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V V
 max  BDC (4.2)
V V
min TDC

Fig. 4.2 - A schematic of each stroke as well as a P-v diagram for an actual four-stroke spark-
ignition engine
Notice that the compression ratio is a volume ratio and should not be confused with
the pressure ratio.
Reciprocating engines are classified as spark-ignition (SI) engines or 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 selfignition temperature. In the next two sections, we discuss the Otto
and Diesel cycles, which are the ideal cycles for the SI and CI reciprocating engines,
respectively.

4.2 Otto Cycle
The Otto Cycle is the idealization for the process found in the reciprocating internal
combustion engines which are used by most automobiles. It is the ideal cycle for spark-
ignition reciprocating engines. The Otto Cycle 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-stroke internal combustion engines. A schematic of each stroke as well as a P-v
diagram for an actual four-stroke spark-ignition engine is given in Fig. 4.3. Initially,
both the intake and the exhaust valves are closed, and the piston is at its lowest position
(BDC). During the compression stroke, the piston moves upward, compressing the air–
fuel mixture. Shortly before the piston reaches its highest position (TDC), the spark
plug fires and the mixture ignites, increasing the pressure and temperature of the
system. The high-pressure gases force the piston down, which in turn forces the
crankshaft to rotate, producing a useful work output during the expansion or power
stroke. At the end of this stroke, the piston is at its lowest position (the completion of
the first mechanical cycle), and the cylinder is filled with combustion products.

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