An ERICSSON engine is a reciprocating thermal motor with external heat supply and separate compression and expansion spaces. It uses a monophasic gaseous working fluid. Unlike the Stirling engine, the ERICSSON engine is equipped with valves around the cylinders to isolate the cylinders from the heat exchangers during the expansion and the compression processes. The ERICSSON engine can be provided with a heat recovery exchanger and it can operate according to a closed or an open cycle. This engine is suitable for low power (up to some kW) thermal energy conversion from renewable energy sources like biomass or solar energy. Dimensionless quantities are defined such as the pressure ratio beta, the temperature ratio theta, the cylinder capacity ratio phi, the relative dead volumes mu(E) and mu(C) the thermal efficiency eta th and the net dimensionless indicated power Pi. The relationships between these quantities are established. The modeling is based on the assumptions of a Joule cycle with internal heat recovery exchanger realized by a perfect gas with constant heat capacity. These relationships allow to determine the pressure in the heater as a function of the temperature ratio and the engine geometrical data. It is shown that there is a well defined operating range for which the engine can produce mechanical energy as a function of the quantities beta, phi, mu(E), mu(C), theta, and irrespective of the fact that the expansion space dead volume is recompressed or not. (C) 2014 Elsevier Ltd. All rights reserved.