| Diesel engine is widely used in commercial vehicles because of its excellent dynamic charicteristic, economy, reliability, etc. While, with the increasing inventory of commercial vehicles, pollution caused by NOx and PM emission drives more and more attention, especially for the heavy-duty diesel engines. To reduce the emission of NOx and PM, China Ⅳ has been carried out around the nation, and part regions like Beijing plan to execute BeijingⅥ(EuroⅥ) legislation when the legislating of ChinaVI is in process. EuroⅥ legislation has already been executed in 2014 in Europe, and the main technology to satisfy the EuroⅥ emission is DOC/DPF+SCR. China is developing the research for the after-treatment system(DOC/DPF+SCR), which sustains the execution of ChinaⅥ legislation. But key technologies of the EuroⅥ emission still keep vacant for the system study of control strategy of DOC/DPF+SCR system. How to raise the DeNOx efficiency of SCR and realize DPF regeneration safely in low load state are main challenges to the application of the coupling use of SCR and DPF to fulfill the EuroVI emission. Due to the low temperature windows of the legislated cycle, SCR efficiency is limited by the catalyst performance. While for the DPF system, lose control of the regeneration temperature can result in unsuccessful regeneration, substrate crack and even finally substrate thaw. Therefore, the thermal management solution and after-treatment temperature control are key points to achieve the EuroⅥ emission with SCR&DPF technology, which are studied in this paper.For EuroVI legislation, temperature is pivotal to improve the SCR efficiency and realize the DPF regeneration, this paper studies the effect of different exhaust temperature management solutions like intake throttle valve\fuel injection advance angle\fuel injection pressure\post injection. The WHTC test results indicated that intake throttle valve combined with post injection could promote the exhaust temperature effectively without sacrificing too much fuel efficiency. In that way, the temperature requirements of SCR and DPF could be fulfilled.The solution of reversible air-flow mixer is proposed to fully use the exhaust heat energy to vaporize the injected ammonia. This structure can accelerate the mixing, broken, pyrolysis and hydrolysis of the ammonia in the exhaust gas, which achieves NH3’s well-distribution on the front catalyst. In that way, the DeNOx efficiency is improved and crystallization risk is reduced. The test bench results indicated that the DeNOx efficiency was increased about 60.3% at WHSC and 69.4% at WHTC compared with the close-coupled structure, meanwhile the formation of deposits was inhibited. Secondly, kinetically chemical reaction model was established to estimate the DeNOx efficiency of WHTC cycle under different exhaust temperature. Based on the reversible air flow mixer, improvement suggestions were given to satisfy the temperature requirements of SCR system through simulation method. Based on this model, the close-loop injection control of ammonia storage was established.Accurate estimation of the soot loading accumulated in DPF has a higher priority than the temperature control for the DPF regeneration. To the question, the DPF soot loading model was built up and validated by bench test. The result showed that error of the soot loading in WHTC cycle could be controlled at about 3.4%, which satisfied the requirement of active regeneration trigger. Meanwhile, the temperature impact on the passive regeneration was studied to support the realization of DPF purely passive regeneration. If the engine state suddenly drops to idle during the DPF regeneration, the peak temperature and the temperature gradient inside DPF can induce the crack of DPF. Therefore, the temperature control of DPF was studied under idle-speed raising and gradient temperature. Results analysis showed that these two methods could effectively prevent DPF crack due to temperature peak.To guarantee that the normal function of the after-treatment system can satisfy the emission limit, the whole thermal management system and control strategy were tested in the vehicle. Based on the PEMS test requested by EuroVI legislation, the DeNOx efficiency was tested under solutions like combining the intake throttle valve with post injection, reversible air-flow mixer and ammonia storage based close-loop control strategy. The effective mean emission of NOx is 0.45g/kWh, which was under the legislation level (0.6g/kWh). For the DPF soot load model and the temperature control of the active regeneration, the mean error between the vehicle test and simulation result was about 4.0g, which was acceptable for the vehicle application. During the active regeneration, the average temperature upstream DOC was raised up to about 320℃ and the lowest temperature was above 300℃ with the introduced thermal management method. This guaranteed that active regeneration could be realized continuously and quickly. The idle speed was controlled at 1100r/min, which was safe enough even the drop-to-idle happened during regeneration. |
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