in the illustrated embodiment of the present invention , the exhaust control valve is shown in conjunction with a single cylinder fuel injected two - stroke cycle , variable speed , crankcase compression engine as used , for example , on motorcycles . however , the exhaust control valve of the present invention has many useful applications in other than the engine shown . the engine 10 includes a cylinder 12 , a cylinder head 14 and a crankcase housing 16 , with a piston , connecting rod and crankshaft ( not shown ). the cylinder 12 includes a combustion chamber 18 , an inlet or intake passage 20 terminating in the inlet port 22 , transfer passages 24 , 24 terminating in transfer ports 26 , 26 and an exhaust port 28 leading to the exhaust passage 30 . in the inlet passage 20 there are reed valves 32 , 32 , a supplement transfer passage 34 and a booster port 36 , similar to those described in my prior u . s . pat . no . 3 , 905 , 341 . as is known , in this type of engine , the piston skirt , not shown , serves as the valve means for opening and closing the inlet port 22 , booster port 36 , transfer ports 26 and exhaust port 28 . in the conventional engine of this type , as the piston is moving toward its bottom dead center position , the upper edge of the piston starts to open the exhaust port 28 at about the 90 ° position of the crankshaft , 0 ° being the crankshaft position at top dead center and 180 ° being the crankshaft position at bottom dead center . upon continued further downward movement of the piston , the transfer ports 26 and booster port 36 start to open at approximately the 120 ° position of the crankshaft . when the piston reaches bottom dead center , all of these above ports are fully uncovered or open . conversely , on the compression stroke of the piston , the transfer ports 26 and booster ports 36 are closed by the piston at about the 240 ° position of the crankshaft and the exhaust port 28 is closed by the piston at the 270 ° position of the crankshaft . thus , in the conventional engine of this type , the effective expansion stroke of the piston and the effective compression stroke of the piston each occur during only a 90 ° rotation of the crankshaft . in accordance with the present invention , valve means 38 are provided to delay fluid flow from the combustion chamber 18 through the exhaust passage 30 during the expansion stroke until after the piston has initially uncovered the upper edge of the exhaust port 28 , but permit such fluid communication prior to , or simultaneously with , the transfer ports 26 being opened . similarly , this same valve means 38 can shut off fluid communication through the exhaust passage 30 prior to the piston closing of the exhaust port 28 . this increases the effective length of the expansion and compression strokes , thereby increasing the power output of the engine as is shown in fig1 a , and 3 , the valve means 38 of the present invention comprises a semi - cylindrical valve element or member 40 , having its substantially flat side a facing upstream , toward the exhaust port 28 . without departing from the present invention , other valve element 40 shapes may be employed , including rectangular , cylindrical , or planar . the valve member 40 reciprocates within , and is channeled by , a corresponding track 42 within the portion of the engine 10 which defines the exhaust passage 30 . as is shown in fig1 and 2a , the lower face of the valve element 40 should be angled on its lower face b so that it withdraws completely from the exhaust passage 30 when the valve is fully retracted to allow unobstructed flow through the exhaust passage 30 . in the embodiment of the present invention shown in fig1 and 2a , a vane 44 is provided across the exhaust passage 30 which divides the exhaust port 28 and the exhaust passage 30 into an upper portion 46 and a lower portion 48 . the vane 44 is positioned so that its lower edge is approximately level with the top of the transfer ports 26 and the booster port 36 . in this embodiment , the valve element 40 in its fully extended position actuates to meet the vane 44 and essentially seal all flow through the upper portion 46 of the exhaust passage 30 . the slide valve member 40 may be driven in any manner which will translate the rotary motion of the crankshaft of the engine 10 into the reciprocating action of the valve 40 . one such mechanism is a linkage system similar to that disclosed in my u . s . patent application ser . no . 096 , 551 , now allowed . however , the preferred form of valve drive for the present invention is described below . as is shown in fig1 a , and 5 , attached to , or integral with , the valve member 40 is a rod 50 which has a rotatable cam follower 52 attached to its distal end . the cam follower 52 is positioned between a first cam element 54 and a second cam element 56 . the two cams 54 and 56 , which are mirror images of each other , should be linked to rotate in a one - to - one relation with each other . this is accomplished in the preferred embodiment by attaching cam 54 to drive gear 58 and cam 56 to driven gear 60 , the two gears 58 and 60 being of identical size and oriented in driven relation to each other . the purpose of employing a second cam element 56 is to assure that the cam follower 52 remains in constant contact with the first cam element 54 throughout the cycle of the valve element 40 . although not preferred , a spring or similar means may be employed to maintain cam follower 52 contact and avoid need for a the second cam element 56 . however , this latter embodiment may not be able to maintain cam and cam follower contact at high engine speeds . the cams 54 and 56 may then be actuated by a direct mechanical link to the crankshaft . as shown , a sprocket 62 is provided on the crankshaft , an identical sprocket 64 is attached to the first cam 54 , and a drive chain 66 is attached between the two sprockets 62 and 64 to drive them in one - to - one relationship with each other . as so arranged , in operation each revolution of the crankshaft produces one revolution of the cams 54 and 56 and , as a result of the corresponding movement of the cam follower 52 , one reciprocation of the valve element 40 . it should be appreciated that various cam shapes may be employed to fine - tune the valve member 40 movement for each engine or each use of an engine . in this manner , the movement of the valve element 40 of the present invention may be altered and adjusted by simply changing cams . additionally , it may be possible in some applications to substitute electronic &# 34 ; cams &# 34 ; which receive electronic signals from the crankshaft and drives a synchronous motor which controls movement of the valve element 40 . in the position as shown in fig1 the slide valve 40 is approximately midway between the top of the exhaust passage 30 and the vain element 44 , and upon rotation of the crankshaft , will extend to close completely the upper portion 46 of the exhaust passage 30 . in the illustrated position , the upper edge of the upward moving piston has just closed the transfer ports 26 and is beginning to compress the gases in the combustion chamber 18 . when the slide valve member 40 continues its outward extension enough to close the upper portion 46 of the exhaust passage 30 , as is shown in phantom in fig2 a , a seal will be provided which prevents the compressing gases in the combustion chamber from entering the exhaust passage 30 , thus avoiding the loss of unburned fuel . in the downward cycle of the piston , the exhaust passage 30 remains blocked , preventing the expanding gases in the combustion chamber 18 from entering the exhaust passage 30 . with this arrangement , the effective expansion force on the piston is extended from 90 ° rotation of the crankshaft to approximately 105 ° to 120 °- 125 ° rotation of the crankshaft . this increases the length of the power stroke of the piston by more than twenty percent . through use of the vane 44 and slide valve 40 of the present invention , an effective seal is possible regardless of whether the valve 40 is placed at the exhaust port 28 itself or further downstream in the exhaust passage 30 , as shown . another embodiment of the present invention is shown in fig2 b . in this embodiment , the vane is eliminated and the valve member 40a extends further into the exhaust passage to or below the level of the top of the transfer ports 26 . ideally , the valve member 40 in this embodiment should seal against the piston , as shown , or should extend to abut the opposite wall of the exhaust passage 30 -- so as to create a complete seal of the exhaust passage 30 . in the embodiment shown , the leading edge 68 of the valve member 40 is contoured to match the curvature of the piston and cylinder 12 in order to improve the seal of the exhaust passage when the valve member is fully extended . however , in either form the benefits of the present invention may still be achieved even without a thorough exhaust passage 30 seal . without departing from the spirit of the present invention , it should be noted that at high engine speeds , it may be necessary to advance opening of the exhaust passage to allow adequate time for scavenging . one such method of advancing opening of the exhaust passage is shown my u . s . patent application ser . no . 096 , 551 , now allowed . further , the present invention may be employed with all forms of fuel delivery systems , including those using conventional carburetors , fuel injectors , blower scavenging or supercharging , and those with various forms of reed valves . the chart of fig4 illustrates with graph a a plot of piston position versus crank angle . graph b is a plot of the movement of the slide valve 38 during the time the exhaust passage 30 is open to the combustion chamber 18 in the illustrated embodiment of the present invention . while the plot of graph b shows the exhaust passage being opened at 105 ° of crank angle and closed at 255 ° of crank angle , there is flexiblity in the design , and the slide valve 38 may be designed to open the exhaust passage 30 anywhere between a 90 ° and 120 ° crank angle and close anywhere up to a 270 ° crank angle . it should be appreciated that the optimal point of opening or closing the exhaust valve depends in large part on many factors peculiar to individual engine design . these include : the type of charge to the engine , the size of the engine , the scavenging flow , and the configuration and shape of the engine . additionally , it is believed that it may be desirable to adjust the degree of valve movement with engine speed and throttle opening , with the valve remaining substantially closed at low speeds and closed throttle and providing full movement at high speeds and wide open throttle . this may be accomplished in any suitable manner , which may include another mechanism in the linkage between the crankshaft and the valve member which controls the extent of valve movement in accordance with engine speed and throttle opening , or by switching electronic cams , as discussed , during operation . while particular embodiments of the present invention have been illustrated and described herein , it should be apparent that changes and modifications may be incorporated and embodied therein within the scope of the following claims .