This paper is the first of a two part study that analyses the technical and financial performance of particular, recuperated engine systems. This first paper presents a thermodynamic study of two systems. The first system involves the chemical recuperation of a reciprocating, spark ignited, internal combustion engine using only the waste heat of the engine to power a steam-methane reformer. The performance of this system is evaluated for different coolant loads and steam-methane ratios. The second system is a so-called 'hybrid' in which not only the waste heat of the engine is used, but also a secondary heat source - the combustion of biomass. The effects of the reformer's temperature and the steam-methane ratio on the system performance are analysed. These analyses show that the potential efficiency improvement obtained when using only the engine waste heat to power the recuperation is marginal. However, results for the hybrid show that although the overall efficiency of the plant, defined in terms of the energy from both the methane and biomass, is similar to that of the conventional, methane fuelled engine, the efficiency of the conversion of the biomass fuel energy to work output appears to be higher than for other biomass fuelled technologies currently in use. Further, in the ideal limit of a fully renewable biomass fuel, the burning of biomass does not contribute to the net CO2 emissions, and the CO2 emission reduction for this second plant can be considerable. Indeed, its implementation on larger internal combustion engine power plants, which have efficiencies of around 45-50%, could result in CO2 emissions that are as much as 10-20% lower than typical natural gas combined cycle (NGCC) power stations. This appears to be a significant result, since NGCCs are commonly considered to have the lowest CO2 emissions of all forms of fossil fuelled, power generation currently in use. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.