Shannon Miller

Thermodynamic requirements for maximum internal combustion engine cycle efficiency. Part 1: Optimal combustion strategy

Abstract 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 constant-internal 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.

Thermodynamic requirements for maximum internal combustion engine cycle efficiency. Part 2: Work extraction and reactant preparation strategies

Abstract 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.

Assessing the feasibility of increasing engine efficiency through extreme compression

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.

Optimal Architecture for Efficient Simple-Cycle Steady-Flow Combustion Engines

Increasing efficiency of steady-flow engines by way of irreversibility minimization has been the underlying objective in the development of a variety of simple, regenerative, and combined cycles. The approach thus far has been to conceptualize new cycles, or choose existing cycles, perform exergy analyses, and make modifications to minimize irreversibility. In this paper, a different approach is taken bydeveloping a thermodynamic framework that defines the principles governing the minimization of irreversibility and uses these principles to deduce an optimal architecture for simple-cycle stationary gas-turbine and propulsion engines. The optimal architecture is thus obtained as the result of the irreversibility-minimization analysis and not by optimization of a preconceived architecture or cycle. The benefit of this approach is that, based on the chosen constraints for the analysis (e.g., polytropic efficiency of compression and expansion processes, blade temperature limits, etc.), the efficiency of the optimal architecture obtained is greater than any preconceived cycle or architecture subject to the same constraints.

Reducing Combustion Irreversibility Through Extreme Compression: Analyzing Device Performance

One promising technique for increasing the efficiency of combustion engines is to perform the reaction at extremely high compression ratios. To manage significantly higher temperatures and pressures, a new engine architecture is required. In addition, critical questions must be answered to address feasibility of operating at these extreme states. To investigate these questions, a single-shot, free-piston laboratory device was built, capable of achieving combustion at compression ratios of 100:1 and greater. This paper quantifies the combined heat- and mass-transfer losses present during air-compression experiments. Since these air-compression losses are also present during combustion experiments, quantifying these baseline operating losses, independent from additional losses during combustion, will help in assessing feasibility of the overall strategy as well as in identifying specific strategies for achieving higher efficiencies.Copyright