed into the cylinder head  4 , so that electrodes of the spark plugs  8  face an ignition chamber  4   a.    
     An exhaust port  10 , opened and closed by the piston  5 , is provided in the surrounding wall of the cylinder block  3 . An exhaust control valve  11  is arranged at an upper edge of an upstream opening of the exhaust port  10 , near the cylinder bore  3   a . The exhaust control valve  11  exerts control in such a manner as to advance the timing of opening of the exhaust port  10 , according to increases in the engine speed. A reinforcing wall  12  is integrally formed with, or alternately attached to, the cylinder block  3 . The reinforcing wall  12  laterally bisects a central part of the upstream opening of the exhaust port  10  and connects a first or upper wall and a second or lower wall. 
     As shown in FIG. 3, the exhaust control valve  11  includes a first valve element  13  and a second valve element  14 , arranged so as to sandwich the reinforcing wall  12 , and a single valve stem  15 . The valve stem  15  is coupled to a first boss  13   a  and a second boss  14   a  of the first valve element  13  and the second valve element  14 , respectively. The end surfaces of the first and second valve elements  13  and  14  are formed as arced surfaces, continuing on from the inner surface of the cylinder bore  3   a , and the first boss  13   a  and the second boss  14   a  abut from the rear of the reinforcing wall  12 . 
     The valve stem  15  is supported at both ends. One end is supported by a bushing  16 . The other end is supported by a ball bearing  17 . The bushing  16  and ball bearing  17  are lined up along the same axis of the valve stem. The bushing  16  is fitted into a pouch-shaped bearing hole  19  provided at a sidewall of the cylinder block  3 . The ball bearing  17  is fitted into a bearing through-hole  20  at the other sidewall of the cylinder block  3 . A central part of the valve stem  15  is formed with angled surfaces  15   a , such a square cross sectional shape. The first boss  13   a  and the second boss  14   a  of the valve elements  13  and  14  engage with the angled surfaces  15   a  of the valve stem  15 , so as to rotate in unison therewith. 
     As shown in FIGS. 3 and 4, rocker valve sections of the first and second valve elements  13  and  14  are formed in a symmetrical manner with respect to the reinforcing wall  12 . The first boss  13   a  and the second boss  14   a  of the valve elements  13  and  14  are formed and positioned in a particular manner, in accordance with the present invention. Namely, a first distance (A) represents a distance, in an axial direction of the valve stem  15 , from the outer end of the boss  13   a , on the pouch-shaped bearing hole  19  side, to the center of the reinforcing wall  12 . Also, a second distance (B) represents a distance, in the axial direction of the valve stem, from the outer end of the boss  14   a , on the bearing through hole  20  side, to the center of the reinforcing wall  12 . The first distance (A) is set to be less than the second distance (B). As a result, both the first boss  13   a  and the second boss  14   a  are arranged in such a manner as to be offset overall from the reinforcing wall  12  towards the bearing through hole  20  side of the exhaust port  10 . 
     A further aspect of the invention is to define the length of the first boss  13   a  as a first length (C), and to define the length of the second boss  14   a  as a second length (D). The first length (C) is set to be greater than the second length (D). As a result, the thermal capacity of the first boss  13   a  is larger than the thermal capacity of the second boss  14   a.    
     As shown in FIGS. 4 and 5, a first driven lever  21  and a second driven lever  22  are attached in a rotatable manner to a second end of the valve stem  15 , protruding outwards from the ball bearing  17 . The first driven lever  21  and the second driven lever  22  are attached using a nut  23 . The first driven lever  21  is equipped with first and second arms  21   a  and  21   b . The second driven lever  22  is equipped with first and second arms  22   a  and  22   b . The first arms  21   a  and  22   a  of the first and second driven levers  21  and  22  are coupled by a pincer spring  24 . The pincer spring  24  is in the form of a twisted coil fitted at the valve stem  15 . The first driven lever  21  therefore rotates the second driven lever  22 , via the pincer spring  24 , so that the exhaust control valve  11  is vertically opened and closed via the valve stem  15 . 
     A lower limit stopper  25  and an upper limit stopper  26  are arranged opposite each other at a lower surface and upper surface of the second arm  22   b  of the second driven lever  22 , respectively. The lower limit stopper  25  and upper limit stopper  26  limit the extent of opening and closing of the exhaust control valve  11 . The lower limit stopper  25  stops a lower surface of the second arm  22   b , so as to define the closed position of the exhaust control valve  11 . The upper limit stopper  26  stops an upper surface of the second arm  22   b , so as to define the open position of the exhaust control valve  11 . The pincer spring  24  can also be modified, so that rotation of the second driven lever  22  is allowed to exceed the extent of rotation of the first driven lever  21 . 
     A centrifugal governor  28  is coupled to the second arm  21   a  of the first driven lever  21  via a link  29 . The centrifugal governor  28  includes a support shaft  32 , supported at the crankcase  2  via a pair of ball bearings  30  and  31 , a governor gear  33 , fixed to the support shaft  32 ; a plurality of centrifugal weights  35 , arranged about the support shaft  32  at a conical recess  34  formed at the side of the governor gear  33 ; a slider  36 , supported in a freely slidable manner at the support shaft  32 , and making contact with the group of centrifugal weights  35  on the opposite side to the governor gear  33 ; and a governor spring  47 , for urging the slider  36  towards the side of the centrifugal weights  35  with a prescribed setting load. A drive gear  48 , fixed to the crankshaft  6 , meshes with the governor gear  33  to bring about driving. 
     An annular rack  50  is attached to the slider  36 , in a freely rotatable manner, via a ball bearing  51 . A pinion shaft  53 , having a pinion  52  meshing with the annular rack  50 , is supported in a freely rotatable manner at the crankcase  2 . A drive lever  54 , fixed to an end of the pinion shaft  53 , is coupled to the second arm  21   b  of the first driven lever  21  via the link  29 . 
     As illustrated in FIG. 4, the governor spring  47  is a single coil spring installed at the outer periphery of the support shaft  32 . The governor spring  47  is equipped with a small pitch coil section  47   a  and a large pitch coil section  47   b . With reference to the above elements, a drive system for driving the valve stem  15  is constituted by the centrifugal governor  28 , the pinion shaft  53 , the drive lever  54 , the link  29 , the first driven lever  21 , the pincer spring  24 , and the second driven lever  22 . 
     During operation, when the engine is running, the governor gear  33  is driven from the drive gear  48  of the crankshaft  6 . The centrifugal weights  35  rotate in accompaniment with this rotation, and push outward of the conical surface of the recess  34 , due to their centrifugal force. The centrifugal weights  35  try to push the slider  36  towards the side of the governor spring  47 . 
     When the engine is running at low speeds, the setting load of the governor spring  47  is larger than the force the group of centrifugal weights  35  exerts on the slider  36 . Because the centrifugal force of the group of centrifugal weights  35  is low, the slider  36  and annular rack  50  are held in the retreated position shown in FIG.  4 . In this position, the drive lever  54  is in an advanced position. In the advanced position, the second driven lever  22  is rotated, via the link  29 , the first driven lever  21 , and the pincer spring  24 , to a position whereby contact is made with the lower limit stopper  25 . In the advanced position, the exhaust control valve  11  is held in a closed position (downward position). As a result, the timing of opening the exhaust port  10  is the most delayed, and at the time of the exhaust stroke, the blowing of new air into the exhaust port  10  from the combustion chamber  4   a  is severely restricted, whereby stability at low running speeds can be achieved. 
     When the engine speed increases, the thrust on the slider  36 , exerted by of the centrifugal weights  35 , is greater than the setting load of the governor spring  47 . The thrust moves the slider  36  until the thrust force balances with the increasing load of the governor spring  47 . The drive lever  54  moves in unison with the slider  36  from the withdrawn position. The second driven lever  22  rotates upwards, via the link  29 , the first driven lever  21 , and pincer spring  24 . Hence, the exhaust control valve  11  is opened (upward position). 
     When the engine E reaches a prescribed high-speed running state, the second driven lever  22  is stopped by the upper limit stopper  26 , and the exhaust control valve  11  is kept in a completely open state. Scavenging of the ignition chamber  4   a  due to new air at the time of the exhaust stroke can then be carried out effectively. Effective scavenging improves the output performance, when the engine is running at high-speeds. Moreover, the scavenging is increased as needed, since the degree of opening of the exhaust port  10  occurs in response to increases in engine speed. 
     The pressure within the exhaust port  10  tends to urge the exhaust control valve  11  in an opening direction, as do the centrifugal weights  35 . Therefore, both the centrifugal weights  35  and the pressure in the exhaust port  10  tend to compress the governor spring  47 . The pressure within the exhaust port  10  increases as the engine speed is increased. Therefore, if the spring constant of the governor spring  47  is constant, when the engine speed increases, the governor spring  47  is compressed more than is necessary due to the influence of the aforementioned exhaust port pressure. If the governor spring  47  is compressed more than is necessary, the exhaust control valve  11  is opened more than is necessary, and the optimum output performance of the engine is not obtained. 
     In the first embodiment of the present invention, the governor spring  47  described above is equipped with a small pitch coil section  47   a  and a large pitch coil section  47   b . The spring constant of the governor spring  47  is then relatively small when the engine is running at low speed, as shown in FIG.  6 . However, when the engine reaches a prescribed high speed, the small spring constant, small pitch coil section  47   a  is compressed into a close contact state, so that just the large pitch coil section  47   b  is operating. When the small pitch coil section  47   a  is compressed, the spring constant of the governor spring  47  becomes large, i.e. the rate of increasing the load with respect to compressive deformation of the governor spring  47  is high. 
     The governor spring  47  is therefore not compressed more than is necessary during high-speed running, even if the pressure within the exhaust port  10  increases. The exhaust control valve  11  can be reliably controlled so as to be opened to an extent corresponding to the high-speed state at this time, and the optimum output performance of the engine can therefore be obtained. It is important to note that the number of parts does not increase, because the governor spring  47  comprises one coil spring, and this contributes towards making the centrifugal governor  28  more compact. 
     Referring back to the exhaust control valve  11 , it is important to note that the exhaust control valve  11  is heated by exhaust gas passing through the exhaust port  10 . The heat dissipating ability of the valve stem  15  deteriorates as the side of the bushing  16  is approached. This is because the bush  16  side end of the valve stem  15  is completely covered by the pouch-shaped bearing hole  19 , into which the bush  16  is inserted. It is therefore easy for heat to become trapped. 
     However, in accordance with the present invention, the first and second bosses  13   a  and  14   a  in their entirety are arranged so as to be offset from the reinforcing wall  12 , which may pass through the center of the exhaust port  10 . The offset is towards the bearing through hole  20 , as illustrated in the Figures by setting the distance (A) to be less than the distance (B). The first and second bosses  13   a  and  13   b  are therefore positioned towards the side of the bearing  15  with superior heat dissipation by this offset portion. As a result, the withdrawal of heat from the bosses  13   a  and  14   a  towards the valve bearing  15  is carried out in an effective manner, and the heat dissipating ability of the first and second valve elements  13  and  14  is promoted, as is their resistance to heat. 
     In addition to the distance (A) being less than the distance (B), by setting the second length (D) less than the first length (C), the thermal capacity of the first boss  13   a  is greater than the thermal capacity of the second boss  14   a . The withdrawal of heat from the first and second bosses  13   a  and  14   a  to the valve stem  15  is therefore uniform, as is the heat resistance of the first and second valve elements  13  and  14 . 
     Now, with reference to FIG. 7, a second embodiment of the governor spring  47  will be described. Same elements are represented by same reference numerals. In the second embodiment, the overall governor spring  47  includes two springs, namely a small pitch coil spring  55  and a large pitch coil spring  56 . The small pitch coil spring  55  and the large pitch coil spring  56  are arranged in series with a retainer  57  sandwiched therebetween. 
     The second embodiment operates in a similar manner to the first embodiment illustrated in FIG.  4 . The spring constant of the governor spring  47  increases when the engine is running at high speed. Therefore, the exhaust control valve  11  is prevented from opening too much or too soon when running at high speed. As a result, the output performance of the engine is increased. 
     It is also possible to use separately made normal equal pitch springs for the small pitch coil spring  55  and the large pitch coil spring  56 , so that a cheap governor spring  47  can be obtained and costs can be reduced. 
     The present invention is not limited to the aforementioned embodiments, and various design modifications are possible without deviating from the spirit of the invention. For example, the governor spring  47  of the first embodiment, having a single spring with two pitch sections, can be replaced by a spring having a successively changing pitch. In other words, one end of the governor spring  47  can have a relatively small pitch and the pitch can continuously increase to a relatively large pitch at the other end of the governor spring  47 . Alternatively, the pitch between coils can be made constant, but the material or material thickness of the spring coils can change along the length of the governor spring  47  to cause a corresponding change in the spring constant as the spring is compressed or expanded. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.