Abstract:
An acceleration device of a carburetor for a two cycle engine with a rotary dual valve which controls air flow through both a scavenging passage and a separate air intake passage each extending through a carburetor body. The carburetor body houses a metering fuel chamber and an air reference chamber defined by a diaphragm between them. Fuel in the metering fuel chamber is discharged through a port into the air intake passage. An acceleration pump has an actuation chamber which communicates with the scavenging passage and a pump chamber which communicates with the air reference chamber and a membrane between them. During engine acceleration the membrane is displaced by a pressure introduced into the actuation chamber to forcibly send air into the air reference chamber from the pump chamber to move, the diaphragm into the metering fuel chamber, and thereby increase the fuel delivered to the air intake passage.

Description:
REFERENCE TO RELATED APPLICATION 
     Applicants claim the priority of Japanese patent application, Ser. No. 11-300118, filed Oct. 21, 1999. 
     FIELD OF THE INVENTION 
     This invention relates to an acceleration device, and more particularly to a carburetor acceleration device for a two-cycle engine. 
     BACKGROUND OF THE INVENTION 
     Fuel from a carburetor for a two-cycle engine is fed via negative pressure into an air intake passage where the fuel mixes with the air and is then drawn into a crankcase. From the crankcase, the fuel-and-air mixture is drawn into a combustion chamber and burned. During engine acceleration the suction, or negative pressure, drawing the fuel and air mixture decreases. Therefore, less fuel is drawn into the air intake passage at a time when more fuel is actually required for smooth acceleration. Consequently, two cycle engines have been known to incorporate auxiliary acceleration pumps which use negative pressure to boost the delivery of fuel during acceleration periods. 
     Air pollutants from the exhaust of the two cycle engine are typically much greater than that of a four-cycle engine, because the two cycle engine does not completely bum the fuel within the combustion chamber. To alleviate some of the air pollutant concerns for two cycle engines, the industry is designing toward a leaner fuel to air mixture, and therefor a cleaner bum. Unfortunately, use of a leaner fuel to air mixture causes fuel starvation during engine acceleration periods. Sudden acceleration from idle of a cold engine may result in a stall due to lack of sufficient fuel. Moreover, use of the common auxiliary acceleration pump which is dependent upon negative pressure, is not responsive for a lean mixture engine because negative pressure is lacking during acceleration periods. 
     SUMMARY OF THE INVENTION 
     An acceleration device of a carburetor provides additional fuel to a two-cycle engine brought on by decreasing negative pressure during acceleration conditions. A carburetor body houses a scavenging passage and an air intake passage opened and closed via a scavenging valve and a throttle valve respectively. The scavenging and throttle valves are preferably integral to a single rotary dual valve and share a common axis of rotation. During steady engine operating conditions, fuel is supplied from a substantially constant pressure fuel supply chamber through a fuel supply tube and into a throttle hole of the throttle valve. The fuel is drawn from the throttle hole via negative pressure of the air intake passage when the intake passage is in communication with the throttle hole. During engine acceleration conditions, additional fuel is pushed into the throttle hole by inward movement of a diaphragm into the fuel supply chamber. 
     Preferably, a membrane disposed between a pump chamber or chamber and an actuation chamber or chamber of an acceleration pump pushes air into or increases the pressure in an air reference chamber housed within the carburetor body and communicating with the diaphragm of the fuel supply chamber. The membrane is actuated when a compressed resilient member, normally held back by a vacuum within the actuation chamber, pushes the membrane into the pump chamber when the vacuum is lost during engine acceleration conditions. The pushed air, in turn, forces the diaphragm into the fuel supply chamber. The vacuum within the actuation chamber is created by a suction from the scavenging passage during steady state engine operation. 
     Objects, features and advantages of this invention include providing a fuel acceleration device which is actuated by a sudden increase in pressure within a carburetor scavenging passage. The acceleration device thereby provides smooth acceleration of a lean burn two cycle engine even during cold operation, improved fuel efficiency and decreased engine emissions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of this invention will be apparent from the following detailed description of the preferred embodiments and best mode, appended claims and accompanying drawings in which: 
     FIG. 1 is a sectional side view of an acceleration device for a two cycle engine according to the present invention; and 
     FIG. 2 is a sectional view of a rotary throttle valve of the acceleration device taken along line  2 — 2  in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring in more detail to the drawings, FIG. 1 is a sectional side view of an acceleration device  10  embodying the present invention. The acceleration device  10  is integral in part with a body  12  of a carburetor for a two-cycle or two stroke engine. The remainder of the acceleration device  10 , is not necessarily part of the carburetor body  12 , and comprises an acceleration pump  14 . The acceleration pump  14  is responsive to air pressure within a scavenging passage  16  extending through carburetor body  12 . The scavenging passage  16  is in communication with a combustion chamber of the engine. Also extending through the carburetor body  12  is an air intake and fuel mixing passage  22  communicating with a crankcase of the two-cycle engine, not shown. 
     Referring to FIGS. 1 and 2, a scavenging valve  18  and a throttle valve  20  coincidingly throttle, open and close, the scavenging and air intake passages  16 ,  22  respectively. Although the scavenging and throttle valves  18 ,  20  may take a variety of forms, such as pivoting plates, preferably they are of a rotary, cylindrical, type extending transversely across the scavenging and air intake passages  16 ,  22  respectively. As rotary valves, the scavenging valve  18  has a scavenging hole  24  and the throttle valve  20  has a throttle hole  26 . The holes  24 ,  26  are generally coincident with and conform to passages  16 ,  22  respectively when in the full open position. Although valves  18 ,  20  may be disposed side by side having parallel axes of rotation, preferably, the valves  18 ,  20  are stacked thereby having a common axis of rotation. In the preferred configuration, the scavenging valve  18  and scavenging passage  16  are generally disposed above the throttle valve  20  and air intake passage  22 . The preferred scavenging valve  18  and the preferred throttle valve  20  together comprise a dual valve  21 . Dual valve  21  has a stepped cylindrical shape for mounting rotatably to the carburetor body  12  generally from above. 
     To a left side, the carburetor body  12  connects to an air-cleaning device via a seal member, and to a right side, the carburetor body  12  connects to a wall of the engine, not shown. At an end of a combustion stroke of an operating two-stroke engine, air is drawn through the scavenging hole  24  and the scavenging passage  16  into the combustion chamber. Also, air is drawn through the throttle hole  16  and the air intake passage  22  into the crankcase of the engine. 
     The acceleration pump  14  translates air pressure changes in the scavenging passage  16  into air volumetric movement within a constant pressure fuel supply mechanism  28  located in the carburetor body  12 . Opening the throttle valve  18  of the air intake passage  22  to accelerate the operating engine results in air pressure changes within the scavenging passage  16 . During acceleration periods, the negative pressure in the scavenging passage  16  decreases, causing the acceleration pump  14  to move air volume into the constant pressure fuel supply mechanism  28 . The fuel supply mechanism  28  uses this air movement to deliver additional fuel into the air intake passage  22 . The acceleration pump  14  thereby assists the fuel supply mechanism  28  in supplying additional fuel to the air intake passage  22  during high fuel demand periods brought on by engine acceleration. 
     As previously stated, when the throttle valve  20  opens, the operating engine accelerates and the existing negative air pressure within the scavenging passage  16  decreases. The decrease in negative air pressure is communicated to an actuation chamber or chamber  30  of the acceleration pump  14 , via a pipe  32 , causing movement of an adjacent membrane  34 . Membrane  34  seals and divides the actuation chamber or chamber  30  from a pump chamber or chamber  36  of the acceleration pump  14 . The actuation chamber  30  is generally defined by a first housing portion  38  and the membrane  34 . The pump chamber  36  is generally defined by a second housing portion  40  and the membrane  34 . The first housing portion  38  rigidly connects and seals to the second housing portion  40 . A resilient member  42  such as a spring is biased against the membrane  34  and acts to move the membrane  34  toward or into the pump chamber  36 , away from the actuation chamber  30  during low negative pressure conditions in the scavenging passage  16  brought on by engine acceleration. 
     During non-accelerating engine conditions, the negative pressure holds or sucks the membrane  34  or spring into the actuation chamber  30 , against the bias of the resilient member or spring  42 . The resilient member  42  may be disposed either within the actuation chamber  30  or the pump chamber  36 . If the resilient member  42  is within the actuation chamber  30 , the negative pressure of the actuation chamber  30  tends to retract or compress the resilient member  42 . However, if the resilient member  42  is in the pump chamber  36 , the negative pressure of the actuation chamber  30  will tend to elongate or expand the resilient member  42 . Preferably, the resilient member  42  is a compressible spring and therefore located in the actuation chamber  30 . 
     Resilient member or spring  42  therefore cooperatively seats between the first member  38  and the membrane  34 . To simplify assembly and to provide operable guidance for the resilient member  42 , a bridge  44  is disposed within the actuation chamber  30 . The bridge  44  is stationary with respect to the first and second housing portions  38 ,  40  and rigidly connects to either the first or second housing portions  38 ,  40 . Preferably, the bridge  44  attaches unitarily to the second housing portion  40 . This way, the resilient member or spring  42  seats between the bridge  44  and the membrane  34  prior to installation of the first housing portion  38  onto the second housing portion  40  over the bridge  44 . 
     When, the operating engine is accelerating and thus requires more fuel, the actuation chamber  30  loses negative pressure. The resilient membrane  34  senses the loss of negative pressure within the actuation chamber  30  and is displaced by the force produced by the resilient member spring  42 . Without the negative pressure causing the membrane  34  to be disposed back into the actuation chamber  30 , the resilient member or spring  42  pushes or forces the membrane  34  into the pump chamber  36  which then transfers air volume into the constant pressure fuel supply mechanism  28 . When resilient member  42  is located in the actuation chamber  30 , the membrane  34  is pushed by resilient member  42 . As stated previously, this is preferable over pulling the membrane  34  which would be the case if the resilient member  42  is located in the pump chamber  36 . 
     An air reference chamber  46  of the fuel supply mechanism  28  accepts the additional air volume through the displacement of a diaphragm  48  into a metering fuel chamber  50 . The volumetric decrease of the metering fuel chamber  50  has the effect of pushing or displacing liquid fuel therein into the air intake passage  22  through a fuel port  52  located in a fuel supply tube  54 . The diaphragm  48  is clamped between an outward member  56  and an intermediate member  58  of the carburetor body  12 . The intermediate member  56  and a face of the diaphragm  48  define the metering fuel chamber  50 . An opposite face of the diaphragm  48  and the outward member  56  define the air reference chamber  46 . The metering fuel chamber  50  is disposed generally between the fuel supply tube  54  and the air reference chamber  46 . 
     The fuel supply tube  54  connects to a bottom part of a valve chamber  60  and communicates with the metering fuel chamber  50  via a check valve. A fuel pump has a membrane  62  generally clamped within the carburetor body  12  and an inlet or suction valve, and an outlet or discharge valve which are not shown. By moving the membrane  62  with pulsation pressure in a crank case of the two cycle engine, fuel in a fuel tank (not shown) is drawn into a pump chamber of the fuel pump and supplied to the metering fuel chamber  50  through the outlet valve and a fuel metering valve actuated by the diaphragm  48 . 
     During non-accelerating engine operating conditions, fuel in the metering fuel chamber  50  is drawn through the fuel supply tube  54 , the fuel port  52 , and into a throttle hole  26  of the throttle valve  20 . The throttle hole  26  is in throttling communication with the air intake passage  22  which is exposed to negative pressure from the crank case of a two cycle or stroke engine. When the amount of the fuel in the metering fuel chamber  50  decreases and the diaphragm  48  moves into the metering fuel chamber  50  via a negative pressure in the air intake passage  22 , a fuel metering valve is opened by a lever associated with the diaphragm  48  and the fuel pump replenishes the fuel in the chamber  50 . In this manner, the fuel in the metering fuel chamber  50  is maintained at a substantially constant level. 
     On the other hand, during acceleration conditions, the fuel in the metering fuel chamber  50  is forcibly sent or discharged through the supply tube  54  into the passage  22  by movement of the diaphragm  48  into the metering fuel chamber  50  caused by air supplied to the chamber  46  by the acceleration pump  14 . This increases the amount of fuel delivery to and thus provides a smooth acceleration of the engine. 
     Dual valve  21  has an integral shaft  66  which extends longitudinally and projects outwardly through a lid  68  of the carburetor body  12 . A throttle valve lever  78  extends radially and is attached to the shaft  66  above the lid  68 . The rotary dual valve  21  is biased to a substantially closed engine idling position by a coil spring  70 . The coil spring  70  encircles the shaft  66  and is received between the lid  68  and the rotary dual valve  21 . One end of the spring  70  engages with the rotary dual valve  21  and the other end engages with the lid  68 . The rotary dual valve  21  is thereby forced to rotate to an idling position, wherein the scavenging and air intake passages  16 ,  22  are partially closed, by the spring  70  with the assistance of a cam mechanism  72 . 
     The cam mechanism  72  comprises a follower  74  upwardly projecting from the lid  68 , and a cam face  76  facing downward from the throttle valve lever  78 . The cam face  76  is urged onto the follower  74  by the force of the spring  70 . When the rotary dual valve  21  rotates in an opening or accelerating direction, the scavenging passage  16  further opens as the scavenging hole  24  rotates, and the air intake passage  22  further opens as the throttle hole  26  rotates. At the same time, a needle valve  80 , supported by the shaft  66  of the rotary dual valve  21  and inserted into the fuel supply tube  54 , is lifted upward by the action of the cam mechanism  72 , thereby further exposing or opening the fuel port  52  of the fuel supply tube  54  to the air intake passage  22 . 
     The lid  68  attaches to the carburetor body  12  by means of a plurality of bolts  82 . An outer sheath of a remote control cable is attached to a wall portion  84  projecting upward from the lid  68 . An inner wire passes through the outer sheath and is connected to the throttle valve lever  78  by means of a swivel. In this manner, the throttle valve lever  78  can be remotely controlled by an operator of a working machine carrying the engine to which the carburetor is connected. 
     A syringe or flexible rubber dome  86  of a manual suction pump is attached to a lower face of the outer member  56  and has a peripheral edge retained by bolts  88  and a holding plate  90 . The dome  86  and the lower face of the outer member  56  generally define a pump chamber  92  in which a mushroom shaped complex valve  94  is received and functions as both a suction valve and a discharge valve. Repeatedly manually pushing and releasing the syringe  86 , prior to starting the engine, causes vaporized fuel and air in the metering fuel chamber  50  to be drawn into the pump chamber  92  through the inlet portion of the complex valve  94 , and then returned to the fuel tank through a shaft portion of the complex valve  94 . Since the metering fuel chamber  50  is subjected to a negative pressure, fuel in the fuel tank is supplied to the metering fuel chamber  50  through the fuel pump and the metering valve. Because such structure has been disclosed in Japanese Publication No. 9-268917 (Application No. 8-1906186 filed Apr. 3, 1996) of an unexamined patent application, for example, a further explanation is omitted here. 
     The operation of the acceleration device  10  in a two-cycle engine according to the invention is described hereinbelow. When the throttle valve lever  78  is rotated in an engine accelerating direction, the scavenging hole  24  with respect to the scavenging passage  16  and the throttle hole  26  with respect to the air intake passage  22  further opens. At the same time, the needle  80  is moved upward by the cam mechanism  72  and the fuel port  52  is further exposed within the air intake passage  22 . The pressure in the scavenging passage  16  becomes almost equal to the atmospheric pressure, and the scavenged air in the scavenging passage  16  enters in the actuation chamber  30  via the pipe  32  so that the membrane  34  is moved into the pump chamber  36  by the force of the resilient member or spring  42 . This movement of the membrane  34  displaces air in the pump chamber  36  to the air reference chamber  46  via a passage  98 . This moves the diaphragm  48  into the metering fuel chamber  50 , and causes fuel in the metering fuel chamber  50  to be discharged into the throttle hole  26  via the check valve and the fuel supply tube  54  which increases the amount of the fuel in the air, providing a smooth acceleration of the engine. When the engine again arrives at steady operation, a strong scavenging negative pressure exists in the scavenging passage  16  which causes the membrane  34  in the acceleration pump  14  to gradually move back toward the actuation chamber  30  against the force of the resilient member or spring  42  and air in the air reference chamber  46  to be drawn into the pump chamber  36 . 
     While the forms of the invention herein disclosed constitute presently preferred embodiments, many others are possible. For instance, the acceleration pump  14  can be an integral part of the carburetor body  12 . With this orientation, the pump chamber  36  and the passage  98  are not required. The air reference chamber  46  is thereby defined directly between the diaphragm  48  and the membrane  34 . Regardless, it is not intended herein to mention all the possible equivalent forms or ramifications of the invention. It is understood that the terms used herein are merely descriptive, rather than limiting, and that various changes may be made without departing from the spirit or scope of the invention.