stunthanger.com
Engine basics => Engine set up tips => Topic started by: Joe Messinger on October 03, 2006, 04:56:28 PM
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Hello,
Am I correct in assuming the purpose of using a hemispherical shaped head on a two stroke engine is to -lower- the compression ratio? Ex.: Fox .35.
If a standard, flat topped piston is used in conjunction with a hemi head, I believe lower compression would result. Perhaps there are other reasons for installing a hemi head on some engines? I imagine controlled performance rather than higher performance is the goal.
Joe
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I assume a control line engine combustion chamber works the same as an automobile engine. The idea behind the hemi head was that the burn under compression would travel equally in all directions providing a more efficient burn...hence a more effctive power as the fuel burns. In the case of the CL engine, it would also lower the compression which would be like adding an additional spacer. I see this as an effective way to adapt a higher compression RC engine for CL use.
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Equal, directional burn would be a benefit of a 2 stroke hemi head. I wonder why hemi heads aren't incorporated in more 2 stroke engine designs? Probably more expensive and not really necessary.
Valve arrangement in a 4 stroke (automobile) hemi engine is very efficient. It reduces valve shrouding and increases power. Wonder if that would work on the 4 strokes we use on model airplanes? Again, probably too expensive and would add quite a bit of weight to the engine.
Joe
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If my memory serves me (and mostly it doesn't) , the 40LA and 46a have a squish band and a hemispherical shaped combustion chamber.
Also, a hemi head doesn't always reduce compression on a Fox .35.
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Most all of the later stunt engines do have hemi heads, as well as the four strokes.