For You Technical Hounds

Paul Brokaw Demonstrates in the “Simplest” Terms that the Ideal Combustion Ratio of 15 to 1 is Based on Weight. We Hope This Holds You

By Paul Brokaw, Photography by Unknown

From the March 1945 issue of Motorcyclist Magazine

I was especially interested in a certain statement made by the undoubtedly capable gentleman writing under the nom de plume of “Shop Foreman,” in a recent issue of the Motorcyclist. Specifically it was the exception he took with Lt. Frank K. McDowell over the question whether the popularly accepted ideal combustion mixture ratio of 15 to 1 was figured in weight or volume.

My interest may be understood when I explain that I have, as an instructor for Ordnance, schooled hundreds of men in the theoretical and practical phases of carburetion. With all respect to “Shop Foreman” I must support Lt. McDowell in his contention that the ratio is based on weight. Furthermore, in the interests of your readers among the military technicians and your technically minded civilian readers, I think that this subject, that is in so frequent controvery, should be cleared up. To do this must necessarily involve some rather dry technical computation. If you will bear with me I will get under way.

In the interests of simplicity we will base our calculations on “Hexane” asit is the chief constituent of all gasolines and for all practical purposes has the same proportions of hydrogen and carbon as found in the other hydro-carbon compounds(gasolines). Below I will list this and other established fundamentals to be used in the succeeding calculations.

1. Hexane (C6H14) is composed of 6 parts of carbon and 14 parts of hydrogen.

2. Dry atmospheric air is composed of 77 parts by weight of nitrogen and 23 parts by weight of oxygen. In addition are water vapor and small amounts of other elements, all of which we may disregard in our calculations as they are impotent in the operation of a gasoline engine.

3. Carbon has an atomic weight of 12, hydrogen of 1 and oxygen 16.

4. When adequate air is mixed with gasoline and ignited, the fuel will combine with the oxygen of the air to form carbon dioxide gas (CO2,) and the hydrogen will combine with the oxygen of the air to form water vapor (H2O).

5. Carbon and oxygen combine in the atomic weight proportions of 12 and 32 respectively. Hydrogen and oxygen combine in the atomic weight proportions of 2 and 16 respectively.

Now referring to paragraphs 1 and 3, we will find the proportions by weight of the two elements in gasoline as follows:

6 X 12 =72 parts of carbon to, 14 X 1=14 parts of hydrogen or expressed in percentage 83.72% carbon to 16.28% hydrogen.

Now referring to paragraph 4 supplemented by paragraphs 3 and 5, let’s write down the reaction occurring after ignition to establish our proportions according to atomic weights.



Proceeding with these proportions it is now a simple task to find the amount of oxygen required for the combustion of one pound of gasoline. Referring to our percentages of carbon and hydrogen established above, we find that in one pound of gasoline we have 0.0837 lbs. of carbon and 0.163 lbs. of hydrogen. Let’s now enter these factors in the above equation:

Now we must remember that the above oxygen is mixed in the atmosphere with nitrogen in the proportion of 23:77 (paragraph 2). Therefore the 3.518 pounds of oxygen above found necessary in the combustion of one pound of gasoline is

mixed with 77/23 X 3.518 equals 11.774 pounds of nitrogen.

Adding 3.518 pounds of oxygen to 11.774 pounds of nitrogen we then have 15.3 pounds of atmospheric air, the amount required for the combustion of one pound of gasoline.

Thus we find that the ratio of 15.3 to 1 is definitely in terms of weight and not volume.

However, we are above dealing with fluid gasoline and “Shop Foreman’’ bases his 15 to 1 proportion with the gasoline in a geaseous state. To convert our above established proportions of air to fluid gasoline, to a proportion of air to vaporized gasoline will require further calculations, and consideration of a few more established findings of chemistry:

1. Gasoline vaporizes at ordinary atmospheric pressures and temperatures, and draws its heat of vaporiaztion from the surrounding air.

By Paul Brokaw
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