Currently, there is no commercially available technology for low temperature (below 100°C) heat conversion into mechanical or electrical energy. The Organic Rankine Cycle (ORC) is a typical technology for low temperature applications. However, ORC plants are rather complex and not economically sound for small-scale applications (<500kW). The isobaric expansion (IE) engine is a promising alternative to ORC plants for low-grade and medium-grade (>40°C) heat conversion. The IE engine works under external heat supply, and thus, almost every type of heat source is accessible. In this work, a mathematical model for transient thermal simulation of recuperative displacer-type IE engines operating with dense working fluids is proposed. The model is based on Nusselt number correlations for the heat transfer prediction and is coupled to the REFPROP 9.1 database yielding thermophysical properties. The first results of a parametric study performed with two different working fluids, namely R134a and carbon dioxide, are presented to show the impact of different operating frequencies on the heat transfer characteristics.