The impact, for example, of swiftly moving gases on a fixed surface ultimately results entirely in turbulent motion, which restores to the gas or to the blade struck all the heat which has disappeared in temperature fall due to adiabetic expansion. What is true of a fixed blade is to some extent also true of the moving turbine blades. A certain proportion of the energy existing in the gas in the form of motion is inevitably lost when this gas comes into contact with any solid surfaces. So much is this the fact, that in designing steam-turbine blades for any type of turbine, the shape of the blades, the shape of the space between the blades - both moving and fixed blades, or fixed jet and moving blades - is of the first importance; and it has only been found by experiment that certain shapes of blades and passages have a much higher efficiency of conversion than other shapes.
In this respect, too, the turbine principle is inferior to the cylinder and piston. In a cylinder, with gases expanding behind the piston, the efficiency of expansion may be considered to be 100 per cent, and even an efficiency of compression in many gas engines is also the same order. I do not here refer, of course, to heat losses due to conduction, or anything of that kind, but to efficiency of adiobatic compression or expansion.
Although the efficiency of expansion is relatively low for gases in steam-turbines, yet the turbine offers a great advantage in total work obtained from steam. This is due to the fact that the turbine avoids initial condensation; and, further, it permits of the utilization of a very long range of expansion at the low-pressure end, which is not available in the case of steam-engines. By saving, therefore, in minimizing initial condensation, and in obtaining added work from pressures wasted in the ordinary steam-engine, the Parsons steam-turbine more than compensates for any inefficiency of expansion as compared with the cylinder engine.
It is well known, however, in turbines of practically all constructions, including that of Mr. Parsons, that the efficiency of the steam-turbines is not so great as that at the low-pressure end. This is partly due to the difficulty of adjusting the velocity of the blades to suit the necessarily varying velocities at different points of the flow of the steam. This, however, is a small difficulty with the steam-turbine, but a very considerable one with the gas-turbine. Compared with cylinder exp msion, I cannot see how it is possible with present knowledge to obtain an efficiency of conversion in a gas-turbine greater than 80 per cent. This is partly due to the high velocity of the issuing hot gases.
To produce an efficient gas-turbine, therefore, on the favorite cycle so much discussed recently, it is necessary first to have, as I have said, a very efficient compressor, a very efficient conversion when the moving gases strike the turbine blades. Using the numbers I have suggested, of 90 per cent efficiency of nozzle expansion, and 80 per cent efficiency of conversion in the turbine, we have a cycle having negative work equal to 0.4 of work in compression, we shall require 0.445 of work put into the compression. On expanding in the nozzle, we shall obtain 0.9 only of the total energy of flame gates in the shape of kinetic energy; and of that 0.9 we shall only get 0.8 returned in the shape of available work by the turbine part. That is, we shall get a total work from the turbine of 0.72; and deducting the negative work, 0.72-0.445=0.276. That is, from a cycle which should give us 0.6 in work, we shall only get 0.275 or about 22 per cent.
The practical efficiency of an engine of this kind will only be 22 per cent, even assuming the high efficiencies of compression and jet expansion which I have mentioned. In my view, no such efficiencies of compression or jet expansion are at present known ; and accordingly there appears no likelihood of the production of any gas-turbine which can rival the reciprocating gas-engine in efficiency and in economy. To produce such a turbine requires the solution of three problems: (1) An efficient turbine compressor, comparable in efficiency with cylinder compression. (2) An efficient nozzle expander with a higher efficiency than 90 per cent. (3) An efficiency of conversion of kinetic energy of the moving gases into work delivered at the turbine spindle of greater than 80 per cent. Either these problems must be satisfactorily solved, or else new materials discovered which will stand temperatures which at present melt firebrick. The outlook, I fear is not hopeful.
From what I have said, you will see that my view of the future of the gas-turbine is not favorable. But, notwithstanding, the subject is so fascinating that many inventors and scientific men will doubtless continue to investigate the problem; and possibly new solutions may be discovered which are not dreamed of today. I am the last man in the world to deprecate daring in any practical and scientific work; but I would advise the junior engineers - members of our Institution - to avoid the subject, except as a scientific study. I fear there is little hope for a young man to make a position and a business success of any internal combustion turbine, so far as present knowledge carries us.