The Thermal and Combustion Efficiencies of a Four-cylinder Petrol Engine.
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Professor W. Watson, D.Sc., F.R.S., in a paper read before the Incorporated Institution of Automobile Engineers at its May meeting in the lecture theatre of the Institution of Mechanical Engineers, Storey's Gate, Westminster, last night, presented a mass of valuable figures which he had deduced from experiments with a fourcylinder Clement-Talbot engine having a bore and stroke of 85 and 120mm. respectively. For the purpose of indicating the pressures within the cylinders of the engine during the tests the author employed a most interesting manograph which, judging by the clearness of the diagrams that were reproduced, is certainly an improvement on both the Schultz and the Carpentier instruments. Sectional views of the marmgraph are reproduced in the paper, a copy of which should most certainly be obtained by every engineer and designer. By the way, we do not think the value of membership of this inktitution is as widely appreciated amongst designers of internal-combustion engines as it deserves to be. The vast amount of research work which has been undertaken in this direction has been the subject of many highly instructive papers during the past two or three sessions, and the proceedings of the Institution are very valuable works of reference.
Professor Watson made three series of tests, with three ratios of compression, the change from one to another being effected by altering the thickness of a packing piece below the flanges of the cylinders. Each piston was fitted with a light aluminium cap, the presence of which, for the '• A " series of tests, reduced the volumetric size of the combustion chamber below that obtaining when neither packing piece nor piston cap was fitted. .
The most interesting feature about these tests is the fact that the experimenter took careful measurements or the quantity of air consumed, in combination with petrol. within the engine's cylinders. The air-intake pipe to the carburetter was connected to a wooden box of 19 cubic feet capacity, and into this box the air was admitted through a circular hole in a thin metal plate. The author deduced, as a result of his tests, that " the efficiency increases as the mixture gets weaker up to very nearly the weakest mixture on which the engine will run regularly.In this connection he observed that " with mixtures containing between 17.5 and 191b. of air to one pound of petrol firing back into the carburetter was liable to occur, while with mixtures either richer or weaker this effect was not observed." With a ratio of air to petrol of 17 to one the maximum efficiency at all speeds was obtained. It is interesting to note a statement made by the reader of the paper to the effect that " combustion is complete for all air to petrol ratios greater than 14." Referring to our table it will be noted that as a basis of comparison for efficiency the "air standard " has been selected. In his explanation of this arbitrary standard, the author went on to say :—" It must be remembered that by no possibility could we have an efficiency even approaching unity even if we had an engine in which there was no loss of heat owing to conduction to the jacket, radiation, etc. Thus, an engine in which there
were no such thermal losses, and which used air as the working substance, the further supposition being made that the specific heats of the air did not change with temperature, would have a thermal efficiency of 0.46 for a compression ratio of 4.7. The efficiency of such an ideal engine is called the air standard efficiency, and the ratio of the actual efficiency of the engine to the air standard is called the relative efficiency."
If we bear in mind that the Gas Engine Research Committee of the Institution of Civil Engineers, in its investigations, only obtained a relative efficiency of 0.69 for an engine with a 14-inch cylinder, and 0.61 for one having a
cylinder inches in diameter, it must be admitted that the results of Professor Watson's tests are very satisfactory. The two engines tested by this Committee of the Institution of Civil Engineers were run at about 150r.p.m., Sc) it would appear that the loss in efficiency due to the small size of the Clement-Talbot cylinders has been, to a certain extent, offset by the increased efficiency due to the higher speed at which the last-named engine was run, The columns giving the mean effective pressures and the cornpres.sion pressures of the charges are interesting, and shoethat, for a mixture containing 14 parts by weight of air to one part of petrol, the mean effective pressure is 831b. per square inch with the compression at 681b. per square inch, while, even with the compression raised to 861b., the mean effective pressure is only increased by 11b.
We should have liked to have had the figures showing the rate of consumption in lb. of fuel per brake-horsepower (b.h.p.) hour instead of in relation to the indicatedhorse-power (i.h.p.), and, although the author has made calculations for the b.h.p., we are not convinced of their accuracy. The absorption of power by the driving of a fan (as was dons during all the three series of tests) is not adapted to the obtaining of b.h.p. results, although it affords an excellent means of making comparative tests.
The determination of engine efficiency on the results of exhaust gas analysis has long been advocated by the president of the Institution, Mr. Dugald Clerk, I.R.S.: it is interesting, therefore, to have the opportunity, presented by Professor Watson. to compare the results deduced from the composition of the exhaust gases with those obtained by the direct measurement of the quantity of air consumed. The ratio of the quantity of air to petrol, as obtained by the analysis of the exhaust gases, should agree with the values obtained by the method of direct measurement; this, however, is not the case.
As a result of his experiments, the author concludes " that it is not safe to deduce the composition of the mixture supplied to a petrol engine from the results of the analysis of the gases made in the ordinary manner," but, by the aid of certain curves which he gives, he does think that " the proportion of air to petrol can be deduced with sufficient accuracy for all practical purposes, and he hopes that the results will be of service to those who have to test carburetters and engines, as. in many cases, it is more convenient to analyse the exhaust gases than to measure the air and petrol."