The flame development pattern of partially premixed combustion(PPC) with split injection strategy was studied on an optical engine fueled with n-heptane and operating at 1200r/min using the high-speed imaging of natural flame luminosity

and the mechanism of controlling combustion reaction rate was analyzed. The results show that when the fuel is injected into the combustion chamber with the single injection strategy before top dead center(TDC)

the flame development is dominated by multipoint auto ignition. When the fuel is injected before TDC with the split injection strategy

the flame development is also dominated by multipoint auto-ignition. Compared to the single injection strategy

the split injection strategy has a little influence on combustion reaction rate because the flame development pattern does not change. When the second injection timing is delayed

the rate of diffusion combustion increases and the peak heat release rate(PHRR) decreases. However

the signal of soot in the combustion images increases dramatically. When the fuel is injected after TDC with the split injection strategy

flame develops in the way of flame propagation

PHRR decreases and no obvious signal of soot is found. The split injection strategy can change the flame development pattern and control the combustion reaction rate while keeping the soot emission low.