The increasingly stringent internal combustion engines emissions regulations, has led to the extended use of after-treatment systems, giving progressively more importance to the engine efficiency optimization. In this framework, the combined modelling and experimental methodologies to perform and analyse the energy balance are key to evaluate the potential of different engine strategies aimed at the consumption optimization and the identification of the improvement paths. This work has been divided into two parts, dealing separately with the development and application of a Global Energy Balance tool. This article corresponds to the first part, which comprises the description of the models required to perform a detailed energy balance and the calibration methodologies followed to achieve accurate energy terms estimation. The models are calibrated based on experimental information, thus, a thermodynamic analysis aimed at defining comparable quantities between experimental and modelled terms is performed. The uncertainty analysis of the tool shows a deviation in the determination of the heat transfer to the coolant and the oil of about +/- 2%, and in terms of fuel energy about +/- 1%. (C) 2018 Elsevier Ltd. All rights reserved.