follows: intake, compression, power, and exhaust. Acycle occurs during two revolutions of the crankshaft.INTAKE STROKE. — The intake stroke begins attop dead center, and as the piston moves down, theintake valve opens. The downward movement of thepiston creates a vacuum in the cylinder, causing a fueland air mixture to be drawn through the intake port intothe combustion chamber. As the piston reaches bottomdead center, the intake valve closes.COMPRESSION STROKE.— The compressionstroke begins with the piston at bottom dead center andrising up to compress the fuel and air mixture. Since boththe intake and exhaust valves are closed, there is noescape for the fuel and air mixture, and it is compressedto a fraction of its original volume. At this point, the fueland air mixture is ignited.POWER STROKE.— The power stroke beginswhen the fuel and air mixture is ignited, burns andexpands and forces the piston down. The valves remainpower stroke ends as the piston reaches bottom deadcenter.EXHAUST STROKE.— The exhaust strokebegins when the piston nears the end of the power strokeand the exhaust valve is opened. As the piston movesupward towards top dead center, it pushes the burntgases, resulting from the ignition of the fuel and airmixture, out of the combustion chamber and through theexhaust port. As the piston reaches top dead center,ending the exhaust stroke, the exhaust valve closes, andthe intake valve opens to begin the intake stroke for thenext cycle.Four-Stroke Cycle Diesel EngineThe four-stroke diesel engine is similar to the four-stroke gasoline engine. They both follow an operatingcycle that consist of intake, compression, power, andexhaust strokes. They also share similar systems forintake and exhaust valves. The components of a dieselclosed so that all the force is exerted on the piston. Theengine are shown in figure 1-6.Figure 1-6.—Four-stroke cycle diesel engine.1 - 4
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