An irreversible dual cycle analysis has been performed. Optimisation of the cycle has been performed for power and thermal efficiency with respect to pressure ratio and temperature ratio. Optimum values of pressure ratio and cut-off ratio of the dual cycle, depending on temperature ratios, have been derived analytically and compared with the results of an experimental study of a low heat rejection indirect injection Diesel engine, whose optimum performance was obtained by retarding the injection timing related to the temperature increase in the combustion chamber. Injection timing was retarded from 38 crank angle (CA) to 32 CA. Optimum performance was obtained at 34 CA. At this injection timing, in comparison to the standard Diesel engine, the decrease in the specific fuel consumption and the increase in the brake thermal efficiency were about 6% and 2%, respectively. (C) 2003 Elsevier Ltd. All rights reserved.