Abstract:
An exhaust gas control mechanism for a two-stroke engine is provided. A first housing chamber is in communication with a discharge window of the cylinder of the engine, while a second housing chamber has the exhaust gases flowing therethrough into an exhaust gas outlet. A flow communication establishes a controlled communication of flow of exhaust gas from the first chamber to the second chamber. A valve is provided for controlling the flow communication, wherein such valve, if initially open, closes upon partial opening of the discharge window and after a given period of time again opens.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an exhaust gas control mechanism for a two-stroke engine. 
     DE-C 464 508 discloses an exhaust gas control mechanism for a two-stroke engine, according to which the exhaust gas muffler is divided into two chambers. The first chamber communicates with the exhaust gas outlet via a large cross-sectional area, while the second chamber communicates with the exhaust outlet via a small cross-sectional area. The entry of exhaust gas into the second chamber is controlled by a valve that is open when the discharge window of the cylinder opens, and then closes as counterpressure builds up in the second chamber, and finally, after reduction of pressure in the second chamber, again opens. This is intended to reduce the operating noise of the two-stroke engine. 
     Since at the time period of the opened transfer window of the two-stroke engine into the first chamber the discharge window is opened, which adjoins a large outlet cross-sectional area at the exhaust gas outlet, it is not possible to prevent the fresh gases or fuel that enter for the rinsing or flushing to be discharged via the discharge window. Thus, considerable portions of unburned fuel/air mixture can be found in the exhaust gas, which is serious with regard to environmental pollution. 
     It is therefore an object of the present invention to provide an exhaust gas control mechanism for a two-stroke engine such that the rinsing losses that increase due to the manner of operation are reduced to a minimum. 
    
    
     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 schematic cross-sectional view through the cylinder of a two-stroke engine to which is flanged an exhaust gas muffler; 
     FIG. 2 is a plan view of a valve disposed between two housing chambers of the exhaust gas muffler; and 
     FIG. 3 is a schematic cross-sectional view through the cylinder and an exhaust gas muffler flanged thereto and including a further exemplary embodiment of an inventive exhaust gas control mechanism. 
    
    
     SUMMARY OF THE INVENTION 
     The exhaust gas control mechanism of the present invention comprises a housing having a first housing chamber, which is in communication with a discharge window of a cylinder of the engine, and a second housing chamber, through which flow exhaust gases into an exhaust gas outlet of the housing; flow communication means are provided for establishing a controlled communication of flow of exhaust gas from the first chamber into the second chamber; and a valve is disposed in the housing for controlling the flow communication means, wherein the valve, if initially opened, closes upon partial opening of the discharge window and after a given period of time again opens. 
     If the discharge window opens at the end of a power stroke, the exhaust gas, which is at a high pressure, flows via the first housing chamber and the valve into the second housing chamber, as a result of which a considerable portion of the exhaust gas can as designed be withdrawn. After a partial opening of the discharge window, the valve closes and separates the housing chambers, which are successively arranged one after the other, so that now for the discharge of the exhaust gases only a smaller volume is still available, which furthermore does not communicate with the exhaust gas outlet. For this reason, an exhaust gas counterpressure builds up in the first housing chamber and approximately at the point in time of the opening of the transfer window of the two-stroke engine is so great that only small portions of the inflowing fresh gas or fuel can still pass via the discharge window into the exhaust gas channel. Thus, the exhaust gas counterpressure that is built up in the first housing chamber counteracts rinsing losses. After a given period of time, the valve again opens the connection or communication between the housing chambers, so that the exhaust gas pressure in the first housing chamber can drop by means of the valve, the second housing chamber and the exhaust gas outlet. This is expediently effected after the fresh gas or fuel flows into the combustion chamber, advantageously at a period in time in which the piston again closes the discharge window. It can be expedient to only then open the flow communication between the chambers when the piston has completely closed the discharge window in the cylinder. 
     Pursuant to a further embodiment of the present invention, the volume of the first housing chamber is less than, and preferably significantly less than, the volume of the second housing chamber. For example, pursuant to one specific embodiment the volume of the first housing chamber is approximately one-fifth to one-tenth the volume of the second housing chamber. 
     The housing chambers are advantageously disposed in a common exhaust gas muffler, so that no additional space is required for the exhaust gas control mechanism. If the drive for the valve is also disposed within the muffler and is driven by exhaust gas pressure, an exhaust gas muffler configured in this way can also be retrofitted as a replacement in delivered two-stroke engines. 
     In order to reduce the resistance to flow during the transfer of the exhaust gases from the first housing chamber into the second housing chamber to a minimum, the flow communication can comprise a plurality of individual flow openings that are disposed in the covering area of the valve member and in the circumferential direction thereof are laterally spaced from one another. In conformity therewith, the valve member also has a plurality of flow openings that are distributed over the periphery thereof and that, when the valve member is viewed in plan, are respectively disposed in the spacing area between the flow openings of the flow communication that lead to the second housing chamber, so that when the valve member rests upon the valve seat the flow openings are closed off by wall portions of the valve member and the flow openings in the valve member are closed off by wall areas of the valve seat. 
     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 schematic illustration in FIG. 1 shows the cylinder  2  of a port-controlled two-stroke engine. The cylinder  2 , together with a piston  3 , delimit a combustion chamber  1  that is provided in the cylinder. The piston  3  travels back and forth in the direction of the arrow  9 , thereby controlling not only a discharge window  5  for the discharge of the exhaust gases, but also a transfer window  26  via which fuel is supplied to the combustion chamber  1 . By means of a transfer channel  36 , which is illustrated in dashed lines, the transfer window  26  communicates with the crank case of the two-stroke engine into which, by means of an intake channel  5 , the fresh gas or fuel as well as the fuel/air mixture necessary for the operation of the internal combustion engine are drawn. The intake channel  25  is also controlled by the piston  3 . 
     The discharge window  5  opens into an exhaust gas channel  40  that is formed in the wall of the cylinder  2  and ends in a flange  41 , to which, in the illustrated embodiment, an exhaust gas muffler  4  is secured. 
     The muffler  4  comprises a housing  46  that in turn comprises two housing portions  42  and  43  that are joined together in an exhaust gas tight manner at a dividing plane  44  by means of fastening screws  45 . 
     Mounted in the housing  46  of the exhaust gas muffler  4  is a partition  28  that is secured in that housing portion  42  that faces the cylinder  2 . The partition  28  divides the interior of the housing  46  into a first housing chamber  7  and a second housing chamber  8 . The volume of the first housing chamber  7 , which adjoins the exhaust gas channel  40 , is less than, and in particular significantly less than, the volume of the second housing chamber  8 . For example, the volume of the first housing chamber  7  can be approximately one-fifth to one-tenth of the volume of the second housing chamber  8 ; in the illustrated embodiment, the second housing chamber  8  is approximately ten times as large as the first housing chamber  7 . 
     Disposed in the partition  28  is at least one flow opening  10 , which forms a flow communication  6  between the first chamber  7  and the second chamber  8 . 
     The flow communication  6  is disposed within a valve seat  31  of a valve  30 , which comprises a valve member  11  that in the illustrated embodiment is formed by a valve plate. The valve plate  11  is disposed within the first chamber  7  approximately parallel to the partition  28  and at a distance B therefrom; the valve plate  11  is centered in the first housing chamber  7  by means of guide or locating elements  12 . These elements  12  are distributed about the periphery of the valve member  10  and are spaced from one another; in the illustrated embodiment, four bellows-like guide elements  12  are provided and are respectively spaced from one another by a circumferential angle of 90°. 
     In the center  33 , the valve member  11 , which is embodied as a valve plate, is secured to the end of a push rod  29  that is disposed along the longitudinal central axis Y of the exhaust gas muffler  4 , and which passes through the partition  28 . The push rod  29  extends through the second housing chamber  8  and is connected to a displaceable wall  39  that, as a reenforced diaphragm plate of a diaphragm  13 , delimits the volume of the second housing chamber  8 . A chamber  48 , which is preferably vented to the atmosphere, is partitioned off between the base  27  of the housing portion  43  and the displaceable wall  39  that is embodied as a diaphragm. A spring  15 , preferably a coil spring, is disposed in the chamber  48 . The coil spring  15  acts upon the valve member  11  in the direction of the arrow  14  with an opening force that holds that end of the push rod  29  that faces the valve against an abutment  38  that is formed by a hood that is permeable to exhaust gas and spans the exhaust gas inlet  38 . 
     As shown in FIG. 2, several flow openings  10  are provided in the partition  28  and together form the flow communication  6  between the chambers  7  and  8 . The flow openings  10  are disposed in the covering area of the circular valve member  11 , i.e. the valve plate, and in the circumferential direction thereof are spaced from one another by the lateral spacing  35 . 
     Distributed in the valve member  11 , which is embodied as a valve plate, over the periphery of the valve are several flow openings  34  which when viewing the valve plate  11  in plan are respectively disposed in the areas  32  between each two flow openings  10  of the partition  28 . The provision of several flow openings  34  ensures a low resistence to flow despite the arrangement of the valve  30 , so that the exhaust gas, which enters the first, low volume housing chamber with high pressure, can pass via the flow openings  34  and the flow openings  10  in a largely unobstructed manner into the larger volume housing chamber  8 . For this purpose, a structurally prescribed, suitable spacing B of the valve member  11  from the partition  28  is provided. 
     If the piston travels downwardly out of its upper dead center position, which is illustrated by dot-dash lines in FIG. 1, the piston first passes over the upper edge of the discharge window  5 , for which reason exhaust gas  49  that is under high pressure flows off through the discharge window  5  and the exhaust gas channel  40  into the first housing chamber  7 , and via the open valve  30  into the second housing chamber  8 . Since the exhaust gas channel  50  that withdraws the exhaust gas from the second housing chamber has a smaller cross-sectional area than the cross-sectional area of the passage of the flow communication  6  between the housing chambers  7  and  8 , an increasing exhaust gas pressure builds up in the second housing chamber  8  until this pressure is so great that it displaces the wall  39 , i.e. the diaphragm plate, counter to the direction of the arrow  14  against the force of the spring  15  until the valve member that is embodied as the valve plate  11  overcomes the spacing B and rests against the valve seat  31  on the partition  28 . In so doing, due to the offset arrangement of the flow openings  34  relative to the flow openings  10 , the flow openings  10  are respectively closed, as a consequence of which the first housing chamber  7  is separated in an exhaust gas tight manner from the second housing chamber  8 . The valve  30  closes off the flow communication  6  approximately in a time period where the piston  3 , which is traveling in the direction toward the lower dead center position, releases the transfer window  26  of the transfer channel  36 . The fresh gases or fuel that flow in rinse the combustion chamber  1 , however cannot escape via the discharge window  5  due to the exhaust gas counter pressure that builds up in the small-volume first housing chamber  7 . Although portions of the fuel pass into the small-volume first housing chamber  7  with the remainder of the exhaust gas, the majority of the fuel remains in the combustion chamber  1 . The loss of rinsing gas that occurs is that much less the smaller is the volume of the first housing chamber  7  that directly adjoins the exhaust gas channel  40 . 
     During the further downward movement of the piston, an exhaust gas pressure is reduced via the exhaust gas channel  50 , whereby as a function of the design of the spring  15 , as the pressure drops below a threshold pressure the push rod  29  is displaced back in the direction of the arrow  14  and the valve  30  is again opened. The exhaust gas counterpressure that has built up in the first housing chamber  7  can be relieved via the valve  30  and the flow communication  6 , whereby approximately at the time period of the renewed opening of the valve  30  the piston  3 , as a result of its upward movement, has nearly completely closed the transfer window  26  and the discharge window  5  in order to compress the fuel that has flowed into the combustion chamber  1  for the next power stroke. In the region of the upper dead center position (see the dot-dashed illustration in FIG. 1) ignition of the compressed mixture is effected by the spark plug  51 , and the piston again travels in the direction of the lower dead center position. 
     The basic construction of the exemplary embodiment illustrated in FIG. 3 corresponds to that of FIG. 1, for which reason the same components have been provided with the same reference numerals. One difference from the embodiment of FIGS. 1 and 2 is that the exhaust gas channel  40  is extended by an essentially cylindrical housing connector  16  into the housing  46  of the exhaust gas muffler  4 . The cylindrical housing connector  16 , together with the exhaust gas channel  40  in the cylinder  2 , form a common exhaust gas channel  17  that leads from the discharge window  5  to the second housing chamber  8 . The common exhaust channel  17  thus forms the smaller volume first exhaust chamber  7 , whereby by reducing the size of the common exhaust channel  17  a minimal volume of the first housing chamber  7  can be provided. 
     The opening  19  of the common exhaust channel  17  into the second housing chamber  8  forms the flow communication  6  between the first housing chamber  7  and the second housing chamber  8 . The opening  19  of the common exhaust gas channel  17 , which extends into the second housing chamber  8 , is closed off by the valve member  11 , whereby the rim of the opening  19  forms the valve seat  31 . In the embodiment illustrated in FIG. 3, the valve member  11  is again embodied as a valve plate, and in particular a valve plate that is pivotable about an axis of rotation  18 ; the edge portion of the valve plate projects beyond the valve seat  31 . The valve member  11 , which is embodied as a valve plate or a valve cover, is urged by a spring  22 , especially a coil spring, into the closing position illustrated in FIG. 3 in which the opening  19  of the common exhaust gas channel  17  into the second housing chamber  8  is closed off. 
     As with the exemplary embodiment of FIG. 1, during operation of the internal combustion engine as the discharge window  5  opens the valve  30  must initially be open in order to convey the high pressure exhaust gas into the larger volume second housing chamber  8 . Approximately in the time period in which the piston oper s the transfer window  26  for the entry of fresh gas or fuel, the valve  30  is to close and to be kept closed over a given period of time until, with the piston traveling upwardly and the discharge window  5  closing, the valve  30  is again opened. 
     To operate the valve  30  of FIG. 3, a rod drive is provided according to which that end of the push rod  29  that extends into the second housing chamber  8  engages against the projecting rim  20  of the valve member  11  in order to open the valve member against the force of the spring  22  for connecting the first housing chamber  7  with the second housing chamber  8 . The push rod  29  is operated by a cam drive  23  that can comprise a cam that is disposed, for example, upon the crank shaft  24 . The contour of the cam  21  is selected such that as the discharge window  5  opens, the flow communication  6  opens; in other words, the valve  30  is opened, and at least shortly after opening of the transfer window  26  the valve is closed by the force of the spring  22  as the push rod  29  travels back. It can be expedient to slowly again open the valve  30  already when the discharge window  5  is still opened, which can be effected by an appropriate incline of the cam. The valve  30  is expediently open when the piston travels in a direction toward the upper dead center position, and the discharge window  5  is substantially or completely closed. The cam drive  23  is disposed externally of the housing  46  of the exhaust gas muffler  4 ; the push rod  29  projects into the second housing chamber  8 . 
     The specification incorporates by reference the disclosure of German priority document 199 56 157.5 of Nov. 23, 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.