Exhaust gas recirculation (EGR) to reduce feed gas NOx emission is common practice in modern diesel engines. Dilution of the intake air with cooled recirculated exhaust gas limits the production of in-cylinder NOx due to a lowering of the adiabatic flame temperature and a reduction in oxygen content of the intake mixture. EGR also reduces the mixture-averaged ratio of specific heats (gamma) of the combustion charge leading to a reduction in the thermodynamic cycle efficiency. This trade-off between minimizing NOx production and maximizing cycle efficiency is of critical importance when calibrating EGR control schemes. Modeling tools that allow a quantitative analysis of this trade-off can be very beneficial in tuning EGR systems over a range of operating conditions. In this study, the systematic development of a model that allows an assessment of the impact of EGR on three parameters, namely (a) the thermodynamic cycle efficiency, (b) the mixture temperatures during the cycle and (c) the mixture-averaged,, Is presented. This is accomplished through a numerical solution of the energy equation while considering the effects of heat loss and temporally varying mixture-averaged values of gamma. Results for a simple phenomenological model relating fuel-burn rate with EGR fraction and the impact of EGR fraction on NOx reduction are also included. Copyright (C) 2008 John Wiley & Sons, Ltd.