The NO_x sensor for diesel applications consists of a sensor probe and a non-detachable sensor control unit. The sensor probe itself contains a ceramic sensor element with an integrated heater. The overview of Sensor Control Unit (SCU) and NO_x sensor probe is shown in Figure 1 . This kind of senor is an electrochemical sensor. Two functional parts build up the NO_x sensor. First, the sensor probe contains cable to the SCU with connector, sensor element and sensor housing with protection tube. Second, the SCU is a controller with hardware (HW) and software (SW). The housing is made of plastics to allow a sealed packaging inside to protect the co-fired ceramics (LTCC). The SCU has to be fixed with a bracket or cable tie.

In the Selective Catalytic Reduction (SCR) system, the NO_x sensor is located in the exhaust pipe, with its sensor element exposed to the exhaust gas [6] . The location is shown in Figure 2 . A NO_x sensor is installed up- and down-stream SCR of the exhaust pipe. The upstream NO_x sensor mainly measures the concentration of NO_x in the diesel exhaust and determines the optimal injection amount of urea water solution (UWS). This calculation has to be done in the Electronic Control Unit (ECU). The downstream sensor mainly detects the catalytic performance of the catalyst. The up- and down-stream sensor is used for closed-loop control of SCR system, which precisely control reduction reaction. Ultimately, the diesel exhaust emission level is controlled.

The basic principle of the SCR system is to use Ammonia (NH₃) as a reducing agent to reduce NO_x into N₂ and H₂O under the action of a catalyst. Although NH₃ is non-toxic, it is a gas with a strong pungent smell and is not convenient to use directly in vehicles. Therefore, the NH₃ required for the reaction is provided by spraying urea aqueous solution into exhaust pipe. The reaction process is as follows.

(NH₂)₂CO + H₂O → CO₂ + 2NH₃

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

2NO₂ + 2NO + 4NH₃ → 4N₂ + 6H₂O

The sensor is used to measure the O₂ and the NO_x concentration with the property of zirconium oxide (ZrO₂) that can conduct O²⁻ at 800˚C in exhaust gas of diesel engines. NH₃ is indirectly measured as an additional contribution to the NO_x signal.

The functional diagram is shown in Figure 3 . The functional parts of the sensors elements are O₂ pumping cell, NO_x pumping cell Nernstian cell, two diffusion barrier and heater circuit [7] . And the main functional parts of the SCU are SBC module that include CAN transceiver and reverse protected voltage regulator, microtroller that accommodates the control logic of the system, O₂ cell control and NO_x cell control the include O₂/NO_x closed loop controller Application Specific Integrated Circuit (ASIC) and heater control with inverse polarity protection (IPP).

When the sensor will start, it is set to “preheat” until the “dew point” bit is sent via CAN. The sensor will start heating up to its working temperature by sending the data “dew point reached”. The message “dew point reached” can only be sent to the sensor if the gas flowing at the protection tube of the sensor does not contain any liquid components. Heating electrodes were heated to 800˚C. The exhaust gas enters the first cell of the sensor, and O₂ pumping electrode pumps O₂ in the out of the cell in form of O²⁻. At this time, the current change that is generated is measured by inner pumping (IP1: control PIN of O₂ cell) and convert into O₂ concentration. The exhaust gas reached the second cell, then NO_x are catalytically reduced to N₂ and O₂ on the electrode. The generated O₂ is pumped out of the second cell in the form of O²⁻ through the second O₂ pumping electrode. The current change generated can be measured IP2 (control PIN of NO_x cell). the concentration of NO_x can be calculated eventually.

It is well known that the development of foreign NO_x sensor technology started relatively early. The mature sensor manufacturers include Bosch and Viteesco Technologies that products are expensive and have a monopoly in the industry. In addition, the technology mastered by well-known suppliers is not open to the public, resulting in slow development of NO_x sensor technology. The research and development of diesel engine treatment system in China started relatively late. After the National IV standard clearly implemented, various diesel engine manufacturers, universities and research institutes began to pay attention to it, and then gradually entered the actual research and development progress of diesel engine treatment system technology.

Tan hong liang, et al, developed a NO_x sensor test bench. The main detection process includes the following steps. Firstly, the gas exhaust simulation needs to be configured. The mixed gas is preheated by the heating tube and then enter the probe of sensor. Secondly, computer control system collects data and analyzes it. By controlling the flower to achieve exhaust gas configuration of different concentrations, the test bench can detect the measurement accuracy of NO_x concentration of the sensor and record relevant data, but it also has the following disadvantages:

Sun wen, et al, designed a NO_x sensor performance detection device, which contains control unit, power supply unit, heating unit, standard gas source unit, temperature control unit, exhaust gas treatment unit. The device used standard gas as the gas source, did not need to configure gas exhaust. It has the advantages of simple structure and convenient operation. There are also the following disadvantages: (1) Need to configure many gases with different concentration values and it is high-cost. (2) The test bench cannot realize automatic of different concentration points and can only be switched manually, resulting in low detection efficiency. According to previous research, significant progress has been made in sensor detection field. Therefore, a low-cost and high-reliability NO_x sensor performance test bench is urgently carried out.

In order to the needs of NO_x sensors accuracy detection in the field of diesel engines, the test bench designed in this paper can be configured with different concentrations of NO_x and N₂. The control range of NO_x is (0 - 2800.56) μmol/mol. The test bench is equipped with a high-concentration NO analyzer (model: CAI) with a range of (0 - 4688.22) μ mol/mol and an accuracy of ±1% FS (Full Scale), which has been calibrated a legal metrology agency to ensure the accuracy and stability of the mixed gas concentration. To achieve mass detection, the control program of the test bench has the characteristics of automatics gas distribution, automatic conversion, auto measurement and recording of various gas concentrations, which effectively improves the detection efficiency. The structural diagram of the test bench is shown in Figure 4 . The test bench includes six major units, such as data processing and display unit, gas mixing unit, analyzer measurement unit, sensor measurement unit, exhaust treatment unit and standard gas source.

The standard gas source consists of multiple gas tanks, which store high-purity N₂ and 95% N₂ + 5% NO mixed gas respectively, and are connected to the gas mixing unit through pipelines. The gas mixing unit includes a pressure regulating value, a gas mass flow meter, a mixer and related pipelines. And the gas mass flow meter receives the target flow signal output by the data processing and display unit, which can automatically adjust the flow of standard gas to achieve flow control of the above three gases. Then the three kinds of gases are mixed in a certain proportion in the mixer and introduced into the analyzer measurement unit. The analyzer measurement unit consist of a NO analyzer and an O₂ analyzer, which can respectively detect the NO_x and O₂ in the mixed gas in real time online and output the results to the next unit. According to the parameters of the three standard gases in the test bench, the calculation method of the flow rate of each standard gas corresponding to the sensor detection point is as follow.

Q_t/(Q_N + Q_t) = C_t/C_m (1)

Q_t—set flow rate of test gas;

Q_N—N₂ flow rate for mixing;

C_t—set concentration of the test gas;

C_m—standard gas concentration.

The detection system allows manual minute adjustment of the flow rates of the three standard gases according to the gas concentration results of the NO_x analyzer and O₂ analyzer to meet the detection point concentration specified by the NO_x sensor. The sensor measurement unit includes measurement chambers and a controller. The data processing unit is used to set the target flow rate of each component gas at different concentrations and transmit to the gas mass flow meter in the gas mixing unit. The unit also includes receiving the detection data of the analyzer measurement unit and the sensor measurement unit. The exhaust gas treatment unit includes an exhaust gas processor to remove harmful gases from the mixed gas and prevent air pollution.

Before the test, the NO analyzer was calibrated with 90.16 μmol/mol and 746.71 μmol/mol standard gas. According to the testing standards JB/T 11880.7-2014, the title is Diesel engines-Selective catalytic reduction (SCR) systems—Part 7 NO_x sensors, Then the detection device was used to test the NO_x accuracy performance of the sensor of Company A and B. The models are EGS-NX Gen2 CV and Generation 3.5a UNI-Volt respectively. And the test results are shown in Table 1 and Table 2 .

By consulting the relevant technical specifications of the product, the test results meet the requirements of the drawings.