An examination of possible reversible combustion at high temperatures and pressures for a reciprocating engine

Abstract

Conventional combustion processes are known to be highly irreversible processes. The potential to obtain useful work from the fuel is degraded during the combustion process. For example, for a reciprocating internal combustion engine, about 20% or more of the potential work from the fuel is destroyed during the combustion process. This potential work is known as availability (a thermodynamic property). The motivation for the current work was to develop a conceptual model of a set of processes related to reciprocating engines that would eliminate this destruction of availability. One conceptual model, proposed by Keenan, suggested that a preselected set of “reactants” could be compressed (at constant composition) to a high temperature and pressure. At this high temperature and pressure, the “reactants” would be in chemical equilibrium. At this point, the “reactants” would be expanded back to the original volume. The expansion process would consist of a “shifting” chemical equilibrium such that the composition during expansion would continue to change. At the end of the compression and expansion, net work would be available without destroying any of the work potential of the fuel. The purpose of the current work was to develop a quantitative model of this concept, and to use the model in a series of computations to examine the effects of temperature, pressure, and other parameters on the work production capability of the concept. The concept was studied for eight different fuels for various conditions. In general, the net work output increased as the temperature, pressure and compression ratio increased. For low compression temperatures and pressures, the concept resulted in a small amount of net work produced without destroying any fuel availability. For sufficiently high compression pressure and temperature (e.g., 10 MPa and 6000 K, respectively), however, the thermal efficiency was ~28% for isooctane and was ~40% for hydrogen and methane, for air as the oxidant, an equivalence ratio of 1.0, and a compression ratio of 18. Although the temperatures and pressures considered are well beyond practical values for the materials and designs of today, the general result of the study is that conditions can be identified to eliminate the combustion irreversibility.