DIESEL ENGINE COMPRESSION STROKE.—The piston is at bottom dead center at the beginning ofthe compression stroke, and, as the piston movesupward, the air compresses. As the piston reaches topdead center, the compression stroke ends (fig. 1-10,view B).DIESEL ENGINE POWER STROKE.— Thepiston begins the power stroke at top dead center. Theair is compressed to as much as 500 psi and at acompressed temperature of approximately 1000°F. Atthis point, fuel is injected into the combustion chamberand is ignited by the heat of the compression. Thisbegins the power stroke. The expanding force of theburning gases pushes the piston downward, providingpower to the crankshaft. The diesel fuel will continue tobum through the entire power stroke (a more completeburning of the fuel) (fig. 1-10, view C). The gasolineengine has a power stroke with rapid combustion in thebeginning, but little to no combustion at the end.DIESEL ENGINE EXHAUST STROKE.— Asthe piston reaches bottom dead center on the powerstroke, the power stroke ends and the exhaust strokebegins (fig. 1-10, view D). The exhaust valve opens,and, as the piston rises towards top dead center, the burntgases are pushed out through the exhaust port. As thepiston reaches top dead center, the exhaust valve closesand the intake valve opens. The engine is now ready tobegin another operating cycle.Multifuel EngineThe multifuel engine (fig. 1-11) is basically afour-stroke cycle diesel engine with the capability ofoperating on a wide variety of fuel oils withoutadjustment or modification. The fuel injection system isequipped with a device called a fuel densitycompensator that varies the amount of fuel to keep thepower output constant regardless of the type fuel beingused. The multifuel engine uses a spherical combustionchamber (fig. 1-12) that aids in thorough fuel and airmixing, complete combustion, and minimizes knocks.NOTE: Because of environmental pollutioncontrols and the development of more efficient dieselengines, the multifuel engine is being phased out.Figure 1-11.—Multifuel engine.1-8
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