This is the first of a two-part study that examines, from the exergy management standpoint, the fundamental thermodynamic requirements for maximizing internal combustion (IC) engine cycle efficiency. The optimal cycle is shown to comprise three distinct engine architectural elements -reactant preparation, combustion, and work extraction from the products -each of which can be analysed separately. This study shows, based on dynamical system optimization, that it is the equilibrium thermodynamics (specifically, the constantinternal energy-volume (UV) product state at the end of combustion) and not chemical kinetics (i.e. reactions taking place during combustion) that ultimately dictates the amount of exergy destroyed due to combustion. The strategy for minimizing this destruction term reduces to carrying out reactions at the highest possible internal energy state -following what may be called the 'extreme state' principle -so as to minimize the corresponding constant-UV entropy change from reactants to equilibrium products. The extreme state principle remains unaltered when system inhomogeneity (from fuel vaporization and mixing with air) and heat loss are accounted for. Based on this optimal combustion strategy, the companion paper examines the remaining elements of the engine cycle (reactant preparation and work extraction) so as to improve overall cycle efficiency.
This is the second of a two-part study that examines, from the exergy management standpoint, the fundamental thermodynamic requirements for maximizing internal combustion (IC) engine cycle efficiency. In Part 1, it is shown that the strategy to minimize exergy destroyed due to combustion reduces to carrying out combustion at the highest possible internal energy state. Based on this optimal strategy, the present paper examines the remaining elements of IC engine architecture -reactant preparation and product expansion (work extraction) -from the standpoint of managing the associated exergy flows to improve overall engine efficiency. When considered on its own, work extraction is maximized when the combustion products expand to the environmental dead state, with zero exergy left in the exhaust. However, this optimality condition is mismatched to post-combustion conditions for most fuel-air systems, and manifests as hot exhaust with high exergy even upon expansion to ambient pressure. Several strategies to alleviate the mismatch, via preparation of the fuel-air mixture before combustion commences, are considered: reactant compression, dilution with exhaust or excess air, and heating or cooling. These strategies entail trade-offs between exergy destruction due to combustion, and exergy transfers in the form of work (compression), matter (dilution), or heat transfer. The consequent effects on optimal IC engine cycle efficiency are systematically analysed and catalogued.
Operating simple-cycle chemical engines at extremely high compression ratios can, theoretically, increase thermal efficiency by nearly a factor of two. To operate at these significantly higher compression ratios, a new engine architecture is required which is inherently compatible with the higher temperatures and pressures present at these conditions. In addition, the design must manage heat transfer, piston–cylinder sealing and friction, the combustion event, and emissions in order to be successful. To test feasibility of this strategy, a single-shot, free-piston device was constructed which operates at compression ratios of up to 100:1. Air-compression experiments are used to characterize the device and its losses – a combination of heat transfer out of the cylinder and mass transfer past the piston sealing rings. Preliminary experiments using a lean, diesel-like combustion strategy are performed with indicated efficiencies ranging from 52 to 60 per cent for compression ratios of 30–100. These high efficiencies indicate initial feasibility and support further research and engineering.
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