Patent Publication Number: US-8113155-B2

Title: Two-cycle engine

Description:
This application is a U.S. National Phase Application under 35 USC 371 of International Application PCT/JP2007/054285 filed Mar. 6, 2007. 
     TECHNICAL FIELD 
     The present invention relates to a two-cycle engine and more particularly to a two-cycle engine including an automatic choke. 
     BACKGROUND ART 
     A small-displacement two-cycle engine is generally used for a portable work machine such as a brushcutter, a chain saw and the like. Such a portable work machine often includes an automatic choke to improve startability of a cold engine. The automatic choke has various structures. For example, the automatic choke may include a solenoid valve integrated with a carburetor. 
     Such an automatic choke includes a starting fuel supply passage which extends from a fuel reservoir of the carburetor and which is apertured in an intake passage of the carburetor, and the solenoid valve as a flow passage opening and closing unit provided in the middle of the starting fuel supply passage. When the solenoid valve is opened, a fuel to be excessively enriched, i.e. a starting fuel, is supplied to the intake passage from the fuel reservoir through the starting fuel supply passage due to a negative pressure in the intake passage. 
     In the two-cycle engine having such an automatic choke, the intake passage of the engine in which a piston valve is employed as an intake method is apertured in a cylinder as an intake port, and is opened and closed by a reciprocating piston. In such a conventional two-cycle engine, when the intake port is opened by moving the piston from a bottom dead center to a top dead center, the negative pressure is applied in the intake passage from the crankcase to supply a fuel to the intake passage from the starting fuel supply passage (for example, see Patent Document 1). 
     [Patent Document] JP-A-2002-339805 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, in such a conventional two-cycle engine, the negative pressure is applied only for a short time after the intake port is opened while the piston ascends to the top dead center until the piston reaches to the top dead center. In addition, for example, when a piston ring is not comfortably fitted into a new engine so that its sealing properties is not sufficiently exhibited, the negative pressure applied in the crankcase is reduced, whereby the negative pressure applied in the intake passage is reduced. In other words, the negative pressure is applied in the intake passage only for a short time while the intake port is opened in the conventional two-cycle engine. In addition, the negative pressure is not sufficiently applied in the intake passage, for example, when the sealing properties is not excellent. Thus, a sufficient amount of the starting fuel is not supplied to the intake passage and the startability is impaired. 
     An object of the present invention is to provide a two-cycle engine having excellent startability. 
     Means for Solving the Problems 
     A two-cycle engine according to an aspect of the invention includes: a starting fuel supply passage for supplying a starting fuel to a crankcase from a starting fuel reservoir; and a flow passage opening and closing unit provided in a midway portion of the starting fuel supply passage, in which the starting fuel supply passage is opened to be constantly in communication with the crankcase. 
     In this arrangement, the starting fuel supply passage is opened to be constantly in communication with the crankcase irrespective of the location of the piston which reciprocates between a bottom dead center and a top dead center. Thus, simultaneously with the generation of a negative pressure within the crankcase when the piston starts to ascend from the bottom dead center, the starting fuel starts to be supplied to the crankcase. Consequently, the starting fuel can be supplied to the crankcase for a relatively long time as compared to a conventional engine. Even when the negative pressure is reduced because of poor sealing properties, a sufficient amount of the fuel can be supplied for starting and startability of the engine can be improved. 
     Further, since the negative pressure is applied only in the starting fuel supply passage and is not applied in the intake passage until the intake port is opened, the negative pressure applied in the starting fuel supply passage is increased so that the starting fuel can be reliably supplied to the crankcase from the starting fuel supply passage. 
     Furthermore, the starting fuel can be supplied at a high speed in response to the pressure from the crankcase since the starting fuel supply passage has a small diameter. 
     The two-cycle engine may include: an intake passage for feeding an air-fuel mixture into the crankcase; a pump for feeding a fuel to be supplied to the intake passage into a fuel reservoir in a carburetor from a fuel tank while being operated by a pressure variation in the crankcase; and a pulsation transfer passage for transferring the pressure variation in the crankcase to the pump, in which the fuel reservoir in the carburetor is the starting fuel reservoir, and the pulsation transfer passage serves as a part of the starting fuel supply passage. 
     In this arrangement, the pulsation transfer passage serves as the part of the starting fuel supply passage. Thus, it is not necessary to provide a dedicated starting fuel supply passage, whereby a structure of the engine can be simplified and a manufacturing cost thereof can be reduced. 
     The two-cycle engine may include: an intake passage for feeding an air-fuel mixture into the crankcase; a pump for feeding a fuel to be supplied to the intake passage into a fuel reservoir in a carburetor from a fuel tank while being operated by a pressure variation in the crankcase; and a pulsation transfer passage for transferring the pressure variation in the crankcase to the pump, in which the starting fuel reservoir is the fuel reservoir in the carburetor, and the starting fuel supply passage is provided separately from the pulsation transfer passage. 
     In this arrangement, since the starting fuel supply passage is provided separately from the pulsation transfer passage, the degree of freedom in design is enhanced. Thus, the most appropriate diameter of the starting fuel supply passage can be decided to provide an excellent response while supplying the starting fuel supply with reliability. 
     The starting fuel supply passage may include a tube that intercommunicates the carburetor and the crankcase. 
     In this arrangement, it is not necessary that a starting fuel supply passage having a complicated shape is provided in the carburetor, and is only necessary that the tube is provided in the carburetor. Thus, a structure of the engine can be simplified and a manufacturing thereof can be facilitated. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross sectional view illustrating a two-cycle engine according to a first exemplary embodiment of the invention. 
         FIG. 2  is a cross sectional view illustrating a main portion of a carburetor in an enlarged manner. 
         FIG. 3  is a cross sectional view illustrating a two-cycle engine according to a second exemplary embodiment of the invention. 
         FIG. 4  is a cross sectional view illustrating a two-cycle engine according to a third exemplary embodiment of the invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     First Exemplary Embodiment 
     A first exemplary embodiment of the invention will be described below with reference to the drawings. 
       FIG. 1  is a cross sectional view illustrating a two-cycle engine  1  according to the first exemplary embodiment of the invention. 
     As shown in  FIG. 1 , the two-cycle engine  1  includes an engine body  2 , a carburetor  4  attached to the engine body  2  through an insulator  3 , and a muffler  5  attached to interpose the engine body  2  between the muffler  5  and the insulator  3 . 
     The engine body  2  includes a cylinder  20 , a crankcase  24  provided on a lower side of the cylinder  20 , a crankshaft  25  supported by the crankcase  24 , and a piston  27  connected to the crankshaft  25  through a connecting rod  26  and slidably inserted into the cylinder  20  with a piston ring  270 . The cylinder  20  includes a combustion chamber  201  above the piston  27  therein. 
     The cylinder  20  includes a cylinder intake passage  21  which is apertured on an inner circumference of the cylinder  20  as an intake port  210  for feeding air-fuel mixture into the crankcase  24  in an intake process. A piston valve method is employed as an intake method of the air-fuel mixture, in which the intake port  210  is opened and closed on an outer circumference of the piston  27 . The cylinder  20  further includes a scavenging passage  28  for scavenging exhaust gas into the combustion chamber  201  in a scavenging process and an exhaust passage  29  for feeding the scavenged exhaust gas into the muffler  5 . A cylinder starting fuel supply passage  22  which is apertured in a groove-shaped portion  220  formed on the inner circumference of the cylinder  20  is provided below the cylinder intake passage  21 . 
     The cylinder starting fuel supply passage  22  communicates with the crankcase  24  through the groove-shaped portion  220  when the piston  27  is located on a side close to a bottom dead center (as shown in  FIG. 1 ), and is apertured in a lower space of the piston  27  within the cylinder  20  to communicate with the crankcase  24  when the piston  27  is located on the other side close to a top dead center. Consequently, the cylinder starting fuel supply passage  22  is opened to be constantly in communication with the crankcase  24  irrespective of the location of the piston  27  which reciprocates between the top dead center and the bottom dead center. 
     The insulator  3  is a synthetic resin member for controlling heat transfer from the engine body  2  to the carburetor  4 . The insulator  3  includes an insulator intake passage  31  that communicates with the cylinder intake passage  21  of the engine body  2 , an insulator starting fuel supply passage  32  that communicates with the cylinder starting fuel supply passage  22  of the engine body  2  on a lower side of an insulator intake passage  31 , and an insulator pulsation transfer passage  33  that is branched from an upper stream (a right side in  FIG. 1 ) of the insulator starting fuel supply passage  32 . A pulsation transfer passage  300  as shown in  FIG. 1  transfers a pressure variation in the crankcase  24  in accordance with a reciprocation of the piston  27  to a later-described pump  7  ( FIG. 2 ) provided in the carburetor  4 . The cylinder pulsation transfer passage  23  of the pulsation transfer passage  300  serves as the cylinder starting fuel supply passage  22 , and the insulator pulsation transfer passage  33  serves as a branch portion of the insulator starting fuel supply passage  32 . 
       FIG. 2  is a cross sectional view illustrating a main portion of the carburetor  4  in an enlarged manner. 
     As shown in  FIGS. 1 and 2 , the carburetor  4  includes a carburetor intake passage  41  which intercommunicates the insulator intake passage  31  and the outside of the engine and which has a venturi-shaped portion on a side close to the outside, a carburetor starting fuel supply passage  42  which intercommunicates the insulator starting fuel supply passage  32  and a fuel reservoir  6  and which is provided below the carburetor intake passage  41 , and a carburetor pulsation transfer passage  43  which intercommunicates the insulator pulsation transfer passage  33  and the pump  7 . 
     In this exemplary embodiment, the cylinder intake passage  21 , the insulator intake passage  31 , and the carburetor intake passage  41  define the intake passage  100 . The cylinder starting fuel supply passage  22  (a cylinder pulsation transfer passage  23 ), the insulator starting fuel supply passage  32  (a branched portion of which is included as the insulator pulsation transfer passage  33 ), and the carburetor starting fuel supply passage  42  define the starting fuel supply passage  200 . The cylinder pulsation transfer passage  23 , the insulator pulsation transfer passage  33 , and the carburetor pulsation transfer passage  43  define the pulsation transfer passage  300 . The diameter of the starting fuel supply passage  200  is smaller than the diameter of the intake passage  100 . 
     As shown in  FIG. 2 , the carburetor  4  has a typical structure for supplying the fuel to the intake passage  100 . Specifically, the carburetor  4  includes: the fuel reservoir  6  for reserving the fuel from a fuel tank (not shown) through an internal flow passage  44  of the carburetor  4  and the inlet pipe  40 ; the pump  7  for feeding the fuel to the fuel reservoir  6  by the pressure variation in accordance with the reciprocation of the piston  2 , the pressure variation transferred from the crankcase  24  through the pulsation transfer passage  300 ; a rotary valve  45  rotated by a throttle with a through hole  450 ; a nozzle  46  communicating with the fuel reservoir  6  for feeding the fuel to the through hole  450  from an opening  460  formed at a tip end of the nozzle  46 ; and a reservoir needle valve  48  for stroking the fuel by opening a communication hole  47  that intercommunicates the internal flow passage  44  and the fuel reservoir  6  after the fuel is fed from the nozzle  46 . The fuel reservoir  6  for reserving the fuel to be supplied to the intake passage  100  serves as a starting fuel reservoir  600  for reserving the starting fuel. 
     Such a carburetor  4  has a solenoid valve  8  as a flow passage opening and closing unit for an automatic choke for opening and closing the starting fuel supply passage  200  (the carburetor starting fuel supply passage  42 ). The solenoid valve  8  includes a movable core  80 , a coil spring  81  that biases the movable core  80  in a left direction in the figure, and a cylindrical coil  82  provided on an outer circumference of the movable core  80 . 
     The starting fuel supply passage  200  is closed by the movable core  80  while the cylindrical coil  82  is not excited. After the cylindrical coil  82  is excited, the movable core  80  is moved against the coil spring  81  in a right direction in the figure so that the starting fuel supply passage  200  is opened. 
     In addition to the solenoid valve  8 , the engine  1  includes a temperature sensor for detecting the temperature of the engine  1 , a controller for controlling the solenoid valve  8  in accordance with a detection result of the temperature sensor as components of the automatic choke. The automatic choke is controlled to work when the temperature of the engine  1  is lower than a predetermined temperature. 
     In the above-described engine  1 , the cylindrical coil  82  is excited and the solenoid valve  8  is opened when the automatic choke works at starting. When the piston  27  starts to ascend from the bottom dead center so that a negative pressure is generated in the crankcase  24 , the negative pressure is applied in the starting fuel supply passage  200  that intercommunicates the fuel reservoir  6  and the crankcase  24 , whereby the starting fuel is fed into the crankcase  24 . 
     Then, the intake port  210  is opened while the piston  27  ascends to the top dead center and the negative pressure in the crankcase  24  is applied in the intake passage  100 , whereby the fuel is fed into the intake passage  100  from the nozzle  46  that communicates with the fuel reservoir  6 . Subsequently, the fuel and a base air of air-fuel mixture are fed into the crankcase  24  to be mixed with the above-described starting fuel, so that the air-fuel mixture including an excessively enriched fuel is supplied in the crankcase  24 . 
     In the engine  1  according to the exemplary embodiment, the starting fuel supply passage  200  communicates with the crankcase  24  through the groove-shaped portion  220  when the piston  27  is located on a side close to the bottom dead center, and communicates with the crankcase  24  through the lower space of the piston  27  within the cylinder  20  when the piston  27  is located on the other side close to the top dead center. In other words, the starting fuel supply passage  200  is opened to be constantly in communication with the crankcase  24 . Thus, simultaneously with the generation of the negative pressure within the crankcase  24  caused when the piston  27  starts to ascend from the bottom dead center, the starting fuel starts to be supplied to the crankcase  24 . As compared to the conventional engine in which the starting fuel is supplied to the crankcase  24  only for a short time after the intake port  210  is opened while the piston  27  ascends and until the piston  27  reaches to the top dead center, the starting fuel is supplied to the crankcase  24  for a relatively long time in the engine  1  according to the exemplary embodiment. Even when the negative pressure applied to the crankcase  24  is reduced due to poor sealing properties, a sufficient amount of the fuel can be supplied for starting. Thus, the startability of the engine  1  can be improved. 
     Until the intake port  210  is opened while the piston  27  ascends, the negative pressure is applied only in the starting fuel supply passage  200  and is not applied in the intake passage  100 . Accordingly, the negative pressure applied in the starting fuel supply passage  200  is increased. Thus, the starting fuel can be reliably supplied to the crankcase  24  from the starting fuel supply passage  200 . 
     In addition, the starting fuel can be supplied at a high speed in response to the negative pressure from the crankcase  24  since the starting fuel supply passage  200  has a small diameter. 
     Since the automatic choke is released after the engine  1  starts, the solenoid valve  8  is closed. The pressure variation in the crankcase  24  due to the reciprocation of the piston  27  is transferred to the pump  7  through the pulsation transfer passage  300  so that the pump  7  starts to work. At this time, the pulsation transfer passage  300  serves as the starting fuel supply passage  200  (the cylinder starting fuel supply passage  22  and the insulator starting fuel supply passage  32  except the branched portion). Thus, it is not necessary to exclusively provide the starting fuel supply passage  200 , whereby a structure of the engine can be simplified and a manufacturing cost thereof can be reduced. 
     Second Exemplary Embodiment 
       FIG. 3  is a cross sectional view illustrating the two-cycle engine  1  according to a second exemplary embodiment of the invention. In the following description, the same members and functional portions as those of the first embodiment will be denoted by the same reference numerals, and the description thereof will be omitted or simplified. 
     In the second exemplary embodiment, the starting fuel supply passage  200  is provided separately from the pulsation transfer passage  300  as shown in  FIG. 3 , unlike the first exemplary embodiment. The engine  1  according to the second exemplary embodiment includes the solenoid valve  8 , the temperature sensor, the controller and the like in the same manner as the first exemplary embodiment. 
     The starting fuel supply passage  200  includes a crankcase starting fuel supply passage  242  which is provided in the crankcase  24  and is apertured directly in the crankcase  24 , the insulator starting fuel supply passage  32  which communicates with the crankcase starting fuel supply passage  242  provided separately from the insulator pulsation transfer passage  33 , and the carburetor starting fuel supply passage  42  which communicates the insulator starting fuel supply passage  32 . The starting fuel supply passage  200  is opened to be constantly in communication with the crankcase  24 , thereby intercommunicating the fuel reservoir  6  and the crankcase  24 . The diameter of the starting fuel supply passage  200  is smaller than that of the pulsation transfer passage  300 . The pulsation transfer passage  300  is the same as that of the first exemplary embodiment, which intercommunicates the crankcase  24  and the pump  7 . 
     In this exemplary embodiment, the starting fuel supply passage  200  is opened to be constantly in communication with the crankcase  24 , thereby intercommunicating the fuel reservoir  6  and the crankcase  24 . Thus, the same advantages can be obtained as in the first exemplary embodiment. In addition, since the starting fuel supply passage  200  is provided separately from the pulsation transfer passage  300 , the degree of freedom in design of the starting fuel supply passage  200  is enhanced in the second exemplary embodiment, as compared to the first exemplary embodiment in which the diameter of a portion serving as the pulsation transfer passage  300  of the starting fuel supply passage  200  is decided in view of a pulsation transfer. Thus, the most appropriate diameter can be provided in view of an excellent response while supplying the starting fuel supply with reliability. 
     Third Exemplary Embodiment 
       FIG. 4  is a cross sectional view illustrating the two-cycle engine  1  according to a third exemplary embodiment of the invention. 
     The engine  1  according to the third exemplary embodiment features that the starting fuel supply passage  200  includes a tube  303  that intercommunicates the carburetor  4  and the crankcase  24 . The starting fuel supply passage  200  has the most appropriate diameter in the same manner as the second exemplary embodiment. 
     In this exemplary embodiment, the starting fuel supply passage  200  having the most appropriate diameter is opened to be constantly in communication with the crankcase  24 , thereby intercommunicating the fuel reservoir  6  and the crankcase  24 . Thus, the same advantages can be obtained as in the first and second exemplary embodiments. In addition, it is not necessary that the carburetor starting fuel supply passage  42  having a complicated shape is provided in the carburetor  4 , and is only necessary that the tube  303  is simply attached to the engine  1 . Thus, its structure can be simplified and its manufacturing can be facilitated. 
     The invention is not limited to the exemplary embodiments described above, but includes other arrangements as long as an object of the invention can be achieved, which includes the following modifications. 
     For example, the starting fuel reservoir  6  provided in the carburetor  4  for reserving the fuel to be supplied to the intake passage  100  may not serve as the starting fuel reservoir  600  for reserving the starting fuel. The starting fuel reservoir  600  may be provided in a dedicated component provided outside the carburetor  4 . In other words, it is only necessary that the cylinder starting fuel supply passage  200  is opened to be constantly in communication with the crankcase  24  irrespective of the location of the piston  27  that reciprocates between the top dead point and the bottom dead point, thereby intercommunicating the starting fuel reservoir  600  and the crankcase  24  while the flow passage opening and closing unit is provided in the middle of the staring fuel supply passage  200 . 
     Although the solenoid valve  8  is used as the flow passage opening and closing unit in the exemplary embodiments, an actuator having a piezoelectric motor and the like may be used as the flow passage opening and closing unit. The flow passage opening and closing unit of the invention may have any structures.