A low pressure direct injection transparent optical engine bench was developed

and the in-cylinder mixture formation process was filmed.Different mixture formation patterns were analyzed in conditions of different combustion chamber geometries

fuel injection timings and engine speeds.The research results show that:guidance of the combustion chamber with decentralized cavity piston is better than that of the proto-engine's combustion chamber;fuel injection timing plays a key role in the mixture formation

and a good mixture formation can be obtained if choosing an injection timing when intake flow is not that strong and the piston moves in the middle position of the cylinder

when the intake flow is strong

an increased engine speed can accelerate the evaporation of the fuel droplets

but the guidance of the combustion chamber is weaked

which results in a bad mixture formation.On the contrary

when the intake flow is weak

higher engine speed can promote the effect of combustion chamber's guidance to the fuel plume.