In the current work, numerical simulations are used to evaluate the effects of detailed reaction chemistry of different primary reference fuel (PRF) blends of iso-octane and n-heptane on heat release in one-dimensional engine simulations. A simplified slider-crank model was used to represent the engine cycle. The contributions of specific reaction classes to ignition and heat release were quantified. Maps of ignition phasing and heat release were created as a function of pressure and temperature to indicate the change in reactivity (defined by the first and second stages of ignition) as a function of state conditions as well as the fraction of heat release associated with the two stages of ignition. For the conditions studied, the reactivity of the second stage of ignition always increased with increasing temperature, i.e. the phasing of autoignition advanced with increasing temperature, whereas the reactivity of the first stage of ignition exhibited negative temperature dependence where increasing temperature delayed the first stage of ignition and decreased the heat release at the first stage of ignition for some conditions. The results show low-temperature chemistry radicals like C-7 RO2 species are not uniquely indicative of low-temperature heat release, but they are formed at earlier times, higher rates and higher concentrations with PRF blends with higher fractions of n-heptane. A modified approach to the Livengood-Wu integral is presented to capture the integrated effects of the compression stroke on the potential for using the first stage of ignition to distribute heat release. The results of the modified ignition integral analysis are presented as a function of engine speed and fuel/air preheat temperature and demonstrate the utility of the approach to design and interpret fueling strategies of fuel blends. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.