has a section offset from the center line of the shaft sothat it “cranks” when the shaft is turned (fig. 1-3).ENGINE CYCLEWhen the piston is at the highest point in thecylinder, it is in a position called top dead center(TDC). When the piston is at its lowest point in thecylinder, it is in a position called bottom dead center(BDC) (fig. 1-4). As the piston moves from top to bottomor from bottom to top, the crankshaft rotates exactly onehalf of a revolution. Each movement of the piston fromtop to bottom or from bottom to top is called a stroke;therefore, the piston completes two strokes for every fullcrankshaft revolution.For an engine to operate, the following sequence ofevents must occur:1. INTAKE: A combustible mixture is pulled intothe cylinder.2. COMPRESSION: The combustible mixture iscompressed into a smaller space.3. POWER: The compressed combustible mixtureis ignited causing it to expand, producing power.4. EXHAUST: The burnt gases are removed fromthe cylinder.The engine repeats this sequence of events over andover again to produce sustained power. One completeseries of these events in an engine is called a cycle.Engines have either a four-stroke cycle or a two-strokecycle; most engines operate on the four-stroke cycle.Four-Stroke Cycle Gasoline EngineIn the four-stroke cycle gasoline engine, there arefour strokes of the piston in each cycle: two up and twodown (fig. 1-5). The four strokes of a cycle are asFigure 1-5.—Four-stroke cycle operation.1-3
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