NATURAL GAS.
Modern gas engines are almost universally fitted with electrical ignition, commonly of the low tension magneto type wherein a magneto with a rocking armature provides current to a low tension make-and-break ignition plug fitted to the combustion chamber of the cylinder. High tension magneto ignition, sub stantially as used in all petrol engines, though of robuster type, —is also becoming more general; thus in 1922 the gas engines of, inter alia, the National, Campbell, and Browett-Lindley companies were fitted with H.T. ignition.
Starting of Gas Engines.—For engines of less than about 3o H.P. no special starting apparatus is usually needed; having retarded the ignition, and turned on the gas supply, the fly wheel is turned by hand as quickly as possible and the engine thus started.
Many gas engines of 30-20o H.P. are started by aid of a small pump fitted to the engine by which an initial charge of gas and air is delivered by hand-power into the combustion chamber, and ignited by "flicking over" the L.T. magneto by hand; many flywheels are furnished with a ring of holes round the rim for insertion of a crow-bar, by which the engine may be "barred" round so as to get the crank-pin in a favourable position for starting, i.e., about 20° beyond the inner dead centre on the firing stroke. Large gas engines are started by compressed air stored in cylindrical steel reservoirs at a pressure of ioo-3oo lb. sq.in. The air compressor is sometimes belt-driven from the en gine, but in very large engines is a separate unit driven by a small auxiliary engine. A special air-starting valve is fitted to the com bustion chamber of the cylinder, and on admission of the com pressed air through this the engine commences to turn, and takes up its working cycle after a few revolutions.
Modern Gas Engines.—In gas engines of less than about 400 H.P. but little essential change has occurred since 191o; there is a steady output by many builders of repute of a nearly standard type of horizontal four-stroke cycle single-acting one- or two cylindered engine using Town's gas or producer gas as fuel. A normal engine of this type, by Crossley Bros. is shown in fig. 5, comprising a water-jacketed cast iron cylinder fitted with a gas tight cast-iron piston driving a crankshaft in the usual manner by means of a connecting rod. The very massive flywheel will be noted ; in this case it is formed with an internally-toothed ring and by means of the hand-wheel and enmeshed pinion shown "barring" round before starting is effected. At the closed end or "breech end" of the cylinder is the combustion chamber con taining at the top the mixture inlet valve and at the bottom the exhaust valve both, of course, cam-operated from the half-speed shaft. Ignition is by low-tension (L.T.) magneto. The engine speed is regulated by a governor which varies the lift of the inlet valve.
Vertical Gas Engines.—The considerable bulk and weight of the horizontal slow-running larger gas engine has caused attention to be given to faster-running multiple-cylindered inverted-verti cal designs, occupying much less floor area, and of diminished size and weight. Prominent among British builders of this type is the National Gas Engine Co. whose designs range from a four cylinder, two-crank, single acting four-stroke inverted vertical tandem engine of 30o B.H.P. at 30o rev. per minute, to a 12-cylin der six-crank similar design of 1,50o B.H.P. running at 200 rev. per minute. One "National" installation of this type aggregates i 1, Soo horse power.
Large Gas Engines.—From about 191 o improvement in the largest types of gas engine have been mainly in matters of detail as, e.g., in the more extended use of refuse material as fuel for the gas producers : in the employment of high tension electrical ignition : and in better cooling and governing arrangements.
The large horizontal slow-speed engine is typified by the four stroke double-acting tandem design as developed by the Nurem berg (M.A.N.), Ehrhardt, Deutz, and Haniel companies in Europe, and built also in Great Britain by the Lilleshall Co., Vickers and Galloways.
The tandem double-acting four-stroke engine possesses the important advantage that every stroke is a working stroke. A view of a 2,50o Horse-power M.A.N. engine, in part section, is shown in fig. 6. The two liberally water-cooled cylinders A,A', are in line with their water-cooled pistons mounted on a common piston-rod (also water-cooled), to one end of which is attached the connecting-rod C driving a single-throw crankshaft D. The three crossheads EEE are adjusted to carry the weight of the piston rod and also of the two heavy pistons, so that these "float" in the cylinders, thus greatly reducing engine friction and conse quent wear. The inlet valves will be noted at the top, and the exhausts at the bottom, of each cylinder. To illustrate perform ance :—Eight engines of this type by Vickers had each a cylinder bore of 43.3 in., a stroke of 47•3 in. and, using blast furnace gas, developed roundly 1,800 B.H.P. at a speed of ioo rev. per minute. One of the largest installations of M.A.N. engines is a 65,000 horse-power plant at Bruckhausen. The type is very reliable and economical; a 2,000 H.P. engine using blast furnace gas has run, day and night, during 19 months. In 13,87o consecutive hours the engine actually ran during 13,687 hours, i.e., 98.6% of the whole time possible; and the stoppages were due to the blast furnace,. and not to the engine.
Large Two-stroke Gas Engines.—The large two-stroke Clerk cycle engine has proved very successful, particularly in the hori zontal designs of Koerting & Oechelhauser.
The Koerting is a single-cylindered double-acting horizontal two stroke engine, and the piston thus receives a working impulse in every stroke. A sectional view of a Koerting cylinder appears in fig. 7. The engine is characterized by the long water-cooled piston AA,--of length fully half that of the cylinder; in its extreme right and left positions the piston over-runs a central belt of exhaust ports BB, and so permits the escape of the burnt gases into the exhaust branch C. At each end of the cylinder is a me chanically operated inlet valve D, The illustration shows the contents of the right-hand end of the cylinder exhausting through BB ; simultaneously the inlet is opened admitting firstly air only, and immediately afterwards the working charge of mixed gas and air. The preliminary air is a scavenging charge; the working charge is delivered to the inlet valve boxes by separate double acting pumps driven by the engine and no mixture of gas and air occurs until they jointly enter the working cylinder; this is an important point in relation to large gas engines. The piston now moving towards the right first cuts off the exhaust ports BB and next compresses the mixture into the combustion chamber where it is fired (by L.T. magneto) as usual, the resulting explosion driving the piston towards the left. The same cycle of operations is performed in the left-hand end of the cylinder, and thus every stroke of the piston is a working stroke. Starting is effected by compressed air stored at 150-300 lb./sq.in.; the air starting valves are shown at EE in fig. 7.
The Oechelhauser Engine.—This successful large two-stroke gas engine works on a modified Clerk cycle ; a diagrammatic view appears in fig. 8. The engine is of the horizontal single-acting type having a cylinder AA open at both ends, containing two pistons BB' working in opposite directions, and driving a common three throw crankshaft. B drives the central crank through an ordinary connecting rod, while B' is fitted with a short piston rod R termi nating in a crosshead Q fitted with side rods PP' and return con necting rods driving the two side cranks. The combined gas and air pump HH is also driven from the crosshead Q as indi cated. Just before it reaches its extreme "out" position piston B over-runs the ring of exhaust ports C; at the same time piston B' over-runs firstly the ring of air inlet ports D and immediately thereafter a second ring of gas ports E. The exhaust gases at once escape through C, assisted by the scavenging charge of air under slight pressure entering through D. This fresh air also cools any residual exhaust gas, so minimizing risk of pre-ignition, and pre venting loss of fresh fuel through the exhaust when the gas ports E open. The pistons next approach one another and compress the entrapped fresh charge between them; at the end of the "in," i.e., compression stroke, the mixture is fired electrically and the work ing out-stroke then follows. The engine thus furnishes one im pulse per revolution. The double-acting pump H is an air-pump on one side of its piston and a gas-pump on the other ; gas and air are pumped separately into the reservoirs L and K respectively, in which a pressure of 5-6 lb./sq.in. is maintained. It will be noted that the gas and air are only mingled actually within the working cylinder. Oechelhauser engines to Messrs. Beardmore's designs range from 40o horse power at 13o rev. per min., with cylinder 24 in. bore and stroke of 3o in., to 2,50o horse power at 8o rev. per min., with cylinder bore of 48 in., and stroke of 6o in.
The Fullagar Engine.—A valuable advance in the double-piston Oechelhauser type of engine was made by H. F. Fullagar about 1912. By associating together two open-ended cylinders he dis pensed with the somewhat cumbrous side-rods and return con necting-rods of the Oechelhauser design, and also was able to use a normal flat two-throw crankshaft. In this way weight and cost were saved and a very compact engine, giving two impulses per revolution, resulted. The essential feature of the Fullagar power unit is a pair of side-by-side open-ended cylinders with their pistons connected across by special diagonal tension rods; the obliquity of these rods is not great, and the side-thrusts arising therefrom are borne by crossheads and (water-cooled) guides. The compressed power charge between, say, pistons•A and B being fired, B is driven downwards thus actuating the left-hand crank, while A is driven upwards, thus simultaneously actuating the right hand crank through a tension rod; hence as the result of each working impulse the crankshaft experiences a nearly simple "torque" ; moreover, the cylinders being open-ended, no stresses are set up in the engine-framing during running; thus the Fullagar engine may be built somewhat light for its power output. As described here, the charges of gas and air are as if sup plied by pumps formed by boxing in the top crossheads; the charge is delivered at a pressure of about 3 lb./sq.in. As with other two stroke cycle engines, the Fullagar may be run in either direction by giving it the necessary initial motion, and suitably timing the ignition.
An early experimental engine of 55o B.H.P. by Allen of Bedford comprised two Fullagar units with cylinders of 12 in. bore, and each piston with a stroke of 18 in. ; normal speed 25o rev. per min. A 3o-hour trial of this engine showed an absolute thermal indi cated efficiency of 37.6% ; the weight of the engine was only 87.5 lb. per B.H.P. which is small for a stationary design. A 2,000 horse-power engine by Belliss and Morcom in 1918 comprised three Fullagar units, with 18 in. cylinders and 27 in. stroke, run ning at 17o rev. per minute. This engine used as fuel coke-oven gas having a net calorific value of 44o B.Th.U. per c.ft. and tests showed a full-load consumption of roundly 2 2 4 c.ft. of gas per B.H.P. hour, corresponding to an absolute brake thermal efficiency of 26%. The Fullagar engine also uses oil fuel in the Diesel man ner, and marine Fullagar-Diesel engines are constructed by Cam mell Laird and others in sets of 3,00o B.H.P. and over.
BIBLIOGRAPHY.-J. E. Junge, Gas Power (1908) ; R. C. Carpenter Bibliography.-J. E. Junge, Gas Power (1908) ; R. C. Carpenter and H. Diederichs, Internal Combustion Engines (1908) ; H. Allen, Gas Engines (19o9) ; H. Guldner, Design and Construction of Internal Combustion Engines, trans. H. Diederichs (191o) ; H. Harder and W. M. Huskisson, Handbook on the Gas Engine (1911) ; D. Clerk and G. A. Burls, The Gas, Petrol and Oil Engine (19'3) ; C. F. Hirshfeld and T. C. Ulbricht, Farm Gas Engines (1913) ; H. Dubbel, High-power Gas Engines (1914) ; H. R. Ricardo, The Internal Com bustion Engine, 2 vo1s. (1922) ; D. Clerk and G. A. Burls, "Thermo dynamics of Internal Combustion Engines," Dictionary of Applied Physics, vol. 1 (1922) ; A. W. and Z. W. Daw, Oil and Gas Engine Power (1923) ; G. A. Burls, "Gas Engines and Gas Producers," Modern Mechanical Engineering, 6 vols., ed. A. H. Gibson and A. E. L. Choriton (1923) .