Research And Development Of Control Unit For SCR After-Treatment System Based On FAI Dosing Injection Unit

Selective catalytic reduction(SCR) technology is considered one of the most effective ways to reduce the NOX emission of diesel engines. Currently, SCR technology has been widely used in Europe and is in the promotion of research and development stage in China. However, the current SCR system still exist some problems: foreign system has high stability and reliability, but the system is complex and expensive; while the quality of domestic product varies greatly, the reagent dosing accuracy cannot be guaranteed.FAI dosing injection unit uses electromagnetic pulse intermittent injection technology for liquid jet, with high precision injection, simple structure, low cost. Because the injection unit is adapted for injection a conductive liquid, it can be use as the SCR system reagent(DEF) dosing unit in diesel exhaust after-treatment system, which will greatly simplify the structure of SCR systems with high dosing accuracy. And it will help promote the use of SCR technology in China.This thesis studied FAI-SCR after-treatment system and its control unit(ACU) based on the FAI dosing injection unit. First, the overall structure design of the FAI-SCR system is introduced, and the working principle and process of two operating modes called "Slave" and "Master" are analyzed, which includes the FAI dosing injection unit, the electronic control unit of FAI-SCR system, NOX sensor, exhaust temperature sensor, the liquid level and temperature sensors, etc. Proposed FAI-SCR system control unit should meet the functional requirements. Secondly, a suitable hardware circuit of the FAI-SCR after-treatment control unit in two working mode was developed concerning the MCU selection, power conditioning, data acquisition, data communication, the solenoid valve drive, the DEF injection control and T3 detection module. And then, the software for the FAI-SCR system control unit was developed, concerning a method of adjusting reagent flux by variable frequency of injection pulse, including the single injection quantity control using T3 feedback correction. The power chip for driving the dosing injection unit is the main component concerning the reliability of the ACU. Power dissipation of the components were measured and analyzed and different types of the power chips were compared and their temperature rise characteristics were tested, which were used to predict anti-heat reliability while keeping the T3 detectable, in order to provide a reference for selection power element performance to meet the higher ambient temperature requirements. Finally, the developed FAI-SCR system were tested on a diesel engine: the accuracy of the dosing injection unit was verified by the ESC mode emission test, and the influence of DEF injection amount on NOX conversion efficiency using the developed FAI-SCR system showed that high NOX conversion efficiency could be realized practically, which also provided a reference for developing the Master mode control strategy to meet different emission requirements.