今年，伊頓 (Eaton) 在德國 (漢諾威) IAA商用車展 (IAA Commercial Vehicles show) 期間展示了一系列適用于重型柴油發(fā)動機的余熱回收及可變氣門正時技術(shù)，這反映了伊頓在節(jié)能減排方面的強大實力。
在今年的車展上，伊頓同時展示了間接和直接兩個類型的余熱回收系統(tǒng)，即基于有機郎肯循環(huán) (ORC) 的間接回收系統(tǒng)和基于“電氣化”設(shè)計的直接回收系統(tǒng)。其中，間接系統(tǒng)是利用熱交換器搭配廢氣系統(tǒng)的組合，間接回收余熱；“電氣化”系統(tǒng)則通過由廢氣驅(qū)動的羅茨壓縮機搭配電機的組合，直接回收能量。
ORC余熱回收系統(tǒng)可以獲得5%左右的燃料經(jīng)濟性提升，但需要配置一個小型外燃活塞發(fā)動機 (ECPE)，系統(tǒng)成本較高。伊頓公司則基于車輛現(xiàn)有流體著手設(shè)計ORC系統(tǒng)，從而避免引入新的流體循環(huán)系統(tǒng)，增加復(fù)雜度。公司第一個考慮的就是發(fā)動機冷卻系統(tǒng)中的乙二醇流體。
正如伊頓車輛集團高級工程總監(jiān)Larry Bennett所言，冷卻液已經(jīng)從發(fā)動機的冷卻循環(huán)中吸取了相當(dāng)多的熱量，那還有潛力從廢氣中吸取更多熱量，并最終為我們的回收貢獻更多能量嗎？“現(xiàn)在，我們絕大多數(shù)的ORC余熱回收系統(tǒng)均是基于車輛現(xiàn)有零部件設(shè)計的。”Bennett表示，“我們已經(jīng)拿到了美國能源部的經(jīng)費。另外，伊頓還有很多重量級合作伙伴，共同完成真正的測試工作，比如帕卡公司 (Paccar) 、殼牌 (Shell Oil) 和密西西比州立大學(xué) ( Mississippi State University ) 等。”
“殼牌正在幫我們研發(fā)一款符合要求的流體。我們要先看該流體能否收集足夠的余熱，再看能否實現(xiàn)轉(zhuǎn)化，最后再加以應(yīng)用，判斷到底能否真正行得通。”
伊頓的一名研究科學(xué)家表示，柴油廢氣液 (DEF) 搭配AdBlue尿素氮氧化物尾氣后處理系統(tǒng)的組合可能是一種非常理想的ORC余熱回收解決方案。
“這里的思路完全一樣，也就是通過沸騰DEF流體開始進行朗肯循環(huán)。”Bennett解釋道，“這個過程中會產(chǎn)生一個有趣的副產(chǎn)物--氨氣。當(dāng)你進行加熱時，氨氣就會出現(xiàn)，而當(dāng)你再次冷卻時，氨氣卻不會立刻回到液態(tài)，我們就可以把這些氨氣儲存起來。”
一旦發(fā)動機冷卻下來又重新啟動時，氮氧化物的產(chǎn)生和處理就成了一個問題。“如果你立刻重新啟動發(fā)動機，則可能產(chǎn)生1噸的氮氧化物氣體，但卻沒有足夠氨氣進行處理。”Bennett表示，“你必須要讓廢氣系統(tǒng)的溫度達到250°C，這樣才能讓液氮汽化，從而開始處理氮氧化物。”
系統(tǒng)中存儲的氨氣必須足夠在發(fā)動機啟動15分鐘內(nèi)處理氮氧化物，直至發(fā)動機上升至正常工作溫度，即讓液氮開始汽化為氣體。Bennett表示，“我們還需要進行大量的研究、模擬和測試，但這看起來似乎可行。”
直接余熱回收系統(tǒng)需要在緊鄰排氣歧管的位置安裝一個羅茨壓縮系統(tǒng)，并利用廢氣氣流驅(qū)動壓縮機轉(zhuǎn)子轉(zhuǎn)動。短期測試結(jié)果顯示，這種方式可以重新回收能量。
Bennett表示，“最初的概念是為系統(tǒng)連接一個電機以回收能量，而后再將回收的能量存儲在電池系統(tǒng)中。”這的確不是最有效的方式，但的確可行。伊頓的研究科學(xué)家認(rèn)為，這種系統(tǒng)可以加快廢氣循環(huán)的速度，并不會受到發(fā)動機轉(zhuǎn)速的限制，因此可以在需要高速廢氣循環(huán)時起到泵的作用。
伊頓之前曾開發(fā)過一款電子輔助可變速率超級增壓器，適用于汽油發(fā)動機。“通過這種設(shè)計，我們可以在不受發(fā)動機轉(zhuǎn)速限制的情況下，獲得不同程度的發(fā)動機增壓。”Bennett解釋說，“對于柴油發(fā)動機而言，這種設(shè)計在控制尺寸和提供瞬時扭矩方面有很多優(yōu)勢。最關(guān)鍵的就是可以不受發(fā)動機轉(zhuǎn)速限制，控制氣流和廢氣流速。”對發(fā)動機廠商來說，這種功能非常具有吸引力。
伊頓聲稱，在我們的模擬測試中，直接余熱回收系統(tǒng)已經(jīng)展示了22%的燃料經(jīng)濟性提升，且同時還能減少氮氧化物的排放。
現(xiàn)階段，可變氣門驅(qū)動技術(shù)尚未在柴油發(fā)動機領(lǐng)域得到廣泛應(yīng)用，但的確具有多種優(yōu)勢?？勺儦忾T驅(qū)動可以起到發(fā)動機壓縮制動的作用，提前或延后關(guān)閉進氣閥均可以降低燃燒溫度和氮氧化物的排放，也就是說，可以提高效率。
“你可以在某個瞬間提前關(guān)閉排氣閥，從而加速渦輪增加器的增壓。”伊頓高級氣門工程經(jīng)理Majo Cecur表示，“你也可以在輕載狀態(tài)下進行氣缸鈍化，從而提高燃料經(jīng)濟性。”
伊頓正在研究多項氣門操作設(shè)計，并尋求通過這些設(shè)計使能更多功能的可能性。
Eaton demonstrated a range of waste heat recovery (WHR) technologies for heavy-duty diesel engines at the IAA Commercial Vehicles show in Hanover, Germany, as well as variable valve timing systems, highlighting their potential to reduce fuel consumption and help reduce emissions.
Both indirect and direct WHR systems were on display. The organic Rankine cycle (ORC) design recovers waste heat indirectly using a heat exchanger with the exhaust system. Alternatively, Eaton’s “electrified” system recovers energy directly using an exhaust-driven Roots compressor in conjunction with a motor/generator.
ORC WHR systems can yield fuel-economy improvements of around 5%, but system cost is high, involving a small external combustion piston engine. Eaton looked at what fluids were already carried on board the vehicle to avoid adding another for the ORC WHR system. The first fluid the company considered was ethylene glycol, already used in engine coolant systems.
As Larry Bennett, director of advanced engineering, Eaton Vehicle Group, observes, the fluid would already be hot from use as an engine coolant, but would it have the potential to add more exhaust heat to it and extract more energy? “Now we can create most of the ORC system with existing componentry,” said Bennett. “We now have a U.S. Department of Energy (DOE) grant. We’re working with Paccar, ShellOil and Mississippi State University, which is where all the actual testing will take place.
“Shell is working on a fluid that has the capability to do this for us. The idea is to see if we can achieve enough waste heat and form this face transition, then utilize it and see if it’s going to work.”
One of Eaton’s research scientists suggested that diesel exhaust fluid (DEF)/AdBlue urea solution for exhaust aftertreatment of oxides of nitrogen (NOx) would be an ideal ORC fluid.
“The idea is exactly the same—you perform the Rankine cycle by boiling the DEF fluid,” explained Bennett. “One of the interesting byproducts is that after you heat it up, ammonia gas comes off and when you cool it down, it doesn’t want to readily return to the liquid state. We can store that ammonia gas.”
Once the engine has cooled down and is restarted, NOx output and treatment is an issue. “If you start it back up, you’re producing a ton of NOx because you don’t have ammonia gas to be able to treat it,” said Bennett. “You need the temperature in the exhaust system to get up to 250°C in order to take the liquid ammonia you’re injecting to get it to vaporize so that you can treat the NOx.”
The ammonia stored from the evaporated DEF should be enough to treat NOx for the first 15 minutes as the engine comes up to operating temperature. “It’s all research, all modeling simulation, but it appears feasible,” he said.
The direct WHR system involves fitting a Roots compressor system right next to the exhaust manifold and using the exhaust gas flow to drive the compressor rotors. Short-term testing shows that the energy can be recaptured.
“The initial concept is to have a motor/generator hooked up to it, then basically take the energy and put it in a battery,” said Bennett. This is not the most efficient way to recapture energy, but it offers another possibility. Eaton’s research scientists believe that this system could be used as a pump when there is a need for high rates of exhaust gas recirculation (EGR). The system could potentially deliver high rates of EGR independent of engine speed.
Eaton had previously developed an electronically assisted variable speed supercharger for use with gasoline engines. “In this application, we can now vary, independent of engine speed, the amount of boost that the engine can get,” Bennett explained. “On a diesel, that has a lot of advantages in the form of downsizing and instant torque. The big thing would be to manage airflow and exhaust flow independent of engine speed.” That capability would be highly attractive to engine manufacturers.
The direct WHR system has shown through simulation a 22% improvement in fuel economy while reducing NOx, Eaton claims.
Variable valve actuation has not been widely used so far with diesel engines, but there are a number of potential advantages. The first is a compression engine brake. Early intake valve closing and late intake valve closing could reduce combustion temperatures and NOx, or improve efficiency.
“You could have early exhaust valve closing for a transient to give faster boost in a turbocharger,” said Majo Cecur, engineering manager, advanced valvetrain, Eaton Vehicle Group. “You could de-activate cylinders in light load conditions so that you could have better fuel efficiency.”
Eaton is investigating valve operation designs that would enable a range of such functions.
Author: John Kendall
Source: SAE Truck & Off-highway Engineering Magazine
羅茨壓縮機搭配電機的組合可以提供一系列余熱回收功能。
伊頓公司價格經(jīng)濟的朗肯循環(huán)余熱回收系統(tǒng)，可將燃料經(jīng)濟系提升5%左右。
伊頓可能有機會通過向柴油發(fā)動機引入可變氣門驅(qū)動技術(shù)，實現(xiàn)壓縮制動功能。