Characterization of NOx sensor performance under oscillating pressure conditions

Sammanfattning: The NOx sensor is essential in the modern after-treatment system. Due to the introduction of regulations from the US, it is necessary to investigate how these sensors operate in conditions of pulsating pressures generated engine - out, after the turbocharger. Therefore, the main objective of the project will be to recreate these pressure conditions by devising, designing and building a test rig where different NOx sensors can be tested under those oscillating pressure conditions.  It is first necessary to understand the operation of NOx sensors and heavy duty systems to identify the pressure conditions to be treated. This project focuses on parameters such as the amplitudes and frequencies of the pulsating pressures, the average pressure on which the oscillations occur and different NOx reference levels. In turn, the presence of O2 in the initial mixture of gas introduced is studied. Using reference gas and not real exhaust gas and not considering the temperature change as a significant parameters are some of the delimitations of this project. A test rig to generate these pressure conditions is designed, built and assembled to house the different NOx sensors and proceed to their testing based on the variation of the previously presented notable parameters. A testing method is devised for the different tests to be performed.  The results of the project reflect that the parameters of frequency and amplitude of the pressure oscillations cause a systematic linear increase in the output signal of the sensor while increasing those parameters, reaching deviations outside the reference concentration. Frequency is the most remarkable parameter. The average pressure in which the oscillations occur increases with higher pressures but it does not cause have a significant effect in the output signal of the sensor compared with the previous treated parameters. When testing various NOx levels, the signal deviation´s percentage is greater at lower initial concentrations. At the same time, a mixture of NOx and O2 was introduced in order to see how O2 affects the sensor´s performance and signal. As a conclusion, it can be said that the sensor signal is more affected with the introduction of O2 in the mixture rather than just introducing NOx gas. Conclusions obtained expose that NOx sensor´s output signal is affected by the generated pressure oscillations by increasing their signal linearly when frequency and pressure amplitudes are increased. Frequency is the parameter that affects most and higher deviations from the reference signal concentration are obtained. Pressure amplitude affects in the same way frequency does but it is not as significant as frequency change. However, there is a proportional deviation change when the pressure amplitude increases with the frequency. Average pressure parameter does not affect significantly the output signal of the sensors.