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The influence of injection duration on the mixture formation and combustion performance of high power multi\|injection gas engine was studied. CFD simulation was employed to study the different effects on the mixture formation by adopting the structures of single elbow and twin elbows individually. On the basis of the result, the twin elbows structure was determined to be used in the experiment. And five different periods of injection duration were particularly designed. CFD simulation and combustion analysis were employed to explore the influence of different injection durations on mixture formation and combustion performance under the conditions of engine working in rated power while other parameters remaining unchanged. In the experiment, the beginning time of each injection duration period was at 40° after inlet TDC. In this situation the exhaust valve was fully closed, natural gas couldn’t directly enter the valve. Moreover, with the quick movement of piston, the injection gas has strong disturbance energy. These all help to promote the combination of natural gas and air. The results of CFD simulation and experiment indicated that under the experimental conditions, when the injection duration extended from 5ms to 10ms, the cycle variation of combustion of the studied engine decreased obviously. That’s because the extended time could guarantee the disturbance energy which was produced by injection gas being fully used, therefore, the mixture formation quality was improved correspondingly. However, when the injection duration was prolonged to 12ms or 15ms, the flow rate of natural gas would be decreased and the injection gas couldn’t enter the cylinder. Thus part of natural gas still stayed in the elbow, which reduced the quantity of the gas that can effectively develop the mixture and make the mixture thinner, which was unfavorable for the combustion.