Stroke 2. Compression of the charge.
Stroke 3. Ignition and expansion of the charge.
Stroke 4. Discharge of the burnt gases.
It is to be observed that these events are those occurring in all present-day four-stroke-cycle gas or gasolene engines.
Otto's Silent Engine.—As stated heretofore, Otto had obtained a patent on a free piston engine in 1866. In company with Langen, Otto formed an engine-building organization—the Gas-Motorem Fabrik Deutz—which is still in existence. Objec tionable features of the Otto and Langen engine caused them to adopt the design proposed by Beau de Rochas. This engine, which was known as the "Otto Silent," created a furor at the Paris Exhibition in 1878. Figure 2 shows a view of one of these early Otto engines, while Fig. 3 is a section through the intake valve. This valve has two passages in it. The port 111 is the air passage, which, on the suction stroke of the engine, is in line with m, the air-suction pipe; Q is, at this time, in contact with a; and n communicates with L, the gas passage. This allows the air and gas to enter the cylinder through a. As the valve con tinues to move to the left on the compression stroke of the piston, N receives a small charge of gas from the gas line d. As the valve moves to the right on its return stroke, the cavity N moves past B, which carries an open flame, and the gas in N ignites. When this passage moves past the passage a, the flame in N ignites the gas in the cylinder. The exhaust valve, not shown, was of the poppet type. The disadvantage lay in the flat admission valve. To keep it against its seat, a strong spring pressure was nec essary; this occasioned rapid wear. However, the engine was so superior to its competitors that thousands were sold all over Europe. It is to be noticed that the cycle used should have been termed the "Beau de Rochas cycle" instead of the Otto, by which latter name this cycle is now universally designated.
This engine, modified to better meet existing conditions, is in daily use all over the world; thousands are sold each year; one American firm in 1917 marketed 30,000 Otto cycle engines in sizes from to 10 h.p., and there are scores of manufacturers who produce from 1000 to 10,000 engines yearly.
Brayton's Constant Pressure Engine.—A few years previous to the design of the Otto silent engine, Geo. Brayton, a Phila delphian, secured patents on an engine that differed from all other internal combustion engines in that, instead of the charge burning at constant volume or instantaneously as in the Otto cycle, the fuel, either gas or liquid, was introduced into the cylinder in such a manner as to cause it to burn at constant pres sure. The design of this engine embodied the use of an engine
cylinder and of a separate charging or compressing cylinder. The gas and air were introduced into this charging cylinder and compressed. At the beginning of the power stroke in the engine cylinder the mixture passed into the cylinder through the intake device, which was in the form of a gauze screen. A pilot light ignited the charge as it blew into the cylinder until cut-off took place at about 10 per cent. of the engine stroke. The flow of the mixture was such that the pressure in the power cylinder did not increase during combustion. After the flow of fuel ceased, the burnt gases expanded as in any engine. This Brayton cycle had much to commend it. It was the most efficient of all cycles operating between the same temperature limits. Its drawback was that, for equal power, it required a much larger cylinder vol ume than did the Otto cycle. The serious objection, both from a manufacturing and operating standpoint, was the excessive size and weight of the engine as well as the complicated mechanims. This cycle was early abandoned in favor of the more simple Otto cycle.
Clerk's Two-cycle Engine.—This engine was the design of Dugan Clerk, and, in place of being four-stroke-cycle, the engine was of the two-stroke-cycle type. The design em bodied the use of a charging cylinder into which the air and gas was introduced and compressed. The engine cylinder used no exhaust valves but, instead, made use of ports about the cylinder. After the power stroke was almost completed, the piston uncovered these ports, allowing the exhaust gases to blow out. At the same time the changing cylinder forced a charge of air and gas into the engine cylinder. This charge assisted in freeing the cylinder of the remaining consumed gases and was compressed by the piston on its return stroke. At the proper time a gas flame, and in some engines a hot bolt on the piston, ignited the charge. This cycle was not so favorably received since it was not as simple as the Otto engine. Sub sequently, many engines were built using this two-cycle principle but compressing the gas and air in the crankcase in preference to the separate charging cylinder. The Clerk engine was the forerunner of the modern two-cycle engine.