BY MR. P. W. WILLANS, M.I.C.E._Scientific American Supplement, No. 647, May 26, 1888

The author described a series of economy trials, non-condensing, made

with one of his central valve triple expansion engines, with one

crank, having three cylinders in line. By removing one or both of the

upper pistons, the engine could be easily changed into a compound or

into a simple engine at pleasure. Distinct groups of trials were thus

carried out under conditions very favorable to a satisfactory

comparison of results.

No jackets were used, and no addition had, therefore, to be made to

the figures given for feed water consumption on that account. Most of

the trials were conducted by the author, but check trials were made by

Mr. MacFarlane Gray, Prof. Kennedy, Mr. Druitt Halpin, Professor

Unwin, and Mr. Wilson Hartnell. The work theoretically due from a

given quantity of steam at given pressure, exhausting into the

atmosphere, was first considered.

By a formula deduced from the [theta] [phi] diagram of Mr. MacFarlane

Gray, which agreed in results with the less simple formulas of Rankine

and Clausius, the pound weight of steam of various pressures required

theoretically per indicated horse power were ascertained. (See annexed

table.)

A description was then given of the main series of trials, all at four

hundred revolutions per minute, of the appliances used, and of the

means taken to insure accuracy. A few of the results were embodied in

the table. The missing quantity of feed water at cut off, which, in

the simple trials, rose from 11.7 per cent. at 40 lb. absolute

pressure to nearly 30 per cent. at 110 lb. and at 90 lb. was 24.8 per

cent., was at 90 lb. only 5 per cent. in the compound trials. In the

latter, at 160 lb., it increased to 17 per cent., but, on repeating

the trial with triple expansion, it fell to 5.46 per cent. or to 4.43

per cent. in another trial not included in the table.

On the other hand, from the greater loss in passages, etc., the

compound engine must always give a smaller diagram, considered with

reference to the steam present at cut-off, than a simple engine, and a

triple a smaller diagram than a compound engine. Nevertheless, even at

80 lb. absolute pressure, the compound engine had considerable

advantage, not only from lessened initial condensation, but from

smaller loss from clearances, and from reducing both the amount of

leakage and the loss resulting from it. These gains became more

apparent with increasing wear. The greater surface in a compound

engine had not the injurious effect sometimes attributed to it, and

the author showed how much less the theoretical diagram was reduced by

the two small areas taken out of it in a compound engine than by the

single large area abstracted in a simple engine. The trials completely

confirmed the view that the compound engine owed its superiority to

reduced range of temperature. At the unavoidably restricted pressures

of the triple trials, the losses due to the new set of passages, etc.,

almost neutralized the saving in initial condensation, but with

increased pressure--say to 200 lb. absolute--there would evidently be

considerable economy. The figures of these trials showed that the loss

of pressure due to passages was far greater with high than with low

pressure steam, and that pipes and passages should be proportioned

with reference to the weight of steam passing, and not for a

particular velocity merely.

The author described a series of calorimetric tests upon a large scale

(usually with over two tons of water), the results of which were

stated to be very consistent. After comparing the dates of initial

condensation in cases where the density of steam, the area of exposed

surface, and the range of temperature were all variables, with other

cases (1) where the density was constant and (2) where the surface was

constant, the author concluded that, at four hundred revolutions per

minute, the amount of initial condensation depended chiefly on the

range of temperature in the cylinder, and not upon the density of the

steam or upon the extent of surface, and that its cause was probably

the alternate heating and cooling of a small body of water retained in

the cylinder. The effect of water, intentionally introduced into the

air cushion cylinder, corroborated the author's views, and he showed

how small a quantity of water retained in the cylinder would account

for the effects observed. At lower speeds surface might have more

influence. The favorable economical effect of high rotative speed,

_per se_, was very apparent.

In a trial with a compound engine, with 130 lb. absolute pressure, the

missing quantity at cut-off rose from 11.7 per cent. at 405

revolutions to 29.66 per cent. at 130 revolutions, the consumption of

feed water increasing from 20.35 lb. to 23.67 lb. This saving of 14

per cent. was due solely to increase of speed. Similar trials had been

made with a simple engine. In one simple trial at slow speed the

missing quantity rose to 44.5 per cent. of the whole feed water.

------------------------------+-----+------------+-------------+------+-------------+-------------+------

admission pressure Lb.| 40 | 90 | 110 | 130 | 150 | 160 | 170

------------------------------+-----+------+-----+------+------+------+------+------+------+------+------

Simple, Compound, or Triple.| S. | S. | C. | S. | C. | C. | C. | T. | C. | T. | T.

admission pressure Lb.|40.88| 92.65|87.54|106.3 |109.3 |130.6 |149.9 |151.9 |158.5 |158.1 |172.5

theoretical mean pressure |98.2 |100 |91.3 |100.7 | 94.8 | 94.2 | 94.6 | 84.54| 95.9 | 85.3 | 85.2

Indicated horse power |16.51| 31.61|28.14| 33.5 | 33 | 36.31| 38.59| 35.69| 39.55| 35.56| 38.45

------------------------------+-----+------+-----+------+------+------+------+------+------+------+------

Simple Lb.|42.76| 26.89| ... | 26 | ... | ... | ... | ... | ... | ... | ...

Compound Lb.| ... | ... |34.16| ... | 21.37| 20.35| 19.45| ... | 19.19| ... | ...

Triple Lb.| ... | ... | ... | ... | ... | ... | ... | 19.68| ... | 19.19| 18.45

per 1 H.P.H. Lb.|34.67| 19.24|19.86| 17.9 | 17.65| 16.25| 15.23| 15.16| 14.87| 14.9 | 14.36

Percentage efficiency |81.1 | 71.5 |82.2 | 68.8 | 82.5 | 80 | 78.3 | 77 | 77.4 | 77.6 | 77.8

------------------------------+-----+------+-----+------+------+------+------+------+------+------+------

high pressure cylinder | ... | ... | ... | ... | ... | ... | ... | 5.33| ... | 6.84| 5.01

Ditto high pressure cylinder | ... | ... | 5 | ... | 9.5 | 11.7 | 15.1 | 14.84| 17 | 12.06| 15.33

Ditto low pressure cylinder |11.7 | 24.8 |15.2 | 29.56| 16.25| 19.1 | 20.6 | 22.12| 21.3 | 22.11| 24.21

in low pressure cylinder |10.4 | 18.83|14.25| 21.53| 16.59| 17.55| 20.69| 18.01| 19.55| 18.81| 19.25

------------------------------+-----+------+-----+------+------+------+------+------+------+------+------

The author compared a series of compound trials, at different powers,

with 130 lb. absolute pressure, and various ratios of expansion, with

a series giving approximately the same powers at a constant ratio of

expansion, but with varying pressures, being practically a trial of

automatic expansion against throttling. Starting with 40 indicated

horse power, 130 lb. absolute pressure, four expansions, and a

consumption of 20.75 lb. of water, the plan of varying the expansion,

as compared with throttling, showed a gain of about 7 per cent. at 30

indicated horse power, but of a very small percentage when below half

power. If the engine had an ordinary slide valve, the greater

friction, added to irregular motion, would probably neutralize the

saving, while if the engine were one in which initial condensation

assumed more usual proportions, the gain would be probably on the side

of variable pressure. Even as it was, the diagrams showed that the

missing quantity became enormously large as the expansion increased.

Judging only by the feed water accounted for by the indicator, the

automatic engine appeared greatly the more economical, but actual

measurement of the feed water disproved this. The position of the

automatic engine was, however, relatively more favorable when simple

than when compound.

In conclusion, the author referred to a trial with a condensing

engine, at 170 lb. absolute pressure, in which the feed water used was

15.1 lb., a result evidently capable of further improvement, and to an

efficiency trial of a combined central valve engine and Siemens'

dynamo, made for the Admiralty, at various powers. At the highest

power the ratio of external electrical horse power to indicated horse

power in the engine was 82.3 per cent. Taking the thermo-dynamic

efficiency of the engine at 80 per cent., that of the combined

apparatus would be nearly 66 per cent.

* * * * *