5.2.7. Reduction of toxicity of the fulfilled gases
Formation of fuel mix and process of combustion
– injection pressure;
– injection time;
– distribution of an aerosol (number of a spray, profile of the sprayed stream, the directions of the sprayed stream);
– fuel injection moment;
– movement of air;
– amount of air.
These factors influence allocation by the engine of harmful substances and fuel consumption. High temperatures of combustion and high levels of concentration of oxygen lead to increase in formation of NOx. Release of soot increases at reduction of amount of air and insufficient formation of fuel mix.
Criteria of the engine
Configuration of the combustion chamber and channel of an air inlet can positively influence reduction of emission of the fulfilled gases. If movement of air in the combustion chamber is carefully coordinated with fuel jets and sprays that promotes effective mixing of air and fuel, full combustion of the injected fuel is reached. Besides, positive effects of homogenization of mix of the air and the fulfilled gas cooled in EGR path are reached. Use of four valves on the cylinder and a turbocompressor with the turbine of variable geometry (VTG) also promotes reduction of toxicity of the fulfilled gases and increase in specific power.
Fuel injection moment
Reduction of temperature of combustion and delay of process of injection of fuel lead to reduction of maintenance of NOx in the fulfilled gases. However, if these parameters are considerably reduced, the maintenance of HC in the fulfilled gases and fuel consumption, also as well as release of soot increases at high loads of the engine.
If the moment of injection of fuel deviates only on 1 ° (bent shaft) optimum value, the maintenance of NOx can increase by 5%, in view of that the deviation on 2 ° can lead (bent shaft) towards an advancing of the moment of injection to increase in pressure in engine cylinders to 10 bars, and the deviation on 2 ° (bent shaft) towards delay can increase on 20 ° temperature of the fulfilled gases. Such high sensitivity demands extreme accuracy at adjustment of the moment of injection of fuel.
Fuel dispersion
Fine dispersion of fuel promotes effective mixing of air and fuel. It makes the significant contribution to reduction of release of soot and HC. High pressure of injection and an optimum geometrical configuration of an opening of a spray lead to good dispersion of fuel.
For an exception of emergence of a visible exit of soot the amount of injectable fuel has to be limited according to amount of the air coming to the engine. It demands supply of excess air, at least, on 10... 40% (X =1,1... 1,4). After closing of a needle of a spray the fuel which remained in a spray opening evaporates and leads to increase in allocation of HC. It means that such harmful volumes have to be minimized.