Abstract:
An exhaust valve assembly for a two stroke internal combustion engine having a cylinder with at least one main exhaust port and one auxiliary exhaust port is disclosed. The assembly comprises a main exhaust valve for at least partially closing the main exhaust port, an auxiliary exhaust valve for at least partially closing the auxiliary exhaust port, an actuator for actuating the main exhaust valve and the auxiliary exhaust valve, the actuator including a rotary shaft having a first gear and a second gear, the rotary shaft being operatively connected to the main exhaust valve via the first gear, the rotary shaft being operatively connected to the auxiliary exhaust valve via the second gear, an electric motor operatively connected to the rotary shaft for rotating the rotary shaft.

Description:
[0001]     The present invention claims priority to U.S. Provisional Application Ser. No. 60/515,694, which was filed on Oct. 31, 2003, the entirety of which is hereby incorporated into the present application by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a exhaust control valve assembly of a two stroke internal combustion engine. More particularly, the invention relates to a technically efficient exhaust control assembly wherein both the main exhaust control valve and the at least one auxiliary exhaust control valve can be controlled by only one actuator.  
         [0004]     2. Discussion of the Related Art  
         [0005]     As is generally known two-stroke (often referred also as two-cycle) engines generally have exhaust and intake ports in the side walls of a cylindrical chamber. The cylindrical chamber and its side walls are usually referred to simply as a cylinder. While the intake ports feed air into the cylinder, the exhaust ports guide burned gas out of the cylinder into the exhaust of the engine. Since the intake and exhaust ports are at a fixed positions on the cylinder wall, the intake and exhaust ports are opened and closed by the piston which reciprocally moves up and down in the cylinder. Ideally however, for optimal operation of the engine, the position of the intake and the exhaust ports would be alterable according to the status of operation of the engine, preferably depending on engine speed and load. It follows from this consideration that the position of the exhaust ports is a compromise which allows for a proper performance of the engine both in low and high engine speed ranges.  
         [0006]     It is known that by altering the vertical position of the upper edge of the exhaust port on the cylinder wall so that at high engine speeds the exhaust can be kept open longer, will considerably improve engine performance.  
         [0007]     In this respect, it is known for example from AT 380 537 B that a main exhaust slide valve can be fitted in the exhaust passage proximate to the cylinder head. This main exhaust slide valve will be supported in a guide opening that is generally radial relative to the cylinder. At its free face end the slide valve forms a control edge that conforms to the cylinder bore, so that when the control valve is in its advanced position, the exhaust outlet cross-section is partially covered in its vertical dimension. This results in a corresponding reduction of the exhaust period because of the delayed opening and earlier closing of the exhaust passage. Thus, the charging of the cylinder in the middle and lower speed ranges is improved.  
         [0008]     In some cases, in order to improve the exhaust conditions at the beginning and at the end of the exhaust gas discharge, auxiliary lateral exhaust outlets, connected through lateral channels to the main exhaust passage, are provided in the cylinder. These auxiliary exhaust outlets may be closed in order to reduce the exhaust period. Typically this is accomplished by cylindrical shutoff valves. These shutoff valves can through the lateral channels and can rotate or slide between a closed position and an open position. They are mechanically connected to the main exhaust valve such that they are actuated thereby.  
         [0009]     One disadvantage of this known exhaust period control system is its technically complex design. Locating rotatable cylindrical shutoff valves in the lateral channels (which must be supported so as to be rotatable), and actuating them via the main exhaust valve (whose pivot axis is offset by 90° relative to the axes of rotation of the shutoff valves) is considerable. Another is the simple fact that the auxiliary (cylindrical shutoff) valves must be driven by the main exhaust valve. It would be desirable to have them be separately driven to allow them to be opened at different speeds.  
         [0010]     EP 0 141 650 A2 describes a lateral auxiliary exhaust outlet which is controlled by means of a shutoff slide valve. The valve is actuated as a function of engine speed, with it only opening at higher speeds. However, this design incorporates no main exhaust valve at all.  
         [0011]     JP 7279675 A describes an engine having both a main exhaust port with a main exhaust valve and an auxiliary exhaust port with an auxiliary exhaust valve. Both valves are actuated by a single motor; the main exhaust via a lever system, the auxiliary exhaust valves via a complicated gear system. This system is expensive and takes up a great deal of space.  
         [0012]     Thus there is a need in the art for an improved exhaust system for a two-stroke internal combustion engine.  
       SUMMARY OF THE INVENTION  
       [0013]     An aspect of embodiments of the invention provides an exhaust control valve assembly for an two-stroke internal combustion engine which is characterised by a technically efficient actuation of the main and the auxiliary exhaust valves.  
         [0014]     Other aspects of embodiments of the invention allow a relatively small installation space, a simple mounting arrangement and a reliable operation of the exhaust control valve assembly.  
         [0015]     Other aspects of embodiments of the invention allow a relatively small installation space, a simple mounting arrangement and a reliable operation of a 2-stroke internal combustion engine.  
         [0016]     Other aspects of embodiments of the invention allow designing a snowmobile, the snowmobile having a very compact and powerful high-performance 2-stroke internal combustion engine as provided by the present invention.  
         [0017]     One aspect of the invention is directed to an internal combustion engine having a crankcase, a crankshaft disposed within the crankcase, a cylinder block connected to the crankcase, the cylinder block having a cylinder disposed therein. The internal combustion engine further having a main exhaust port in fluid communication with the cylinder, an auxiliary exhaust port in fluid communication with the cylinder, a piston movably disposed within the cylinder and being operatively connected to the crankshaft and an exhaust valve assembly. The exhaust valve assembly of the internal combustion engine comprising a main exhaust valve for at least partially closing the main exhaust port, an auxiliary exhaust valve for at least partially closing the auxiliary exhaust port and an actuator for actuating the main exhaust valve and the auxiliary exhaust valve. The actuator including a rotary shaft having a first gear and a second gear, the rotary shaft being operatively connected to the main exhaust valve via the first gear, the rotary shaft being operatively connected to the auxiliary exhaust valve via the second gear, an electric motor operatively connected to the rotary shaft for rotating the rotary shaft. Said internal combustion operating on a two-stroke principle.  
         [0018]     By the present invention both of the main exhaust and the at least one auxiliary exhaust valve can be controlled by one single actuator. According to a preferred embodiment the actuator comprises a rotatable shaft on which a number of gears or gear segments are disposed. By this arrangement a direct response of the exhaust valves on the actuation, for instance by a servomotor, can be ensured. Compared to prior art which teaches the use of levers or pneumatic devices the inventive arrangement provides the most direct response of the exhaust control valves to their actuation. Furthermore by using, for instance a steppermotor to rotate the rotatable shaft, it is possible to provide an unlimited number of positions of the exhaust valves. This is clearly an advantage over the pneumatic devices taught by the prior art, as those can only provide a retracted and an extended position of the control valves. As should be appreciated by those skilled in the art, the invention is particularly useful for high performance engines, such as those used on motorcycles, snowmobiles, personal watercrafts or other vehicles of the recreational business.  
         [0019]     By actuating the main and the auxiliary exhaust valves by gears or gear segments the man skilled in the art can overcome all prior solutions dealing with complicated levers and pneumatic actuation of the exhaust control valves. Having the gears for the main and the auxiliary exhaust control valve arranged on a single rotatable shaft per cylinder, it is possible to control the position of-the main exhaust control valve and the auxiliary exhaust control valve at the same time by simply rotating the shaft, by an appropriate servo motor or another device capable of rotating the shaft, preferably according to the demand of an engine control unit.  
         [0020]     According to a preferred embodiment of the present invention the main exhaust port and the auxiliary exhaust port are disposed laterally on the cylinder, as is common on most two-cycle internal combustion engines.  
         [0021]     According to a preferred embodiment of the present invention the main exhaust valve is directly connected with the first gear and wherein the auxiliary valve is directly connected with the second gear. With this embodiment a very reliable and compact design can be accomplished.  
         [0022]     According to another aspect of the invention the main exhaust valve and the at least one auxiliary exhaust valve each comprise a toothed rack, wherein the toothed rack of the main exhaust valve meshes with the teeth of the first gear and wherein the toothed rack of the at least one auxiliary exhaust valve meshes with the teeth of the second gear.  
         [0023]     By the combination of the gears mounted on the rotary shaft and the toothed racks of the main exhaust valve and the at least on auxiliary exhaust valve a reliable and efficient connection can be established. Compared to solutions known from the prior art which teach the use of levers or linkages using either wires or cables for connecting the exhaust valves to the rotatable shaft, the proposed connection is more reliable and minimises the need of readjustment.  
         [0024]     According to a further aspect of the invention each of the main exhaust valve and the at least one auxiliary exhaust valve comprises a valve body, the valve body of the main exhaust valve being operatively connected with the rotary shaft via the first gear, the valve body of the at least one auxiliary exhaust valve being operatively connected with the rotary shaft via the second gear.  
         [0025]     According to another aspect of the invention the exhaust valve assembly further comprises an exhaust valve assembly casing, the exhaust valve assembly casing having a first linear guiding for guiding the valve body of the main exhaust valve and at least one second linear guiding for guiding the valve body of the at least one auxiliary exhaust valve.&lt; 
         [0026]     According to a further preferred embodiment of the present invention the proposed way of connecting the main and the auxiliary exhaust valve via gears with the actuator has proven to be especially useful when applied on so called sliding valves. These sliding valve are guided along linear guides, preferably designed in the exhaust valve assembly casing. Compared to rotating exhaust valves the sliding valves can easily be moved between a retracted and an extended position. Furthermore the bearing of a linearly sliding valve is significantly easier than the bearing of a rotating exhaust valve used elsewhere in the prior art.  
         [0027]     According to a preferred embodiment of the present invention the exhaust valve assembly casing is at least partly formed by the cylinder block.  
         [0028]     According to another preferred embodiment of the present invention the first gear has a first number of teeth A and the second gear has a second number of teeth B and wherein B is greater than A.  
         [0029]     It has been discovered that at a point of transition to high engine speed it is especially beneficial to open the auxiliary exhaust valves before the main exhaust valve. From the prior art it is known to provide a pneumatic actuator which retracts the auxiliary exhaust valve before the main exhaust valve. This is done by connecting the main exhaust valve to the auxiliary exhaust valve over a shoulder engaging a corresponding notch in the main exhaust valve. Since the notch provides an extended clearance the main exhaust valve is retracted only with a time delay with regard to the auxiliary exhaust valve. It has been further discovered that as a matter of fact not the chronology of the movement of the main exhaust valve with respect to the auxiliary exhaust valve positively influences the performance of the internal combustion engine but rather that the opening speed of the main exhaust port with respect to the auxiliary exhaust port is significant. According to a preferred embodiment of the present invention a different lift of the main and the auxiliary exhaust valve can be provided by using gears with different numbers of teeth. Hence in order to let the auxiliary exhaust valve open faster than the main exhaust valve, a second gear with. a higher number of teeth respectively with a greater radius is used. Increasing the distance between the point of contact between the gear and the rack of the exhaust valve with respect to the axis of the rotatable shaft increases the speed of the, preferably linearly, moving exhaust valve respectively increases the distance of movement of the exhaust valve with respect to the angle of rotation of the shaft.  
         [0030]     According to another preferred embodiment of the present invention the first gear has a first radius R 1  and the second gear has a second radius R 2  and wherein R 2  is greater than R 1 .  
         [0031]     According to a preferred embodiment of the present invention at least one of the first or the second gear is at least partly made of plastic. It has been found that using gears or gear segments made of plastic reduces the overall weight of the control valve assembly while ensuring superior reliability and low wear of the gearing on the exhaust valves.  
         [0032]     Another aspect of the invention is directed to an internal combustion engine comprising a crankcase, a crankshaft disposed within the crankcase, a cylinder block connected to the crankcase, the cylinder block having a first and a second cylinder disposed therein. The internal combustion engine further comprising a first main exhaust port in fluid communication with the first cylinder, a second main exhaust port in fluid communication with the second cylinder, a first auxiliary exhaust port in fluid communication with the first cylinder, a second auxiliary exhaust port in fluid communication with the second cylinder and a first piston movably disposed within the first cylinder and a second piston movably disposed within the second cylinder, wherein the first and the second piston are operatively connected to the crankshaft. The internal combustion engine further comprising an exhaust valve assembly, the exhaust valve assembly comprising a first main exhaust valve for at least partially closing the first main exhaust port, a second main exhaust valve for at least partially closing the second main exhaust port, a first auxiliary exhaust valve for at least partially closing the first auxiliary exhaust port, a second auxiliary exhaust valve for at least partially closing the second auxiliary exhaust port, and an actuator for actuating the first and second main exhaust valve and the first and second auxiliary exhaust valve. Said actuator including a rotary shaft having a first and a second main gear and a first and a second auxiliary gear, wherein the rotary shaft is operatively connected to the first main exhaust valve via the first main gear and to the second main exhaust valve via the second main gear. The rotary shaft further being operatively connected to the first auxiliary exhaust valve via the first auxiliary gear and to the second auxiliary exhaust valve via the second auxiliary exhaust valve. The actuator further comprising an electric motor operatively connected to the rotary shaft for rotating the rotary shaft. Said internal combustion engine operating on a two-stroke principle.  
         [0033]     It is a particular advantage of the gear-driven exhaust control valve assembly as herewith presented that by the use of just one single rotary shaft and the corresponding gears or gear segments disposed thereon the main and auxiliary exhaust valves of at least two cylinders can be controlled at the same time.  
         [0034]     According to a preferred embodiment of the present invention the first main exhaust port and the first auxiliary exhaust port are disposed laterally on the first cylinder, and wherein the second main exhaust and the second auxiliary exhaust port are disposed laterally on the second cylinder.  
         [0035]     According to another preferred embodiment of the present invention the first main exhaust valve is directly connected with the first main gear and the second main exhaust valve is directly connected with the second main gear. According to a fuirther preferred embodiment the first auxiliary valve is directly connected with the first auxiliary gear and the second auxiliary valve is directly connected with the second auxiliary gear.  
         [0036]     According to a preferred embodiment of the present invention at least one of the first and the second main gears and the first and the second auxiliary gears is shaped as a gear segment.  
         [0037]     According to a preferred embodiment of the present invention each of the first and the second main exhaust valves and each of the at least first and second auxiliary exhaust valves comprises a toothed rack, wherein the toothed rack of the first main exhaust valve meshes with the teeth of the first main gear and the toothed rack of the second main exhaust valve meshes with the teeth of the second main gear and wherein the toothed rack of the at least one first auxiliary exhaust valve meshes with the teeth of the first auxiliary gear and the at least one second auxiliary exhaust valve meshes with the teeth of the gears of the second auxiliary gear.  
         [0038]     According to a preferred embodiment of the present invention the first main exhaust valve comprises a first main valve body and the second main exhaust valve comprises a second main valve body. Furthermore according to another preferred embodiment of the invention the at least one first auxiliary exhaust valve comprises a first auxiliary valve body and the at least one second auxiliary exhaust valve comprises a second auxiliary valve body. According to these different embodiments the first main valve body being operatively connected with the rotary shaft via the first main gear, the second main valve body being operatively connected with the rotary shaft via the second main gear and the first auxiliary valve body being operatively connected with the rotary shaft via the first auxiliary gear and the second auxiliary valve body being operatively connected with the rotary shaft via the second auxiliary gear.  
         [0039]     According to a preferred embodiment of the present invention the exhaust valve assembly further comprises an exhaust valve assembly casing, the exhaust valve assembly casing having a set of first linear guidings for guiding the first and the second main valve body and a set of second linear guidings for guiding the first and second auxiliary valve body.  
         [0040]     According to a preferred embodiment of the present invention the exhaust valve assembly casing is at least partly formed by the first and the second cylinder.  
         [0041]     According to a preferred embodiment of the present invention the first and the second main gear each have a first number of teeth A and the first and the second auxiliary gear each have a second number of teeth B and wherein B is greater than A.  
         [0042]     According to a preferred embodiment of the present invention the first and the second main gear each have a first radius R 1  and the first and the second auxiliary gear each have a second radius R 2  and wherein R 2  is greater than R 1 .  
         [0043]     According to a preferred embodiment of the present invention at least one of the first or the second main or auxiliary gears is at least partly made of plastic [what type of plastic?].  
         [0044]     It is the objective of the present invention to avoid the shortcomings outlined above and to so configure a device for controlling the exhaust period of a cylinder of a two-cycle internal combustion engine that advantageous drive conditions can be ensured, given a simple structure and a small installed size.  
         [0045]     The present invention achieves this objective in that both the position of the main exhaust slide valve for the main exhaust outlet and the position of the auxiliary exhaust slide valve of at least the one auxiliary exhaust outlet can each be adjusted by a rack and pinion drive system, each system comprising a rack and a pinion.  
         [0046]     By providing a main exhaust slide valve for the main exhaust outlet and a auxiliary exhaust slide valve for at least one auxiliary exhaust outlet, with the set position of the main exhaust slide valve and of the auxiliary exhaust slide valve each being adjustable by way of a rack and pinion drive system, it is possible to ensure advantageous drive conditions for the exhaust slide valves with a simple construction and a small installed size. As a result, the greatest possible peak torque in the upper speed range is achieved while, at the same time, torque is increased in the lower and middle speed ranges. By introducing a plurality of set positions for the exhaust slide valves, for example, a middle position, it is possible to improve the torque for the middle speed range even further. If the set position of the exhaust slide valves, in particular in the partial load range of the internal combustion engine, is matched continuously to the particular load on the internal combustion engine, it is possible to achieve a significant improvement of emission values, this involving a complex control system.  
         [0047]     Particularly simple construction conditions for the adjusting drive for the exhaust slide valves will be achieved if the pinions of the main exhaust slide valve of the main exhaust outlet and of the auxiliary exhaust slide valve of a least one auxiliary exhaust outlet are mounted rigidly on a common shaft, since in this case the position of the exhaust slide valves for the main exhaust outlet and at least one auxiliary exhaust outlet can be adjusted by a common adjusting device, for example, a servo-motor.  
         [0048]     In order to make it possible to open the auxiliary exhaust passages faster, for example, than the main exhaust passage, it is recommended that the rack and pinion drives of the main exhaust slide valve of the main exhaust outlet and the rack and pinion drive of the auxiliary exhaust slide valve of at least one auxiliary exhaust outlet be set up with different gear ratios; it has proved to be particularly advantageous if the gear ratio used for the rack and pinion drive for the main exhaust slide valve is smaller, in particular one-half, that of the gear ratio of the rack and pinion drive for at least one slide valve.  
         [0049]     If the main exhaust slide valve and at least one auxiliary exhaust slide valve are adjusted in directions that the parallel to each other, it becomes much simpler to couple the main exhaust slide valve and at least one auxiliary exhaust slide valve. The main exhaust slide valve and the least one auxiliary exhaust slide valve can, in an advantageous manner, be actuated by the common adjusting device, for example, against the action of a spring, it being possible, in principle, to make the spring act in the direction of the operating position or the starting position.  
         [0050]     The auxiliary exhaust slide valve for the auxiliary exhaust outlet can be guided in a bore that is parallel to the through guide for the main exhaust slide valve in the exhaust passage wall, said bore passing through the auxiliary exhaust passage, it being preferred that in this case at least one auxiliary exhaust slide valve is a shutoff slide that selectively closes or releases the auxiliary exhaust passage.  
         [0051]     The pivoted position of the shafts is preferably adjusted by an electronic control unit as a function of the engine speed and/or the load on the engine, by way of an adjusting system, so as to permit advantageous pre-setting over a wide range. The adjusting drive comprises a servomotor that can be connected selectively through an actuating cable to the shaft so as to transmit a drive, or directly or through gearing to the shaft.  
         [0052]     These and other aspects, advantages and features of the invention will become apparent in view of the following detailed description of the invention. 
     
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
       [0053]     In the drawings, which form part of this original disclosure, like elements are represented by like reference numerals.  
         [0054]      FIG. 1 : A device according to the present invention for controlling the exhaust period of one cylinder of a two-cycle internal combustion engine, in axial cross-section through the cylinder in the area of the main exhaust passage;  
         [0055]      FIG. 2 : the slide valves of the device according to the present invention, together with the adjusting drive as viewed from the front;  
         [0056]      FIG. 3 : the slide valves, together with the adjusting drive, in a cross section on the line III-III shown in  FIG. 2 ;  
         [0057]      FIG. 4 : the slide together with the adjusting drives as shown in  FIG. 2  and  FIG. 3 , in plan view;  
         [0058]      FIG. 5 : an exploded view of another preferred embodiment of the invention;  
         [0059]      FIG. 6 : a cross-section of the assembly at the connecting surface between the upper and lower casing.  
         [0060]      FIG. 7 : a top view of a 2-stroke internal combustion engine using the inventive exhaust valve assembly.  
         [0061]      FIG. 8 : a schematical drawing of a snowmobile using the 2-stroke internal combustion engine of  FIG. 7   
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0062]     Referring to FIGS.  1  to  4 , the cylinder  1  of a two-cycle internal combustion engine has an exhaust passage  3  that adjoins a main exhaust outlet  2  and, located symmetrically to this main exhaust outlet  2 , lateral auxiliary exhaust outlets  4  that are connected to the exhaust passage  3  through of lateral channels  5 . The wall  6  of the exhaust passage  3  that is proximate to the cylinder head forms a passage opening  7  for the main exhaust slide valve  8 , which has at its unattached face end a control edge  9  that conforms to the cylinder bore. This main exhaust slide valve  8  can move between a position in which it is withdrawn into the passage opening  7  and a position in which it is advanced against the cylinder jacket, at an acute angle to the cylinder axis  10  and almost perpendicular to the wall  6  of the exhaust passage  3  that is proximate to the cylinder head.  
         [0063]     Receiving bores for the auxiliary exhaust slide valves  11  that are associated with the lateral channels  5  and which are parallel to the passage guide  7  for the blocking slide  8  are provided in the area of the lateral channels  5 ; these can be moved in their associated receiving bores between a position in which the release the side channel  5  and the position in which they block the side channels  5 . A rack and pinion drive that comprises a pinion  13 ,  14  and a rack  12  is provided to adjust the set position of the main exhaust slide valve  8  for the main exhaust outlet  2  and the set position of the auxiliary exhaust slides  11  of the auxiliary exhaust outlets  4 ; the pinions  13 ,  14  of the main exhaust slide valve  8  for the main exhaust outlet  2  and the auxiliary exhaust slides  11  of the auxiliary exhaust outlets  4  are arranged on a common shaft  15  so as to rotate in unison with said shaft  15 . The gear ratios of the rack and pinion gearing for the main exhaust slide valve  8  of the main exhaust outlet  2  and the auxiliary exhaust slides  11  of the auxiliary exhaust outlets  4  are different, with the diameter of the pinion  14  being approximately half that of the pinion  13 .  
         [0064]     The rotated position of the shaft is preferably adjustable by way of an electronic control unit as a function of engine speed and of the load on the engine, this adjustment be made by way of an electric motor, so that the adjusted or set position of the slide  8 ,  11  can be pre-set over a wide range. However, a motor could also be mounted at a location on the engine that provides sufficient space and connected to the shaft through a Bowden cable so as to transmit a driving force.  
         [0065]     FIGS.  5  to  FIG. 7  show another particular embodiment of the present invention. In this preferred embodiment shaft  16  is mounted in a exhaust valve assembly casing  17 , which comprises a bottom casing  18  and a top casing  19 . The bottom and the top casing are connected via screws  20 , whereby the screws  20  are further used for mounting the exhaust valve assembly casing  17  on the cylinderblock of the internal combustion engine  49 . According to a preferred embodiment a first gasket  21  can be disposed between the bottom casing and the top casing and a second gasket  22  between the bottom casing and the cylinders  50  of the internal combustion engine  49  for ensuring optimal tightness of the assembly. The shaft  16  carries a first and a second main gear  23  and  24  as well as four auxiliary gears  25 ,  26 ,  27  and  28 . Auxiliary gears  25  and  26  as well as the first main gear  23  are assigned to the exhaust port of the first cylinder  51  of the internal combustion engine  49 , while auxiliary gears  27  and  28  as well as the second main gear  24  are assigned to the second cylinder  52  of the internal combustion engine. The auxiliary gears  25 ,  26 ,  27  and  28  as well as the first and second main gear  23  and  24  are fixed on the shaft  16  by screws  29 . These screws  29  ensure that the auxiliary gears and the main gears rotate in unison with the shaft  16 .  
         [0066]     The exhaust valve assembly further comprises a first main exhaust valve  30  acting on the main exhaust port of the first cylinder of the internal combustion engine and a second main exhaust valve  31  acting on the main exhaust port of the second cylinder of the internal combustion engine. As can be clearly seen in  FIG. 5 , the first and second main exhaust valves each comprise a toothed rack  32  and  33  as well as valve bodies with defined control edges  34  and  35 . The first and second main exhaust valves are guided in the exhaust valve assembly casing  17  as well as in the cylinder via linear guides. The toothed racks  32  and  33  mesh with the first and the second main gears  23  and  24  such that a rotation of the shaft  16  will result in a linear movement of the first and the second main exhaust valve  30  and  31 . As can be clearly seen in  FIG. 5 , there is a hole provided on each of the toothed racks  32  and  33  of the main exhaust valves. A first insert  53  is placed in the first main exhaust valve, whereas a second insert  54  is placed in the second main exhaust valve. Both inserts protrude from the holes such that the prevent contact of the top casing  19  and the toothed racks  32  and  33  when the main exhaust valve is in the fully retracted state. According to a preferred embodiment of the present invention the main gears are made of plastic, preferably of polyamide or one of its derivatives.  
         [0067]     The exhaust valve assembly further comprises auxiliary exhaust valves  36  and  37  acting on the auxiliary exhaust ports of the first cylinder of the internal combustion engine and auxiliary exhaust valve  38  and  39  acting on the auxiliary exhaust ports of the second cylinder of the internal combustion engine. As can be clearly seen in  FIG. 5 , each of the auxiliary exhaust valves  36 ,  37 ,  38  and  39  comprises a toothed rack  40 ,  41 , 42  and  43 . The auxiliary exhaust valves are guided in the exhaust valve assembly casing  16  as well as in the cylinder via generally linear guides. The toothed racks  40 ,  41 ,  42  and  43  of the auxiliary exhaust valves mesh with the auxiliary gears  25 ,  26 ,  27  and  28  such that a rotation of the shaft  16  will result in a linear movement of the auxiliary exhaust valves  36 ,  37 ,  38  and  39 . According to a preferred embodiment of the present invention the auxiliary gears are made of plastic, preferably of a thermoplastic elastomer for instance based on co-polyester.  
         [0068]     According to a preferred embodiment of the present invention the shaft  16  is connected with a pulley wheel  44  to rotate in unison with it. A Bowden cable  45  is used to connect the pulley  44  with a second pulley  46 , as can be seen on  FIG. 7 . The second pulley  46  is operatively connected to an electric motor  47 , preferably a servo-motor, which is controlled by an ECU  48  according to the current status of the internal combustion engine  49 .  
         [0069]     The adjusting mechanism of the exhaust valves according to the present invention offers clear advantages as compared to the prior art, which provides for resetting the slide valves by way of a spring mechanism. Thus, on the one hand, in this way any possible and undesirable movement of spring-loaded slides and thus the generation of undesirable noise can be prevented and, on the other hand, active setting of the slides into the desired intermediate positions, between an open and a closed position, can be ensured.  
         [0070]     The inventive exhaust valve assembly of the present invention is suitable for use on a snowmobile  110  such as that shown in  FIGS. 10 .  FIG. 10  shows a snowmobile incorporating the present invention and is identified generally by the reference numeral  110 . The snowmobile  110  includes a forward end  112  and a rearward end  114 , which are defined consistently with the travel direction of the vehicle. The snowmobile  110  further includes a chassis  116  which normally includes a rear tunnel  118 , an engine cradle portion  120  and a front suspension assembly portion  122 . An 2-stroke internal combustion engine  124  which is schematically illustrated by broken lines is mounted to the engine cradle portion  120  of the chassis  116 . A ski and steering assembly (not indicated) is provided, in which two skis  126  are positioned at the front end  112  of the snowmobile  110 , and are attached to the front suspension assembly portion  122  of the chassis  116  through a front suspension assembly  128 . The front suspension assembly  128  includes ski legs  130 , supporting arms  132  and ball joints (not shown) for operatively joining the respective ski legs  130 , supporting arms  132  and a steering column  134 . The steering column  134  at its upper end is attached to a steering device such as handlebars  136  which is positioned forward of a driver (not shown) and behind the engine  124  to rotate the ski legs  130  and thus the skis  126 , in order to steer the vehicle.  
         [0071]     At the front end  112  of the snowmobile  110  there are provided fairings  138  that not only protect the engine  124 , but can also be decorated to make the snowmobile  110  more aesthetically pleasing. A windshield  140  may be connected to the fairings  138  near the front end  112  of the snowmobile  110  or may be attached directly to the handlebars  136 . The windshield  140  acts as a wind screen to lessen the force of air on the driver while the snowmobile  110  is traveling. A seat  142  is provided on the rear tunnel  118  to position the driver and any rider of the vehicle.  
         [0072]     An endless drive track  144  is positioned at the rear end  114  of the snowmobile  110 , and is disposed under rear tunnel  118 , being operatively connected to the engine  124  via CVT  162 . CVT  162  includes a drive pulley  164  rotating with an output shaft  123  of the engine  124 , a driven pulley  166  as well as an endless drive belt  168  connecting the drive pulley  164  with the driven pulley  166  in a manner as well known by the man skilled in the art. One skilled in the art would recognize that the driven pulley  166  could be directly connected to a front drive axle  170  or it could be connected to the front drive axle  170  via a gear reduction unit (not shown) in order to rotate the endless drive track  144 .  
         [0073]     Thus, the endless drive track  144  is driven to run about a rear suspension assembly  146  for propulsion of the snowmobile  110 . The rear suspension assembly  146  includes a pair of slide rails  148  in sliding contact with the endless drive track  144 . The rear suspension assembly  146  also includes one or more shock absorbers  150  which may further include a coil spring (not shown) surrounding the individual shock absorbers  150 . Front and rear suspension arms  152 ,  154  are provided to attach the slide rails  148  to the chassis  116 . One or more idler wheels which include a pair of rear idler wheels  156  and a pair of other idler wheels  158 , are also provided in the rear suspension assembly  146 .  
         [0074]     Modifications and improvements to the above-described embodiments of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the present invention is therefore intended to be limited solely by the scope of the appended claims.