The combustion characteristics of a 2.0 L gasoline direct injection(GDI)engine were studied at the engine's medium and low speeds and part load operation conditions

and compared with that of PFI engine. The effects of port tumble ratio on GDI engine's combustion characteristics were also analyzed. The results show that

with the same engine compression ratio and operation condition of 2 000 r/min

0.2 MPa brake mean efficient pressure

the combustion velocity of GDI engine is slower than that of PFI engine. At the engine's medium and low speeds

increasing inlet port tumble ratio can enhance airflow intensity and combustion velocity. However

when engine speed increases to 3 000 r/min

big tumble ratio deteriorates combustion in cylinder. The flap added to the inlet port can increase the air flow velocity and the combustion process of the fuel air mixture at engine low speed conditions

so that improve the engine's low speed thermal efficiency. The CFD results of 2 000 r/min

0.2 MPa BMEP condition show that closing the inlet flap can intensify the in cylinder air flow and increase the uniformity of fuel air mixture.