Exhaust valve assembly

Abstract

See as attached

Description

Technical FieldThe present disclosure generally relates to an exhaust valve assembly for a two-strokeinternal combustion engine.BackgroundTwo-stroke internal combustion engines are widely utilized in various applications dueto their high power-to-weight ratio, mechanical simplicity, and low manufacturing cost.However, these two-stroke internal combustion engines inherently suffer from asignificant operational challenge that limits their efficiency and higher pollutantemissions. A known issue with the two-stroke internal combustion engines is the lossof fresh air-fuel mixture during a scavenging process. In conventional designs, wheremotion of a piston passively controls ports, a simultaneous opening of a transfer and anexhaust port allows a portion of the unburnt air-fuel mixture to escape. This "short-circuiting" of the fresh air-fuel mixture may result in reduced fuel efficiency, increasedhydrocarbon emissions, and lower power output.Numerous active control mechanisms, such as complex camshaft-driven valve systems,have been proposed to optimize exhaust port timing. But these active controlmechanisms often add significant mechanical complexity, cost, and potential reliabilityissues, thereby offsetting the inherent simplicity of a two-stroke design. Thus, there isa need for a simple, robust, and cost-effective active exhaust valve system that caneffectively prevent scavenging losses without introducing the complexity of prior artsolutions.SummaryThe object of the present invention is therefore to take account of the above-mentioneddisadvantages and to suggest an improved exhaust valve assembly for a two-strokeinternal combustion engine. This object is achieved by a sliding valve of the exhaustvalve assembly having features of the independent claim. Advantageous embodimentsand developments are stated in the dependent claims.According to a first aspect, an exhaust valve assembly for a two-stroke internalcombustion engine is disclosed. The exhaust valve assembly has a crankshaft and anexhaust port. The exhaust valve assembly includes a sliding valve having a plate witha slot matched to a geometry of the exhaust port. The sliding valve is configured toreciprocate linearly between an open position, in which the slot is aligned with theexhaust port, and a closed position, in which the plate blocks the exhaust port. Theexhaust valve assembly further includes a first gear configured to be mechanicallydriven by the crankshaft. The exhaust valve assembly further includes a parallel,secondary crankshaft disposed parallel to the crankshaft. The exhaust valve assemblyfurther includes a second gear mounted on the secondary crankshaft and meshing withthe first gear. The exhaust valve assembly further includes a lever arm mechanicallylinked between the secondary crankshaft and the sliding valve, such that the rotation ofthe secondary crankshaft causes the lever arm to convert its rotational motion into thelinear reciprocating motion of the sliding valve.The exhaust valve assembly of the present disclosure includes the sliding valve. Thesliding valve has the plate with the slot matched to the geometry of the exhaust port.The sliding valve is configured to reciprocate linearly between the open position andthe closed position. The exhaust valve assembly may allow the exhaust port to besealed at the precise moment required to trap the maximum amount of fresh fuel-airmixture within a cylinder. The motion of the sliding valve can be effectuated througha transmission mechanism. In this embodiment, the transmission mechanism isprovided as the first gear configured to be mechanically driven by the crankshaft. Thismechanical arrangement provides a robust and reliable means to synchronize the valvemovement with the crankshaft of the two-stroke internal combustion engine, enablingdynamic control of the exhaust port timing to improve an engine performance.According to an embodiment, the first gear is configured to be mounted coaxially onthe crankshaft. This configuration may provide a direct and compact power take-offfrom the primary rotational source of the two-stroke internal combustion engine,simplifying the overall assembly and ensuring precise transmission and control of thetiming of the two-stroke internal combustion engine exhaust valve assembly.According to an embodiment, the first gear and the second gear establish apredetermined, fixed gear ratio to synchronize the reciprocating motion of the slidingvalve with the rotation of the crankshaft. This may ensure a consistent and non-variablesynchronization between the exhaust valve assembly and the crankshaft, which iscrucial for reliability of the operation of the two-stroke internal combustion engine andpredictable performance characteristics.According to an embodiment, the predetermined, fixed gear ratio is a one-to-one ratio.The one-to-one gear ratio may simplify the mechanical design and may ensure that thesecondary crankshaft completes one full rotation for every rotation of the crankshaft,allowing for straightforward timing of the exhaust valve assembly's single open-closecycle per revolution of the two-stroke internal combustion engine.According to an embodiment, the secondary crankshaft is driven by an electric motorto move the exhaust valve assembly in synchronous to the crankshaft. Using theelectric motor may allow for more flexible and programmable control over the timingof the exhaust valve assembly, enabling dynamic adjustments based on load and speedthat are independent of a fixed mechanical linkage.According to an embodiment, the sliding valve is actuated by a cam or an eccentricmechanism. The cam or the eccentric mechanism is mounted on the crankshaft and isconfigured to directly actuate the sliding valve. This direct-drive arrangement maysimplify the exhaust valve assembly by eliminating intermediate gears and shafts. Theinclusion of the cam or the eccentric mechanism may result in a more compact, cost-effective, and reliable system with fewer moving parts and lower frictional losses.According to an embodiment, a distal end of the lever arm is pivotally connected toboth the secondary crankshaft and the sliding valve. This pivotal connection mayprovide a simple and effective mechanical interface for converting the rotationalmotion of the secondary shaft into the linear reciprocating motion required by thesliding valve.According to an embodiment, the exhaust valve assembly is configured to move thesliding valve to the closed position during an upward stroke of a piston within the two-stroke internal combustion engine. By closing the exhaust port during the upwardstroke of the piston, more of the fresh fuel-air mixture is trapped within the cylinder,which may increase the effective compression ratio and improve torque and efficiencyof the two-stroke internal combustion engine.According to an embodiment, the sliding valve reaches the closed position during thetime when the piston is at BDC to when the piston closes the transfer ports of the two-stroke internal combustion engine during its upward stroke. This timing may ensurethat no fresh charge is lost through the exhaust port, maximizing the amount of trappedcharge in the cylinder for combustion and thereby enhancing power and fuel economyof the two-stroke internal combustion engine.According to an embodiment, the sliding valve is disposed within a guide channelformed into the exhaust port, the guide channel constraining the movement of thesliding valve to the linear reciprocating motion. The use of the guide channel mayensure precise, repeatable, and stable linear movement of the sliding valve. The guidechannel may also prevent unwanted lateral motion and ensure a proper seal when thesliding valve is in the closed position.According to an embodiment, the exhaust valve assembly is integrated with an exhaustduct of the two-stroke internal combustion engine, and the sliding valve is configuredto traverse in the guide channel in a wall of the exhaust duct. Integrating the assemblywith the exhaust duct may provide compact and robust construction, simplifyingarchitecture of the two-stroke internal combustion engine and may ensure the slidingvalve operates in close proximity to the exhaust port it controls.According to a second aspect, an exhaust valve assembly for a two-stroke internalcombustion engine having a crankshaft and an exhaust port is provided. The exhaustvalve assembly includes a sliding valve having a plate with a slot matched to ageometry of the exhaust port. The sliding valve is configured to reciprocate linearlybetween an open position and a closed position. The exhaust valve assembly furtherincludes a secondary crankshaft disposed parallel to the crankshaft. The exhaust valveassembly further includes a lever arm fixed to the secondary crankshaft andmechanically linked to the sliding valve, such that rotation of the secondary crankshaftis converted into the linear reciprocating motion of the sliding valve. The exhaust valveassembly further includes an electric motor configured to drive the secondarycrankshaft. The exhaust valve assembly further includes a control unit configured tooperate the electric motor in synchronization with the rotation of the crankshaft. Themotion of the sliding valve may be effectuated through a transmission mechanism. Inthis embodiment, the transmission mechanism is provided as the electric motorconfigured to drive the parallel shaft. The control unit may execute sophisticatedtiming maps, allowing the motion of the sliding valve to be precisely tailored to anyoperating condition. This electro-mechanical configuration may offer highly flexibleand programmable control of the sliding valve timing, allowing the performancecharacteristics of the two-stroke internal combustion engine to be dynamicallyoptimized in real-time to suit varying operating conditions.According to a third aspect, a two-stroke internal combustion engine includes a cylinderdefining an exhaust port. The two-stroke internal combustion engine includes a pistondisposed to reciprocate within the cylinder. The two-stroke internal combustion engineincludes a crankshaft mechanically linked to the piston. The two-stroke internalcombustion engine further includes the exhaust valve assembly of the first aspect. Byincorporating the exhaust valve assembly, the two-stroke internal combustion enginemay overcome the fundamental efficiency and emissions drawbacks of conventionaltwo-stroke designs. The exhaust valve assembly may deliver the high power-to-weightratio expected from the two-stroke engine while also providing the fuel economy andlow emissions typically associated with more complex and heavier four-stroke engines.The details of one or more examples of the disclosure are set forth in the accompanyingdrawings and the description below. Other features, objects, and advantages of thedisclosure will be apparent from the description and drawings, and from the claims.Brief Description of DrawingsExemplary embodiments disclosed herein may be more completely understood inconsideration of the following detailed description in connection with the followingfigures. The figures are not necessarily drawn to scale. Like numbers used in thefigures refer to like components. However, it will be understood that the use of anumber to refer to a component in a given figure is not intended to limit the componentin another figure labeled with the same number.Fig. 1 is a schematic perspective view of an embodiment of an exhaust valve assembly,showing a sliding valve in an open position, according to an embodiment of the presentdisclosure;Fig. 2 is a schematic perspective view of an embodiment of the exhaust valve assemblycorresponding to Fig. 1, showing the sliding valve in a closed position, according toanother embodiment of the present disclosure;Fig. 3A is a schematic view of an embodiment of the exhaust valve assembly using acam, according to an embodiment of the present disclosure; andFig. 3B is a schematic view of an embodiment of the exhaust valve assembly using aneccentric mechanism, according to another embodiment of the present disclosure.Detailed DescriptionIn the following description, reference is made to the accompanying figures that forma part thereof and in which various embodiments are shown by way of illustration. Itis to be understood that other embodiments are contemplated and may be made withoutdeparting from the scope or spirit of the present disclosure. The following detaileddescription, therefore, is not to be taken in a limiting sense.FIG. 1 is a schematic perspective view of a two-stroke internal combustion engine 100,according to an embodiment of the present disclosure. The two-stroke internalcombustion engine 100 includes a cylinder 126 defining an exhaust port 105. Theexhaust port 105 may be an aperture or an opening in a wall of the cylinder 126 throughwhich burnt gases are expelled. The two-stroke internal combustion engine 100 furtherincludes a piston 128 disposed to reciprocate i.e., move up and down within the cylinder126. The two-stroke internal combustion engine 100 further includes a crankshaft 104mechanically linked to the piston 128, for instance, by a connecting rod, to convert thelinear reciprocating motion of the piston 128 into the rotational output of the engine100. The term "two-stroke internal combustion engine 100" is interchangeably referredhereinafter as "the engine 100". The engine 100 may be utilized in a motorcycle, apersonal watercraft, a drone, an unmanned arial vehicle, or a piece of handheld powerequipment like a chainsaw.The two-stroke internal combustion engine 100 further includes an exhaust valveassembly 102. The exhaust valve assembly 102 is integrated with an exhaust duct 106of the two-stroke internal combustion engine 100. The exhaust valve assembly 102 forthe two-stroke internal combustion engine 100 has the crankshaft 104 and the exhaustport 105.The exhaust valve assembly 102 further includes a first gear 114 configured to bemechanically driven by the crankshaft 104. The first gear 114 is configured to bemounted coaxially on the crankshaft 104. For example, the first gear 114 may be aspur gear mounted coaxially on the crankshaft 104, such that as the crankshaft 104rotates, it directly imparts its rotational motion to the first gear 114, causing them torotate together as a single unit.The exhaust valve assembly 102 further includes a parallel, secondary crankshaft 116disposed parallel to the crankshaft 104. For example, the secondary crankshaft 116may serve as an intermediary actuator shaft to facilitate the transfer of rotational motionbetween the two shafts, i.e., the crankshaft 104 and the secondary crankshaft 116, viaa gear train.The exhaust valve assembly 102 further includes a second gear 118 mounted on thesecondary crankshaft 116 and meshing with the first gear 114. For example, the secondgear 118 is fixedly mounted on the secondary crankshaft 116, and its teeth areconfigured to interlock with teeth of the first gear 114. This meshing engagementcreates the gear train that transfers the rotational motion from the first gear 114 to thesecond gear 118, thereby causing the secondary crankshaft 116 to rotate insynchronization with the main crankshaft 104.The exhaust valve assembly 102 includes a sliding valve 108 having a plate 110 with aslot 112 matched to a geometry of the exhaust port 105. For example, if the exhaustport 105 is rectangular, the slot 112 may also be rectangular and of a similar or identicalsize to allow for an unobstructed flow of exhaust gases when the exhaust port 105 andthe slot 112 are aligned. In some embodiments, the plate 110 may be a substantiallyflat metal component. The motion of the sliding valve 108 can be effectuated througha transmission mechanism. In this embodiment, the transmission mechanism isprovided as the first gear 114 configured to be mechanically driven by the crankshaft104.The first gear 114 and the second gear 118 establish a predetermined, fixed gear ratioto synchronize the reciprocating motion of the sliding valve 108 with the rotation ofthe crankshaft 104. The predetermined, fixed gear ratio is a one-to-one ratio. Forexample, the gear ratio may be the one-to-one ratio, where the first gear 114 and thesecond gear 118 have an identical number of teeth. This may ensure that for everysingle rotation of the crankshaft 104, the secondary crankshaft 116 also completesexactly one rotation, thereby guaranteeing that the position and movement of the slidingvalve 108 are always in precise mechanical synchronization with the angular positionof the crankshaft 104.The sliding valve 108 is disposed within a guide channel 124 formed into the exhaustport 105. The guide channel 124 constrains the movement of the sliding valve 108 tothe linear reciprocating motion. The sliding valve 108 is configured to traverse in theguide channel 124 in a wall of the exhaust duct 106. For example, the guide channel124 may include a set of grooves machined into the walls of the exhaust duct 106adjacent to the exhaust port 105. The edges of the sliding valve 108 engage with andslide within these grooves, which may prevent any lateral or rotational movement andensures the sliding valve 108 follows a precise linear path as it reciprocates.The sliding valve 108 is configured to reciprocate linearly between an open position,in which the slot 112 is aligned with the exhaust port 105 and a closed position, inwhich the plate 110 blocks the exhaust port 105. For example, in the open position,the slot 112 is aligned with the exhaust port 105, creating the unobstructed passage forthe exhaust gases to exit the cylinder 126. In the closed position, the solid portion ofthe plate 110 is moved to cover and block the exhaust port 105, thereby sealing thecylinder 126 from the exhaust system. In the illustrated embodiment of FIG. 1, thesliding valve 108 is in the open position.The exhaust valve assembly 102 further includes a lever arm 120 mechanically linkedbetween the secondary crankshaft 116 and the sliding valve 108, such that the rotationof the secondary crankshaft 116 causes the lever arm 120 to convert its rotationalmotion into the linear reciprocating motion of the sliding valve 108. A distal end 122of the lever arm 120 is pivotally connected to both the secondary crankshaft 116 andthe sliding valve 108. For example, a proximal end of the lever arm 120 may berotationally connected to the secondary crankshaft 116, while its distal end 122 ispivotally connected to the sliding valve 108. As the secondary crankshaft 116 rotates,the rotational end of the lever arm 120 travels in a circular path, which in turn drivesthe sliding valve 108 in a linear, up-and-down motion within its guide channel 124.FIG. 2 is a schematic perspective view of the two-stroke internal combustion engine100 of FIG. 1, according to another embodiment of the present disclosure. The exhaustvalve assembly 102 is further configured to move the sliding valve 108 to the closedposition during an upward stroke of the piston 128 within the two-stroke internalcombustion engine 100. The sliding valve 108 reaches the closed position during thetime when the piston (128) is at BDC to when the piston (128) closes the transfer ports(not shown) of the two-stroke internal combustion engine 100 during its upward stroke.For example, as the piston 128 travels upwards from bottom dead center, thesynchronized rotation of the secondary crankshaft 116 drives the lever arm 120 toprogressively raise the sliding valve 108 and seal the exhaust port 105. In a furtherembodiment, the sliding valve 108 reaches the closed position during the time whenthe piston (128) is at BDC to when the piston (128) closes the transfer ports of theengine 100 during its upward stroke. For example, the piston 128 may be at BDC at 0degrees after bottom dead center (ABDC) and closes the transfer port at 30 degreesABDC, and the sliding valve 108 is timed to become fully seated and block the exhaustport 105 at 25 degrees ABDC, ensuring that the fresh air-fuel mixture that has enteredthe cylinder 126 is fully trapped before significant compression begins.In some embodiments, the exhaust valve assembly 102 includes an electric motor (notshown) configured to drive the secondary crankshaft 116. In some embodiments, thesecondary crankshaft 116 is driven by the electric motor to move the exhaust valveassembly 102 in synchronous to the crankshaft 104. The exhaust valve assembly 102further includes a control unit (not shown) configured to operate the electric motor insynchronization with the rotation of the crankshaft 104. The motion of the sliding valve108 can be effectuated through the transmission mechanism. In this embodiment, thetransmission mechanism is provided as the electric motor configured to drive thesecondary crankshaft 116. For example, the electric motor may be a servo motor or abrushless direct current motor or a stepper motor that may receive command from thecontrol unit. The control unit receives real-time position data from a sensor (not shown)on the crankshaft 104 and operates the electric motor to rotate the secondary crankshaft116 in a precisely corresponding motion, thereby achieving a programmable andsynchronized actuation of the sliding valve 108.The control unit may include one or more processors. In some examples, the processormay include one or more devices, circuits, and / or processing cores configured toprocess data, such as computer program instructions. The processor may include, forexample, one or more of a general-purpose processor (e.g., ARM-based processor), adigital signal processor (DSP), a programmable logic device (PLD), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), etc.The control unit may include a memory. The memory may include any type of tangiblevolatile or non-volatile computer-readable memory device. The memory may alsoinclude computer storage media. The memory may store computer readableinstructions that, when executed by the processor of the control unit, cause theprocessor to perform computer-implemented steps or functions according to theinstructions.FIG. 3A is a schematic view of an embodiment of the exhaust valve assembly 102 usinga cam 130, according to an embodiment of the present disclosure. FIG. 3B is aschematic view of an embodiment of the exhaust valve assembly 102 using an eccentricmechanism 132, according to another embodiment of the present disclosure. Referringto FIGS. 3A and 3B, the sliding valve 108 is actuated by the cam 130 or the eccentricmechanism 132. The cam 130 or the eccentric mechanism 132 is mounted on thecrankshaft 104 and is configured to directly actuate the sliding valve 108. For example,the plate 110 of the sliding valve 108 may be directly actuated by the crankshaft 104.This may be achieved by mounting either the cam 130 or the eccentric mechanism 132directly onto the crankshaft 104. As the crankshaft 104 rotates, a profile of the cam130 or an offset of the eccentric mechanism 132 may impart a precise, linearreciprocating motion to the sliding valve 108. This direct-drive arrangement may serveas a mechanically simpler alternative to the second gear 118 (as shown in FIGS. 1 and2) and the lever arm 120 (as shown in FIGS. 1 and 2). The inclusion of the cam 130 orthe eccentric mechanism 132 may reduce the overall number of components, whichmay lower the manufacturing costs, decrease potential points of failure, and reducefrictional losses, thereby enhancing the reliability and mechanical efficiency of theexhaust valve assembly 102.In the present detailed description of the preferred embodiments, reference is made tothe accompanying drawings, which illustrate specific embodiments in which theinvention may be practiced. The illustrated embodiments are not intended to beexhaustive of all embodiments according to the invention. It is to be understood thatother embodiments may be utilized, and structural or logical changes may be madewithout departing from the scope of the present invention. The following detaileddescription, therefore, is not to be taken in a limiting sense, and the scope of the presentinvention is defined by the appended claims.Unless otherwise indicated, all numbers expressing feature sizes, amounts, andphysical properties used in the specification and claims are to be understood as beingmodified by the term "about". Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the foregoing specification and attached claims areapproximations that can vary depending upon the desired properties sought to beobtained by those skilled in the art utilizing the teachings disclosed herein.Although specific embodiments have been illustrated and described herein, it will beappreciated by those of ordinary skill in the art that a variety of alternate and / orequivalent implementations can be substituted for the specific embodiments shown anddescribed without departing from the scope of the present disclosure. This applicationis intended to cover any adaptations or variations of the specific embodimentsdiscussed herein. Therefore, it is intended that this disclosure be limited only by theclaims and the equivalents thereof.List of Elements100 Two-Stroke Internal Combustion Engine102 Exhaust Valve Assembly104 Crankshaft105 Exhaust Port106 Exhaust Duct108 Sliding Valve110 Plate112 Slot114 First Gear116 Secondary Crankshaft118 Second Gear120 Lever Arm122 Distal End124 Guide Channel126 Cylinder128 Piston130 Cam132 Eccentric Mechanism.

Claims

1. An exhaust valve assembly (102) for a two-stroke internal combustion engine (100) having a crankshaft (104) and an exhaust port (105), the exhaust valve assembly (102) comprising: a sliding valve (108) having a plate (110) with a slot (112) matched to a geometry of the exhaust port (105), wherein the sliding valve (108) is configured to reciprocate linearly between: an open position, in which the slot (112) is aligned with the exhaust port (105), and a closed position, in which the plate (110) blocks the exhaust port (105); a first gear (114) configured to be mechanically driven by the crankshaft (104); a parallel, secondary crankshaft (116) disposed parallel to the crankshaft (104); a second gear (118) mounted on the secondary crankshaft (116) and meshing with the first gear (114); and a lever arm (120) mechanically linked between the secondary crankshaft (116) and the sliding valve (108), such that the rotation of the secondary crankshaft (116) causes the lever arm (120) to convert its rotational motion into the linear reciprocating motion of the sliding valve (108).

2. The exhaust valve assembly (102) of claim 1, wherein the first gear (114) is configured to be mounted coaxially on the crankshaft (104).

3. The exhaust valve assembly (102) of claim 1, wherein the first gear (114) and the second gear (118) establish a predetermined, fixed gear ratio to synchronize the reciprocating motion of the sliding valve (108) with the rotation of the crankshaft (104).

4. The exhaust valve assembly (102) of claim 3, wherein the predetermined, fixed gear ratio is a one-to-one ratio.

5. The exhaust valve assembly (102) of claim 1, wherein the secondary crankshaft (116) is driven by an electric motor to move the exhaust valve assembly (102) in synchronous to the crankshaft (104).

6. The exhaust valve assembly of claim 1, wherein the sliding valve (108) is actuated by a cam (130) or an eccentric mechanism (132), the cam (130) or the eccentric mechanism (132) being mounted on the crankshaft (104) and configured to directly actuate the sliding valve (108).

7. The exhaust valve assembly (102) of claim 1, wherein a distal end (122) of the lever arm (120) is pivotally connected to both the secondary crankshaft (116) and the sliding valve (108).

8. The exhaust valve assembly (102) of claim 1, wherein the exhaust valve assembly (102) is configured to move the sliding valve (108) to the closed position during an upward stroke of a piston (128) within the two-stroke internal combustion engine (100).

9. The exhaust valve assembly (102) of claim 7, wherein the sliding valve (108) reaches the closed position during the time when the piston (128) is at BDC to when the piston (128) closes the transfer ports of the two-stroke internal combustion engine (100) during its upward stroke.

10. The exhaust valve assembly (102) of claim 1, wherein the sliding valve (108) is disposed within a guide channel (124) formed into the exhaust port (105), the guide channel (124) constraining the movement of the sliding valve (108) to the linear reciprocating motion.

11. The exhaust valve assembly (102) according to claim 1, wherein the exhaust valve assembly (102) is integrated with an exhaust duct (106) of the two-stroke internal combustion engine (100), and the sliding valve (108) is configured to traverse in the guide channel (124) in a wall of the exhaust duct (106).

12. An exhaust valve assembly (102) for a two-stroke internal combustion engine (100) having a crankshaft (104) and an exhaust port (105), the exhaust valve assembly (102) comprising: a sliding valve (108) having a plate (110) with a slot (112) matched to a geometry of the exhaust port (105), wherein the sliding valve (108) is configured to reciprocate linearly between an open position and a closed position; a secondary crankshaft (116) disposed parallel to the crankshaft (104); a lever arm (120) fixed to the secondary crankshaft (116) and mechanically linked to the sliding valve (108), such that rotation of the secondary crankshaft (116) is converted into the linear reciprocating motion of the sliding valve (108); an electric motor configured to drive the secondary crankshaft (116); and a control unit configured to operate the electric motor in synchronization with the rotation of the crankshaft (104).

13. A two-stroke internal combustion engine (100), comprising: a cylinder (126) defining an exhaust port (105); a piston (128) disposed to reciprocate within the cylinder (126); a crankshaft (104) mechanically linked to the piston (128); and the exhaust valve assembly (102) according to any one of claims 1 to 11.