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INLET

per minute by an 8-cylindered single-acting 4-stroke Diesel engine of crosshead type, aggregating 2,500 indicated H.P. ; the first voyage was made in Feb. 1912. The sister ship " Christian X."was similarly engined. In May 1913 the largest Diesel- engined vessel afloat was the " Siam," built and engined by Bur- meister & Wain, at Copenhagen, having a displacement of 13,200 tons; this boat is fitted with two 4-stroke engines aggregating 3,1 50 1.H.P., twin screws, and attained 12-4 knots on her trial run. The great success of the geared steam turbine has made it a formidable competitor of the marine Diesel engine; nevertheless, steady progress continues to be made, and it is noteworthy that some of the largest British engineering and shipbuilding' com- panies were in 1921 engaged in Diesel-engined ship production. Thus, during 1920, Messrs. Harland & Wolff launched seven ocean-going Diesel ships, viz. five at Glasgow, one at Greenock, and one at Belfast; while Messrs. Barclay Curie, Doxford & Vickers also built one each ten in all; and in the United States five or six large oil-tankers with Diesel engines were also built. In May 1921 there were in hand in Great Britain among others: (i) A large vessel for the Ocean Steamship Co., Liverpool, of 15,000 tons displacement and 13 knots speed; this was to be

fitted with two large 8-cylin- dered Diesel engines by Bur- meister & Wain, each develop- ing 3,200 1. H. P.; the daily fuel consumption was estimated not to exceed 20 tons. These en- gines were of the same type and size as those already fitted in four i4,ooo-ton " Glen " liners built by Harland & Wolff;

(2) two i4,ooo-ton boats, also by Harland & Wolff, for the Holland-America line, in each of which twin-screw Diesel machinery aggregating 6,400 I. H.P. was to be installed of the same design as those in the Ocean Company's liner;

(3) The British India Co.'s vessel " Domala " with engines by the North British Diesel Co.; these were 8-cylinder 4- stroke engines of 26j-in. bore, and 47-in. stroke running at 96 revs, per minute, and giving about 2,350 I.H.P.; this was the first large Diesel-engined passenger liner.

Great attention was in'i92i being devoted to improved de- signs, and some very interesting Diesel engines of special type had recently appeared. Thus Messrs. Doxford had produced a 2-stroke inverted vertical 3,000 H.P. engine of the" Oechelhauser" 2-piston type. Messrs. Cammell Laird have developed the " Fullagar " engine for marine use; this is a modified Oechelhauser made in "units" each comprising two Oechelhauser cylinders placed side by side (fig. n) with their pistons connected diagonally by long tie-rods as indicated; the obliquity of the rods is small, and the side thrusts are resisted by the cross-heads and guides shown above A and C in the illustration.

The Cammell Laird-Fullagar engine works on the 2-stroke Diesel cycle, the oil fuel being injected between the pistons near the point of maximum compression by a high-pressure air blast; as the inlet and exhaust ports are at opposite ends of the cylinders excellent scavenging is obtained.

This engine is light and low in cost relatively to its power output; it is of high thermodynamic efficiency ; and the frame is almost wholly relieved from stress in working. The 1,000 H.P. marine engine comprises two " units," i.e. four open-ended tubular cylinders,

EXHAUST

FIG.

eight pistons, and a 4-throw crankshaft. The cylinders are i8J in. in diameter, the stroke of each piston is 25 in., and the engine runs at 110-115 revs, per minute.

Two of these engines had by 1921 been installed in a cargo vessel for Messrs. Brocklebank ; trials made in April 1921 were said to have shown the extremely low consumption of only 0-39 Ib. of oil per B.H.P. hour. These engines drive their own 3-stage air compressors, scavenging pumps, and circulating water and lubricating oil pumps.

A 4,000 H.P. marine engine of this type was in May 1921 being built by Messrs. Cammell Laird, in a 4-cylinder unit ; each cylinder is 26 in. diameter, stroke 42 in., and speed 90 revs, per minute.

Messrs. Swan, Hunter & Wigham Richardson had also recently produced a design of 2-stroke 4-cylinder " Neptune-Diesel " engine of special type.

The F.I.A.T. Co. (Turin) were building large 2-stroke Diesels in sizes up to a 6-cylinder marine type developing 3,200 H.P.

A large number of small multi-cylindered quick-revolution Diesel engines were fitted in submarines during the war by Thornycroft, Vickers, White, etc. This class includes engines having up to 12 cylinders, and run usually at from 400 to 500 revolutions per minute. They are now built in power up to 1,300 B.H.P., frequently with 8 cylinders, of both 4-stroke and 2-stroke type.

Group 3 (b). Semi-Diesel Engines. Within the period 1910-21 a large group of engines appeared, which it has become usual to term." semi-Diesel " engines. The very high compression pres- sure of the normal Diesel engine necessitates not only a heavy and costly design but in addition the maintenance of an ex.tremely high pressure in the air reservoirs for supplying the fuel oil blast. Designers have accordingly devoted considerable atten- tion to the problem of producing engines (i) having a lower com- pression pressure than the Diesel engine, and (2) avoiding the necessity of high pressure air blast reservoirs by injecting the fuel oil into the cylinder by mechanical means through an " atom- izer," or spraying device. Great success has been attained with but little sacrifice in fuel consumption efficiency.

In March 1919 the Diesel Engine Users' Association adopted the following useful definitions of Diesel and semi-Diesel engines respectively:

Diesel Engine.

" A Diesel engine is a prime mover actuated by the gases resulting from the combustion of a liquid or pulverized fuel injected in a fine state of subdivision into the engine cylinder at or about the con- clusion of a compression stroke. The heat generated by the com- pression to a high temperature of the air within the cylinder is the sole means of igniting the charge. The combustion of the charge proceeds at, or approximately at, constant pressure." Semi-Diesel Engine.

" A semi-Diesel engine is a prime mover actuated by the gases resulting from the combustion of a hydrocarbon oil. A charge of oil is injected in the form of spray into a combustion space open to the cylinder of the engine at or about the time of maximum compres- sion. The heat derived from an uncooled portion of the combustion chamber, together with the heat generated by the compression of the air to a moderate temperature, ignites the charge. The combus- tion of the charge takes place at, or approximately at, constant volume."

In the semi-Diesel engine definition it will thus be seen that there is no limitation made as to the mode in which the charge of fuel oil is injected, and that the essential features are (i) the practically instantaneous introduction of the fuel oil charge, giving approximately a " constant volume " explosion; and (2) the use of a " hot bulb " for aiding vaporization and ignition, whence these engines are sometimes styled " Hot-bulb Diesels." As the " Hot-bulb " engine was invented by Mr. Stuart Akroyd (1886-90), the " Akroyd-Diesel " would have been a more appropriate name for this class of engine.

Nearly all recent semi-Diesel engines are of the two-stroke type with mechanical or " solid " injection of the fuel oil, i.e. no high- pressure air blast; a usual device comprises a small force-pump operated by a quick-acting or " steep " cam which causes the small charge of oil delivered by it to forcibly raise a spring-closed needle valve in the spraying nozzle through from o-oi to 0-02 of an inch; the charge of oil thus enters the hot-bulb in tire form of a well- diffused fine spray. Several recent designs include simple air com- pressors whose function it is to supply an air jet to improve cylinder scavenging and assist in the cooling of pistons and cylinder walls, thus rendering recourse to the somewhat crude " cylinder water- drip " unnecessary during prolonged full-load running.

An instructive series of diagrammatic sections through nine representative semi-Diesel designs is given in fig. 12 '; seven of the engines are of the two-stroke type, the " hot-bulb " being shown.

1 By kind permission of The Diesel Engine Users' Association.