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520 With improvements in the H.T. magneto, the recently introduced American " Impulse Starter " may be found to prove a simple, com- pact, and low-priced solution of the starting problem ; briefly, this comprises a spring introduced between the engine and magneto and so arranged that on turning the starting handle the spring is at first wound up, the magneto armature remaining stationary. At an arranged instant the locking device is released, and the armature at once " flicks over " very suddenly, thus producing an intense igniting spark.

Fuels. Petrol was still in 1021 the principal fuel, though benzol, either alone or mixed with petrol in varying proportions, is now used so far as available; alcohol had not yet come into use, though great efforts were being made to render it generally available. Mixtures of petrol and benzol, or benzol alone, can be used in existing engines usually with no change in adjustment, but with alcohol special designs will become necessary. The cost of petrol to the consumer rose steadily largely due to the war from 1910 to 1921. In 1910 the price per gallon in the London district was gd. and 6d. tax, total is. 3d. ; in August 1920 it rose to the very high figure of 45. 3d. ; by June 1921 it had fallen to roundly 35. This great increase, added to the heavy vehicle taxation of i per Treasury-rated horse-power ( = 0-4 X Bore 2 X No. of Cyls.) tended to some extent to discourage the use of the private motor vehicle, but this might be regarded as a temporary check only.

The motor-cycles of 1921 may be considered to v have nearly attained perfection; swift, comfortable, very economical of fuel, reliable, fitted often with 3-speed gears, " kick " starter, free engine, electric lighting, and many other refinements, they were veritably " cars " in miniature, and continually increased in favour with the motoring public of all ages. In 1920 no fewer than 186,200 licences for motor bicycles were taken out in Great Britain alone.

M

FIG.

A great increase is also observable in that singular, though convenient, makeshift vehicle the motor-cycle and side-car; this is probably largely attributable to the prevailing high cost of cars. With hardly an exception, motor-cycle engines are all air- cooled; the principal feature of note is the large increase in the number of engines of the 3-port 2-stroke type; the disadvantage of a lower fuel economy than that of the 4-stroke engine is,

with many riders, more than compensated for by their great simplicity and low first cost.

A diagrammatic section through one of these very useful little engines is given in fig. 13; it comprises the usual air-cooled cylinder A, piston B, connecting-rod C, and crankshaft E>; the piston has a " lumped " crown to deflect upwards the entering stream of fresh mixture as indicated. The crank-chamber is completely enclosed, and to start the engine it is caused to rotate by the driver; the piston rises, producing a partial vacuum in the crank-chamber until its lower edge uncovers the port K when an explosive mixture from the carburettor immediately rushes in ; on its downward stroke the piston first covers the port K, and thereafter compresses the charge of ex- plosive mixture in the crank-chamber until its upper edge uncovers the port F when the mixture, at a slight pressure, immediately passes up the passage shown into the space above the piston. Simulta- neously the used gases are discharged through the exhaust port E, which is uncovered by the piston shortly before F. On the following up-stroke the piston first shuts off the ports E and F, and then com- presses the charge into the upper portion of the cylinder; at the instant of maximum compression it is exploded by a sparking plug in the usual manner, and the piston is at once driven downwards; near the end of the down-stroke the burnt gases escape through E, at the same time that the next fresh charge is entering through F. and the rycle is then repeated indefinitely. Thus every downward stroke is a working stroke; the engine is valveless; the only moving parts are the piston, connecting-rod, and crankshaft ; and the engine will run equally well in whichever direction it may be started, a feature of value in its application to small motor boats and launches which are readily reversed by slowing down the engine and then suddenly advancing the ignition.

A small compression release valve M is usually fitted in the top of the combustion chamber, which is held open by hand-operated gear to facilitate the operations of starting and stopping. In motor bicycles these engines are commonly run at from 2,000 to 3,000 revs. per minute; they are very reliable, and require no attention beyond the occasional removal of the deposit of oily carbon which forms on the piston crown and walls of the combustion chamber.

Engines for Aircraft. In principle these are all high-speed petrol engines of the four-stroke or, in rare instances, two-stroke type, characterized by their extreme lightness relatively to their power output. Fig. 14 shows three standard types of engine to scale, each of 75 B.H.P., and enables relative sizes and weights to be readily compared; from this illustration the great engineering achievement embodied in the "Aero Engine" can to some extent be appreciated.

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a

FIG. 14. (a) 75-B.H.P., Single-cylinder Horizontal Engine. 200 R.P.M. Weight 200 Ib. per B.H.P. Total 15,000 Ib. (6) 75-B.H.P., 6-cylinder, Vertical, Water-cooled Aero Engine. 1200 R.P.M. Weight 5J Ib. per B.H.P. Total 410 Ib. (c) 75-B.H.P., 7-cylin- der, Rotary, Air-cooled Aero Engine. 1200 R.P.M. Weight 2j Ib. per B.H.P. Total 205 Ib.

Aero engines are conveniently grouped in five classes, viz., Horizontal Engines, Radial Engines, Diagonal or " V " Engines, Vertical Engines, and Rotary Engines. The horizontal aero engine is now obsolete. A classification of seventy-six aero