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tion, is taken to be the temperature. The temperature of the residual exhaust gases cannot easily be determined directly with any degree of accuracy. The methods used by Professor Thurston and Dr. Slaby will hardly bear criticism ; it may

be useful to know in an engine of a certain size that a pot of zinc placed with due precautions in the exhaust pipe will melt, whereas a pot of antimony will not; but this has only a remote connection with the temperature of the residuals remaining in the compression space.

In order to determine the temperature of the exhaust gases during the return stroke in these experiments the exhaust pipe was entirely removed, and diagrams were taken with a carefully adjusted indicator to a scale of 1 lb.=\, inch, showing the exhaust back pressure both with full load and running light, precautions being taken to insure constancy of speed. Fig. 2 shows one of the diagrams so obtained, in which a is the exhausting curve at full load, and b is the curve obtained when running light. The greatest back pressure observed is about 2 lb. above the atmosphere, consequently we may assume that the density of the gases is proportional to their back pressure without making any serious error. Now the temperature of the no load gases-air--is known to be about 50° C.= 323° absolute, then the temperature of the exhaust gases absolute

323 1 will be the temperature varying inversely as the density. Taking the mean of four diagrams, we get 1432° absolute =,1159° C. This cannot be far from the truth, and it is about what would be expected when it is considered that the expansion curve towards the end of the working stroke becomes practically isothermal, which means that heat is being supplied by after combustion more rapidly than it is being lost through the cylinder walls; it seems improbable, therefore, that the temperature would fall very substantially during the discharging stroke.

The actual volume of the charge at atmospheric pressure is somewhat less than the total contents of the cylinder, and is given by the point of intersection of the compression curve and atmospheric line on the charging diagram-see fig. 2. The volume of residuals remaining in the cylinder is slightly greater

a

than that of the compression space. Taking the volume of charge at atmospheric pressure as 100 units, then that of compression space = 32:6 and volume of residues unexpelled = 33-3. Now the volume of the whole of the charge at atmospheric

50 by 100 pressure and exhaust temperature would equal

14.7

33:3 340, or the proportion of residuals in the charge = 9

340 per cent. The composition of the charge at full load and no load is given on tables I. and II.

Fig. 207 shows the temperature of the various points of the cycle plotted as curves, the full load curve showing a higher temperature at all points of the cycle, owing to the initial admixture of the hot residues to the charge. The dotted line C is the temperature curve for a true adiabatic, and the position of the actual curve above or below this shows the balance or deficit of heat due to after combustion and loss through the cylinder walls ; during the first part of the stroke the latter predominating, and vice versâ. These curves are calculated on the assumption that the mixture behaves as a perfect gas both before and after ignition, and that no change of mean molecular weight takes place during combustion. Some measurable inaccuracy is thus doubtless introduced, but it is usual to make these assumptions to avoid complex calculations. Table III. gives the temperature at the various points in the cycle.

The theoretical efficiency of an Otto engine of these proportions is 36 per cent., the difference between this and the 24 per cent. actually obtained being mostly due to the loss of heat through the water jacket. Table V. shows the distribution of energy utilised and lost.

TABLE I

Composition of Charge

Cub, in.

Per cent.

No Load
Gas
Air

203:3
2962:7

6.42 93.58

100.00

Temperature Curve
of Expansion

load
6 - full load.

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Temperature Curves

Compression.
16

a. no load.

b- full load.
-OC

Faint Lines, shew actual diagrams
Thick Line shews curve from which
temperatures are calculated.

Q- no load
20
40 50 60 70 80

100 Vols.per cent

b. full load. 3166 çu.in.

SWAIN ENS
FIG.207.- - INDICATOR DIAGRAMS AND ESTIMATE TEMPERATURES

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Scale of Temperature

Centigrade 100 200 300 400 500 600 700 800 900 1000 1100 1200-1300* 1400* 1500 1600 1700*1800* 1900 2000 2100

0°C

273 Absolute Zdro

"The ENGINEER

TABLE II

Per cent.

8.58 Gas.

Composition of Charge
Full load
Cub. in.

Cub. in.
Gas.
203:3

203.3 Air for combus.

tion of above 4 1236.0
gas

2006.95 Excess of air 659.7

Air

75.25 Residues from 231.0

Products previous charge consist

of com- 155.75 155.75 98 per cent.

ing of

bustn.&c)

84.84 Total air.

.

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Summary of Brake and Indicator Tests at City of Birmingham

Corporation Gasworks, Saltley

Brake circumference 17 ft. exactly.
Cylinder diameter 12 in., stroke 193 in.
December 22.

January 8.
1
2
3

4 Duration of test

i hour å hour

1 hour lh. 2m. Mean revolutions per minute

154:7 156 207.25 170 Mean nett load .

232.5

248 278.75 261:3 Brake horse-power

18.5
19.9 29.8

22.95 Mean explosions per minute

64.3

64
96:1

79.1 Mean pressure of cards

67

67
67

60•3 Indicated horse-power

24.4
24:3
36.6

27.15 Mechanical efficiency

•76
.82
•814

.84 Gas consumed, in cubic foot per hour (ignition excluded)

405-5 409.5 648.75 481.25 Gas per B.H.P. hour (ignition excluded)

21.9
20.6 21:3

21.05 Gas per I.H.P. hour (ignition excluded)

16.6
16.85 17.7

17.75

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The above results, obtained without any attempt atóscavenging,' are exceedingly good.

To measure the temperature of the exhaust gases, the composition of the charge, the indicator introduced by Mr. Lanchester is a new departure.

CHAPTER XXXVII

PRIVATE ELECTRIC LIGHTING

WITHIN the past few years electricity has forced its way very rapidly and become almost general for lighting purposes. The dynamo is the machine used to generate electricity; and amongst the various forms of prime movers the gas engine is taking an important place after the steam engine.

Circumstances invariably decide which of the various forms of gas engine is the best to adopt; but as at times the question of comparative cost of production and distribution, both initial and continuous, has to be taken into consideration, the author

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