In a recent paper (Santillán et al 2001 J. Phys. D: Appl. Phys. 34 2068–72) the local stability of a Curzon–Ahlborn–Novikov (CAN) engine with equal conductances in the coupling with thermal baths was analysed. In this work, we present a local stability analysis of an endoreversible engine operating at maximum power output, for common heat transfer laws, and for different heat conductances α and β, in the isothermal couplings of the working substance with the thermal sources T1 and T2 (T1 > T2). We find that the relaxation times, in the cases analysed here, are a function of α, β, the heat capacity C, T1 and T2. Besides, the eigendirections in a phase portrait are also functions of τ = T1/T2 and the ratio β/α. From these findings, phase portraits for the trajectories after a small perturbation over the steady-state values of internal temperatures are presented, for some significant situations. Finally, we discuss the local stability and energetic properties of the endoreversible CAN heat engine.
In this work, we calculate the fractal dimension of heart interbeat time series of some healthy young and elderly individuals. As has been found by means of other methods (detrended fluctuation and spectral analyses), we also find that interbeat series of healthy young subjects can be characterized by only one scaling exponent and a crossover behavior in it is observed with aging. By means of a zoom over the hinges of the crossover region, interesting effects of aging are presented. Our results with real interbeat time series are reasonably reproduced by using a simple model based on combinations of noisy first-order autoregressive series.
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