Figure 1-13.—Two-stroke cycle diesel engine.Two-Stroke Cycle Diesel EngineA two-troke diesel engine (fig. 1-13) shares thesame operating principles as other internal combustionengines. It has all of the advantages that other dieselengines have over gasoline engines.A two-stroke diesel engine does not produce asmuch power as a four-stroke diesel engine; however, itruns smoother than the four-stroke diesel. This isbecause it generates a power stroke each time the pistonmoves downward; that is, once for each crankshaftrevolution. The two-stroke diesel engine has a lesscomplicated valve train because it does not use intakevalves. Instead, it requires a supercharger to force airinto the cylinder and force exhaust gases out, becausethe piston cannot do this naturally as in four-strokeengines.The two-stroke diesel takes in air and dischargesexhaust through a system called scavenging.Scavenging begins with the piston at bottom deadcenter. At this point, the intake ports are uncovered inthe cylinder wall and the exhaust valve is open. Thesupercharger forces air into the cylinder, and, as the airis forced in, the burned gases from the previousoperating cycle are forced out (fig. 1-14).COMPRESSION STROKE.— As the pistonmoves towards top dead center, it covers the intakeports. The exhaust valves close at this point and sealsthe upper cylinder. As the piston continues upward, theair in the cylinder is tightly compressed (fig. 1-14). Asin the four-stroke cycle diesel, a tremendous amount ofheat is generated by the compression.POWER STROKE.— As the piston reaches topdead center, the compression stroke ends. Fuel isinjected at this point and the intense heat of thecompression causes the fuel to ignite. The burning fuelpushes the piston down, giving power to the crankshaft.The power stroke ends when the piston gets down to thepoint where the intake ports are uncovered. At about thispoint, the exhaust valve opens and scavenging beginsagain, as shown in figure 1-14.Valve TrainThe operation of the valves in a timed sequence iscritical. If the exhaust valve opened in the middle of theintake stroke, the piston would draw burnt gases into thecombustion chamber with a fresh mixture of fuel andair. As the piston continued to the power stroke, therewould be nothing in the combustion chamber that would1-10
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