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four pistons operating upon each crank-pin. The angular arrange- ment adopted is considered by the builders to give the most con- venient sequence of working impulses. Two exhaust and two inlet valves are provided in each cylinder head, placed at such an angle that the combustion chamber is approximately spherical in form; each row of cylinders has its own overhead half-speed cam-shaft operating the valves through rockers. Four carburettors are fitted, mounted on facings on the front (propeller) end of the crank-case. Ignition is by four high-tension magnetos.

FIG. 15.

The peremptory demands of the war compelled the rapid development of aircraft of all kinds, and aeroplanes driven by two, three, four and even more engines soon became necessary. In 1914 the British depended mainly on the French for aero engines, principally of the " Gnome " and " Renault " types, but towards the close of the war British aero engines in both quality and number surpassed all others. At the end of 1918 the aggregate H.P. of the British aero engines was 7,094,000, and of this huge total 4,143,000 was contributed during 1918 alone.

Progress in the commercial applications of aircraft is slow, but will certainly continue, and increase; its vital importance in warfare renders it essential that adequate encouragement be afforded to enable it to be developed in all directions.

Group 5. Special Types, (a) The Humphrey Pump. This is an internal combustion pump, simple in principle, and of high efficiency. Its mode of working will be understood by fig. 17.

It consists essentially of a U-tube AA'A" containing water, one leg of which, A, is closed; within this closed end a mixture of gas and air is introduced, compressed, and exploded, thus setting the water column in oscillation; the water thus rises in A", and some is discharged through B into the upper reservoir as indicated. C is a lightly spring-supported inlet valve which opens automatically, admitting a charge of fresh mixture, when the water-column in descending reduces the pressure in A to below that of the atmo- sphere; on the return oscillation C at once closes, and the fresh charge is compressed in A, and fired by the ignition plug S at the instant of maximum compression; explosion at once occurs, and the water in A is driven rapidly downwards, with corresponding rise in A" and discharge throfq;h B. Towards the end of the working stroke the fall of the water in A causes the resultant pressure upon the suction valve E to become vertically downwards; E thereupon opens, admitting a fresh supply of water to the U-tube from the lower reservoir, and simultaneously, by a simple link-work, re- leases a pawl holding uo the exhaust valve D which at once falls by its own weight, permitting the burnt gases to discharge into the atmosphere. The exhaust valve D is placed at the lower end of a

short pipe projecting downwards into the combustion chamber as shown, and remains open during the return oscillation of the water- column in A until the water-level reaches and closes it, the pawl then automatically locking it in readiness for the next cycle.

The residual burnt gases are next compressed by the still rising water column in A, which is thus brought to rest. On the succeeding downward oscillation the pressure in A rapidly falls below that of the atmosphere, whereupon the automatic inlet valve C opens and admits a fresh charge; the sequence of operations is then repeated.

The pump as described thus works upon the " 4-stroke " cycle, but is also, suitably modified, arranged to work on the 2-stroke cycle; a full account of this very ingenious application will be found in Mr. Humphrey's paper in the Proc. Jnst. Mech. Eng. for Dec. 1909. It will be noted that, excepting the valves, there are no moving parts, the momentum of the water-column being utilized to charge and compress in the working cylinder, and obtain the fresh supplies of water to be pumped. The four strokes of the cycle, as above de- scribed, are all unequal, the working stroke being the longest; this is thermodynamically an advantage.

Mr. Humphrey has produced designs of pumps of this type ca- pable of working with a suction, and for lifts of as much 33300 feet.

A very interesting installation of Humphrey pumps is that at the Chingford reservoir of the Met. Water Board, where are five large pumps, each of which delivers 40,000,000 gallons of water daily into the reservoir from the River Lea. Each pump cylinder is 7 ft. in diameter, and develops from 250 to 300 horse- power. The pumps use gas supplied by anthracite-burning gas-producers, and the consumption per actual pump-horse- power-hour is stated to be about 0-9 Ib. of anthracite only.

(ft) The Internal Combustion Turbine. The exceedingly diffi- cult problem of the internal combustion turbine has continued to receive attention; the chief difficulty encountered has been that of the extremely high temperature (i5oo-2oooC.) of burning gas in relation to the metals employed in construction.

The late M. Rene Armengaud succeeded in obtaining 300 B.H.P. from a petrol internal combustion turbine of constant- pressure type by reducing flame temperature at efflux to about 4ooC. by the addition of large quantities of steam; this may accordingly be equally well regarded as a highly superheated steam turbine. About 3 Ib. of petrol were required per B.H.P.

FIG. 17.

hour, which is fully five times as much as is needed by a modern petrol engine of normal type. M. Karovodine has also built a small turbine in which explosions from atmospheric pressure oc- curring in rapid succession drive a small impulse wheel; this turbine was very small, giving only 1-6 B.H.P. at 10,000 revs. per minute, and the fuel consumption was very high. It is con-