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The New Zealand Railways Magazine, Volume 2, Issue 11 (March 1, 1928)

Theory of Combustion — (Continued) — Combustion And The Locomotive Firebox

page 30

Theory of Combustion
Combustion And The Locomotive Firebox

The complicated action and re-action met with in burning coal may be divided into three processes. First, the conversion of coal into gases; second, the burning of the gases; and third, the separation and disposal of the ash and refuse. None of these processes are easily carried out in the ordinary locomotive firebox where bituminous coal is used.

The conversion of coal into gases can be managed without difficulty if it were possible to hold all of the coal on the firebars until gasification was completed; but under ordinary working conditions from 5 to 15 per cent, of the coal is picked up by the draught (or blast) and ejected at the smoke stack as cinders or sparks. Under bad conditions as much as 25 per cent, can be lost in this manner.

The loss of solid coal through the firebars and drop-grates generally averages from 1 to 2 per cent., but my observations lead me to believe it is nearer the vicinity of 10 per cent.—not through a badly designed fire-grate, but owing to careless and indifferent firing.

Under the most favourable conditions and at low rates of combustion, about 94 per cent, of the solid combustible matter is converted to gases. At medium rates about 85 per cent, is so converted, and at high rates only 75 per cent.

The burning of the combustible gases in the spaces above the fire is also attended with difficulties—due to lack of air, imperfect mixing of the gases, and insufficient combustion chamber volume (that is, the firebox isn't big enough).

The amount of air supplied through the grates depends upon the air opening through the ashpan and finger bars, the thickness of the fuel bed, the nature of the coal (as to size and ash and clinker forming contents), and above all, to the skill of the fireman. But even if all these conditions are favourable there is a gradual decrease in the air supplied as the rate of combustion increases. At high rates of combustion the supply of air is generally deficient.

Under ordinary conditions, with the firebox fitted with a deflector plate and a brick arch, the loss due to the escape of unburned gases varies from 2 to 10 per cent. The losses are much greater without the brick arch, as the saving of from 10 to 16 per cent, effected by the arch is largely due to the decrease in the amount of combustible matter that escapes unburned in the form of gases, coal dust, sparks and cinders.

The separation and disposal of the ash and clinker forming impurities is an unsolved problem which is largely responsible for the low average daily mileage of our locomotives. The frequent necessity for cleaning fires, dropping fires through the drop-grate (which often causes the tubes to leak) is due largely to imperfection in firebox design and to methods of burning coal that our great designers are always endeavouring to overcome.

The foregoing remarks will convey some idea, of the complicated structure of coal and the complexities of the problems yet to be solved in this direction. I trust, too, that firemen may be encouraged to study the problem of combustion. The firebox with which they are working becomes vastly more efficient for its purpose when an intelligent hand guides the coal and air supply.


One pound of coal—good coal—is nearly one pound of carbon, and one pound of carbon properly burnt results in the liberation of 14,650 heat units or B.T.U's. (B.T.U. means British. Thermal Unit, the amount of heat required to raise one pound of water one degree Fahrenheit.) A unit of heat will raise the temperature of one pound of water one degree Fahrenheit.

In order that the importance of an ample and uniform air supply (and the necessity of intimately mixing the air and the coal gas by every possible means), may be grasped, the following; figures are given:—

One cubic inch of air contains 443 billion billion (443,000,000,000,000,000,000) molecules; 93 billion billion of these are oxygen molecules. One molecule of oxygen is required to burn one atom of carbon. One cubic inch of air, therefore, contains oxygen sufficient to burn 93 billion billions of carbon atoms—this number of atoms page 31 being contained in a piece of coal no larger than a pin's head.

Those wishing to study more widely the problem of combustion will find the works of Millikan, Lodge, Comstock and Troland, Crowther, Rislieu, Deeley, Jones, Roscoe and Schorlemmer, Thomson, Campbell, Gibson, the U.S.A. Bureau of Mines and Professors Goss, Cranford, Fry and Dr. Brislee, of considerable interest and importance.

Locomotive Fuel Economy.

The first comprehensive and thorough study of this subject was made by the “American Engineer” (news journal) and appeared in April, 1908.

On November 20th, 1908, the International Fuel Association (to bring about economy in the use of fuel) was organised at Chicago with a membership of 35. Since the formation of the Association every railway has taken up this most important subject. There are to-day some who think that fuel economy is a new-fangled idea and a fad, but it is time we all woke up to the fact that fuel economy is the most important factor in railway operation costs. When the cost of coal is high it often depends on the amount of coal saved or wasted whether or not there be dividends.

(Photo W. W. Stewart.) The New Plymouth Express Leaving Wellington.

(Photo W. W. Stewart.)
The New Plymouth Express Leaving Wellington.

We must bring up the work of the fireman to a higher plane and teach the chemistry of the firebox. To obtain the best, results enginemen must understand the theory which underlies the combustion of coal, and they must combine this theory with practice. It must be remembered that drivers and firemen are, for the greater part of their time, beyond the limits of direct supervision. If rules and regulations for controlling the use of coal are laid down for their guidance the reasons should be thoroughly explained, so that such rules, etc., not only appeal to their intelligence, but inspire interest in carrying them into effect.

The system of imparting instruction by those in charge of locomotives is by means of lectures, by the personal help of the older and more experienced enginemen, and by the inspectors on the footplate.

The function of the supervising officers is to see that the best use is made of the coal. The basis of any efforts directed towards securing greater fuel economy is the recognition that the human element is by far the most important factor.

Lectures and verbal instructions supply the means whereby some men will take full advantage of any knowledge offering.

It will usually be found that those who scoff and jeer in this respect will (when they realise that the “sit still” attitude solves no problems) in the end fall into line and assist the men who are trying to improve methods in this vitally important matter.

I could write many pages illustrating the methods adopted in giving publicity to the fuel economy campaign on many railways in American and Great Britain. Figures, lectures, and pamphlets, photographs and pictures—in fact no system of moden publicity is left untried to keep up the interest in this subject, and I am confident that, were we paying £2 a ton for coal (as some railways do) instead of what we are paying, we should have to step quick and lively along this road of fuel economy. But because our coal is cheap, we should not be careless in its use. In a recent year we used (on the South African Railways) the enormous amount of 2,153,970 tons of coal. It is obvious, therefore, that we have a great oportunity of saving the country a considerable sum of money by the intelligent use of coal.

The man who economises in the use of pins, or who uses an envelope a second time, deserves well of his employer. Likewise in the use of coal, it is within the power of the locomotive section of the railways to make savings in the big things—coal, oil, etc.

Everyone and everything connected with the operation of a locomotive or the movement of a train has some effect, direct or indirect, in the fuel consumption.

Fuel economy and efficient operation, under ordinary conditions, are practically synonymous.

The basis of a successful fuel campaign is the fact that the average man is anxious to do his best and thus to protect his reputation and his home. To secure the best results from the enginemen there requires to be a painstaking page 32 and thorough campaign of education which, will reach each man and inspire him to the proper performance of his duties.

Significance of a Pound of Coal.

“In a first-class stationary plant one pound of coal will produce nearly one horse-power for one hour, but in a modern superheater locomotive it will only produce one horse-power for twenty to twenty-five minutes (says Professor Goss). One pound of coal used in a goods locomotive will provide enough energy to carry one ton fifteen or sixteen miles, and in a modern train it will be fed to the boiler every 52ft. of distance travelled; in other words, if coal were fed to the boiler continuously it would take a rod of coal ⅜ in, square constantly fed into the firebox.”

Early Fairlie N.Z. Locomotive.

Early Fairlie N.Z. Locomotive.

From the exhaustive tests of Professor Goss the actual distribution of fuel consumed on the average locomotive on a division where no interest is being taken in fuel economy may be stated as follows:—

(1) Stand-by losses, consisting of fuel used in keeping steam while the engine is standing idle, in starting fires preparatory to taking out on runs and fuel in firebox at end of runs 20%
(2) Losses due to vapourising the moisture contained in the coal 5%
(3) Wasted on the ground and stolen 1%
(4) Losses due to unconsumed gases escaping through the smoke stack 10%
(5) Loses due to unconsumed fuel in cinders and sparks 10%
(6) Losses due to unconsumed fuel in ashes 3%
(7) Losses due to radiation, leakage of steam and miscellaneous sources 6%
(8) Utilised in effective work 45%

Now let us examine each item and study where the enormous loss of 55% can be reduced.

(1) Stand-by Losses 20%

This loss would be considerably reduced by the co-operation of the shed, engine, and traffic staff. The shed staff should carefully examine each engine as it arrives and promptly report any driver who has not a low fire on completion of trip. The traffic staff should not order an engine until prepared to make use of it, and by arranging crossings (single line working), that will obviate trains being kept standing in station yards and sidings for an unnecessary length of time. It is absurd to talk fuel economy to a fireman when the shedmen and foreman let them bring in engines from a trip with a ton or more unburned coal in the firebox (practically all of which is wasted in the ashpit when the fire is cleaned), and in other ways permit fuel to be wasted.

I calculate that on the South African railways there is enough coal wasted through the safety valves and ashpans and around sheds to run a division.

(2) Vapourising Moisture—A loss of 5%.

This is a loss practically beyond the control of the engine crew, but study of this point under Theory of Combustion will help.

(3) Wasted on the Ground and Stolen—A Loss of 1%.

The loss sustained under this head is due chiefly to bad loading of the tender and carelessness on the part of the fireman, resulting in coal rolling off on to the road. Strict supervision by the drivers would have the necessary effect in this direction.

(4) Losses Due to Unconsumed Gases Escaping Through the Smoke Stack, 10%.

I trust what I have already said under the Theory of Combustion will help the firemen in realising that a pronounced saving could easily be brought about by them. But on the accepted principle that theory is only the servant of practice I would urge upon the drivers their duty to carefully watch the firemen and coach them in the fine art of firing. Many of the older drivers are too apt to forget the period of their own apprenticeship—that they themselves probably were taught much that they know to-day by the kindly help of the drivers with whom they served in days gone by.

(5) Losses Due to Unconsumed Fuel in Cinders and Sparks, 10%.

Much of this loss can be reduced by intelligent firing. (This item was dealt with under the Theory of Combustion.)

(6) Losses Due to Unconsumed Fuel in Ashes, 3%.

Here is something to think about. Absolute waste through the finger bars and ashpans. How we should squeal if we were paying for the coal ourselves. Shedmen and foremen can put a stop to this around the shed, and so can the drivers along the road. The firemen can easily save the whole of this loss by exercising care in cleaning and dropping fires.

No green coal should ever go into the ashpan.

(To be continued.)