Air standard Dual Combustion cycle
The combustion in case of actual S.I. and C.I.Engines do not takes place at constant volume and constant pressure respectively. In order to more closely to these actual cycle, the dual combustion or limited pressure cycle has been developed as shown in Fig A and B on (p-v) and (T-S) diagram in which the heat is supplied partly at constant volume and partly at constant pressure.
The air standard efficiency of this cycle can calculated as follows :
For isentropic process (4-5)
T5 = T4(V5/ V4) γ−1
Above expression shows that the efficiency increase with increasing compression ratio and decrease in cut ratio.
It can be noted from Equation A that a value of pressure ratio α > 1 results in an increased efficiency for given value of r and ρ.
Hence the efficiency of the dual cycle lies between the Otto and Diesel cycles for given value of compression ratio.