Kwee-Yan Teh

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.

Thermodynamic analysis of fermentation and anaerobic growth of baker’s yeast for ethanol production

Thermodynamic concepts have been used in the past to predict microbial growth yield. This may be the key consideration in many industrial biotechnology applications. It is not the case, however, in the context of ethanol fuel production. In this paper, we examine the thermodynamics of fermentation and concomitant growth of bakers yeast in continuous culture experiments under anaerobic, glucose-limited conditions, with emphasis on the yield and efficiency of bio-ethanol production. We find that anaerobic metabolism of yeast is very efficient; the process retains more than 90% of the maximum work that could be extracted from the growth medium supplied to the chemostat reactor. Yeast cells and other metabolic by-products are also formed, which reduces the glucose-to-ethanol conversion efficiency to less than 75%. Varying the specific ATP consumption rate, which is the fundamental parameter in this paper for modeling the energy demands of cell growth, shows the usual trade-off between ethanol production and biomass yield. The minimum ATP consumption rate required for synthesizing cell materials leads to biomass yield and Gibbs energy dissipation limits that are much more severe than those imposed by mass balance and thermodynamic equilibrium constraints.

An Optimal Control Approach to Minimizing Entropy Generation in an Adiabatic IC Engine With Fixed Compression Ratio

Entropy generation due to combustion destroys as much as a third of the theoretical maximum work that could have been extracted from the fuel supplied to an engine. In this paper, an optimal control problem is set up to minimize the entropy generation in an adiabatic internal combustion engine, with the piston velocity profile serving as the control input function. The compression ratio of the engine is fixed, thereby imposing a constraint on the piston motion. The switching conditions for the optimal bang-off-bang control is determined based on Pontryagins maximum principle. In thermodynamic terms, the optimal solution reduces to a strategy of equilibrium entropy minimization. This result is independent of the underlying combustion mechanism.© 2006 ASME

Optimizing Piston Velocity Profile for Maximum Work Output From an IC Engine

In this paper, an optimal control problem is set up to maximize the work output from a piston engine via piston motion shaping. The empirical heat transfer correlation and global reaction kinetics used in the engine model result in a stiff, non-smooth dynamical system. The algorithm used to transcribe this system to a constrained nonlinear program is described. The optimal solution is obtained using the gradient-based optimization solver SNOPT. At optimality, the total heat transfer loss is lower due to the shorter time spent at elevated temperatures after combustion, leading to a modest increase (less than 3%) in work output as compared to a conventional slider-crank piston engine.Copyright

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.

Identification of Optimal Architecture for Efficient Simple-Cycle Gas Turbine Engines

Increasing efficiency of gas turbine 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 we take a fundamentally different approach by developing a thermodynamic framework that defines the principles governing the minimization of irreversibility and uses these principles to deduce an optimal architecture for simple-cycle gas turbine engines. No engine cycle/design is assumed in the beginning. The benefit of this approach is two-fold. First, it explains the factors affecting irreversibility in gas turbine engines. Second, it defines an optimal architecture for simple-cycle engines based on the chosen constraints (e.g., polytropic efficiency of compression and expansion processes, blade temperature limits, etc.) having an efficiency greater than any preconceived cycle/architecture with the same constraints.Copyright

An Optimal Control Approach to Minimizing Entropy Generation in an Adiabatic Internal Combustion Engine

Entropy generation due to combustion destroys as much as a third of the theoretical maximum work that could have been extracted from the fuel supplied to an engine. In this paper, an optimal control problem is set up to minimize the entropy generation in an adiabatic internal combustion engine. The optimal bang-bang control is shown to be a function of the pressure difference between the instantaneous thermodynamic state of the system and its corresponding equilibrium thermodynamic state. At optimality, the entropy difference between these two thermodynamic states is shown to be a Lyapunov function. Therefore, the equilibrium serves as the attractor for the optimal state trajectory. The results are independent of the underlying reaction mechanism, which may be highly nonlinear

Thermodynamic Analysis of Fermentation and Anaerobic Growth of Baker’s Yeast

Thermodynamic concepts have been used in the past to predict microbial cell yield under various growth conditions. Cell yield may be the key consideration in some industrial biotechnology applications. It is not the case, however, in the context of biofuel production. In this paper, we examine the thermodynamics of fermentation and concomitant growth of baker’s yeast in continuous culture experiments under anaerobic, glucose-limited conditions, with emphasis on the yield and efficiency of ethanol production. We find that anaerobic metabolism of baker’s yeast is very efficient; the process destroys less than 7% of the total chemical exergy supplied to the fermentation reactor. However, the exergy of ethanol secreted constitutes less than 60% of the in-flowing exergy, or 75% that of glucose fed to the continuous culture. Effects of varying the specific adenosine 5′ -triphosphate (ATP) consumption rate, which is the fundamental parameter that quantifies the energetic requirements for cell growth and maintenance, are also examined.Copyright