4 in. Distance from centre of leading to centre of centre wheels. 7 0 Distance from centre of centre wheels to centre of trail NAME OF Table 3.-DIMENSIONS, WEIGHTS, AND CAPACITIES OF RAILWAY WAGONS ON DIFFERENT LINES. London and ins. ins. tons. cwt. Ditto. London, Brighton, and South Super-elevation of Rails on Railway Curves.-All moving bodies have a tendency to continue their motion in a straight line; therefore, when a railway train moves in a curve the centrifugal force produced by the velocity urges the carriages towards the outer rail, which it is necessary to elevate above the inner one in order to counteract the centrifugal force. The super-elevation of the outer rail required for trains of different speeds is given in the following table : Table 4.-SUPER-ELEVATION OF THE OUTER RAIL ON RAILWAY Curves. Until within the last few years a strong prejudice existed amongst engineers against the use of gas engines. This arose from the unsatisfactory working of the early engines of this kind. Since the introduction of the "Otto" gas engine this prejudice has been gradually disappearing. The great success of the "Otto" engine, since its introduction by Messrs. Crossley Bros., of Manchester and London, shows that whilst the steam engine has almost reached its limit of improvement, the gas engine offers a possibility of improvement in efficiency and economy, about double that attainable by the steam engine. Apart from their superior economy, gas engines possess many advantages over small steam engines. No boiler is required for a gas engine; it can be started at a moment's notice on lighting a gas jet and turning a fly wheel, and stopped by turning the gas off. No extra insurance is charged by the leading insurance companies. Gas engines can be fixed in the upper storeys of a building without danger from fire. They are more regular in speed and more easily managed than a steam boiler and engine. * The Author is indebted for this information to Mr. Robert Wilson, 24, Poultry, London. In instituting a comparison of the consumption of fuel in a gas engine and a steam engine, nothing may be more misleading than to take the cubic feet of gas and the pounds of coal respectively, per horse-power when the engines are working up to full power. A steam engine, and a boiler that consumes 5 lb. coal per horse-power when working up to 12 horse-power may consume fully one half that amount when driving the engine itself only. But in the case of a gas engine the quantity of gas required to drive the engine only is butto of that required to drive it with all its work on. In varying and intermittent work, the quantity of fuel consumed varies according to the work done much more nearly in a gas engine than in a steam engine. It is this fact, so often overlooked, which accounts for the unexpected superior economy of the gas engine. The following explanation of the manner in which a gas engine works may not be altogether out of place. It is known to every one that if a gas tap is left turned on overnight and the gas is allowed to escape into the room, the striking of a match or bringing a lighted candle into the room will probably cause a violent and disastrous explosion. A mixture of from 7 to 12 parts of air to 1 of gas may be ignited at atmospheric pressure. Such a mixture forms an explosive compound which may prove dangerous when filling a room, but may be made a very useful servant when confined in a cylinder with a movable piston. If ignited under pressure considerably above that of the atmosphere, still weaker mixtures can be employed. This is one reason why compression engines like the "Otto" are more economical than the non-compression engines, which are not, as a rule, made in sizes over 2 horse-power. The "Otto" may be regarded as the parent or prototype of gas engines over 2 horse-power, and as the makers guarantee an economy of from 25 to 70 per cent. over other engines it may still be considered the best. In general appearance it resembles a horizontal steam engine, but here the resemblance ceases. It is single-acting, the cylinder being open at the front end. The engine acts alternately as a pump for drawing in and compressing its charge, and as a motor for utilising this charge when fired. The fly-wheel makes two complete revolutions for every charge of gas admitted. The first outstroke draws in the compound charge; by the first instroke the charge drawn in by the previous outstroke is compressed to about one-third its volume; at the end of this first instroke or the beginning of the second outstroke, the compressed charge is ignited, when the expanding gases propel the piston to the end of the stroke; and the second instroke expels the products of combustion and completes the cycle of operations which are continually repeated when the engine is working up to its full power. When the engine is working within its power, the gas is temporarily cut off by the governor, and the engine simply works as a pump for drawing in, compressing and expelling the air. The "Otto" engine differs also in other respects from all other gas engines previously made. The charge to be ignited is not a unifor.n mixture of gas and air, but consists of a compound charge of incombustible gas, i.e. combustion products or air next the piston combining gradually with a mixture of gas and air that becomes stronger and more readily ignitable as it reaches the point where it is fired. The effect of this so-called stratification is that whilst the charge is as easily ignited as at uniform charge of highly explosive mixture, the presence of a large quantity of diluent, causes the combustion of the complete charge to be effected gradually. The result of this is most important. In the first place it prevents the sudden shock that occurs when a uniform mixture is ignited, and which is a sure indication of waste. In the second place, it ensures the pressure being sustained to the end of the stroke. The "Otto" is the first gas engine in which the whole length of the stroke has been utilised for propelling the piston. The initial pressure of the gas in the cylinder when ignited at the beginning of the stroke is about 170 lbs. per square inch. The gases expand to a pressure of about 35 lbs. at the end of the stroke. The average pressure is about 70 lbs. per square inch on the piston. The consumption of gas varies from 18 cubic feet per indicated horsepower in the largest sizes of engines to 25 cubic feet in the smallest. With gas at 45. per 1000 cubic feet, this corresponds to a working cost of about one penny per indicated horse-power per hour. For engines working up to 20 horse-power, the cost of working is generally greatly in favour of a gas engine with coal and gas at the respective prices prevailing in London. For engines of larger size, in order to compete in economy with steam where coal is comparatively cheap, gas other than that supplied for lighting must be employed, such as the Dowson Economic Gas. This is a so-called water gas, and can be made at a cost of three-pence per 1000 feet. In an "Otto" using this Dowson gas, the consumption is as low as 1 lb. of anthracite coal per indicated horse-power per hour,-an economy of working not yet reached by a small steam engine. Table 5.-THE "OTTO" ENGINE IS MADE OF THE Following Sizes : |