How to figure cfm exhaust from small engine?

1307 Views 12 Replies 7 Participants Last post by undeRGRound, Feb 26, 2021

mr_flintstone

Discussion Starter · Feb 14, 2021

I’m trying to figure the cubic feet per minute of exhaust gas from a 420 cc generator at 3600 RPM. If my thought process is right, it should be 420CC x 1CF/28317CC x 3600RPM/2 ~ 27 CFM for a 4-stroke engine and about 54 CFM for a 2-stroke engine.

Now I realize that the internal volume of exhaust gas is still under a little pressure inside the cylinder. How much... I don’t know. So, I’ll figure it expands another 20% as it exits the exhaust pipe for around 32 CFM/4-stroke and 64 CFM/2-stroke.

The reason I ask is that I have placed my generator inside a small shed with a 1300 CFM fan drawing air through the shed from one side and bottom, and out the other side. I’m having trouble getting my clamp to hold my tailpipe extension on that extends through the wall to the outside. If my math is right, the fan should draw plenty fresh air into the shed for complete combustion, and ventilate all the exhaust with plenty fresh air to spare without the need for the exhaust extension to the outside.

Anybody care to weigh in on this subject? Am I missing anything?

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engine2

· #10 · Feb 17, 2021

Rachgier said:
Blue loctite "breaks" at 50 ft-lbs and all colors of loctite "melt" at 550°F. Average exhaust temps run between 300° and 500° F. He'll be fine with blue.

I think you might be a little shy on the exhaust temp. Depends on the load but I think you're looking at 750-1300° on an air cooled engine. It'll melt nylocks, use pinch nuts (or double nut). Steel starts to glow red at 900°.

As far as air flow in the shed I have a 16' X 20' shed that is fairly tight that had mice last fall. Fired up the 318 JD with a 18hp Onan in there and shut the door. Took an hour to use up the oxygen and kill the Onan. I'm not sure if the lack of oxygen or carbon monoxide killed the mice but haven't seen them since. :bounce:

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engine2

· #11 · Feb 17, 2021

Maybe get rid of the temp sensor. Plug the fan into the genset. If the gen is running so is the fan, you can't over cool an air cooled engine. Simpler is better. If for some reason the fan fails open the door.

Also I'm not a fan of synthetic oil in air cooled engines BUT if your in a situation like Texas is now I would use synthetic. Trying to start an air cooled engine at 0° with regular oil will be tough.

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