Study On Emission And Control Strategy Of Diesel Methanol Dual Fuel Engine

As a clean combustion method,diesel/methanol dual fuel combustion（DMDF）has attracted more and more attention from researchers at home and abroad.And our previous study shown that DMDF could simultaneously reduce the emission of PM and NO_x.Coupled with simple after-treatment,DMDF engine could meet the request of ChinaⅤemission regulation.While the emission rule is becoming more and more stringent,which is a great challenge for DMDF engine.Therefore,by using two DMDF engines,this study aims at further controlling pollutant emissions.Combination with in-cylinder combustion,we systematically studied the coupling effect of engine parameters such as methanol substitution percent（MSP）,diesel main injection strategy,diesel pre-injection strategy,intake air temperature,and EGR on PM and NO_x.Based on these researches,we further studied the after-treatment control strategies of PM and NO_x.And non-urea selective reduction technology and diesel particle filter（DPF）regeneration strategy was proposed for DMDF engines.The main results of this study are as followed:The characteristics of soot and particulates under different loads were thoroughly explored.The results shown that,under the high load conditions,the influence of MSP on Soot and PN was affected by the engine intake air temperature.Under the condition of high inlet air temperature,Soot and PN gradually increased with the increase of MSP.The higher the temperature was,the more Soot and PN were produced.Under the low inlet air temperature condition,Soot and PN gradually decreased with the increase of the MSP.The main reason for this phenomenon was that different intake air temperatures lead to different combustion modes within the cylinder.In addition,when the MSP was small,the increase of methanol had a negative effect on Soot and PN.For the controlling of PM emissions,when the DMDF engine was operating at high load condition,higher intake air temperatures should be avoided,and the corresponding MSP should not be less than 20%,especially for lower speeds conditions.Under medium and low load conditions,the effect of methanol on Soot and PN was significantly enhanced by delaying fuel injection time.Which could balance the relationship between Soot and NO_x.In addition,experimental studies shown that the linear relationship between dry smoke and opacity smoke was stronger under pure diesel mode,while the linearity was weaker under DMDF mode.The experiment explored the effect of different engine parameters on DMDF engine.Studies shown that,DMDF was more adaptable to EGR.When EGR was introduced into DMDF engine,NO_x was gradually reduced and at the same time it did not cause significant deterioration of PM,effectively balancing the relationship between PM and NO_x.In addition,when pre-injection was adopted,the pre-injection angle should not be too large.And there is a suitable interval.Although the Soot did not change much when this interval was exceeded,the total number of PN,especially nuclear particles,could increase significantly.Non-urea selective catalytic reduction（SCR）route was proposed:methanol-SCR coupled DMDF to reduce NO_x emissions.The results of Synthetic gas and engine benches proved that methanol-SCR+DMDF was feasible to control NO_x emission.As a theoretical basis,syngas bench tests shown that by using different types of catalysts,methanol could effectively reduce NO.At low temperature（less than 300°C）stage,the conversion of NO by methanol over Mo/γ-Al₂O₃catalyst systems was highest.But in the high temperature stage,its efficiency greatly reduced to a low level.On the contrary,at high temperature stage（more than 300°C）,the conversion of NO by methanol over Co/γ-Al₂O₃ andγ-Al₂O₃ catalyst systems was highest,but in the low temperature stage its efficiency was very low.The combined catalyst Co/γ-Al₂O₃+Mo/γ-Al₂O₃ could simultaneously take into account the high and low temperature catalytic efficiency.Although the catalytic efficiency at low temperature was low,when a certain amount of H₂ was added to the gas distribution,the catalytic efficiency at low temperature can be significantly optimized.The engine test results shown that the conversion efficiency of methanol-SCR was different under different engine loads.Due to the restriction of the active site of the catalysts,the catalytic conversion efficiency of NO_x first increased with the increase of methanol and then tended to be stable.At the same time,due to the actual exhaust conditions,the NO_x conversion efficiency measured by the engine bench was lower than that measured by synthesis gas bench.The results of experiment study shown that using methanol-SCR coupled DMDF could effectively reduce NO_x emissions at different loads and speeds,and NO_x reduction could reach 90%under high loads.The experiment explored the control strategies of PM and DPF regeneration strategies for DMDF engine.Results shown that the particle oxidation catalyst（POC）coupled with DMDF could improve its capture efficiency.On the contrary,the DMDF had little effect on DPF,but the capture efficiency of DPF was still higher than the capture efficiency of POC coupled with DMDF.Compared to conventional diesel engines,the ratio of NO₂/PM of DOC downstream in DMDF was relatively high,it was easier to achieve passive regeneration of the DPF,which was conducive to prolonging the active regeneration cycle.For active regeneration,the DMDF also had significant advantages over the pure diesel mode,mainly because that DMDF coupling with DOC could greatly improve the inlet temperature of DPF,especially in the condition of low load and speed.In order to control the secondary pollution discharge during active regeneration,the control strategy of pre-injection following a main injection was adopted.Experiments show that this strategy could reduce the emission of pollutants such as methanol,formaldehyde and THC.