In order to reduce NOx (nitrogen oxides) emissions, N-2 (nitrogen) was introduced as dilution gas to dilute mixture with a specially-designed injection device. The impacts of varying N-2 DR (dilution ratio) on the combustion and the exhaust emissions were investigated, including engine heat release rate, indicator diagram, NOx, CO (carbon monoxide), THC (total hydrocarbon) emissions and so on. For this study, a modified 6.6 L CNG (compressed natural gas) engine was tested and N-2 was injected into the end of intake manifold by a specially-designed device. The results showed that N-2 dilution has a significant influence on the combustion and the exhaust emissions. With the rise of N-2 DR, the maximum of pressure in cylinder and the maximum of heat release rate exhibited decrease trends, the centre of heat release curve showed a moving backward tendency. Higher N-2 DR exhibited lower NOx (17-81%) emissions, but higher emissions of THC (3-78%) and CO (1-28%). The change of BSFC (brake specific fuel consumption) can be ignored with N-2 DR no more than 167%. Satisfactory results can be obtained, with lower NOx (31%) emissions, lower BSFC (0.5%), and relatively higher THC (6%) and CO (1%) emissions, when N-2 DR is 67%. (C) 2014 Elsevier Ltd. All rights reserved.