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Old 31-08-2011, 08:56   #1
tsl
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Calculating CFM for a Diesel Engine

I'm sure this has been discussed before but my search did not pull anything up.
I intend to install a walker airsep on my Universal 5444(Kabota) V-1902 engine in an O'day 39. I'm also installing new Sound down soundproofing and intend to seal up the engine room as much as possible.

The Walker Airsep I intend to install is designed for gen sets and small sailboat auxiliaries up to 350 CFM. Walker Engineering - Product Detail

I googled how to calculate cfm for engines and got many formulations and calculations all offering results often magnitudes apart. Some use fuel consumtion and the air fuel ratio others go with volumetric efficiency yet others seem to take into account everything including the rotational spin of the earth.
I found a slick chart that looks impressive and if the author spent half as much time in with the formula as he did with the graphics it should be most accurate. The answer I got was 90 CFM for the 44 HP engine. Seems a bit low.
Engine room ventilation , Based on this result and some overkill I intend to use 4" vent hose with two intakes at 4" and one powered exhaust at 4". 240 CFM fan.

Inputs:
HP 44
No. of Cylinders 4
Bore X Stroke 3.3X3.25
Displacement Cu. In. 115
Max RPM 3000
Compression Ratio 21.1
Rotational speed of the earth 1 rpd
Don't know fuel consumption/hr.

What do you think am I anywhere close?
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Old 31-08-2011, 09:02   #2
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Re: calculating cfm for a diesel engine

I get @200 CFM @3,000 RPM, so you should be fine. Make sure the intakes are from either the bilge or cabin area to eliminate salt spray intake. Do not put the exhaust in the cockpit in case (or when) you fill it with water.
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Old 31-08-2011, 09:12   #3
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Re: calculating cfm for a diesel engine

Don't forget these engines only draw air in everyother rotation,(4 stroke cycle) I came up with about 100 CFM. But it will be much less than that because they are not 100% volumetric efficient.
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Old 31-08-2011, 09:15   #4
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Re: calculating cfm for a diesel engine

Great point about the salt spray intake but putting it in the cabin ruins my soundproofing plan.
I'm planning to put the Exhaust out the stern.
The O'day has grid pan that I could tap into as long a it does not reverberate throughout the boat.
The boat will be in the great lakes for a few years but I want to do things right. I'll open to other intake suggestions
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Old 31-08-2011, 11:02   #5
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Re: Calculating CFM for a Diesel Engine

Matt is correct on the CFM, I had detroit on my brain
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Old 31-08-2011, 11:14   #6
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Re: Calculating CFM for a Diesel Engine

Love those Detroits too. Had one eat my note book back in trade school when it ran away. (Note to self, a note book will foul a turbochager if you use it in an attempt to choke down an 8V92....)
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Old 31-08-2011, 11:18   #7
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Re: Calculating CFM for a Diesel Engine

Engine data doesnt say how much?
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Old 31-08-2011, 11:35   #8
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Re: Calculating CFM for a Diesel Engine

See Caterpillar’s excellent paper on Engine Room Ventilation:
http://www.gregorypoole.com/products...ENTILATION.pdf
http : //www . gregorypoole.com/products/electricpowergenerator/Documents/LEBW4971-02%20ENGINE%20ROOM%20VENTILATION.pdf See also, some earlier CF discussions:

Engine Room Blower

Engine Ventilation: Skip it?

Bilge/Motor Room Blower Exhaust Port?
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Old 02-09-2011, 06:56   #9
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Re: Calculating CFM for a Diesel Engine

Thanks all for the help. Follow the rest of the project on O'day 39 project boat - SailboatOwners.com
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