Fig. 2Clerk Two-stroke Cycle Engine
The following begins 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 van der Gugten, 2633 Ware Street, Abbotsford, B.C., Canada V2S 3E2, who thought our readers would find them of interest.
Historical Summary.Space permits of a very brief reference only to the early history of the internal-combustion engine; for a full account the reader may consult Vol. I of Sir D. Clerk's The Gas, Petrol, and Oil Engine. Probably the earliest instance of the use in Britain of an explosion to obtain mechanical effect is that of the primitive cannon used by Edward III about A.D. 1327. Three hundred and fifty years later C. Huygens, followed by Papin and d' Hauteville, endeavored to obtain continuous motion from successive explosions of gunpowder in a cylinder fitted with a piston; and much later still, viz. in 1820, Farish of Cambridge constructed a small engine intended to be driven by gunpowder; this explosive is, however, quite unsuited for use in internal-combustion engines for both thermal and practical reasons.
In 1820 Cecil, also of Cambridge, made what was probably the first actually working gas-engine, using as his fuel an explosive mixture of hydrogen and air; thereafter appeared in succession Brown's 'gas-vacuum' engine, Wright's engine, Barnett's engines, and the very singular 'free-piston' engines of Barsanti and Matteucci and of Otto and Langen; and in 1860 the once popular, though very uneconomical, Lenoir engine, which may be fairly described as a double-acting steam-engine using a mixture of coal-gas and air in lieu of steam; Hugon (1865) effected improvements in the details of Lenoir, but its still high fuel consumption caused its abandonment in favour of the much more economical, though mechanically objectionable, Otto and Langen free-piston type. It should be mentioned also here that in 1873 there appeared in America the theoretically efficient 'constant pressure' Brayton gas-engine, whose success was prevented by practical difficulties; the modern highly economical Diesel oil-engine illustrates the principle embodied in the Brayton of burning fuel at (approximately) constant pressure during its admission to the combustion chamber.
The final step in the evolution of the modern four-stroke cycle internal-combustion engine was made in 1862 when de Rochas first laid down explicitly the procedure to be adopted to obtain maximum economy in the working of gas-engines, comprising suction, compression, explosion and expansion, and exhaustall performed within the working cylinder.
A four-stroke single-acting engine of the simplest type is illustrated diagrammatically in Fig. 1. It includes an open ended cylinder fitted with a closely fitting piston which drives a crank by means of the usual type of connecting-rod; in the upper end, or 'combustion chamber', of the cylinder a pocket is formed containing the inlet valve A, and exhaust valve B, the latter being forcibly raised at suitable intervals by a cam C and roller-ended tappet-rod D. Suppose the engine to be turning, and the suction stroke about to commence. The piston in its descent creates a partial vacuum above it, the inlet valve A accordingly opens and fresh mixture passes into the cylinder; this is the suction stroke. When the piston has reached the bottom of its stroke and commenced to return the inlet automatically closes, and the upper charge is compressed into the upper part of the cylinder and valve pocket; this is the compression stroke. The mixture is exploded by the ignition plug at, or near, the instant of greatest compression, and the piston is next driven downwards, thus performing the expansion or working stroke. Near the bottom of this stroke the cam C lifts the exhaust valve B, and the burnt gases escape into the atmosphere; B remains open throughout the whole of the succeeding or exhaust stroke; this cycle is repeated indefinitely so long as the engine continues running. The cam-shaft is driven by gearing at half the crank-shaft speed, and is accordingly frequently referred to as the 'half-time' shaft. In all modern engines the inlet valve is also cam-operated, an increased volumetric efficiency and increased speed being thus obtained.
The Otto Silent Gas-Engine.It was not, however, until 1876 that Dr. Otto produced his world-famous 'silent' gas-engine, working exactly upon the four-stroke cycle as laid down by de Rochas fourteen years earlier. Otto realized the de Rochas cycle in a practical and most successful manner and employed flame ignition. The gas consumption of his engines was at once found to be much lower than had ever previously been attained, averaging only from about 24 to 30 c. ft. of coal-gas per brake horse-power hour. The introduction of the 'Otto silent gas-engine' marked the beginning of the era of the internal-combustion engine as a prime mover of world-wide importance.
The Otto silent gas-engine was taken up in England by Messrs. Crossley Brothers, Ltd., of Manchester, and progress was rapid. In 1878 a gas-engine of 3 h.p. was regarded as large; in 1881 a 20 h.p. engine was considered as remarkable; in 1898 the largest gas-engine built was of 220 i.h.p. The high cost of coal-gas as a fuel probably restrained the growth of engines, but the possibility of utilizing the hitherto wasted blast furnace gases for power purposes first demonstrated by Thwaite in 1895, and the evolution of the gas producer by J.E. Dowson and others between 1878 and 1903, provided engineers with suitable gases for power purposes which could be cheaply and easily produced in very great quantities. These discoveries provided the necessary stimulus to the construction of very large engines.
Thus in 1899 the Socit Cockerill of Seraing (Belgium) had in operation a single-cylindered single-acting four-stroke engine of 51.2 inch bore and 55.13 inch stroke running on blast-furnace gas at 90 r.p.m., and developing 600 b. h.p.
At the end of 1910 the Nuremberg Company (M.A.N) had in operation double-acting gas-engines aggregating roundly 450,000 b.h.p., and including two-cylindered tandem units developing 2500 b.h.p.. While in 1922 the Premier Gas Engine Company Ltd. of Sandiacre (Notts.), had designed an eight-cylindered two-crank double-acting four-stroke engine of 6000 h.p. (pp. 50,51).
The Two-stroke Cycle.In the Otto, or Beau de Rochas, or 'four-stroke' cycle the utilization of the same cylinder alternately for pumping and power output, though exceedingly convenient as a simplification in design, introduces the disadvantage that a working impulse is obtained once only in every four strokes of the piston; the crank-shaft speed is consequently far from uniform unless one, or frequently two, very heavy flywheels are fitted which by their momentum maintain the rotation only slightly diminished during the exhaust, suction, and compression periods.
Attention was accordingly quickly directed to the problem of increasing the frequency of the impulses, and as early as 1878, Sir D. Clerk produced his first two-stroke cycle engine, quickly improved upon in a second design of which an example was exhibited at the Paris Electrical Exhibition of 1881. This engine is illustrated diagrammatically in Fig. 2. A is the power cylinder containing at its outer end exhaust ports E, , overrun by the piston C when near the end of its out-stroke. The pump-cylinder B is, fitted with a piston D driven by a crank about 90° in advance of the power crank; on its out-stroke D draws into B a charge of mixed gas and air through the sliding valve H and pipe, W. When D commences its in-stroke the charge in B becomes slightly compressed; as soon as its pressure exceeds that in the power cylinder A it is delivered into the combustion chamber G through an automatic inlet valve. The power piston next returns, first cutting off the ports E, E, and next causing the automatic inlet valve to close; the entrapped fresh charge is then compressed into the chamber G, fired, and the working out-stroke follows.
Thus every out-stroke of C is a working stroke, and the engine accordingly gives one impulse every revolution.
The working impulses being thus twice as frequent as in a four-stroke engine, an ideal two-stroke cylinder should develop twice as much power as a four-stroke of the same bore, stroke, and speed. Practically, however, the two-stroke cycle suffers from drawbacks which prevent the realization of this ideal; thus in the four-stroke cycle the inlet valve is opened slightly before the end of the piston stroke, and remains open throughout the whole suction stroke and usually for a short period thereafter; the charging of the cylinder thus continues during about 220 of crank-shaft revolution. On the other hand, in the two-stroke engine charging must be effected during the short interval elapsing between the uncovering and re-covering of the ports E, E, which ordinarily occurs in about 80 of crankshaft revolution; hence the duration of charging in the four-stroke is about three times as great as in the two-stroke engine. Though this drawback is somewhat reduced by providing large inlet area, the two-stroke engine is nevertheless in general incapable of such effective charging as the four-stroke, and more power is also absorbed in the charging operation.*
Again, in the four-stroke engine, the exhaust valve is open during about 240° of crank-shaft revolution, and the burnt gases are moreover positively expelled by the piston during the whole exhaust stroke, the combustion chamber alone remaining filled with burnt gas at, or often slightly below, atmospheric pressure. In the two-stroke the exhaust has to be effected while the crank-shaft turns through about 80°, and the whole cylinder remains filled with burnt gases which, while being assisted in their exit by the incoming fresh charge, heat it, thus reducing its density and diminishing the 'volumetric efficiency' of the engine. Further, it frequently happens in small two-stroke engines that there is some loss of fresh charge by direct passage through the exhaust ports; in large engines this is avoided by means described later. Clerk improved the scavenging bf his engine and avoided loss of fresh mixture through the exhaust ports by forming his combustion chamber G (Fig. 2) in long, conical shape, and admitting the charge under slight pressure at the apex. As early as 1884 his engines were built by Messrs. L. Sterne & Company Ltd., in sizes ranging from 2 to 12 HP, running at 200 to 130 RPM, and showing for that period considerable economy in fuel consumption, ranging from 29 c. ft. of coal-gas per indicated horse-power hour in the smallest, to about 20 c. ft. in the largest sizes.
The two-stroke engine, with suitable modifications, has long been firmly established, both in the very largest types of gas-engine represented by the Koerting and Oechelhauser and Diesel designs, and also, in a simplified form next to be described, in immense numbers as the power unit of many motor-boats and of the very popular two-stroke motor-bicycle.
The Day Gas-engine.The very important simplification just referred to was made by Day in 1891, and consisted in using the crank chamber as a charging pump, thus dispensing altogether with the separate pump B of Fig. 2.
The Day gas-engine is illustrated diagrammatically in Fig. 3 in two forms, that on the left showing the 'two port' type used e.g. in the well-known 'Bolinders' marine engine, while the right-hand view illustrates the three-port type as universally employed in motorcycle applications.
Referring firstly to the two-port type, the crank-chamber is made quite airtight, and the ascent of the piston accordingly creates in it a partial vacuum causing mixed gas and air to enter through the non-return valve shown; on the descent of the piston this charge is compressed to four or five pounds per square inch pressure. When near the bottom of its stroke the piston first overruns the exhaust port shown, permitting the discharge of the burnt gases, and immediately afterwards uncovers the inlet port, whereupon the compressed fresh charge from the crank-chamber at once enters the cylinder, and assists in the removal of the exhaust gases. The 'hump' shown on the piston deflects the entering stream of fresh mixture upwards, and thus minimizes loss by short-circuiting through the exhaust port. The piston next ascends, cuts off the ports, and compresses the entrapped charge into the top part of the cylinder, where it is fired at or about the instant of greatest compression, and the working stroke follows.
The two-port type requires a valve, but the three-port engine is entirely valveless. The ascent of the piston causes a partial vacuum in the crank-chamber as before; when near the top of its stroke, its lower edge overruns the port shown, and a charge of gas and air immediately rushes into the crankchamber. The piston next descending-cuts off this charging port, and the mixture is compressed in the crank-chamber, the subsequent action being exactly as in the two-port type. Thus every down-stroke is a working stroke, and in the three-port form the engine is of the simplest possible character, the only moving parts being the piston, connecting-rod, and crank-shaft. It will be observed that these engines will run equally well in either direction; this is a valuable property in marine applications, where ready reversibility is essential, and they are accordingly largely used for the propulsion of motor-launches, particularly in America.
For a fuller account of the history of the evolution of the modern internal-combustion engine, and of the labours of many other distinguished engineers, as Ackroyd, Atkinson, Daimler, Diesel, Robson, & c., reference must be made to the large special treatises, as, e.g., those of Clerk* and of Bryan Donkin. The theoretical aspect of these engines is also dealt with in a separate article of this work entitled 'Applied Heat', Vol. IV, p 123. We proceed therefore to briefly refer to the gaseous mixtures now ordinarily employed as fuel by gas-engines.
Gaseous FuelsThe principal gaseous fuels used in Great Britain are:
(1) town's gas, often termed 'coal-gas'; (2) producer gas; (3) coke-oven gas; (4) blast-furnace gas. In America (5) natural gas is also used.
1. Town's gas obtained by the distillation of coal in closed retorts, and subsequently enriched by the addition of other gases, and particularly of 'water-gas' (consisting mainly of carbon monoxide and hydrogen), is still the principal gaseous fuel of the very numerous small-powered stationary gas-engines so widely used for miscellaneous industrial purposes; it is an excellent fuel for gas-engines of any size, but its relatively high cost restricts its use in general to the smaller-powered units. Until recently companies were compelled to supply gas of a standard illuminating power, but the introduction of the incandescent mantle, and the largely increased use of gas for heating and power purposes, rendered its heating value of the first importance. Accordingly in 1920 an Act was passed empowering companies to sell gas at a price proportioned to its heat value, 100,000 British units of heat*, termed one 'therm,' being taken as the unit of supply. Thus, in 1922, the heating values of the gas supplied by the principal London companies were: Of the South Metropolitan Gas Company: 560 B.Th.U. per cubic foot. Of the Gas, Light & Coke Company: 500 B.Th.U. per cubic foot. Of the Commercial Gas Company: 475 B.Th.U. per cubic foot.
To ascertain the heat supplied in therms to a consumer one has therefore to multiply the number of cubic feet of gas used by the heating value per cubic foot, and divide by 100,000. Town's gas is a mixture, in somewhat variable proportions, of the combustible gases hydrogen, carbon monoxide, methane (CH4), and other hydrocarbons, with small quantities of the incombustible gases carbon dioxide, oxygen, arid nitrogen.
2. Producer gas is a mixture of gases formed by the combustion of almost any carboniferous fuel with a limited supply of air, and consists principally of carbon monoxide, hydrogen, and carbon dioxide, with a large quantity of atmospheric nitrogen.
In the most usual form of 'suction producer' anthracite or coke contained in a cylindrical fire-brick lined 'producer' is maintained incandescent by air drawn through it by the engine suction. Steam is also injected into the fuel, and the issuing mixture of hot and smoky gases after being cooled and cleaned is mixed with a suitable quantity of fresh air and supplied to the engine. For further details of this important subject the reader should refer to the article on 'Gas Producers' in this volume. Producer gas is now used all over the world, a great variety of refuse material being utilized as fuel as, e.g., peat, sawdust, straw, coconut shells, cotton and sunflower seeds, tea prunings, rice husks, etc. The heat value per cubic foot varies with the fuel from which the gas is made; using anthracite or coke in a suction producer an average value is about 130 B.Th.U., the gas usually requiring for complete combustion rather under 1 cu. ft. of air per cubic foot.
Its heat is thus very much less than that of town's gas; nevertheless, as explained here under, it forms a most satisfactory fuel for gas-engines; using gas coke a suction producer may be expected to give roundly about 180,000 c. ft. of gas per ton of coke burned.
3. Coke-oven GasIn the production of coke for use in blast-furnaces, foundries, and general metallurgical processes, large quantities of gas (about 10,000 c. ft. per ton of coke made) of high heat value are evolved. The gas varies considerably in composition, and consists of a mixture of the same gases as producer gas, but is characterized by its high hydrogen and methane content, the hydrogen varying from about 30 percent to 60 percent and the methane from about 20 percent to 25 percent of the whole volume. Such gas requires for complete combustion from 3 to 4 c. ft. of air per cubic foot, and has a heat value ranging from about 350 to 450 B.Th.U. per cubic foot. Though a valuable fuel for gas-engines, its variable constitution and high hydrogen content introduce some practical difficulties in its use, but these are met by providing special means of rapidly adjusting the admission of gas and air, while the tendency to pre-ignition due to the high proportion of hydrogen may be overcome by Clerk's method of admitting a small quantity of cooled exhaust gases with each fresh charge*.
4. Blast-furnace GasIn 1895 the late Mr. B. H. Thwaite showed that the hitherto waste gases produced in large quantities by blast-furnaces could be used in gas-engines, and his discovery greatly stimulated the development of the very large gas-engines now becoming common. The gas is of low heat value, averaging only about 100 B.Th.U. per cubic foot, and requiring for complete combustion about 0.75 c. ft. of air per cubic foot. Roundly, some 180,000 c. ft. of gas are ordinarily produced per ton of fuel burnt, the average composition in Great Britain being (by volume) carbon monoxide 25 percent, hydrogen 2 percent, nitrogen 66 percent, carbon dioxide 6 percent, traces of methane, etc., 1 percent. The gas must, of course, be cooled and cleaned before use.
5. Natural GasVery frequently associated with the oil deposits, vast reservoirs of natural gas exist over a wide area in the United States and elsewhere, as, e.g., the neighborhood of the Caspian Sea; a small supply also exists in Great Britain at Heathfield, in Sussex, and it has been thought that considerable quantities are probably obtainable in this locality. In the United States the amount obtained annually is immense; e.g., in 1903 over 240,000 millions of cubic feet were used for heating, lighting, and power purposes. Natural gas often contains from 90 to 95 percent methane (CH4), with small quantities of ethane and olefines, and traces of water and oil; it is an ideal gas for use in internal-combustion engines. Considered as methane, its heat value is very high, viz. 1070 B.Th.U. per cubic foot, and requires per cubic foot for complete combustion 9.5 c. ft. of air; actually its heat value is usually in the neighborhood of 950 B.Th.U. per cubic foot.
Average Heat Values of the usual Gaseous Mixtures
B.Th.U. of Heat per Cubic Foot at 60° F. and 760 mm.
Cubic Feet of Air per Cubic Foot of Gas for just Complete Combustion.
B.Th.U. of Heat per Cubic Foot of Mixture with Air.
Average petrol (as vapour)
Suction-producer gas from anthracite or coke }
In Table 1 some average figures relating to the above-mentioned gaseous fuels are collected together for general reference; figures for average petrol are included for comparison. It will be observed that, notwithstanding the large variation in the heat value per cubic foot of gas, the heat value per cubic foot of mixture with air varies between the much narrower limits of 57 to 95, due to the greatly differing volumes of air necessary to cause complete combustion in the several cases. It is mainly for this reason that the poorer gases prove so useful as engine fuels.
Further, excepting in the very special, relatively small petrol-engines of motors and aircraft, experience has shown that in order to avoid trouble from overheating of pistons and cylinders it is necessary to limit the amount of heat supplied to the engine. A convenient method of estimation is to take the heat supplied per cubic foot of working stroke swept by the piston, and for gas-engine cylinders British practice in general allows only about 50 B.Th.U. per swept cubic foot with cylinders up to 20 in. in diameter, falling to from 35 to 40 B.Th.U. per swept cubic foot with 30-inch cylinders. This restriction limits the practically attainable (indicated) mean effective pressure in such engines during continuous running, to from about 65 to 55 lb. per square inch. The reduction in heat supply is obtained by increasing the admixture of air; thus, with town's gas, usual proportions are 9 to 10 volumes of air to 1 of gas; and similarly in other cases. See equations 8 and 9 and Table III of page 31.
This concludes the first two chapters of the Gas Engine section of the 1923 edition of Modern Mechanical Engineering. Additional chapters will follow in future issues of GEM.