2017年9月，Euro 6c輕型發(fā)動機排放限值規(guī)定正式出臺后，無論是汽油機還是柴油機都將面臨一系列挑戰(zhàn)。其中的一個重要難題就是要求柴油發(fā)動機的選擇性催化還原（SCR）催化劑加快起燃速度。
在當(dāng)前的系統(tǒng)中，一臺柴油機的SCR催化劑需要花費10-12分鐘才能達到起燃溫度。這在很大程度上是由尾氣后處理系統(tǒng)的封裝所決定的。
在當(dāng)前的Euro 6b體系中，柴油顆粒捕集器（DPF）可能是發(fā)動機下游的第一臺后處理設(shè)備，DPF的尾端和SCR催化劑的前端之間相隔50-60 cm（19.6 – 23.6英寸）。而Euro 6c則規(guī)定起燃時間必須縮減至3-4分鐘。不僅是Euro 6c，美國的Tier 3法規(guī)也同樣如此規(guī)定，甚至要求起燃必須在2分鐘或更短的時間內(nèi)解決。而將SCR和DPF之間的距離縮短至10 cm（4英寸）左右可能解決這一問題。
但這么做有一個問題，那就是無論是DPF還是SCR催化劑都距離發(fā)動機太遠了，因此無法保證在要求的時間內(nèi)達到要求的工作溫度。在2015年IAA法蘭克福車展上，天納克公司公司提出了一種新的解決方案：將DPF和SCR合二為一，而具體做法則是為DPF涂上SCR催化劑。
雙渦流技術(shù)
“我們把這個方案稱為SDPF，也就是涂了SCR催化劑的DPF，”天納克公司清潔空氣公司高級工程與熱端研發(fā)核心科學(xué)執(zhí)行主任Frank Terres解釋道。
“如果你想把SCR和DPF合并起來，你還需要把投加點移到柴油氧化催化劑（DOC）和SDPF中間的縫隙那里，” Terres指出。“通常這兩片基質(zhì)中間有1 cm的縫隙，但現(xiàn)在因為你得在這個小縫里面加料，所以縫隙必須擴大。在封裝的限制下，擴大1 cm都很困難。你擴大了幾厘米，可能不到10cm，接著注射尿素添加劑的整個流程都需要在小洞里完成。”
這個流程包括注射、氧化、蒸發(fā)、水解尿素并將其轉(zhuǎn)化成氨，并在SDPF上均勻涂抹。Terres承認這的確不簡單。
為了在這么小的一片空間里將尿素溶液和尾氣混合，天納克公司公司開發(fā)出了一種解決方案：全新的雙渦流系統(tǒng)。該系統(tǒng)有助于促進上述物質(zhì)的混合，即便混合空間如SDPF般狹小。
“我們可以制造一段渦流，同時噴射尿素，接著用缸壁來控制液滴的接觸，而液滴會在缸壁上蒸發(fā)，”他解釋道。
除了冷啟動狀態(tài)下必須縮短起燃時間外，新的歐洲真實駕駛排放（RDE）測試還提出了另一些要求。“你的系統(tǒng)必須在所有的實際駕駛情況中運行，”Terres指出。針對該要求，天納克公司將尿素噴射器放在了催化劑系統(tǒng)中一個溫度高得多的位置上。
此外，天納克公司還采用將尿素從缸體流向噴射器和后部的方法來冷卻噴射器。通過這種方法，無論尾氣系統(tǒng)的環(huán)境溫度升得多高，噴射器中的尿素溫度都能維持在40ºC (104ºF)左右。
設(shè)計DOC和SDPF的串聯(lián)系統(tǒng)需要足夠大的空間，因此只能設(shè)計一種平行系統(tǒng)。也就是說，需要將柴油氧化催化劑和渦輪增壓器直接相連，再將SDPF與其平行放置。一個臺階型的混合器可以提供與串聯(lián)系統(tǒng)混合器相同的功能。
全新的橢圓形GPF可塞進狹小空間
另外，天納克公司還在法蘭克福車展上展出了專為Euro 6c設(shè)計的橢圓形的汽油顆粒捕集器（GPF），Euro 6c規(guī)定了2017年9月1日之后直噴汽油發(fā)動機的顆粒數(shù)量。此外，Euro 6c還將規(guī)定一項全新的測試循環(huán)，適用于歐盟所有的發(fā)動機測試。天納克公司專門為此設(shè)計了一款橢圓形GPF，使其得以嵌入柱形GPF無法進入的地方。
“我們碰到的難題是，在GPF中使用的基質(zhì)的洞孔比柴油過濾器更多，因此它們更加脆弱，”Terres表示。“這使得整個封裝流程特別困難。”
天納克公司技術(shù)、戰(zhàn)略和商務(wù)發(fā)展副總裁Tim Jackson闡述了GPF研發(fā)背后的思考。
“我們必須優(yōu)化發(fā)動機的燃油經(jīng)濟性和CO2排放，在此基礎(chǔ)上，再用成本劃算的后處理系統(tǒng)來清理剩余的尾氣，”他表示。“我們不能為了達到排放標(biāo)準(zhǔn)而犧牲CO2指標(biāo)和燃油經(jīng)濟性?，F(xiàn)在全球各地都面臨著CO2減排的問題，因此我們的想法應(yīng)該能得到更多共鳴。”
和柴油顆粒過濾器不同的是，GPF是一種可以自我再生的裝置，只要達到特定溫度便可再生。而且這種再生還是持續(xù)不斷的。針對這種特性，天納克公司發(fā)明了兩種封裝方法。該裝置既可在現(xiàn)有的三元催化劑之外安裝，也可以將三元催化劑涂在GPF的基材上，以形成天納克公司所稱的“四元催化劑”。
“由于這個過濾器的流動特性更好，氣體污染物的轉(zhuǎn)換效率更高，因此你可以縮小整體體積并節(jié)約一部分貴金屬材料。” Frank Terres指出。
作者：John Kendall
來源：SAE《汽車工程雜志》
翻譯：SAE上海辦公室
Tenneco unveils emission-control solutions for Euro 6c
Euro 6c emissions limits will present a number of challenges for light duty engine emissions, both gasoline and diesel when the regulations come into force in September 2017. One significant hurdle is speeding light-off of diesel selective catalytic reduction (SCR) catalysts.
In current systems it can take up to 10 or 12 minutes for a diesel SCR cat to reach its light-off temperature. This is determined to an extent by the packaging of the exhaust aftertreatment system.
In a current Euro 6b system the diesel particulate filter (DPF) may be the first aftertreatment device downstream of the engine, with a separation of 50 to 60 cm (19.6 to 23.6 in) between the rear of the DPF and the front of the SCR cat. Under Euro 6c, the light-off time would have to be shortened to between three to four minutes. The same will apply to US Tier 3 regulations, which will require a light off time of two minutes or less. Reducing the distance between the SCR and DPF to around 10 cm (4 in) could help.
The problem then is that neither the DPF nor the SCR cat is close enough to the engine to ensure that both have reached the required working temperature in the required time interval. Tenneco’s solution, on display at the 2015 IAA Frankfurt Show, is to combine the two devices in one. In this case the DPF is treated with an SCR coating.
Double-swirl does it
“That is what we call an SDPF, an SCR coated DPF”, explains Frank Terres, Executive Director, Core Science, Advanced Engineering and Hot End Development, Tenneco Clean Air.
“If you move the SCR portion into the DPF, you also need to move the dosing point to the gap between the diesel oxidation cat (DOC) and the SDPF," noted Terres. "Normally there is 1 cm between those two substrates but now you need to increase this gap because you need to dose in this small gap. Every centimeter is a pain because you have package constraints. So you give your few centimeters, maybe less than 10cm and the whole processing of the urea additive that you inject needs to be done in this small cavity."
Processing means injection, atomization, evaporation, hydrolysis and transforming into ammonia and then distributing everything homogeneously on the SDPF–a challenge, he admits.
Tenneco has developed a solution to the problem of mixing the urea solution with the exhaust gases in such a small space. The company’s new double swirl system is able to promote the mixing even in a very compact mixing zone such as in the SDPF.
“We generate a swirl and inject the urea, then have a controlled contact of the fluid droplets with the wall, where they can evaporate,” he explained.
In addition to the shortened light-off time from a cold start, the new European Real Driving Emissions (RDE) test also presents challenges. “You need to have a system that performs under all realistic driving conditions,” Terres noted. The urea injector is positioned in a much hotter area of the catalyst system than on previous designs.
Tenneco also uses the urea supply to cool the injector by circulating the flow from the tank to the injector and back. This ensures that the urea temperature in the injector is around 40ºC (104ºF) despite the high surrounding temperature of the exhaust system.
There may be insufficient space for an in-line DOC/SDPF design. In this case some form of parallel design would be necessary. This might involve connecting the diesel oxidation catalyst directly to the turbocharger, with the SDPF in parallel to it. A step-shaped mixer offers the same functionality as the mixer in the in-line system.
New oval GPF fits tight spaces
Tenneco also launched its oval-shaped gasoline particulate filter (GPF) at Frankfurt, anticipating the EU Euro 6c regulations, which will introduce a particulate number standard for direct injection gasoline engines from 1 September 2017. Euro 6c will also introduce a new test cycle for all EU engine testing. The oval shape has been designed so that it can fit into spaces where a cylindrical shape might not fit.
“The challenge for us is that the substrates used in a GPF have a higher porosity than a diesel filter and as a result they are more fragile," said Terres. "That makes the whole canning process very challenging."
Tim Jackson, Tenneco's Executive Vice President Technology, Strategy and Business Development, explained the thinking behind the development of the GPF.
"Let’s optimize the engine for fuel economy and CO2 emissions and let’s use the affordable aftertreatment to clean up the remaining emissions," he said. "Let’s not sacrifice CO2 and fuel economy to meet criteria [emissions]. Now that all the continents are faced with CO2 reduction goals, I think that message resonates a little stronger.”
The GPF is a self-regenerating device, unlike diesel particulate filters, which need to reach a particular operating temperature for regeneration to take place. The GPF is a continuously regenerating device. Tenneco has devised two ways of packaging it. The unit can either be included in addition to the existing three-way catalyst or the three-way catalyst coating can be applied to the GPF substrate to form what Tenneco terms a four-way catalyst.
“Because of the better flow characteristics in this filter, the efficiency of the gaseous pollutants conversion is much better, so you could make the total volume smaller and reduce the amount of precious metals used," noted Frank Terres.
Author: John Kendall
Source: SAE Automotive Engineering Magazine
天納克公司的雙渦流混合器專為Euro 6c柴油后處理標(biāo)準(zhǔn)設(shè)計。
在這張雙渦流混合器的剖面圖中可以看到，尿素噴射在氧化催化劑和SDPF間的一個狹小空間內(nèi)進行。
天納克公司設(shè)計的橢圓形汽油顆粒過濾器不僅適用于狹小空間的封裝，還能滿足Euro 6c的要求。