per hour, as expressed in carbon value. If we take this weight as expressing the unit of economical efficiency for steam engines, the comparative efficiency of your plant is shown in the following table: COMPARATIVE EFFICIENCY AS REGARDS FUEL CONSUMPTION PER I.H.P. PER HOUR In connection with the above figures, it may be stated that they are strictly net, as regards your plant, for full allowance has been made in the way of fuel-equivalent for the steam and fan blast, and in computing the indicated horse-power from the indicator diagrams. 'I have purposely confined myself in the foregoing comparison to the relative weight of fuel alone.' CHAPTER XXXVI GAS ENGINE TESTS THE author was pleased to receive an invitation from Professor Capper, to be present on January 10 and 11, 1895, when a 7 H.P. NOM. engine by Messrs. Crossley Bros. was tested in his laboratory at King's College, London, for purely scientific purposes. I have Professor Capper's permission to publish the following report : In gas engine trials it has hitherto been usual to assume that the temperature of the explosive mixture during constant volume ignition varies directly as the pressure. As this treatment does not seem altogether justifiable, I have recently carried out a number of trials in my laboratory in which complete gas and exhaust analyses have been made for me by Mr. G. N. Huntley, A.R.C.S., F.I.C.S., and the temperatures and heat additions.have been calculated from the constants so obtained. The following full-power trial will illustrate the method. This trial was carried out on January 10, Mr. W. Norris, M.I.Mech. E., being present. The engine was run in working condition, the last time it was disconnected for cleaning being some eight months before the trial. Engine. The engine is a 7 NOM. H.P. Crossley engine with tube ignition and loaded ball governor. It was built in 1892, and has been in constant use driving the workshops and running experimental trials ever since. Its cylinder is 8.5 inches diameter x 18 inches stroke. The clearance volume is 2467 cubic feet. Trial. The trial on January 10 lasted forty minutes, indicator diagrams and all other observations being taken every five minutes. Indicator Diagrams.—A Wayne indicator, calibrated under steam upon my mercury column, was used. For the power diagrams a spring and for the pumping stroke a spring was employed. From the indicator diagrams (see fig. Card taken at 3.04 pm. Atmospheric Line INDICATOR CARD FULL SIZE PRESSURE, 120 lbs. to the In. FIG. 200 200) which approached nearest to the average of the whole number taken, a mean diagram has been drawn on an enlarged scale, shown in fig. 201. A pumping diagram is shown, actual size, in fig. 202. Brake. A simple rope brake passed once completely round the flywheel, and to one end was attached a spring balance and to the other a dead load. The flywheel has an effective circumference of 17.458 feet. Gas Meter. The gas was measured by a standard meter made by Messrs. Alexander Wright & Co. for the Society of Arts motor trials. It now belongs to my laboratory and can be accurately read to of a cubic foot. The ignition gas was measured through a smaller meter, which can be estimated too of a cubic foot. Sampling. The gas was sampled by Mr. Huntley over water through a bye-pass close to the meter, a continuous sample being taken for the whole period of trial. For the exhaust a continuous stream was sucked through a small pipe by a vacuum pump, the sample being taken at right angles to the floor of the main stream. The whole arrangement was devised by Mr. Huntley, and a very perfect average sample of gas and exhaust could thus be obtained. Water.—The jacket water was run to waste over a syphon pipe, and was measured in calibrated tanks before it entered the jackets. The inlet and outlet temperatures were taken on standardised thermometers. Counters.-Both revolutions and explosions were recorded by mechanical counters. Power Developed. The mean pressure from the nine diagrams is 60.3 lbs. per square inch, and the explosions being 84.7 per minute, the gross I.H.P. works out to 13.2. The mean pressure during the pumping stroke is 1.6 lb. per square inch, which should be subtracted from the 60·3 lbs. above to give the true effective mean pressure. The net effective H.P. would then be 12.8. With a net load of 126 lbs. and 172.1 revolutions per minute the brake H.P. = 11.47. Mechanical Efficiency.-The mechanical efficiency will then be 89.6 per cent. reckoned on the net I.H.P., and 86.8 per cent. reckoned on the gross I.H.P. Air used. The air consumed was not directly measured, but its amount can be very closely calculated by the following indirect method. The gas per explosion was '0592 cubic foot, and the temperature of the meter being 51° Fahr. and the pressure of gas 14.75 lbs. per square inch, its specific volume would n × (51 + 461) The value of n obtained from the analysis be 14.75 x 144 ⚫0592 of the gas is 128.1, and the specific volume therefore would equal 30.95 cubic feet per lb. The weight of gas used per explosion is therefore = .00191 lb. On entering the cylinder this gas would be raised in temperature to a point which may be assumed to be midway between the outlet temperature of the jacket water and the meter temperature; here 60° to 89° Fahr., or 550° Fahr. above absolute zero. The pressure at the end of the suction stroke measured upon the pumping cards is 13.1 lbs. per square inch. The specific volume of the gas on entering the cylinder is therefore 128.1 x 550 = 37.35 cubic feet per lb., and the volume occupied 13.1 x 144 by the gas = ·00191 × 37·35 = ·07145 cubic foot. The total volume (cylinder + clearance) being 8377 cubic foot, there remains 837707145 cubic foot to be occupied by air and exhaust products from the last explosion. The exhaust products will amount approximately to the volume of the clearance = 2467 cubic foot, and their specific volume being (from the value of n obtained from the exhaust analysis, 556 × 550 = 16·21 cubic feet per lb.; their weight 13.1 x 144 •2467 16.21 viz. 556) will be = ⚫01521 lb. The air will therefore occupy 8377-(07145 +2467) 5196 cubic foot. Its specific volume will be 53.2 × 550 13.1 x 144 = 15·51 cubic feet per lb., and its weight, The total weight of the charge will therefore be (00191 015203349) = .05061 lb. |