enclosed end of a cylinder has two openings. One of theopenings, or ports, permits the mixture of air and fuel toenter, and the other port permits the burned gases toescape from the cylinder. The two ports have valvesassembled in them. These valves, actuated by thecamshaft, close off either one or the other of the ports,or both of them, during various stages of engineoperation. One of the valves, called the intake valve,opens to admit a mixture of fuel and air into the cylinder.The other valve, called the exhaust valve, opens to allowthe escape of burned gases after the fuel-and-air mixturehas burned. Later you will learn more about how thesevalves and their mechanisms operate.The following paragraphs explain the sequence ofactions that takes place within the engine cylinder: theintake stroke, the compression stroke, the power stroke,and the exhaust stroke. Since these strokes are easy toidentify in the operation of a four-cycle engine, thatengine is used in the description. This type of engine iscalled a four-stroke-Otto-cycle engine, named after Dr.N. A. Otto who, in 1876, first applied the principle ofthis engine.INTAKE STROKEThe first stroke in the sequence is the intake stroke(fig. 12-4). During this stroke, the piston is movingdownward and the intake valve is open. This downwardmovement of the piston produces a partial vacuum inthe cylinder, and air and fuel rush into the cylinder pastthe open intake valve. This action produces a resultsimilar to that which occurs when you drink through astraw. You produce a partial vacuum in your mouth, andthe liquid moves up through the straw to fill the vacuum.COMPRESSION STROKEWhen the piston reaches bottom dead center at theend of the intake stroke (and is therefore at the bottomof the cylinder) the intake valve closes and seals theupper end of the cylinder. As the crankshaft continuesto rotate, it pushes the connecting rod up against thepiston. The piston then moves upward and compressesthe combustible mixture in the cylinder. This action isknown as the compression stroke (fig. 12-4). In gasolineengines, the mixture is compressed to about one-eighthof its original volume. (In a diesel engine the mixturemay be compressed to as little as one-sixteenth of itsoriginal volume.) This compression of the air-fuelmixture increases the pressure within the cylinder.Compressing the mixture in this way makes it morecombustible; not only does the pressure in the cylindergo up, but the temperature of the mixture also increases.POWER STROKEAs the piston reaches top dead center at the end ofthe compression stroke (and is therefore at the top of thecylinder), the ignition system produces an electric spark.The spark sets fire to the fuel-air mixture. In burning,the mixture gets very hot and expands in all directions.The pressure rises to about 600 to 700 pounds per squareinch. Since the piston is the only part that can move, theforce produced by the expanding gases forces the pistondown. This force, or thrust, is carried through theconnecting rod to the crankpin on the crankshaft. Thecrankshaft is given a powerful twist. This is known asthe power stroke (fig. 12-4). This turning effort, rapidlyrepeated in the engine and carried through gears andshafts, will turn the wheels of a vehicle and cause it tomove along the highway.EXHAUST STROKEAfter the fuel-air mixture has burned, it must becleared from the cylinder. Therefore, the exhaust valveopens as the power stroke is finished and the piston startsback up on the exhaust stroke (fig. 12-4). The pistonforces the burned gases of the cylinder past the openexhaust valve. The four strokes (intake, compression,power, and exhaust) are continuously repeated as theengine runs.ENGINE CYCLESNow, with the basic knowledge you have of the partsand the four strokes of the engine, let us see whathappens during the actual running of the engine. Toproduce sustained power, an engine must repeatedlycomplete one series of the four strokes: intake,compression, power, and exhaust. One completion ofthis series of strokes is known as a cycle.Most engines of today operate on four-strokecycles, although we use the term four-cycle engines torefer to them. The term actually refers to the four strokesof the piston, two up and two down, not the number ofcycles completed. For the engine to operate, the pistoncontinually repeats the four-stroke cycle.TWO-CYCLE ENGINEIn the two-cycle engine, the entire series of strokes(intake, compression,in two piston strokes.power, and exhaust) takes place12-5
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