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Specific heat of Coal Gas at constant volume = Cv. = 5196 B.T.U.
to raise Weight Calorific value
Cal value-18950 BTUs
Specific heat at constant pressure = Cp = 2646 B.T.
THE ‘FORWARD ' GAS ENGINE
Consumption and Temperature Trials
The ‘Forward’ Otto gas engine, which ‘The Engineer,' April 6, 1894, illustrates on page 293, built by Messrs. T. B. Barker & Co., of Birmingham, has recently undergone a series of brake and indicator tests, the results of which are given in the annexed table. These trials were made at the instance of the city of Birmingham gas department—to whom the engine was supplied—and conducted by Mr. Morrison, the assistant engineer of the Saltley Gas Works, in conjunction with Mr. F. W. Lanchester.
This engine works on the now almost universally adopted Otto cycle, with independent feed and exhaust valves and gas valve actuated by hit-and-miss mechanism controlled by a centrifugal governor. The ignition is effected by a tube igniter without a timing valve of any description. A glance at results of these trials and the indicator cards taken will show how superfluous such an adjunct appears to be. The makers consider that an ignition valve adds to the complexity of an engine without compensating advantages. This engine is of 30 maximum brake horse-power, and has a cylinder 12 inch diameter by 20 inch stroke. The crankshaft is of forged Siemens-Martin steel, 4; diameter at journals. The brasses are phosphor bronze throughout, and ample bearing surfaces are allowed, the mean pressures during the working stroke being as follows: Piston pin, 535 lbs. per square inch; crank pin, 375 lbs. per square inch ; main bearings, 100 lbs. per square inch. The piston packing consists of four cast iron rings 3 inch wide, fitted between junk rings in the usual manner, each ring having a baffle groove turned in its surface # inch wide, dividing the bearing surface of the ring into two bands + inch wide ; the pressure exerted by the ring on the walls of the cylinder liner amounts to 5 lbs. per square inch of surface.
This engine is fitted with a Lanchester multiple impulse self-starter, of which we gave a description in Volume lxxii., No.
1860. This starter is arranged not only to give the initial impulse necessary to set the engine in motion, but gives a series of low-pressure impulses at the commencement of the working strokes until sufficient speed is acquired to overcome the compression. In fig. 1, plate 206, line a shows the expansion curve of initial explosion, lines b b b show subsequent series of lowpressure explosions, ignition being effected in all cases by the starting igniter; the tube igniter only comes into operation when the exhaust roller is thrown off relief cam. Referring to the tabulated results of the trials, it will be seen that a gas consumption as low as 21 feet per brake horse-power hour has been obtained. This, we are informed, is 1 foot less than the makers guarantee, and is one of the best results ever recorded to our knowledge. The gas consumption per indicated horse-power hour ranges from 16% feet to 17% feet. We are not aware that as low figures as these have been previously obtained with an engine of this size. The calorific value of the gas is taken as 576,730 foot-pounds per cubic foot at 14.7 lbs. pressure and O.C.; this is the value calculated by Mr. Dugald Clerk from the analysis of Dr. P. Frankland of 17} candle-power Birmingham gas. This, in the absence of anything more recent, may be taken as approximately correct. Corrections are made for temperature and pressure. With the object of determining the composition of the charge and its temperatures at various points of the cycle, some further experiments were afterwards carried out by Mr. F. W. Lanchester at Messrs. Barker's works. These experiments were directed to determine, firstly, the temperature of the charge entering the cylinder before admixture with the residuals; secondly, the temperature of the residuals themselves. The temperature of the charge was recorded by a thermometer placed in a hole specially drilled in the feed valve box so as to project to within 1% inch of the valve, the bulb of the thermometer being protected from radiation by a cylindrical paper screen; the actual readings with the engine at nearly full load ranged up to 44°C. The further increase of temperature which would take place in passing the valve and port would in all probability bring this up to 50° C., which, for purposes of calcula