of Aimé Witz, Professor in the Faculty of Sciences of Lille, in those of Dugald Clerk, of London, F. Grover, of Leeds, and Otto Güldner, of Munich, and in those of the American authors, Goldingham, Hiscox, Hutton, Parsell and Weed, etc. The new tendencies in the construction of large engines may be regarded as an interesting verification of the forecasts of these men—forecasts which coincide with the opinion long held by the author. Aimé Witz has always been an advocate of high pressures and of increased piston speed. English builders who made experiments in this direction conceded the beneficial results obtained; but while they increased the original pressure of 28 to 43 pounds per square inch employed five or six years ago to the pressure of 85 to 100 pounds per square inch nowadays advocated, the Germans, for the most part, have adopted, at least in producer-gas engines, pressures of 114 to 170 pounds per square inch and more.
High Compression.—In actual practice, the problem of high pressures is apparently very difficult of solution, and many of the best firms still seem to cling to old ideas. The reason for their course is, perhaps, to be found in the fact that certain experiments which they made in raising the pressures resulted in discouraging accidents. The explosion-chambers became overheated; valves were distorted; and premature ignition occurred. Because the principle underlying high pressures was improperly applied, the results obtained were poor.
High pressures cannot be used with impunity in cylinders not especially designed for their employment, and this is the case with most engines of the older type, among which may be included most engines of English, French, and particularly of American construction. In American engines notably, the explosion-chamber, the cylinder and its jacket, are generally cast in one piece, so that it is very difficult to allow for the free expansion of certain members with the high and unequal temperatures to which they are subjected (Fig. 22).
Some builders have attempted to use high pressures without concerning themselves in the least with a modification of the explosive mixture. The result has been that, owing to the richness of the mixture, the explosive pressure was increased to a point far beyond that for which the parts were designed. Sudden starts and stops in operation, overheating of the parts, and even breaking of crank-shafts, were the results. The engines had gained somewhat in power, but no progress had been made in economy of consumption, although this was the very purpose of increasing the compression.
High pressures render it possible to employ poor mixtures and still insure ignition. A quality of street-gas, for example, which yields one horse-power per hour with 17.5 cubic feet and a mixture of 1 part gas and 8 of air compressed to 78 pounds per square inch, will give the same power as 14 cubic feet of the same gas mixed with 12 parts of air and compressed to 171 pounds per square inch.
"Scavenging" of the cylinder, a practice which engineers
of modern ideas seem to consider of much importance, is better effected with high pressures, for the simple reason that the explosion-chamber, at the end of the return stroke, contains considerably less burnt gases when its volume is smaller in proportion to that of the cylinder.
In impoverishing the mixture to meet the needs of high pressures, the explosive power is not increased and in practice hardly exceeds 365 to 427 pounds per square inch. With the higher pressures thus obtained there is consequently no reason for subjecting the moving parts to greater forces.
