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(一)国四的热闹与爱科的门道

2022-11-24 08:35:55 来源:爱科中国 阅读量:10750 评论

导读:环境污染问题日益凸显,那么为了我们赖以生存的大气环保,对排放作出限制就势在必行。凡事要实行也必须有个标准,那么我们就从标准看起。

  【农机网 明星企业】1892年,狄塞尔(Diesel)成功发明内燃式柴油机,这也就是1000多年前的唐朝有位狄大人一直说的“必有隐情”的引擎。为了表彰狄塞尔的伟大功勋,不仅多国发行了他的人物邮票,也更是直接以他的名字代表了“柴油”,以此纪念。
 
  100多年一来,柴油机的技术水平不断提高,这为人民生活水平的不断提高,可谓是发挥了巨大贡献。时至今日,制约柴油机的好像不再是功率、热效率、压缩比等等问题,而偏偏是排气,你说此事蹊跷不蹊跷。
 
  环境污染问题日益凸显,那么为了我们赖以生存的大气环保,对排放作出限制就势在必行。凡事要实行也必须有个标准,那么我们就从标准看起。
 
非道路移动机械用柴油机排气污染物排放指南(部分)
 
  在说国几之前,我们先看看全球,站的高才能看的远。目前全球主流的排放有三种:
 
  第一种欧洲,这也是我们一直的主要参照。
 
  第二种美国,他们执行自己的一套标准,强大到一定程度自己就是标准了。
 
  第三种还处于奔放状态几乎无标准,比如非洲。
 
  从上面的表上大家看阿拉伯数字能简单的看出来,我们的国四≈欧四,但加入了PN这一项呢又是在向欧五靠拢了。欧洲从2019年开始实行欧五,有消息称我们2027年会有国五,争取逐步赶上。不过目前我们的邻居韩日实行的还是欧四,所以至少在这一块的排放上,我们国四后对韩日马上就硬气了,不像现在的世界杯,我们只能做纯看客。
 
  阿拉伯数字看完以后,就该看英文字母了,事物总是这么的循序渐进,然后方能渐入佳境。先进行名词解释:
 
  CO:这个大家应该都认识,一氧化碳是煤气的主要成分,可以使人窒息中毒。
 
  HC:烃(tīng ),碳氢化合物,它也是柴油本身的重要组成部分。它不是一种物质,是一类物质的统称。既然是一类物质,那么它的危害也是多种多样,主要是会致癌。
 
  NOx:氮氧化物,于环境来说它会形成酸雨或者和HC结合形成光化学烟雾。对人直接来说,主要会引起呼吸系统疾病。有时候我们听到的“笑气”也属于氮氧化物。
 
  PM:这个一直比较热门,于柴油机来讲主要就是碳烟微粒。如果小时候烧过灶头的会有直观感受,危害也就不用讲了。
 
  PN:这个说的是上面PM的数量,就是有多少个微小颗粒。不仅要以质量来控制颗粒的总量,还要要求颗粒数量也不能超,两手抓,两手硬。这也是欧五和欧四的最主要区别,我们一步到位在国四上直接享受了。
 
  既然有这么多的东西有这么多的危害,那怎么办呢。无非两点,要么发动机本身少排要么排出来后再进行处理,而国四的后处理系统就是在这样的背景下应运而生的。
 
  上图是目前常规的全套后处理方案,并且顺序不能乱,一定是1234,但并不是说所有的发动机都一定同时用上了这四个方式。
 
  目前主要是两种路线,马力稍微小一点的采用123,马力大点的就得是234。我们可以再回上去看下标准表,毕竟不同马力段国四排放还是有差异的,比如小于130KW的排放中NOx的标准相对还是要低一些的。
 
  既然是两种方案,那肯定是有区别的。我们先看下123这个方案,其中采用中的EGR模块,俗称废气再循环,循环这个词比较好听,但我觉得不甚恰当,这里叫做废气再回炉可能更直观。简单讲就是让燃烧后的废气其中一部分继续进入缸套再次参与燃烧。这样的话,对排放污染物肯定有帮助,也就是排气管肯定开心些。有人开心就有人不开心,对发动机本身而言,就难说了。
 
  首先,它会增加油耗,一般来讲5%左右肯定是有的。另外也会一定程度上影响性能,有动态响应试验表明,在加载1S后,EGR车型输出扭矩要比普通车型低16%。其次,由于其没有采用SCR,那么为了防止产生过多的NOx,必须控制燃烧温度不能过高。因为温度越高、压力越大,氧气越充分,NOx越多。
 
  可问题是,地球人都知道,在一定工况条件下,温度越高、压力越大,氧气越充分,功率才越大。不管是柴油机还是飞机火箭都是如此。不仅仅如此,如果排气温度相对偏低,也不利于DOC和DPF的工作,如下图能明显看到,红色区域为DOC的高效率区,而这些区域都在相对的高温区。DPF也是如此,其被动再生的温度必须维持在250以上,如果不能维持温度,那么较容易被碳烟微粒堵塞。之前沸沸扬扬某汽车的堵塞有很大因素可能就是温度不够。
 
DOC的工作效率与温度关系图
 
  (纵轴为效率/横轴为温度)
 
  最后,凡事有弊也必有利,只在权衡间。由于123方案,没有SCR这个装置,也就不存在这个部件的故障,整车价格会相对低些。另外不需要加尿素,心理上的使用成本会低些,请注意“心理上”。毕竟人的认知除了客观上,心理上有时候会占据上风。就像歌里唱的“明明知道不可以,却偏偏自己骗自己”。大家要多做“明明”,少做“偏偏”,因为输不起。
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