Research And Application Of Frequency Modulated High Accuracy SCR Dosing System For Diesel Engine

SCR(Selective Catalyst Reduction)is a very important technology used for the deduction of NOx(Nitrogen Oxide)to meet China stage 4 and stricter emission regulations for diesel engines.As the core component of SCR system,SCR DEF(Diesel Exhaust Fluid)dosing system had got wide research and application.But there are still some problems: a.Liquid dosing and injection technology art and some core parts are greatly dependent on foreign manufacturers.b.Dosing accuracy is hard to meet the requirement of regulations stricter than China stage 6.c.Liquid dosing and OBD(On-board diagnosis)are highly dependent on sensors.d.Higher spray distribution uniformity is required.e.Low degree of miniaturization and integration.Aim at solving above problems,in this paper,a newly developed sleeve mode DEF dosing unit,which has completely independent intellectual property rights,was presented.It breaks the long-term monopoly of foreign liquid metering technology system and dependence on foreign core components.The volume of the unit is 1/20 of traditional pumps'.In view of the character that the dosing accuracy of the dosing unit is largely affected by the environment state change,a whole stage control model based on parameter of T3 was built to achieve high dosing accuracy because T3 directly reveals the change of dosing amount.Liquid dosing experiment results proved that the dosing error could be controlled in the range of ±3% under the feed-back adjustment of control model.Besides,the frequency modulation method didn't affect the dosing accuracy.An air-assisted dosing module was built based on the DEF dosing unit.An orifice was set in the compressed air path to increase the air velocity to “critical state of sound velocity” and then the pressure in mixing chamber was stabilized.Thus the dosing accuracy of the module of ±3% error was achieved.Besides,optical diagnosis methods were carried out for the analysis of the both macro and micro spray characteristics under different operating frequency.Results showed that the increase of the operating frequency leads to wider spray cone angle and the increase of SMD(Sauter Mean Diameter).Exorbitant frequency even results in the pure liquid spray pulsation with deteriorated spray quality.An airless dosing module was also built based on the DEF dosing unit.A jet vecuum based on Bernoulli equation was designed to form the back pumping,which can effectively clean up the DEF in the tube to avoid blocking and expansion damage caused by DEF freezing.Similar optical experiment results revealed the different spray characteristics with those of air-assisted module.The spray angle and SMD ddin't change with the increase of operating frequency due to the stable state in each dosing cycle.Moreover,in view of the abnormal cyclic dosing amount variation caused by some failures,the sensor-free diagnosis of some typical failures can be realized by the creative utilization of T3 parameter.This improves system reliability and decreases the cost.CFD(Computational Fluid Dynamics)approach was carried out to compare the properties of frequency modulation and PWM modulation on spray distribution uniformity and deposit prevention.Results showed that,frequency modulation has large improvement on DEF uniformity in exhaust system and has remarkable positive effect on SCR conversion efficiency,DEF consumption and prevention of DEF crystallization.The research results of this paper have been successfully applied to the exhaust aftertreatment products of China stage 5 and 6 diesel engines in batches,creating obvious economic and social benefits.