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  1. #1
    ABBOTT's Avatar
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    PURPOSE OF THE POWERVALVES

    A stroke is the action of a piston travelling the full length of its cylinder. In a 2-stroke power valve system, one of the two strokes combines the intake and the compression stroke, while the other stroke combines the combustion and exhaust stroke.

    As the piston travels upward in the cylinder, it creates a vacuum in the crankcase; this draws fresh air and atomized fuel from the carburetor through a hole in the crankshaft. As the piston continues travelling upward, transfer ports that are responsible for delivering the fresh air-fuel mixture to the cylinder are closed off, thus trapping the combustible mixture. As the piston reaches the top of the cylinder, the mixture in the cylinder is compressed to the point of ignition.

    The second stroke begins once ignition has taken place. The combustion-or power—stroke begins as the air-fuel mixture is ignited. The burning fuel creates pressure in the cylinder above the piston and forces it downward. As the piston passes the midpoint of the downstroke, the exhaust port to the side of the cylinder starts to open and initiates the flow of burned fuel out into the expansion chamber or muffler through the manifold.

    As the piston is forced downward, positive pressure builds up in the crankcase where the air-fuel mixture remains from the previous intake-compression stroke. Shortly after the exhaust port is uncovered by the downward travel of the piston, the transfer ports begin to be uncovered. The transfer ports act as a passage through which the air-fuel mixture moves from the crankcase into the cylinder above the piston. The rush of the fresh air-fuel mixture as it enters the cylinder helps to push out residual exhaust gases. Once the piston reaches the bottom of the stroke, the second cycle is completed and the process is repeated.

    Engineering design improvements

    The only moving parts inside a 2-stroke engine are the crankshaft, the connecting rod and the piston. This means 2-strokes are very simple engines. Because there is a combustion-stroke whenever the piston travels downward, they are capable of producing tremendous power. It is the same simplicity in design, however, that makes a 2-stroke engine less fuel-efficient. At the bottom of the power stroke, the transfer ports, which deliver fresh fuel, are open at the same time as the exhaust port. This allows a significant amount of fresh fuel to run straight through the engine without ever being available for power production. Properly designed exhaust systems help minimize the amount of raw fuel loss in the exhaust process, but a 2-stroke engine will always waste some precious fuel.

    Many producers of 2-stroke performance bikes fit them with the exhaust valve systems. A valve is normally situated alongside the exhaust port. To make a 2-stroke engine high-powered, the cylinder is given large ports, particularly the exhaust port. The main problem with this is that it makes the engine produce very little power at low RPM (revolution per minute) and have a high fuel consumption.

    2-STROKE ENGINE>>


    4-STROKE ENGINE>>YOU HAVE TO CLICK ON THE PIC TO SEE IT MOVE!!


  2. #2
    Moderator RX951's Avatar
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    good post

  3. #3
    I think John that you are consufindd Power Valves which change the exhaust port timing with reed valves on the 2 strokes and inake and exhaust valves on the 4 strokes. Regardless of it its a great document.

  4. #4
    ABBOTT's Avatar
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    yea its the wrong picture---i tried to find the right animated pic but had some trouble.

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