Studies of unconventional heat engine design, operation and efficiency: four-step quasi-Carnot cycles with a linear P/V transition

Abstract

Although the ideal Carnot engine rejoices in the largest efficiency of any heat engine operating between given temperature extremes, alternative closed-loop four-step cycles in which a variable-temperature, straight-line transition replaces the Carnot high-temperature isothermal expansion appear ostensibly to offer a higher thermodynamic efficiency. This arises from the fact that higher maximal temperatures are attained; the efficiency of the equivalent Carnot engine operating between such temperature extremes reinstates Carnot supremacy by slightly transcending the unconventional engine's efficiency. Our investigations stress the importance of the second, as well as the first, law of thermodynamics, and consider operating characteristics quantitatively for specimen engines of various design.

Unconventional cycles and multiple adiabatic points

“…Importantly, in all these cases, δQ is either positive or negative along the whole process. Nonetheless, in the socalled unconventional (albeit idealized) cycles, δQ has a smooth, continuous reversal of direction (signal) [1][2][3][4][5][6][7][8] , changing from endo to exothermic. In this so-called adiabatic point, as the name indicates, the process P (V ) is tangent to an adiabatic curve 2 and δQ = 0.…”

“…Importantly, in all these cases, δQ is either positive or negative along the whole process. Nonetheless, in the so-called unconventional (albeit idealized) cycles, δQ has a smooth, continuous reversal of direction (signal) [1][2][3][4][5][6][7][8], changing from endo-to exothermic. Where it happens, the so-called adiabatic point, as the name indicates, the process described by the behavior of the pressure as a function of the volume, P(V ), is tangent to an adiabatic curve [2] (that, for example, for a monoatomic ideal gas, is given by the Poisson equation PV 5/3 =constant) and δQ=0 (the notation δQ denotes an inexact differential).…”

An approach for determining thermal performance of the unconventional lobeP–Vcycles

“…Most studies and developments of the unconventional cycles are focused on demonstrating and understanding thermodynamic heat cycles such as triangle cycle [1][2][3][4], quasi-Carnot cycle with a linear P -V transition [1,5], circular cycle [2][3][4][5][6][7][8] and elliptical cycle [6] in the P -V plane. It is important to know the fact that the presentation of the P -V cycle may lead to a miscalculation of thermal efficiency if the wrong location of points in the P -V plane is used to evaluate the Q in and Q out [1,2,4].…”

Analysis of an unconventional cycle as a new comparison standard for practical heat engines: the circular/elliptical cycle inT-S diagram

“…The purpose of MPD analyses in the literature for the endoreversible and irreversible heat engines is to find the design parameters that lead to minimum engine size for a given power. Kaufman and Sheldon (1997) investigated the relation between the thermal efficiency and engine size for standard reversible Carnot and quasi-Carnot engines employing a straight-line transition in place of the Carnot isothermal expansion step. Recently, Marcella and Sheldon (2000) applied both the first and the second law of thermodynamics to the determination of the characteristics of an ideal diatomic gas heat engine that operates via a closed loop circular cycle in a P -V diagram.…”