In this study, a comparative performance analysis and optimisation based on maximum power and maximum thermal efficiency criteria have been performed for irreversible Dual and Diesel cycles. Optimal performance and design parameters, such as pressure ratio, cut-off ratio and extreme temperature ratio, of the cycles has been derived analytically and compared with each other based on maximum power (MP) and the corresponding thermal efficiency criteria. The effects of the internal irreversibilities of the cycles on overall performance in terms of isentropic efficiencies for the compression and expansion processes are also investigated. The obtained results may provide a general theoretical tool for the optimal design and operation of real engines. (C) 2004 Elsevier Ltd. All rights reserved.