Heat Engines running upon a Non-Ideal Fluid Model with Higher Efficiencies than upon the Ideal Gas Model

TL;DR

This study investigates Otto and Diesel heat engines using a van der Waals non-ideal fluid model, demonstrating that such engines can surpass ideal-gas efficiencies while respecting the Carnot limit, highlighting the importance of working substance properties.

Contribution

It introduces a detailed efficiency analysis of non-ideal fluid models in heat engines, showing potential for higher efficiencies than ideal-gas models, especially at higher densities.

Findings

Non-ideal fluid engines can exceed ideal-gas efficiencies.

Maximum Carnot efficiency remains the same for both models.

Higher non-ideality density leads to increased efficiency in low-temperature regimes.

Abstract

We consider both Otto and Diesel heat engine cycles running upon the working substances modeled by the van der Waals fluid as a simple non-ideal gas model. We extensively perform the efficiency study in these model engines. Then we find that this "real" engine model can go beyond its ideal-gas counterpart in efficiency, whereas as well-known, the maximum Carnot efficiency is the same for both ideal and non-ideal gas engines. In fact, the higher the density of non-ideality is, the higher efficiency tends to be found, especially in the low-temperature regime, but with more shrinkage in the range of physically allowed compression ratio, determined by the Carnot upper bound, namely, the Second Law of thermodynamics. Our findings imply that in addition to the engine architectures and bath temperatures, the properties of working substances should also be taken into consideration in the performance study of heat engines, the theoretical model aspects of which have not sufficiently been discussed so far. It is straightforward that our methodology may also apply for other non-ideal fluid models.