The history of the modern gas engine commenced with the invention of the Otto motor in 1876, but the most rapid and extensive development has been subsequent to the expiration of the Otto patents. Prior to this the ingenuity of inventors was chiefly employed in devising methods of construction which, while not infringing those patents, would be of a practical and economical character, affording a prospect of successful competition with the Otto engine.
As regards the latter, its makers had continued steadily to improve the details of the engine, and especially so by the abandonment, in 1888, of the slide valve. This part of the engine was always its weak point, for the slides could never be kept in order for long together, and it was found necessary to supply them in duplicate, in order that the engine might not be standing whilst they were re-faced. This difficulty was obviated by the substitution for the slides of lift valves, the surfaces of which, not being subject to the same wear, will keep tight for an indefinite period. They also possess the advantage of effecting a much more instantaneous admission than was possible under the old method. Although this improvement did much to encourage the use of the gas engine as a motor, yet, owing to the protection afforded by the patents covering the Otto principle, and the consequent fact that engines of this type could only be obtained from the licensed makers, the superiority of the Otto cycle was constantly called in question, and new types of engines, claiming advantages over it, were constantly brought before the public.
Before proceeding to indicate the lines on which the modern gas engine has been developed, it may be useful to describe briefly the principal types which led up to it.
EARLY TYPES
Mr. Dugald Clerk, himself the inventor of an ingenious gas engine, classified all gas motors under three types: (a) engines ignited at constant volume, but without previous compression; (b) engines igniting at constant pressure, with previous compression; (c) engines igniting at constant volume, with previous compression.
To the first of these types belonged the Lenoir engine, the Hugon engine, the Bisschop engine, and the Otto and Langen engine. The last of these was a ' free-piston' engine, in which a vertical piston was driven upwards by the explosion, and, in descending under the pressure of the external atmosphere, engaged with and revolved the flywheel shaft. The second type was exemplified in the Drayton engine, which, although never assuming practical importance, was at one time made in an improved form by Messrs. Simon, of Nottingham. In this, the combustion of the gas and air, issuing from the orifice of a separate chamber in which it was previously compressed, was made to take place gradually, and thereby a constant pressure in the cylinder was maintained. To the third type belong all the more modern forms of gas engine, in which a constant volume of an explosive mixture, forming the charge, is first compressed and then exploded in the cylinder of the engine, causing an initial maximum pressure, which is afterwards reduced by expansion of the gases.
This type of engine, previous to the expiration of the Otto patents, was represented by two distinct classes, viz., the Otto engine, in which the cycle of operations in the cylinder occupies four piston strokes, giving an impulse once only every alternate revolution, and those having for object the realization of a working stroke in each revolution.
The Otto is based on what is known as the Beau de Rochas cycle, patented in France in 1862. Its sequences are well known, but for the sake of clearness it may be well to recapitulate them. A mixture of gas and air of definite volume is drawn into the back end of the cylinder by one forward stroke of the piston, at the end of this stroke the admission valves are closed, and on the return stroke the mixture is compressed at the end of the cylinder, where a certain clearance, or back chamber, is provided for the purpose. On the next outward stroke ignition takes place, and the pressure generated by the explosion gives the working impulse to the piston. The fourth stroke of the cycle, viz., the second return stroke, is devoted to expelling the waste products.
Those makers of gas engines who were not entitled to use the Otto patents endeavored to solve the difficulty by compressing the charge in a separate chamber, instead of in the engine cylinder. By this means they were enabled to obtain an impulse at each outward stroke of the piston, inasmuch as the operations of drawing in and compressing the next charge were performed during the active or explosive stroke and the next return stroke of the engine piston. This was the method adopted in Clerk's engine. In the ' Robson' engine, made by Messrs. Tangyes, Limited, the operations of charging and compressing were performed in the front end of the engine cylinder, which was closed like that of a steam engine. In the original 'Stockport' engine a second cylinder was used for this purpose, the front end of the trunk of the engine being formed into a second piston working in the charging cylinder. Another engine embodying the same idea was the Atkinson 'Differential' engine. From all these, results were obtained which made them by no means to be despised as competitors of the Otto; but, unfortunately, the makers in few instances could escape the charge of infringement of the Otto patents. Mention must also be made here of the Griffin six-cycle engine. This engine embodied the Otto cycle in the first four strokes, but the fifth and sixth were devoted to drawing in and expelling a charge of pure air, which completely displaced the burnt gases.
With the expiration of the Otto patents, however, the
raison d'être of all engines on other than the Otto principle fell to the ground. The fact is that the Otto system is at once the simplest and most economical of application; all other types of engine which necessitate separate appliances for the preparation and compression of the charge being more complicated in construction and more costly to manufacture. At the present day they have all been abandoned, and their makers have adopted the Otto cycle. The recent development of the gas engine has been essentially on Otto lines, and is an approximation of all to one standard type.
The first requisite in the successful development of the gas engine being economy in fuel consumption, it is natural that the attention of inventors should have been primarily directed towards the production of greater efficiency. In the earliest gas engines one or other of two defects invariably existed, either their fuel consumption was extravagant, or the power exerted was very small in proportion to their size. The former defect was owing to the fact that the differences of pressure and temperature at the beginning and end of the cycle were small, and consequently little useful effect was developed by the combustible. Non-compressing engines of the Lenoir type worked with low initial pressure and very incomplete expansion. A large amount of gas was subjected to combustion, but a very small percentage of the heat evolved was converted into mechanical work. The efficiency of the Lenoir engine was only 4 per cent. The Bisschop engine, and the Otto and Langen engine were more economical, because, although non-compressing, the time and space afforded for expansion were much greater. On the other hand, they exerted a very small power for their size and weight.
The introduction of the principle of compression saved the gas engine from being relegated to that limbo towards which all hot-air engines have hitherto gravitated. It rendered possible the utilization of a much larger percentage of the energy developed by providing the means for extensive expansion of the products of combustion, at the same time that it concentrated the forces at work by producing a higher initial pressure and temperature than were previously possible. With the introduction of the Otto engine the consumption of gas was reduced from 25 to 35 cubic feet per indicated horsepower per hour. From 1880 to 1890, by modifications in the proportions of gas and air, and other improvements, the consumption was reduced to about 25 cubic feet for the smaller and 20 cubic feet for the larger engines per effective horsepower.
THE SCAVENGING SYSTEM
It has however, been reserved for the past year's improvements to bring about a further marked reduction in the gas consumption by the adoption of what is known as the 'scavenging' principle. This principle is not new in itself, having been carried out, as has already been stated, in the Griffin six-cycle engine, which, however, involved the disadvantage of prolonging the cycle by two additional strokes, thus diminishing by one third the power of the engine. In the original Otto a portion of the waste gases always remained behind, and, becoming mixed with the succeeding charge, diluted the mixture and weakened the force of the explosion. A slow explosion is manifestly incompatible with a high initial pressure, and the value of the latter having been recognized, an improvement has recently been introduced by Messrs. Crosby Brothers, by which the spent products are swept out at the close of the cycle without recourse to two additional strokes of the piston.
This is effected by opening the air-admission valve a quarter of a revolution before the discharge valve is closed. This latter open at three fourths of the semicircle on the forward working stroke, and is not closed until one quarter of the semicircle on the next forward or suction stroke has been performed. It, therefore, remains open for three fourths of an entire revolution. The air-admission valve opens before the piston has returned to the back end of the cylinder, at three fourths of the semicircle on the return discharge stroke. A long vertical discharge pipe is attached to the engine, and the rush of the spent gases up this pipe causes a vacuum by which air is sucked through the engine and up the pipe, sweeping all the spent products before it. The result of this is that when the discharge valve closes and the gas admission valve opens, the space behind the piston is filled with pure air, and a correctly proportioned mixture is formed, the explosion of which produces an impulse of full and uniform strength at each working stroke. This is the joint invention of Mr. Frank Crossley and Mr. James Atkinson, who is now associated with Messrs. Crossley Brothers.
Besides this improvement, an increase in the amount of compression has also been effected. Formerly from 40 lb. to 60 lb. pressure per square inch was usually obtained in compressing the charge. This is now increased to 80 lb. or 90 lb. The effect of this, together with the purity of the charge, has been to raise the initial to about 300 lb. per square inch.
The economy resulting from these improvements is remarkable. The gas consumption in the new engine is reduced to 16.48 cubic feet per effective horse power, as against 23.87 cubic feet in the older engines. The actual efficiency of the new engine is stated by Mr. Atkinson to be 28.26 per cent., as compared with 22 per cent, actual efficiency in those of the older type. It may here be stated that by actual efficiency is meant the ratio of the heat converted into work to the total heat developed by the combustion of the gas. When coal gas is used some 6 per cent, is required to be deducted to arrive at the net heat effect utilized from coal. The mechanical efficiency is the ratio of the effective or brake horse power to the indicated horse power. This, in the best engines, has been brought up to 86 per cent.— that is to say, 14 per cent, only of the power exerted is spent in working the engine itself.
IGNITION AND GOVERNING
Improvements in valve and governing gear have played an important part in the development of the modern gas engine. The substitution of independent lift valves for the old slide has already been referred to. This enabled makers to work the gas and air admission separately, and so better to regulate the proportions of gas forming the explosive mixture. In the modern engine the air is admitted at every cycle without alteration, and only the gas supply is interrupted by the governor. The governing of the Otto engine has always been effected by Messrs. Crossley Brothers, on the hit-and-miss principle. In the engines the governor, which was of the centrifugal type, determined whether or not the gas-valve lever should be struck by the cam on the side shaft. Two types of governors are used in the later engines, a centrifugal for the larger and an inertia governor for the smaller sizes. In the largest engines a lever carrying a roller and a steel blade is actuated by a cam at every revolution of the side shaft, and a disc is moved by the governor in a vertical direction into such a position that when struck by the blade it opens the gas valve. When, by increased speed, the governor rises, the position of the disc is so altered that it is not struck by the blade, and the valve remains closed.
A modification of this plan is adopted with engines of a moderate power. The governor moves in a horizontal direction, a disc, set loose on a spindle, which is attached to a bell crank lever actuating the gas valve, and according to the position into which the disc is moved by the governor, it is struck, or not, by a narrow step on the cam of the side shaft. The inertia governor applied to small engines consists of a small weight and blade, pivoted on a bell-crank lever, which tends to lag behind as the engine increases in speed, and hits or misses the valve spindle. Means are adopted in addition, whereby the discharge valve can be opened at every in-stroke of the piston, so as to do away with compression at starting. The cam actuating the discharge is made to slide on the shaft, and at one side has a narrow projection or step forming a secondary cam opposite the primary one. When, by moving a lever, the cam is shifted to the starting position, the secondary cam acts on the discharge as well as the primary one.
The governing of the engines made by Messrs. Tangyes, Limited, is done by the movement of a roller along a pin on the gas valve lever, which brings it opposite a grooved cam on the side shaft of the engine. In order to prevent snipping of the gas-valve (
i. e., partial contact between the cam and the roller, and subsequent slipping off of the latter), a second (knife-edged) cam is used, which, coming in contact with a knife-edged collar on the roller, fixes its position when opening the gas valve, exactly opposite the cam.
The abandonment of the slide necessitated an alteration in the method of ignition. Messrs. Crossley Brothers introduced the system of tube igniters. These were first made of wrought iron, closed at the upper end, and placed vertically in a cylindrical casing. The lower end communicates with the cylinder, and the tube is kept red hot by the flame of a Bunsen burner. These tubes have a short life, and become corroded, so that the heat of the flame is unable to penetrate the metal and keep it sufficiently hot to ignite the charge. Messrs. Crossley experimented with tubes of cast iron, nickel and platinum alloys, and other mixtures, but have finally adopted porcelain tubes, which, while possessing the advantages of being quickly heated and of costing little for renewal, are practically indestructible by heat. They are now generally placed horizontally in a rectangular casing, one end of the tubes fitting over a nozzle communicating with the cylinder, and the other closed by a pin held tight by a screw or spring.
An important feature in the modern igniting arrangements is the timing valve. This valve opens communication with the cylinder at the moment when ignition is required, and allows a small portion of the charge to be injected into the tube, where it fires back and explodes the charge. Some makers dispense with the timing of the valve, and make it open automatically by the pressure of the charge. Under these conditions, however, it is difficult to prevent premature ignitions, which may cause a very severe strain to be thrown upon the crankshaft and cylinder of the engine. The French makers of gas engines mostly adhere to ignition by the electric spark, which was the method adopted in the Lenoir engine.
STARTING GEAR
The development of the gas engine is in nothing more marked than in the increase of their size and power; many single-cylinder engines are now being made in this country of 100 horse power effective. Messrs. Tangyes, Limited, have just completed one large single-cylinder engine, indicating 180 horsepower with Dowson gas. They have also lately made one with a cylinder 24-in. diameter, and a stroke of 30 in., to run at 150 revolutions per minute, with a piston speed of 750 ft. per minute. The trials of this engine with producer gas have given an indicated horse power of 196, and with Welsh anthracite coal a fuel consumption not exceeding 0.8 lb. per indicated horse power can be guaranteed. The exhaust valve would require to be of such large area, and as it would have to lift against a terminal pressure of 40 lb. per square inch, that the strain upon the pins, levers, and cams would be very great, it was considered advisable to use two valves instead of one, the areas of which are in the proportion of 2 to 1. Both valves are actuated by the same lever, but the smaller one is raised first, and held up for a little time to reduce the pressure before the large valve rises.
Another engine has lately been constructed by Messrs. Dick, Kerr, and Co., of Kilmarnock, of 270 indicated horsepower (with Dowson gas); but this is a double-acting engine. On the Continent the firm of Matter, in Rouen, has constructed a single simplex engine of 300 horsepower, devised by Messrs. Delamare, Debouttville, and Malaudin, which does not differ in principle or design from the Otto type.
In large engines the opening of the heavy gas valves used with producer gas causes excessive wear on the narrow-edged cams. To obviate this Messrs. Tangyes have adopted in their latest engines a method by which the gas valve lever actuated by the cam does not act direct on the valve spindle, but does so through a tumbler, which is kept out of a gear by a spring or weight. One end of a light secondary lever is connected to the tumbler, and the other end is provided with a roller sliding on a pin, and moved by the governor, as in the ordinary sized engines. This roller is actuated by the usual grooved cam and the main gas-valve lever by a broad cam at every revolution of the side shaft, in the same way as the air-valve lever. The secondary lever brings the tumbler into position, so that the main lever can raise the valve.
The demand for large engines has rendered necessary the application of self-starting gear. Engines up to twelve horse power nominal can be easily started by hand, but above this power starting appliances are desirable, and in large engines are absolutely essential. There are several ingenious methods adopted, all of them having for their object the introduction of an explosive mixture into the cylinder (either with or without compression), and by the firing of this charge to start the engine. Messrs. Crossley use a hand pump to charge a receiver, and when communication is opened between this and the cylinder, the mixture enters and replaces the air previously contained therein. A few strokes of the pump raise the pressure to about 1 lb. above the atmosphere, and the mixture is then fired at the receiver and the combustion propagated in the cylinder. Messrs. Andrews, of Stockport, employ a different method. In their engines gas is introduced into the cylinder through a small valve kept open by a spring. The gas displaces part of the air, and in due course forms, with the remainder, an explosive mixture, driving out the air before it, which, on passing through the incandescent tube, fires back and starts the engine. The pressure of the explosion automatically closes and locks the spring valves by which the gas is admitted and the air etc., discharged. In Messrs. Tangyes' engines, Pinckney's self-starter is used. By this method an explosive mixture is introduced under pressure into the cylinder, the crank being prevented from revolving by a spring, which releases it when ignition occurs, the degree of compression being regulated by a safety valve on the pump. In starting large engines, or when it is required to start several engines at once, Messrs. Tangyes now use a mild steel reservoir, into which the explosive charge is pumped to a pressure of 80 lb., and from which the cylinders are charged. Messrs. Fielding and Piatt use a small reservoir, which is charged with compressed air only. In starting, a charge of gas is introduced into the cylinder, and the air from the reservoir is mixed with it, thus forming a compressed mixture. The charge is fired at the igniting tube after the piston has commenced to move under the influence of the compression. Messrs. Dick, Kerr, and Co. have departed from the plan followed by most makers in their large engine, which is started by steam from the small boiler used for making Dowson gas.
DOUBLE CYLINDERS AND DOUBLE ACTION
The construction of double-cylinder engines has been a source of some diversity in form. Messrs. Crossley Brothers first commenced in 1890 to make pair cylinder engines, with double cranks placed at an angle of 180 deg. to one another. The cycles in the two cylinders being alternate, an impulse was obtained at each revolution, first in one and then in the other cylinder. This plan has now been relinquished in favor of placing the cylinders with the pistons facing one another, and working on one crank. In this form there is less strain on the crankshaft, but the impulses are not so equally divided, inasmuch as two working revolutions follow one another, and are followed by two negative revolutions. Messrs. Andrews & Co., in their latest large engine, adopted the tandem form. The cylinders are placed one behind the other, the piston of the hinder-most cylinder working, by means of crossheads and coupling rods, on the same crank as that of the forward cylinder. The cycles are alternate, and an impulse is obtained at each revolution from one or the other cylinder. This engine is 400 horse power effective (200 to each cylinder). It was made for Messrs. Henry Spicer and Sons, and is erected at their paper mills at Godalming.
"It has been stated that the recent development of the gas engine is an approximation to one standard type. Whilst this is so in the main, there are some singular exceptions. The double-acting engine of Messrs. Dick, Kerr & Co. is an attempt to reduce the form of the gas engine to that of the steam engine. In this engine the cylinder is closed at both ends and has a piston rod, in all respects similar to that of a steam engine, working through a stuffing box in the front cylinder cover. The engine works on the Otto cycle at both ends of the cylinder, and its action is identical in effect with that of the end-to-end double-cylinder engine of Messrs. Crossley Brothers. Messrs. Scott Brothers, of Halifax, are the makers of what is called the Duplex Otto engine. This engine has two pistons working in opposite directions in one cylinder, under one and the same impulse. The back piston acts by two side connecting rods, on cranks placed at 180 deg. with that of the forward piston.
So far as the author is aware, no compound engine has yet been made in this country, but Professor Aimé Witz, in his latest volume on the gas engine, describes one invented in America, bearing the name of Connelly, in which it is attempted to embody this principle. In this engine two cylinders of different dimensions are employed, and, whilst the explosion takes place in the smaller cylinder, the expansion of the products is completed in the larger cylinder.
USE AND COST
The purposes for which gas engines are used are becoming rapidly more extended and various. Machinery of all kinds is now driven successfully with them, and their application for electric lighting purposes is becoming an important consideration, especially for taking up the day load in central stations. At the Blackpool Tower, gas engines to the extent of 500 horse power effective, are used for electric lighting, and at the new Midland Railway Station, Leicester, gas engines, in the aggregate of 300 horse power, have been put down for the same purpose. Gas engines are used at the Birmingham Waterworks for pumping purposes, and Messrs. Crossley have now on hand for the River Wear Commissioners a set of three high-speed engines, each of 120 horse power effective, for driving centrifugal pumps, to which they will be coupled direct. A similar set of engines is being made by Messrs. Tangye for the same purpose.
Gas engines have already been tried in this country for driving tramway locomotives, and on the Continent there are several instances where they are successfully applied for this purpose. They have also been used for propelling boats, and a company has been formed at Havre to work boats on the Seine with gas engines, the fuel being ordinary town gas compressed in cylinders. The number of gas engines actually in use at the present day is hardly to be realized, unless special attention is given to the subject. Messrs. Crossley have turned out 24,400 Otto engines, Messrs. Andrews give 7,000 as the number they have made, whilst the German Otto Company have made some 8,000.
At the present day there are some dozen or more makers of Otto engines in this country alone. In Manchester there were, at the close of the last financial year, 966 gas engines on the books of the Gas Department, and the consumption due to these in the year was 123,000,000 cubic feet of gas.
So far as the smaller engines are concerned, every addition to the purposes for which stationary gas motors are used, means an increase in the business of the gas manufacturer, but under present conditions this is not so with regard to large engines. So long as gas is sold in our towns at the usual lighting rates, large engines will be more cheaply worked with Dowson gas. Messrs. Crossley state that it costs them 2½d. per thousand cubic feet to make Dowson gas, and they require to use five times as much of it as of Manchester city gas to produce an equal horse power effect. According to this, it would be necessary to sell town gas at 1s. 0½d., in order to compete with Dowson gas; but inasmuch as other advantages ensue from using gas of high calorific value, such as reduced size of engine and less cost for attendance and wear and tear, the author believes at a price of 1s. 6d. per thousand cubic feet town gas would be used in preference to Dowson gas. He is supported in this opinion by the fact that at Sunderland, where gas is supplied very cheaply in large quantities, many high-power engines at the docks and elsewhere are working with town gas.
At the engineering works of Mr. John Dickenson in that town, ten gas engines, with an aggregate of 400 horse power effective, are supplied with town gas at Is. 6d. per thousand cubic feet, and the proprietor states that there is a considerable saving over the use of the steam engines which he formerly employed. At Messrs. Andrews' works, at Stockport, a gas engine of 100 horsepower indicated drives the workshop machinery. It is fed with Dowson gas, and can develop 67 brake horsepower, but its present load does not exceed 40 horse power. The Dowson producer consumes 26 to 28 cwts. of Welsh anthracite coal, at 22s. 6d. per ton delivered, per week of 60 working hours. Taking the mean consumption at 27 cwts., the consumption per horse power per hour works out at 1.26 lb.
The cost of working this engine will be as follows:
| 27 cwts. of coal | | £1 15 4½ |
| Attendant for producer | | £1 5 0 |
| Total | | £3 0 4½ |
If fed with town gas the consumption would be 20 cubic feet per horsepower per hour or 48,000 cubic feet for the 60 hours. At Is. 6d. per thousand cubic feet the cost would be £3 12s. per week. No attendance would be chargeable, as the engine, once started, will work on all day with occasional inspection in connection with the water circulation and oil feeds. The difference of 11s. 7½d. per week (or less than £30 per annum) is certainly not too much to allow for repairs to, and interest on, the Dowson plant. If instead of this unnecessarily large engine one of 40 horse power were employed, with a gas consumption of 16½ cubic feet per horse power, the fuel consumption would not exceed 40,000 cubic feet per week, costing, at 1s. 6d. per thousand cubic feet, £3 or below the cost of working the Dowson plant.
Information Sources
- Industry Magazine Oct 1895, pages 612-616 (Part 1)
- Industry Magazine Nov 1895 pages 658-662 (Part 2)
- Industry Magazine Dec 1895 pages 709-712 (Part 3)