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Meeting future exhaust emission and fuel consumption standards for passenger cars will require refinements in how the combustion process is carried out in spark ignition engines. A direct injection system reduces fuel consumption under road load cruising conditions, and stratified charge of the air-fuel mixture is particularly effective for lean combustion. This paper describes an approach to improve combustion stability for direct fuel injection gasoline engines. Effects of spray characteristics (spray pattern and diameter) and air flow motion on the combustion stability were investigated. Spray patterns were observed by the laser sheet scattering method and 3-dimensional laser doppler velocimetry. Mixture behavior in the combustion chamber was observed by the laser-induced fluorescence method using an excimer laser and single cylinder optical engine. It was found that the spray pattern for a pressurized condition affects the combustion stability and smoke generation.

The mixture formation and combustion process in direct fuel injection engines was analyzed using the simulation program developed by the authors. The following conclusions were obtained. (1) The swirl air motion generated in the cylinder plays an important rule for carrying fuel vapor around the spark plug. (2) The combustion period becomes shorter due to reduction of attachment of fuel on the piston when using the skewed spray. (3) There is an empirical correlation between homogeneity of the in-cylinder mixture and the engine torque obtained experimentally. (4) Generation of a uniform mixture in the cylinder is important to improve combustion efficiency.

This paper describes a combustion stability control using fuel injection control for a gasoline homogeneous charge compression ignition (HCCI) engine. First, using a single-cylinder engine we examined the influence that fuel injection and air/fuel mixture had on HCCI engine auto-ignition and combustion. This was achieved by visualization experiment of in-cylinder air/fuel mixture with fuel injection as a parameter. Next, the effect of the fuel injection control was evaluated by using a 4-cylinder HCCI engine. We proposed the following concept for a gasoline HCCI combustion control: internal-EGR (I-EGR) is applied to either internal EGR control of each-cylinder or a multi-cylinder control scheme using a variable valve event and timing system, and the fuel injection is applied to each cylinder control while keeping the mixture homogeneous.

Mixture formation technology for multipoint fuel injection systems in spark ignition engines has been reviewed regarding reduced exhaust emissions, fuel consumption and improved engine performance. In conventional systems, under light load conditions, the mixture of fuel to suction air is not uniform due to a short injection pulse width against a long duration of suction stroke. Under heavy load conditions, fuel spray is apt to be deflected by the air flow through the intake port and the injected fuel clings and remains onesidely on the cylinder wall during the combustion cycle. Under cold start conditions, the fuel on the intake manifolds and ports is not evaporated quickly enough so that it is evaporated in the cylinder after the temperature rises due to the compression stroke. A lot of fuel is injected to compensate for the small evaporation rate.

The effect of fuel injection atomization on engine performance has been known to improve fuel economy and to reduce emissions. Hitachi America, Ltd. Research and Development along with Hitachi Research Laboratory in Japan have studied the effects and the operation of the air assist injection system which was developed and studied to help meet future Low Emission Vehicles (LEV) regulations and also Ultra Low Emission Vehicles (ULEV) regulations. The system consisted of newly designed air assist injectors having a spray angle of 15° at 170 kPa (absolute air pressure) with 370 kPa (absolute fuel pressure). The air assist injector generates highly atomized fuel droplets by swirling the fuel clockwise and the air counterclockwise. The fuel and air flowing in opposite directions collide, thereby producing particles around 30 μm in size at 274 kPa air pressure. These characteristics improve cold start, cold coolant conditions and fully warm engine conditions.