Four-cycle gas engine. I, admission valve. O, exhaust valve.
Gas Engines.
The first gas engines used gas and air at ordinary atmospheric pressure; at due intervals the charge was exploded by a glowing hot tube exposed by a slide-valve, or, according to the practice now general, by an electric spark of the jump variety. In 1862 De Rochas patented, and in 1876 Otto built, an engine on a model still in favor. Its cardinal feature is the compression of each charge. In the field of steam practice, we know how great economy is realized by beginning work with high pressures. A similar gain attends the compression of gases in a cylinder before explosion; whatever their pressure before ignition, it is trebled or quadrupled by ignition, returning a handsome profit on the work of compression, The four-cycle operation devised by De Rochas proceeds thus:—First, by drawing in a mixture of gas and air in due percentages during an outward stroke of the piston. Second, this charge is compressed by an inward piston stroke. Third, the compression charge is ignited, preferably by an electric spark, when the piston moves outward by virtue of a pressure initially extreme. Fourth, the exhaust valve opens and the spent gases are ejected as the piston returns to complete its cycle. As but one of the four piston journeys is a working stroke, it is necessary to employ a heavy flywheel to equalize the motion of the engine. When two or more engines are united, their piston rods are so connected to a common shaft as to distribute the working strokes with the best balancing effect. With four engines their piston rods may be arranged at distances apart of 90 degrees, so that one working stroke is always being exerted. This plan is adopted for the gasoline engines of automobiles so that they are served by fly-wheels comparatively small.
In his work on the gas engine, Professor F. R. Hutton discusses the advantages and disadvantages of that motor.[41] By his kind permission his main conclusions may be thus summarized, first as to advantages:—
[41] “The Gas-engine: a treatise on the internal-combustion engine using gas, gasoline, kerosene, or other hydro-carbon as source of energy.” By F. R. Hutton, professor of mechanical engineering in Columbia University. New York, John Wiley & Sons. $5.00.
The heat energy acts directly upon the piston, without intervening appliances. Fuel economy is greater than with steam, because there is no furnace or chimney to waste any heat. No fuel is wasted in starting the motor, or after the engine stops. The bulk, weight and cost of a furnace and boiler are eliminated, as well as their losses by radiation. A gas motor has a portability which lends itself to important industries, as logging and lumbering. It may be started at once, with no delay as in getting up a fire under a boiler; when the fuel-supply is cut off, the motor stops and needs no attention: these are important in automobile practice. Gas engines are gainfully united to systems of gas storage so that a producer may be run at high efficiency when convenient, and its gas held in holders till needed: this is helpful when a plant is worked overtime, or is liable to stresses of extreme demand at certain hours of the day. Incident to this is the advantage of subdividing power units in a large plant: each motor may receive its gas in pipes without loss, to be operated at will. The rapidity of flame propagation renders possible a high number of shaft rotations per minute, so that a multi-cylinder engine weighs little in comparison with its power. There is no liability to boiler explosion, or trouble from impurities deposited by water in a boiler. There is no exposed flame or fuel-bed requiring attention. The mechanism of the motor is simple, and its moving parts are few. A gas or oil engine furthermore enjoys a combustion which is smokeless. The fuel requires no diluting excess of air, with its cooling effect and incidental waste of energy. Dust, sparks and ashes are avoided, with diminished risk of fire. Liquid or gaseous fuel can be served by pumps or blowers so that the cost of handling is avoided.
As to disadvantages:—In a four-cycle engine there is but one working stroke in four piston traverses. In a two-cycle engine there is one working stroke in two traverses. For a given mean pressure the cylinder of a gas engine must be larger than a double acting steam cylinder. In single cylinder gas engines the crank effort is irregular; hence a heavy fly-wheel is required, or, a number of cylinders must be joined together, adding much weight. The motor does not start by the simple motion of a lever or valve. It has to be started by an auxiliary apparatus stored with energy enough to cause one working stroke. A steam engine may be overloaded to meet brief demands for extra power: not so with a gas engine. The extreme temperatures of the cylinder require cooling systems by air or water, adding weight and involving waste of energy; these temperatures furthermore may seriously distort the mechanism while rendering lubrication difficult and uncertain. Explosions of some violence may occur in exhaust pipes and passages, unless the engine is carefully adjusted and operated. Imperfect combustion clogs the working parts with soot or lampblack, especially injuring the ignition appliances. Initial pressures are so high as to cause vibration and jar. Governing is not easy, since explosion is all but instantaneous. The normal motor runs at maximum efficiency only when running at a certain speed. To vary that speed is much more troublesome and wasteful of energy than with the steam engine.
Gas engines united to gas producers have been employed with success on shipboard. This field, with its high premium on fuel reduction, which means more space for cargo, is likely to be largely developed in the near future. Soon, also, we may expect locomotives to exhibit a like combination with profitable results.
Steam and Gas Engines Compared.
During 1904 and 1905 the U. S. Geological Survey compared at St. Louis a steam engine with a gas engine, each of 250 horse-power, using 24 varieties of lignites and bituminous coals. The steam engine was of a simple, non-condensing, unjacketed Corliss type, from the Allis-Chalmers Company, Milwaukee. The gas engine was a three-cylinder, vertical model from the Westinghouse Machine Company, Pittsburg. Its gas was supplied by a Taylor gas producer furnished by R. D. Wood & Company, Philadelphia, of the design illustrated on page 460.