تجزیه و تحلیل عملکرد مقایسه ای چرخه های دوگانه و دیزلی غیر قابل برگشت تحت شرایط قدرت حداکثر

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.

During the last decade, several authors have conducted optimisation studies for heat engines based on endoreversible and irreversible models by considering fine time and finite size constraints and friction losses under various heat transfer modes, mainly linear and non-linear ones [1], [2], [3], [4], [5] and [6]. Much interest has been recently focussed on optimisation of the air standard Otto, Diesel and Dual cycles [4], [5], [6], [7], [8] and [9]. In these optimisation studies, optimal design and operation parameters under maximum power (MP) conditions were investigated. Usually in these studies, power and thermal efficiency were chosen for the optimisation criteria, and the design parameters at MP and/or at maximum thermal efficiency were investigated [10]. Bhattacharyya [4] proposed a simplified irreversible model for an air-standard Diesel cycle by using the finite time thermodynamic approach. In his study, global thermal and friction losses are lumped into an equivalent friction term, which is linear in the piston velocity. Chen and coworkers [7] extended the same technique to an irreversible air standard Dual cycle model by considering a friction like term loss during finite time. Anggulo-Brown et al. [8] and [11] optimised an irreversible Otto cycle model to obtain higher power output and efficiency. Other studies [12] and [13] analysed endoreversible internal combustion engine cycles using finite time thermodynamic techniques. However, these studies excluded internal irreversibilities, making them less suitable for practical applications. No comparative performance analyses of irreversible internal combustion engine Dual and Diesel cycles under a MP criterion in terms of isentropic efficiency and extreme temperature ratio appear to have been published in the open literature yet. The isentropic efficiency terms for the compression and expansion processes of the cycle were considered in a similar manner to the definitions of compressor isentropic efficiency for the compression process and turbine isentropic efficiency for the expansion process occurring in a gas turbine system [10]. The extreme temperature ratio was used to account for the effect of combustion on engine performance. It is important how the performance changes with increase of the extreme temperature ratios because of its importance for ceramic engines. The isentropic efficiencies for the compression and expansion processes can be used to account for the trends for all the irreversibilities, such as the friction effect on performance. A comparative performance analysis has been performed with the aim of understanding this effects under the MP criterion. Further study could be of benefit to the analysis of an irreversible Dual cycle considering finite time thermodynamics with the aim of accounting for trends of the heat transfer effects on overall performance of the engine.

A comparative performance analysis based on the MP criterion has been performed for the irreversible Dual and Diesel cycles. It is found that the MP and the corresponding efficiency of the Dual cycle are always higher than those of the irreversible Diesel cycle. When the isentropic efficiencies are taken as ηci=ηei=1, the MP and the corresponding efficiency of the Diesel cycle become equal to the MP and the corresponding efficiency of the Dual cycle. As the isentropic efficiencies decrease, the difference in MP and the corresponding efficiency of the irreversible Dual and Diesel cycles increases. This analysis may provide guidelines for performance evaluation and improvements of real Diesel engines.