Abstract:
A manually guided implement having an internal combustion engine, especially one embodied as a two-stroke engine, is provided, and also includes a crankcase and a carburetor. The implement has a fuel-conveying system for supplying fuel to the carburetor. Downstream of the carburetor, via a power line and a metering device disposed in the primer line, the fuel-conveying system is connected to an area that in the intake phase communicates with the crankcase. The metering device is controlled by the pressure in the crankcase.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a manually guided implement having an internal combustion engine, especially one embodied as a two-stroke engine. 
     To achieve a high output and to fulfill the applicable exhaust gas standards, manually guided implements having an internal combustion engine and a carburetor are provided with an appropriate setting of the fuel/air ratio, which is adapted to the hot-running condition of the internal combustion engine. When starting the internal combustion engine after it has been inoperative for a long period of time, and in particular under cold environmental conditions, the mixture provided by the carburetor is too lean and can lead to difficulties in starting the engine. 
     To improve the starting conditions, especially the cold-start conditions of the internal combustion engine, arrangements are known for enriching the mixture during the starting phase. For example, an increased underpressure can be produced in the intake channel of the carburetor via a starter valve that is disposed in the intake channel ahead of the butterfly valve. Due to the increased underpressure, an increased quantity of fuel is drawn in relative to the quantity of air that is drawn in by the internal combustion engine. The mixture becomes richer, thereby improving the starting conditions. However, a drawback of this arrangement is the increased cost of construction, especially with regard to the kinematic control of the starter valve. 
     It is also known, for enriching the fuel/air mixture, to provide a so-called primer pump, via which, prior to the start or during the starting process, a predetermined quantity of fuel can be injected into the intake line of the internal combustion engine via a primer line. As a result, a sufficient quantity of fuel is already available during the first rotation of the crankshaft during the starting process. To avoid difficulties during the metering of the fuel via a primer pump, embodiments are known according to which the pump is integrated into the carburetor. By means of a pressure line that is connected with the crankcase of the internal combustion engine, the primer pump is shut off when the internal combustion engine starts. Such a carburetor arrangement has a complex construction and is difficult to adjust with regard to the quantity of fuel that is to be injected. 
     It is therefore an object of the present invention to improve the starting characteristics of the internal combustion engine of a manually guided implement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This object, and other objects and advantages of the present invention, will appear more clearly from the following specification in conjunction with the accompanying schematic drawings, in which: 
     FIG. 1 is a cross-sectional view of one exemplary inventive arrangement of an internal combustion engine having a carburetor and a primer line; and 
     FIG. 2 is a schematic illustration of details of one variation of the arrangement of FIG.  1 . 
    
    
     SUMMARY OF THE INVENTION 
     It is proposed to provide the primer line with a metering device for fuel, via which the fuel-conveying system of the implement can be connected with the crankcase or with an area, such as a transfer channel, that communicates with the crankcase in the intake phase. The metering device can thereby be controlled by the pressure in the crankcase. With such an arrangement, the carburetor does not have to be modified, or at most has to be modified only slightly. By controlling the metering device via the pressure in the crankcase, the quantity of fuel that is to be injected prior to or during the starting process can be metered precisely, whereby as a consequence of the varying pressure distribution in the crankcase during starting of the internal combustion engine, the additional supply of fuel can be automatically reduced or stopped. By supplying the fuel directly into the crankcase or, for example, into a transfer channel, the fuel can be made available close to the combustion chamber, as a result of which undesired condensation effects on cold engine components, for example in the region of the intake channel close to the carburetor, are reduced. 
     Pursuant to an expedient further development, a fuel pump is disposed in the fuel line that leads from the fuel tank to the carburetor. By means of this fuel pump, the carburetor is supplied with an adequate quantity of fuel regardless of its position. In this connection, on the pressure side of the fuel pump the primer line is branched off from the fuel line. With such an embodiment, no further pump elements or the like are necessary for conveying the fuel through the primer line, so that the overall arrangement of the fuel enrichment can be kept very simple. In particular, for this purpose a diaphragm pump is provided in the housing of the carburetor, as a result of which the structural expenditure is limited to an appropriate branching off of the primer line. 
     Pursuant to one advantageous further development, the metering device is controlled by the pressure in that portion of the primer line that is disposed between the metering device and the crankcase. In particular when the metering device is embodied as a diaphragm control valve, the reciprocal effect between the valve control function and the oscillating pressure in the crankcase is thereby effected via only one line, so that an additional expenditure, for example for a control line, a mechanical drive, or the like can be eliminated. In this connection, the diaphragm valve is expediently provided with respective first and second chambers that are separated from one another by a diaphragm, whereby the first chamber forms a portion of the primer line. During or after the starting process, a valve element that can be closed off by the diaphragm takes care, as the need arises, for a supply or an interruption of additional fuel as a function of the control pressure that occurs in the crankcase and that is transferred by the primer line. The control via an elastic diaphragm is resistant to wear and has a straightforward construction. In this connection, the valve element is expediently in the form of a short cylindrical piece against the sealing rim of which the diaphragm rests. With this arrangement, a mechanical expenditure for pivotably or displaceably guided mechanical valve parts is avoided, and a reliable sealing function is ensured. 
     In conjunction with a check valve in the primer line between the crankcase and the metering device, fuel is conveyed into the first chamber of the diaphragm control valve via the fuel pump and the valve element. From there, fuel droplets are drawn in during the starting phase of the internal combustion engine by the underpressure in the crankcase. The subsequently rising underpressure in the crankcase effects a closing of the check valve, thereby avoiding a back pumping. After the internal combustion engine starts, the pressure in the crankcase drops during the intake phase in such a way that the diaphragm is pressed onto the sealing rim against the conveying pressure of the fuel pump. As a result, the fuel supply via the primer line is reliably prevented after the internal combustion engine starts; the internal combustion engine draws in a fuel/air mixture having a preset fuel/air ratio in a desired manner essentially exclusively via the carburetor. 
     The second chamber of the diaphragm control valve is expediently provided with an air supply opening via which the valve is connected in particular with the clean air side of an air filter that is disposed ahead of the carburetor. The diaphragm control valve thereby operates independent of the environmental pressure with regard to its setting. In this connection, the first and second chambers are expediently interconnected via a flow control device via which a pressure equalization can be effected between the two chambers at an only low underpressure in the crankcase and a correspondingly low intake volume stream through the primer line. With an appropriately set flow control device, a fuel supply can thereby also be effected via the primer line even after starting of the internal combustion engine at only low speed. This can be expedient in particular if the engine has already started, yet is not running smoothly. As the speed of the internal combustion engine increases, the appropriately set flow control device no longer effects an adequate pressure equalization between the two chambers, as a result of which the diaphragm closes the valve and terminates the primer process. 
     Pursuant to one expedient variation, the flow control device is adjustable. At appropriately cold environmental conditions, the cross-sectional area of the flow control device can be opened as wide as necessary, as a result of which even after the engine starts, at average speeds additional fuel flows through the primer line for enriching the mixture. Along with the metering device, the primer line assumes a function comparable to that of a starter valve. As a result, the carburetor can have a simple construction due to the elimination of a starter valve. 
     A shut off valve can be provided in the primer line upstream of the metering device. By actuating the shut off valve, it is possible even during a rough operation of the implement, where impact and vibrational stresses are encountered, to reliably prevent a supply of fuel via the primer line, which supply would not be desired during operation of the implement. 
     Further specific features of the present invention will be described in detail subsequently. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings in detail, illustrated in the schematic cross-sectional view of FIG. 1 is an internal combustion engine  2  in the form of a two-stroke engine  1 . The internal combustion engine  2  can also be a mixture-lubricated four-stroke engine or the like. In the region of the cylinder  39 , the internal combustion engine  2  is connected with a carburetor  4 , whereby air is drawn in, in the direction of the arrows  34 , by means of an intake channel  33  that leads through the carburetor  4 . A fuel-conveying system  5  includes a fuel tank  11  that is filled with fuel  7  and that is connected with the carburetor  4  via a fuel line  10 . A fuel pump  13 , which is disposed in a housing  12  of the carburetor  4 , is embodied as a diaphragm pump  40  and for its operation is connected via a pressure line  30  with a crankcase  3  of the internal combustion engine  2 . By means of the pressure line  30 , the diaphragm pump  40  is acted upon by an oscillating pressure in the crankcase  3 . The fuel pump  13  can also be an electrical pump or the like. 
     The fuel  7  is conveyed to a fuel nozzle  38  via the diaphragm pump  40  and a control chamber  41  by means of a fuel valve  35  that is controlled by a diaphragm  36 . The fuel nozzle  38  is disposed in a venturi section  37  of the intake channel  33 . As a consequence of the venturi section  37 , during intake of combustion air along the arrows  35  there results an underpressure, as a result of which fuel is drawn in through the fuel nozzle  38  and is combined with the air to form a fuel/air mixture. 
     Provided downstream of the fuel nozzle  38  is a butterfly valve  31  for controlling the flow volume and hence the power of the internal combustion engine  2 . Disposed upstream of the fuel nozzle  38  and the venturi section  37  is a choke valve  32  by means of which the pressure in the venturi section  37  can be reduced further and hence the fuel/air mixture can be enriched for a cold start. In addition, upstream of the arrows  34  the carburetor  4  is connected to the clean air side  23  of an air filter  24  that is not shown in detail. 
     An area  9 , which in the intake phase of the internal combustion engine  2  is in communication with the crankcase  3 , is connected via a primer line  6  with the fuel-conveying system  5 . In the illustrated embodiment, the area  9  is the crankcase  3  itself, and can also be a transfer channel or intake channel of the two-stroke engine  1  or an intake channel or crankcase of a mixture-lubricated four-stroke engine. On the pressure side of the diaphragm pump  40 , the primer line  6  is branched off from the fuel line  10 , and can also, for example, be supplied from a separate pump or can be connected directly with the fuel tank  11 . Provided in the primer line  6  is a metering device  8  which in the illustrated embodiment is a diaphragm control valve  15 . Depending upon the application, an embodiment as a mechanically, electrically, or magnetically controlled valve can also be expedient. A shut-off valve  28  is disposed in the primer line  6  upstream of the metering device  8 . A check valve  27  is provided in a portion  14  of the primer line  6  between the crankcase  3  and the metering device  8 . 
     The diaphragm valve  15  comprises respective first and second chambers  17 , 18 , which are separated from one another by a diaphragm  16 . In this connection, the first chamber  17  forms a portion  19  of the primer line  6 . The second chamber  18  is provided with an air supply opening  25  to which is connected an air line  29 . The second chamber  18  of the diaphragm control valve  15  is connected via the air line  29  with the clean air side  23  of the air filter  24  in a flow-conducting manner. With an appropriate configuration of the metering device  8 , it would also be possible to eliminate the choke valve  32 , as in the embodiment of FIG.  2 . 
     Details of the arrangement of FIG. 1 are schematically shown in FIG. 2, according to which the fuel pump  13 , in the form of the diaphragm pump  40 , comprises an arrangement of two check valves  43  and a diaphragm  42 . By means of the diaphragm pump  40 , fuel  7  is conveyed out of the fuel tank  11  to the control chamber  41  via a fuel filter  44  and the fuel line  10 . A pressure equalization line  45 , which branches off from the pressure side of the diaphragm pump  40 , is provided for withdrawing fuel vapor from the fuel tank  11  and hence for the establishment of a pressure equalization in the fuel tank  11 . On the pressure side of the fuel pump  13 , the primer line  6  branches off from the fuel line  10  to the crankcase  3 . Provided in the primer line  6  is the diaphragm control valve  15  with its two chambers  17 , 18 . A shut-off valve  28  is disposed in the primer line  6  upstream of the diaphragm control valve  15 . 
     The two chambers  17 , 18  are interconnected via an adjustable flow control device  26 . Depending upon the application, the flow control device  26  can also have a fixed value. In the region of the first chamber  17 , the diaphragm control valve  15  is provided, as a valve element  20 , with a short cylindrical piece  21  that is in communication with the primer line  6  and against the circumferential sealing rim  22  of which the diaphragm  16  can come to rest in a sealing manner. It can also be expedient to embody the valve element  20  with a pivotable or linearly displaceable valve body. 
     A check valve  27  is provided in the portion  14  of the primer line  6  that is disposed between the diaphragm control valve  15  and the crankcase  3 . In this arrangement, the diaphragm control valve  15  can be controlled by the pressure in the portion  14  in that during the intake phase of the internal combustion engine  2  (FIG. 1) fuel droplets are drawn in through the portion  14  from the first chamber  17 . The underpressure that thereby results in the chamber  17  draws the diaphragm  16  against the sealing rim  22  and thereby closes off the valve element  20 . By means of the flow control device  26 , air filtered by the air filter  24  can flow out of the second chamber  18  and through the air supply opening  25  into the first chamber  17 , as a result of which, at low speeds of the internal combustion engine  2  (FIG.  1 ), the fuel pump  13  can convey against the pressure of the diaphragm  16  on the sealing rim  22 . At high speeds, the underpressure in the first chamber  17  generates a sufficiently high pressure of the diaphragm  16  on the sealing rim  22 , whereby the underpressure in the first chamber  17  is no longer equalized by the flow control device  26  when the latter is appropriately set. Consequently, the valve element  20  is closed for normal operation. 
     The specification incorporates by reference the disclosure of German priority document DE 101 14 866.6 filed Mar. 26, 2001. 
     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.