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Rotax air consumption

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paul.the.aviator(at)gmail
Guest

Posted: Sat Apr 24, 2010 1:36 pm Post subject: Rotax air consumption

Hi all,



I was wondering if anyone has come across any Rotax 914 data that will tell me the airflow into the engine for various power settings.  I have looked on a few of the auto enthusiasts web sites and for the follow parameters I get:

 

  0 centigrade

  5100 RPM 

  30"



I get 87,700 cfm



Does that sound about right ?

  [quote][b]

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rparigoris

Joined: 24 Nov 2009
Posts: 796

Posted: Sat Apr 24, 2010 3:20 pm Post subject: Re: Rotax air consumption

Hi Paul

I am not quite sure how to deal with turbo, venturi restriction and exhaust restriction but your number of 87,700 cfm seems quite off. 



Lets take a simple look at the 914 as a simple air pump not developing any pressure. We need to keep in mind that since it is a four stroke, you only intake the displacement once every other revolution. Thus you only intake 1/2 of the 1211cc displacement every revolution (two full breaths of the four) or 605.5cc per revolution.



Now lets take 5100rpm and multiply it 605.5cc to give us the cc per minute of air pumped, then multiply it by conversion number of .000035314 to convert cc to cf. That gives us a tad over 109 cfm.



Not sure what you are after, but a 650cfm carb is quite a big beast compared to the ability of two type 64 Bing carbs. I can't imagine how large an engine would be that could handle a 87,700cfm carb, but then again you are looking for something else? Exhaust design? Or?



You could always measure the exhaust of engine running at that power setting. Take several sheets of large plastic and make an enclosure. Calculate internal area and plumb your exhaust to it and time to fill.



Now if you are talking about how many cfm of air the propellor could flow at 30" and 5100rpm with your Airmaster "ground loop" adjustible prop, perhaps the 87K is a lot closer. A 10hp motor running a 30" prop may push perhaps 20 or 25K of air per minute at a very low pressure. My basis for this estimate is my homebuilt balloon is 95,036cf and I can fill it in under 4 minutes, but then there is some spill out from the 15 foot diameter mouth.



Hope that helps.



Ron Parigoris

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craigb(at)onthenet.com.au
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Posted: Sat Apr 24, 2010 7:02 pm Post subject: Rotax air consumption

I  think you are way off there, typical V8 car engine, with after market  performance carb are set up for

 between 600 and 800cfm, you might be out by a factor of  1000, 87cfm would seem a bit low though.

 [quote]   --

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rampil

Joined: 04 May 2007
Posts: 870

Posted: Sun Apr 25, 2010 12:43 pm Post subject: Re: Rotax air consumption

Try it stoichiometrically:



Air flow must be proportional to fuel flow



Use a ratio of 15:1 (just easier that 14.7 reference value)

 so 1 gallon per hour (6 lbs) is 15*6 = 90 pounds/air per hour



Dry air at 1 STP = 0.0807 lbs/ft^3



so 90lbs of air = 1115 ft^3 per hour per gph = 18.6 ft^3 per min



Typical Rotax Fuel Flow =5 gph = 93 Ft^3/min Air flow



Air flow into manifold should not depend on turbo settings or anything

else directly except keeping mixture at proper ratio.

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