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Of late direct injection engines are replacing carburetted and port injected engines due to their high thermal efficiency and fuel economy. One of the reasons for the increased fuel economy is the ultra lean mixture with which the engine operates under low loads. Under the low load conditions, the air fuel ratio of the mixture near the spark plug is close to stoichiometric values while the overall mixture is lean, which is called stratified mixture. In order to achieve this, proper air motion during the late stages of compression is a must. Quality of the mixture depends on the time of injection as well as the type of fuel injector and mixture preparation strategy used. Engines employing air guided mixture preparation are considered as the second generation engines. For understanding the efficient mixture preparation method, three types of flow structures like base (low tumble), high tumble and inclined swirl are created inside the engine cylinder using shrouds on the intake valves.

Of late direct injection engines are replacing carbureted and port-injected engines due to their high thermal efficiency and fuel economy. One of the reasons for the increased fuel economy is the ultra-lean mixture with which the engine operates under low loads. Under the low load conditions, the air-fuel ratio of the mixture near the spark plug is near stoichiometric values while the overall mixture is lean, which is called stratified mixture. In order to achieve this, proper air motion during the late stages of compression is a must. Quality of the mixture depends on the time of injection as well as the type of fuel injector and mixture preparation concept used. Engines employing air-guided mixture preparation are considered as the second-generation engines. The air-guided mixture preparation method uses either swirl or tumble in preparing the mixture.

Numerical simulations were performed in a four stroke pent-roof SI engine under cold flow conditions at three different speeds (1000 rpm, 2000 rpm and 3000 rpm) to investigate the flow characteristics like swirl, tumble, turbulence, mass inducted and the formation of liquid film during cold start condition. The suction and compression strokes were simulated at the three speeds mentioned. Results show that swirl and tumble are increased from 1000 rpm to 2000 rpm whereas the increase is very low between the speeds 2000 rpm and 3000 rpm. The computational model was validated against the experimental data available in the literature. There is a slight increase in the mass of air inducted as the speed goes up. Further investigation with liquid film modelling reveal, presence of liquid film mass on the cylinder and intake region. Around 34% of fuel quantity converted to film mass in the port wall region of intake while using port fuel injection (PFI) at the entry of Siamese port.

The combustion and flow results of experimental and numerical investigations conducted on a stationary direct injection engine are presented in this paper. Experiments are conducted at a speed of 1500 rpm with the standard hemispherical piston at full load condition. A three dimensional finite volume model of the baseline engine with hemispherical piston (HCC) was created and the reacting flow simulation for it provided good agreement with experimental data. Further simulations are conducted for the reacting flow with modified pistons of shallow combustion chamber (SCC) and torroidal combustion chamber (TCC) configurations. In the design perspective, the geometric dimensions of the pistons are chosen so as to maintain the same piston bowl volume. From the numerical study, tt was revealed that both modified pistons generate higher pressure, with TCC piston producing a highest pressure for the same quantity of fuel injected.