"10%" Nitro Fuel

by
R. W. Stuart

So you have plopped down your dollars for fuel, maybe "Super Slush 10" or similar, and now you wonder what you really got. What exactly is in this stuff and is it really 10% nitromethane. Nitro is added to provide a little extra oxygen- beyond that which can be induced through the carb- to provide more complete oxidation of the fuel and extra output (more horsepower).

At present I am using a 4 cycle fuel, home brew, made : 
                 Nitro      10%      12.8 oz.
                 Klotz oil  10%      12.8 oz.
                 Castor oil 10%      12.8 oz.
                 Methanol   70%      89.6 oz. 
                 --------   ----     -------------
                  Gallon    100%     128 oz.

So far- so good! Notice that the oil content is 20% oil as specified by Saito, which may be a bit conservative (oily). Better to be a little overlubed than not for those lean or overreved "occasions".

The fuel-air ratio for methanol is 1 to 7.14 while the fuel- air ratio for nitromethane is 1 to 3.57 (based on the evaporated volume of each fuel). Each of these ratios provide exactly the amount of air to burn the fuel and this perfect fuel-air ratio does not waste unburned fuel and does not waste heat by heating excess air which is not used for burning (perfect on-ratio combustion is called stoichiometric combustion and is never achieved in actual practice and the ratio is generally fuel rich in our little engines and air rich in most industrial and domestic applications). Variations in air density, as experienced on cold days, will require resetting the needle to get good engine operation.

The horsepower limit for a given fuel (methanol) in a model engine will be limited by the combustion air which can be induced into the engine. Note that nitro only uses half the amount of air methanol uses, and when burned releases available oxygen which helps burn the methanol. Net result is more hp, more rpm, with more air induction and thus more hp. Also, the operating temperature rises as does the cost of high nitro fuel ($30.00-$40.00 per gallon for pure nitro).

Now the "punch line"- how much nitro in the home brew "10%" fuel I use? It is not 10%. Note that the fuel contains 12.8 oz nitro, 25.6 oz oil and 89.6 oz methanol. The oil does not contribute to combustion, so we throw it out of the calculation for nitro. Now the combustion components are only methanol and nitro which contribute to combustion and the ratio of nitro to methanol (12.8 oz divided by 89.6 oz) is 0.142 or 14.2%. I doubt that even the fuel manufacturers have thought of such things, but the effective nitromethane is 14.2 %. Next time I talk to a fuel provider, I will ask if his 10% is based on total volume or on effective combustion volume.

~~ BE SAFE~~

~~~ Stuart ~~~
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