TYPES OF ENGINES. DESIGN OF IGNITION DEVICES General.—Although the crude oil engines should properly be divided into the three classes: Diesel, or high-compression; semi-Diesel, or medium-compression; and low-compression, or vaporizer engines, there is a strong tendency on the part of many builders of the low-pressure engines to call their product semi-Diesel engines. As discussed in previous chapters, the word "semi-Diesel" does convey a distinction as regards the limits of compression carried on the engine. Any engine whose compression pressure does not reach at least 250 lbs. per sq. inch • and which depends on some manner of hot surface for the vapori zation of the fuel before ignition, quite properly falls within the third class that is covered by the term "low-compression engines." It is quite possible to use another distinguishing mark whereby an engine may be correctly labeled. If the heat is received at constant volume, the engine falls in the third class. There is no theoretical reason why a high-compression engine should not be built to receive the heat at constant volume. It is impossible, practically, since the pressure of explosion would run so high as to wreck the engine.
Probably the greatest volume of crude oil engine business, both in number of .ttrfits and in horsepower, is in the low-pressure type. Various designs of this class of engines have been built in this country for fifteen years or more; and the total number of installations is large. In the year of 1916, one company sold more than 50,000 h.p. in sizes ranging from 10 h.p. to 200 h.p. The majority of these units are installed in small mills, factories and light plants where the attendants are usually unskilled. It speaks volumes for the simplicity of the low pressure engines that they can run day in and day out under such conditions. It is this simplicity of design that causes the low-compression engine to prove actually more economical than the Diesel in total, operating costs in . a great many plants.
The low-compression engines are all modifications of the original Hornsby-Akroyd engine. While this engine was of the four-cycle principle, only one manufacturer, the De La Vergne Machine Co., still adheres to the four-stroke-cycle; all the other manufacturers adopted the two-stroke-cycle as being simpler and less costly to build. This design is quite similar to the well known two-cycle gasolene engine wherein the crankcase is enclosed and acts as a compressor to furnish the necessary air to scavenge-the cylinder of the burnt charge, although in the oil engine nothing but air is compressed in the crankcase.
Two-stroke-cycle Engine. Method of Operation.—Figures 243 to 246 show this type of engine with the crank at different angles and the probable process of mixing and firing of the fuel charge. It is impossible to ascertain the exact state of affairs transpiring in the cylinder, and these figures are at best but an attempt to give the most probable course of events. In Fig. 243, after the bulb on the cylinder has been heated by a torch, not shown, the fuel is sprayed upon the red-hot lip of the bulb, which projects into the cylinder space. This fuel, on igniting, drives the piston forward. At a point in the stroke of the piston, as indicated in Fig. 244, the exhaust port is uncovered by the piston, and the
burnt gases, which still have considerable pressure, rush out through the exhaust pipe. Figure 245 shows the air port un covered by the piston and the pure compressed air from the crank case displacing the exhaust gases and filling the cylinder with a fresh air charge. In Fig. 246 the piston has compressed this air charge to from 70 to 120 pounds, and the fuel is beginning to spray into the combustion space. The above sequence of action is followed in all two-strokelcycle surface ignition oil engines, although each builder has certain modifications of the cylinder and the hot-bulb design.
Theory of Combusfion.—The operation of the low-compression engine is based on the fact that the heavier oils will not self ignite in the presence of air having a temperature corresponding to 90 to 150 pounds compression pressure, but if this oil strikes a hot surface it will break up into hydrocarbons of a less complex series; in other words, it will "crack." These light oils will vaporize readily and will ignite at a temperature much lower than is required to ignite the heavy original oil. Since the com bustion of the fuel involves a process of distillation of the oil into oils of lightei gravity before burning, the time necessary for complete combustion is somewhat longer than in the gasolene engine, where the distillation process has been completed at the refinery, or in the Diesel engine, where the atomization of the heavy oil breaks it up into particles small enough to be ignited by the high temperature of the compressed air charge. Con sequently, if the engine speed is kept above 200 r:p.m., the oil must be injected at such a point before dead-center as to allow it sufficient time to "crack," vaporize and burn. At 200 r.p.m., or 400 strokes, the time consumed in making a power stroke is but .15 second; if the combustion or explosion takes place while the piston covers one-fifth of its stroke, the time interval is only .03 second. To allow the oil time to undergo the process of distillation and ignition by the time the piston reaches dead-center, the current practice is to begin the injection when the crank is 30 degrees or more ahead of rear dead-center, allowing the injection to cease before the piston completes this compression stroke. Since practically all the oils used contain a percentage of lighter constituents that will vaporize without going through the process of "cracking," and burn at a low tem perature, there is a tendency of this part of the oil to ignite before the piston reaches dead-center and while the compression pressure is still low. To avoid this, builders depend upon one of two factors—they either design the combustion chamber in such a manner as to allow the fresh oil vapors to remain un mixed with the fresh air until the piston is approximately 10 degrees from dead-center, or they provide for the injection of a small amount of water at each cycle; this water tends to keep down the temperature in the cylinder during compression, there by precluding the preignition of the lighter portions of the oil. Stratification of the gases and air as aimed at in the first method does occur in some designs, although it is far from successful in all.