DIESEL ENGINE COMPRESSION STROKE.-

The piston is at bottom dead center at the beginning of the compression stroke, and, as the piston moves upward, the air compresses. As the piston reaches top dead center, the compression stroke ends (fig. 1-10, view B).

DIESEL ENGINE POWER STROKE.- The piston begins the power stroke at top dead center. The air is compressed to as much as 500 psi and at a compressed temperature of approximately 1000F. At this point, fuel is injected into the combustion chamber and is ignited by the heat of the compression. This begins the power stroke. The expanding force of the burning gases pushes the piston downward, providing power to the crankshaft. The diesel fuel will continue to bum through the entire power stroke (a more complete burning of the fuel) (fig. 1-10, view C). The gasoline engine has a power stroke with rapid combustion in the beginning, but little to no combustion at the end.

DIESEL ENGINE EXHAUST STROKE.- As the piston reaches bottom dead center on the power stroke, the power stroke ends and the exhaust stroke begins (fig. 1-10, view D). The exhaust valve opens, and, as the piston rises towards top dead center, the burnt gases are pushed out through the exhaust port. As the piston reaches top dead center, the exhaust valve closes and the intake valve opens. The engine is now ready to begin another operating cycle.

Multifuel Engine

The multifuel engine (fig. 1-11) is basically a four-stroke cycle diesel engine with the capability of operating on a wide variety of fuel oils without adjustment or modification. The fuel injection system is equipped with a device called a fuel density compensator that varies the amount of fuel to keep the power output constant regardless of the type fuel being used. The multifuel engine uses a spherical combustion chamber (fig. 1-12) that aids in thorough fuel and air mixing, complete combustion, and minimizes knocks.

NOTE: Because of environmental pollution controls and the development of more efficient diesel engines, the multifuel engine is being phased out.

Figure 1-11.-Multifuel engine.

Figure 1-12.-Spherical chamber.

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.