Two-Stroke Cycle Diesel Engine

Figure 1-13.—Two-stroke cycle diesel engine. Two-Stroke Cycle Diesel Engine A two-troke diesel engine (fig. 1-13) shares the same operating principles as other internal combustion engines. It has all of the advantages that other diesel engines have over gasoline engines. A two-stroke diesel engine does not produce as much power as a four-stroke diesel engine; however, it runs smoother than the four-stroke diesel. This is because it generates a power stroke each time the piston moves downward; that is, once for each crankshaft revolution. The two-stroke diesel engine has a less complicated valve train because it does not use intake valves. Instead, it requires a supercharger to force air into the cylinder and force exhaust gases out, because the piston cannot do this naturally as in four-stroke engines. The two-stroke diesel takes in air and discharges exhaust through a system called scavenging. Scavenging begins with the piston at bottom dead center. At this point, the intake ports are uncovered in the cylinder wall and the exhaust valve is open. The supercharger forces air into the cylinder, and, as the air is forced in, the burned gases from the previous operating cycle are forced out (fig. 1-14). COMPRESSION STROKE.— As the piston moves towards top dead center, it covers the intake ports. The exhaust valves close at this point and seals the upper cylinder. As the piston continues upward, the air in the cylinder is tightly compressed (fig. 1-14). As in the four-stroke cycle diesel, a tremendous amount of heat is generated by the compression. POWER STROKE.— As the piston reaches top dead center, the compression stroke ends. Fuel is injected at this point and the intense heat of the compression causes the fuel to ignite. The burning fuel pushes the piston down, giving power to the crankshaft. The power stroke ends when the piston gets down to the point where the intake ports are uncovered. At about this point, the exhaust valve opens and scavenging begins again, as shown in figure 1-14. Valve Train The operation of the valves in a timed sequence is critical. If the exhaust valve opened in the middle of the intake stroke, the piston would draw burnt gases into the combustion chamber with a fresh mixture of fuel and air. As the piston continued to the power stroke, there would be nothing in the combustion chamber that would 1-10