The performance of an air standard Otto-cycle is analyzed using finite-time thermodynamics. In the irreversible cycle model. the non-linear relation between the specific heat of the working fluid and its temperature, the friction loss computed according to the mean velocity of the piston, the internal irreversibility described by using the compression and expansion efficiencies, and the heat-transfer loss are considered. The relations between the power output and the compression ratio, between the thermal efficiency and the compression ratio, as well as the optimal relation between the power output and the efficiency of the cycle are indicated by numerical examples. Moreover, the effects of internal irreversibility, heat-transfer loss and friction loss on the cycle performance are analyzed. The results obtained in this paper may provide guidance for the design of practical internal-combustion engines. (c) 2007 Elsevier Ltd. All rights reserved.