NOTES.-A, B, C, D, tandem engines at electrical stations A, Frankfort a/M.; B, Zurich; C. Mannheim; D, Mayence. E. F. tandem engine with intermediate superheater: E, Metallwarenfabrik. Geislingen, Würtemberg; F, Neue Baumwoll-Spinnerei, Hof, Bavaria. G, H, engines at electrical stations, Berlin G, Moabit station, horizontal 4-cyl.; H, Louisenstrasse, 4-cyl. vertical.

A 1500 30.5 and

to 49.2X59.1 1800

85 130 356 26.4 850 13.3 14.90 21.30 0.895 0.851 132 428 26.4 842 12.05 13.52 19.48 0.891 0.842 122 482 26.6 1719 12.42 13.24 18.72 0.939 0.903

B 1050 26.8 and 100 108 455 26.8 1167 13.10 13.77 19.72 0.951 0.904 to 43.3X51.2

1250

C 800 24 and
to 40.4X51.2
1000

83 136 357 28
134 356 28

D 950 26 and
to 42.3X51.2
1150

481 13.00 14.68 21.30 0.886 0.830 750 13.10 14.14 20.35 0.926 0.877 135 356 27.6 1078 14.10 14.95 21.30 0.932 0.892 135 547 28 515 11.32 12.70 18.69 0.894 0.824 132 533 27.8 788 11.52 12.38 17.90 0.931 0.875 134 545 27.2 1100 11.88 12.50 17.92 0.951 0.902

86 130358 28.2 1076 14.10 14.82 21.25 0.951 0.902 129 358 28 1316 14.50 15.10 21.55 0.960 0.915 132 496 28.31071 11.73 12.33 17.70 0.951 0.903 1021 15.37 16.30 23.40 0.943 0.893

do., non-cond'g 136 527

Relative Economy of Compound Non-condensing Engines under Variable Loads.-F. M. Rites, in a paper on the Steam Distribution in a Form of Single-acting Engine (Trans. A. S. M. E., xiii. 537). discusses an engine designed to meet the following problem : Given an extreme range of conditions as to load or steam-pressure, either or both, to fluctuate together or apart, violently or with easy gradations, to construct an engine whose economical performance should be as good as though the engine were specially designed for a momentary condition-the adjustment to be complete and automatic. In the ordinary non-condensing compound engine with light loads the high-pressure cylinder is frequently forced to supply all the power and in addition drag along with it the low-pressure piston, whose cylinder indicates negative work. Mr. Rites shows the peculiar value of a receiver of predetermined volume which acts as a clearance chamber for compression in the high-pressure cylinder. The Westinghouse compound single-acting engine is designed upon this principle. The following results of tests of one of these engines rated at 175 H.P. for most economical load are given :

WATER RATES UNDER VARYING LOADS, LBS. PER H.P. PER HOUR.

Horse-power.
Non-condensing.
Condensing

210 170 140 115
22.6 21.9 22.2 22.2
18.4 18.1 18.2 18.2

100 80 50 22.4 24.6 28.8 18.3 18.3 20.4

Efficiency of Non-condensing Compound Engines. (W. Lee Church, Am. Mach., Nov. 19, 1891.)-The compound engine, non-condensing, at its best performance will exhaust from the low-pressure cylin der at a pressure 2 to 6 pounds above atmosphere. Such an engine will be limited in its economy to a very short range of power, for the reason that its valve-motion will not permit of any great increase beyond its rated power, and any material decrease below its rated power at once brings the expansion curve in the low-pressure cylinder below atmosphere. In other words, decrease of load tells upon the compound engine somewhat sooner, and much more severely, than upon the non-compound engine. The loss commences the moment the expansion line crosses a line parallel to the atmospheric line, and at a distance above it representing the mean effective pressure necessary to carry the frictional load of the engine. When expansion falls to this point the low-pressure cylinder becomes an air-pump over more or less of its stroke, the power to drive which must come from the high-pressure cylinder alone. Under the light loads common in many industries the low-pressure cylinder is thus a positive resistance for the greater portion of its stroke. A careful study of this problem revealed the functions of a fixed intermediate clearance, always in communication with the high-pressure cylinder, and having a volume bearing the same ratio to that of the high-pressure cylinder that the high-pressure cylinder bears to the low-pressure. Engines laid down on these lines have fully confirmed the judgment of the designers.

The effect of this constant clearance is to supply sufficient steam to the low-pressure cylinder under light loads to hold its expansion curve up to atmosphere, and at the same time leave a sufficient clearance volume in the high-pressure cylinder to permit of governing the engine on its compression under light loads.

Economy of Engines under Varying Loads. (From Prof. W. C. Unwin's lecture before the Society of Arts, London, 1892.)-The general result of numerous trials with large engines was that with a constant load an indicated horse-power should be obtained with a consumption of 11⁄2 pounds of coal per indicated horse-power for a condensing engine, and 134 pounds for a non-condensing engine, figures which correspond to about 134 pounds to 2% pounds of coal per effective horse-power. It was much more difficult to ascertain the consumption of coal in ordinary every-day work, but such facts as were known showed it was more than on trial.

In electric-lighting stations the engines work under a very fluctuating load, and the results are far more unfavorable. An excellent Willans noncondensing engine, which on full-load trials worked with under 2 pounds per effective horse-power hour, in the ordinary daily working of the station used 71⁄2 pounds per effective H.P. hour in 1886, which was reduced to 4.3 pounds in 1890 and 3.8 pounds in 189!. Probably in very few cases were the engines at electric-light stations working under a consumption of 41⁄2 pounds per effective H.P. hour. In the case of small isolated motors working with a fluctuating load, still more extravagant results were obtained.

At electric-lighting stations the load factor, viz., the ratio of the average load to the maximum, is extremely small, and the engines worked under very unfavorable conditions, which largely accounted for the excessive fuel consumption at these stations.

In steam-engines the fuel consumption has generally been reckoned on the indicated horse-power. At full-power trials this was satisfactory enough, as the internal friction is then usually a small fraction of the total. Experiment has, however, shown that the internal friction is nearly constant, and hence, when the engine is lightly loaded, its mechanical efficiency is greatly reduced. At full load small engines have a mechanical efficiency of 0.8 to 0.85, and large engines might reach at least 0.9, but if the internal friction remained constant this efficiency would be much reduced at low powers. Thus, if an engine working at 100 indicated horse power had an efficiency of 0.85, then when the indicated horse-power fell to 50 the effective horse-power would be 35 horse-power and the efficiency only 0.7. Similarly, at 25 horse-power the effective horse-power would be 10 and the efficiency

0.4.

Experiments on a Corliss engine at Creusot gave the following results: Effective power at full load. 1.0 0.75 0.50 0.25 0.125 Condensing, mechanical efficiency.. 0.82 0.79 0.74 0.63 0.48 Non condensing, " 0.86 0.83 0.78 0.67 0.52

66

At light loads the economy of gas and liquid fuel engines fell off even more rapidly than in steam-engines. The engine friction was large and nearly constant, and in some cases the combustion was also less perfect at light loads. At the Dresden Central Station the gas-engines were kept working at nearly their full power by the use of storage-batteries. The results of some experiments are given

Brake-load, per cent of full Power.

100

75

59

20

1212

Gas-engine, cu. ft.

of Gas per Brake

H.P. per hour.

22.2

23.8

28.0

40.8

66.3

below:
Petroleum Eng.,
Lbs.of Oil per
B.H.P. per hr.

0.96

1.11

1.44

2.38

4.25

Steam Consumption of Engines of Various Sizes.-W. C. Unwin (Cassier's Magazine, 1894) gives a table showing results of 49 tests of engines of different types. In non-condensing simple engines, the steam consumption ranged from 65 lbs. per hour in a 5-horse-power engine to 22 lbs. in a 134-H.P. Harris-Corliss engine. In non-condensing compound engines, the only type tested was the Willans, which ranged from 27 lbs. in a 10 H.P. slow-speed engine, 122 ft. per minute, with steam-pressure of 84 lbs. to 19.2 lbs. in a 40-H.P. engine, 401 ft. per minute, with steam-pressure 165 lbs. A Willans triple-expansion non-condensing engine, 39 H.P., 172 lbs. pressure, and 400 ft. piston speed per minute, gave a consumption of 18.5 lbs. In condensing engines, nine tests of simple engines gave results ranging only from 18.4 to 22 lbs., and, leaving out a beam pumping-engine running at slow speed (240 ft. per minute) and low steam-pressure (45 lbs.), the range is only from 18.4 to 19.8 lbs. In compound-condensing engines over 100 H.P., in 13 tests the range is from 13.9 to 20 lbs. In three triple-expansion engines the figures are 11.7, 12.2, and 12.45 lbs., the lowest being a Sulzer engine of 360 H.P. In marine compound engines, the Fusiyama and Colchester, tested by Prof. Kennedy, gave steam consumption of 21.2 and 21.7 lbs.; and the Meteor and Tartar triple-expansion engines gave 15.0 and 19.8 lbs.

Taking the most favorable results which can be regarded as not exceptional, it appears that in test trials, with constant and full load, the expenditure of steam and coal is about as follows:

These may be regarded as minimum values, rarely surpassed by the most efficient machinery, and only reached with very good machinery in the favorable conditions of a test trial.

Small Engines and Engines with Fluctuating Loads are usually very wasteful of fuel. The following figures, illustrating their low economy, are given by Prof. Unwin, Cassier's Magazine, 1894.

COAL CONSUMPTION PER INDICATED HORSE-POWER IN SMALL ENGINES. In Workshops in Birmingham, Eng.

It is largely to replace such engines as the above that power will be distributed from central stations.

Steam Consumption in Small Engines.

Tests at Royal Agricultural Society's show at Plymouth, Eng. Engineering, June 27, 1890.

Steam-consumption of Engines

Various Speeds.

(Profs. Denton and Jacobus, Trans. A. S. M. E., x. 722)—17 × 30 in. engine, non-condensing, fixed cut-off, Meyer valve.

STEAM-CONSUMPTION, LBS. PER I.H.P. PER HOUR.

Figures taken from plotted diagram of results. Revs. per min....

8 12 16 20

cut-off, lbs...

66

39 35 32
39 34 31
39 36 34

30

29.3

24 32 40 48 56 72 88 29 28.7 28.5 28.3 28 27.7 28.4

28 27.5 27.1 26.3 25.6 30.8 29.8 29.2 28.8 28.7

STEAM-CONSUMPTION OF SAME ENGINE; FIXED SPEED, 60 REVS. PER MIN. Varying cut-off compared with throttling-engine for same horse-power and boiler-pressures:

Cut-off, fraction of stroke 0.1 0.15 0.2 0.25 0.3 0.4 0.5 0.6 0.7 0.8 Boiler-pressure, 90 lbs... 29 27.5 27 27 27.2 27.8 28.5

60 lbs... 39 34.2 32.2 31.5 31.4 31.6 32.2 34.1 36.5 39

THROTTLING-ENGINE, % CUT-OFF, FOR CORRESPONDING HORSE-POWERS. Boiler-pressure, 90 lbs... 42 37 33.8 31.5 29.8 60 lbs... 50.1 49 46.8 44.6 41

....

....

....

Some of the principal conclusions from this series of tests are as follows: 1. There is a distinct gain in economy of steam as the speed increases for 2, %, and 4 cut-off at 90 lbs. pressure. The loss in economy for about 4 cut-off is at the rate of 1/12 lb. of water per H.P. for each decrease of a revolution per minute from 86 to 26 revolutions, and at the rate of 5% lb. of water below 26 revolutions. Also, at all speeds the 4 cut-off is more economical than either the % or cut-off.

2. At 90 lbs. boiler-pressure and above cut-off, to produce a given H.P. requires about 20% less steam than to cut off at % stroke and regulate by the throttle.

3. For the same conditions with 60 lbs. boiler-pressure, to obtain, by throttling, the same mean effective pressure at % cut-off that is obtained by

cutting off about, requires about 30% more steam than for the latter condition.

High Piston-speed in Engines. (Proc. Inst. M. E., July, 1883, p. 321). The torpedo boat is an excellent example of the advance towards high speeds, and shows what can be accomplished by studying lightness and strength in combination. In running at 22% knots an hour, an engine with cylinders of 16 in. stroke will make 480 revolutions per minute, which gives 1280 ft. per minute for piston-speed; and it is remarked that engines running at that high rate work much more smoothly than at lower speeds, and that the difficulty of lubrication diminishes as the speed increases.

A High-speed Corliss Engine.-A Corliss engine, 20 × 42 in., has been running a wire-rod mill at the Trenton Iron Co.'s works since 1877, at 160 revolutions or 1120 ft. piston-speed per minute (Trans. A. S. M. E., ii. 72). A piston-speed of 1200 ft. per min. has been realized In locomotive practice.

The Limitation of Engine-speed. (Chas. T. Porter, in a paper on the Limitation of Engine-speed, Trans. A. S. M. E., xiv. 806.)-The practical limitation to high rotative speed in stationary reciprocating steam. engines is not found in the danger of heating or of excessive wear, nor, as is generally believed, in the centrifugal force of the fly-wheel, nor in the tendency to knock in the centres, nor in vibration. He gives two objections to very high speeds: First, that "engines ought not to be run as fast as they can be;" second, the large amount of waste room in the port, which is required for proper steam distribution. In the important respect of economy of steam, the high-speed engine has thus far proved a failure. Large gain was looked for from high speed, because the loss by condensation on a given surface would be divided into a greater weight of steam, but this expectation has not been realized. For this unsatisfactory result_we have to lay the blame chiefly on the excessive amount of waste room. The ordinary method of expressing the amount of waste room in the percentage added by it to the total piston displacement, is a misleading one. It should be expressed as the percentage which it adds to the length of steam admission. For example, if the steam is cut off at 1/5 of the stroke, 8% added by the waste room to the total piston displacement means 40% added to the volume of steam admitted. Engines of four, five and six feet stroke may properly be run at from 700 to 800 ft. of piston travel per minute, but for ordinary sizes, says Mr. Porter, 600 ft. per minute should be the limit.

Influence of the Steam-jacket.—Tests of numerous engines with and without steam-jackets show an exceeding diversity of results, ranging all the way from 30% saving down to zero, or even in some cases showing an actual loss. The opinions of engineers at this date (1894) is also as diverse as the results, but there is a tendency towards a general belief that the jacket is not as valuable an appendage to an engine as was formerly supposed. An extensive résumé of facts and opinions on the steam-jacket is given by Prof. Thurston, in Trans. A. S. M. E., xiv. 462. See also Trans. A. S. M. Ë., xiv. 873 and 1340; xiii. 176; xii. 426 and 1340; and Jour. F. I., April, 1891, p. 276. The following are a few statements selected from these papers.

The results of tests reported by the research committee on steam-jackets appointed by the British Institution of Mechanical Engineers in 1886, indicate an increased efficiency due to the use of the steam-jacket of from 1% to over 30%, according to varying circumstances.

Sennett asserts that "it has been abundantly proved that steamjackets are not only advisable but absolutely necessary, in order that high rates of expansion may be efficiently carried out and the greatest possible economy of heat attained."

Isherwood finds the gain by its use, under the conditions of ordinary practice, as a general average, to be about 20% on small and 8% or 9% on large engines, varying through intermediate values with intermediate sizes, it being understood that the jacket has an effective circulation, and that both heads and sides are jacketed.

Professor Unwin considers that "in all cases and on all cylinders the jacket is useful; provided, of course, ordinary, not superheated, steam is used; but the advantages may diminish to an amount not worth the interest on extra cost."

Professor Cotterill says: Experience shows that a steam-jacket is advantageous, but the amount to be gained will vary according to circumstances. In many cases it may be that the advantage is small. Great caution is necessary in drawing conclusions from any special set of experiments on the influence of jacketing.