TYPES. ACTION. ADJUSTMENT. REPAIRS Injection Design of the Original Diesel Engine.—Dr. Diesel in his first patent application, made no mention of any mechan ism for the forcible injection of the fuel. The fuel contemplated was coal dust, and this was to be deposited in a pocket of the rotary valve V, which, in revolving, dropped the charge into the cylinder. A schematic outline of this fuel-conveying apparatus is shown in Fig. 109.
This design was never followed in an actual engine since it was early seen that it would not prop erly deliver the fuel charge. Fur therm,- re, the plan of using coal dust was abandoned in favor of oil. Dr. Dies,:. in conjunction with the M. A. . Co. of Germany, adopted the idea of employing a blast of air to break up the oil charge and deliver it into the cylinder.
Injection Action.—For the bene fit of those unacquainted with the functioning of the fuel injection valve the following brief explanation is given. The charge of fuel oil is pumped into a receptacle called the fuel valve by some form of pump. An air compressor delivers a charge of pure air at about 900 lbs.
gage to this injection device. At the proper moment in the engine's cycle the needle valve of the injection device, or fuel valve, is opened, connecting the cylinder with the fuel supply.
The high-pressure air then rushes into the cylinder, carrying the oil charge with it. This fuel, as it is forced through a device called the atomizer, which is located in the valve housing or cage; is broken up into fine fog-like particles that will ignite when intermingled with the cylinder charge of air, which is at a high temperature. Figure 110 shows such an elementary form of fuel valve where D is the valve stem; A, the oil line; C, the air line; B, the atomizer; and the opening into the cylinder.
It is evident that the fuel valve has two main functions. First, it must allow the oil charge to be introduced into the engine cylinder or combustion chamber at the proper time. Second, it atomizes or breaks up the stream of oil in such a thorough manner as to occasion ignition. The cylinder tem perature is high; the value correspond ing to 550 lbs. compression pressure should be at least 1000° Fahrenheit, when the engine is cold; after warming up, the temperature would be well above 1400° Fahrenheit. If the oil
charge was injected in a solid mass into this highly heated air, the oil would vaporize and burn but at a very slow rate. The air would not be thoroughly mixed with the oil, and the combustion would occur only on the surface of the oil mass, in exactly the same manner as a pool of oil burns when ignited. In this is seen an example of the so-called "surface ignition" employed in hot-bulb engines. The objection to this method lies in the slow rate of combustion and in the loss of fuel which escapes as unburned gas vapors. If the oil charge is separated into many minute particles, more surface area is presented to the air. If this oil separation process is to be successful, the fuel valve must be provided with some means whereby the oil is broken into particles and mixed with the injection air before the oil enters the engine cylinder.
The theory of the present-day Diesel contemplates the intro duction of the fuel at a rate which will allow the combustion to be carried on at constant pressure. If the oil charge was in jected at a high rate, the combustion would partake of the nature of an explosion; the indicator card would then resemble an Otto-cycle engine, and the combustion line would be in the form of a sharp peak, somewhat like Fig. 215. If the combustion is to be at a constant pressure rate, the flow of oil through the fuel valve must be gradual. The two main offices the valve performs are that of "braking," or offering a resistance to the oil flow, and that of thoroughly atomizing the charge. The latter would not be difficult of achievement if there were no other considerations entering into the problem. The same con ditions prevail as to the "braking" action. If this "braking" effect is obtained by the imposition of a series of improperly designed baffle plates or disks, the air pressure required to force the oil through these resistances may become so great as to make the method impractical. The desirable fuel valve is one that thoroughly atomizes the fuel and exercises a control, or a "brak ing" action, over the fuel charge without any great loss of air pressure through the valve. Many designs have been used, all of which fail to completely fulfill the above conditions.