工程師們可能認為，汽車的研發(fā)歷史已有一個多世紀(jì)之久，汽車動力總成中各個部件的熱量處理問題一定已經(jīng)得到了有效解決。但現(xiàn)在限另一個問題又凸顯出來——如何保證車輛尾氣的溫度不高不低，恰好符合人們的要求？這對商用車和非公路設(shè)備的影響尤為明顯。
如何控制尾氣溫度？這個新挑戰(zhàn)不僅將直接影響車輛的排氣系統(tǒng)，而且也與汽車行業(yè)老生常談的幾個需求分不開：效率、法規(guī)、尺寸、成本。如何在這些相互矛盾的需求之間達到平衡？這是一個問題。
熱管理專業(yè)技術(shù)公司 Zircotec 集團總經(jīng)理 Terry Graham 警告說，即將實行的全球排放新標(biāo)準(zhǔn)，將給車輛的發(fā)動機、催化器和柴油顆粒過濾器 (DPF) 提出新的優(yōu)化要求，而這均有可能導(dǎo)致車輛尾氣溫度的升高。
此外，汽車行業(yè)還必須面對另一個挑戰(zhàn)。Graham 表示，“為了保證渦輪增壓器的效率和響應(yīng)能力，我們還必須將尾氣的溫度控制在一定水平之上。Zircotec 認為，有時，排氣系統(tǒng)的內(nèi)部溫度可能將從 500°C 上升至超過 700°C。在某些情況下，通過加裝催化器和過濾系統(tǒng)來優(yōu)化車輛性能，可能會增加排氣系統(tǒng)的尺寸，影響周邊其他不耐熱的車輛系統(tǒng)，進而給車輛的整體封裝帶來挑戰(zhàn)。”
減少熱量傳遞
Zircotec 公司正在尋找各種可能辦法，減低熱量在整個排氣系統(tǒng)中的傳遞，包括使用排氣歧管以減少高溫廢氣向其他外部系統(tǒng)傳遞熱量的機會，從而維持尾氣溫度。這種設(shè)計可能對車輛的冷啟動和系統(tǒng)預(yù)熱時間產(chǎn)生重大影響，還能降低外殼結(jié)構(gòu)的工作溫度。由于對材料的熱性能需求更低，因此廠商可以使用一些更加經(jīng)濟的選擇。
Graham 說，“即使是用于支撐排氣系統(tǒng)的金屬夾子，也可能成為熱量損失的主要來源之一。為了解決這個問題，Zircotec 與一家供應(yīng)商合作研發(fā)了一種陶瓷涂層解決方案。”
相關(guān)安全法規(guī)已經(jīng)對車輛尾氣溫度進行了限定。然而，除了易燃性外，商用車輛的高溫尾氣更有可能給行人帶來傷害。Graham 認為，目前一些組件的最高外部溫度為 480°C (896°F)，而未來可能被強制限制到 70°C (158°F) 左右。
“傳統(tǒng)的解決方案，是在可能受到影響的區(qū)域增加隔熱層，但如果使用傳統(tǒng)材料，這個隔熱層通常必須要 70毫米以上，才能真正實現(xiàn)熱阻隔。” Graham 表示，“在許多應(yīng)用中，除非重新走線，否則排氣系統(tǒng)根本騰不出空間安裝這種厚度的隔熱層。此外，這種做法還有可能引發(fā)一系列連鎖反應(yīng)，給車輛的其他系統(tǒng)和整體封裝帶來影響。當(dāng)然還會增加成本。”
為了縮短車輛預(yù)熱的時間，并保證關(guān)鍵系統(tǒng)可以在有需要時得到充足熱量，汽車的熱管理系統(tǒng)通常需要非常嚴(yán)密的控制。
然而，Graham 估計，目前的設(shè)計僅能保證尾氣中剛好有足夠的熱量驅(qū)動這些系統(tǒng)，而我們需要開發(fā)一種新的方法。他說，“為了保證效率，我們的一個客戶希望保證尾氣在排氣管的溫度不低于排出發(fā)動機時的 87%。要做到這一點，我們必須仔細分析排氣系統(tǒng)中各個部件的熱量傳遞情況。”
后處理系統(tǒng)
為了顯著提高尾氣凈化效率，Zircotec 公司的另一個關(guān)注點是后處理系統(tǒng)的熱管理性能。Graham 表示，Zircotec的技術(shù)可以用于 DPF 和催化器系統(tǒng)的內(nèi)外表面，從而提高系統(tǒng)內(nèi)部氣體的溫度，保證熱量可以順利傳遞、集中至有需要的地方，并同時通過優(yōu)化隔熱性能，降低傳遞到系統(tǒng)之外的熱量。Zircotec 公司還積極與多家一級供應(yīng)商客戶合作，研發(fā)模塊化催化器單元的隔熱系統(tǒng)，為未來需求做好準(zhǔn)備。
為了縮小安裝尺寸，Zircotec 公司對 ThermoHold 陶瓷隔熱材料的應(yīng)用進行了大量調(diào)查對比，包括采用和不采用整體傳統(tǒng)金屬隔熱屏的情況。通過引入一些小型氣隙，此類解決方案可以針對不同工作環(huán)境和具體情況進行優(yōu)化，從而優(yōu)化隔熱性能。
Graham 說，“隔熱板可以在密閉空間內(nèi)提供效果顯著的熱保護效果，是一種極具性價比的解決方案。”
Graham 補充說，優(yōu)化排氣歧管的熱管理性能是公司的長期重點項目之一。為了實現(xiàn)這一目標(biāo)，公司已經(jīng)與一家主要汽車制造商和多家尾氣處理系統(tǒng)組件供應(yīng)商展開合作，將通過兩種“非常特殊”的手段，驗證在歧管內(nèi)部應(yīng)用 ThermoHold 陶瓷涂層的效果。
目前，原型開發(fā)已經(jīng)完成，也已經(jīng)有一家歧管供應(yīng)商采用了 Zircotec 的陶瓷材料。Graham 表示，根據(jù)早期試驗和相關(guān)的測試結(jié)果看來，“前景非常光明”。
Automotive engineers may have thought that after a century or more, the problems of handling heat in all parts of the powertrain had been solved. But another is emerging that particularly affects commercial vehicles (CVs) and off-highway equipment: keeping exhaust heat only where it is wanted.
The new challenge affects exhaust systems and involves a combination of the auto industry’s decades-old conflicting demands: efficiency, legislation, packaging and cost.
Terry Graham, managing director of thermal management specialist Zircotec Group, warns that expected upcoming global emissions standards will require improvements to the engine, catalyst and DPF (diesel particulate filter), each of which is likely to increase exhaust gas temperature.
But there is an added challenge, said Graham: “It’s a desire to keep heat in the exhaust to maintain turbocharger efficiency and response. Zircotec believes that internal temperatures in some cases will rise from 500°C to levels in excess of 700°C. In some instances, obtaining higher performance from catalysts and filters may require an increase in size, putting pressure on overall packaging as hot exhaust system components encroach on other systems that could potentially be vulnerable to heat damage.”
Reducing heat transmission
So Zircotec is now looking at ways of reducing heat transmission throughout the entire structure of the exhaust, including the exhaust manifold, thus reducing heat transfer from the hot exhaust gas to the outer containment to help maintain exhaust gas temperatures. This would also potentially have a significant impact on cold-start and the time taken for systems to warm up. It should also cut the operating temperature of the containment structure, reducing the thermal demands on the material and allowing more economic choices.
“Even the metal clamps holding the exhaust system can be a major source of heat loss. To overcome this, Zircotec is working with a supplier on ceramic coatings options to reduce this effect,” revealed Graham.
Safety legislation already limits external temperatures. Apart from flammability, the exhaust on a CV is more exposed to pedestrians. The maximum external temperature, currently as high as 480°C (896°F) on some components, could be mandated down to just 70°C (158°F), believes Graham.
“The traditional solution would be to add insulation around the affected areas, but conventional materials would typically need to be up to 70 mm thick to achieve the thermal barrier performance required,” he said. “There just isn’t the room for such a thickness in many applications without re-routing the exhaust line and introducing knock-on effects that would compromise other systems and overall packaging, to say nothing of the cost involved.”
On modern vehicles, heat management is often carefully controlled to reduce warm-up times and ensure sufficient heat is delivered to key systems.
But Graham estimates that the point is approaching where there is only just enough high-grade heat in the exhaust gas to drive these systems, so a different approach is required: “For reasons of efficiency, one of our clients is aiming to maintain the exit gas temperature at the tailpipe to no less than 87% of the temperature at which it leaves the engine. To achieve this we have to analyze the heat transmission through each individual component of the exhaust system.”
Aftertreatment considerations
A further key focus for Zircotec is on thermal management of the aftertreatment systems to significantly improve clean-up efficiency. Graham said Zircotec’s technology can be used inside and around the DPF and catalyst to increase the internal gas temperature and ensure heat is delivered and focused where it is needed, yet simultaneously reduce external heat transmission through improved thermal barrier performance. Together with one of its Tier 1 customers, the company is working on the insulation of a modular catalyst unit that would meet future requirements.
To reduce the packaging space required, Zircotec is investigating the use of its ceramic ThermoHold-based heat shield material, both with and without an integral conventional metal heat shield. By incorporating small integral air gaps, solutions have been devised that can operate in different environments and orientations to improve thermal barrier performance.
“The heat shields can deliver significant heat protection within a confined space, providing a highly cost-effective way to meet packaging requirements,” stated Graham.
He added that one of Zircotec’s significant long-term projects is to improve the thermal management of the exhaust manifold. To achieve this, the company has a collaborative program in place with a major vehicle manufacturer, and with exhaust component suppliers, via two “very different” approaches to examine the incorporation of ThermoHold ceramic coating within the manifold.
Prototypes have been produced, with the Zircotec ceramic-based material used and applied by a manifold supplier. Graham says early trials, and associated test results, are “very promising.”
Author: Stuart Birch
Source: SAE Automotive Engineering Magazine
Zircotec 公司排氣系統(tǒng)組件的等離子涂層。
Zircotec 公司總經(jīng)理 Terry Graham 表示，為了滿足即將到來的新排放標(biāo)準(zhǔn)要求，車輛的尾氣溫度可能會有升高。
Zircotec 涂層組件的質(zhì)量控制流程。
