The combustion phenomena of fuel injection in sub- and super-critical environments was studied by using a constant volume bomb system. The high-speed camera and shadow method were used to collect the combustion image information. The differences in the spray combustion characteristics with the change of ambient pressure and injection pulse width were studied at the same super-critical temperature. The n-hexane and ether were used as fuels in the experiment. Results show that at sub-critical pressure

the combustion time of the two fuels with different injection pulse widths increases with the rise of ambient pressure

while the spray flame maintains a relatively stable morphology and brightness in the later stage of combustion development

which is in line with the traditional spray combustion process. When using n-hexane as fuel

the combustion flame at the critical pressure point has the shortest length and regular morphology. The flame morphology at super-critical pressure trends to be stable with a small fluctuation

and the flame length is slightly shorter than at sub-critical pressure. When the fuel injection pulse width changes

the combustion time will show a different trend near the critical pressure. As for the ether fuel

the combustion time first increases with the rise of ambient pressure

and then decreases after reaching its maximum value at critical pressure. In addition

the flame morphology is slightly different under different pressure conditions. The differences in the combustion phenomena are mainly due to the differences of physical properties and fracture evaporation mechanism of fuels in sub and super-critical environments.