THE DIESEL ENGINE Cycle of Events.—The readers who are familiar with the opera tion of a gasolene engine should easily grasp the cycle of events occurring in the cylinder of the Diesel engine. In Fig. 4, a to d, are shown four conditions existing in the engine cylinder at various points of the piston's stroke, while Fig. 4e indicates the portion of the stroke as covered by each event.
In Fig. 4, drawing a covers the suction or admission stroke of the piston. The admission valve J has opened at the point A just before dead-center. The valve J remains open from the point A to the point B. This admission stroke is shown in Fig. 4e and • Fig. 4f; in the latter the indicator card shows this line as being below the atmospheric pressure line xy. The air ac tually enters the cylinder under suction pressure. In drawing b, Fig. 4, the admission valve J has closed at B, and the pure air charge is compressed by the piston up to the point G, which is top dead-center. This process is covered by the compression line BC on the indicator card in Fig. 4f. The clearance volume is very small, and the maximum or final compression pressure rises to some 500 to 550 lbs. per sq. inch. The work done on the air charge in compression causes the temperature to ascend to about 1100° Fahrenheit.
At the point C in drawing c, Fig. 4, the injection valve opens, and a charge of fuel is blown into the cylinder by means of a blast of high-pressure air. The injection valve is designed to cause the rate of flow through the valve to be "braked" so that the in jection is not instantaneous but takes place while the engine crank turns through a considerable angle. In drawing c, Fig. 4, the injection of fuel starts when the crank is at C and ends when the crank is at D. In Fig. 4f the line CD represents the admission period, and the desired condition is attained when the line CD is practically horizontal, showing that the rate of heat addition is such that there is no increase in the cylinder pressure.
The injection and the combustion of the fuel ceasing at the point D, the piston continues to the end of its stroke under the influence of the expanding gase. Before the completion of the
stroke, the exhaust valve L °pans when the crank is at E. This allows the gases to rush out`through the exhaust passage. The exhaust valve continues to remain open until the piston again ascends to the top of the cylinder, expelling all the exhaust gases. This part of the cycle is shown in drawings d and e as continuing from E to F; in Fig. 4f this forms the exhaust line EF. Before the exhaust valve L closes, the admission valve opens at A, allow ing a fresh air charge to be inducted into the cylinder during the stroke shown in drawing a.
These events complete the cycle of the four-stroke-cycle Diesel. From a practical viewpoint the differences between the Diesel and the gas engine are that in the Diesel nothing but pure air is compressed in the cylinder and that the fuel is forced into the cylinder slowly, causing the combustion to be gradual; in the gas engine both the gaseous fuel and the air are compressed, and the combustion takes the form of an explosion.
Schematic Layout of the Diesel Engine.—Figure 5 embodies the schematic arrangement of the essential mechanism of a Diesel engine. In this particular instance the engine is of the horizontal type operating on the four-stroke-cycle principle. The engine crank is represented with its center at 0, and the crank revolves clockwise. To the crankshaft is geared a lay shaft A which revolves at half-engine speed. On this layshaft are mounted the cams used to actuate the exhaust, admission and fuel injection valves, which are operated in the sequence out lined in Fig. 4. The fuel pump P is driven off the layshaft, the fuel charge being under control of the governor Q. The fuel is deposited in the injection valve C, out of which it is forced by the air charge at the proper moment. The air blast is supplied by the air compressor D, which is driven by a crank on the end of the engine shaft. In this diagram the air line is not supplied with a receiver or air bottle, and the air is delivered directly from the compressor to the fuel valve.