Exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) have become important technologies to reduce the NO x emission of heavy-duty diesel engines and meet the increasingly stringent emission regulations. This paper studied the effect of EGR combined with SCR on the NO x emission characteristics of a heavy-duty diesel engine based on the engine bench test. The results showed that the NO reduction rate of EGR-coupled SCR increased with the increase of engine load, and the effect was no longer significant when the NO reduction rate exceeded a certain limit under the same working conditions. EGR combined with SCR has little effect on NO 2 emission reduction, and the increase of engine speed can significantly improve the efficiency of the NO 2 reduction rate at 75 and 100% load. 25% opening of the EGR valve (OEV) and 50% OEV have very similar effects on the NO x reduction rate when the engine speed is at a low level. Compared with low engine speeds, increased OEV or ammonia NO x molar ratio (ANR) had a more obvious effect on the NO x reduction rate at high engine speeds. SCR combined with low valve-opening EGR had a more significant effect on the NO x reduction rate. The increase of OEV led to the increase of fuel consumption rate, but the effect on the fuel consumption rate decreased gradually with the increase of diesel engine speed. Meanwhile, this study optimized the matching relationship between OEV and ANR based on the data of the genetic algorithm, which provides a theoretical research method and application basis for diesel engine-matching of EGR and SCR.

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<div class="section abstract">With the continuous upgrading of emission regulations, NOx emission limit is becoming more and more strict, especially in the cold start phase. Passive NOx absorber (PNA) can adsorb NOx at a relatively low exhaust temperature, electrically heated catalyst (EHC) has great potential to improve exhaust gas temperature and reduce pollutant emissions of diesel engines at cold start conditions, while experimental research on the combined use of these two kinds of catalysts and the coupling mode of the electrically heated catalyst and the aftertreatment system under the cold start condition are lacking. In this paper, under a certain cold start and medium-high temperature phase, the exhaust gas temperature and emission characteristics of PNA, EHC and aftertreatment system under different coupling modes were studied. Results showed that the average inlet temperature of diesel oxidation catalyst (DOC) and selective catalytic reduction catalyst (SCR) integrated into diesel particulate filter (SDPF) increased by 246.35 ℃ and 126.09 ℃ respectively under the coupling mode of PNA+EHC+DOC+ SDPF+SCR during cold start phase. The inlet temperature of DOC was not affected, and the average inlet temperature of SDPF increased by 200.52 ℃ under the coupling mode of PNA+DOC+EHC+SDPF+SCR. In terms of emissions, the effective NOx adsorption range of PNA reached more than 140s under the two coupling modes. During cold start phase, under the coupling mode of PNA+EHC+DOC+SDPF+SCR, the conversion efficiencies of carbon monoxide (CO), total hydrocarbons (THC) and nitrogen oxides (NOx) were 97.58%, 94.62% and 78.81% respectively, compared with EHC off, it had increased by 55.84%, 55.84% and 55.84% respectively. Under the coupling mode of PNA+DOC+EHC+SDPF+SCR, the conversion efficiencies of CO, THC and NOx were 58.68%, 87.50% and 95.42% respectively, compared with EHC off, it had increased by 15.91%, 11.70% and 51.41% respectively. EHC coupled front DOC was more conducive to CO and THC emission control, and EHC coupled rear DOC was more conducive to NOx emission control. The SDPF pressure drop corresponding to EHC coupled front DOC was greater than that of EHC coupled rear DOC, the PN conversion efficiency of EHC coupled front or rear DOC was more than 99%. </div>

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