The recent development of thin tubular solid oxide fuel cells (SOFCs), microturbines and Stirling engines has inspired design studies of the integration of a SOFC-heat engine (HE) system within a car. The total power system consists of a SOFC-HE power generation unit, a power storage (battery) system, a power management system and electric motors at the wheels. The sizes of the HE and the SOFC stack are to be matched by the start-up requirements. The use of micro tubes allows a very high power density of the stack. The thermodynamic calculation of the cycle gives the actual design values for the study and indicates further steps for system optimisation. The first SOFC-GT layout lead to an electric efficiency of 45% for the cycle used as a base for a design study [The Design of Stationary and Mobile SOFC-GT Systems, UECT, 2001]. The design study shows that the space available in a mid-class car allows the integration of such a system including space reserves. A further improvement of the system might allow an electric efficiency of more than 55%. The integration of a Stirling engine instead of the microturbine is a second possibility and the object of an ongoing study. This was motivated by interesting results from the development of solar powered Stirling engines. Generally, the analyses show that the optimal match of the SOFC and the HE will be a key issue for any engineering solution.