CN106605048B - 排气后处理装置中的NOx还原控制方法 - Google Patents

排气后处理装置中的NOx还原控制方法 Download PDF

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CN106605048B
CN106605048B CN201580038103.XA CN201580038103A CN106605048B CN 106605048 B CN106605048 B CN 106605048B CN 201580038103 A CN201580038103 A CN 201580038103A CN 106605048 B CN106605048 B CN 106605048B
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exhaust gas
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长冈大治
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Abstract

提供一种即使排气温度低也能够无障碍地进行NOx还原的排气后处理装置中的NOx还原控制方法。在排气管(20)中配置氧化催化剂(25)和LNT催化剂(26)，反复在空燃比为稀燃时吸附或吸收NOx、空燃比为富燃时还原NOx的排气后处理装置中的NOx还原控制方法中，在排气温度低时，进行远后喷射或排气管喷射，使HC预先吸附到氧化催化剂，在富燃时使排气温度上升，使吸附到氧化催化剂(25)的HC脱离而将由LNT催化剂(26)吸附的NOx还原。

Description

排气后处理装置中的NOx还原控制方法
技术领域
本发明涉及使用NOx吸收还原型催化剂的排气后处理装置，尤其涉及使排气后处理装置的氧化催化剂等吸附HC并将其用于NOx还原的排气后处理装置中的NOx还原控制方法。
背景技术
作为柴油引擎的排气后处理装置，DOC(柴油氧化催化剂(Diesel OxidationCatalyst)；氧化催化剂)、DPF(柴油微粒过滤器)、NOx吸收还原型催化剂(LNT：稀燃NOx捕获(Lean NOx Trap)或NSR：NOx存储还原(NOx Storage Reduction)系统等已经实用化。
NOx吸收还原型催化剂是在氧化铝(Al2O3)等催化剂承载体上承载Pt和Pd等贵金属催化剂、Na、K、Cs等碱金属、Ca、Ba等碱土类金属、Y、La、Ce等稀土类等具有NOx吸收功能的吸收材料的催化剂，根据排气中的氧浓度，发挥NOx吸收和NOx排放/净化的两个功能。
基于通过三元催化剂功能将该NOx还原的NOx吸收还原型催化剂(以下称作LNT催化剂)的净化系统，在如通常运转状态这样排气中的氧浓度高的条件(稀燃(lean)空燃比)下，排气中的NO通过Pt和Pd等贵金属催化剂等被氧化为NO2，吸收材料将其作为硝酸盐(Ba(NO3)2)吸收并将NOx净化。
但是，若继续吸收NOx，则由于硝酸盐饱和而丧失吸收材料的吸收功能，因此改变运转条件，在低氧浓度的条件(富燃(rich)空燃比)下进行EGR(Exhaust GasRecirculation：排气再循环)、燃料的远后喷射(post injection)、排气管喷射，形成富燃状态，将燃料在贵金属催化剂上还原，从而使排气中生成CO、HC、H2，将排放的NOx进行还原而净化。
这样的基于LNT催化剂的净化系统中三元催化剂功能发挥作用，即在空燃比稀燃时(氧浓度高的条件)吸附或吸收NOx，在富燃时吸附或吸收的Nox从Ce或Ba释放，排气中的HC、CO和NOx通过三元催化剂功能而成为无害的气体。
现有技术文献
专利文献
专利文献1:(日本)特开2009-002179号公报
专利文献2:(日本)特开2001-050034号公报
专利文献3:(日本)特开2008-240704号公报
发明内容
此时，若相对于排放的NOx，反应所需的HC、CO量较少，则NOx的一部分不被还原而直接作为NOx被排放。
通常HC通过缸内的远后(post)喷射或对排气管的HC给料而被添加到排气中，通过温度或DOC中的催化剂反应而被分解并被供应给催化剂。
但是，在排气和催化剂温度为低温时(200℃以下)，由于供应的未燃燃料被分解为HC需要花费时间，因此富燃时的NOx还原效率降低，NOx容易漏失。
若达到某种程度的高温(例如250℃以上)，则HC的分解加速，容易有助于NOx还原，富燃时的NOx漏失也减少。
该稀燃和富燃的切换如下这样进行：基于设置在LNT催化剂的出入口的NOx传感器的检测值，推定NOx吸附量，或者，根据基于引擎的运转状态即燃料喷射量的NOx浓度和该排气流量，基于图来求出NOx量，并将该NOx量根据运转状态来累积，从而推定NOx吸附量，在NOx吸附量超过了设定值时，从稀燃切换为富燃，在NOx还原量成为阈值以下时，从富燃切换为稀燃。
但是，如上所述，存在若排气温度为200℃以下而低于催化剂活性温度，则即使从稀燃切换为富燃也无法充分进行NOx还原的问题。
因此，本发明的目的在于提供一种解决上述课题，即使排气温度低，也能够没有障碍地进行NOx还原的排气后处理装置中的NOx还原控制方法。
用于解决课题的手段
为了达到上述目的，本发明是一种排气后处理装置中的NOx还原控制方法，所述排气后处理装置在排气管中配置氧化催化剂和LNT催化剂，并反复在空燃比为稀燃时吸附或吸收NOx、在空燃比为富燃时还原NOx，所述NOx还原控制方法的特征在于，在排气温度低时，进行远后喷射或排气管喷射，使HC预先吸附到氧化催化剂，在富燃时使排气温度上升，使吸附到氧化催化剂的HC脱离而将由LNT催化剂吸附的NOx还原。
附图说明
图1是实施本发明的排气后处理装置中的NOx还原控制方法的装置的概略图。
图2是表示本发明的排气后处理装置中的NOx还原控制方法的流程图。
具体实施方式
以下，基于附图详述本发明的优选的一个实施方式。
图1表示基于LNT催化剂的排气后处理装置10。
在引擎E的吸排气系统中连接有涡轮增压器11和EGR管12，从空气滤清器13吸入的空气由涡轮增压器11的压缩机14压缩并被压送到吸气通道15，从引擎E的吸气岐管16被供应到引擎E内。在吸气通道15中设有用于调节输送给引擎E的空气量的吸气阀17。
从引擎E排出的排气从排气岐管18被排出到涡轮增压器11的涡轮19并同时驱动涡轮19，并且被排气到排气管20。
吸气岐管16和排气岐管18上连接EGR管12，EGR管12上连接用于冷却从排气岐管18到吸气岐管16的排气的EGR冷却器21，并且连接用于调节EGR量的EGR阀22。
排气后处理装置10如下这样构成：在涡轮19的下游侧的排气管20上设有排气管喷射器23，在该排气管喷射器23的下游侧的排气管20上所形成的罐装容器24内，依次罐装DOC(氧化催化剂)25、LNT催化剂26、DPF27。
在DOC25的上游侧设有DOC前的排气温度传感器28，在LNT催化剂26的入口侧设有DOC后的排气温度传感器29，在出口侧设有NOx传感器30。
引擎E由ECU32进行运转的整体控制。在ECU32中形成NOx吸附量推定部件33、HC吸附量推定部件34、NOx还原量推定部件35。
ECU32进行在空燃比稀燃状态下由LNT催化剂26进行NOx吸收的稀燃循环，以及在NOx吸收率降低时通过缸内的远后喷射或者通过图示的排气管喷射器23脉冲地喷射燃料HC，从而在空燃比富燃状态下进行NOx还原净化的富燃循环。
该稀燃和富燃的切换是在稀燃燃烧时由NOx吸附量推定部件33推定LNT催化剂26所吸附的NOx吸附量，在该NOx吸附量达到设定值时，切换为富燃燃烧。
NOx吸附量推定部件33根据基于NOx浓度和排气流量的NOx量图等，求出从引擎的运转状态起排出的NOx量，将其进行累积从而推定LNT催化剂26中的NOx吸附量，或者基于NOx传感器30的检测值而计算NOx吸附量。
ECU32在NOx吸附量小于设定值时通过稀燃循环控制引擎的燃烧，并在NOx吸附量为设定值以上时，通过富燃循环控制引擎的燃烧。
另外，在本发明中，ECU32在稀燃循环时，在由DOC前的排气温度传感器28检测到的排气温度小于催化剂活性温度(约200℃)时，通过远后喷射或排气管喷射器23喷射燃料。
由此，未燃燃料被DOC25吸附，由该DOC25分解为HC并被吸附。该HC吸附量通过HC吸附量推定部件34基于远后喷射量或由排气管喷射器23喷射的燃料喷射量而计算。
此外，在富燃循环中，NOx还原量推定部件35计算通过HC还原NOx时的NOx还原量，并在NOx还原量成为阈值以下时，切换为稀燃循环。
在通常的富燃中，在低温时，由于HC分解要消耗时间，因此催化剂的NOx还原效率会降低，容易发生NOx的漏失。
在本发明中，在低温时通过远后喷射或排气管喷射而对排气管20供应燃料(HC)，使其预先吸附到DOC25。该预先吸附到DOC25的HC在超过排气温度200℃时从DOC25脱离，容易利用于NOx还原，即使在低温下，富燃时的NOx还原也会进行，能够减少NOx漏失。
这样，低温富燃时的NOx还原通过利用预先吸附到催化剂的HC，从而引擎侧能够专注于排气的升温和设立富燃空燃比，因此能够期待通过升温来提高还原效率的效果。
DOC25和LNT催化剂26在200℃以下的低温时，具有吸附NOx和HC的作用。因此，在稀燃燃烧中，在排气为低温时，在排气中供应HC，并且使其主要吸附到DOC25，同时未被吸附的HC由LNT催化剂26吸附。
若该HC的吸附量过多则在升温时存在异常升温的情况，故由HC吸附量推定部件34推定HC吸附量，并设置该HC吸附量的阈值，若达到了阈值，则停止远后喷射或排气管喷射，从而防止过分吸附。
若超过DOC25或LNT催化剂26的活性温度(例如200℃)，则进行富燃还原。此时的富燃的目的在于，通过远后(POST)喷射使燃料在缸内燃烧而使排气温度升温，以及使排气成为富燃空燃比。即，由于目的不在于以往那样的向排气管供应HC，因此关于喷射定时，使远后喷射接近主喷射(例如45°BTDC以内)。富燃时的扭矩为了与通常燃烧匹配，而对主喷射量和远后喷射量进行校准，从而使得没有异常感。此时，作为燃烧辅助，也可以使用预热塞通电。
若升温的排气到达催化剂表面，则分子运动变得活跃，被吸附到DOC25的HC或被吸附到LNT催化剂26的NOx被从催化剂释放，在富燃空燃比下还原NOx。
若进行数次的富燃，则催化剂的温度成为高温(250℃以上)，HC也全部被释放而利用于NOx还原，因此基于由HC吸附量推定部件34推定的HC吸附量，若脱离排放的HC减少，则变更富燃的燃烧方法，使远后喷射推迟(例如曲柄角150°BTDC)，从引擎向催化剂供应HC。由于催化剂为高温，因此HC的分解也在早期进行，所以通过来自引擎的HC供应也能够进行富燃时的NOx还原。
接着，通过图2的流程说明上述NOx还原控制方法。
在步骤S10中，控制开始后，在步骤S11中，在DOC前的排气温度T1<催化剂活性温度(约200℃程度)时，通过远后喷射或向排气管的HC给料，来向排气气体中供应HC。
接着，在步骤S12中，判断是否DOC前的排气温度T1＞催化剂活性温度(约200℃程度)，在DOC前的排气温度T1小于催化剂活性温度(条件不符合；否)时，返回步骤S11，一边继续向排气气体中供应HC，一边累积该HC，并推定向催化剂的HC吸附量。在该步骤S11、S12中，若HC吸附达到阈值(设定值)，则停止供应HC。
在步骤S12的判断中，若DOC前的排气温度T1超过催化剂活性温度(约200℃程度)(条件符合；是)，则转移到步骤S13，在此期间若根据NOx吸附量等而从ECU发出实施富燃的指示，则进行富燃还原。此时，喷射的定时由于不是为了对排气管供应HC、而是为了使排气温度上升而进行的，因此使远后喷射接近主喷射(例如曲柄角45°以内)地进行，大致以后喷射的状态进行喷射。此外，在该情况下，也可以通过预热塞通电进行燃烧辅助而使排气温度升温。
接着，在步骤S14的判断中，判断是否DOC后的排气温度T2＞燃料分解温度(250℃以上)，在DOC后的排气温度T2没有超过燃料分解温度时、或HC吸附量没有超过阈值时(条件不符合)，返回到步骤S13，继续远后喷射，从而提高排气温度，并且通过步骤S13，由吸附的HC进行NOx还原，其间，在步骤S14中，在DOC后的排气温度T2超过燃料分解温度，并且HC吸附量为阈值以下时(条件符合)，在步骤S15中，变更富燃燃烧方法，推迟远后喷射(例如曲柄角150°BTDC)，采用通常的远后喷射，从引擎对催化剂供应HC，从而进行富燃还原。
接着，在步骤S16的判断中，判断NOx还原量是否达到阈值以下，在没有达到时(条件不符合)，返回到步骤S15而继续富燃还原，在NOx还原量达到阈值以下时(条件符合)，在步骤S17中结束控制，并再次返回最初的控制开始。
这样，本发明在排气温度低时，预先进行远后喷射或排气管喷射，使未燃燃料吸附到DOC25等，由此，吸附的未燃燃料即使环境温度低也被催化剂分解为HC，在富燃还原时，通过将排气温度设为200℃以上，从而HC脱离，能够通过该HC将NOx还原，并且能够防止低温时的富燃还原下的NOx漏失。

Claims (4)

1.一种排气后处理装置中的NOx还原控制方法，所述排气后处理装置在排气管中配置氧化催化剂和比所述氧化催化剂靠下游侧的LNT催化剂，并反复在空燃比为稀燃时吸附或吸收NOx、在空燃比为富燃时还原NOx，所述NOx还原控制方法的特征在于，
所述氧化催化剂在排气温度低于催化剂活性温度时吸附HC，并推定向所述氧化催化剂的HC吸附量，若HC吸附量达到阈值，则停止供应HC，并在所述排气温度为所述催化剂活性温度以上时使所吸附的HC脱离，
在所述排气温度低于所述催化剂活性温度时，进行燃料的远后喷射或燃料的排气管喷射，使HC预先吸附到氧化催化剂，在使HC吸附于所述氧化催化剂后，在排气温度超过所述催化剂活性温度并且根据NOx吸附量而发出实施富燃的指示时，使排气温度上升，使吸附到氧化催化剂的HC脱离而将由LNT催化剂吸附的NOx还原。
2.如权利要求1所述的排气后处理装置中的NOx还原控制方法，其中，
在排气温度为200℃以下时，进行远后喷射或排气管喷射，在排气温度超过所述催化剂活性温度、基于LNT催化剂中的NOx吸附量而将空燃比设为富燃时，接着主喷射进行远后喷射，或者通过预热塞通电而使排气温度上升，使吸附到氧化催化剂的HC脱离从而将NOx还原。
3.如权利要求2所述的排气后处理装置中的NOx还原控制方法，其中，
在氧化催化剂中的HC吸附量成为阈值以下时，使远后喷射推迟，或者进行排气管喷射而将HC供应给LNT催化剂，进行通常的NOx还原。
4.如权利要求3所述的排气后处理装置中的NOx还原控制方法，其中，
在NOx还原量成为阈值以下时，将空燃比设为稀燃而吸附或吸收NOx。
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