One major drawback about premixed charge compression ignition (PCCI) engines is incomplete combustion and producing high levels of carbon monoxide (CO) emission in lean operating conditions. This issue becomes even more serious when fuels with high octane number, e.g. natural gas, are used. In the present work, the combustion process of premixed charge natural gas engines is studied by means of numerical tools. The studied engine concepts are: conventional homogenous charge compression ignition (HCCI), HCCI with a pre-chamber and PCCI with a pre-chamber. The numerical model is validated against the experimental data and then, combustion characteristics and engine emissions in various operating regions, are discussed and compared. The results reveal that the PCCI case has the earliest start of combustion. It is also shown that in the ultra lean operating conditions (fuel equivalence ratio of 0.1) decreasing the intake temperature from 450 K results in an efficiency increment for the PCCI engine, while for the HCCI case this causes misfiring and more CO emission. Nevertheless, the PCCI engine has higher levels of NO emission compared to the HCCI cases and only in the ultra lean condition this level reaches amounts less than the acceptable limits set by Euro 6 emission regulations. (C) 2016 Elsevier Ltd. All rights reserved.