Internal Combustion Engines

"INTERNAL COMBUSTION ENGINES. - It is usual now, as a matter of terminology, to deal comprehensively with gas engines and oil engines under the general description of " internal combustion engines," and the present article gives some account of the new developments that have been made between 1910 and 1921; in the earlier volumes of this work (IIth ed.) a full account will be found (under GAS Engine, 11.495 seq., and OIL Engine, 20.35 seq.) of the history of these very important engines up to 1910, together with a statement of the engineering thermo dynamics of the subject.

Gas and oil engines in 1921 might conveniently be grouped as follows: Group I. Large Gas Engines (a) Of horizontal, slow-speed, double-acting type, of both 4stroke and 2-stroke cycles.

(b) Of horizontal and vertical medium-speed, single-acting manycylindered type, usually 4-stroke.

Group 2. Medium Gas Engines Usually of horizontal single-acting, Ior 2-cylindered type, working on the 4-stroke cycle.

Group 3. Heavy Oil Engines (a) Of large Diesel design, both 4-stroke and 2-stroke.

(b) Of " semi-Diesel " type, both 4-stroke and 2-stroke.

(c) Of the low-compression hot-bulb, or Akroyd type, and normal heavy oil engines with vaporizers.

Group 4. " Light Oil " Engines Small quick-revolution usually multi-cylindered engines of the 4-stroke Daimler, or 2-stroke Day type; all single-acting, and usually vertical.

Group 5. Special Types (a) The Humphrey Pump.

(b) The Holzwarth Turbine.

(c) The Still Engine.

Group r (a). Large Gas Engines

The period 1910-21 saw a considerable increase in the number of large engines of the slowrunning horizontal type. Considerable difficulties were encountered at first with large gas engines as the necessity of providing very complete cylinder cooling arrangements was not clearly realized, and cracked and seized pistons, failures of valves, and ruptured cylinders were not infrequent. These difficulties had by 1921 been completely overcome, and these large engines work with the utmost regularity and freedom from trouble.

The type under discussion - frequently referred to as the " Nurnberg " or " M.A.N." engine, on account of the important part taken in its development by the Maschinenfabrik Augsburg-Nurnberg A. G. - is illustrated in section in fig. 1 and in external appearance in fig. 2.

The engine shown is a 4-stroke, or " Otto " cycle, tandem, doubleacting, single-crank, " blowing " engine of M.A.N. type constructed by the Lilleshall Co., Ltd., of Shropshire, England. Rated at 1,200 B.H.P. and running at 90 revolutions per minute on blast furnace gas, it is capable of compressing 26,000 cub. ft. of free air per minute to a pressure of 8 lb. per sq. in. above atmosphere.

Within each of the working cylinders A and B is a piston F 35 in. in diameter and having a stroke of 434 in.; the pistons are mounted upon a common piston rod which terminates towards the right in ?1 ?? \\\\\\ ???

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l FIG. I 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-,1 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, 222 ft. in diameter, is provided to ensure a sufficiently uniform motion of the engine. The long piston rod carrying the two working pistons and the air pump piston is borne on four crossheads as shown; the pistons " float " in their respective cylinders, 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 lb. per sq. in. above atmosphere; for the pistons, piston rod, and valves, the water pressure necessary is about 55 lb. 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.

.1G. 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 commencement 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 provided 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 horsepower. The Schalke Mining Co. have three Haniel-Lueg twintandem 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 cylinders; these are 42 in. in diameter, with a piston stroke of 60 in. and the speed is 90 revs, per minute. Thus each working cylinder 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 Yokohama. Each engine is direct-coupled to a 1,50o-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: - The cylinders are 474 in. in diameter and the stroke is 51 1 i in.; at too revs. per minute each engine has an output of roundly 2,500 B.H.P. The crank-pin is 231n. in diameter; the crankshaft at the fly-wheel end is no less than 324 in. in diameter; and the fly-wheel is about 22 ft. in diameter and 39.4 in. in rim-width; an illustration of this enormous wheel, which weighs about too 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, anti each complete engine about 400 tons, including the fly-wheel.

110.3.

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 2,50o,000 H.P. supplied by engines of the large, slow-running, tandem, horizontal, double-acting type.

Group r (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 Flu. 4.

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

The National Gas Engine Co., Ltd., of Ashton-under-Lyne, took a leading position in the development of the vertical type of this class, and in 1910 erected a special factory for their exclusive manufacture.

An illustration of a 1,500 B.H.P. " National " vertical gas engine direct-coupled to an alternating current generator is given in fig. 4, while fig. 5 shows a transverse section through one of the six pairs of c y linders. It will be seen that this 1,500 B.H.P. engine comprises twelve cylinders arranged in six vertical tandem single-acting pairs AA and BB respectively (fig. 5); the pistons EE are rigidly connected together by a stout piston rod, and from the lower piston the six-throw crankshaft D is driven through a connecting-rod CC. The lower part of the upper cylinder AA is closed, and in this air is compressed during the downward stroke of the pistons thus " softening " the running. The engine works on the four-stroke cycle, and the inlet and exhaust valves and gas, air, and exhaust passages are clearly indicated. These engines work on coal gas, Producer gas, coke oven gas or blast furnace gas, and no explosive mixture exists outside the engine itself. The cylinders are well water-jacketed, but the pistons, though not water-cooled, are so designed as to facilitate the conduction of the heat away from their crowns to the surrounding cylinder walls.

Engines of this type are built in a series ranging from a 4-cylinder Two-crank design of 300 B.H.P. running at 300 revs. per minute to a 12-cylinder six-crank design of 1,500 B.H.P., 200 revs. per minute.

Many fairly large plants had by 1921 been installed, among which may be mentioned one of 11,500 H.P. at Palmer's Shipbuilding Co., Ltd., Jarrow; one of 1r,000 H.P. at the Government Factory, Lanquith; one of 10,500 H.P. at the Partington Steel Co.'s Works, Irlam, nearManchester; and one of 4,500 H.P. for -the Midland Coal & Iron Co., Ltd.

As a typical example of the horizontal type of multi-cylindered single-acting large gas engine, the design adopted by the Premier Gas Engine Co., Ltd., of Sandiacre, Notts., is taken. An important installation of this type is that of the Hoffmann Mfg. Co., Ltd., at Chelmsford, which commenced operations in March 1919.1 This installation comprises six 500 B.H.P. four-cylinder fourcrank horizontal Premier engines running at 190 revs. per minute, each direct-coupled to a 360 K.W.-generator; thus here 3,000 H.P. is produced from 24 cylinders. Gas is supplied by a Lynn pressure producer plant with ammonia recovery. An examination of the results of six months' working in ordinary service showed that the overall thermal efficiency of this plant, i.e. the ratio of heat of For details see Patchell, Journ. Inst. of Elec. Engrs., June 1920.

electrical energy to heat of coal supplied, had the very high value of 1 99, i.e. 19.9%.

Group 2. Medium-powered Gas Engines. - Included in this group are the very numerous engines principally of the singlecylindered four-stroke horizontal single-acting type using coal gas or Suction Producer gas as fuel, and employed for a great variety of purposes by the smaller class of power consumers. In H.P. they range from 2 or 3 up to (two-cylindered) designs of about 300. Messrs. Crossley Bros., Ltd., of Manchester, produce annually a large number of engines of this type in a series ranging from 31 H.P. to 260 H.P.; up to the end of 1920 this firm alone had built over 80,000 of these engines.

Many other important firms and companies are also engaged in this industry, among whom may be mentioned Messrs. Brotherhood, Browett-Lindley, The Campbell Co., Davey Paxman & Co., Fielding & Platt, Gardner, Grice, Hindley, The National Co., The Premier Co., Ruston & Hornsby, The Stockport Co., Tangye, The Brit. Westinghouse Co., etc.

A typical combination of Suction Producer and gas engine is illustrated in fig. 6. Through an incandescent zone of anthracite or coke contained in the Producer or " Generator " a mixture of air and steam is drawn by the suction of the engine when at work. This air and steam in passing through the hot zone is decomposed, and issues from the generator as a very hot, smoky mixture consisting mainly of nitrogen, carbon monoxide, carbon dioxide, and hydrogen.

A rough average composition by volume is as follows: nitrogen 55%; carbon monoxide 22%; hydrogen 15%; Carbon dioxide 6%; miscellaneous hydrocarbons, free oxygen, etc., 2 Such a mixture of gases is termed " Producer Gas " and has a (lower) calorific value of 125 to 130 B. Th. U. per cub. ft.; about 200,- 000 cub. ft. are produced per ton of anthracite consumed.

The hot and smoky gas in the case illustrated passes first through a chamber fitted with a baffle plate, and water-sealed at the bottom; here the grosser impurities are deposited; thence it goes past a 2-way valve which permits its escape through a chimney into the atmosphere when the engine is at rest; when running, however, the gas is drawn by the engine suction through a large cylinder filled with small coke over which a spray of water is constantly played. This is termed the " coke scrubber," and here the gas is cooled and freed from dust and tarry impurities; thence finally it passes into a reservoir, and so to the engine.

The illustration shows in section the normal type of horizontal 4stroke cycle engine fitted with one or two massive fl y -wheels which by their momentum maintain the required degree of uniformity of rotation of the crankshaft.

Vehicle and Marine Applications. - In 1918-21 attention was again given to the problem of propelling motor road vehicles by small producer-gas engines, and a certain measure of success was obtained by Mr. D. J. Smith and others. experiments Were in 1921 being continued.` Marine Producer-Gas Engines. - In 1904 Herr Capitaine fitted a tug at Frankfort-on-Main with a 70 H.P. engine and anthracite producer plant. In 1905 Messrs. Thornycroft fitted the launch " Emil Capitaine " with a 60 H.P. engine and producer, and, later, the barge " Duchess " with a similar 30 H.P. plant.

In 1908 Messrs. Beardmore fitted H,M.S. " Rattler, " 715 tons displacement, with an experimental Soo H.P. gas engine running at 120 revs. per minute on gas supplied by an anthracite producer. In 1910 the sailing boat " Castell san Nicolan " was fitted with an auxiliary 60 H.P. Gardner anthracite suction power plant.

The principal difficulty encountered with marine suction-gas plants was in obtaining efficient manoeuvring power; progress was made, though slowly, and it is of interest to record that the first producer-gas-engined ship crossed the Atlantic in 1919. The largest marine producer plant in 1921 was that of the American vessel " Wilhelmina," and comprised a 350 B.H.P. engine supplied by two suction gas producers.

The Dutch " Van Rennes " producer-gas engines, built in sizes up to a maximum of 200 H.P. have been fitted to a number of small cargo-boats employed in coastal service; these engines are readily reversible, and it is claimed that their fuel consumption is, roundly, 1 lb. of anthracite per H.P. hour.

A good class ocean-going cargo steamer may be taken as of about 8,000 tons dead weight, with steam engines of about 2,000 I.H.P. running at 65 revs. per minute and using 1: lb. of coal 2 Vide Proc. Inst. Auto. Engrs., xiv., 1919-20.

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Shaker Crate per I.H.P. hour; this statement clearly defines the state of development of the marine producer-gas engine in 1921.

Group 3 (a). Diesel Engines

The Diesel engine has now be come established as a reliable prime-mover, having a very low consumption of fuel. A typical illustration is in fig. 7.

7.

Within a very strong water-jacketed cast-iron cylinder BB slides a long, heavy cast-iron piston AA driving a crankshaft C by means of the connecting-rod D. The cylinder head EE is a deep, well waterjacketed casting containing four valves, viz. the air inlet, fuel inlet, exhaust, and air starting valve; the latter is not shown.

On the downward stroke of the piston air only is drawn into the cylinder through the air inlet valve; during the following upward stroke this air is compressed to a pressure of 450 to 500 lb. per sq. in. with accompanying great rise of temperature. At or near the moment of greatest compression, and continued during the first 20° to 30° of crankshaft revolution, the necessary small charge of fuel oil is blown - in the form of a very fine and uniformly diffused mist - into this compressed and heated air. Spontaneous ignition or " explosion " of the mixture instantly takes place, but the fuel supply is so regulated that the pressure is but little, if at all, increased by the explosion, the end aimed at being to cause combustion to take place at as nearly as possible constant pressure; a typical diagram is reproduced in fig. 8.

FIG. 8.

On the cessation of the fuel injection the exploded charge expands at rapidly falling pressure during the working stroke; towards the end of this stroke the exhaust valve opens, and continues open until the completion of the succeeding up-stroke, the burnt gases thence escaping into the atmosphere; this completes the cycle. The engine is thus of the " four-stroke " type, receiving one working impulse in each two revolutions of the crankshaft, the order of operations being (I) suction of air; (2) compression of air; (3) working stroke; (4) exhaust.

The valves are operated by rocking levers actuated from a halfspeed overhead cam-shaft H, driven from the crankshaft.

Compressed air reservoirs - primarily charged and thenceforward maintained by the engine itself - containing air stored at a pressure of from 750 to I,000 lb. per sq. in., are used for starting the engine, and providing the air blast for the fuel oil injection into the cylinder.

Many Diesel engines are also built operating upon the 2-stroke cycle, and fig. 9 shows a typical design in section.

Here there is no exhaust valve, but towards the end of the downstroke the piston over-runs exhaust ports AA formed in the cylinder walls whence the burnt gases escape into the atmosphere. Simultaneously an air pump, operated by the engine, delivers a charge of fresh air at a pressure of 3-5 lb. per sq. in. into the cylinder through an air valve, or valves, in its head. This air hastens the discharge of the burnt gases, and is compressed on the return up-stroke of the piston at or near the end of which the fuel oil is injected, as before, and the working stroke then follows. This type has been largely developed by Carels Bros. of Ghent; it will be noted that every downstroke is a working stroke, but due to the less perfect scavenging of the exhaust, and to the power absorbed by the air pumps, the power output is found to be only from I Z to 14 times that of an equal 4-stroke cycle engine instead of twice as much as would be realized in an ideal case.

A second type of 2-stroke Diesel engine, specially developed by Sulzer Bros. of Winterthur, is valveless excepting for the fuel injection valve and air starting valve. In this type fresh air, at slight pressure, enters through ports on one side of the lower part of the FIG. 9.

cylinder at the same time that the burnt gases are escaping through ports on the other side, substantially as in the small two-stroke Da y -type petrol engine, but without crank-chamber compression, as described later (see fig. 13). The fuel consumption of the 4stroke cycle Diesel is roundly 0.4 lb. per B.H.P. hour, and of the 2-stroke of either type about 0.45 lb. per B.H.P. hour.

Land Installation of Diesel Engines

Steady progress was made between 1910 and 1921, and at the end of 1920 upwards of 100 plants existed in the United Kingdom of capacity ranging from 50 to 6,000 B.H.P., and aggregating more than 50,000 B.H.P. Outside England more than twenty important installations had by 1921 been erected, or extended, aggregating fully 25,000 B.H.P.; these are spread over the world, being found in Egypt, India, Ceylon, Burma, Malay States, Hong-Kong, S. Africa, Australia and N. and S. America. The principal makers in Great Britain are Mirrlees, Bickerton, & Day; Willans & Robinson; Hick, Hargreaves & Co.; The Brit. Westinghouse Co.; Swan, Hunter & Wigham Richardson; and Thornycroft.

Noteworthy installations are (1) that of the Charing Cross & City Electricity Supply Co., Ltd., London, which includes ten Sulzer Diesel engines aggregating 6,000 B.H.P. Of these, four are of soo B.H.P. and four of 600 B.H.P. 3-cylinder engines, all running at r50 revs. per minute: the remaining two are 4-cylinder Boo B.H.P. engines running at r50 revs. per minute; this installation was completed in 1912. (2) At the Southend-on-Sea Electricity Works there is a total of 3,900 B.H.P. supplied by two high-speed 6-cylinder Koerting Diesel engines of 4 50 B.H.P. each, running at 450 revs. per minute, and four 750 B.H.P. 6-cylinder " M.A.N." Diesels running at the same speed; these engines commenced running in 1920. (3) At Letchworth, Herts, the Electricity Supply Station contains six Diesel engines aggregating 1,90o B.H.P.; the first of these commenced work in 19ro and the sixth in 1916. (4) The great majority of Diesel engines are of the inverted vertical type; an interesting exception is that of the plant at Kingston-on-Thames Electric Power Station where are installed one 400 B.H.P. 4-cylinder 4-stroke M.A.N. horizontal Diesel engine running at 190 revs. per minute, and one 500 B.H.P. 2-cylinder, 2-stroke M.A.N. horizontal Diesel running at 165 revs. per minute; these engines commenced work in 1913. Of large installations outside England may be mentioned that of the Hong-Kong Electric Co. with an aggregate of 3,060 B.H.P. supplied by seven Sulzer Diesel engines; the first of these, two 3-cylinder engines each of 300 B.H.P., started work in 1908, and the last two - of 4-cylinder 540 B.H.P. each - in 1914. An external view of a 500 B.H.P. 4-cylinder, 4-stroke standard Diesel engine is shown in fig. 10.

FIG. If/ Marine Diesel Engines. - Marine Diesel engines differ in no essential respect from the land type. The majority of Dieselengined vessels existing at the end of 1920 were fitted with 4stroke cycle crosshead type engines largely on account of the known reliability of the 4-stroke engine in land installations. Two-stroke Diesel engines were at first made with cast-steel cylinder covers which not infrequently failed by cracking; this difficulty has been entirely overcome by making the cylinder covers of a suitable grade of cast iron, and the single-acting 2stroke engine will probably become the preferred type..

For the very large engines required by war-ships the doubleacting 2-stroke design is most suitable, and several had already been built in 1921 though there was not yet an instance of one fitted in a vessel; the cooling problem had not yet been quite satisfactorily solved. The 2-stroke engine possesses the advantages of reduced weight, reduced space occupied, greater simplicity in reversing gear, and lower first cost as compared with the 4-stroke type. Up to the end of 1920 a maximum of r,800 H.P. had been attained in a single-acting, and 2,750 H.P. in a double-acting 2-stroke Diesel cylinder.

The first ocean-going passenger ship propelled by Diesel engines was the East Asiatic Co.'s 12-knot boat " Selandia," carrying a dead-weight cargo of about 7,400 tons, Copenhagen to Bangkok. There were twin screws each driven at 140 revs.

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 Dieselengined vessel afloat was the " Siam," built and engined by Burmeister & Wain, at Copenhagen, having a displacement of 13,200 tons; this boat is fitted with two 4-stroke engines aggregating 3,150 I.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 companies 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 Curle, 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: (r) A large vessel for the Ocean Steamship Co., Liverpool, of 1 5,000 tons displacement and 13 knots speed; this was to be fitted with two large 8-cylindered Diesel engines by Burmeister & Wain, each developing 3,200 I.H.P.; the daily fuel consumption was estimated not to exceed 20 tons. These engines were of the same type and size as those already fitted in four 14,000-ton " Glen " liners built by Harland & Wolff; (2) two 14,000-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 b y the North British Diesel Co.; these were 8-cylinder 4stroke ' engines of 262-in. bore, and 47-in. stroke running at 96 revs. per minute, and giving about 2,330 I.H.P.; this was the first large Diesel-engined passenger liner.

Great attention was in 1921 being devoted to improved designs, 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. 11) 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 r,000 H.P. marine engine comprises two " units," i.e. four open-ended tubular cylinders, eight pistons, and a 4-throw crankshaft. The cylinders are 181 in. in diameter, the stroke of each piston is 25 in., and the engine runs at to-115 revs. per minute.

Two of these engines had b y 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. J9 lb. of oil per S.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-c y lindered quick-revolution Diesel engines were fitted in submarines during the war by Thornycroft, Vickers, White, etc. This class includes engines having up to 12 c y linders, and run usually at from 400 to 500 revolutions per minute. The y 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 (h). 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 pressure of the normal Diesel engine necessitates not only a heavy and costly design but in addition the maintenance of an extremely high pressure in the air reservoirs for supplying the fuel oil blast. Designers have accordingly devoted considerable attention to the problem of producing engines (1) having a lower compression 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 " atomizer," 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 b y 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 conclusion of a compression stroke. The heat generated by the compression 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 compression. 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 combustion 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 (1) 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 highpressure air blast; a usual device comprises a small force-pump operated b y a quick-acting or " steep " cam which causes the small charge of oil delivered by it to forcibl y 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 the form of a welldiffused fine spray. Several recent designs include simple air compressors 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 waterdrip " unnecessary during prolonged full-load running.

An instructive series of diagrammatic sections through nine representative semi-Diesel designs is given in fig. 12 1; seven of the engines are of the two-stroke type, the " hot-bulb " being shown.

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

FIG. 11.

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.

a b c e g la FIG. 12. - Two-stroke Cycle: - (a) " Beardmore," (b) "Bolinders," Ec) " Petter," (d) " Ailsa Craig," (e) " Campbell," (f) " Kromhout," (g) " Robey." Four-stroke Cycle: - (h) " Cross," (i) " Hein." The Swedish " Bolinders " engine (agents, Messrs. James Pollock & Sons, London) has for y ears been successfully and largely applied to the propulsion of fishing-boats and coastal craft. As early as Dec. 1911 the small (65-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 semiDiesels, 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 account of their simplicity, relatively low cost, and ability to use as fuel, with good economy, most of the heavy petroleum fuel oils varying from o 8 to 0.9 in specific gravity and 130° F.-250°F. 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 lb. 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 Vaporizer-type Engines

There is no change of any fundamental importance to record regarding this group; mostly of the singlecylindered horizontal type, their use is established in many cases where 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, aircraft, 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 multitudinous 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 remarkable 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 lb. to 9 to 12 lb. 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 to 95 lb. per sq. in. approximately. Compression pressures employed were from about 70 to 120 lb. 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-cylindered 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. 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 engines 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 remainder 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 ver y convenient electrical self-starting equipment with which so many vehicles are now supplied; 1 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 hilly advanced.

1 For a valuable review see The Autocar for Oct. 23 1920.

2 Proc. Inst. Auto. Eng., 1919-20, P. 374 With improvements in the H.T. magneto, the recently introduced American " Impulse Starter " may be found to prove a simple, compact, 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 1921 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 9d. and 6d. tax, total 1 s. 3d.; in August 1920 it rose to the very high figure of 4s. 3d.; by June 1921 it had fallen to roundly 3s. This great increase, added to the heavy vehicle taxation of L1 per Treasury-rated horse-power (= 0.4 X Bore 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 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.

FIG. 13.

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 aircooled; 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 D; 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 explosive 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. Simultaneously 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 compresses 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 cycle 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.

I FIG. 14. - (a) 75-B.H.P., Single-cylinder Horizontal Engine. 200 R.P.M. Weight 200 lb. per B.H.P. Total 15,000 lb. (b) 75-B.H.P., 6-cylinder, Vertical, Water-cooled Aero Engine. 1200 R.P.M. Weight 51 lb. per B.H.P. Total 410 lb. (c) 75-B.H.P., 7-cylinder, Rotary, Air-cooled Aero Engine. 1200 R.P.M. Weight 24 lb. per B.H.P. Total 205 lb.

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 Some Data Relating To Typical British Aero Engines In 1919 (From Lord Weir, of Eastwood) engines in 1910 by Burls' showed that 10 were horizontal, 12 radial, 25 diagonal, 24 vertical or " straight " and 5 rotary. With few exceptions the horizontal, diagonal, and vertical engines were water-cooled, the radial mostly air-cooled, and the rotary all air-cooled. Horse-power ranged from 15 to 130, but in aeroplane service from 60 to 120 was usually found. The weight per B.H.P. even in 1910 ranged from slightly under 2 lb. in the " Gnome " type of engine, illustrated in fig. 15, to as much as 7 lb. in water-cooled types in the remaining classes.

In July 1919, Lord Weir of Eastwood' gave a table (see above) of data relating to current typical British aero engines; it will be seen that vertical, rotary, radial and diagonal designs are all represented, the last-named predominating. The largely increased power of the 1919 engines is noteworthy; in 1910 the practical maximum used was about 130; in Lord Weir's table the maximum is 610 B.H.P. The 1919 engines show also a very satisfactorily low consumption of petrol and lubricating oil.

Considerable reduction has also been effected in the weight per B.H.P. of the water-cooled diagonal-type engines, which range from 34 lb. down to only 22 lb. In the engines of 1910 the average piston speed was, roundly, ',too ft. per minute; the average of the eight engines in the above table is roundly 1,800 ft. per minute, a substantial and noteworthy increase.

The power of aero engines is usually stated at ground level; with increase of altitude the power output diminishes owing to the lessened density of the air; if at ground level a full power of too H.P. be obtained, at 5,000 ft. elevation this falls to about 82, and at 10,000 ft. to only about 68. To provide for this loss some aircraft engines have been designed for partly throttled running only at ground level, full throttle being only used when working in rarefied air at a suitable altitude; in these engines large compression ratio and forced induction are usually adopted, and full power output is never at 1 "Aero Engines," G. A. Burls (Charles Griffin).

2 Proc. of N.E. Coast Inst. of Engineers and Shipbuilders. tempted at the ground level. In other cases a blower is provided to deliver additional air to the cylinders at high altitudes; exhaustdriven turbines of high efficiency have been developed by Rateau for this supercharging; by this means a nearly constant pressure may be maintained in the cylinders at the end of the suction stroke, with consequent constant power output at all heights.

In radial, as in rotary, engines a star-wise arrangement of the cylinders is adopted, all the pistons operating upon one or two cranks only, but the cylinders, are stationary and the crankshaft rotates. Designs have appeared including 3, 5, 7, 9, 10 and 14 cylinders, the two last in two planes.

The' 9cylindered air-cooled radial engine of the Cosmos Co. developed 450 H.P. with the extremely low weight of but 1.47 lb. per horse-power; but large radial engines are open to objection on account of the increased head resistance involved in their use. With vertical or " straight " engines, i.e. those in which the cylinders are arranged as in the normal motor-car engine, weight per horsepower is found to diminish up to about four cylinders; thereafter, the larger crankshaft and heavier crank-case necessary to provide adequate stiffness tend to cause the weight per horse-power to increase; designers have accordingly associated cylinders together in two or more rows, and the " diagonal " engine with two or more pistons operating on each crank-pin is thus frequently met with in recent high-powered engines.

Weight is also saved and mechanical efficiency increased by operating the valves directl y from overhead cam-shafts. Aluminium alloys are also largely used for pistons and cylinders, the latter being fitted with thin steel or cast-iron working barrels.

In their 450-H.P. engine 'Messrs. Napier have three rows each of four cylinders,-twelve in all,-three connecting-rods being attached to each crank-pin; the weight is thus reduced to only 22 lb. per B.H.P. " wet." The latest design of this form in 1921 was a sixteen-cylindered r,000-H.P. aero engine, illustrated in external view in fig. 16 (Plate). In this engine there are four rows or " banks," each of four cylinders; the cylinders are separate, each being machined from a solid steel forging, with water-jackets formed of light steel pressings welded on.

The vertical angle between the axes of the cylinder rows is 522°, side angle 90°, and angle at base 1272 °; the engine may be regarded as formed of two eight-cylinder 90° diagonal engines placed back-toback, and jointly actuating a " flat four-throw " crank-shaft, with four pistons operating upon each crank-pin. The angular arrangement adopted is considered by the builders to give the most convenient sequence of working impulses. Two exhaust and two inlet valves are provided in each cylinder head, placed at such an angle ithat 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 4evelopment of aircraft of all kinds,, and aeroplanes driven by 'Iwo, 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, .hut 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 tightly 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 atmoSphere; 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 throc'gh 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, releases a pawl holding up 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 watercolumn 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. Inst. Mech. Eng. for Dec. 190o. 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 described, are all unequal, the working stroke being the longest; this is thermodynamically an advantage.

Mr. Humphrey has produced designs of pumps of this type capable of working with a suction, and for lifts of as much as 300 feet.

A very interesting installation of Humphrey pumps is that at the Chingford reservoir ofthe 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 horsepower. The, pumps use gas supplied by anthracite-burning gas-producers,. andtheconsumption per actual pump-horsepower-hour is, stated to be about o. 9 lb. of anthracite only.

(b) The Internal Combustion Turbine

The exceedingly difficult problem of the internal combustion turbine has continued to receive attention; the chief difficulty encountered has been that of the extremely high temperature (r50o-2000 C.) 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 constantpressure type by reducing flame temperature at efflux to about 400 °C. by the addition of large quantities of steam; this may accordingly be equally well regarded as a highly superheated steam turbine. About 3 lb. of petrol were required per B.H.P.

Lower 17.

hour, which 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 occurring 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 FIG. 16. - END AND Side View Of 16-Cylindered woo-H.P. Aero Engine.

sidered by many engineers that a combination of the steam and gas turbine will be found to be a satisfactory solution.

Herr Holzwarth has, however, devoted many years to the production of the gas turbine alone, and has made noteworthy progress.' The Holzwarth turbine unit comprises a small combustion chamber supplied with a mixture of gas or oil vapour and air by a suitable pump, through mechanically operated inlet valves.

The mixture is delivered under small pressure and ignited by a H.T. magneto. The resulting gases, at high temperature and pressure, then discharge through a spring-controlled flap valve termed the " nozzle valve," and issuing from the nozzle impinge on the vanes of the rotor. Having passed the vanes, the gases enter an exhaust chamber wherein a partial vacuum is maintained by a suitable exhauster. Shortly after ignition the nozzle valve is slowly closed mechanically, sufficient time being allowed for a gust of scavenging air to be passed through the combustion chamber, which is thus cooled and cleared in readiness for the next working charge; this air-gust also cools the nozzle and rotor vanes.

Thus the action is intermittent, and three valves and charging and exhausting pumps are required. In an actual turbine several such units are disposed around a turbine wheel, or " rotor." Towards the end of 1920 the first gas turbine, using oil fuel, was built in Germany to a definite order by Herr Holzwarth. This turbine is direct-coupled to an alternator, and is stated to develop 500 B.H.P. with an overall thermal efficiency of 26%.

(c) The " Still " Engine

This is a combination of an internal combustion engine and a steam engine. The working cylinder at one end uses a combustible mixture of gas or oil vapour and air, and at the other end of steam produced from the exhaust of the " internal combustion " end of the engine. The quantity "of steam generated from the heat of the exhaust is stated to be about 7 lb. per B.H.P. hour at full load.

An experimental " Still " engine tested by Prof. Vernon Boys gave an average mean effective pressure on the 4-stroke internal combustion side of 90 lb. per sq. inch, and from the steam side of 14 lb. per sq. inch; thus the effect was equivalent to that of a normal 4stroke internal combustion engine giving a mean effective pressure of 90 - (2 X14) =118 lb. per sq. inch, as the " steam " end has a 2-stroke cycle.

A high thermal efficiency is claimed, and it was stated in May 1921 2 that official trials recently carried out on a 350 B.H.P. experimental " Still " engine under Lloyd's inspection had shown such favourable results that Messrs. Scott's Shipbuilding & Engineering Co., the licensees, had decided to standardize designs of 6-cylinder marine sets of 2,000 B.H.P. using oil fuel in the manner of a 2-stroke cycle Diesel engine,. with " solid," i.e. mechanically sprayed, injection of fuel into the cylinders. These engines would be started and reversed by steam. The development of this combination of internal combustion engine and steam engine was one which in 1921 was being followed by engineers with much interest. (G. A. Bu.)

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