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Carb / mixer design questions for hit and miss engine

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Roger B:
Thank you for posting the update  :ThumbsUp:  :ThumbsUp: It is interesting that the lower the compression ratio with an atmospheric inlet valve the harder it is to properly purge the cylinder. With a mechanically operated valve no problem  :thinking:

Vixen:
 :ThumbsUp: :ThumbsUp: :ThumbsUp:

Thanks for posting your results. Now we all have some real numbers to work from.

Mike

Misterg:

--- Quote from: Roger B on April 03, 2021, 08:13:43 PM ---It is interesting that the lower the compression ratio with an atmospheric inlet valve the harder it is to properly purge the cylinder. With a mechanically operated valve no problem  :thinking:

--- End quote ---

I think that the 'problem' comes from the difference in purging between free running and governed. It's such a slow revving engine, with conservative valve timing that I'm sure there's little/no effect from gas momentum, so the piston just works like a big syringe. When free running, any residual gas in the cylinder is the exhaust gas from the previous combustion stroke, so after the intake stroke, the gas in the cylinder is 80% fresh air/fuel mixture + 20% "inert" exhaust left over from the previous cycle. (I think 20% is  right for a 4:1 compression ratio?)

If the engine is governed by locking out the exhaust valve, the engine draws air back through the open exhaust valve when coasting which dilutes the residual exhaust gas in the cylinder; the mixture is expelled as the piston rises, then more air is drawn in on the next coasting stroke, etc. so that after a few revolutions of coasting, the cylinder contains mostly fresh air. When the governor trips back in, and there is an induction stroke, the cylinder contains 80% fresh air/fuel mixture drawn through the inlet valve + 20% residual air, meaning that the combustion conditions will be different than the free-running case (much leaner if the mixture hasn't been adjusted).

Increasing the CR narrows the difference - that's the way I'm thinking of it, anyway!  ;D


Kim:
Though I don't have any IC experience, I have found your experiment and this whole discussion on compression ratio and exhaust purging just fascinating.  Thank you for posting your experiment and for the whole discussion around it.

Kim

90LX_Notch:
This has been a great post.  I have been wondering/considering adapting this for my “Tiny”.

-Bob

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