Nitrogen oxide sensor (NOx sensor) is an important component of SCR (selective catalytic reduction reaction) system of diesel engines above the fourth national standard. Since heavy truck engines burn at high temperature and high pressure and produce more harmful nitrogen oxides, heavy trucks will add a reactor with rare metal coating on the exhaust pipe, and then add ammonia (NH3) to let nitrogen oxides and ammonia react in the reactor, so as to reduce the harmful nitrogen oxides in the emission. Since it is a hazardous substance through catalytic reaction, and the national law stipulates the corresponding emission indicators, it is necessary to monitor the catalytic efficiency of SCR system. In this way, we can always know whether the emission of the diesel engine meets the standard and whether there are problems in the aftertreatment system and combustion system. The NOx sensor is the component used to monitor the catalytic efficiency. Its quality affects the power of the engine and even monitors the health of the engine.
Operating conditions of NOx sensor: when the ignition switch is turned on, the NOx sensor will be heated to 100 ℃. Then wait for the ECM to send a "dew point" temperature signal (dew point temperature means that there will be no moisture in the exhaust system that can damage the NOx sensor after this temperature); At present, the dew point temperature is set to 120 ℃, and the temperature value is the value measured by referring to the outlet temperature sensor of EGP; When the sensor receives the dew point temperature signal from ECM, the sensor will heat itself to a certain temperature (up to 800 ℃). Note: at this time, if the sensor head contacts water, the sensor will be damaged; After heating to the working temperature, the sensor starts normal measurement; The sensor sends the NOx value to the CAN bus, and the engine ECM monitors the NOx emission through this information.
The NOx sensor of the exhaust treatment system has a variety of installation positions, depending on different engine applications. However, Cummins usually installs it in the silencing part of EGP. Flushing the nitrogen and oxygen sensor in operation with water will cause damage to the sensor. The NOx sensor assembly is a fully intelligent device, which is composed of three parts: sensor head, control module and connecting cable. The sensor currently provided by Cummins can only be used to measure the value of NOx at the outlet. The sensor communicates with the body ECM through the CAN bus, and it also monitors the fault of the NOx sensor system. The whole NOx sensor assembly is a part, and no part can be replaced independently!
The NOx sensor transmits the NOx value in the exhaust gas to the ECM in real time after normal operation. ECM does not judge whether the exhaust is qualified by monitoring the real-time NOx value, but detects whether the NOx value in the exhaust exceeds the standard through a set of NOx monitoring program. The following conditions must be met to run NOx monitoring: there is no relevant fault code of cooling water temperature sensor; No relevant fault code of ambient pressure sensor; The temperature of cooling water is higher than 70 ℃; The altitude is less than 1600 meters. Under normal operating conditions, the amount of nitrogen to be injected is 380L • engine / 6ml • H 600 to 3800 ml/h 4cy engine； • engine speed range • 1400 - 1700 RPM 6 cylinder engine. • 1400 – 1750 rpm 4 cylinder engine； The NOx sensor gives a relatively stable reading value; Stable injection quantity for at least 15 seconds. After the above conditions are met, the ECM will take two samples, and then average the values of the two samples. The average value obtained will be compared with the set limit value. If the average value obtained is lower than the limit value, the routine monitoring of NOx is completed once. If the average value obtained is higher than the limit value, it will enter the additional monitoring program.
I have said so much about its working principle and working method, so how to maintain it at ordinary times? 1. Because the nitrogen and oxygen sensor has vent holes and pressure balance holes, and the exhaust pipe is generally located behind the cab, it is easy to water in this position, and the whole nitrogen oxide sensor will be damaged after water inflow. Generally, 80% of the damaged nitrogen and oxygen sensors are caused by this reason. Therefore, this part must be wrapped with waterproof things at ordinary times. 2. Fixed in a place with small vibration, the NOx sensor itself has anti vibration design. If the vibration is large, the NOx sensor will not work normally. 3. To prevent the water intake and exhaust system, because the probe part of the NOx sensor is of ceramic structure, it will heat itself to a very high temperature during operation. If it encounters water, it will cause the probe part to burst. 4. Don't fiddle with it. Some repairmen will fiddle with the NOx sensor in order to burn less urea for post-treatment or eliminate some faults. However, it is easy to damage it.