Abstract:
An exhaust valve assembly has a valve actuator, a first valve part (FVP) operatively connected to the actuator, a second valve part (SVP), and at least one auxiliary valve operatively connected to the second valve part. When the actuator is in a first position, the FVP is in a fourth position, the SVP is in a seventh position and the at least one auxiliary valve is in a ninth position. When the actuator is in a second position, the FVP is in a fifth position, the SVP is in the seventh position and the at least one auxiliary valve is in the ninth position. When the actuator is in a third position, the FVP is in a sixth position, the SVP is in an eighth position and the at least one auxiliary valve is in a tenth position. The second position is intermediate the first and third positions.

Description:
CROSS-REFERENCE 
       [0001]    The present application claims priority to U.S. Provisional Patent Application No. 61/860,641, filed Jul. 31, 2013, the entirety of which is incorporated herein by reference. 
     
    
     FIELD OF TECHNOLOGY 
       [0002]    The present technology relates to an exhaust valve assembly suitable for use with an exhaust port of a two stroke internal combustion engine. 
       BACKGROUND 
       [0003]    In order to ensure that two-stroke engines have a high power capacity at high speeds, a high volumetric efficiency is required and the charge losses must be minimized This can be accomplished by an early and therefore higher opening of the exhaust passage into the cylinder. The adjustment of the exhaust port, to obtain maximum power capacity of the engine at high speeds involves, in the medium speed range, not only an appreciable decrease of the useful stroke, but also a large increase of the charge losses. As a result, the torque decreases and the specific fuel consumption increases greatly. A higher torque in conjunction with lower fuel consumption can be obtained, at lower engine speeds, only if the opening of the exhaust port happens later in the down stroke of the piston which means that the exhaust port must be at a lower position than it is at high engine speeds. 
         [0004]    For this purpose it is known to provide a valve in the exhaust port which is movable between a full flow position and a flow restricting position. When in the flow restricting position, the end of the valve is substantially flush with the peripheral surface of the cylinder bore. In this flow restricting position, the exhaust port is effectively lowered in relation to the down stroke of the piston. The valve is adjustable to vary the relative height of the exhaust port as is required by the given operating conditions of the engine. 
         [0005]    U.S. Pat. No.  7 ,484,482 B1, issued on Feb. 3, 2009 to Mayringer, entitled “Valve Assembly for a Two-Stroke Engine”, discloses a valve assembly having a two-part valve provided, in part, in a main exhaust port and auxiliary valves provided, in part, in auxiliary exhaust ports. The valve assembly also has an actuator to which the valves are connected. The actuator is movable between a lowered position, an intermediate position and a raised position. The two-part valve has a first valve part connected to the actuator and movable with the actuator between the lowered position, the intermediate position, and the raised position. The auxiliary valves are connected to the first valve part and are movable with the first valve part and the actuator between the lowered position, the intermediate position, and the raised position. The two-part valve has a second valve part that is movable between a lowered position and a raised position. When the first valve part is in its lowered position or its intermediate position, the second valve part is in its lowered position. When the first valve part is in its raised position, the second valve part is in its raised position. 
         [0006]    Although the valve assembly of Mayringer provides an intermediate valve position used at medium engine speeds which is an improvement over the prior two position valves, the auxiliary valves partially open the auxiliary exhaust ports when in this intermediate position. This is because the auxiliary valves are connected to and movable with the first valve part. Maintaining the auxiliary exhaust ports fully closed when the first valve part is in the intermediate position would further improve fuel consumption. 
         [0007]    Therefore, there is a need for an exhaust valve assembly having auxiliary exhaust valves that close the auxiliary exhaust ports at medium engine speeds. 
       SUMMARY 
       [0008]    It is an object of the present technology to ameliorate at least some of the inconveniences present in the prior art. 
         [0009]    According to one aspect of the present technology, there is provided an exhaust valve assembly for a two-stroke internal combustion engine having a valve actuator movable between a first position, a second position and a third position, the second position being intermediate the first and third positions, a two-part valve having a first valve part and a second valve part, the first valve part being operatively connected to the valve actuator, and at least one auxiliary valve operatively connected to the second valve part. The first valve part is in a fourth position when the valve actuator is in the first position. The first valve part is in a fifth position when the valve actuator is in the second position. The first valve part is in a sixth position when the valve actuator is in the third position. The fifth position is intermediate the fourth position and the sixth position. The second valve part is in a seventh position when the valve actuator is in any one of the first and second position. The second valve part is in an eighth position when the valve actuator is in the third position. The at least one auxiliary valve is in a ninth position when the second valve part is in the seventh position. The at least one auxiliary valve is in a tenth position when the second valve part is in the eighth position. 
         [0010]    In some implementations of the present technology, a distance traveled by the at least one auxiliary valve between the ninth and tenth positions is greater than a distance traveled by the first valve part between the fourth and sixth positions. 
         [0011]    In some implementations of the present technology, the fourth, seventh and ninth positions are flow restricting positions of the first valve part, the second valve part and the at least one auxiliary valve respectively. The sixth, eighth and tenth positions are full flow positions of the first valve part, the second valve part and the at least one auxiliary valve respectively. The fifth position is an intermediate position of the first valve part. 
         [0012]    In some implementations of the present technology, a lever pivotally connects the at least one auxiliary valve to the second valve part. 
         [0013]    In some implementations of the present technology, the lever has at least one arm having a first end and a second end, at least one first shaft connected to the first end of the at least one arm, the at least one first shaft being pivotally connected to the at least one auxiliary valve, at least one second shaft connected between the first and second ends of the at least one arm, the at least one second shaft being pivotally connected to the second valve part, and at least one third shaft connected to the second end of the at least one arm, the at least one third shaft abutting a portion of the valve actuator. 
         [0014]    In some implementations of the present technology, the at least one third shaft abuts at least one post extending from the valve actuator. 
         [0015]    In some implementations of the present technology, the at least one arm is V-shaped. The at least one second shaft is connected at the corner of the V-shaped arm. 
         [0016]    In some implementations of the present technology, the second valve part has at least one pair of fingers. The at least one second shaft is received between the at least one pair of fingers. 
         [0017]    In some implementations of the present technology, when the first valve part moves from the fifth position to the sixth position, the first valve part abuts at least one of the fingers to move the second valve part from the seventh position to the eight position and the at least one auxiliary valve from the ninth position to the tenth position. 
         [0018]    In some implementations of the present technology, when the first valve part moves from the fifth position to the sixth position, the first valve part abuts a portion of the second valve part to move the second valve part from the seventh position to the eight position and the at least one auxiliary valve from the ninth position to the tenth position. 
         [0019]    In some implementations of the present technology, at least one spring is provided for biasing the second valve part toward the seventh position. 
         [0020]    In some implementations of the present technology, the at least one auxiliary valve is two auxiliary valves. The first valve part and the second valve part are disposed between the two auxiliary valves. 
         [0021]    In some implementations of the present technology, the valve actuator is biased toward the second position. 
         [0022]    In some implementations of the present technology, the valve actuator is a pneumatic actuator. 
         [0023]    According to another aspect of the present technology, there is provided an internal combustion engine having a crankcase, a crankshaft disposed in the crankcase, a cylinder block connected to the crankcase, the cylinder block having a main exhaust passage and at least one auxiliary exhaust passage, a cylinder defined in the cylinder block, a piston movably disposed within the cylinder and being operatively connected to the crankshaft, and a valve assembly according to one or more of the above aspect and implementations connected to the cylinder block. When the first valve part is in the fourth position, the first valve part extends a first distance in the main exhaust passage. When the first valve part is in the fifth position, the first valve part extends a second distance in the main exhaust passage, the second distance being less than the first distance. When the first valve part is in the sixth position, the first valve part is withdrawn from the main exhaust passage. When the second valve part is in the seventh position, the second valve part extends a third distance in the main exhaust passage. When the second valve part is in the eighth position, the second valve part is withdrawn from the main exhaust passage. When the at least one auxiliary valve is in the ninth position, the at least one auxiliary valve extends a fourth distance in the at least one auxiliary exhaust passage. When the at least one auxiliary valve is in the tenth position, the at least one auxiliary valve is withdrawn from the at least one auxiliary exhaust passage. 
         [0024]    In some implementations of the present technology, the fourth distance is greater than the second and third distances. 
         [0025]    According to another aspect of the present technology, there is provided a method of operating an exhaust valve assembly of a two-stroke internal combustion engine. The valve assembly includes a valve actuator, a two-part valve having a first valve part operatively connected to the valve actuator and a second valve part, and at least one auxiliary valve operatively connected to the two-part valve. The method comprises: moving the valve actuator between a first position, a second position and a third position, the second position being intermediate the first and third positions; moving the first valve part to a fourth position when the valve actuator is moved to the first position; moving the first valve part to a fifth position when the valve actuator is moved to the second position; moving the first valve part to a sixth position when the valve actuator is moved to the third position, the fifth position being intermediate the fourth position and the sixth position; moving the second valve part to a seventh position when the valve actuator is moved to any one of the first and second position; moving the second valve part to an eighth position when the valve actuator is moved to the third position; moving the at least one auxiliary valve to a ninth position when the second valve part is moved to the seventh position; and moving the at least one auxiliary valve to a tenth position when the second valve part is moved to the eighth position. 
         [0026]    For purposes of the present application, the term “ambient pressure” means a pressure of the surrounding fluid, the term “positive pressure” means a pressure which is greater than the ambient pressure, and the term “negative pressure” means a pressure which is less than the ambient pressure. 
         [0027]    Implementations of the present technology each have at least one of the above-mentioned object and/or aspects, but do not necessarily have all of them. It should be understood that some aspects of the present technology that have resulted from attempting to attain the above-mentioned object may not satisfy this object and/or may satisfy other objects not specifically recited herein. 
         [0028]    Additional and/or alternative features, aspects and advantages of implementations of the present technology will become apparent from the following description, the accompanying drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    For a better understanding of the present technology, as well as other aspects and further features thereof, reference is made to the following description which is to be used in conjunction with the accompanying drawings, where: 
           [0030]      FIG. 1  is a partial cross-sectional view of a two-stroke engine having an exhaust valve assembly; 
           [0031]      FIG. 2  is a partial cross-sectional view of a cylinder block, a cylinder head and the exhaust valve assembly of the engine of  FIG. 1 ; 
           [0032]      FIG. 3  is a perspective view of an engine having exhaust valve assemblies like the exhaust valve assembly of the engine of  FIG. 1  and a pressure control device; 
           [0033]      FIG. 4A  is a front elevation view of the exhaust valve assembly of the engine of  FIG. 1  with a first valve part, a second valve part and auxiliary valves in flow restricting positions; 
           [0034]      FIG. 4B  is a right side elevation view of the exhaust valve assembly in the position shown in  FIG. 4A ; 
           [0035]      FIG. 4C  is a partial cross-sectional view of the exhaust valve assembly in the position shown in  FIG. 4A  shown on the engine of  FIG. 1 ; 
           [0036]      FIG. 4D  is a cross-sectional view of an actuator of the exhaust valve assembly of  FIG. 4A  shown in a position corresponding to the position of the exhaust valve assembly shown in  FIG. 4A ; 
           [0037]      FIG. 5A  is a front elevation view the exhaust valve assembly of  FIG. 4A  with the first valve part in an intermediate position and the second valve part and the auxiliary valves in flow restricting positions; 
           [0038]      FIG. 5B  is a right side elevation view of the exhaust valve assembly in the position shown in  FIG. 5A ; 
           [0039]      FIG. 5C  is a partial cross-sectional view of the exhaust valve assembly in the position shown in  FIG. 5A  shown on the engine of  FIG. 1 ; 
           [0040]      FIG. 5D  is a cross-sectional view of the actuator of the exhaust valve assembly shown in a position corresponding to the position of the exhaust valve assembly shown in  FIG. 5A ; 
           [0041]      FIG. 6A  is a front elevation view the exhaust valve assembly of  FIG. 4A  with the first valve part, the second valve part and the auxiliary valves in full flow positions; 
           [0042]      FIG. 6B  is a right side elevation view of the exhaust valve assembly in the position shown in  FIG. 6A ; 
           [0043]      FIG. 6C  is a partial cross-sectional view of the exhaust valve assembly in the position shown in  FIG. 6A  shown on the engine of  FIG. 1 ; and 
           [0044]      FIG. 6D  is a cross-sectional view of the actuator of the exhaust valve assembly shown in a position corresponding to the position of the exhaust valve assembly shown in  FIG. 6A . 
       
    
    
     DETAILED DESCRIPTION 
       [0045]    An exhaust valve assembly  10  has a valve actuator  12 , a two-part valve  14  connected to the actuator  12  and auxiliary valves  16  ( FIG. 4A ) connected to the valve  14 . As shown in  FIG. 1 , the exhaust valve assembly  10  forms part of a two-stroke engine  18 . It should be understood that the term “two-stroke engine” includes an engine having at least one cylinder. The engine  18  comprises a crankcase  20  and a cylinder block  22  connected to the crankcase  20 . A cylinder  24 , defined in the cylinder block  22 , has a main exhaust port  26 . An exhaust passage  28  communicates with the main exhaust port  26 . Auxiliary exhaust ports  30  are disposed on each side of the main exhaust port  26  so as to be symmetrical to the main exhaust port  26 . The auxiliary exhaust ports  30  are connected to the exhaust passage  28  by way of auxiliary passages  32 . An admission port  34  is defined in the cylinder block  22 . The crankcase  20  has an internal chamber  36  communicating with the admission port  34 . A crankshaft  38  is disposed in the internal chamber  36  of the crankcase  20 . A piston  40  is connected to the crankshaft  38  via a connecting rod  42  and reciprocates in the cylinder  24  during operation of the engine  18 . The piston  40  is adapted to open or close the main exhaust port  26 , the auxiliary exhaust ports  30  and transfer ports  44 . 
         [0046]    When the engine  18  is operating at low or medium speeds, the main exhaust port  26  and the auxiliary exhaust ports  30  should not be exposed prematurely by the piston  40 , as the latter moves downwardly. Such a premature exposure of the main exhaust port  26  and the auxiliary exhaust ports  30  is prevented by the valve  14 . The valve  14  is slidably mounted in a guide channel  46  having a longitudinal direction that is approximately radial with respect to the cylinder  24  and extends at an acute angle to the axis of the main exhaust passage  26 . Auxiliary guide channels (not shown) are provided parallel to the guide channel  46  in the area of the auxiliary passages  32  to receive the auxiliary valves  16 . 
         [0047]    Turning to  FIGS. 4A to 4C , the two-part valve  14  will now be described. The two-part valve  14  includes a first valve part  50  and a second valve part  52 . The first valve part  50  and the second valve part  52  each have an edge  54 ,  56  ( FIG. 5A ) respectively, shaped so as to match the shape of the cylinder  12 . In operation, as in  FIG. 4C , the second valve part  52  is supported and is disposed above the first valve part  50 . The first valve part  50  has an integrally formed connector  58  which connects the first valve part  50  to the actuator  12 . The second valve part  52  has two pairs of fingers  60 , the lower ones of which are engaged by the first valve part  50  as the first valve part  50  is moved to a full flow position, as described in greater detail below. A pair of springs  62  is provided between the second valve part  52  and a bottom of the valve actuator  12 . 
         [0048]    As mentioned above, the exhaust valve assembly  10  also has auxiliary valves  16  for restricting the flow of exhaust gases in the auxiliary exhaust passages  32 . The auxiliary valves  16  are separate from and movably connected to the two-part valve  14  via a lever  64 . The lever  64  has two arms  66  that are connected to each other at one end by a shaft  68 . The arms  66  have a generally obtuse V-shape. Shafts  70  extend inwardly from the corners of the V-shaped arms  66 . Each shaft  70  has a bushing  71  disposed around it. The bushing  71  has a generally rectangular outer perimeter. Each bushing  71  is received between a corresponding pair of fingers  60  such that the shafts  70  can pivot inside their respective bushings  71  in the space between the two fingers  60  of their corresponding pairs of fingers  60 . Shafts  72  extend outwardly from the ends of the arms  66  opposite the ends of the arms  66  where the shaft  68  is provided. Each shaft  72  has a bushing  73  disposed around it. The bushing  73  has a generally rectangular outer perimeter. The bushings  73  are received in oblong apertures  74  defined in the upper ends of the auxiliary valves  16 . In the present implementation, a distance from the central axis  76  of the shaft  68  to the central axes  78  of the shafts  70  is greater than a distance from the central axes  78  of the shafts  70  to the central axes  80  of the shafts  72 . Also, in the present implementation the angle between a line passing through the central axis  76  of the shaft  68  and the central axes  78  of the shafts  70  and a line passing through the central axes  78  of the shafts  70  and the central axes  80  of the shafts  72  is between  140  degrees and  150  degrees, but other angles are contemplated. 
         [0049]    Posts  82  extend from a bottom of the actuator  12 . The posts  82  each have an arcuate lower surface against which the shaft  68  abuts. As a result, when the second valve part  52  moves from its flow restricting position to its full flow position, the shafts  70  move with the second valve part  52 . This movement of the shafts  70  causes the lever  64  to pivot about the central axis  76  of the shaft  68 , thereby moving the auxiliary valves  16  from their flow restricting positions to their full flow positions as will be described in greater detail below. 
         [0050]    The valve actuator  12  will now be described with respect to  FIG. 4D . The valve actuator  12  has a valve housing  90 . A first end portion of a diaphragm  92  is connected via a first annular spring  94  onto the valve housing  90 . A pressure chamber wall  96  is connected to a second end portion of the diaphragm  92  via a second annular spring  98 . The diaphragm  92  and the pressure chamber wall  96  define therebetween a pressure chamber  100  which has a variable volume. The connector  58  of the first valve part  50  passes through the valve housing  90  and is connected to the pressure chamber wall  96  so as to move therewith. A ring  102  and a sealing ring  104  are disposed between the valve housing  90  and the connector  58  to guide the connector  58  as it moves with the pressure chamber wall  96  and to prevent exhaust gases from entering the pressure chamber  100  from the guide channel  46 . A spring  106  is disposed inside the pressure chamber  100  around the connector  58  between the pressure chamber wall  96  and the ring  102 . The spring  106  biases the first valve part  50  toward an intermediate position, shown in  FIGS. 5A to 5C , described in greater detail below. It is contemplated that the spring  106  could be located elsewhere and still bias the first valve part  50  toward the intermediate position, such as between the first valve part  50  and a bottom of the valve housing  90 . It is also contemplated that the spring  106  could be omitted completely and that the diaphragm  92  could be self-biasing so as to bias the first valve part  50  toward the intermediate position, by properly shaping the diaphragm  92  and by making it out of an appropriate material. A cover  108  ( FIG. 4A ) is connected to the valve housing  90  by bolts  110  to enclose the pressure chamber wall  96  and diaphragm  92  assembly therebetween. 
         [0051]    The valve housing  90  is connected to the cylinder block  22  via bolts (not shown). A seal (not shown) is disposed between the valve housing  90  and the cylinder block  22  to prevent exhaust gases from leaving the exhaust passage  28  via the guide channel  46 . A port  112  ( FIG. 4C ) of the pressure chamber  100  disposed on the valve housing  90  fluidly communicates the pressure chamber  100  with at least one pressure source, via a pressure control device, as described below. Another port  114  ( FIG. 4C ) is used to ventilate the room surrounding the pressure chamber  100 . 
         [0052]      FIG. 3  illustrates one possible implementation of a pressure control device for fluidly communicating the pressure chambers  100  of two valve actuators  12  on an engine  18 ′ with a plurality of pressure sources. During operation of the engine  18 ′, positive and negative pressures are created inside the crankcase  20 . A first line  116  fluidly communicates the crankcase  20  with a first solenoid valve  118 . A first one-way valve (not shown) is disposed in the first line  116  to only permit a positive pressure to be supplied to the first solenoid valve  118 . A second line  120  fluidly communicates the crankcase  20  with a second solenoid valve  122 . A second one-way valve (not shown) is disposed in the second line  120  to only permit a negative pressure to be supplied to the second solenoid valve  122 . Lines  124  fluidly communicate the first and second solenoid valves  118 ,  122  with the ports  112  of the pressure chambers  100  of the valve actuators  12 . By selectively opening and closing the solenoid valves  118 ,  122 , it is thus possible to supply one of a positive pressure and a negative pressure to the pressure chambers  100  of the valve actuators  12 . A third valve (not shown) selectively fluidly communicates the pressure chambers  100  with the air surrounding the engine  18 ′ so as to supply an ambient pressure to the pressure chambers  100 . It is contemplated that a pressure equivalent to an ambient pressure could be supplied to the pressure chambers  100  by supplying controlled amounts of both the positive and the negative pressures by using solenoid valves  118  and  122 . An electronic control unit (not shown) determines which pressure is to be supplied to the pressure chambers  100  based on the engine speed, as described below, and opens and closes the valves  118 ,  122  accordingly so as to obtain a desired position of the valves  14 ,  16  of the valve assembly  10 . It is contemplated that the electronic control unit could also determine which pressure is to be supplied to the pressure chambers  100  based on other operating conditions of the engine  18 ′ or of components associated with the engine  18 ′, such as, for example, a degree of throttle opening or a rate of acceleration of the engine  18 ′ or a combination of two or more of the engine speed, the degree of throttle opening, the rate of acceleration. Other pressure control devices are contemplated, such as those disclosed in U.S. Pat. No. 6,244,227 B1, issued Jun. 12, 2001 to Matte, entitled “Valve Assembly Using Pressurized Medium for Controlling Operating Conditions of a Two-Stroke Engine”. 
         [0053]    It is contemplated that the pneumatic actuator  12  described above could be replaced with another type of actuator having three positions. For example, the actuator  12  could be a three position solenoid. 
         [0054]    The operation of the valve assembly  10  will now be described in association with  FIGS. 4A to 6D . Line  126  in  FIGS. 4A ,  5 A and  6 A is a line passing through the points where the ends of the edge  54  of the first valve part  50  meet the cylinder  24  when the first valve part  50  is in its flow restricting position shown in  FIG. 4A . Line  126  has been added simply to illustrate the movement of the valves  14 ,  16  relative to the cylinder  24 . 
         [0055]    As previously mentioned, at low engine speeds, it is desirable to restrict the flow of the exhaust gases through the exhaust port  26  and auxiliary exhaust ports  30 . Thus, at low engine speeds, the valve  14  is moved to the position shown in  FIGS. 4A to 4C . To move the valve  14  to this position, a negative pressure is supplied to the pressure chamber  100 . Since the negative pressure is sufficient to overcome the bias of the spring  106 , the volume of the pressure chamber  100  is decreased as shown in  FIG. 4D . This causes the spring  106  to be in compression and the pressure chamber wall  96  moves the first valve part  50  in a flow restricting position in the exhaust port  26 . The second valve part  52  is held in a flow restricting position by the bias of springs  62 . The auxiliary valves  16  which move with the second valve part  52  via the lever  64  are also held in their flow restricting positions in the auxiliary exhaust passages  30 . 
         [0056]    At medium engine speeds, it is desirable to restrict the flow of the exhaust gases through the exhaust port  26 , but to a lesser degree than at low engine speeds. It has also been found that it is desirable to restrict the flow of exhaust gases through the auxiliary exhaust ports  30  to the same degree as at low engine speeds. Thus, at medium engine speeds, the valve  14  is moved to the position shown in  FIGS. 5A to 5C . To move the valve  14  to this position, an ambient pressure is supplied to the pressure chamber  100 . Since the ambient pressure is insufficient to overcome the bias of the spring  106 , the volume of the pressure chamber  100  is changed to reach a volume where the spring  106  is slightly compressed as shown in  FIG. 5D . The spring  106  is slightly compressed due to the bias applied by springs  62  on the first valve part  50  via the lower fingers  60  of the second valve part  52 . The pressure chamber wall  96  moves the first valve part  50  in an intermediate position in the exhaust port  26 . The intermediate position is intermediate the full flow position ( FIG. 6A to 6C ) and the flow restricting position ( FIG. 4A to 4C ) of the first valve part  50 . The second valve part  52  is held in its flow restricting position by the bias of springs  62 . As such, the auxiliary valves  16  are also held in their flow restricting positions in the auxiliary exhaust passages  30 . 
         [0057]    At high engine speeds, it is desirable that the exhaust gases flow freely through the exhaust port  26  and auxiliary exhaust ports  30 . Thus, at high engine speeds, the valve  14  is moved to the position shown in  FIGS. 6A to 6C . To move the valve  14  to this position, a positive pressure is supplied to the pressure chamber  100 . Since the positive pressure is sufficient to overcome the bias of the spring  106 , the volume of the pressure chamber  100  is increased as shown in  FIG. 6D . This causes the spring  106  to be in extension and the pressure chamber wall  96  moves the first valve part  50  in a full flow position where it is withdrawn from the exhaust port  26 . As it is moved to its full flow position, the first valve part  50  pushes against the lower fingers  60  of the second valve part  52  and moves the second valve part  52  to a full flow position where it is withdrawn from the exhaust port  26 . When the second valve part  52  is in its full flow position the springs  62  are in compression. As the second valve part  52  moves to its full flow position, the fingers  60  lift the shafts  70  of the lever  64  with the second valve part  52 . As a result, since the shaft  68  abuts the posts  82 , the lever  64  pivots about the central axis  76  of the shaft  68  thereby causing the auxiliary valves  16  to move to full flow positions where they are withdrawn from the auxiliary exhaust ports  30 . 
         [0058]    The ranges of engine speeds corresponding to low, medium and high engine speeds providing the above described valve positions depend on the specific configuration of the engine and its associated components and on the desired performance characteristics of the engine. 
         [0059]    Due to the geometry of the arms  66  of the lever  64 , the distance travelled by the auxiliary exhaust valves  16  between their flow restricting positions shown in  FIGS. 4A to 4C  and  5 A to  5 C and their full flow positions shown in  FIGS. 6A to 6C  is greater than the distance travelled by the first valve part  50  between its flow restricting position shown in  FIGS. 4A to 4C  and its full flow position shown in  FIGS. 6A to 6C . Therefore, for implementations having equivalent displacement of the first valve part  50 , since the auxiliary exhaust valves  16  in the implementation described above can be displaced by a greater amount than if they were connected to the first valve part  50  to move therewith, as in U.S. Pat. No. 7,484,482 for example, the exhaust ports  30  and exhaust passages  32  can have a greater height for the same dimension of auxiliary valves  16  than in implementations where the auxiliary valves  16  are connected to the first valve part  50  to move therewith. Therefore, in addition to improving performance at medium engine speeds by maintaining the auxiliary exhaust valves  16  in their flow restricting positions at medium engine speeds, the exhaust valve assembly  10  can also improve performance at high engine speeds by allowing auxiliary exhaust ports  30  and passages  32  of bigger size to be provided in the engine  18 . 
         [0060]    Modifications and improvements to the above-described implementations of the present technology 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 technology is therefore intended to be limited solely by the scope of the appended claims.