A diesel engine follows a dual combustion diesel cycle which is similar to the diesel cycle but differs in that the combustion process occurs within two sub$-$processes: constant volume heat addition followed by constant pressure heat addition. Suppose an engine takes in air at $1\mathrm{.}01$ bar, $20$ C$,$ maximum cycle pressure is $69$ bar and compression ratio is $18.$ Assuming that the heat addition during constant volume process is equal to the heat addition during constant pressure process, calculate the thermal efficiency of the engine.