汽車排放的廢氣主要由一氧化碳(CO)、碳氫化合物(HC)和氮氧化合物(NOX)等組成，它們在空氣中積累到一定程度後在太陽光線的作用下，氮氣化合物和碳氫化合物會起反應，生成含有二氧化氮(NO2)和臭氧(O3)的光化學煙霧，這兩種物質均難溶於水，被吸入人體會長驅直入到肺部，濃度大時可引起中毒性水腫，進入血液可形成變性血紅蛋白，使組織缺氧，對人們健康危害極大。1955年美國洛杉磯市曾被光化學煙霧籠罩幾天，幾千人受害，三百人死亡。另外，氫氧化合物進入大氣後會形成酸雨，危害生態環境。所以，隨著汽車數量不斷增加，世界各國日益重視汽車廢氣的排放問題。從70年代以來，歐美各國針對汽車排放問題相繼制定了法律條例，控制汽車廢氣排放。這種政府行為促進了汽車排氣淨化裝置的發展和應用，一種比較簡便和有效的催化式排氣淨化器也就應運而生了。
催化式排氣淨化器的關鍵在於「催化」，也就是利用催化劑對汽車的廢氣進行淨化，將廢氣中的有害物質轉化為無害物質。早在70年代中期，美國已經實行了這種方法，以後被各國汽車業廣泛使用，到目前為止仍是最有效的淨化方法。催化式排氣淨化器有氧化型、雙床型、三元型等多種型式，其中最常用的是三元型催化式淨化器。歐共體規定從1993年1月開始，在歐共體各國出售的汽油發動機新車一律要配置三元型催化式淨化器。
三元型催化式淨化器的外形象一個排氣消聲器，實際上也起到消聲器的作用。殼體用耐高溫的不銹鋼製成，內部的蜂巢式通道上塗有催化劑，催化劑的成份有鉑、鈀和銠等稀土金屬，當汽車廢氣通過淨化器的通道時，一氧化碳和碳氫化合物就會在催化劑鉑與鈀的作用下，與空氣中的氧發生反應產生無害的水和二氧化碳，而氮氧化合物則在催化劑銠的作用下被還原為無害的氧和氮。所謂三元型催化式淨化器是指汽車廢氣只要通過淨化器本身，就可同時將廢氣中的三種主要有害物質轉化為無害物質的一種高效率淨化器。為了充分發揮三元催化劑的效率，必須要將汽車發動機的空燃比(充入氣缸的空氣量與進入氣缸燃油量的重量比值)要接近理論上的比值，其空燃比只能在很小的範圍內變動，否則就不能同時對廢氣中的三種有害物質進行淨化。所以，三元型催化式淨化器要與車上的電子計算機控制系統連在一起使用，用氧傳感器檢測排氣中的氧濃度，將信息反饋到計算機，再由計算機去控制空燃比。
三元型催化式排氣淨化器是安裝在汽車發動機的排氣裝置上，其淨化效率十分高，可以淨化90%以上的有害物質，是現代轎車上一種新的裝置。當然，催化式排氣淨化器也不是全能的，它只能適用於無鉛汽油做燃料的汽車，因為使用含鉛汽油，廢氣中的鉛就會覆蓋住催化劑，使淨化器停止工作而不起任何作用，俗稱「中毒」。因此，汽車使用三元型催化式排氣淨化器的前提條件有二個：一是要用無鉛汽油，二是發動機要使用電控燃油噴射裝置，這樣，三元型催化式排氣淨化器才能起到淨化效果。而且，三元型催化式排氣淨化器的技術較高，蜂巢式通道上的催化劑塗層如果展開的話，足有兩個足球場面積那麼大，製造工藝相當嚴格，但由於有良好的廢氣轉化效果，因此得到廣泛的應用。最近，北京執行了新的輕型汽車排放標準，上海及其它一些大城市也將會公佈類此汽車排放標準，三元型催化式排氣淨化器在中國將會被大量使用.作者: uyk80x0z 時間: 2017-11-27 17:53 標題: colored yeti cups Friday How do I get started faster
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