CN101600862A - 压缩点火式内燃机的排气净化装置 - Google Patents

压缩点火式内燃机的排气净化装置 Download PDF

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CN101600862A
CN101600862A CNA2008800039860A CN200880003986A CN101600862A CN 101600862 A CN101600862 A CN 101600862A CN A2008800039860 A CNA2008800039860 A CN A2008800039860A CN 200880003986 A CN200880003986 A CN 200880003986A CN 101600862 A CN101600862 A CN 101600862A
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selective reduction
temperature
reduction catalyzer
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CN101600862B (zh
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利冈俊祐
小田富久
伊藤丈和
伊藤和浩
田内丰
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Abstract

在内燃机上，在内燃机排气通路内配置NOX选择还原催化剂(15)，在NOX选择还原催化剂(15)上游的内燃机排气通路内配置氧化催化剂(12)。在内燃机起动时，由HC供给阀(28)向氧化催化剂(12)供给HC，利用HC的氧化反应热使NOX选择还原催化剂(15)升温。此时，使NOX选择还原催化剂(15)的温度上升到HC从NOX选择还原催化剂(15)脱离的HC脱离温度范围。

Description

压缩点火式内燃机的排气净化装置
技术领域
本发明涉及压缩点火式内燃机的排气净化装置。
背景技术
已公知一种内燃机，其在内燃机排气通路内配置NOX选择还原催化剂，在NOX选择还原催化剂上游的内燃机排气通路内配置氧化催化剂，向NOX选择还原催化剂供给尿素，利用由尿素产生的氨选择性地还原排气中含有的NOX(例如参照特开2005-23921号公报)。在该内燃机中，氨吸着于NOX选择还原催化剂，吸着的氨与排气中含有的NOX反应，使NOX被还原。
可是，在该内燃机中，在内燃机起动时向氧化催化剂供给HC，利用HC的氧化反应热使NOX选择还原催化剂升温的情况下，若为了将NOX选择还原催化剂早期地暖机而供给大量的HC，则在氧化催化剂中未被氧化掉的HC流入NOX选择还原催化剂，并附着于NOX选择还原催化剂。可是，如果HC附着于NOX选择还原催化剂，则氨不能吸着于NOX选择还原催化剂，这样一来就产生NOX净化率降低的问题。
与此相对，若减少HC的供给量以避免HC附着于NOX选择还原催化剂，即避免NOX选择还原催化剂遭受HC中毒的话，则NOX选择还原催化剂升温需要时间，这样一来在该情况下会产生NOX净化率降低的问题。
发明内容
本发明的目的在于，提供在内燃机起动时可得到良好的NOX净化率的压缩点火式内燃机的排气净化装置。
根据本发明，提供一种压缩点火式内燃机的排气净化装置，其在内燃机排气通路内配置NOX选择还原催化剂，在NOX选择还原催化剂上游的内燃机排气通路内配置氧化催化剂，向NOX选择还原催化剂供给尿素，利用由该尿素产生的氨选择性地还原排气中含有的NOX，其中，在内燃机起动时向氧化催化剂供给HC，利用HC的氧化反应热使NOX选择还原催化剂升温，此时，使NOX选择还原催化剂的温度上升到HC从NOX选择还原催化剂脱离的HC脱离温度范围。
通过这样地使NOX选择还原催化剂的温度上升到HC脱离温度范围，可消除NOX选择还原催化剂的HC中毒，这样一来可得到良好的NOX净化率。
附图说明
图1是压缩点火式内燃机的总体图；图2是表示压缩点火式内燃机的另一实施例的总体图；图3是表示氧化速度和脱离速度的图；图4是表示暖机控制的时间图；图5是用于进行暖机控制的流程图。
具体实施方式
图1示出压缩点火式内燃机的总体图。
参照图1，各标号分别表示如下：1-内燃机主体、2-各气缸的燃烧室、3-用于向各燃烧室2内分别喷射燃料的电子控制式燃料喷射阀、4-吸气歧管、5-排气歧管。吸气歧管4借助于吸气导管6与排气涡轮增压器7的压缩机7a的出口连结，压缩机7a的入口借助于吸入空气量检测器8与空气滤清器9连结。在吸气导管6内配置由步进电动机驱动的节气门10，而且在吸气导管6周围配置用于将在吸气导管6内流动的吸入空气冷却的冷却装置11。在图1所示的实施例中，内燃机冷却水被导入冷却装置11内，由内燃机冷却水将吸入空气冷却。
另一方面，排气歧管5与排气涡轮增压器7的排气涡轮7b的入口连结，排气涡轮7b的出口与氧化催化剂12的入口连结。在该氧化催化剂12的下游，与氧化催化剂12相邻地配置用于捕集排气中含有的粒子状物质的微粒过滤器13，该微粒过滤器13的出口，借助于排气管14与NOX选择还原催化剂15的入口连结。氧化催化剂16与该NOX选择还原催化剂15的出口连结。
在NOX选择还原催化剂15上游的排气管14内配置尿素水供给阀17，该尿素水供给阀17借助于供给管18、供给泵19与尿素水罐20连结。贮藏于尿素水罐20内的尿素水，利用供给泵19从尿素水供给阀17喷射到在排气管14内流动的排气中，利用由尿素产生的氨((NH2)2CO+H2O→2NH3+CO2)将排气中含有的NOX在NOX选择还原催化剂15中还原。
排气歧管5和吸气歧管4，借助于排气再循环(以下称为EGR)通路21相互连结，在EGR通路21内配置电子控制式EGR控制阀22。另外，在EGR通路21周围配置用于将在EGR通路21内流动的EGR气体冷却的冷却装置23。在图1所示的实施例中，内燃机冷却水被导入冷却装置23内，由内燃机冷却水将EGR气体冷却。另一方面，各燃料喷射阀3借助于燃料供给管24与共轨25连结，该共轨25借助于电子控制式的喷出量可变的燃料泵26与燃料箱27连结。贮藏于燃料箱27内的燃料，利用燃料泵26供给到共轨25内，供给到共轨25内的燃料，借助于各燃料供给管24供给到燃料喷射阀3。另外，在排气歧管5上配置用于向排气歧管5内供给氢、即供给HC的HC供给阀28。在图1所示的实施例中，该HC包括轻油。
电子控制单元30包括数字计算机，具备由双向性总线31相互连接的ROM(只读存储器)32、RAM(随机读取存储器)33、CPU(微处理器)34、输入端口35以及输出端口36。在氧化催化剂12上安装有用于检测氧化催化剂12的床温的温度传感器45，在NOX选择还原催化剂15上安装有用于检测NOX选择还原催化剂15的床温的温度传感器46。这些温度传感器45、46和吸入空气量检测器8的输出信号通过各自对应的AD转换器37输入到输入端口35。
另外，在油门踏板40上连接产生与油门踏板40的踏下量L成比例的输出电压的负荷传感器41，负荷传感器41的输出电压通过对应的AD转换器37输入到输入端口35。而且，在输入端口35上连接每当曲轴旋转例如15°就产生输出脉冲的曲轴转角传感器42。另一方面，输出端口36借助于对应的驱动电路38与燃料喷射阀3、节气门10的驱动用步进电动机、尿素水供给阀17、供给泵19、EGR控制阀22、燃料泵26以及HC供给阀28连接。
氧化催化剂12担载了例如铂之类的贵金属催化剂，该氧化催化剂12起到将排气中含有的NO转化为NO2的作用和使排气中含有的HC氧化的作用。即，NO2的氧化性比NO强，因此若使NO转化成NO2，则在微粒过滤器13上捕获的粒子状物质的氧化反应得到促进，并且，在NOX选择还原催化剂15中的由氨带来的还原作用得到促进。另一方面，NOX选择还原催化剂15，如上述那样附着HC时，氨的吸着量减少，因此NOX净化率降低。因此，通过这样地利用氧化催化剂12将HC氧化，来避免在NOX选择还原催化剂15上附着HC，即避免NOX选择还原催化剂15产生HC中毒。
作为微粒过滤器13，可以使用没有担载催化剂的微粒过滤器，也可以使用担载了例如铂之类的贵金属催化剂的微粒过滤器。另一方面，NOX选择还原催化剂15，由在低温下具有高的NOX净化率的氨吸着型的Fe沸石构成。另外，氧化催化剂16，担载了例如由铂制成的贵金属催化剂，该氧化催化剂16起到将从NOX选择还原催化剂15漏出的氨氧化的作用。
图2示出压缩点火式内燃机的另一实施例。在该实施例中，微粒过滤器13配置于氧化催化剂16的下游，因此在该实施例中，氧化催化剂12的出口借助于排气管14与NOX选择还原催化剂15的入口连结。
若NOX选择还原催化剂15的温度未上升某种程度，则不进行NOX的选择还原作用，即未活化，因此在内燃机起动时必须尽可能快地将NOX选择还原催化剂15活化。因此，在本发明中，在内燃机起动时向氧化催化剂12供给HC，利用HC的氧化反应热使NOX选择还原催化剂15升温。该HC的供给，例如可以通过在排气行程中向燃烧室2内喷射燃料来进行，也可以通过向内燃机排气通路内供给HC来进行。在图1和图2所示的实施例中，该HC的供给通过由HC供给阀28喷射轻油来进行。
然而，在内燃机起动时，在氧化催化剂12中，未必能够使所供给的全部的HC氧化。对于该情况，参照图3(A)进行说明。图3(A)示出氧化催化剂12的床温TO与HC的氧化速度MO(g/秒)即与每单位时间可氧化的HC的量的关系。
由图3(A)可知，在氧化催化剂12的床温TO为约200℃以下时，即氧化催化剂12没有活化时，氧化速度MO为零，因此此时，流入氧化催化剂12内的HC从氧化催化剂12逃过去。另一方面，在氧化催化剂12已活化时，每单位时间流入氧化催化剂12的HC的量，比由氧化催化剂12的床温TO确定的氧化速度MO少时，全部的流入的HC在氧化催化剂12中被氧化，每单位时间流入氧化催化剂12的HC的量，比由氧化催化剂12的床温TO确定的氧化速度MO多时，超过氧化速度MO的部分的HC从氧化催化剂12中逃过去。
从氧化催化剂12中逃过去的HC，流入NOX选择还原催化剂15，附着于NOX选择还原催化剂15上。然而，该附着的HC，通过使NOX选择还原催化剂15升温，可从NOX选择还原催化剂15脱离。接着关于该情况，参照图3(B)来说明。
图3(B)示出NOX选择还原催化剂15的床温Tn与HC的脱离速度Md(g/秒)即与每单位时间从NOX选择还原催化剂15脱离的HC的量的关系。如图3(B)所示，当NOX选择还原催化剂15的床温Tn超过约350℃时，脱离速度Md上升，在图3(B)中，由TF表示的约350℃为脱离开始温度。因此若使NOX选择还原催化剂15的床温Tn上升到脱离开始温度TF以上，则可使HC从NOX选择还原催化剂15脱离。
为使NOX选择还原催化剂15早期地升温，由HC供给阀28供给大量的HC即可。然而，若供给大量的HC，则HC从氧化催化剂12中逃过去，NOX选择还原催化剂15会产生HC中毒。然而，如果使NOX选择还原催化剂15的温度上升到脱离开始温度TF以上的HC脱离温度范围，则可消除HC中毒。因此在本发明中，在内燃机起动时使NOX选择还原催化剂15的温度上升到HC从NOX选择还原催化剂15脱离的HC脱离温度范围。
接着参照图4对本发明涉及的NOX选择还原催化剂15的暖机控制进行说明。
内燃机一起动，大量的未燃HC就从燃烧室2排出，因此如图4所示，在内燃机起动时，从燃烧室2排出的HC的量GO暂时性地变高。通常，此时，氧化催化剂12没有活化，因此该排出的HC从氧化催化剂12中逃过去。该逃过去的HC附着于NOX选择还原催化剂15上，因此由图4可知，逃过去的HC量W加算到NOX选择还原催化剂15的HC附着量∑HC中。
接着，若氧化催化剂12的床温TO超过活化温度TX，则从HC供给阀28开始供给HC，该HC供给量的变化以GI表示。即，使HC供给量GI一点一点地减少，以使氧化催化剂12的床温TO平滑地接近于目标的温度。由于HC供给量GI为大量，因此大量的HC从氧化催化剂12中逃过去，但氧化催化剂12的床温TO越上升，在氧化催化剂12中被氧化的HC量越增大，因此如图4所示，逃过去的HC量W随着时间迁移逐渐地减少。
另一方面，由于NOX选择还原催化剂15被在氧化催化剂12中升温了的排气加热，因此在图4中，如实线所示，比氧化催化剂12晚地进行温度上升。在NOX选择还原催化剂15的床温Tn低于脱离开始温度TF时，逃过去的HC量W加算到HC附着量∑HC中，因此附着量∑HC逐渐增大。然而，NOX选择还原催化剂15的床温Tn一超过脱离开始温度TF，就开始HC从NOX选择还原催化剂15脱离的作用，因此HC附着量∑HC逐渐减少。
接着，若HC附着量∑HC为设定值HCX以下，则停止HC的供给。即，在该实施例中，计算附着于NOX选择还原催化剂15上的HC量，在算出的HC量∑HC为预定的设定值HCX以下时，停止HC的供给。
另外，NOX选择还原催化剂15的床温TO的可上升的最大极限温度，考虑到热老化，为650°左右。因此，在本发明的实施例中，在内燃机起动时，使NOX选择还原催化剂15的温度上升到350℃～650℃的范围内。
另一方面，在图4中，由虚线表示的NOX选择还原催化剂15的床温Tn的变化，显示了以往的升温控制时的变化。在内燃机中，在暖机运行中，怠速运行持续进行时，存在NOX选择还原催化剂15的温度最终收敛的收敛温度Tf，该收敛温度Tf为200℃～250℃左右。在以往的升温控制中，NOX选择还原催化剂15的床温Tn，如虚线所示，以朝向该收敛温度Tf平滑地上升的方式变化。
与此相对，可知在本发明中，在内燃机起动时使NOX选择还原催化剂15的床温Tn升温到比收敛温度Tf高100℃以上的350℃以上。
接着对图5所示的暖机控制程序进行说明。另外，该控制程序通过每隔一定时间的中断来执行。
参照图5，首先，起初在步骤50中计算排出HC量GO。相应于该内燃机的运行状态而变化的排出HC量GO预先存储于ROM32内。接着在步骤51中，判断氧化催化剂12的床温TO是否超过了活化温度TX。当TO≤TF时，进入步骤52，逃过去的量W设为排出HC量GO，接着进入步骤59。
与此相对，在TO＞TX时，进入步骤53，算出HC供给量GI。接着在步骤54中，由HC供给阀28进行HC供给控制。接着在步骤55中，基于氧化催化剂12的床温TO，计算图3(A)中所示的氧化速度MO。接着在步骤56中，判断氧化速度MO是否大于排出HC量GO与供给HC量GI之和(GO+GI)。当MO≥GO+GI时，进入步骤57，逃过去的量W设为零，接着进入步骤59。与此相对，当MO＜GO+GI时，进入步骤58，逃过去的量W为GO+GI-MO，接着进入步骤59。
在步骤59中，基于NOX选择还原催化剂15的床温Tn，计算图3(B)中所示的脱离速度Md，接着在步骤60中，在HC附着量∑HC中加上逃过去的量W，从HC附着量∑HC减去脱离速度Md，由此算出HC附着量∑HC。接着在步骤61中，判断HC附着量∑HC是否在减少中，当为减少中时，进入步骤62，判断HC附着量∑HC是否低于设定值HCX。在∑HC＜HCX时，进入步骤63，停止HC的供给。
本发明中表示数值范围的“以上”和“以下”均包括本数。

Claims (4)

1、一种压缩点火式内燃机的排气净化装置，其在内燃机排气通路内配置NOX选择还原催化剂，在该NOX选择还原催化剂上游的内燃机排气通路内配置氧化催化剂，向该NOX选择还原催化剂供给尿素，利用由该尿素产生的氨选择性地还原排气中含有的NOX，其中，在内燃机起动时向所述氧化催化剂供给HC，利用HC的氧化反应热使NOX选择还原催化剂升温，此时使NOX选择还原催化剂的温度上升到HC从NOX选择还原催化剂脱离的HC脱离温度范围。
2、根据权利要求1所述的压缩点火式内燃机的排气净化装置，其中，在所述NOX选择还原催化剂升温时，使该NOX选择还原催化剂的温度上升到350℃～650℃的范围内。
3、根据权利要求1所述的压缩点火式内燃机的排气净化装置，其中，在暖机运行中，怠速运行持续进行时，存在NOX选择还原催化剂的温度最终收敛的收敛温度，在所述NOX选择还原催化剂升温时，使该NOX选择还原催化剂的温度上升到比该收敛温度高100℃以上的温度。
4、根据权利要求1所述的压缩点火式内燃机的排气净化装置，其中，计算附着于所述NOX选择还原催化剂的HC量，在算出的HC量为预定的设定值以下时，停止HC的供给。
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