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The New Zealand Railways Magazine, Volume 2, Issue 12 (April 1, 1928.)

Theory of Combustion — (Continued) — Losses Due to Radiation, Leakage of Steam and Miscellaneous Sources, 59 per cent.

page 36

Theory of Combustion

Losses Due to Radiation, Leakage of Steam and Miscellaneous Sources, 59 per cent.

See that the valve and steam connections are tight, and do not blow the steam through the safety valves or cylinder cocks. If drivers will only book all engine defects in the repair book and see to it that the shed staff do their share in carrying out repairs, the above loss could be wiped out. There is nothing more discouraging to a good fireman than an engine that will not steam properly; to a poor fireman, however, it is a somewhat more serious matter. A badly steaming engine causes him to forget everything he has ever heard or known about the correct principles of firing, and makes him believe that he is no longer a fireman—merely a heaver of coal, and he acts on that belief. The average man, overtaken by the feeling that he has a good excuse for making a failure, fails with ease.

It is essential to stop steam leaks. Steam waste includes not only that which is wasted past the piston and valve glands, packing rings, valve rings or seats, but that steam which is wasted because of poor valve setting and excessive engine friction.

Extracts from the Presidential Address by Mr. R. H. Whitelegg to the Institution of Locomotive Engineers, 1922.

“You will readily understand the necessity of strict control over this all important commodity (coal) upon the railways when I inform you that on fifteen English railways the consumption for the year 1921 was close on 8,500,000 tons and on five of the Scottish railways fully 1,500,000 tons were consumed. If it were possible, by economic working, to reduce the poundage per mile by only 11b., the saving for these twenty companies would amount to no less a sum than £377,000.” Further, Mr. Whitelegg goes on to say:—

“There are some firemen who use regularly a few pounds of coal per mile in excess of their fellow workmen. They are costing their company (in the excess coal they are burning) a sum equal to their yearly wage, for, of course, the man who wastes doesn't stop at coal.”

Again, I find Mr. Hill, the Chief Mechanical Engineer of the Great Eastern Railway of England, appealing to enginemen to save at least one shovelful of coal (weighing 14½ lb.) per mile, and stating that, if this were done, the Great Eastern Railway would save £312,000. Staggering figures!

In April 1923 the General Manager of the Great Western Railway of England told his enginemen that “One shovelful of coal saved per trip would mean a saving to the G.W.R. of 10,000 tons of coal per annum.”

I am only quoting these cases to illustrate my contention that enginemen, by studying the principles of combustion and the problem of fuel economy, could, of they so wished, save a considerable poundage of coal on almost every trip, which saving, spread over a period of a year, would represent a big saving in money.

The Brick Arch and its relation to Fuel Economy.

The advantages claimed for the use of the brick arch are:—(1) Fuel saving; (2) smoke abatement; (3) tube protection and reduction in tube repairs; (4) improvement of steaming qualities under demands for maximum power; (5) reduction of engine failures from leaking tubes and low steam pressure; (6) reduction in tube stoppage; (7) reduction in clinkering or honeycombing of the tube plate; and (8) the beneficial effect on the life of a set of tubes and of the tube plate.

Against these advantages can only be set the cost of maintenance of the brick arch. We have all of us got past that point where we questioned the advantages of the brick arch; we know, from practical experience, that the brick arch does give the results claimed for it. A large portion of the saving effected by the brick arch is due to the intimate mixing of the combustible gases and oxygen brought about by the arch. A thorough mixing of the gases in the firebox is impossible without a brick arch. Many of the fine fuel particles that break off the coal in the fuel bed are so light that they are often picked up by the draught and whirled page 37 out of the firebox (in company with the fine coal dust that never reaches the fuel bed) in a partly burned condition—unless they are “baffled” and thrown down again by striking the brick arch. In order to secure perfect combustion of the gases, all flame must be burnt out entirely before reaching the tube plate. Now this can only be accomplished by “baffling” them in such a manner that none can reach the tubes without passing around and over the brick arch. Under average working conditions (with the firebox equipped with an arch) the loss due to the escape of unburned gases may be between 2 and 10 per cent. The losses, however, are much greater without the arch, as the saving of 10 to 16 p.c. effected . by the arch, is largely due to the decrease in the amount of combustible material that escapes unburned in the form of gases, coal dust, sparks and cinders.

“Mere length of combustion chamber counts for little compared with some device for thoroughly mixing the gases of the flame stream (says Dr. Breck-enridge). One good mixing wall or ‘baffle’ is probably worth more than many feet of undisturbed flow.”

This is not said to discount the significance of the combustion chamber length, but to emphasise the importance of mechanically mixing the different strata of the gas stream.

In the Auckland Province.Photos. J. F. Davey Top: Rotorua express leaving Auckland—Ab. engines. Bottom: Suburban-train nearing Ellerslie—Wab. Engine.

In the Auckland Province.
Photos. J. F. Davey
Top: Rotorua express leaving Auckland—Ab. engines. Bottom: Suburban-train nearing Ellerslie—Wab. Engine.


Superheating is of course a form, and a very important form, of fuel economy. You are all familiar with superheater engines and the principle of superheat, which not only increases the steaming capacity of the boiler, but at the same time permits the use of larger cylinders because the superheat prevents the cylinder condensation. The steam and coal economy obtained with superheater locomotives which is equivalent to a corresponding increase of the boiler capacity has made it possible in many cases to reduce the boiler pressure.

Professor Goss on “The Use of Superheated Steam in Locomotive Service”:—

“Neither steam nor coal consumption is materially affected by considerable changes in boiler pressure, a, fact which justifies the use of comparatively low pressures in connection with superheat.”

Before we got the use of superheat in boilers the pressures were being gradually put up, whereas they are now down to 180/200 for passenger engines and 160/180 for goods engines.

The economical results obtained with superheater locomotives show considerable savings in coal. In passenger engines it is not so much the coal saving which makes superheater engines so popular, but the increase in power obtained and the better way in which the superheater engine handles the train. The great feature of a superheater engine is that its efficiency increases with the demand for power. If an ordinary wet engine is forced, its efficiency decreases on account of the increased wetness of the steam furnished by the boiler. The super-heater, however, on the contrary, improves with increased demands, since the degree of superheat increases in proportion with the power the engine has to develop.

Professor Goss says:—

“In operation the degree of superheat increases with the increased rate of power, which tends to conserve the steam supply as the demand for power is increased.”

This flexibility is one of the main features that distinguishes the superheater from the wet and from the compound engines; and is, besides the coal and labour saved, the principal cause why superheater engines are such favourites with the men handling them.

(To be continued.)