Abstracts from an address by A. Dougal Clark, Prest. of the Junior Institute of Engineers, London.
The wonderful success obtained by your distinguished Past-President, the Hon. C. A. Parsons, and his many able followers with the steam turbine in its various forms, has naturally attracted the attention of engineers to the apparently analogous problem of the internal combustion turbine. Accordingly, much mathematical and engineering ability has been recently devoted to the subject so far, I am sorry to say, without concrete result. In this subject, as yet, the dreams of the theorist obstinately decline to realize themselves in tangible iron and steel.
I have not been able to find any gas-turbine in a state of effective rotation doing useful work, though I have noted many statements in the Press to the effect that sorne wonderful German, French or Italian gas-turbine had worked, or was about to work in such manner as to relegate the ordinary cylinder and piston gas-engine to the museum with which many engineers used to threaten the steam-engine. One gas-turbine only has really rotated within my own direct knowledge. It was designed by Mr. F. W. Lanchester of Birmingham, to operate with exhaust gases from one of the petrol engines used in his well-known motor-cars. He assured me a few days ago that it really rotated at a high speed, and made a loud shrieking noise, but only gave, he said, a total brake horse power equal to that capable of being evolved by two bluebottle flies. This power he did not consider satisfactory.
Speaking seriously, it does seem remarkable that so much interest should be taken by so many able men without any sort of result in practice. Why is this? I propose tonight to answer the question in so far as I can. It appears to me that most of those who have written on gas-turbines and have even designed and patented them, have given too little weight to certain differences between the steam and internal combustion engine problem. Many, indeed, have assumed that the solution of the gas-turbine problem is the easier of the two, and that few difficulties exist which have not already been met and conquered by Mr. Parsons in the steam turbine.
Many distinguished men have been of this opinion ; and even Mr. Parsons himself, so early as his first turbine patent, appears to nave been of the opinion that the hot gas or internal combustion turbine presented practically the same problem as the steam-turbine. In his specification Mr. Parsons clearly intended to apply his invention to the gas-turbine as w«ll as the steam-turbine; and he outlined the fundamental idea of nearly all subsequent proposals of gas-turbines. Many other inventors have followed him; but I may only mention two well-known names - those of Ferranti and
Stodola. Both have proposed turbines similar to his, with more or less elaboration, as well as other modifications intended to overcome certain difficulties.
In most of the recent discussions upon gas-turbine problems, it has been recognized that the temperatures possible to the cylinder gas-engine are impossible for the gas turbine. It has been fully proved by many investigators, including myself, that the temperature quite common in ordinary gas-engine practice ranges as high as 2000° C, though in the best practice, for most economical results, 1500° C, or 1600 C. appears to be an upper limit. With these temperatures a first" class modern gas-engine of about 40 h. p. will give an indicated efficiency of 35 per cent. At the same time the negative work of the cycle is so low that the mechanical efficiency of the engine may be as high as 86 per cent, or even more. If one realizes what the temperature of 2000° C. means, it becomes very evident that no tnrbine constructed either on the lines of Par-sons or Laval could possibly be made to work with continuous supply of such gases; as 2000° C. is considerably above the melting-point of platinum. It is much higher than the temperature at which cast-iron flows from the blast furnace or, indeed, the temperature of the interior of the blast furnace. Any blades of iron, steel, or, in fact, of any other material, even fire-brick itself, becomes fluid or semi-fluid at the temperature. It is obviously hopeless, therefore, to attempt in the gas-turbine temperatures which are quite feasible in the cylinder-engine. This fact as I have said, is accordingly said, by those who take a favorable view of the gas-turbine, that it is necessary to supply the turbine with gases at a much lower temperature. Mr. Neilson fixes the temperature of 700° C. as one which steel turbine blades would probably stand without too rapid deterioration. I fear that on this point I must differ from him, because in my experience, oxidation of steel and even iron, is a fairly rapid process at this temperature. Nothing new has been proposed as to the thermo - dynamic cycle of the gas-turbine; so that all reasoning upon efficiencies de pends upon the deductions already made from inter, nal combustion engine practice.
Seeing the impossibility of consuming a turbine with materials to stand a high temperature, many have proposed to convert high temperature into kinetic energy, so that instead of having work stored up in the gas in the form of heat, the heat shall disappear and the energy of the heat be transformed into motion of the gaseous particles at a high velocity. Such proposals, then, include the compressing of a gaseous mixture to (say) 50 or 60 lbs. above atmosphere; the igniting of that mixture within a combustion chamber at constant pressure; and the expansion of the mixture through an expanding jet of the Laval type so as to drop the temperature and obtain its equivalent in kin. etic energy or velocity of the gaseous particles. The rapidly moving particles at the relatively low pressure and temperature are then allowed to impinge upon rapidly rotating blades of sickle co-figuration; and they are supposed to give up their energy of mo. tion to the blades, and so expend work upon the turbine. This appears to be the most feasible of all the gas-turbine proposals, so I will proceed to examine it little more minutely.