Effect of specific heat variations on irreversible Otto cycle performance

•Specific heats varied with temperature nonlinearly are considered.•Irreversible Otto cycle model is built up using finite-time thermodynamics.•Internal irreversibility, friction and heat transfer loss are considered.•Optimal ecological function performance is studied.•Cycle performances with three specific heat models of working fluid are compared. Considering internal irreversibility loss, friction loss and heat transfer loss, an irreversible Otto cycle model is built up by using finite-time thermodynamics with air standard assumption. Using the various irreversible losses in the cycle to compute the entropy generation rate, the optimal ecological function performance of the cycle is studied when the specific heats of working fluid are nonlinear relation with its temperature. Some important expressions, including ecological function, entropy generation rate, efficiency and power output, are obtained. The cycle ecological function performances with constant specific heats, specific heats changed with linear and nonlinear relations of its temperature are compared. Moreover, the impacts of internal irreversibility loss, friction loss and heat transfer loss on ecological function performance are analyzed. The results show that optimization of the exergy-based ecological function not only represents a compromise between the power output and the rate of entropy generation but also represents a compromise between the power output and the thermal efficiency, the specific heat models have no qualitative effect and only have quantitative effect on the performance characteristics of ecological function versus power output and ecological function versus efficiency, and the ecological function, power output and efficiency decrease with the increase of heat transfer, friction and internal irreversibility losses.