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Expansion ChamberAn expansion chamber is an exhaust system used on two stroke cycle engines to enhance horsepower output. It makes use of the energy left in the burnt exhaust exiting the cylinder to aid the filling of the cylinder for the next cycle. It is the two stroke equivalent of the tuned pipes (or headers) used on four stroke cycle engines. How it works The high pressure gas exiting the cylinder flows in the form of a"wave" as all disturbances in fluids do. If this wave encounters any change in cross section or temperature it will reflect a portion of its strength in the opposite direction to its travel. For example a high pressure wave encountering an increase in area will reflect back a low pressure wave in the opposite direction. A high pressure wave encountering a decrease in area will reflect back a high pressure wave in the opposite direction. An expansion chamber makes use of this phenomenon by varying its diameter (cross section) and length to cause these reflections to arrive back at the cylinder at the desired times in the cycle. There are three main parts to the expansion cycle.
When the exhaust port first opens the exhaust flows out powerfully due to its own pressure so no help is needed from the expansion chamber and so the first portion of the pipe is constant or near constant with a divergence of 0 to 2 degrees. This section of the system is called the head pipe. By keeping the head pipe near constant, the energy in the wave is preserved until needed later in the cycle.
Once the exhaust pressure has fallen to near atmospheric level the piston uncovers the transfer ports. At this point energy from the expansion chamber can be used to aid the flow of fresh mixture into the cylinder. To do this the expansion chamber is increased in diameter so that the out going high pressure wave reflects a negative pressure wave back toward the cylinder. This negative pressure arrives in the cylinder during the transfer cycle and greatly increases the flow of fresh mixture into the cylinder and can even suck fresh mixture out into the headpipe. This part of the pipe is called the divergent(or diffuser)section.
When the transfer is complete the piston is on the way back up on its compression stroke but the exhaust port is still open, an unavoidable problem with the two stroke design. To help prevent the piston pushing fresh mixture out the open exhaust port a strong high pressure wave from the expansion chamber is timed to arrive during the compression stroke. The port blocking wave is created by reducing the diameter of the chamber. This is called the convergent section. The outgoing high pressure wave hits the narrowing convergent section and reflects back a high pressure wave to the cylinder which arrives in time to block the port during the compression stroke and can push back into the cylinder any fresh mixture drawn out into the head pipe. Combined with the high pressure wave there is a general rise in pressure in the chamber caused by deliberately restricting the outlet with a small tube called the stinger. The stinger restricts flow out of the chamber to cause higher pressure during the compression cycle and empties the chamber during the compression/power stroke to ready it for the next cycle.
All these events need to be synchronized with the engine port timings and speed. An expansion chamber tuned for 8,000 rpm will not deliver the proper wave timings at 4,000 or 11,000 rpm. In fact it is possible to actually cause a power loss outside its tuned range. The length of the pipe determines at what time the waves arrive back at the cylinder. Longer pipes require more time for the waves to traverse and so will be tuned to a lower rpm than a shorter pipe. The shorter the pipe the higher the rpm it is tuned to. The diameter of the center or dwell section determines the ratio of scavenging suction to port blocking pressure. The fatter the pipe the harder it sucks but the weaker the blocking pressure. Thinner pipes will scavenge less but block the port very strongly. The optimum diameter is related to the quality of the transfer port layout and its scavenging efficiency. A variety of devices are used to try to extend the tuned range of the expansion chamber. Pipes that slide like a trombone adjust the timing to match the rpm changes of the running engine. Devices that control the exhaust port timing to extend the tuned range of the expansion chamber. Valves that open at certain speeds to absorb or dump waves arriving at undesirable times. Links - http://www.liebold.com/rumipages/espansioni/twostroke.html
-Contains a nice animation. - http://www.sae.org/servlets/productDetail?PROD_TYP=BOOK&PROD_CD=R-161
-The authoritative source book for two stroke engine information.
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