Fuel Nozzle of the Koerting Diesel Engine Showing Operating Cam and Lever, and Compressed Air Connection.
Since the fuel is introduced gradually into the combustion chamber the combustion pressure rises very slowly so that it is not an explosive engine in any sense of the word, the combustion pressure rising steadily from the compression pressure to the maximum in proportion to the supply of fuel. In the ordinary type of gas engine with a compression pressure of from 60 to 70 pounds per square inch the pressure rises abruptly to about three and one-half times the compression pressure, with a correspondingly rapid drop in the pressure on the expansion stroke. In the Diesel engine the drop of pressure in expansion is much more gradual, the indicator diagram expansion curve being nearly horizontal. The uniform pressures thus obtained result in smooth action and even driving power, obtained with no other type of engine.
Fuel Pump of Koerting Diesel Engine with Operating Cam.
As the fuels used vary from the lightest hydrocarbons to the heaviest crude oils, there are many types of oil injection valves in use, the valves being in general divided into two classes, those in which the oil is vaporized mechanically by the pressure of a force pump, and those in which the fuel is vaporized by the atomizing effect of compressed air. Atomization by compressed air is however, the most common method since less trouble is experienced with the air pumps than with the liquid force pumps. The compressed air is supplied by pumps that are either operated by the main engine or by an independent compressor engine.
The fuel valve is a plug screwed into the cylinder containing an inwardly opening check valve in the inward end. The hole in the center of the plug receives the oil charge under a few pounds pressure from the tanks, during the compression stroke of the engine, and at the end of the compression stroke, a blast of air at a pressure of about 250 pounds above the compression pressure blows it into the cylinder in the form of a fine spray. Injection valves of the forced feed type consist of a plug with a small passage and a needle valve for regulating the spray. Fuel is pumped into the valve at about 250 pounds above the compression pressure of the engine by a small single acting pump which is built so that the length of the stroke may be adjusted to meet the load. In practice the length of stroke is regulated by the governor, so that the full contents of the pump are delivered at full load, and a reduced amount with a short stroke at small loads. On issuing from the fuel nozzle, the liquid strikes a gauze screen by which it is broken up into very fine spray.
Fluidity is practically the only factor that governs the quality of fuel that may be used with the engine, since exceptionally heavy oils and tars cannot be successfully sprayed. In Fig. 9 is shown a cross-section of a Diesel engine cylinder in which the center valve in the cylinder head is the fuel valve, and the valves to the right and left are the air inlet and exhaust valves respectively. The two latter valves correspond to the inlet and exhaust valves of the Otto cycle engine.
Compressed air is used in starting the engine, which is admitted to the cylinder through an auxiliary valve which is operated by a starting cam on the cam shaft. By this mechanism, high pressure air is furnished to the cylinder during a portion of the working stroke, turning it over on the first few revolutions as a common air engine. As soon as the engine picks up speed, the starting valves are thrown out of operation, and the engine proceeds on its regular working cycle with the oil fuel.
When used for marine purposes in sizes over 100 horse-power, where it is not possible to use reverse gears, the Diesel engine whether of the two stroke cycle or four stroke cycle type must be made reversible. This may be accomplished by either of two methods, first, by changing the angular position of the cams in regard to the piston position, and second by using two sets of cams, one being for right hand rotation and the other for left hand. When a single cam is used, the relation of the cam shaft on which the oil pump cams and oil valve cams are located, is advanced or retarded in respect to the crank shaft by means of sliding the two spiral gears that drive the cam shaft, over one another, in a direction parallel to their axes. The spiral gears are moved back and forth by a hand controlled reverse lever. This type is used principally on the two stroke cycle type of engine as there are not so many factors to contend with as on the four stroke cycle.
With double cams, the system almost invariably used with the four stroke cycle engine, the cams may be mounted either on one shaft, or the ahead cams on one cam shaft and the reverse cams on another. When two shafts are used they are arranged so that either set of cams may be swung under the valve lifters by swinging the shafts in a radial direction by brackets. The single type of cam shaft is usually moved back and forth in a direction parallel to its axis, the ahead cams coming under the valve lifts at one position, and the reverse cams at the other. In the four stroke cycle Diesel it is evident that not only the relations of the oil pump and oil valves must be changed in respect to the piston position but the relations of the air inlet and exhaust valves must be changed as well. This necessitates double cams for the inlet and exhaust valves in order to reverse rotation.