TABLE A. Details of the Principal Engines Available in 1918 for Service.
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Country
Engine
Type
H.P.
Weight
R.P.M.
Wt. per H.P.
Great Britain
Beardmore
6 cyl. W.C.
170
592
1350
4-85
Green
6 ' W.C
170
585
1350
3-99
"
12 ' Vee W.C
300
990
1300
3-85
Rolls Royce Eagle
12 ' Vee W.C. .
360
947
1800
3'i8
Falcon . . .
12 Vee W.C
275
715
2000
3-15
" Napier Lion .
12 ' broad-arrow
456
850
1925
2-41
Sunbeam Arab ....
8 ' Vee W.C
220
524
2IOO
2-93
Maori ....
12 " Vee W.C
280
720
2IOO
3-32
Siddeley Puma . . . .
6 " W.C.
290
635
1650
2-74
B.H.P
6 " W.C
254
604
I4OO
2-93
R.A.F
12 " VeeA.C
1 60
639
I8OO
4-00
B.R.I. . . . ' .
9 " Rotary A.C. .
150
410
1250
2-78
B.R.2 A.B.C. Dragonfly
9 ' Rotary A.C. . 9 ' Radial A.C.
224 294
496 651
1200 1650
2-21 2-22
Cosmos Mercury
14 " Radial A.C.
315
582
I8OO
I-8 4
France .
Hispano Suiza ....
8 cyl. Vee W.C
217
484
2OOO
2- 7 8
" 11
8 " Vee W.C
315
558
2OOO
2-33
Renault
12 " Vee W.C
245
924
1300
4-32
Lorraine Dietrich
8 " Vee W.C
215
834
1450
3-4
Canton Unne ....
9 " Radial W.C. .
255
840
1300
4-13
Anzani
10 ' Radial A.C. . .
125
522
I2OO
4-17
Le Rhone
9 ' Rotary A.C.
130
330
1250
2-54
Clerget
9 ' Rotary ....
125
37
1250
2-96
Mono-Gnome ....
9 ' Rotary .
105
260
I2OO
2-48
it "
9 ' Rotary ....
154
313
1300
2-03
Italy .
Fiat
6 cyl. Vertical W.C. .
317
910
I6OO
3-42
"
12 " Vee W.C
400
805
226O
2-57
Isotta Fraschini ....
6 " Vertical W.C. .
190
574 .
1450
3-57
14 11
6 " Vertical W.C. .
300
662
1650
2-76
Tosi
12 " Vee W.C
410
904
I6OO
2-76
Spa
6 " Vertical W.C. .
230
507
I6OO
2-76
Anzani
12 ' Radial A.C.
too
386
1320
3-86
Germany
Austro Daimler ....
6 cyl. Vertical W.C. .
200
728
I4OO
4-19
Benz
6 " Vertical W.C. .
163
592
I2OO
4-19
"
6 " Vertical W.C. .
235
846
1400
4-17
Maybach
6 " Vertical W.C. .
200
it
6 " Vertical W.C. .
300
891
1400
3-52
Mercedes
6 " Vertical W.C. .
164
618
I4OO
4-3i
6 " Vertical W.C. .
252
93
I40O
4-36
(2) A greater uniformity of temperature throughout the cylinder, and therefore less tendency to distortion.
(3) Generally, greater reliability and higher efficiency.
These advantages could justly be claimed over the earlier types of air-cooled engines ; to-day they are less clear. Thus the first claim is only justified where great attention is paid to the design and arrangement of the jackets and circulating systems. Measurements confirm claim (2), but also show that the lack of uniformity is not necessarily a serious matter, while troubles from overheated exhaust valves have recently been more prevalent on water-cooled than on the modern air-cooled type.
For the air-cooled engine is claimed :
(1) Smaller weight per H.P. of the complete power unit.
(2) No danger of water freezing on gliding from great heights, or when standing.
(3) Reduced vulnerability in war service and easier installation.
(4) Special adaptability for use under widely differing atmospheric temperatures, and for the tropics.
(5) Better adaptability for application of some supercharging device to give constant power at heights.
Claim (l) is a matter of demonstration, the usual weight allow- ance for water-cooling being 0-6 Ib. per H.P. while the very best is 0-4 Ib. per H.P. Claim (2) is admissible to the extent that if freezing be prevented by the use of some other liquid, such as a mixture of alcohol and water, the alcohol evaporates unless the temperature of the fluid is kept below about 70 C. which increases the radiator size.
There is undoubtedly a future for the air-cooled engine of the fixed-cylinder type up to a certain size of cylinder. What this limit of size may be is not known at present. Cylinders of 8-in. bore by lo-in. stroke developing over too H.P. have been made and proved to be possible, and investigations on cylinders up to 10 in. in diameter are in progress. Twelve 6-inch cylinders would give 600 H.P., a useful size at present, and an 800 or 900 B.H.P. air-cooled engine is certainly feasible.
Design of Air-Cooled Cylinders. The useof aluminium alloyforthe cylinder heads has largely conduced to these results. In a normal design the middle portion of the head is the hottest point because the flow of cooling air and the placing of fins at this point is impeded by the inlet and the exhaust valve ports and valve gear. Most of the heat has to be conducted outwards till dissipated from the periphery of the combustion head, and the aluminium alloy effects this well, both because its conductivity is 3-5 times greater than the steel, and
because being 0-4 of the density of steel it may be used in ample thickness.
Such a cylinder must be of composite construction, since the valve seats and the working surface of the cylinder barrel must be of some harder material than aluminium. The valve seats may consist of rings of steel or of bronze, and these may be either cast or expanded into position. Tests appear to favour a steel barrel with integral cooling fins, screwed into an aluminium head for diameters as large as eight inches.
Arrangement of Cylinders. Aero engines may be subdivided according to the arrangement of their cylinders, into the following types:
(1) Single line engines suitable for water-cooling.
(2) Vee engines suitable for water- or air-cooling.
(3) Broad arrow engines suitable for water-cooling.
(4) Radial engines fixed cylinders ; air-cooling.
(5) Rotary engines suitable for air-cooling.
The general arrangement of these types is shown in fig. 22.
The straight line engine (a), with six or eight cylinders in line, offers a low hea,d resistance and is accessible. On the other hand its fore-and-aft length is large. The crank-shaft and crank-case are long, and hence the type is heavy.
In the Vee type engine (6) two lines of cylinders are used inclined to each other to form a Vee in elevation, and the corresponding port and starboard cylinders operate a common crank-pin. Weight is saved on crank-shaft, case, and valve gear.
In the Broad Arrow (c) three lines of cylinders are used as above with further weight saving.
In the Radial engine (d) economy of crank-shaft and case is carried to its logical conclusion. The cylinders are mounted in one plane at equal angular intervals around the crank-shaft. All the connecting rods operate on a single crank-pin. The small fore-and-aft length of the engine helps the aeroplane designer but its considerable diameter may hamper him.
To obtain explosion impulses at equal intervals throughout each revolution an odd number of cylinders must be used, the usual number being seven or nine. Where a larger power is required two rows of cylinders may be used, operating a two-throw crank-shaft. The radial cylinders may be stationary or rotating. In the latter case the airscrew is mounted on a continuation of the rotating crank- case. The rotating cylinder engine is quite unsuited for water- cooling. Although the radial engine with fixed cylinders is not well
