Diaphragm-type carburetor for a two-cycle engine that operates with layered scavenging

A diaphragm-type carburetor is provided for a two-cycle engine, in a manually guided implement, that operates with layered scavenging. Formed in the carburetor housing is an intake channel portion in which is disposed a butterfly valve that is pivotably held by a shaft. Opening into the intake channel portion are fuel-conveying channels supplied from a fuel-filled control chamber that is formed on a longitudinal side of the intake channel portion in the carburetor housing and is separated from a compensation chamber by a diaphragm. Air for combustion is additionally supplied to the engine via an air channel formed in a functional component of the carburetor fixed on the housing thereof on a longitudinal side of the intake channel portion. The air channel is disposed approximately parallel to the intake channel portion and is guided from that end face of the carburetor that faces an air filter to the connecting side of the carburetor that faces the internal combustion engine. Disposed in the air channel is a throttle member that is rotatably held by a shaft, which is coupled together with the shaft of the butterfly valve by means of a transmission connection.

BACKGROUND OF THE INVENTION
 The present invention relates to a diaphragm-type carburetor for an
 internal combustion engine that operates with layered scavenging,
 especially for the two-cycle engine in a manually guided implement, such
 as a power chainsaw, a cut-off machine, a brush cutter, or the like.
 Formed in the carburetor housing is an intake channel portion in which is
 disposed the butterfly valve that is pivotably held by a shaft and in the
 vicinity of which fuel-conveying channels open into the intake channel
 portion. Such fuel-conveying channels are supplied from a fuel-filled
 control chamber that is formed on a longitudinal side of the intake
 channel portion of the carburetor housing and is separated from a
 compensation chamber by a control diaphragm. An air channel that
 additionally supplies air for combustion is formed in a component secured
 to the carburetor housing and has a throttle member that is rotatably held
 by a shaft and that is adjustable by being coupled with the position of
 the butterfly valve.
 A carburetor of this general type is known from the Japanese publication JP
 09268917 A. Disposed between the air filter and the carburetor housing is
 an intermediate piece that extends the intake channel portion and from
 which branches a connecting line that opens into the transfer port between
 crank case and combustion chamber of the connected two-cycle engine.
 During operation of the internal combustion engine, fuel-air mixture is
 drawn into the crank case via the carburetor, while via the connecting
 line into the transfer ports adjacent to their inlet openings fuel-free
 air for combustion is drawn into the combustion chamber. Therefore, when
 the transfer ports are opened, first combustion air by itself that was
 previously collected in the transfer ports flows into the combustion
 chamber and displaces the exhaust gases found therein to the exhaust
 valve. The fuel-free air for combustion follows the fuel/air mixture that
 flows out of the crank case into the combustion chamber. Such a layered
 scavenging lowers the scavenging losses that are unavoidable with
 two-cycle engines. Unfortunately, arranging an intermediate flange between
 the air filter and the carburetor for branching off the air for combustion
 by itself leads to a lengthening of the overall length, which creates
 problems where the space conditions are limited. Especially for portable,
 manually guided implements, the installation space provided for
 accommodating the carburetor and the air filter is very limited, for which
 reason the conversion of the layered scavenging is particularly
 problematic for such applications.
 It is an object of the present invention to provide a diaphragm-type
 carburetor for an internal combustion engine that operates with layered
 scavenging, which such a carburetor having a short overall length while
 maintaining a small overall size.

SUMMARY OF THE INVENTION
 The diaphragm-type carburetor of the present invention is characterized
 primarily in that the component secured on the carburetor housing is
 secured on a longitudinal side of the intake channel portion, in that the
 component is a functional component of the carburetor, in that the air
 channel extends approximately parallel to the intake channel portion from
 that end face of the carburetor that faces an air filter to the connecting
 side of the carburetor that faces the internal combustion engine, and in
 that the shaft of the throttle member and the shaft of the butterfly valve
 are coupled together by means of a transmission connection.
 Arranging the air channel in a functional component of the carburetor that
 is disposed on a longitudinal side of the carburetor housing leads to a
 compact construction, so that the inventive carburetor can also be used
 where the space conditions are limited. With the inventive carburetor the
 shaft of the throttle member and the shaft of the butterfly valve can be
 coupled together via a straightforward transmission connection in a manner
 that is dependent upon position, whereby this transmission connection is
 also provided on the carburetor housing on a longitudinal side of the
 intake channel portion.
 The functional component is expediently embodied as the cover of the
 compensation chamber, so that in place of the cover otherwise used with
 diaphragm-type carburetors for the compensation chamber, now the
 functional component is utilized. Despite the provision of a further
 component this leads to a small overall size as measured transverse to the
 intake channel, so that the carburetor is particularly suitable for use in
 portable, manually guided implements.
 The transmission connection is advantageously disposed externally of the
 carburetor housing and connects one end of the butterfly valve shaft with
 the adjacent end of the shaft of the throttle member in the air channel.
 In so doing, the transmission connection is driven by the butterfly valve
 shaft and is advantageously configured in such a way that the butterfly
 valve shaft traverses a free play path in the opening direction of the
 butterfly valve before the shaft of the throttle member in the air channel
 is taken along. This ensures that during idling and in the lower partial
 throttle range when the air channel is opened the fuel/air mixture that
 flows into the combustion chamber is not unintentionally made lean. Only
 after the butterfly valve has reached a structurally prescribed opening
 position, in which a greater quantity of fuel/air mixture flows into the
 crank case of the internal combustion engine, is the throttle member of
 the air channel opened. The configuration of the free play determines the
 angular range over which the butterfly valve can be adjusted without
 actuating the throttle member.
 Further specific features of the present invention will be described in
 detail subsequently.
 DESCRIPTION OF PREFERRED EMBODIMENTS
 Referring now to the drawings in detail, the diaphragm-type carburetor 1
 illustrated in FIGS. 1-3 serves for supplying fuel-free air for
 combustion, and a fuel/air mixture, to an internal combustion engine 2.
 The internal combustion engine is a two-cycle engine that operates with
 layered scavenging and is preferably used in manually guided implements.
 Such manually guided, portable implements can be embodied as power
 chainsaws, cut-off machines, brush cutters, blowers or the like.
 The diaphragm-type carburetor essentially comprises a carburetor housing 3
 in which is formed a continuous intake channel portion 4 having a Venturi
 section 5. A throttle or butterfly valve 6 is disposed downstream of the
 Venturi section 5 in the intake channel portion 4. The butterfly valve 6
 is pivotably held via a shaft 7 that is mounted in the carburetor housing
 3. A fuel-conveying main channel 8 opens into the Venturi section 5
 upstream of the butterfly valve 6. Fuel-conveying idling ducts 9 open into
 the intake channel portion 4 in the vicinity of the butterfly valve.
 The channels and ducts 8, 9 are supplied with fuel from a fuel-filled
 control chamber 10, which in turn is supplied with fuel from a
 non-illustrated fuel tank, via a fuel line, by a fuel pump 50 that is
 driven by the fluctuating crankcase pressure of the two-cycle engine.
 The control chamber 10 is formed in the carburetor housing 3 and by means
 of a control diaphragm 11 is separated from a compensation chamber 12 that
 is connected by means of a bore 60 with the atmosphere or by means of a
 channel 70 with the clean air chamber 17 of an air filter 16. The control
 chamber 10, the control diaphragm 11, and the compensation chamber 12 are
 provided in the carburetor housing 3 on a longitudinal side 13 of the
 carburetor housing 3 that is approximately parallel to the intake channel
 portion 4 (FIG. 1).
 In addition to the intake channel portion 4, by means of which a fuel/air
 mixture is supplied to the internal combustion engine 2, an air channel 15
 is provided and has a throttle member 14. The air channel 14 acts as a
 bypass to the intake channel portion 4 and connects the clean air side of
 the air filter 16 with non-illustrated transfer ports of the internal
 combustion engine 2. In the illustrated embodiment, the air channel 15 is
 disposed approximately parallel to the intake channel portion 4, and is
 provided in a functional component 18 of the diaphragm-type carburetor 1.
 The functional component 18 is fixedly connected to the carburetor housing
 3 and forms the cover 19 of the compensation chamber 18, whereby the
 control diaphragm 11 is preferably held in a clamped manner between the
 carburetor housing 3 and the functional component 18 that is installed as
 the cover 19.
 The air channel 15 extends from that end face 20 of the diaphragm-type
 carburetor 1 that faces the air filter 16 to the connecting side 21 of the
 carburetor that faces the internal combustion engine 2. In the illustrated
 embodiment, an end 22 of the functional component 18 that faces the air
 filter 16 projects into the air filter housing; similarly, the air channel
 15 extends into the clean air chamber 17 of the air filter.
 Advantageously, an end 23 of the functional component 18 also extends over
 the connecting side 21, whereby in the vicinity of the end 23 the air
 channel 15 is larger than at the end 22 that faces the air filter 16. In
 the illustrated embodiment, at that end 23 of the functional component 18
 that extends beyond the connecting side 21 the air channel 15 has an inner
 diameter D and by means of a transition portion 124 adjoins a portion of
 the air channel 15 that leads to the air filter 16 and that has the
 smaller diameter "d".
 Disposed in that portion of the air channel 15 that has the larger diameter
 D is the throttle member 14, which is in the form of a valve and that is
 pivotably held by a shaft 24 that is mounted in the functional component
 18.
 In the illustrated embodiment, the functional component 18 is
 monolithically embodied with the cover 19 of the compensation chamber 12
 and is disposed on the longitudinal side 13 of the carburetor housing 3.
 In a similar manner, the functional component 18 can also be
 monolithically embodied with the cover 25 of the fuel pump 50 and can be
 arranged on the longitudinal side 26 of the carburetor housing 3.
 As shown in FIGS. 3-5, in the illustrated embodiment the butterfly valve 7
 and the shaft 24 of the throttle member 14 of the air channel 15 are
 disposed approximately parallel to one another; however, an angular
 disposition relative to one another can also be expedient.
 One end of the butterfly valve shaft 7 carries an actuating lever 27 that
 is connected in a non-illustrated manner with a gas lever or the like for
 adjusting the butterfly valve 6. In particular, the actuating lever 27 is
 fixedly disposed at an end 28 of the butterfly valve shaft 7 and is spring
 loaded in the closing direction of the butterfly valve 6 by means of a
 return spring 29.
 The other end 28' of the butterfly valve shaft 7 projects out of the
 carburetor housing 3 and ends at an imaginary plane 30 at which an end 31
 of the shaft 24 of the throttle member 14 in the air channel 15 ends. As
 can be seen in particular in FIGS. 3 and 4, the end 28' fixedly carries a
 lever 32 while the end 31 of the shaft 24 of the throttle member 14
 carries a lever 33. The levers 32, 33 are interconnected by means of a
 tie-rod 34, one end of which pivotably engages the lever 33 and the other
 end of which is disposed in an elongated slot 36 in the other lever 32,
 with this slot extending approximately in the direction of rotation 35.
 The transmission connection 40 that is thus formed between the shaft 24 of
 the throttle member 14 and the butterfly valve shaft 7 is driven by the
 butterfly valve shaft 7, resulting in a coupling of the throttle member 14
 with the butterfly valve 6 that is a function of position. The starting
 position of the butterfly valve shaft 7 and the shaft 24 of the throttle
 member 14 illustrated in FIG. 4 is in each case determined by springs. The
 return spring 29 acts upon the butterfly valve shaft 7 in a closing
 direction of the butterfly valve 6; in a similar manner, a coil spring 37
 acts upon the shaft 24 of the throttle member 14 and determines the
 closing position of the valve-type throttle member 14 in the air channel
 15.
 In the idle position of the internal combustion engine 2 shown in FIG. 2,
 the two valves 6, 14 are in the closed position. In this position, the
 valves 6 and 14 form an angle 38 of about 12.degree. to 18.degree.
 relative to the longitudinal central axis 39 and 41 of the channels 15 and
 4.
 Starting from the idle position shown in FIG. 2, the butterfly valve 6 is
 opened by pivoting the actuating lever 27 in the opening direction 35, so
 that a greater quantity of fuel-air mixture is conveyed to the internal
 combustion engine 2. The speed of the internal combustion engine
 increases. As soon as the butterfly valve 6, i.e. the butterfly valve
 shaft 7, in the opening direction 35 has passed through a free play extent
 42 that is determined by the length of the slot 36, then by means of the
 tie-rod 34 the lever 33 is also pivoted in the opening direction 35, as a
 result of which by means of the shaft 24 the valve-type throttle member 14
 in the air channel 15 is carried along in the opening direction 35. In
 additional to the fuel/air mixture, air for combustion by itself, which is
 expediently collected previously in the transfer ports from the crank case
 to the combustion chamber, is conveyed to the internal combustion engine
 via the air channel 15. For this purpose, as shown in FIG. 6, a branch
 element 43 can be disposed at the end 23 of the functional component 18;
 the branching air supply channels 44 and 45 of the branch element 43 open
 into the corresponding transfer ports.
 Starting from the illustrated idle position of the butterfly valve 6,
 during the start of pivoting of the butterfly valve 6, i.e. of the lever
 32 that is connected with the butterfly valve shaft 7, in the opening
 direction 35, the position of the shaft 24 of the throttle member 14
 initially remains unchanged due to the free play 42; the throttle member
 14 keeps the air channel 15 closed. This ensures that during idling as
 well as in the lower low-running range of the internal combustion engine,
 not too much air is supplied, which would result in making the mixture
 leaner than desired. Only after the butterfly valve 6 in the intake
 channel portion 4 conveys a greater quantity of fuel/air mixture is the
 free play 42 traversed, so that during a further movement in opening
 direction 35 by means of the transmission connection 40 the shaft 24 with
 the throttle member 14 is also pivoted in the opening direction. In
 addition to the fuel/air mixture, in the middle partial throttle and full
 throttle ranges air for combustion by itself is conveyed via the air
 channel 15 to the transfer ports. The air for combustion is expediently
 supplied to the intake channel portion 4 and the air channel 15 via a
 common air filter 16.
 In the embodiment of a transmission connection 40 illustrated in FIG. 5,
 the lengths L1 and L2 of the levers 32' and 33' are such that their free
 ends 46 and 47 can come into abutment with one another. The facing
 longitudinal edges of the levers 32' and 33' have a cam contour 48 or 49,
 the configuration of which effects a constructive coupling of the
 inter-associated position of the butterfly valve 6 and throttle member 14.
 If in the embodiment illustrated in FIG. 5 the butterfly valve shaft 7 with
 the butterfly valve 6 is moved in the opening direction 35 against the
 force of the spring 29, then during idling and in the lower low-running
 range the shaft 24 of the throttle member 14 in the air channel 15 remains
 unactuated until the free play extent 42 between the free end 47 of the
 lever 32' and the free end 46 of the lever 33' is overcome. When the cam
 contour 48 of the free end 47 comes to rest against the cam contour 49 of
 the free end 46, the butterfly valve 6 in the intake channel 4 is already
 in a partial throttle position. Upon further opening of the butterfly
 valve 6, the shaft 24, and hence the throttle member 14, are taken along
 in the opening direction 35, whereby the regulating path is determined by
 the cam contour 48 and 49 of the longitudinal edges of the levers 32' and
 33'. By means of these cam contours, a desired opening characteristic of
 the throttle member 14 in the air channel 15 can be structurally described
 as a function of the position of the throttle valve 6 in the intake
 channel portion 4 of the carburetor 1.
 The specification incorporates by reference the disclosure of German
 priority document 199 18 719.3 of Apr. 24, 1999.
 The present invention is, of course, in no way restricted to the specific
 disclosure of the specification and drawings, but also encompasses any
 modifications within the scope of the appended claims.