Still the car engine of given cylinder capacity remained appreciably heavier than the contemporary rotary, until care- ful studies in 1916-17-18 were made to increase the output per unit of cylinder volume, and the thermal efficiency.
The volumetric efficiency was increased by improving the design of the inlet pipes, valves, and valve gearing, and the combustion space of the cylinder. The thermal efficiency and the mean effective pressure were increased by augmenting the compression. Since high compression is only practicable with a compact and symmetrical combustion chamber the L-headed cylinder was replaced by the overhead valve-cylinder. More- over, since high compression necessitates good cooling of the cylinder, the water-cooled engine gained a distinct relative ad- vantage over the earlier air-cooled engines which were, in general, inadequately cooled. As a result of these steps in the detail design, the brake mean effective pressure was raised from the 75 to 95 Ib. usual on cars, to as high as 130 Ib. per sq. in. in the best modern aero engines, while at the same time the petrol consumption was reduced to approximately 0-45 Ib. per B.H.P. hour, a value some 40% better than that of the average car engine.
In many cases the output was also improved by increasing the speed of the engine. The speed of the rotary engine was limited to about 1,200 revolutions per minute, by the stresses due to centrifugal force. In the fixed cylinder engine, however, much higher rotational speeds could be adopted by attention to the balance of the moving parts, and to the design of the bearings. These speeds now range from 1,400 to 2,100 revolutions per minute, reduction gears being used for the airscrew drive in the case of the larger and less rapidly flying aeroplanes.
The resultant weight economy was considerable. Thus the 300 H.P. Hispano-Suiza water-cooled Vee, rotating at 2,000 r.p.m., weighed only 1-80 Ib. per H.P. and the 450 H.P. Napier " Lion " of 1921 only 1-89 Ib. per H.P. In each case these weights include that of the propeller boss, but not that of the radiator and its water, which would add approximately 0-55 Ib. per H.P.
These advances in the car type of aero engine were accom- panied by improvements in the specialized type. In 1912 the radial engine with fixed cylinders was represented by a few examples of which the 9-cylinder, water-cooled " Salmson " developing no H.P., the 6-cylinder, water-cooled " Laviator "
up to 6 in. and up to 50 B.H.P. per cylinder, give an output and fuel-consumption of similar order to those from the best water- cooled cylinders.
No air-cooled engine with these large cylinders reached the stage of production in quantity during the war. A number of British radial engines were, however, developed in 1918, and of these the " A. B.C. Dragonfly," having nine steel cylinders, giving 300 H.P. and weighing 2-22 Ib. per H.P., and the 450 H.P. " Cosmos Jupiter," having nine steel cylinders with an aluminium patch containing the inlet and exhaust ports bolted to each head, and weighing 1-42 Ib. per H.P., are worthy of mention.
As compared with these it will be recalled that the 150 Mono- Gnome of the same date weighed 2-03 Ib. per H.P.
A i2-cylinder Vee experimental engine with aluminium cylin- ders was built at the Royal Aircraft Factory in 1916-7 and gave excellent results in flight and on the test bed. This developed 210 H.P. and weighed 3-0 Ib. per H.P.
Prior to 1914 the American aero engine was mostly of the car type, and was outdistanced during the first two years of the war by the more intensive development in those countries active- ly engaged. At that time the 160 H.P. Curtiss was probably the most outstanding engine in America, and when the United States declared war in 1917 her need for high-powered aero engines became acute. In May 1917 it was decided, in confer- ence with the Allied Mission in the United States, to design and build the Liberty engine, of which an 8-cylinder model was completed for test on July 3 1917. This was not put into production, as advices from France indicated that demands for increased power would render it obsolete before it could be produced in quantity. Efforts were then concentrated on a 12- cylinder model, the first of which passed its so-hour test on Aug. 25 1917. This engine is a water-cooled Vee, originally developing 400 H.P. and weighing 2-0 Ib. per H.P. More recent improve- ments have increased the output to 510 H.P. and reduced the dry weight per H.P. to 1-75 Ib. or about 2-3 Ib. with cooling water and radiator.
The progress in the average aero engine in service between 1910 and 1918, in power, weight, and efficiency, is shown in the following table. The main details are abstracted from the report of the American National Advisory Committee for Aeronautics in 1918:
Weight per H.P.
Average petrol (Ib. per B.H.P.)
1915 1916 1917 1918
309 437 5'2 570 603
5-7" 3-9 3-7 3-1
developing 80 H.P., and the 6 and 10 cylinder, air-cooled " Anzani " developing 60 and 100 H.P. are among the most noteworthy. The Salmson was developed at a later stage as a i4-cylinder, two-row engine of 200 H.P. and the Anzani as a 20- cylinder, four-row engine of 200 H.P. These engines were French, but since 1914 British designers have greatly advanced the science of the air-cooled engine.
The fixed radial engine has a number of features of superiority over the rotary. It enables a normal type of carburetter and of piston to be used; it eliminates the large windage losses; while since the cylinders are not exposed to centrifugal stresses aluminium alloys can be used. This light and highly conducting metal has greatly helped air-cooling. Owing to the greater ease of installation of the air-cooled engine in an aeroplane, the absence of a fragile radiator liable to freeze on descent from great heights, as well as to its adaptability to work in the tropics, much attention was paid during the war to the design of air- cooled cylinders. A composite construction using aluminium alloy for cylinder heads was evolved at the Royal Aircraft Factory, Farnborough, between 1915 and 1921, with the result that air-cooled cylinders became available which, for diameters
Since the water-cooled engines cannot function without radiator and water, an addition of 0-55 Ib. per H.P. has been made in their case to render Table A comparative. The weights after deduction of 0-55 Ib. are actual measurements, and include those of the propeller boss and of the gear, if any. In cases where the respective makers produce a series of engines of different powers, only representative examples have been quoted.
During the latter part of the war, the demand for engines of large H.P. for bombing aeroplanes and dirigibles led to the production of many experimental engines, which were available by 1921, e.g. the 800-900 H.P. Sunbeam Coatalen, the 850 H.P. Fiat, the 1,000 H.P. Lorraine Dietrich, and the 1,000 H.P. Napier " Cub."
Types of Engines. Of the total heat from the fuel, 25 % to 35 % passes through the walls and piston and must be dissipated by water- cooling or direct air-cooling if the normal operation of the engine is to be maintained.
Water or air-cooling have their respective advantages and dis- advantages.
For the water-cooled engine is claimed:
(l) A lower cylinder-wall temperature; a reduced tendency to the burning of exhaust valves ana pistons; and more effective lubri- cation.