On-board Detection Of Urea Concentration In SCR System By Magnetically Coupled Resonators

The selective catalytic reduction( SCR) is a standard aftertreatment technique to meet current engine emission regulations, to eliminate oxynitride(xNO) of exhaust gas. In order to ensure engine emissions compliance, insufficient and improper reductant in tank is force to be detected by on-board diagnosis(OBD) legislations. Generally, current urea solution concentration detection methods must contact with urea solution directly. Due to the corrosion and crystallization properties of the urea solution, there are some problems in the practical application of the current detection methods. In this project, a non-contact method is proposed to detect the Ad Blue residual and quality in tank by magnetically coupled resonators, the specific work of this paper is as follows:The detection method has been validated from principle、simulation and test in this paper. In the first, the electromagnetic characteristics of urea solution has been analyzed, and obtain that conductivity and permittivity increase with the urea concentration increases, and the rate of change in conductivity is greater than the dielectric constant. Considering the low conductivity of the urea solution and raising the detection sensitivity, we decided to measure the characteristics of the complex permittivity of the solution to the cavity resonance frequency to detect urea solution margin and measure the perturbation characteristic of the complex permittivity of solution to impedance characteristics of the cavity to detect urea solution quality.Based on the above basic principles, the coil structure has been improved. In this paper, different detection coil structure, detection performance and detection sensitivity simulated through ANSYS, and designed urea solution level and concentration of onboard testing program that using the self-resonant frequency of the coil to detect the liquid level changes and using the amplitude of the return loss using a coil(11S) to detect the change in concentration of the solution changes. In order to improve the sensitivity of detection of the magnitude of 11 S, a driving coil added on the basis of the detection coil. So, can adjust the relative position of the drive coil between the detection coil to achieve the detection coil impedance matching, thereby greatly enhancing the electromagnetic signal strength of the system and improving the signal to noise ratio of the detection signal.In order to verify the reasonableness of the above test program, a test platform built for related programs and different liquid level, frequency characteristics in concentration detection coil and impedance characteristics tested through network analyzer. The results show that: when the urea solution level increases, the coil self-resonance frequency decreases obviously, eventually stabilized and when urea solution is at low level, the coil self-resonant frequency is more sensitive. Experimental and simulation results show the same performance both in numerical and trends. For the urea concentration testing, match concentration of 32.5% urea solution impedance first, when the concentration of the solution is reduced, the amplitude of the coil reduced, and the magnitude of the change in the concentration of the urea solution more sensitive near 32.5%, the experimental results are consistent with the simulation results on the trend, which proves the feasibility of detection schemes.