Study on the effect of water addition on combustion characteristics of a HCCI engine fueled with natural gas

•Effect of water addition on natural gas HCCI combustion characteristics was studied.•Water addition controls the start of combustion under different loads.•Water addition reduces the ringing intensity and expands the operation range to run on high load condition.•Coordinated intake temperature and water addition is easier to realize controlled combustion.•Internal mechanism of important species was proposed.