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a crosshead, whereby it is attached to the crankshaft D by means of the usual type of connecting-rod. On the left of the working cylinders is a large double-acting " blowing " cylinder, or air pump, CC, with a bore of 85! in. ; this is a conspicuous feature in fig. 2.

The valves are operated by mechanism driven by the crankshaft, and a large fly-wheel E, 22 j ft. in diameter, is provided to ensure a sufficiently uniform motion of the engine. The long piston rod carry- ing the two working pistons and the air pump piston is borne on four crossheads as shown; the pistons " float " in their respective cylin- ders, thus minimizing engine friction and wear, as all the weight is carried on these four external crossheads.

It will be observed that the cylinders and cylinder covers are well water- jacketed ; the pistons, piston rod, and exhaust valves are also water-cooled. For the cylinders and their covers the cooling water is supplied at a pressure of about 15 Ib. per sq. in. above atmosphere ; for the pistons, piston rod, and valves, the water pressure necessary is about 55 Ib. per sq. in., due to their reciprocating motion. Close to each cylinder is fitted an open water tank into which the various cooling-water pipes discharge in full view of the attendant. Each discharge is fitted with a regulating valve and thermometer, whereby the cooling-water temperature from the several parts of the cylinders, pistons, etc., may be separately adjusted while the engine is running.

FIG. 2.

Special oil pumps are provided for cylinders, stuffing-boxes, and exhaust valves, so that each of these parts is lubricated directly, with provision for separate adjustment. Surplus oil is collected in a sump, filtered, and returned to an oil supply tank above the engine.

Enough has been said to show that in its present form this type of large gas engine is well designed in every detail, and it has proved itself a very safe, economical, and reliable power producer.

The engine illustrated above is installed in the works of the Barrow Haematite Steel Co., and was built by an English engineering firm; the type has, however, made most progress on the European continent. Thus, from 1908 up to the commence- ment of the World War, the M.A.N. Co. and their licensees had built over 300 of these engines, aggregating 500,000 B.H.P. The M.A.N. Co. had by 1921 installed engines in 22 stations, three of which exceeded 27,000 B.H.P. each, six exceeded 15,000 B.H.P. each, and the remaining thirteen were all over 10,000 B.H.P. in capacity. Messrs. Thyssen & Co. have engined a power station at Bruckhausen which has a capacity of 65,000 horse-power. At the Neunkirchen Works, 14,000 H.P. is pro- vided by 2-stroke cycle double-acting gas blowing engines. It is stated that 2,000 H.P. per cylinder is obtained from these engines. At Heinitz, Saarbrucken, there is an installation of nine Ehrhardt-Sehmer engines, aggregating about 16,500 horse- power. The Schalke Mining Co. have three Haniel-Lueg twin- tandem engines totalling 12,000 horse-power.

Of American installations may be mentioned: The Indiana Steel Co.'s plant at Gary, Ind., where, in one engine house in units of about 3,700 H.P., is an aggregate of fully 60,000 H.P. supplied by M.A.N. type engines built by the Allis-Chalmers Co. The San Mateo Power Co. have 21,600 H.P. supplied by four 5,400 H.P. horizontal double-acting twin-tandem engines built by the Snow Steam Pump Co. ; each, engine thus comprises four cylin- ders; these are 42 in. in diameter, with a piston stroke of 60 in. and the speed is 90 revs, per minute. Thus each working cylin- der supplies 1,350 horse-power. At the Carnegie Steel Co.'s Ohio works there are four very large blowing engines of 3,000 H.P. each, capable of dealing jointly with 200,000 cub. ft. of air per minute. An important installation is that (1921) at Kamata, Japan, where four large M.A.N. type engines by the Lilleshall Co., Ltd., operating on Recovery Producer gas, supply electrical energy required to work the railway between Tokio and Yoko- hama. Each engine is direct-coupled to a i,5oo-K.W. alternator the pole-pieces of which are mounted in the rim of the fly-wheel.

An idea of the size of these huge engines may be formed from the following particulars :

in

The cylinders are 47J in. in diameter and the stroke is 51 J in.; at loo revs, per minute each engine has an output of roundly 2,500 B.H.P. The crank-pin is 23! in. in diameter; the crankshaft at the fly-wheel end is no less than 32 J in. in diameter; and the fly-wheel is about 22 ft. indiameter and 39-4 in. in rim-width; an illustration of this enormous wheel, which weighs about loo tons, is given in fig. 3 ; its great energy of rotation reduces the coefficient of fluctuation of engine-speed at full load to less than 1/250 as required for parallel running with alternators.

Each cylinder, complete, weighs 25 tons, and each complete engine about 400 tons, including the fly-wheel.

FIG. 3.

Exact data of output during the World War and afterwards are unobtainable, but it is considered that in 1921 there was an aggregate of roundly 2,500,000 H.P. supplied by engines of the large, slow-running, tandem, horizontal, double-acting type.

Group i (b). English designers up to 1921 had not much favoured the large water-cooled-piston double-acting engine, preferring the greater simplicity of the single-acting cylinder

with uncooled piston; this led to the development of an impor- tant class of vertical and horizontal medium-speed single-acting four-stroke multi-cylindered engines.