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.

2. A unit of gasoline at a given temperature and pressure has a vapor density of 3.25 or in other words, weighs 3.25 times an equal unit of air at the same pressure and temperature. Now, inasmuch as the air drawn into the cylinder is the vehicle that carries the atomized gasoline from the carburetor, the vaporization of the gasoline is the result of the heat drawn from the vehicular air, so the above established proportions would definitely apply.

3. The established weight of a cu. ft. of air at 29.92 inches of mercury (ordinary atmospheric pressure) and 32 degrees Fahr. is 0.0878 pounds.

By applying these factors to our accepted ratio of 15.3 pounds of air to 1 pound of gasoline we can readily determine our ratio by volume:

Here, then is our ideal combustion ratio of air to gasoline (vaporized) and figured by volume: 189.57 cu. ft. air to 3.81 cu. ft. vaporized gasoline, or a ratio of 49.76 to 1.

I might add that comparison by volume is not used because, in greater part, vaporization of the gasoline does not occur until the air-gasoline mixture has entered the cylinder. At this time the air picks up the heat from the cylinder wall and combustion chamber and imparts it to the gasoline. When excessive vaporization occurs in the carburetor or manifold, engine efficiency is impaired and we have a condition known as vapor lock. Actually the carburetor is an atomizing instrument and not a vaporizer.

Some of the old-timers and no doubt “Shop Foreman” will recall the wick type carburetor used on some of the most early model motorcycles. This type of carburetor did employ the principle of vaporization but for obvious reasons couldn’t supply the demands of a flexible, high turn-over engine.

I hope that “Shop Foreman” takes no offense in all this. We all seek correct information and “Shop Foreman’s” column is itself a most valuable source of reliable information.

Questions & Answers

Question-I notice as I read through your Q&A that there is very little said about 101’s. I used to own a Crocker 61 Overhead, there is very little said about a Crocker also.-James M. Conant.

Answer-We have probably ridden 100,000 miles on 101 Scouts but from your letter we can’t tell just what you want to know. We will give you any specific details about the 101 or the Crocker if you will just tell us what you want to know. Instruction books are furnished by most factories and we suggest you contact them. However, if you will refer to the November issue of the Motorcyclist, you will find a number of questions answered about a Crocker 61.

Question-I would be obliged if you could furnish me with information regarding the bore, the angle of the valves and their diameters, and the timing of the lubricator pump of the 1930 “Super X” Motorcycle. -G. Earl Goddard.

Answer-The Super X45 was practically a “square” motor and if my memory does not fail me, it was 3” bore by 3/16 stroke. I believe the valve angle was 45 degrees and the tappets should set intake .004, exhaust .006 with a cold motor. The spark timing should be 3/8” lead on fully advanced spark. The oil pump is a plunger type and does not require timing.

Question-I have a 41 74 OHV Harley-Davidson and the valve guides are in bad shape. I talked it over with a few riders around here and what I would like to know, what do you think about making my valve guides out of cast iron or a good grade of steel? Which do you think would work the best? -Edwin Popp.

Answer-If it is at all possible you should replace your valve guides with the regular factory replacement part. If it is absolutely impossible to buy factory parts you can make a satisfactory guide that may last the duration out of cast iron. Factory guides for your motor are a special heat-treated steel.

Question-Some time ago I wrote to you for information on the German BMW and Zundapp solo motorcycles, but I have heard nothing further. I would appreciate it very much if you could give me some information on these machines as I am greatly interested in them. -James L. Weaver.

Answer-Sorry, but we still do not have the information you want on these machines. Inasmuch as both are German machines, it is not possible for us to contact the manufacturers as we would do if they were American or British machines.

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By Paul Brokaw
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