Otto Engine.— Otto's aim was to use the en tire stroke for the explosion and expansion. To obtain this he built an experimental machine in which the engine turned forward for the in take; then the valves were closed and the fly wheel turned backward to compress the mix ture, which was then ignited by hand. The effect was surprising. The ignited mixture, on account of the compression, exploded with such violence that the wheel rotated for some time at high speed. This was the more surprising, since, when the engine was operated without compression, it ran at a low speed. So by these experiments, 1861-62, Otto arrived at the com pression of the charge before explosion, igni tion at the dead-centre and the accomplishment of the charging, compression, combustion and discharging events in one cylinder — in short, he came onto the cycle which is the very foundation of the internal combustion engine development. Great mechanical difficulties, however, prompted Otto to drop his work on this engine and to devote his time to the de velopment of the free piston engine described above. Fifteen years later, in 1877, Otto again took up the work on the four-stroke cycle and succeeded in perfecting the engine so favorably known throughout the world. One of the first new Otto engines is shown in Fig. 55. It is a has opened the gas valve, g, so that gas and air enter the cylinder simultaneously at the end of the stroke. Now the slide valve, b, closes the port, i, and the charge is compressed into the clearance space by the returning piston. Dur ing this time the slide valve, b, has moved to the right, and the ignition port, h, is opposite the port, i, so that the flame is carried through, i, into the clearance space, a. Ignition occurs on dead centre and combustion takes place at almost constant volume. At the end of the ex pansion stroke the valve, c, is opened and ex haust occurs during the next stroke. The burnt gases in the combustion chamber cannot be ex pelled, but remain and become a part of the next charge. Governing is by the "hit and miss') principle. The carrying out of the four-stroke cycle in the Otto engine is mechanically almost perfect. The over-all efficiency was about 15 per cent in small sized engines, increasing with horizontal, one-cylinder, four-stroke cycle gas engine, consisting of cylinder with water jacket, piston, piston-rod, crosshead, connecting-rod, crankshaft, flywheel, frame and timing gears. The cylinder has a clearance space, a, of about two-thirds of the piston displacement. The en t-ance of the gas and air, as well as the ignition, is controlled by the slide valve, b. Exhaust takes place through the valve, c, both valves be ing operated from the half-speed shaft, f, by means of the crank, d, and the cam, e, respec tively. The gas valve, g, is operated by the cam g'. At the beginning of the intake stroke the exhaust valve, c, is closed, while the slide valve, b, has moved its air canal to communicate with the channel, 1, of the cylinder head. Air alone is drawn in for the first part of the stroke, which is supposed to force the burnt gases of the clearance space against the receding piston. In the meantime the valve, 1, has moved far enough to open the gas inlet, while the cam, 0', capacity of cylinder and improvement of con struction. To-day Otto engines show over-all efficiencies of 28 per cent to 30 per cent. In 1878 the largest engine sold was 4 horse power; 1880, 20 horse power; 1885, 80 horse power; 1889, 100 horse power; 1893, 200 horse power; 1900, 1,000 horse power; in 1912, 6,000 horse power in the double-acting four cylinder units were delivered. Fig. 56 illustrates a double acting, four cycle, high-powered gas engine of American make.
Holzwarth Gas Turbine.— The recent great success of the steam turbine has aroused not less interest in the internal combustion turbine win, while from a thermodynamical point of view the problem of the gas turbine is not worse and not better than that of the recipro cating gas engine, however, the mechanical aspects offer less encouragement. The com
bustion of fuel in the gas turbine may take place at constant volume without or with com pression of the charge or at constant pressure. The only gas turbine of considerable size which has yet appeared is the one systematically de veloped by Hans Holzwarth. It is of the ex plosion (constant volume), compressed charge type with a nominal output of 1,000 horse power at 3,000 revolutions per minute, gas and air are separately compressed by compressors and forced into a number of combustion cham bers arranged annularly around a vertical shaft, on which a horizontal turbine wheel is fixed. Each combustion chamber contains gas and air inlet valves, spark plugs and a nozzle valve. The latter is closed during the charging period. The explosion pressure forces it open, thus enabling the high-tension gases to pass through the small nozzle on the turbine wheel at high velocity. After having given up their energy to the wheel the burned gases escape into the at mosphere. Governing is effected by throttling the gas or by cutting out the spark. The over all efficiency is said to be 4 per cent. This low value is mostly due to the mechanical complica tion inherent in this type of internal combus tion engine.
Humphrey Gas Pump.— During the Brussels Exposition in 1910 there was exhibited a new type of pumping engine, using the work de veloped by the explosion of a gas-air mixture for the lifting of water. It is known as the Humphrey Gas Pump. Since that time this gas pump has gained a world-wide reputation. It has been introduced into this country and greatly improved by American designers. Its operation can best be understood by reference to Fig. 56. The pump consists of a vertical gas cylinder, A, with inlet and outlet valves, B and C. These valves interlock with each other. On the water side of the pump there is a suction pipe, D, a number of suction valves, S, and a pressure pipe, E, connecting the cylinder with the pressure tank, F. The water column, G, forms a gas-tight piston. To start the engine the combustion chamber must be filled with an explosible gas-air mixture, having the pressure, px. This mixture is ignited and the pressure, due to the combustion, suddenly increases to pi. While this takes place the volume will scarcely change so that combustion practically occurs at constant volume. The water column, owing to the increased pressure on its surface, is rapidly accelerated and the gases expand to volume v. and pressure p..
As soon as the gas pressure has become less than that of the atmosphere the exhaust valve on the top of the cylinder and the suction valves on the water inlet tank begin to open auto matically. The inflowing water follows the moving water column and fills the gas cylinder up to volume v., thus partly replacing the burned gases. The hydrostatic pressure from the water tank reverses the water column closing the water inlet valves and forcing the inert gases out through the exhaust valve. Reaching the exhaust valve, the water seals the outlet and consequently the remaining gases are compressed to the volume v. and the pres sure. p.. Now the water column reverses again. Re-expansion of the compressed gases takes place and the pressure falls below that of the atmosphere. The mixture inlet valve, B, opens and a new charge is taken in until the volume v. is filled. The water column reverses again and compresses the charge to volume v. and pressure p2. Ignition occurs and the cycle is started over again. The engine works on the four-cycle principle, the expansion and com pression occurs, theoretically, along adiabatic curves, and the cycle is carried on by the oscilla.;. tion of the water column due to the changes of pressure. The action of the pump is not altered if instead of delivering into the ele vated tank it is discharged into an air vessel. The pump has the advantage of being capable of handling enormous quantities of water. In the large pumps installed near the city of Lon don, 15 tons of water are discharged at each working stroke.