How to figure cfm exhaust from small engine?

1305 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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RACHGIER

· #2 · Feb 14, 2021

You're over thinking it.
1. How big is the shed?
2. How long does it take the fan to fully exchange the air in the shed?
3. Your vent fan should be up high, not down low. You want to draw a vacuum, not push positive pressure, to vent effectively.

There's a reason fire fighters cut holes in the roof THEN pop windows in the fire floor. Heat and exhaust gases concentrate up high then start to bank down. So focus on forcefully ejecting the exhaust gases through your upper vent and the cool clean air will naturally refill the space below the thermal blanket.

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RACHGIER

· #5 · Feb 14, 2021

MaryB said:
swap to a locking nut on the clamp... or stake it with a punch once tight but this will make service more difficult backing the nut off the damaged threads. Or drill it and safety wire...

Or just use loctite, like a normal person.

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RACHGIER

· #6 · Feb 14, 2021

@mr_flintstone you'll be fine with or without the exhaust vent. I would be more concerned with heat buildup than anything else. Plus, venting the exhaust means the fan can fail and you won't starve the motor.

Also, t not a shed, it's more like a box or enclosure. When you said shed, I was thinking something walk-in sized.

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RACHGIER

· #8 · Feb 15, 2021

ajole said:
Loctite is defeated by heat, unless you use the high temp version, so make sure you get the right one.

Better solution is double nuts and decent lock washers.

But realistically...I doubt you need an exhaust at all, whether the fan runs or not.

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.

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RACHGIER

· #12 · Feb 17, 2021

engine2 said:
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:

No, exhaust gas temps on gasoline engines run between 300° and 500° but points of constriction in the exhaust system itself can increase the temperatures up to 1200°. However it is extremely rare to see temperatures that high in normally operating systems. Catalytic converters generally reach temps up to 750°, but since we're talking a generator, not a vehicle engine, I doubt we have to worry about a cat overheating.

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