Thermodynamics

494 | ThermodynamicsPMEPW net = MEP(V max – V min )V minV maxVTDCW netBDCFIGURE 9–12The net work output of a cycle isequivalent to the product of the meaneffective pressure and thedisplacement volume.Reciprocating engines are classified as spark-ignition (SI) engines orcompression-ignition (CI) engines, depending on how the combustionprocess in the cylinder is initiated. In SI engines, the combustion of theair–fuel mixture is initiated by a spark plug. In CI engines, the air–fuelmixture is self-ignited as a result of compressing the mixture above its selfignitiontemperature. In the next two sections, we discuss the Otto andDiesel cycles, which are the ideal cycles for the SI and CI reciprocatingengines, respectively.9–5 ■ OTTO CYCLE: THE IDEAL CYCLEFOR SPARK-IGNITION ENGINESThe Otto cycle is the ideal cycle for spark-ignition reciprocating engines. Itis named after Nikolaus A. Otto, who built a successful four-stroke enginein 1876 in Germany using the cycle proposed by Frenchman Beau deRochas in 1862. In most spark-ignition engines, the piston executes fourcomplete strokes (two mechanical cycles) within the cylinder, and thecrankshaft completes two revolutions for each thermodynamic cycle. Theseengines are called four-stroke internal combustion engines. A schematic ofeach stroke as well as a P-v diagram for an actual four-stroke spark-ignitionengine is given in Fig. 9–13(a).End ofcombustionP atmPPIntakevalve opensTDC3IgnitionExpansionCompressionExhaustIntakeExhaust valveopensBDCvAir–fuelmixtureCompressionstrokePower (expansion)stroke(a) Actual four-stroke spark-ignition engineq inExhauststrokeExhaustgasesAir–fuelmixtureIntakestrokeq outq inAIR(2)AIR(2)–(3)AIR(3)AIR2IsentropicIsentropic41q outTDC BDC vIsentropiccompression(1)(b) Ideal Otto cyclev = const.heat additionIsentropicexpansion(4)v = const.heat rejection(4)–(1)FIGURE 9–13Actual and ideal cycles in spark-ignition engines and their P-v diagrams.