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All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> RC Fuels >> Check my fuel math?
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Check my fuel math? - 9/21/2012 6:35 AM   
Kmot



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From: Northridge, CA, USA
Status: offline 		I need fuel with 7% oil by volume.

I have Powermaster 15% Sport fuel, which is 18% oil by volume.

My math is as follows:

128 oz of fuel =
86 oz of methanol
23 oz of oil
19 oz of nitromethane

If I take 16 oz of the fuel, I have:

10.725 oz of methanol
2.875 oz of oil
2.4 oz of nitromethane

The above is still 18% oil by volume.

Now, by my calculations, if I add 30 oz of pure methanol to the 16 oz of this Powermaster fuel, I will have:

46 oz of 7% oil by volume, fuel.

Which would be:

40.725 oz of methanol
2.875 oz of oil
2.4 oz of nitromethane

Anyone see anything wrong with my math?

Thanks!

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RE: Check my fuel math? - 9/21/2012 12:00 PM   
1QwkSport2.5r



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quote:

ORIGINAL: Kmot

I need fuel with 7% oil by volume.

I have Powermaster 15% Sport fuel, which is 18% oil by volume.

My math is as follows:

128 oz of fuel =
86 oz of methanol
23 oz of oil
19 oz of nitromethane

If I take 16 oz of the fuel, I have:

10.725 oz of methanol
2.875 oz of oil
2.4 oz of nitromethane

The above is still 18% oil by volume.

Now, by my calculations, if I add 30 oz of pure methanol to the 16 oz of this Powermaster fuel, I will have:

46 oz of 7% oil by volume, fuel.

Which would be:

40.725 oz of methanol
2.875 oz of oil
2.4 oz of nitromethane

Anyone see anything wrong with my math?

Thanks!


You have 88.5% methanol, 6.25% oil, 5.2% nitromethane. in 46oz of mixed fuel, you would need 3.22oz of oil to have 7%. I would just subtract the small amount of methanol to bring the oil up. If you dropped the methanol by 6oz to 34.725, you would have 86.8% methanol, 7.2% oil, 6% nitromethane which gives you 40oz of mixed fuel instead of 46oz.

When mixing in odd quantities, I use graduated cylinders and measure in mL instead of fluid ounces as its easier to be more precise.

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*Real* glow fuel is made with Methanol, Nitromethane, and Castor oil.

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RE: Check my fuel math? - 9/21/2012 3:59 PM   
hauckf


 

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From: Decatur, AL, USA
Status: offline 		And the answer is........ Add 201 oz methanol to a gallon of the Powermaster fuel, or 47 oz of methanol to 30 oz of the Powermaster to get a final product containing 7% oil and 5.8% nitro. In other words, multiply the starting amount of Powermaster by 1.57, and that is how much methanol to add to it.

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RE: Check my fuel math? - 9/21/2012 5:30 PM   
Kmot



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From: Northridge, CA, USA
Status: offline 		Thanks for the replies guys!

Let me try to understand this.

You are dividing the 46 oz total fuel by the oil content of 2.875 oz to get the oil by volume of 6.25%, correct?

The way I look at it, the 46 oz total is not the figure to use, but instead the 40.725 oz of methanol is. Because that is actually how much methanol there is. When this is calculated with 2.875 ounces of oil, the oil by volume becomes 7%, does it not?

Why am I unable to understand this?

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RE: Check my fuel math? - 9/21/2012 5:59 PM   
hauckf


 

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From: Decatur, AL, USA
Status: offline 		To calculate the percent by volume on any component of the fuel, you divide the amount of that component in the fuel by the total amount of fuel, not the amount of methanol. In your original post, you multiplied the percent of oil in the gallon (18%) by total amount of fuel (128 oz), to get the amount of oil in the gallon, right? If I gave you 46 oz of fuel and told you it was 7% oil, wouldn't you calculate the amount of oil in that 46 oz of fuel the same way?

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RE: Check my fuel math? - 9/21/2012 8:04 PM   
1QwkSport2.5r



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quote:

ORIGINAL: hauckf

To calculate the percent by volume on any component of the fuel, you divide the amount of that component in the fuel by the total amount of fuel, not the amount of methanol. In your original post, you multiplied the percent of oil in the gallon (18%) by total amount of fuel (128 oz), to get the amount of oil in the gallon, right? If I gave you 46 oz of fuel and told you it was 7% oil, wouldn't you calculate the amount of oil in that 46 oz of fuel the same way?

This is right on the money. Well said. Its no different if someone buys a gallon of premix and adds oil to bring the oil content up. By doing this, the other components will change concentrations since the overall amount of liquid is being increased by "X" amount of ounces. 20% oil in 1 gallon of 10% nitro fuel ends up being 25.6oz oil, 12.8oz nitro and 89.6oz methanol. Add 2 ounces of oil to this mixture increased the overall volume of liquid by 2 ounces and thus divide each component by the new volume of 130 ounces. 27.6 ounces of oil is now 21.2%. The nitro drops to 9.8% and methanol drops to 68.9%. Obviously this wont add up to 100% necessarily because of the calculator rounding up or down. You get the idea though...



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RE: Check my fuel math? - 9/21/2012 9:47 PM   
Kmot



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From: Northridge, CA, USA
Status: offline 		DING!

Now I get it. Thanks!

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RE: Check my fuel math? - 9/21/2012 10:00 PM   
1QwkSport2.5r



Posts: 3741
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quote:

ORIGINAL: Kmot

DING!

Now I get it. Thanks!

Quite welcome.

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*Real* glow fuel is made with Methanol, Nitromethane, and Castor oil.

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RE: Check my fuel math? - 9/21/2012 10:02 PM   
hauckf


 

Posts: 220
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Status: offline 		Glad we could help. Now I have a question: What is fuel with 7% oil used for?

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RE: Check my fuel math? - 9/22/2012 1:44 AM   
Kmot



Posts: 10743
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From: Northridge, CA, USA
Status: offline 		Will be testing a special intake manifold for the ASP radial engine. Like the Seidel and Moki engines, it allows the use of 7% oil fuel.

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RE: Check my fuel math? - 9/22/2012 2:27 AM   
Old Fart


 

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Status: offline 		Tom can you keep us up to date on the manifold? i've run a friends 770 and it's been hard to keep the lower cylinders lit evenly till we added 7% unleaded petrol to the mix.
Cheers,peter

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RE: Check my fuel math? - 9/22/2012 2:52 AM   
rockin daddy


 

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From: Fredericktown, PA, USA
Status: offline 		Hey Old Fart:

I have a Saito 325 and an ASP 400 sitting waiting for this slow old builder(me)  to finish the planes.  Have been mixing my own fuel for many years.  Usually use it in my Saito 4-strokes and a few 2-strokes I fly on occasion.  I use 20% oil(10% castor and 10% Klotz), and  10 % Nitro.   I can mix fuel for less than $8.00/ gal.   I have heard of using 5~ 7% gasoline to give the glow engines a little "kick".   Have you had a bit of experience with the addition of gasoline to the glow fuel mix??? 

Have a couple of twin Saito 4-cycle motors that sometimes one of the cylinders will loose the "fire" when I throttle down to half when on the back side of a loop and the motor continues to run at 1/2 speed, running on just one cylinder.   The only way to get the dead cylinder to relight is to chop the throttle a bit and the McDaniel glow plug controller fires up the glow plugs.

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RE: Check my fuel math? - 9/23/2012 6:08 PM   
Kmot



Posts: 10743
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From: Northridge, CA, USA
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quote:

ORIGINAL: Old Fart

Tom can you keep us up to date on the manifold? i've run a friends 770 and it's been hard to keep the lower cylinders lit evenly till we added 7% unleaded petrol to the mix.
Cheers,peter

Will do. I intend to post a full build-up and test report.



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RE: Check my fuel math? - 10/11/2012 6:49 AM   
Kmot



Posts: 10743
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From: Northridge, CA, USA
Status: offline 		Peter,

I ran the engine today on the blended 7% oil fuel and the custom intake manifold. The engine runs just fine on this fuel. No smoke in the back yard either!

Report and video coming soon.

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RE: Check my fuel math? - 11/2/2012 9:14 AM   
Donairplane


 

Posts: 150
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From: Aurora, OR, USA
Status: offline 		Try this site when mixing fuel.  http://www.nitrorc.com/fuelws/allmix.asp

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All Forums >> Glow Engines, Gas Engines, Fuel &amp; Mfg Support Forums >> RC Fuels >> Check my fuel math?
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