FRICTION OF ENGINES.—PrOf. R. H. Thurston, in papers read before the American Society of Mechanical Engineers (Trans., vols. viii, ix, x). has called attention to the fact that the variation of load in steam-engines is not productive either of the method or of the amount of engine-friction that has been commonly assumed by earlier authorities on that subject.
Later experiments by Prof. R. C. Carpenter and Mr. G. B. Preston, of Sibley College, lead to the conclusions, as stated by Prof. Thurston. that the most important item of friction waste, in every instance, is that of lost energy at the main bearings. In every case it amounts to one third or one half of all the friction resistance of the engine, or from about 5 to 10 per cent of the whole power of the engine in the cases examined, the higher figures being given by the condensing, the lower by the non-condensing engines, except that the first experiment, with the straight-line engine, gives as high a figure as the condensing engines—a fact due, however, rather to the exceptionally low total than to exceptionally high friction on the main shaft. The second highest item is, in all cases apparently, the friction of piston and rod, the rubbing of rings and the friction of the rod-packing. This is a very irregular item, and amounts to from a mininnun of 20 per cent to some higher but undetermined quantity. The third item in order of importance is the friction of valve, in the case of the engines having unbalanced valves. This is seen to be hardly a less serious amount than the frictions of shaft and of piston. But it is further seen at once that this is an item which may be reduced to a very small amount by good design, as is evidenced by the fact that, in the straight-line engine, it has been brought from 20 to 2.5 per cent by skillful balancing. Ninety per cent, therefore, of the friction of the unbalanced valve is avoidable or remediable. The importance,
of this fact is readily perceived when it is considered that not only is it a serious direction of lost work and wasted power and fuel, hut that the ease of working of the valve is a matter of supreme importance to the effective operation of the governing mechanisth in this class of engines. No automatic engine can govern satisfactorily when the valve is unbalanced, and is certain to throw much load on the governor. The frictions of crank-pin, of cross head, and of eccentrics. are the minor items of this account they are comparatively unim portant.
Cylinder Condensation in- Stationary Engines—Single-Cylinder.—Mr, G. H. Burros gives the following figures representing the proportion of feed-water which, with tight valves and piston, will be accounted for by the indicator at different cut-offs, for factory-engines as com monly used with unjacketed cylinders exceeding 20 in. in diameter, supplied with dry but not superheated steam. In many cases, however, leakage through the valves or by the piston in creases materially the percentage of waste; so that if the wastes in this table are exceeded it can be inferred at once, unless the engine speed is extremely low, that the excess is due to this cause: In ordinary practice there is a rough rule, agreeing nearly with that above, applicable be tween 21 and 5 expansions, slight leakage only it may be stated thus: The total amount of loss due to condensation per stroke is a constant amount equal to' 5 per cent of the feed-water employed at quarter cut-off.
See also a paper by Major T. English. 11. E.. in Proc. Inst. 31 E., September, 1887. Prof. R. H. Thurston, in his Manual of the S'tearn-Engine, compares the statements of different authorities on this subject.