新設(shè)計、新材料將是下一代電動汽車動力電機的關(guān)鍵。
如今，汽車電氣化需求穩(wěn)步增長，可能創(chuàng)造數(shù)十億甚至上百億美元的業(yè)務(wù)，汽車行業(yè)對此已經(jīng)開始為汽車傳動系統(tǒng)的轉(zhuǎn)型布局。
福特公司表示，公司計劃在 2022 年前推出大約 40 款電動車型，其中包括 16款全新的純電動汽車。本田預(yù)計，到 2030 年，電動汽車將占該公司全球銷售額的三分之二。通用汽車公司計劃到 2023 年在全球范圍內(nèi)推出 20 款電動車型。傳統(tǒng)豪車品牌法拉利也加入了汽車電氣化的大軍。我們不難從這些全球主要汽車制造商的行動中看出，推進系統(tǒng)的電氣化幾年后將不再是新事物。此后，各家廠商將專注于發(fā)展可以提高推進效率的設(shè)計、工藝和制造等關(guān)鍵競爭優(yōu)勢。
舉個例子，為了在電推進系統(tǒng)的完整價值鏈上（即從電池組到動力電子組件再到電機）掌握主動權(quán)，博世在 2019 年初接手了其與戴姆勒合資的電機研發(fā)公司 EM-motiv 的全部所有權(quán)和控制權(quán)。博世認為，僅對電動汽車的系統(tǒng)性熱管理這一項進行優(yōu)化，即有潛力將電動汽車的續(xù)航里程提高 20%。
博世公司在最近的一次發(fā)布中表示，“歸根結(jié)底，電氣化交通出行取得突破的關(guān)鍵在于以低廉的成本提供可以滿足需求的續(xù)航里程。”
定制設(shè)計
電動汽車的評價標準往往集中于電池容量，然而驅(qū)動電機也同樣重要，重要程度甚至堪比傳統(tǒng)汽車的發(fā)動機。事實上，電機功率和效率是相互關(guān)聯(lián)的，因此根據(jù)不同車輛推進系統(tǒng)定制不同功率和效率的電機至關(guān)重要，但也同時需要大量投資。
目前，汽車行業(yè)中主要采用“永磁”和“感應(yīng)”兩種主要交流牽引電機，兩者各有利弊。許多汽車制造商和供應(yīng)商都青睞于工作效率更高的永磁電機，其中幾家最著名的制造商包括本田、豐田、通用和寶馬。
對比來說，交流感應(yīng)電機的效率較低，但也憑借更高的最高功率輸出贏得了一些支持者，比如電動汽車領(lǐng)頭企業(yè)特斯拉。目前，特斯拉的高性能大尺寸 Model S 和 Model X 系列均采用了交流感應(yīng)電機，但最近推出的緊湊型 Model 3 卻采用了永磁驅(qū)動電機。
過去，大多數(shù)觀點認為感應(yīng)電機更適合尺寸更大、重量更重或?qū)π阅芤蟾叩碾妱悠嚒Ｊ聦嵣?，永磁電機的應(yīng)用也并非僅限于一些尺寸更小且更注重效率的車型。目前，盡管電動汽車初創(chuàng)公司 Rivian 并未透露旗下 R1T 電動皮卡和 R1S 運動多功能車的具體規(guī)格，但一位公司發(fā)言人向《汽車工程雜志》證實，這兩款車型的每個車輪均配備了獨立的電機，且這些電機采用了永磁設(shè)計。
探索新材料
通常來說，永磁電機生產(chǎn)經(jīng)常受制于重稀土元素的供應(yīng)，正因如此，一些高產(chǎn)量制造商一直對永磁電機“敬而遠之”。目前，全球重稀土元素供應(yīng)主要來自中國。據(jù)估計，中國的稀土儲量占全球的35-40%，比如釹和鏑，而這兩種稀土元素均對各類磁性產(chǎn)品的生產(chǎn)非常重要。
通常來講，汽車牽引電機中使用的磁體需要較高的矯頑力，也就是磁體在車輛常見高溫環(huán)境下保持磁性的能力，而磁體的這種性能主要與其中含有的稀土材料有關(guān)。通常來說，磁體中 30% 的原材料都是稀土。
2016 年年中，本田汽車有限公司和大東鋼鐵有限公司（Daido Steel）共同發(fā)布了全球首款專為電動汽車量產(chǎn)而研發(fā)的新型磁體材料—熱變形釹。該材料將首次用于 2017 款 2017 FreedSport Hybrid 混合動力小貨車搭載的新型永磁牽引電機。
具體來說，這種新型熱變形釹材料不需要注入鏑或鋱等“重”稀土元素，即可優(yōu)化對牽引電機至關(guān)重要的一項指標——高耐熱性。
本田最新款 Insight 和 Accord 雅閣混合動力車型均采用了旗下第三代雙電機（牽引電機和發(fā)電機）混合動力設(shè)計；據(jù)本田透露，這兩臺電機所用磁體均不使用重稀土金屬。據(jù)稱，Insight 混合動力汽車的牽引電機可提供129 hp 和267 N·m的動力輸出。
無獨有偶，豐田也曾在去年表示，公司已經(jīng)開發(fā)了一種釹元素含量顯著降低的新型耐熱電機磁鐵。豐田在新聞稿中表示：“這種新型磁體可在高溫條件中使用，且其中釹稀土元素的用量明顯減少。”
豐田補充道，這種新型磁體并不使用“高耐熱釹磁體所必需的”鋱或鏑元素，“我們采用了成本更低的鑭和鈰進行替代，從而減少了磁體中釹的用量。”
豐田工程師認為，鑭和鈰的儲量豐富且成本低廉，但同樣可以幫助磁體保持高耐熱性和高矯頑力。
美國能源部高級研究項目機構(gòu) ARPA-e 也啟動了“REACT 關(guān)鍵技術(shù)中的稀土替代材料計劃”（Rare EarthAlternatives in Critical Technologies），以開發(fā)成本更低、供應(yīng)更可靠的稀土替代品。過去十年里，REACT 項目已資助了數(shù)個使用非稀土磁體的電動汽車研發(fā)項目。
隨著電機技術(shù)的不斷發(fā)展，未來的研發(fā)重點必定在于提高效率、功率和可靠性。本田、通用汽車（Bolt 電動汽車）等公司已經(jīng)從為定子繞組配備的方截面導(dǎo)線得到了可靠的結(jié)果—方形導(dǎo)線的“排布”更加高效，可以提高給定區(qū)域的密度。一些消息來源稱，繞組技術(shù)也會對電機的功率輸出和效率產(chǎn)生重大影響。
安裝設(shè)計
在純電動汽車應(yīng)用中，牽引電機通常用于驅(qū)動車軸，另外在某些情況下也會直接驅(qū)動某個單獨車輪，比如 Rivian 的電動汽車設(shè)計。不過，牽引電機在混動汽車中的應(yīng)用場景則更加多樣。
在早期的“輕”電氣化設(shè)計中，開發(fā)人員通常將牽引電機/發(fā)電機單元安裝在發(fā)動機曲柄的前部，也就是所謂的 P0 位置，具體由驅(qū)動軸連接。電機可以逐漸移回至傳動系統(tǒng)，逐步對發(fā)動機曲軸或驅(qū)動輪施加更大影響。P3 位置將電機集成到變速器中，而 P4 位置則意味著電機將驅(qū)動一根并未與內(nèi)燃機機械相連的軸。
據(jù)了解，Rivian 電動汽車的輪內(nèi)牽引電機外殼來自 Protean Electric 公司，其 Pd16 和 Pd18 車輪電機系統(tǒng)已經(jīng)與輪輞集成封裝，永磁同步電機則直接集成在外部轉(zhuǎn)子內(nèi)。電源和控制電子單元也進行了集成。Protean 的車輪電機瞄準自動駕駛大巴的應(yīng)用場景，Pd18 則用于 Local Motors 的“Olli”自動駕駛穿梭大巴。
作為傳統(tǒng)汽車向電動汽車過渡的必要橋梁，電機的重要性不言而喻，因此各大廠商均發(fā)力電機設(shè)計，推出了功能非常完善的電機?，F(xiàn)階段，很多電機均可以通過不同“P”級，實現(xiàn)不同程度的效率和性能提升，取得比如驅(qū)動去耦“航行”、扭矩“填充”等功能，從而緩解發(fā)動機增壓滯后或換擋卡頓等問題，通過車輛“電子軸”實現(xiàn)車輛的全輪驅(qū)動。
New designs and materials are key to the next generation of electric machines for EV propulsion.
With the momentum to expand vehicle electrification increasing steadily, the industry is beginning to arrange the pieces for its multi-billion-dollar transformation of powertrain development.
Ford said it intends to have some 40 electrified models in showrooms by 2022, including 16 all-new battery-electric vehicles. Honda projects electrified vehicles will account for two-thirds of the company’s global sales by 2030. General Motors plans 20 electrified models globally by 2023. Even Ferrari is joining the march. As the list grows, it’s clear that in a few years, propulsion-system electrification no longer will be news per se. The dialogue will then shift to key differentiators in design, engineering and manufacturing that impact efficiency.
Bosch, for instance, in early 2019 assumed full ownership and control of its EM-motiv electric-motor development joint venture with Daimler, as the supplier seeks to manage the full value chain of electric propulsion—from battery pack to power electronics to motors. Optimization of system thermal management alone, the company believes, can increase an electric vehicle’s (EV’s) range by as much as 20%.
“In the end,” said Bosch in a recent release, “affordable [driving] range is the key to helping electromobility achieve a breakthrough.”
Tailored designs
For EVs, the discussion often focuses on battery capacity, but the drive motor is as much a factor as the engine is in a conventional powertrain. Electric-machine power and efficiency are mutually related—and how those characteristics are tailored for automotive propulsion is a matter of widening engineering investment.
The two primary types of alternating-current (AC) traction motors, permanent-magnet and induction, have advantages and limitations for automotive applications. Many automakers and suppliers have favored permanent-magnet motors because they typically are inherently more efficient. Honda, Toyota, GM and BMW, as well as many major suppliers, currently use permanent-magnet motors in production vehicles.
AC induction motors may be preferable if high power output is a factor, but they are less efficient. Tesla, which many consider a bellwether of EV technology and development, uses AC induction motors for its larger and more performance-oriented Model S and Model X vehicles, but elected permanent-magnet drive motors for its most recent (and smaller) Model 3.
Many in the past have viewed induction motors as more aligned with EVs that are either larger and heavier or are focused on high performance, but permanent-magnet motors are not limited to smaller, efficiency-focused vehicles. Although EV startup Rivian has disclosed scant specifics regarding its intriguing new platform for its R1T electric pickup truck and R1S sport-utility, a company spokesperson did confirm to Automotive Engineering that its drive motors—one for each wheel, combined in a unique integrated twin-motor/transmission housing for the front and rear—are permanent-magnet design.
Materials quest
High-volume manufacturers have been wary of permanent-magnet motors because of their traditional reliance on heavy rare-earth elements. The preponderance of these materials currently comes from China, which is estimated to hold 35-40% of the world reserves of rare earths such as neodymium and dysprosium. Both are critical to all manner of magnetic products.
Magnets used in automotive traction motors typically aim for high coercivity, or the ability to maintain magnetization, at the high temperatures that can be common in automotive applications. The rare-earth materials impart added coercivity; often around 30% of the elements used in magnets are rare earths.
In mid-2016, Honda Motor Co. and Daido Steel Ltd. announced the first production application of a new magnet material for EVs. That material was hot-deformed neodymium and it was first used for a new-design permanent-magnet traction motor for the 2017 Freed Sport Hybrid compact minivan.
The hot-deformed neodymium doesn’t require infusion with dysprosium or terbium “heavy” rare earths to achieve the high heat-resistance characteristic vital to traction motors.
Honda’s latest Insight and Accord Hybrid models employ the third generation of the company’s dual- motor (traction motor and generator) hybrid design; the magnets for both motors, the company said, use no heavy rare-earth metals. For the Insight Hybrid, the traction motor develops a claimed 129 hp and 197 lb·ft (267 N·m).
In a similar vein, Toyota said last year it had developed a new neodymium-reduced, heat-resistant magnet for electric motors. “The new magnet uses significantly less neodymium, a rare-earth element, and can be used in high-temperature conditions,” the company said in a release.
The new magnets use no terbium or dysprosium “necessary for highly heat-resistant neodymium magnets,” Toyota said, adding, “A portion of the neodymium has been replaced with lanthanum and cerium, which are low-cost rare earths, reducing the amount of neodymium used in the magnet.”
Use of lanthanum and cerium—both abundant and low-cost rare earths— enables high heat resistance to be maintained and loss of coercivity minimized, Toyota engineers believe.
In the U.S., the Advanced Research Project Agency-Energy (ARPA-e, a part of the U.S. Dept. of Energy) started its REACT (Rare Earth Alternatives in Critical Technologies) program to develop low- cost, reliable alternatives for rare earths. The REACT program in the last decade has helped fund several development efforts for EV motors using non-rare-earth magnets.
Deeper engineering of every aspect of motor design is certain to improve efficiency, power and reliability. Honda, GM (in its Bolt EV) and others have gleaned solid results from using square-cross-section wire for stator windings because it was determined the square wire “nests” more effectively, providing increased density for the given area. And winding technique, some sources say, also can have a significant impact on motor output and efficiency.
Placement options
For pure EVs, traction motors typically drive an axle, or in some cases such as Rivian’s, individual wheels. But for hybridization, there are numerous choices for where in the drivetrain the electric motor can do its work.
Early efforts for “mild” electrification have placed motor/generator units to act on the front of the engine crankshaft, typically linked by a drive belt, for a so-called “P0” location. The electric machine can be progressively moved back in the drivetrain, generally to impart increasing degrees of influence on the engine crankshaft or the drive wheels. A P3 location integrates the electric machine into the transmission, while a P4 location insinuates an electric motor driving an axle not mechanically connected to the combustion engine.
The case for in-wheel traction motors is made by Protean Electric, whose Pd16 and Pd18 wheel-motor systems are packaged with the road wheel rim—the permanent-magnet synchronous machine is contained in the outer rotor. Power and control electronics are also integrated into the units. Protean is aiming its wheel motors at autonomous shuttle applications, and the Pd18 is used in ‘Olli,’ Local Motors’ self-driving shuttle.
As a bridge to EVs, industry sources project increasingly sophisticated designs for incorporating electrification into conventional drivelines. As electric motors progress through the various “P” stages, the corresponding benefits are efficiency- and performance-enhancing features such as drive-decoupling “sailing,” torque “fill” to mask lag in engine boost and smooth gear changes, as well as all-wheel-drive via fully-contained “e-axles.”
