Study On The Strategy Of SCR System Temperature Control And Fuel Injection Quantity Diagnosis In Diesel Engines

With the attention to environmental issues and increasingly stringent emission regulations, the diesel engine emission control technology has become more and more important. Aimed at decreasing the main pollutants in diesel exhaust, which are mainly particles(PM) and nitrogen oxides(NOx), two types of emission control technologies are commonly used in worldwide. The first type is to reduce PM through optimization of diesel combustion, then adopt selective catalytic reduction technology(SCR) to remove NOx. The second type is to reduce NOx through the exhaust gas recirculation technology(EGR), then use diesel particulate filter technology(DPF) to remove PM. Considering the fuel economy and the fuel characteristic in China, SCR technology has become the main emission control technology of heavy-duty diesel engines in China. However, domestic research on the key techniques of the SCR system is not a patch on the research abroad. Domestic research on the control strategy of SCR control unit is not enough, restricting the localization of SCR technology.The core objective of SCR control strategy is to control urea injection quantity precisely. The first step is to calculate required urea quantity according to the NOx conversion efficiency of SCR convertor and the NOx emission at the convertor inlet. The second step is to adjust and limit the required urea quantity based on ammonia storage model, which is the actual urea injection quantity. The last step is to use closedloop control strategy to correct the urea injection quantity with the NOx emission at the convertor outlet. Therefore, the estimation of NOx conversion efficiency and the acquirement of inlet NOx emission are key contents in the urea injection control strategy.Because the conversion efficiency of SCR convertor is mostly up to the catalyst temperature, acquiring the catalyst temperature precisely is significant for the control strategy to estimate the NOx conversion efficiency. In the current control strategy, the temperature at SCR convertor inlet is used to replace the catalyst temperature. However, it was found experimentally that the temperature at SCR convertor inlet is not consistent with the catalyst temperature in the process of operating condition transition. Therefore, the catalyst temperature estimation in the current control strategy is not accurate, which will make the NOx conversion efficiency estimation has deviation. As a result, urea injection quantity is not accurate, causing the actual NOx conversion efficiency of SCR systems inadequate and an amount of ammonia leakage. In order to solve the problem, temperature control model of SCR systems was built in this paper. In the model, reaction rates of the main SCR reactions were based on the LH mechanism. According to the conservation of mass and energy, the temperature at SCR convertor outlet was derived. At last, the catalyst temperature was estimated as the average of temperatures at SCR convertor inlet and outlet, which improves the accuracy of the SCR catalyst estimation.Acquiring the NOx emission at SCR convertor inlet is another key control strategy in urea injection control. The NOx emission at SCR convertor inlet is consistent with that of the engine outlet. In the strategy of SCR control unit, original NOx emission from engines is got through a MAP based on two parameters, the engine speed and the fuel injection quantity. However, the set fuel injection quantity of Electronic Control Unit(ECU) is commonly used in the MAP, which is probably not consistent with the actual fuel injection quantity because of the engine aging and the fuel system worn. Consequently, the engine original NOx emission acquired by the MAP is not the actual one. In this paper, the fuel injection quantity diagnosis model was built so as to get the accurate NOx emission at engine outlet. In the model, the process of fuel combustion in cylinder, NO generation under high burning temperature, SCR reactions in the SCR convertor were analyzed. The fuel injection quantity was deduced theoretically by means of the oxygen concentration measured by the NOx sensor at SCR convertor outlet. The actual fuel injection quantity was diagnosed by comparing the fuel quantity calculated in the model and that acquired from ECU.The temperature control model of the SCR system and the fuel injection quantity diagnosis model were verified on a diesel engine test bench. The temperature at SCR convertor outlet calculated in the model and that measured by the temperature sensor were compared under ESC operating conditions and the transmitted operating conditions. The results show that the relative error of the calculated temperature is below 5% at most of ESC conditions. Under transient condition, the curve of the calculated temperature is consistent with that of the measured temperature. The fuel injection quantity diagnosis model were tested experimentally under the conditions of different engine speeds and torques. The results show that the relative error of calculated fuel injection quantity is less than 5% under most operating conditions. When the engine torque is greater than 1500N·m, the relative error is below than 2%. Therefore, the two models developed in this paper is credible and can be applied in realtime engine control.