In order to study the nitrous-oxide emissions of HCNG engines a numerical NO. prediction model has been developed and coupled with the most accepted quasi-dimensional combustion model of SI engines. The experiments have been performed at the different hydrogen volumetric percentage (0-40%), ignition timing (22-30 BTDC), A/F ratio (1-1.73) and manifold-absolute pressure (60-137 kPa) at various engine load and speed. In the next phase of study, the simulation has been performed at the different operating conditions and various HCNG blends. The two NOx mechanism: the thermal NOx and prompt NOx mechanism have been used to predict the NOx emission. The presented Model III of NOx prediction is validated by the previous models (Model I by Dawyer et al. and Model II by Kornbluth et al.) and experimental results. The NO packet and three-zone approach are designed and incorporated to Model III, which is based on the temperature duration of NOx formation. The NO curve has been transformed into NO packets through the temperature difference of NOx formation. Whereas, the width of the packet is equal to the ratio of crank angle duration to the total combustion duration. The total combustion duration is divided into three zones (phases). In order to check the accuracy of the model, the percentage error of NOx emission has been evaluated. Three NOx prediction models have been compared, and the model III is the best one. The error range of the model III is within +/- 15%. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.