Our readers are well informed in the history of the attempts which have been made to substitute air for steam as an expansive agent in engines. With the commencement of these efforts the name of Capt. Ericsson wiU ever stand as one of the earUest pioneer investigators, and, should the success which is now claimed for the combination of an- and steam, applied to the same purpose, be fully realized, that share of the honor attending it will be due to him, justly claimed by those who help to point out the way by which others may mount to success. To the mechanical engineer the paper bearing the above title, read before the British Association at Exeter, will be one of the most interesting of any of the able and valuable contributions to the transactions of that distinguished body. We can give only a brief review of this paper at this time, but we may perhaps refer to it again at a favorable opportunity. The first part of the paper was devoted to a review of the data by which it has been satisfactorily established that not more than one tenth of the eatire heat of coal is on the average utilized by steam engines. The author, Mr. Richard Eaton, of Nottingham, England, then discusses the practical diflculties encountered in the effort to substitute heated air for steam, the principal of which is, as our readers are already aware, the effect of highly heated air upon such metals as may be economically employed in the construction of machines. lie then proceeds to give a brief history of the new Aerosteam motor, which avails itself of air expansion, using at the same time steam, which removes the difficulty above mentioned. Mr. George Warsop, of Nottingham, as the son of an air gun maker there, was born with aerial ideas, and although his only education was received at a Sunday school, and he was sent to work at ten years of age, he turned that education to such good account that before he was twenty he had in leisure moments secretly constructed an air engine. Later in life it was his privilege, while a working mechanic in New York, during his engagement with Mr. Ericsson, to observe the weak points in the system of that highly gifted and persevering inventor, and after years of research to supply the dficiences by a marvelously simple system of mechanism which, as far as present experience goes, promises complete success by means which, happily for the cause of economy and progress, seem compatible alike with physical scienco and mechanical construction. In the first attempts at practically carrying out the system, the arrangement adopted was an ordinary high pressure engine with vertical boiler as used where fuel is cheap. An air pump is added, which is put in operation by the action of the steam engine. Thus, cold air is taken in by the air pump and is forced on in its compressed state through an air pipe, which, in the case before us is conducted first within the exhaust, then in a coiled form down the funnel of the boiler, then past the fire, and finally past a self-acting clack valve at the bottom of the boiler into the boiling water itself, rising naturally through the water, the air is intercepted and subdivided by diaphragms of metal gage. Thus a twofold service is rendered by the contact of the elements, the water be-corjing aerified and deprived of its cohesion and prompted to a free ebullition, while the air on rising above the water is saturated by the steam, and the two together pass on to their duty in the cylinder where saturation assists lubrication. The agitation of the water prevents scahng. The machine thus constructed, but having two air pumps, and with cam motions applied to the valves as also to the poppet valves of the working cylinder, gave the following results, results which it must be admitted were sufiiciently discouraging to have deterred the inventor and his associates from proceeding further in the matter, but for their faith in the intrinsic soundness of the system, and perseverance in carrying it to a practical issue. The work had to be done under disadvantages of various kinds, on inconvenient premises, which centuries back were a farm house standing within the ancient walls of Nottingham, and until the protection of the patent laws had been obtained, the original apparatus was I carefully guarded in an unsuspected attic. In this form of the apparatus the power obtained by the increased volume of the air forced in by the pump, did not compensate for that consumed in forcing it into the boiler. At the same time there were encouraging indications which led to further experiment. One of the air pumps being discarded, experiments were made witn waste holes in the barrel of the other pump, to ascertain what proportion of air admitted to the boiler compensated for compression. It was found that about ten per cent of the effective consumption of fiuid in the working cylinder gave much better results. At the same time tho cam motions were discarded and the pumps left to their own unaided action. In this form it is claimed that a gain in work done by the combined air and steam engine was made of 42'5 per cent. Here, although a very remarkable relative economy was apparent, it became obvious on consideration that danger of mistake would arise in assuming this economy as absolute, inasmuch as the duty performed, when contrasted with that obtained from engines of standard types, actuated by steam, was manifestly low, and it seemed probable that, as by judicious improvement in details, the duty was made to approximate more closely to fair steam engine duty, this relative economy might fall off considerably, inasmuch as there would be less margin to economize upon. With a view of testing this point, and also for the satisfaction of railway engineers, of conducting experiments at locomotive pressures, a thorough remodeling of the whole apparatus vras effected. The tappet motions were thrown aside in favor of the usual slide valve arrangement, working with a moderate amount of expansive action. The former wasteful vertical boiler was discarded in favor of a more economical one of the compound or Cornish multi-tubular description, so as to obtain a better evaporative duty from the coal consumed. The radiating surfaces of the cylinder pipes were re-clothed, and the feed water heated by the exhaust steam. Instead of exposing the air pipe to the direct heat of the furnace, as in the former case, the air became thoroughly heated on its passage from the pump to the boiler at a tem perature of from 500 to 600 Fah., by being conducted through suitable coils and pipes through the exhaust steam in the heater, and the waste heat in the boiler flues and uptake. When these changes were made a gain of 47 per cent over steam only, was claimed on an even pressure trial, and a gain of nearly 80 per cent on an open valve trial, a step in advance so huge that it staggers belief. We, shall watch future experiments in this field svith the utmost interest in the hope that they may be sucofessful, and that at last some decided progress in the conversion of heat into work has been made. Not that there has been no progress, but what has been made has been slow and painful, compelling, as it were, only a small fraction more of the heat which we know is constantly eluding us, to fall into line and do work. But 30, 40 per cent is something to make an engineer suspend his breath, aye, and his belief too, until the plairf proof is before him that the results claimed are really secured. An illustrated description of this apparatus will he found in another column.
This article was originally published with the title "Aero-Steam Engines" in Scientific American 21, 13, 201-202 (September 1869)