In an earlier experimental effort, the change in internal energy of non-ideal carbon dioxide was measured by noting the temperature drop when its volume rapidly expanded with the sudden opening of a valve from one to two compressed gas cylinders. It was determined that an empirical equation for the change in internal energy of a non-ideal fluid was more accurate than previous methods used for estimating the change in internal energy by estimating the change in entropy. It was followed up with a numerical model, utilizing both non-ideal argon and carbon dioxide, to demonstrate that the internal energy of a non-ideal fluid is a function of the integration of the temperature-dependent intermolecular Van der Waals force; an attractive intermolecular force serves to reduce the variability of molecular speeds and positions, and thus reducing the effective entropy. With this empirical equation, a theoretical ideal Stirling cycle heat engine that exceeds the Carnot efficiency was realized by utilizing non-ideal carbon dioxide as a working fluid. This enhanced thermodynamic efficiency was achieved by utilizing a non-ideal working fluid, where the Van der Waals attractive intermolecular forces serve to reduce the overall pressure, thereby reducing both the colder compressive work input and the hotter expansive (and beneficial) work output. As the attractive Van der Waals forces are stronger at colder temperatures, the reduction in work input is greater than the reduction of work output, resulting in a net increase in mechanical energy output and an increase in thermodynamic efficiency. A patent-pending (USPTO 2026/0078714-A1) practical demonstration of this macroscopic engine cycle was build and tested. This effort demonstrated how a better understanding of the internal energy of non-ideal fluids can have a real impact of the efficiency of practical mechanical heat engines.
https://www.nature.com/articles/s41598-022-11093-z
ArXiv: 2004-03584