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Compression Ratio required

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HobbyNut15/12/2010 16:34:18
113 forum posts
93 photos
My Farm engine is complete, has spark, and some compression. I had read that these engines do not like too much compression, and I have read recently that you need "good" compression. The original plans had the piston stopping about 3/8" down from the head, and there was little to no compression. I moved the cylinder and head so that there was 1/2 that amount of space, and it now shows "some" compression. It will not run, I can get a slight feel of firing if I use some butane as a fuel, but not so that it will run any more than 1 fire, and it is not enough power to turn it over 2 revs. It will not even fire with petrol or white gas.
With the spark plug out, I can flick the flywheel and it will turn over about 3-4 rev's. With the plug in , the same flick will only turn it over about 1 1/2 rev's.
Any suggestions, from anyone.?
I posted some of this under Beginners Questions, but didn't get any responses.
blowlamp15/12/2010 17:29:04
1245 forum posts
82 photos
Compression ratio is Swept volume + Unswept volume divided by Unswept volume. So a cylinder of swept capacity of 100cc plus combution chamber space of 15cc will have a compression ratio of about 7.66:1 which is low by modern standards, but might be a reasonable figure for you to start from.
If you still don't have good compression once the theoretical is right, then it's time to check the basics such as valves and clearances and the fit of piston and rings etc.

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