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All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> RC Fuels >> Facts and figures needed!
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Facts and figures needed! - 11/30/2006 5:52:47 PM   
usted


 

Posts: 5
Joined: 8/19/2006
From: , UNITED KINGDOM
Status: offline 		I am looking in to model engines, especially nitro powered 2-stroke engines as to how they work, mainly because I am looking to do general engineering at university.

However, I also am looking at the chemical engineering aspects as well.


Anyway, the question is, what is the usual (if there is one) fuel/air ratio, with a .18 engine running on 20% nitro fuel?

Another thing, does anyone know the brake specific fuel consumption? I have googled for the answer, but I can't find anything at all.

One more - the Engine Volumetric Efficiency?


p.s. I know that these values will vary depending upon the setup, but any values would be gratefully recieved.
Also, please tell me if the figures do not exist, so I don't waste my time looking for them!!
       Post #: 1

RE: Facts and figures needed! - 11/30/2006 10:01:57 PM   
hpi apollo



Posts: 2639
Joined: 5/13/2006
From: Georgetown, KY, USA
Status: offline 		http://www.nitrorc.com/ how they work

there is no specific fuel/air ratio, its all in the tuning, engines are designed with different materials, layouts, etc, so there really isnt the same aar/fuel ratio

as far as fuel consumption, it depends on the engine and how it is tuned, with what fuel it is running, you can have the same engines and run different fuels, one might go farther per tank than the other

i hope that at least helps a little bit


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RE: Facts and figures needed! - 12/1/2006 1:24:39 AM   
downunder



Posts: 3171
Joined: 10/10/2002
From: Adelaide, AUSTRALIA
Status: offline 		Probably your best starting point for some figures regarding fuels is at http://www.holdfastmac.com.au/techcharts.html

You'll see it's almost impossible to give a definite answer to air/fuel ratio when using nitro because of its huge allowable range while still giving maximum power but as a very rough figure with 20%nitro you're probably looking at around a 4:1 A/F ratio.

BSFC is almost never a concern with model engines so it's never referenced although it'd be simple to calculate. The only time it's useful (although once again there are never any figures for it) is in CL team racing where very low fuel consumption while still giving a lot of power is required.

I've never seen any figures for volumetric efficiency but I'd take a guess it'd probably be around 60% or so. But all 2 strokes (and 4 strokes as well) have their peak VE at exactly the peak of the torque curve. Which is actually backwards because peak torque occurs at peak VE .


(in reply to hpi apollo)
       Post #: 3

RE: Facts and figures needed! - 12/1/2006 4:08:36 PM   
usted


 

Posts: 5
Joined: 8/19/2006
From: , UNITED KINGDOM
Status: offline 		Thankyou for taking your time to reply - much appreciated.

I sort of thought that the values would differ greatly, but your information is still a great help.

Thanks again


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       Post #: 4

RE: Facts and figures needed! - 12/1/2006 11:16:16 PM   
Jim Thomerson



Posts: 2792
Joined: 10/9/2002
From: Austin, TX, USA
Status: offline 		The optimum air to fuel ratio for methanol alone is said to be 7.14 air to 1 methanol (by weight, I suppose). So figuring you have so much oil in the fuel which may or may not burn, some or all, and the oxygen released by the nitomethane, Doununder's 4 to 1 may be close to right. All very interesting.

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RE: Facts and figures needed! - 12/2/2006 1:31:33 AM   
downunder



Posts: 3171
Joined: 10/10/2002
From: Adelaide, AUSTRALIA
Status: offline 		Thinking about it, you could get a close approximation of the A/F ratio for 20% nitro by simply running an engine on both zero nitro and then 20% and timing the run. Methanol has a stoichiometric ratio of about what Jim said but gives maximum power at between 6.5 and 4.5 which is a fairly wide range. If an engine was run on zero nitro and peaked out just before the point it started to lose revs then it could be assumed it was running with an A/F of close to the 6.5 ratio. So in a test, if it ran for 6.5 minutes on zero nitro then (after retuning) it ran for 4.25 minutes with 20% nitro (with the same volume of fuel of course) then it's A/F ratio would be 4.25:1, or near enough.

One trap though. The oil content of both fuels would have to be identical and use only castor to make certain that no oil was being burnt to contaminate the readings.

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       Post #: 6

RE: Facts and figures needed! - 1/3/2007 12:17:25 AM   
iraiam


 

Posts: 163
Joined: 8/13/2006
From: Lakewood, CO, USA
Status: offline 		If I remember correctly the A/F ratio for Nitromethane alone is like 1.8 pounds of air to burn 1 pound of nitromethane, and methanol alone is 7:1

not sure if that helps but good luck

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RE: Facts and figures needed! - 1/3/2007 9:00:35 PM   
Sport_Pilot



Posts: 7652
Joined: 1/21/2002
From: Acworth, GA, USA
Status: offline
quote:

ORIGINAL: iraiam

If I remember correctly the A/F ratio for Nitromethane alone is like 1.8 pounds of air to burn 1 pound of nitromethane, and methanol alone is 7:1

not sure if that helps but good luck



That is ideal for maximum efficiency. However because nitromethane can be used as a monopropellent;you get more power, not efficiency, the richer it is run. However when there is some methanol or oil mixed there is a limit, so you get a very broad needle.

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       Post #: 8

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