engine, which was the pioneer of all the Hot-Bulb or "Surface Ignition" engines, was an English product. It is still being manufactured in England and in the United States, though the design has been consider ably improved.
This engine was of the four-stroke-cycle principle, the valves being placed in a valve chamber at one side of the cylinder. The cylinder head included a vaporizer or uncooled bulb. The oil was injected into the bulb and, owing to the intense heat contained in the walls of the bulb, was vaporized. At the end of the compression stroke the heat of the bulb, increased by the heat caused by the compression in the cylinder, was high enough to ignite the fuel. The engine was an explo sive or constant-volume engine and followed the Otto cycle in its action.
Since the engine used the four-stroke-cycle, the air charge was compressed in the engine cylinder. The compression pressure seldom exceeded 60 lbs. per sq. inch. While this was as high a pressure as the first engine could use due to preignition troubles, later modifications of this engine have the compression as high as 140 lbs. per sq. inch. The variation in compression depends on the time of depositing the fuel charge in the vaporizer. In the early models the oil entered the cylinder very early in the compression stroke, while the later designs have the oil injection occurring during the last 45 degrees before dead-center.
The hot-bulb or vaporizer engine came into general favor based on its ability to successfully burn kerosene (termed paraffine in England) and on the simplicity of design, which made it far superior to the ordinary four-stroke-cycle gasolene or carburetor engine.
The trend during the past fifteen years is toward a two stroke-cycle engine using the crankcase or front of the cylinder as a scavenging air compressor. This present-day engine is, in respect to the method of fuel ignition and combustion, the original Hornsby-Ackroyd; the chief departure from this orig inal design is the change from four-stroke-cycle to two-stroke cycle. This change lowers the manufacturing costs and in some respects simplifies the operating details. In the number
of builders and in the total horsepower capacity of these two cycle hot-bulb engines the United States easily ranks first. In England and on the Continent more attention was given the vaporizer engine, and it was as late as 1910 before the hot-bulb came into favor in England. At the present time they are finding a field of usefulness in stationary work and in small vessels, such as fishing boats, tugs and cruising yachts.
The Diesel practice has always been for the inventor or engine builder to construct his engine and then from its operation deduce the thermodynamic principles under which it operated. Dr. Diesel, a German engineer, designed an engine to operate on the constant temperature cycle. In this engine the air charge on the compression stroke was to be compressed to a pressure such that the resulting temperature was above the ignition temperature of the fuel. The fuel was to be introduced into the cylinder at a rate such as to cause the heat released by the combustion to exactly equal the work per formed on the moving piston. This would cause the cylinder temperature to remain constant during the period of fuel intro duction extending over some 10 per cent. of the piston stroke. After "cuttoff " the gases were to expand adiabatically without absorbing or losing any heat save that consumed in doing work; the exhaust was to be at constant pressure. The engine was originally designed without water-jacket cooling since there was to be no heat loss.
The original patent, dated 1892, outlined an engine wherein the fuel used was to be pulverized coal or coal dust. This coal was to be stored in a hopper immediately above the engine cylinder head. Between the hopper and the cylinder was inter posed a rotary valve having a cavity or pocket. The valve received a charge of coal dust and in rotating in its seat came in communication with the cylinder. The coal dust then dropped into the combustion space as the piston reached the end of the compression stroke. The fuel charge was varied to suit load conditions through the governor control of the valve movement.