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In this study, we focused on premixed charge compression ignition (PCCI) combustion with dual fuel of carbon monoxide (CO) and diesel fuel. The effects of CO as fuel on PCCI combustion characteristics were investigated based on engine tests and numerical simulations. As a result, it was found that the combustion became mild and combustion phase was retarded with increasing amount of CO supply. The results suggest that the combustion phase can be controlled by optimizing amount of CO supply, and moreover that the operating regions of PCCI combustion will be extended. In addition, it became clear that the supply of CO on PCCI combustion could be an alternative method for introducing large amount of EGR and lowering the compression ratio. On the other hand, the supply of CO led to an increase in unburnt CO emission and a deterioration in thermal efficiency. The numerical simulation results showed that it is effective to simultaneously increase the effective compression ratio and the EGR ratio in order to overcome these disadvantages without increasing NOx emission. Therefore, there is a possibility that PCCI combustion with dual fuel of CO and diesel fuel will possibly achieve high thermal efficiency.