The gasoline internal combustion engine [petrol engine] operates in a cycle that consists of six operations, out of which four are called strokes. Although processes of the cycle are actually irreversible, and friction and heat losses are present, we may assume the entire cycle as an idealised quasi- static cycle, as shown in the figure.
This cycle is known as the "Air-standard Otto cycle. Throughout this idealised cycle the working substance is treated as if it were an ideal gas of adiabatic exponent \[\gamma = 1.4\] alone, ignoring the fuel vapours and combustion products.
$I.$ "Isobaric intake stroke" in which the volume increases from zero to \[{V_a}\] at atmospheric pressure into the cylinder.
$II.$ Adiabatic compression stroke: The inlet valve is closed and the gasoline - air mixture is compressed adiabatically from \[{V_a}\] to \[{V_b}\] . Here \[r = \dfrac{{{V_a}}}{{{V_b}}}\] is known as the compression ratio of the engine \[\left[ {r = 6} \right]\]. If
\[r > 7\] then the temperature \[{T_b}\] that is reached at the end is sufficiently high to ignite the fuel mixture before the occurrence of spark. Such pre-ignition causes "knocking" and loss of power.
$III.$ Constant volume ignition: A spark is produced and mixture of air-gasoline vapour is ignited.
$IV.$ Adiabatic expansion or Power stroke: The heated air and combustion products expand against and drive the piston. This is the stroke that delivers power to the crankshaft.
$V.$ Constant volume exhaust: At the end of power stroke the exhaust valve is opened; this causes a sudden partial escape of cylinder gases. No piston motion takes place in this part of the cycle.
VI. Isobaric exhaust stroke: The moving piston drives out the residual cylinder gases to restore the initial condition of the engine. The ignition of air- gasoline vapour takes place at:

A. a
B. b
C. c
D. d