November j 3, 1914. THE COLLIERY GUARDIAN. 1019 THE THERMAL AND MECHANICAL EFFICIENCY OF THE GAS ENGINE, AND ITS INFLUENCE UPON THE FUTURE.* By A. Vennell Coster. At the present moment, when all our industries are affected either in men or material by the devastating European War, and when the country’s unfettered energies are being directed to capture German markets and German manufactures, it is imperative that we should remember that their commercial conquests have primarily and continuously received an impetus not only from their superior financial and industrial bank- ing systems, but also from the enormous savings they have effected in production by the utilisation of waste gases and fuels for power purposes. They have not V 1 n Era. 1.—Crossley Gas Engine. 350 B.H.P. at 170 revolutions per minute. hesitated, in spite of initial blunders and costly experi- ments, knowing that the gas engine can obtain a com- bined superior thermal and mechanical efficiency over every other prime mover, particularly when driven by these waste resources; waste resources which in this and other parts of our country, are waiting to help us back to a great commercial supremacy. Samples of waste products are placed before you to prove to you what Messrs. Crossley Brothers Limited have already done in this direction. But the thermal advantages of the gas engine are not confined to the utilisation of waste products only. Although Germany has undoubt- edly forestalled us in obtaining the use of immense power reserves, 'yet on examination we find that even with this advantage her prestige is more apparent than real. And although large German gas engines have been imported into this country, of which there are several noteworthy examples in this district, in the majority of cases the principles underlying their design and construction have been pirated from us. We will go a step further, and say that the fantastic halo of prestige that has for 50 years been gathering about German manufactures is rapidly being dissipated. In a large colliery works in. South Wales, where there are several large German gas engines in units up to 1,600- horse power, the piston rods have to be renewed about every 18 months. These engines run continuously, supplying electrical current for the general uses of power and light about the colliery. The chief engineer there informed me that during a period of 18 months these rods are dismantled, turned, and trued-up three times, until , the diameter is so reduced that the rods are of no further service. And yet, in spite of the heavy annual repairs necessary for these engines., they handsomely pay by producing power directly from the waste gases of their coke ovens. Evidently German design and material in these rods, with their elaborate stuffing boxes and metallic packings, have not proved any approach to perfection. Recently, when in correspond- ence with Messrs. J. J. Habershon and Sons, of Rother- ham, regarding the rods and metallic packings supplied for their large gas engine, directly driving their rolling mills, their reply was as follows :— The piston rods are polished like silver, and we cannot find that there is on the average more than in. between that part of the rod which is clear of the packings and that which is running through them. As a rule we take out one stuffing box every four weeks, and find this sufficient. Our engine has been running on double shifts since May 1913, rolling down into this steel strips billets at the rate of 15 to 20 tons a shift.” Here we have a concrete example of British supremacy in design and manufacture in large gas engines, and one which must give a higher mechanical efficiency and reliability. The second example is in reference to the German gas engine design and construction of their breech ends and pistons, which constantly fail, so much so that it is generally necessary to hold the complete * From a lecture delivered before the Rotherham T.I. Engineering Society, November 7, 1914. machined castings in reserve; whereas, in the British design of these parts (and in this connection we par- ticularly refer to the “ Crossley ” breech end) the reli- ability and cost of repairs of the large British made gas engine and plants are about level with those of similar power steam engine installations, and in smaller sizes the gas engine has double the reliability and half the cost of upkeep when compared with the steam engine. In Belgium, at the great works of Messrs. Coekerill and Company, near Liege, all their power plant is developed from blastfurnace gas, and allowing for an annual depreciation equal to 13 per cent, of the whole cost of the installation, working hours per year, 4,380, corresponding to a load factor of 50 per cent., the total cost per kilowatt, including depreciation, equals 0-183d. In all metallurgical industries in this country it is now high time that the utilisation of these waste gases should be energetically considered and the opportunity given to British firms to supply reliable and economical engines and plants, and obtain similar or better result. In a blastfurnace of 250 tons per day capacity, and consuming 10 tons of coke per hour, the Waste gas pro- duced is sufficient to generate 10,000 b.h.p. by gas engines. In a coke oven of 200 tons per 24 hours day capacity, requiring 10 tons of coal per hour, enough waste gas is generated to develop 1,800 to 2,000 b.h.p. in gas engines; and, further, by the utilisation of the waste coke (coke smalls) in a gas producer, a further 900 b.h.p. are obtained; so that altogether close upon 3,000 b.h.p. are available continuously from this regenerative coke oven plant. And it has been proved that a fijrna.ee giving a production of 10 tons of raw iron, the waste gas, if put into gas engines, produces 10,000 b.h.p., but if put into steam boilers produces only 4,000 b.h.p., or 25 per cent, in favour of the gas engines. The problem of the gas engineer is to utilise a larger proportion of the heat within the cylinder; our experi- ence shows that on the average one-third to one-fourth of the total heat supplied per effective b.h.p. is lost in the external cooling medium, or water jacket, usually 27 per cent.; whereas in the exhaust about 35 per cent, is lost, and 26 per cent, turned into actual work, the remainder being unaccounted for. Assuming a liberal consumption of heat per horse power hour as 12,000 B.T.U., the exhaust carries off 4,200 B.T.U., the water jacket 3,250 B.T.U., and the b.h.p. accounts for 3,120 B.T.U. per b.h.p. per hour. From these figures it can be seen that both the exhaust and jacket heats can be put to useful purposes, and assist in raising the thermal powers of the engine. Turning our attention to gas engine design, the extra- ordinary thermal advantage of this engine over the steam engine lies in the fact that the fuel combustion actually takes place within the engine cylinder in the combustion chamber. As already stated, a large pro- portion of the heat is lost to jacket and exhaust, and therefore it has been our constant aim that the working cycle should be completed in such a manner so as to expose its working fluid to the least possible cooling surface, and that the exposure should be for the shortest possible time, and also that the mean temperature during exposure should be as low as possible. The ideal form of combustion chamber is spherical, but we have found it is practically impossible to maintain this form in the gas engine, because the most economical ratio between cylinder diameter and piston stroke pre* eludes this form. A slow moving piston is most uneconomical, a minimum speed of 600 ft. per minute to a maximum of 900 ft. per minute being the usual limits, controlled by the weight of the moving parts and reciprocations per minute. A rapidly moving piston assists in creating eddying currents throughout the burning gases, thereby intensifying not only a thorough mixing of the gases and rapidity of combustion, but reducing the relative importance of conduction losses through the cylinder walls, and assisting in maintain- ing a higher pressure throughout the power stroke. Although a high compression tends to a better com- bustion of the power charge, in practice it is not advis- able to approach within the region of temperature in which spontaneous explosion might result. Herein lies the inherent difference between the high compression oil engine, in which the temperature of compression is ample for the purpose of igniting the sprayed oil injected at or about the commencement of power stroke, the injection being accurately timed to prevent any pre- ignition troubles. In gas engines it is not wise to exceed 1801b. compression pressure, unless some special means are taken for reducing the resultant high temperatures. Thermal efficiency depends also upon the completeness of combustion within the cylinder, the curve of expansion upon the indicator card lies above the adiabatic curve, which proves that, although the ignition may appear on the card instantaneously, after- burning invariably takes place as can be easily proved by removing the exhaust pipe; when on the opening of the exhaust valve, flames are projected from the engine cylinder. This after-burning adds somewhat to the loss of thermal efficiency in the gas engine, and requires further patient research. Complete combustion is what we have aimed at. Experience has taught us the best settings of gas and air and exhaust valves to suit varying speeds, so as to obtain the best running and fuel efficiencies, and rarely to-day we see the waste of unburnt fuel in the exhaust, particularly of oil engines. In our present examination of the subject we wish to go deeper, and we ask : Is complete combustion possible in the gas engine? Our reply is : That as the thermal losses due to incomplete combustion are comparatively trivial when compared with the losses into the cylinder jacket and exhaust, it is not of much utility to the power user to go much further in this direction. While the thermal efficiency may be somewhat reduced by this after-burning, the mechanical efficiency is very considerably improved. Referring particularly to the modern Crossley gas engine, having the Otto cycle and patent throttle gear, which determines the weight of the power charge to accurately suit the load; its mechanical efficiency or the difference between the indicated horse power within the cylinder and the work actually available at the crank shaft is usually not less than 85 per cent.; so that, whereas the thermal efficiency of the gas eligine con- sidered in relation to actual horse power is about twice as good as the steam engine, it would be even more than this, but for the reason that the gas engine of the Otto or any cycle requires a pump to deliver the power charge into the cylinder, and then to compress it before the power stroke. The negative power necessary for this pump, whether the pump is internal or external to the engine cylinder, accounts almost wholly for this differ- ence in efficiencies as between the gas and steam engines. The chief points in our construction which result in the valued reputation we possess are as follow :— (1) The explosion chamber has a minimum cooling sur- face in relation to its capacity, is symmetrical in form, and arranged as a simple extension from the cylinder liner, and thereby ensures the best thermal efficiency and freedom from ignition troubles. (2) There is only one joint exposed to the explosion pres- sure, and by the special method of fixing the jointing flange of the liner with that of the water jacket, water pressure in the cylinder jacket is prevented from obtaining access to the joint packing. This single joint can be also tightened up by means of external bolts or studs, whose nuts are available to the spanner while the engine is running. (3) There are no fixing, studs or bolts within the water of the water jacket, where they would be liable to corrosion. Fig. 2.—Crossley Waste Wood Gas Plant. 350 B.H.P. (4) There are no dry valve covers, which not only are liable to joint troubles, but are a frequent cause of pre- ignition of the power charge during the suction or compres- sion strokes. In our modern engine the vertical admission valve block forms the cover for the exhaust valve below it. (5) The head of the exhaust valve, which otherwise is liable to overheating, is thoroughly cooled by the cold incoming power charge admitted above it. (6) The engine has an extended bedplate, supported for its whole length upon a concrete foundation, which is not only rigid and substantial, but eliminates vibration. (7) Self-lubricating crank shaft and side shaft hearings, forced lubrication to the cylinder liner and exhaust valve guide, and centrifugal lubrication to the crank pin. Sight feed lubrication to all other working parts; and also com- plete arrangements for rotating and catching all spent oil, so as to prevent all waste of oil and deterioration of the concrete foundations. (8) All the materials of construction are subject to severe physical and chemical tests, and after manufacture are care- fully checked to limit gauges, and to the Brunel test for hardness and general suitability; thereby not only ensuring interchangeability of parts, but also the highest possible per- fection in the finished engine. (9) The valve gear for operating the air, gas, admission governing, and exhaust mechanism is controlled and