August i i, 1892] 



NATURE 



357 



between i860 and 1865. Mr. Isherwood showed that in engines 

 such as those then in use in the United States Navy, with the 

 large cylinders and low speeds then prevalent, any expansion of 

 the steam beyond three times led, not to an increased economy, 

 but to an increased consumption of steam. Very little later 

 than this M. Him undertook, in 1871-5, his classical researches 

 on the action of the sleaoi in an engine of about 150 indicated 

 horse-power. Experiments of greater accuracy or complete- 

 ness, or of greater insight into the conditions which were im- 

 portant, have never since been made, and Hirn with his assis- 

 tants, MM. Hallaner and Dwelshauvers Dery, has determined, 

 once for all, the whole method of a perfect steam-engine trial. 

 M. Hirn was the first to clearly realize that the indicator gives 

 the means of determining the steam present in the cylinder 

 during every period of the cycle of the engine. Consequently, 

 superheating in ordinary cases being out of the question, we 

 have the means of determining the heat present and the heat 

 already converted into work. The heat delivered into the 

 engine is known from boiler measurements, combined with 

 calorimetric tests of the quality of the steam, tests which Hirn 

 was the first to undertake. The balance or heat unaccounted 

 for is, then, a waste or loss due to causes which have to be 

 investigated. Hirn originated a complete method of analysis 

 of an engine test, showing at every stage of the operation the 

 lieat accounted for and a balance of heat unaccounted for; and 

 he latter proved to be a very considerable quantity. 



Meanwhile theoretical writers, especially Rankine and Clau- 

 dius, had been perfecting a thermodynamic theory of the steam- 

 engine, based primarily on the remarkable and irrefragable 

 principle of Carnot. The result of Hirn's analysis was to show 

 that these theories, applied to the actual steam-engine, were 

 liable to lead to errors of 50 or 60 per cent., the single false 

 assumption made being that the interaction between the walls 

 of the cylinder and the steam was an action small enough to be 

 negligeable. 



In this country Mr. Mair Rumley, following Hirn's method, 

 made a series of experiments on actual engines with great care 

 nnd accuracy and completeness. All these experiments demon- 

 strated the fact of a large initial condensation of steam on the 

 walls of the cylinder, alike in jacketed and unjacketed engines. 

 This condensed steam is re-evaporated partially during expan- 

 sion, but mainly during exhaust, and serves as a mere carrier of 

 heat from boiler to condenser, in conditions not permitting its 

 utilization in producing work. 



It became clear from Hirn's experiments, if not from the 

 earlier experiments of Isherwood, that for each engine there is 

 a particular ratio of expansion for which the steam expenditure 

 pei- horse-power is least. Professor Dery has since deduced 

 from them that the practical condition of securing the greatest 

 efficiency is that the steam at release should be nearly dry. In 

 producing that dryness the jacket his an important influence. 

 In spite of much controversy amongst practical engineers about 

 the use of the jacket, it does not appear that any trustworthy 

 experiment has yet been adduced in which there was an actual 

 loss of efficiency due to the jacket. In the older type of com- 

 paratively slow engines it is a rule that the greater the jacket 

 condensation the greater the economy of steam, even when the 

 jacket condensation approaches 20 per cent, of all the steam 

 used. It appears, however, that as the speed of the engine 

 increases, the influence of the jacket diminishes, so that for any 

 engine there is a limit of speed at which the value of the jacket 

 becomes insignificant. 



Among steam-engine experiments directed specially to deter- 

 mine the action of the cylinder walls, those of the late Mr. 

 Willans should be specially mentioned. Mr. Willans' death 

 is to be deplored as a serious loss to the engineering profession. 

 His steam-engine experiments, some of them not yet published, 

 are models of what careful experiments should be. They are 

 graduated experiments designed to indicate the effect of changes 

 in each of the practically variable conditions of working. They 

 bowed a much greater variation of steam consumption (from 46 

 to 18 lbs. per indicated horse-power hour) in different con- 

 ditions of working than, I think, most practical engineers sus- 

 pected, and this has been made more significant in later experi- 

 ments, on engines working with less than full load. The first 

 series showed that in full load trials the compound was superior 

 to the simple engine in practically all the conditions tried, but 

 that the triple was superior to the compound only when certain 

 limits of pressure and speed were passed. 



As early as 1878 Prof. Cotterill had shown that the action of 



NO. I 189, VOL. 46] 



a cylinder wall was essentially equivalent to that of a very thin 

 metallic plate, following the temperature of the steam. The 

 exceedingly rapid dissipation of heat from the surface during 

 exhaust especially being due to the evaporation of a film of 

 water initially condensed on its surface. In permanent rc-^ime the 

 heat received in admission must be equal to that lost after cut 

 off. In certain conditions it appeared that a tendency would 

 arise to accumulate water on the cylinder surfaces, with the 

 effect of increasing in certain cases the energy of heat dissipa- 

 tion. Recently Prof. Cotterill has been able to carry much 

 further the analysis of the complex action of condensation and 

 re-evaporation in the cylinder, and to discriminate in some 

 degree between the action of the metal and the more ambiguous 

 action of the water film. By discarding the less important 

 actions, Prof. Cotterill has found it possible to state a semi- 

 empirical formula for cylinder condensation in certain restricted 

 cases which very closely agrees with experiments on a wide 

 variety of engines. It is to be hoped that, with the data now 

 accumulating, a considerable practical advance may be made in 

 the clearing up of this complex subject. There are, no doubt, 

 some people who are in the habit of depreciating quantitative 

 investigations of this kind. They are as wise as if they recom- 

 mended a manufacturer to carry on his business without attend- 

 ing to his account books. Further, the attempt to obtain any 

 clear guidance from experiments on steam-engines has proved a 

 hopeless failure without help from the most careful scientific 

 analysis. There is not a fundamental practical question about 

 the thermal action of the steam-engine, neither the action of 

 jackets or of expansion or of multiple cylinders, as to which 

 contradictory results have not been arrived at, by persons 

 attempting to deduce results from the mass of engine tests with- 

 out any clear scientific knowledge of the conditions which have 

 affected particular results. In complex questions fundamental 

 principles are essential in disentangling the results. Inter- 

 preted by what is already known of thermodynamic actions, 

 there are very few trustworthy engine tests which do not fall into 

 a perfectly intelligible order. There is only one known 

 method, not now much used, by which the cylinder condensation 

 can be directly combated. Thirty years ago superheating the 

 steam was adopted with very considerable increase of economy. 

 It is likely that it was thought by the inventor of superheating 

 that an advantage would be gained by increasing the tempera- 

 ture range. If so, his theory was probably a mistaken one. For 

 th° cooling action of the cylinder is so great that the steam is 

 reduced to saturation temperature before it has time to do work ; 

 but the economy due to superheating was unquestionable, and 

 was very remarkable considering how small a quantity of heat 

 is involved in superheating. The heat appears to diminish the 

 cylinder wall action so much as almost to render a jacket un- 

 necessary. The plan of superheating was abandoned from 

 purely practical objections, the superheaters then constructed 

 being dangerous. Recently superheating has been tried again 

 at Mulhouse by M. Meunier, and his experiments are interesting 

 because they are at hiifher pressures than in the older trials and 

 with a compound engine. It appears that even when the super- 

 heater was heated by a separate fire there was an economy of 

 steam of 25 to 30 per cent, and an economy of fuel of 20 to 25 

 per cent., and four boilers with superheating were as efficient as 

 five without it. 



It may be pointed out as a point of some practical importance 

 that if a trustworthy method of superheating could be found, the 

 advantage of the triple over the compound engine would be 

 much diminished. For marine purposes the triple engine is 

 perfectly adapted. But for other purposes it is more costly than 

 the compound engine, and it is less easily arranged to work 

 efficiently with a varying load. 



There does not seem much prospect of exceeding the efficiency 

 attained already in the best engines, though but few engines are 

 really as efficient as they might be, and there are still plenty of 

 engines so designed that they are exceedingly uneconomical. 

 The very be>t engines use only from 12 to 13 lbs. of steam per 

 indicated horse-power hour, having an absolute efficiency 

 reckoned on the indicated power of 16 per cent., or reckoned on 

 the effective power, 13 per cent. The efficiency, including the 

 loss in I he boiler, is only about 9 per cent. But there are in- 

 ternal furnace engines of the gas-engine or oilengine type in 

 which the thermal efficiency is double this. 



In his interesting address to this Section in 1878, Mr. Easton 

 expressed the opinion that the question of water-power was one 

 deserving more consideration than it had lately received, and he 



