Reciprocating engines are classified as spark-ignition (SI) enginesor
compression-ignition (CI) engines, depending on how the combustion
process in the cylinder is initiated. In SI engines, the combustion of the
air–fuel mixture is initiated by a spark plug. In CI engines, the air–fuel
mixture is self-ignited as a result of compressing the mixture above its self-
ignition temperature. In the next two sections, we discuss the Ottoand
Diesel cycles,which are the ideal cycles for the SI and CI reciprocating
engines, respectively.9–5 ■ OTTO CYCLE: THE IDEAL CYCLE
FOR SPARK-IGNITION ENGINESThe Otto cycle is the ideal cycle for spark-ignition reciprocating engines. It
is named after Nikolaus A. Otto, who built a successful four-stroke engine
in 1876 in Germany using the cycle proposed by Frenchman Beau de
Rochas in 1862. In most spark-ignition engines, the piston executes four
complete strokes (two mechanical cycles) within the cylinder, and the
crankshaft completes two revolutions for each thermodynamic cycle. These
engines are called four-strokeinternal combustion engines. A schematic of
each stroke as well as a P-vdiagram for an actual four-stroke spark-ignition
engine is given in Fig. 9–13(a).494 | Thermodynamics
Wnet = MEP (Vmax – Vmin)Vmin Vmax VMEPPTDC BDCWnetFIGURE 9–12
The net work output of a cycle is
equivalent to the product of the mean
effective pressure and the
displacement volume.
qinqout4321Patm
PPCompression
strokePower (expansion)
strokeAir–fuel
mixture(a) Actual four-stroke spark-ignition engine(b) Ideal Otto cycleIsentropic
compressionAIR
(2)(1)End of
combustionExhaust valve
opensIgnitionTDC BDCIntakeExhaustIntake
valve opensExpansionCompressionIsentropicIsentropicAIR(4)–(1)Air–fuel
mixtureAIR
(2)–(3)Exhaust
strokeIntake
strokeAIR
(3)(4)Exhaust
gasesIsentropic
expansionv = const.
heat additionv = const.
heat rejectionqin qoutvTDC BDC vFIGURE 9–13
Actual and ideal cycles in spark-ignition engines and their P-vdiagrams.