 A mass of air (working fluid) is drawn into the cylinder, from 0 to 1, at atmospheric pressure (constant pressure) through the open intake valve, while the exhaust valve is closed during this process.The intake valve closes at point 1.Piston moves from crank end (BDC, bottom dead centre and maximum volume) to cylinder head end ("TDC", top dead centre and minimum volume) as the working gas with initial state 1 is compressed isentropically to state point 2, through compression ratio .