THE GAS TURBINE
Page 43
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Some Questions
and Answers
By Major W. H. Goddard,
IN effect, a gas turbine is an oil engine, in that it obtains its power from the expansion of the products of combustion of oil fuel burning in the presence of the oxygen contained in the highly compressed hot air in the combustion chamber. Instead, however, of the expansion taking place intermittently, as it does in an engine cylinder, it is a rotary process taking effect on a bladed turbine wheel to produce continuous and smooth energy.
In, the case of a four-stroke oil engine, the compression takes place once in every four strokes of the piston, and the compression chamber is self-contained in the upper part of the cylinder. In the gas turbine the combustion chamber is separate, as also is the compressor, so that, including the turbine from which the power is obtained, there must be at least three main components: (I) A combustion chamber, which receives compressed air from the compressor in which oil fuel is being continually sprayed and burned. (2) A compressor driven by the turbine. (3) The turbine itself, in which the products of combustion are expanded to produce the power.
A unit on the foregoing lines represents the basic and simplest form of gas turbine, and it was from this that the modern and more complicated machine was evolved.
In its basic form, the gas turbine is most inefficient, as considerable heat is lost to the atmosphere from the turbine exhaust and, with only one stage of compression, no great efficiency is possible.
It soon became obvious to those responsible for its development that it was necessary to arrange for two, or even three, stages of compression, which meant using aq equivalent number of compressors. This, again, demanded some form of cooling between the stages of compression in order to reduce the great volume of air, whilst still retaining some portion of the heat and raising the pressure.
Thus there was brought into being what is known as the intercooler, in which the hot air from the first compressor is passed through a nest of tubes, which are air-cooled, and then to the second compressor for raising the pressure. With the addition of the intercooler, the efficiency is raised by 5-6 per cent.
Still further to improve the efficiency, the regenerator or heat-exchanger was introduced. This component is placed between the second compressor and the combustion chamber, and employs the heat from the turbine exhaust to preheat air from the second compressor before it enters the combustion chamber.
The increased efficiency obtained from its use is from 7-9 per cent,, so that with the intercooler, the overall efficiency figure has been raised by some 12-15 per cent.
With a plant consisting of three compressors, three turbines, three combustion chambers, two intercoolers and one regeneratiar, an efficiency figure of 34 per cent. is claimed to be possible. Comparing the gas turbine with the compact and self-contained internalcomb ustion reciprocating engine, it is a composite multiple plant, the efficiency of which is dependent upon the number of units employed, as well as on the temperature of the gas delivered to the turbine. It also depends, to some extent, on the temperature of the air being drawn in, as the cooler it is, the greater will be the efficiency.
There is no doubt that the road transport industry in general, is interested in the possibility of the development of the gas turbine for use in road vehicles, and therefore is anxious to learn all it ean about it. By way of settling some of the matters which must be uppermost in the minds of operators, I have set out a number of questions, to which I have appended the answers, based on my knowledge of the gas turbine at the present stage of development, (1) Is it possible in the confined space available on a road vehicle to fit the complete multiple plant which appears to be necessary if the gas turbine is to show reasonable thermal efficiency, that is, sufficient to enable it favourably to compete with present prime movers?
It is not possible.
(2) Can the best possible combination of turbines, compressors, combustion chambers and heat-exchangers, which could be accommodated on a goods or passenger vehicle, show a degree of thermal efficiency comparable with that of a modern high-speed oil engine?
No.
(3) Is the consumption of fuel as closely proportional to the load demanded of the engine as is the case with the oiler, bearing especially in mind that over a few days' work it is estimated that the average call on the engine for power is probably under 50 per cent, of its possible maximum?
No.
(4) Bearing in mind that, because of many serious problems, no small gas turbine has yet been produced or even designed, is it not evident that great difficulty will be experienced in producing a turbine of 100-130 h.p. for road vehicles?
It will be extremely difficult, even impossible, to produce a gas turbine even as small as 100 h.p.
(5) Is it possible so to reduce the present disproportionate percentage of total installed power which is absorbed by the compressors, that the thermal efficiency would approach that of the oil engine?
The principle on which the gas turbine functions demands an enormous volume of air to be highly compressed and thus it would seem that this serious disadvantage is unavoidable.
(6) Can the present difficulties of producing the requisite torque characteristics for road transport purposes, be overcome without recourse to complicated and expensive auxiliary apparatus?
No.
(7) One of the, at present, unsolved problems of the gas turbine is that of arranging for adequate and efficient control for road transport work, the vital necessity is that the driver must have an instantaneously responsive control over his prime mover. Can this problem be solved satisfactorily? No solution at present appears to be in sight.
(8) The volume of the exhaust gases is said to be six times as great as that of an internal-combustion engine of equal power. As this gas issues at a temperature of 850-900 degrees F., would not this be an insuperable difficulty in a congested traffic area? If one visualizes oneself seated in an open car close behind a gasturbine lorry, the answer is quite obvious.
(9) To reduce the extremely high speed of the gas turbine to that of the road wheels of a vehicle travelling at, say, 30 rn.u.h., would call for a special type of reduction gear occupying considerable space. Whilst not actually impossibk, would this not be considered a disadvantage?
This is a matter of mechanical design, and it could be accomplished, but at great expense.
To sum up the subject, and quite apart from the specific questions and answers which I have set out, it appears to be reasonable to state that, whilst the gas turbine undoubtedly will be successfully employed as power units for marine and, possibly, locomotive purposes, in addition to its ideal use for the jet propulsion of aircraft, for road transport purposes itis fundamentally unsuitable.