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Old 06-10-2011, 10:03 PM   #3 (permalink)
Hal
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Join Date: May 2009
Location: Washington state
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Two-strokes are inherently less efficient than four-strokes because of the way the ports are arranged - there is a significant amount of time when both the intake and exhaust ports are open and the exhaust closes after the intake. As a result a good portion of the incoming charge is lost out the exhaust. There are all manner of devices to try to minimize the effect, such as expansion chambers, slide valves and so on but they (other than expansion chambers) start adding a lot of complexity and the initial advantage of the design - low parts count, light weight, etc - start to get lost. The exception to that being direct injection...
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