Excerpts from MODERN MECHANICAL ENGINEERING
The following is the third in a series of articles from the 1923 edition of Modern Mechanical Engineering, on the subject of gas engines. The original articles were sent to us by Jan vander Gugten, 2633 Ware Street, Abbotsford, B.C., Canada V2S 3E2, who thought our' readers would find them of interest. The first two installments appeared in our April and May 1997 issues.
Engines in general are required to maintain a considerable degree of uniformity in their speed under normal working conditions, and in this respect the single-cylindered single-acting four-stroke engine is at a serious disadvantage, as a working impulse occurs only, at best, once in every four strokes; the momentum of massive flywheels thus has to maintain rotation during at least three-fourths of the running time. This inherent drawback is, however, reduced to a practically negligible amount: (a) by using multi-cylindered single-acting engines; (b) by using tandem or twin-tandem double-acting engines; and (c) by using two-stroke single-or double-acting engines; and in nearly all cases with the addition of a massive flywheel.
Speed Fluctuation. The coefficient of fluctuation of speed of an engine is defined as the ratio of the difference between the maximum and minimum angular velocity of the crank-shaft per cycle to its mean angular velocity. The permissible value of this coefficient depends upon the nature of the work performed by the engine; usual values in a number of typical cases are given here-under:
Nature of Service
Coefficient of Speed Fluctuation.
Driving machine tools
Driving textile machinery
Driving C.C. dynamos
Driving spinning machinery
Driving direct-coupled alternators in parallel
Cf. R. E. Mathot, Construction and Working of I.C. Engines (Constable), or Clerk and Burls, The Gas, Petrol, and Oil Engine, Vol. II (Longmans).
For a full account see Clerk and Burls, Vol. II, Chapter IV.
So long as the external resistances overcome by the engine remain unchanged, cyclic speed fluctuation can be reduced to any desired extent by providing sufficient flywheel inertia; for a detailed consideration of this question reference must be made to the larger special treatises.
When the external resistance is changeable, it is usually necessary to provide that the engine speed shall not be permitted to vary from the normal by more than a small amount, and some system of governing thus becomes essential.
The governor itself is almost always of the centrifugal type of which the well-known 'conical pendulum' of Watt is the parent; such governors have been greatly developed in recent years by 'British and German engineers, among whom may be mentioned Hartnell, Prll, Beyer, Hartung, and Rost.
In all these centrifugal action, due to varying engine speed, provides motion which is applied to vary the quantity or quality of the explosive charge admitted to the engine cylinder in one or more of the following three principal ways:
1. By completely cutting off the gas supply in one or more cycles 'hit-or-miss' governing.
2. By partially cutting off the gas supply, thus varying the quality of the mixture, the mass of the charge remaining unchanged.
3. By varying the opening of the mixture valve, thus varying the quantity of charge admitted to the cylinder, keeping the ratio of gas to air unchanged.
'Hit-or-miss' Governing. In this method when engine speed increases one or more charges of gas are completely cut out, so that no working stroke occurs until the speed falls again to about its normal value. It was for long almost universally employed in stationary gas-engines, and is still widely used in engines of up to about 100 b.h.p. (as, e.g., by the Campbell Company) in cases where extreme uniformity of speed is unnecessary. It possesses the advantage of giving economical fuel consumption both at light and at full loads, and is mechanically simple and reliable.
A simple and effective hit-or-miss arrangement is illustrated in Fig. 7. The 'pecker' P normally opens the gas valve H through the 'pecker block' D, suspended by a rod C from a lever A which is pivoted at B; obviously a very small rise of the governor balls suffices to lift D out of the way of the pecker P, and when this occurs the gas valve remains closed. The governor is easily and quickly adjusted by varying the spring compression by aid of the milled nuts N on the top of the governor spindle.
Quality Governing With this method of governing, the mixture becomes progressively weaker as the load on the engine diminishes, though the compression pressure is constant since the mass of the charge remains unchanged. The reduction in the gas admitted is usually affected either: (1) by shortening the duration of opening of the gas valve, the air admission remaining constant; or (2) by throttling the gas supply throughout the suction stroke, with constant air admission. Often the gas valve is not opened until part of the suction stroke has been performed, thus providing a rich and readily ignitable mixture near the firing plugs in the combustion chamber.
At light loads the weak mixtures supplied to the engine were at first difficult to ignite and burned slowly; heat losses were thus often much increased, and the charge was also often not fully burned before release occurred. Combustion was occasionally so slow as to persist during the exhaust stroke and explode the succeeding fresh charge, thus causing what is termed as 'back fire'. Both these drawbacks are practically overcome by delaying the opening of the gas valve until part of the suction stroke has been performed.
By simply stepping the pecker block as shown in the lower portion of Fig. 7, a crude form of quality governing is readily obtained. With a stepped pecker block the pecker, when raised by the action of the governor, moves the gas valve not only later but also through a smaller lift, and finally, when the speed-change is considerable, misses it altogether.
The Premier gas-engines are governed on the 'quality' method; a centrifugal spring-loaded enclosed governor driven from the crankshaft by skew gearing controls a throttle valve in the gas supply. A difficulty gas-engine designers have to overcome is that of the considerable variation in the quantity and composition of the gas supplied to the engine, particularly when only one suction producer is employed, and pro -vision for manual adjustment of the mixture is therefore necessary. In the Premier engines a hand-adjusted master-throttle determines the total air supply to the engine, while separate throttles to the cylinder enable the mixture strength of each to be independently regulated.
Quantity Governing. In this method, the mass or 'quantity' of the working charge is reduced as the engine load diminishes, but the ratio of air to gas is kept unaltered; the compression pressure is accordingly also then reduced. Sir Dugald Clerk observes:
'The reduction in the compression pressure causes some diminution of thermal efficiency at such times; but as the compression and expansion curves rise and fall together the variation of crank-pin effort is not unfavourably affected. With poor gas of small hydrogen' content high compression pressures (175 lb. per square inch and above) may safely be used, thus giving increased economy and more rapid and complete combustion of the charge, and in such cases quantity governing is most satisfactory, since the compression at light loads remains still high enough to ensure ready ignition of the mixture...the frictional resistances of the engine are also reduced by the diminished compression. On the whole the balance of practical advantage in general favors governing by this method rather than by that of quality.' Quantity governing is largely employed, as, e.g., in the Browett-Lindley, Crossley, Hornsby-Stockport, and Tangye gas-engines. The exceedingly simple and effective 'shifting fulcrum' device used in the quantity governing of the Crossley gas-engines is shown in Fig. 8, wherein the rocking lever fulcrum is in the position for light load; at full load the governor moves the fulcrum towards the right, causing the two arms of the rocker to become nearly equal, thereby increasing the lift of the mixture inlet valve. The gear being in full view of the attendant, inspection of the position of the fulcrum indicates at once the amount of load on the engine. With this governor the permanent speed variation between no load and full load is only about 2percent from the mean. Messrs. Tangye also use a shifting fulcrum device, but of somewhat different type.
The governing arrangement used in the 'National' gas-engines ingeniously combines all three methods, viz. of hit-or-miss, quality, and quantity; it is illustrated diagrammatically in Fig. 9. The gaseous mixture is admitted to the combustion chamber of the cylinder through the charge inlet valve A; this valve is operated by its cam in an unvarying manner at all loads, opening about 15° before inner dead centre, and closing about 55° after outer dead centre of the crank-pin. The air valve B is a circular disc' of somewhat smaller diameter than the hole in the casing in which it works, so that even when the gas-valve C remains seated it is still possible for air alone to enter the cylinder through the charging valve A.
The cam D on the side shaft actuates the air-valve spindle and through this the gas valve also as indicated through the agency of the two levers E and F and the governor-controlled plate H, the lever E is pivoted at K, and when the cam D is out of action the serrated ridge of the lever E is clear of the plate H, and during such times the governor is free to raise or lower this plate agreeably with its speed of rotation. When the speed increases, H is raised and the opening of the gas and air valves is then reduced, and vice versa; in this way the lift of these valves may be varied from ? inch at light loads to inch at full load.
The cam D is so shaped that the cutoff point of the gas remains nearly constant; the gas is cut off early enough to allow the combustible mixture in the space below the charging valve to be sucked into the combustion chamber before this valve closes. This is necessary in order that this space may be filled with air which then alone enters the cylinder at the commencement of the next suction stroke, thus cooling the residual exhaust gases before fresh inflammable mixture enters the combustion chamber.
At full load the plate H occupies its lowest position, and a full charge of gas and air is admitted. For somewhat smaller loads the gas supply is reduced while the air supply suffers but little diminution; the mass of the charge and accordingly the compression pressure are therefore but little affected, and the governing is of the quality type. At lighter loads the air valve B is nearer to its housing, and the ratio of gas to air admitted then remains more nearly constant while the mass admitted is reduced; the governing is then of the quantity type. Finally, at very light loads the plate H may be raised so high that it is occasionally missed altogether by the serrated ridge of the lever E, and the governing is then of the hit-or-miss type.
For gas-engines of less than about 30 b.h.p. no special starting apparatus is usually needed. When preparing to start, attention should be given: (1) To the oiling system. All lubricators and oil reservoirs should be replenished, oil wicks adjusted, and sight-feed lubricators started. (2) To the cooling water system. The cock on the water-supply-pipe to engine should be opened, drain cocks closed, etc. (3) The inlet, exhaust, and throttle valves should be moved by hand to ascertain that they are not 'gummed up.' (4) The driving belt should be on the loose pulley. (5) If a compression relief be fitted, the exhaust valve roller should be placed opposite the narrow or 'relief cam. (6) The ignition should be fully retarded. (7) The gas should be turned on only when ready to start, ascertaining first that the gas-supply pipe is free from air and charged with pure gas up to the engine by turning on the vent pipe and lighting the test burner until it burns well and steadily. (8) The flywheel should next be turned by hand as quickly as possible until the engine starts, pulling downwards on the rim and away from the engine; the feet should never be placed on the flywheel spokes. (9) After a few explosions have occurred the ignition may be slightly advanced and the relief cam put out of action. (10) When full speed is attained and the engine is warmed up the ignition should be advanced to the normal working position, and the air and gas regulators adjusted. Finally the load may be imposed on the engine.
With engines of over about 30 b.h.p. some form of special starting apparatus is, in general, necessary. Flywheels arc usually furnished with a series of holes round the rim for the insertion of a crowbar, or an internally-toothed ring operated by a small pinion and hand-wheel or small motor, by which the engine may be 'barred' round so as to place the crank-pin in a favorable position for starting, i.e. 15° to 20° beyond the inner dead centre on the firing stroke.
Many medium large gas-engines, from 30 or 40 b.h.p. up to about 200 b.h.p. are started by means of a small pump fitted to the engine, by which an initial charge of gas and air is pumped by hand into the combustion chamber and ignited by 'flicking over' the magneto by hand. The explosion thus obtained imparts sufficient motion to the engine to enable it to take up its normal working cycle. A small quantity of petrol is often pumped in with the air, as an alternative to gas, and this gives a more powerful starting impulse. Such starters are very effective, and are used, e.g., in the National', Anderson-Grice, and Brotherhood gas-engines.
With large gas-engines the single impulse furnished by the hand-pump method is insufficient to effect a start, and accordingly compressed air, stored in cylindrical steel reservoirs, is employed for starting purposes. With new engines the reservoir is sent out charged, hut thereafter its pressure may be maintained: (1) from the engine cylinder and piston through a special delivery valve; or (2) from a small air-compressor belt-driven by the engine; or (3) by a small auxiliary engine and air-compressing pump. Starting by compressed air is very simple; a cam opening a small compressed-air supply valve in the combustion chamber, during the earlier part of the firing stroke, is put in action. The compression relief cam of the engine is also put in action and the ignition retarded; the engine is then barred round until the crank-pin is in the starting position (i.e. just well over the inner dead centre on the firing stroke); and the gas is turned on. On opening the air reservoir cock the engine at once commences to turn, and usually takes up its normal working cycle after three or four air impulses. The air-valve cam is then moved out of action and the air cock shut; the reservoir is made of capacity sufficient to provide a large number of engine impulses in case of difficulty in starting occurring.
Having started, the relief cam is put out of action, the ignition advanced, and the mixture adjusted as usual, the load being finally put on the engine when everything is well warmed up.
The air is stored in the reservoirs at a pressure usually of from 100 to 250 lb. per square inch; and with multi-cylindered engines air starting gear is often fitted only to some of the cylinders, as, e.g., in the four-cylindered Premier and Browett-Lindley gas-engines, where two only of the cylinders are so fitted. In the case of the 400 b.h.p. four-cylindered Browett-Lindley engines, air for starting purposes, compressed to 250 lb. per square inch, is stored in three steel cylinders or 'air bottles' each 16 inch diameter and 8 feet long.
The adequate cooling of gas-engines has proved a problem of very great difficulty, and it is still necessary to limit strictly the supply of heat to large engines in order to avoid trouble from overheating (p.11).
Of the whole heat supplied to a gas-engine, roundly from 25 to 35 per cent usually appears in the cooling water. Taking the heat supply in normal everyday working as 10,000 B.Th.U. per brake horse-power hour, it is clear that the cooling water must carry away from 2500 to 3500 B.Th.U. per brake horsepower hour. Assuming the rise of temperature of the water to be a 60° F. in passing through the engine, it will accordingly be necessary to pass through the jackets, etc., from 4 to 6 gall. per brake horse-power hour in normal full load working; when the cooling water is run to waste this is not difficult to arrange, the outlet temperature being kept at about 120° F. as, e.g., in small engines cooled from town mains. With larger engines the more usual practice is to provide an overhead storage tank from which water is supplied to the cylinder jackets under a gravity head of not less than about 20 ft. (approximately 10 lb. per square inch pressure); for engines of up to say, 100 b.h.p. and in temperate climates the 'thermo siphon' system of cooling is often employed, but in such cases the outlet temperature of the water is higher and may be 130° F. to 140° F., though it should not exceed the latter value, while the incoming water may be fully 100° F. in temperature. The rise of temperature being less than as previously assumed, more water must be circulated through the jackets, and hence it is common to find provision made for passing fully 10 gall. of water through per brake horse-power hour, with a water storage capacity of 25 to 35 gall. per (maximum) brake horse-power of engine. With large engines the water is usually pump-circulated, a cooling tower being often installed from the top of which the heated water from the engine falls through the air in fine streams. Thus a large 600-b.h.p. 'Simplex' engine working on blast-furnace gas at the Ormes by Iron Works, Middles borough, is cooled by water from a tank built over the engine-house, and delivered under a head of about 60 ft. On leaving the engine it passes into a small tank containing a float so arranged that should the circulation fail the float sinks and stops the engine through the governing gear; from this small tank the water passes to a reservoir, whence it is pumped up to the top of a 'Klein' open-type water cooler fixed above the elevated tank. The makers recommendation was that about 12 gall. of water should be passed through the jackets per brake horsepower hour, i.e. about 7000 gall. per hour total.
Soft water is preferable, but the available water is commonly of some degree of hardness which in time causes the formation of deposits in the jacket and cylinder liner; this can be largely prevented by using a specially large cooling tank, and by treating the water, when chalk-hard, with common soda, 1 lb. being added per 250 gall. of water in the tank, once a month. When tanks and coolers are used, only the loss by evaporation has to be made up, and this may be estimated at from 0.2 to 0.4 gall. per brake horse-power hour. Some further references to cooling details occur in the description of actual engines given later.
With every engine a part of the work done by the gases in the cylinder is expended in overcoming the internal resistances of the engine itself; of these resistances piston friction constitutes by far the greatest item, and accounts, under normally good running conditions, for fully 50 percent of the difference between the indicated horse-power and the brake horse-power. Piston friction is thus large, and it is also variable, being very dependent upon the condition of fit of the piston and rings in the cylinder, on the nature and extent of the lubrication, temperature of the jacket water, and age of the engine. In all gas-engines careful provision is accordingly always made for the adequate lubrication of the pistons. In small horizontal engines an adjustable glass sight-feed drip lubricator is commonly fixed on the top of the cylinder, frequently near the open end, and delivers oil on to the piston, which is furnished with grooves down which it runs, thus reaching all parts of the surface. With large gas-engines the oil is force-fed to the cylinder lubricator by a small oil-pump usually driven from the half-speed shaft. The arrangement employed in the Tangye engines, for example, is shown in Fig. 10: A is a small oil-pump cam-operated from the half-speed shaft or 'side-shaft' B; the pump takes oil from the reservoir C through a suction duct hand-regulated by the milled nut D, and delivers it as shown into the top of the engine cylinder E.
Gudgeon bearings of horizontal engines are commonly lubricated by an adjustable visible drip-feed lubricator delivering oil by aid of a 'wiper' into a short open trough fitted on the top of the small end of the connecting-rod; often also the gudgeon bearing is oiled by the piston lubricator through a hole in the upper part of the piston, as shown in Fig. 12.
Crank-pins are usually and effectively oiled by a sight-feed lubricator in conjunction with a centrifugal oiling ring attached to the crank web as clearly indicated in Fig. 11, illustrating the method used in, e.g., the Crossley, Tangye, Campbell, and Anderson-Grice engines. The oil delivered into the ring is discharged by a centrifugal action, through the duct shown, to the outer surface of the crank-pin.
Main crank-shaft bearings are most commonly oiled by 'ring' lubrication, also illustrated in Fig. 11. The ring runs loosely on the shaft and is driven round by it, thus continuously raising oil from the reservoir in the base of the bearing and delivering it on the upper surface of the shaft. A drain plug is fitted in the oil reservoir enabling it to be emptied and cleaned out when necessary. Side-shaft bearings are also often ring-lubricated, though ordinary cotton wick lubricators are frequently fitted, while, in small engines, simple oil holes suffice; the gear wheels driving the side-shaft are usually run in an oil-bath. All rollers, pins, and valve-spindles should be lightly oiled; if the gas used produces a tarry deposit, exhaust valve spindles should be lubricated with a mixture of oil and ordinary paraffin to prevent 'sticking up'. Engine bedplates and crank-pits are so arranged as to form troughs in which waste oil may collect, thus preventing the engine foundations from becoming oil-soaked.
Many of the lubricating details referred to above may be studied in situ in later illustrations herein.
Modern Gas-engines For long almost the only type of gas-engine built was the single-cylindered single-acting four-stroke horizontal design, and this is the type still most largely used from the smallest powers up to as much as 250 b.h.p. in factories, mills, and for general industrial purposes where a cyclic speed fluctuation as high as 4 to 5 percent is permissible. In 1922, for example, Messrs. Anderson-Grice, Campbell, Crossley, National Company, Premier Company, Ruston-Hornsby, Tangye, etc. in Great Britain were all building ranges of single-cylindered horizontal engines from 1 to 250 b.h.p., and of double-cylindered, and coupled, horizontals ranging from 50 to 500 b.h.p., together with a few four-cylindered horizontal designs ranging from about 250 to 600 b.h.p. per unit; all these were of the open crank-chamber single-acting four-stroke type with un cooled pistons, and running at speeds ranging from 450 to 600 r.p.m. in the smallest sizes to 150 to 160 r.p.m. in the largest. British gas engineers have not, so far, favored the extra complication involved in the double-acting cylinder with water-cooled piston, but have consistently adhered to the single-acting un cooled piston type; the largest un cooled British pistons are found in certain Crossley engines of 26 inch cylinder bore, and in some of the large vertical engines of the National Company which have 24 inch cylinders.
Large horizontal engines of the Continental type have been built in Great Britain to a limited extent by Messrs. Beard more, Mather & Platt, Galloways, Richard sons West garth, Vickers, and the Lilles hall Company, this latter firm having produced some fine examples of the Nuremberg or 'M.A.N.' type.
' National ' Vertical Gas-engines, 1922
Stroke in Inches.
Approximate Weights, Tons.
Vertical gas-engines were developed rapidly from about 1900 onwards, prompted largely by the necessity of saving floor space and weight in multi-cylindered single-acting engines; one of the first attempts to produce engines of this type was made by Messrs. Burt, of Glasgow, in 1894. High-powered quick-speed multi-cylindered enclosed verticals were common in 1922, prominent British builders at that date being:
1. Messrs Browett-Lindley, who manufactured a range having two, three, tour, or six cylinders, with an output from 60 to 750 b.h.p. per unit, and running at from 450 to 200 r.p.m.. At the Waterloo Colliery, near Leeds, two of their four-crank, four-cylinder enclosed vertical engines are installed, each driving a 250 kw. generator. Each of these engines comprises four single-acting four-stroke cylinders of 19-inch bore and 20-inch stroke, capable of a maximum output of 400 b.h.p. at 250 r.p.m.
2. The Campbell Gas Engine Company has also built vertical gas-engines since about 1904; in 1922 their standard types included two-cylindered engines of 50 to 125 b.h.p. running at 300 to 250 r.p.m.; and four-cylindered designs giving from 100 to 500 b.h.p. at from 300 to 200 r.p.m.
3. The National Gas Engine Company has devoted special attention to the production of high-powered vertical engines, especially of the enclosed, single-acting, four-stroke, tandem type, thus giving a working impulse on every downstroke of every crank-pin; one of these important engines is illustrated and described later (p.46). The standard vertical National engines of 1922 were built in two sizes, viz. of 18-inch stroke and 24-inch stroke respectively, the up -per cylinder of each tandem pair having a bore of 23 inches, and the lower of 22 inches; these engines are suitable for use with town gas, coke-oven gas, or producer gas; with a poor fuel, as e.g., blast furnace gas, extra large cylinders were fitted in order to maintain the full rated output.
Table II gives some instructive data relating to these engines.
Two cylinders, in tandem, act upon each crank. The normal full brake horse-power is that which the engines are capable of maintaining continuously; the 'overload' is 10 percent greater and should only be imposed during short emergency periods. The powers as stated are, moreover, for engines working at sea-level and in a temperate climate; when the plant is installed above sea-level a deduction of about 3 percent should be made from the power rating for every 1000 ft. of height. In Table II the initial temperature of the gaseous mixture before entering the engine is also assumed as 60 degrees F.; in hot countries some power loss is incurred by reason of the diminished density of the mixture when supplied to the engine at a higher temperature; the loss from this cause may be estimated as 1 percent of the sea-level rating for every increase of 5° F. above the standard temperature of 60° F.
The normal full revolution speeds given in Table II are also maxima values for continuous running; it is well, in order to minimize wear and tear, to arrange wherever possible to run ordinarily at from 5 percent to 10 percent below the 'normal full' values. The flywheels referred to in the table are as used with engines driving direct-current electrical generators, air-compressors, etc.; when alternating-current generators are being driven the necessary steadying rotational inertia is usually furnished by the generator rotor itself, and in such cases only a coupling at the crank-shaft end, or a light wheel for barring round the engine, is necessary. The total 'flywheel effect' required in parallel running is, however, in general settled by the suppliers of the alternating-current generators.