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A critical examination of the type will be found in a paper by Mr. J. Richardson read before the Diesel Engine Users' Association on Oct. 25 1918, and reproduced in Engineering of the same date.

Messrs. Beardmore were in 1921 building this type of engine up to 600 B.H.P. and were considering a design to give 1,000 B.H.P.

FIG. 12. Two-stroke Cycle: (a) " Beardmore," (6) "Bolinders," [c) " Fetter," (d) " Ailsa Craig," (e) " Campbell," (/)" Kromhout,"
 * ) "Robey." Four-stroke Cycle: (h) "Cross," (i) " Hein."

The Swedish " Bolinders " engine (agents, Messrs. James Pollock & Sons, London) has for years been successfully and largely applied to the propulsion of fishing-boats and coastal craft. As early as Dec. 1911 the small (6s-ft.) vessel " Lingueta," fitted with a 30- H.P. Bolinders engine, ran from Weymouth to Pernambuco (Brazil). In 1912, the vessel " Isleford " was fitted with a 4-cylinder 320- B.H. P. Bolinders engine; one of the latest cases (1921) is that of the Duke of Westminster's yacht " Belem," propelled by two 4-cylinder Bolinders engines each of 240 B.H.P. Of this type alone it is stated that over 650,000 B.H.P. had already in 1921 been supplied for various services; and as many other firms build 2-stroke semi- Diesels, the aggregate of this type must reach a very high figure.

The 2-stroke Petter semi-Diesel is also now largely employed in land service.

Semi-Diesel engines are becoming increasingly frequent on ac- count of their simplicity, relatively low cost, and ability to use as fuel, with good economy, most of the heavy petroleum fuel oils varying from 0-8 to 0-9 in specific gravity and J3O F.-25OF. in flash point; the range at present is thus from ordinary kerosene to Texas fuel oil.

Semi-Diesel engines are usually started by compressed air stored in reservoirs at about 200 Ib. per sq. in. ; prior to starting the hot-bulb is blow-lamp-heated for 10 to 15 minutes.

Group 3 (c). Hornsby-Akroyd, and Normal Heavy Oil Vapor- er-type Engines. -There is no change of any fundamental iportance to record regarding this group; mostly of the single- ylindered horizontal type, their use is established in many cases vhere the requirements call for only comparatively small powers.

Group 4. Quick-revolution "Light Oil " Engines. This group includes the " Petrol Motors " now universally applied to the propulsion of road vehicles of all types, motor launches, air- craft, and small miscellaneous services.

Invented by Gottlieb Daimler, about 1887, and first applied seriously to road transport by Messrs. Panhard & Levassor, 1890, that modern miracle the "Petrol Motor" had in 1921, in the short space of thirty years, profoundly affected the conditions of civilized life in both peace and war. Distributed now over the whole world it is, par excellence, the motor for the multitudi- nous daily wants of humanity where large power is not required, and on land, in the air, on and under water, in agriculture, domestic service, and in the miscellaneous smaller departments of industry it finds universal application. Not the least remark- able of the features of these wonderful little engines is their high thermal efficiency, as much as 28% of the whole heat of the petrol not uncommonly being obtained at full load.

In his presidential address to the Inst. of Auto. Engineers in 1910, Dr. F. W. Lanchester stated that the 4-cylindered and 6-cylindered petrol engine had even then reached a degree of perfection that would have been regarded as impossible of attainment at the commencement of the century; the weight per H.P. developed at full load had been reduced from about 30 Ib. to 9 to 12 Ib. only (exclusive of fly-wheel); he commented also upon the absence of vibration and efficient silencing of the 1910 engines. Car engines of 1910 were, with few exceptions, of the 4-cylindered vertical type, with bore ranging from 3 to 5 inches, stroke from 3 to 6 inches, and speed, when developing 90% of their maximum power, of from 750 to 1,500 revolutions per minute, the corresponding brake mean effective pressure ranging from 65 togslb. persq. in. approximately. Compression pressures employed were from about 70 to 120 Ib. per sq. in., absolute. Very full details will be found tabulated in the Proc. Auto. Engineers for 1910-11, vol. v., pp. 180 et seq.

Between 1910 and 1921 the advance made was purely in refinements of detail, with no change in leading principles of action. Six-cylindered engines for road vehicles show a slight increase in number; and in smaller and lower-priced cars, due to post-war cost increases, there has been some tendency observed to the production of two-cylindered horizontal, or " V," car engines, a few of these being of the air-cooled type. The 4-cyl- indered vertical engine, water-cooled, still largely predominates, as is shown by the following analysis of engines fitted to motor vehicles exhibited at Olympia in the autumn of 1920: Engines of Motor Vehicles, 1920.

Number of Cylinders

Number of Engines

Per cent of Engines

2

I 8

12

25 203

59 13 i

301

8-3 67-5 19-6

4'3 o-3

IOO-O

Of these 301 engines, 292 were water-cooled and only 9 air-cooled.

The provision of detachable heads is a noteworthy improvement in design. In 1910 they were almost unknown; in 1920, of the 301 en- gines examined, 133 had detachable heads. Valve location has undergone but little change, 227 engines in 1920 having the usual side-by-side arrangement; the overhead valve type showed a small increase, 48 engines being thus arranged. Sixteen engines had sleeve valves, six engines valves on opposite sides of the cylinders, involving two cam-shafts, and four with the inlet valve vertically over the exhaust. The exceedingly reliable high-tension magneto ignition still predominated, 247 engines being thus fitted; the re- mainder had either battery ignition or a combination of both H.T. and battery.

The revival of battery ignition is a consequence of the introduction of the very convenient electrical self-starting equipment with which so many vehicles are now supplied ; ' of 262 cars examined in 1920 no fewer than 245 were thus equipped. Recent improvements in H.T. magneto designs enable these machines to " spark " at very much reduced speeds; Messrs. Young & Warren 2 mention a magneto which will spark regularly across a 5-5-mm. air gap at about 60 revs, per minute only, with the timing lever fully advanced.

1 For a valuable review see The Autocar for Oct. 23 1930.

2 Proc. Inst. Auto. Eng., 1919-20, p. 374.