This paper deals with an innovative natural gas (NG) combined cycle cogeneration system (150-kW(e), 192 kW(t)). The system is made up of a combination of two interconnected combined heat and power (CHP) systems: a reciprocating internal combustion engine cogenerator (ICE CHP) as the topping cycle and a Rankine cycle cogenerator (RC CHP) which operates as the bottoming cycle on the exhaust gases from the ICE. The expander technology chosen for the Rankine cycle prime mover is a reciprocating single expansion steam engine with three cylinders in a radial architecture. The ICE is an automotive derived internal combustion engine with a high part-load electrical efficiency, due to a variable speed operation strategy and reduced emissions. The main design parameter of the Rankine bottoming cycle and of the steam engine are defined in this paper, and the electrical and heat efficiency of the new CHP unit are determined and discussed at full and part-load. Some design criteria and energy balance considerations are introduced and some technical solutions are analysed and studied. (C) 2008 Elsevier Ltd. All rights reserved.