随着全球二氧化碳排放标准的日益严格，并且世界上一些人口最密集的城市亟需解决用车需求问题，电驱动桥产品将迎来发展的黄金时期。目前，由于全电动车型仍处于酝酿阶段，因此电动驱动桥看上去是一个将当前车型电动化的理想选择。

电驱动桥系统也称为电动全轮驱动系统（eAWD），指的是一种装在一个车桥结构中的机电系统，其中包含一个电机、动力电子元件和一种相当于变速箱或差速器的装置。与传统内燃机（ICE）或混合动力系统联合工作时，该系统能够为原本没有电力驱动的车桥增添电驱动力，因此被称为eAWD系统。
 

通过将电子组件、电机与差速器集成为一体， 电驱动桥系统可以提升各种级别汽车的封装灵活性、电气化程度以及性能，起到了出色的整合作用。由于几乎所有交通领域都必须采取一定形式的电气化，以满足二氧化碳排放限值，因此电驱动桥这一创新设备一下变得炙手可热起来。
 

然而一位一级供应商直接表示：“我们简直要忙坏了。”
 

几位接受本文作者采访的工程师们均表示，电驱动桥较强的适应性带动了整个技术的快速发展。不过，许多一级供应商们也逐渐发现“模块化”不等于“插件”。对供应商而言，人们最感兴趣的模块化技术，同样增加了技术迭代的难度，也带来了系统集成方面的新挑战。

Dana公司推出的新型eS5700r 电驱动桥系统，能够直接取代现有的中型卡车与公交车悬架。安装有该款电驱动桥系统的新款东风卡车于近期正式在中国投入运营。本电驱动桥系统由液冷电机、逆变器、盘式制动器构成，并且仅重673磅（305千克）。（图片来源：Dana公司）

电驱动桥背后的推动力
 

尽管某些地区的市场因政治问题而出现动荡，但二氧化碳排放标准的严格化，是全球的普遍趋势。最近，一些主要城市的零排放区正在制定各项举措，以推动纯电动车的普及。而电驱动桥是一种可以同时解决多个问题的理想部件。
 

德纳（Dana）公司电气化与混合动力部门高级主管Michael Lembke表示，二氧化碳排放标准是推动汽车电气化的最主要原因，不过“各个国家的标准都不一样”。中国的影响力非常大，因为它强制规定了电动汽车的比例。Lembke指出：“中国不仅规定了车队的二氧化碳排放限值，它还规定在OEM生产的汽车中，必须有8%采用动力电池。”
 

尽管中国近期出台的法规会推动电驱动桥应用的普及，但其他市场上更有可能出现电动汽车的需求上升。
 

吉凯恩传动系统公司（GKN Driveline）高级工程副总裁Theo Gassmann表示：“我们在中国的业务将在未来几年中高速增长。但欧洲与美国也不落后，所有顶尖OEM都在加大对电动车及插电式混合动力汽车的研发与投资力度。二氧化碳减排计划的出台、当地污染问题的加重以及零排放区的设立，都大大推动了各地区的电气化发展。”
 

据IHS Markit 市场调研公司汽车技术领域首席分析师Graham Evans称，电驱动桥行业将迎来显著增长，并指出：“我们预测，2017-2023年，轻型车用电驱动前桥的复合年均增长率将达到35%，后桥将达到46%。”
 

 根据IHS 公司的预测，电驱动前桥系统的产量将从2017年的75万台增长到2023年的450台以上，而同时，电驱后桥/全轮驱系统的产量也将分别从32万台/11万台增长到300万台/250万台。而随着零排放区在全球主要城市及地区的设立，电驱动桥产品的商业应用将使其产量进一步高于预期。
 

Dana公司轻型车工程副总裁Seth Metzger表示：“如果我们只考虑商业领域，城市内使用的工作车辆和配送车辆是需要全电动的。而那些来往于城市间的车辆在进城后需要在一段时间内切换为纯电模式。”

快速迭代与成本问题
 

对于汽车制造商们而言，幸运的是，电驱动桥产品能够适应现有的汽车结构，帮助解决上述汽车二氧化碳排放方面的问题。来自Dana公司的Lembke解释说：“由于时间紧迫，现在很多OEM们都要求在他们现有汽车结构（包括发动机、变速箱、传动轴、排气系统与油箱）的基础上，直接安装一个eAWD系统。”
 

极强的适应性，是电驱动桥技术广受欢迎的主要原因。麦格纳动力总成系统公司（Magna Powertrain）产品管理与传动系统部门全球主管Walter Sackl解释说：“很多的汽车制造商在生产汽车时，都已经形成了自己固有的汽车平台与汽车结构，而采用电驱动桥部件后，我们能将原有的汽车结构直接转化为混合动力系统结构。并且，电驱动桥的尺寸往往可以轻松地装进普通汽车。”
 

电驱动桥技术发展迅速，它的应用还使得是原有汽车平台的安装工作相较于几年前变得更为方便。GKN集团的Gassmann解释说：“每一款新的电驱动桥产品都会增添新的特征，并且集成度也更高、布置更为紧密，也更为先进。最开始的电驱动桥有多个组成部分：一台与变速箱相连的电机，一台位于后备箱的逆变器，所有部件都由电缆连接。而现在的系统整合程度很高，逆变器、电机和变速箱基本整合在一起，组成一个完整的部件。”
 

任何新的系统在投入运营时都会遇到一定的工程挑战，电驱动桥系统也不例外。电驱动桥系统在推出时遇到的最大的一个挑战在于，由于推动电动车产业发展的是二氧化碳排放法规，而非消费者需求，因此OEM难以从电动车市场中获利。
 

Dana 公司的Metzger表示：“高昂的成本是制约电动车发展的一大障碍。许多OEM都坦言，纯电动车难以获利。但随着电驱动桥产量的增加，市场份额也会提升，并相应带动电池、电机与逆变器成本的下降。这些部件的成本最后都会降低，因为OEM必须获利才能生存。” 
 

要想降低成本，除了提升电驱动桥的产量外，还需要降低原材料的价格压力。IHS Markit公司的Evans表示：“电驱动桥是电动车中第二昂贵的部件，而鉴于目前激烈的市场竞争，单位成本应该很快就会下降了。”
 

Evans 指出，电机使用的稀土金属原料是导致价格居高不下的主要原因。不过他也表示，“现在已经有很多公司开始开发无稀土电机，这样供应链上的这一限制将不复存在，也不会阻碍技术发展了。”
 

除了成本方面的挑战外，所有接受《汽车工程》杂志采访的供应商们都指出，噪声、振动与不平顺性（NVH）问题也是电动汽车所面临的一个新挑战。
 

ZF电动交通部门驱动桥系统产品/项目管理主管MichaelWetzel 指出：“由于电动车很安静，因此只要汽车传动装置、电子元件以及电机发出一点噪音，你就能听得很清楚。而电动车的部件布置又较为紧密，功率重量比较高，因此难以同时采用所有的NVH对策。” 
 

要从系统层面上解决NVH问题，就需要先进的仿真模拟工具加以辅助。GKN的Gassmann解释说：“从一开始，你就要牢记系统中每个部件以及这些部件的组合方式。然后，你需要确保基本部件要素都准确地布置到位，这样才能保证汽车无噪声地高效运行。这其中最关键的就是要选择合适的润滑油与轴承，也就是说要采用先进的润滑技术，以尽可能减少发动机运转过程中的摩擦损失。” 
 

电驱动桥系统输入转速较高也是造成电动车NVH问题难解决的原因之一。一般高性能内燃机的最高转速只能达到8000 rpm，而相比之下，新一代电驱系统的转速可达20000 rmp。Gassmann表示：“它和传统系统截然不同，同时也带来了艰巨的NVH难题。”
 

电驱动桥系统推广所面临的另一大挑战在于系统布置形式的复杂性。来自Magna动力总成系统公司的Sackl表示：“传统内燃机总共只有六种布置方式，比如前横置发动机前轮驱动，前纵置发动机后轮驱动等等。而电驱动桥系统的布置形式则复杂得多，可能会产生42种甚至更多的布置形式。”

OEM与供应商合作，加强系统集成

在此之前，零部件供应商主要专注于零部件的机械制造，而现在则需要与OEM客户们共同合作，以应对集成系统、尤其是集成系统软件研发制造方面的挑战。
 

Gassmann 也承认说：“OEM们现在开始同一级供应商合作，致力于集成式电驱系统的开发。这对我们供应商而言既是巨大的机遇，也是巨大的挑战。不过这也意味着我们供应商需要做出重大的转变，因为我们原先主攻的是机械零部件的整合，以及变速箱的制造等等，但现在我们却需要担负起整个系统集成的工作。”
 

麦格纳的Sackl 表示：“过去是机械部件决定汽车的功能，而现在我们开始重点研发制造像电控离合器这样的电子组件。因此，我们需要在现有汽车结构的基础上再加入控制软件，这使得整个系统部件的生产流程变得复杂起来。”
 

Sackl继续解释说：“同以往相比，一级供应商所提供的产品的范围也在不断扩大。现在，我们所供应的大部分都是软件类的部件产品，并且软件类部件研发的工程人员，也已经占到了我们工程人员总数的近一半之多。”

 
48伏电气系统 vs. 高压电气系统

48伏电驱动桥系统的应用前景如何？我们采访的大部分供应商都表示，虽然许多新型舒适性技术与底盘技术的运行仍需要48伏系统的支持，但未来的电驱系统却需要更高的电压支撑。麦格纳的Sackl解释说：“48伏系统最主要的功能是向汽车的其它系统供电，但高压电气系统的主要功能则是为汽车提供动力。”
 

ZF的Wetzel对此表示赞同，他指出：“我们目前提供的大部分电驱动桥产品都是与高压电气系统相兼容的，因为这一系统下电驱动桥产品的输出功率可达50 kw及以上，这为用户在内城区驾驶纯电动车提供了可能。”
 

GKN的Gassmann也表示：“我们认为48伏电驱动桥系统的市场前景并不大，因为这一系统所能提供的扭矩很有限。如果你想在市中心驾驶电动车，并且想让汽车保持足够的扭矩、毫不费力地驶过路缘或是爬上停车场的斜坡，那么汽车只能以低速行驶。”
 

“但如果你更注重于二氧化碳减排目标的实现，那么就需要确保电动车能够高速行驶，否则就发挥不了电动车的二氧化碳减排能力以及电驱动桥在提升汽车驱动性能方面的优势了。因此，要实现这一目标，就必须增加整个混合动力系统的设计成本，将传统的变速箱升级为双速变速箱，但这就又与原先的48伏低成本混合动力驱动桥系统的设计原则相违背了。”Gassmann继续解释道。

汽车动力学与二氧化碳减排

作为零部件供应商，最重要的是生产出客户想要的产品，而不应只是一味专注于提高产品的效率。将电驱动桥系统添加到现有汽车平台中，除了能减少汽车二氧化碳的排放量、减小ICE的尺寸外，更为重要的是是能够增提升车辆的性能。
 

GKN的Gassmann 指出：“虽然电驱动桥行业的发展必然会对机械式全轮驱动系统的市场造成一定的冲击，但这不代表我们有所损失。相反，我们还获利了，因为电驱动桥系统的价值远高于机械式全轮驱动系统。”
 

 Dana公司的Metzger表示：“大部分向我们询问高性能系统报价的客户都会问及汽车的扭矩矢量分配功能。从这里就可以看出，虽然燃油经济性是一个方面，但很多客户所关注的已不仅仅是汽车的燃油经济性这么简单，而是更注重汽车的整体性能以及驾驶乐趣。”
 

麦格纳的Sackl也表示：“我们的主要目标是实现汽车动力学性能以及减排能力的提升，二者缺一不可。最终，我们将以增加客户驾驶乐趣为主攻方向，提升汽车动力学性能，让客户真正享受到驾驶的乐趣。”

电驱动桥行业迎来快速发展期

无论最终推动电动车产业发展的是排放法规，还是消费者需求，电驱动桥行业都将迎来重大变化。接受《汽车工程》杂志采访的主要供应商们也都预测，电驱动桥行业将迎来巨变。
 

Dana公司的Lembke 表示：“看看过去5年中电气化行业发生的改变就知道，电气化行业正经历着内燃机与变速箱过去25年中所经历的变革。每个月都有OEM宣布:‘到X年，我们混合动力汽车或纯电动汽车的生产量将达到……’。尤其是在中国以外的地区，这样的现象每周都有发生。”
 

GKN传动系统部门的客户与项目工程副总裁StevenLaChance表示：“有了全轮驱动技术后，我们解决了很多系统集成方面的问题，这很棒。但电驱动桥技术更让人振奋，属于工程师们的激动人心的时代终于到来了。”

In the quest for ever-lower CO₂ emissions, and more immediate needs to retain vehicle access in some of the world’s most-populous cities, the e-axle has found its calling. With most dedicated, fully electrified platforms still gestating in OEM product pipelines, the e-axle seems an automotive component born to electrify current architectures.
 
Often described as eAWD, the e-axle is an electro-mechanical propulsion system contained in an axle structure housing an electric motor, the power electronics and some form of gearing/differential. Used in conjunction with a conventional ICE or hybrid powertrain, an e-axle can add electric propulsion to an unpowered vehicle axle, hence the eAWD moniker.
 
Consolidation of electronics, motor and differential makes the e-axle an ideal “bridge” component in terms of packaging flexibility, electrification and performance gains across vehicle segments. As nearly every transport segment will require some form of electrification to remain viable within the CO₂ emissions glidepath, the e-axle is an ingenious and in-demand solution.
 
As one Tier 1 noted dryly: “We're very busy in the space.”
 
Engineers interviewed for this article note that the e-axle business is booming, thanks to the technology’s adaptability. However, many Tier-1 suppliers are discovering that “modular” does not equal “plug-in.” The same interest that’s creating such demand is also fueling iteration complexity, along with new integration challenges for suppliers.
 
What’s the draw?
Even with certain market regions in flux due to political shifts, ever-stricter reductions in global CO₂ emissions continue. More recently, zero-emission zones in major cities are creating demand for products that can run electric-only. The e-axle is a single component that can provide multiple solutions.
 
Michael Lembke, senior director of electrification and hybridization at Dana, says that CO₂ regulations are still the biggest draw but, “they vary significantly by country.” China’s role is large, as it’s mandating electric vehicle percentages. “They're not just mandating fleet CO₂; they're telling the OEMs eight percent of your fleet have to be battery electric,” Lembke noted.
 
Though the latest Chinese regulations will boost e-axle applications, other markets are seeing increased electrified need.
 
“It's not just China, where we have the biggest growth rate over the next couple of years,” explained Theo Gassmann, VP of advanced engineering for GKN Driveline. “All the premium OEMs in Europe and in the U.S. are investigating and investing in electric vehicles and plug-in hybrids. The CO₂ agenda, local pollution issues, zero emissions zones—all these are pushing hard on electrification.”
 
According to Graham Evans, a principal analyst in automotive technology at IHS Markit, the e-axle segment will see significant growth. “For our light-vehicle forecast from 2017 to 2023, we see compounded annual growth of 35% for front e-axles and 46% for rear e-axles,” Evans noted.
 
The IHS light-vehicle forecast includes front–e-axle applications growing from 750,000 units in 2017 to more than 4.5 million by 2023. During the same time frame, rear/AWD e-axle applications are expected to increase from 320,000/110,000 to more than 3 million/2.5 million, respectively. With major cities and other localities continuing to adopt zero-emission zones, commercial applications should only add to those projections.
 
“If you think about the commercial segment, the way people do work in a city and deliver goods, those vehicles would need to be fully electric,” said Seth Metzger, VP light-vehicle engineering at Dana. “Vehicles that move into the city and move out, they will have to go into a full-electric operating mode for some period of time.”
 
Rapid iteration, cost challenges
Fortunately for vehicle manufacturers, e-axles are a solution available now—and can be adapted to current architectures. “Many OEMs need to take an existing vehicle architecture, a vehicle that has been designed for an engine, transmission, prop-shaft, exhaust system and the fuel tank,” Dana’s Lembke explained. “And within that given architecture—because time is of the essence—fit an electro-mechanical propulsion system into that same vehicle architecture.”
 
Such adaptability is key to e-axle adoption. “Companies have legacy platforms, legacy architectures to be considered,” explained Walter Sackl, global director of product management, driveline systems at Magna Powertrain, “and the e-axle is one of the units which can transform an architecture into a hybrid system. The nice thing is that an e-axle usually fits into the packaging space of a standard vehicle.”
 
E-axle technology is also evolving swiftly, making the latest applications, even within legacy platforms, more easily packaged from even a few years ago. “Every new e-axle application has new features, more integration, and it’s more compact, more sophisticated than the generation before,” GKN’s Gassmann said. “When we started, it was an electric motor bolted to a transmission, an inverter somewhere in the trunk, with a thick cable connecting everything. Now we are talking about integrated systems with an inverter, motor and gearbox pretty much bolted together and it’s one turn-key unit.”
 
Any new system faces engineering challenges and the e-axle has its own set of hurdles. One of the largest is OEM customers trying to gain share in a segment that’s being driven more by regulation than consumer demand.
 
“Cost is extremely challenging,” admitted Dana’s Metzger. “A lot of the OEs have been upfront that they’re not making money on battery electrics. As we get more volume it could help [market] share and reduce the costs of batteries, motors and inverters. Ultimately, we have to drive those costs down, because the OEMs have to be able to make money.”
 
Volume will be key, as well as reducing pricing pressure on some of the raw materials going into e-axles. “It's the second most expensive component within an EV, and all this competition means that the cost per unit is going to come down fairly quickly,” said Evans from IHS Markit.
 
He said the primary concern is in the rare-earth metals within the motor. “There’s a lot of interest in developing motor applications that lean away from the use of rare-earth metals,” Evans explained, “so this supply-chain constraint won’t exist and won’t hamper growth.”
 
Beyond cost challenges, all the suppliers who spoke with AE mentioned NVH, which is an entirely new aspect in electrified platforms.
 
“Because electric cars are nearly silent inside and out, you tend to hear every noise from the gears, electronics and e-motor,” noted Michael Wetzel, head of product/project management axle drive systems, e-mobility division, ZF Group. “And as compact as the components are, you’ve a high power-to-weight ratio, and that makes it complicated to get all the NVH measures implemented.”
 
To address NVH on a system level, advanced simulation tools are needed. “From the very first approach, you have to keep in mind the individual components and their interactions,” said GKN’s Gassmann. “Then you have to have the experience to get the basic ingredients right so it’s quiet and efficient. It's about oil, it's about bearings, it's about advanced lubrication concepts to minimize churning losses.”
 
There are also the e-axle’s inherent high-speed inputs. While peak speeds in a high-performance ICE may be up to 8000 rpm, peak input speeds in the next-generation electric-propulsion system will approach 20,000 rpm—making them "a totally different animal and very challenging for NVH,” Gassmann asserted.
 
Another common challenge in the expanding e-axle business is managing complexity. “We have a history of around six powertrain arrangements in the traditional ICE world – front transverse-engine with FWD, front longitudinal-engine with RWD and so on,” said Magna’s Sackl. “When we transfer into those future architectures, we need to handle a large number of different architectures—maybe 42 and beyond.”
 
Closer OEM integration
For suppliers used to solving mechanical issues, this can mean working more closely with their OEM customers and with greater integration challenges, particularly on the software front.
 
“The OEMs are now pushing the whole system integration effort through the first-tier suppliers, which is a great opportunity but a big challenge as well,” acknowledged Gassmann. “It's a huge shift, because we used to do the mechanical integration and the gearbox stuff, but now we’re actually doing complete system integration.”
 
“In the past, a mechanical solution decided the function,” explained Magna’s Sackl. “Now, we’re transferring to electronics—this includes electronically controlled clutches for example—which makes things more complex because the software controls need to be embedded in the structure.
 
“The portion of the Tier-1’s scope is by far larger than it was,” he continued. “Now we deliver a large portion of the software function associated with it. So for us, a large portion of our engineering staff—almost 50 percent—are dedicated just to software function.”
 
48-volt vs. high-voltage
Where does 48-volt fit into the e-axle equation? Most of the suppliers we spoke to see continued growth of 48-volt to power a host of new comfort and chassis technologies, but the future of electrified propulsion will require higher voltages. “The main goal for 48-volt is to generate power for other systems,” Magna’s Sackl explained, “which is completely different from the high-voltage approach, where the primary aim is to propel the car.”
 
“We’re currently working mainly on high-voltage systems,” ZF’s Wetzel echoed. “In the high-voltage systems, you have power outputs of 50 kilowatts or more, and this gives the possibility to drive fully electric in the inner cities.”
 
“We don’t see a huge market for an e-axle with 48-volt, because the issue is with the torque it can deliver,” GKN’s Gassmann said. “You either have something that can provide some EV-drive capability downtown, with enough torque to get over the curb or up the gradient in the carpark, but then the system will only be available at lower speed.
 
“If you go for CO₂ [reduction], you need the system to operate at higher speeds as well, otherwise you’re losing all the CO₂ recuperation and boosting potential,” Gassmann continued. “To accomplish both, you have to add complexity to the transmission with a 2-speed, which is then spoiling the whole approach, because 48-volt is considered low-cost hybridization.”
 
Vehicle dynamics + CO₂
One of the key roles as suppliers, is helping create products that consumers want to buy, and not just via efficiency. With existing vehicle platforms, engineers can continue to reduce CO₂ and the size of the ICE with e-axle boosting. The real draw, however, will come from added capability and performance.
 
“Electric axles will probably eat some of the all-wheel-drive mechanical market, that's for sure,” GKN’s Gassmann noted. “And it’s not lost business; it’s actually gaining business, because an e-axle adds significantly more value than mechanical all-wheel-drive.”
 
“Where we've received RFQ’s for performance cars, the majority of those have had torque vectoring as a request, where you're not just purchasing it for fuel economy,” said Dana’s Metzger. “Of course, that’s a benefit—but it's also really for performance and the joy of driving.”
 
“Vehicle dynamics, plus CO₂, is our main goal. For us it’s not either or,” said Magna’s Sackl. “At the end of the day, we are positioning ourselves very much in the space of ‘fun to drive,' with vehicle dynamics to make consumers happy.”
 
Development accelerates
Whatever the draw, be it regulatory or consumer driven, there’s enormous activity in the e-axle field. The key players AE spoke with all noted the near-seismic shift in the industry.
 
“If you look at the pace of change, what has happened in electrification in the last five years, it’s equivalent to what happened with combustion engines and transmissions in the last 25 years,” opined Dana’s Lembke. “There's not a month that goes by where an OEM isn’t announcing, ‘by the year X, we’re going to have so many hybrids or so many battery electrics in production. We seem to see that almost every week, particularly out of China.”
 
Noted Steven LaChance, VP Customer & Program Engineering, GKN Driveline: “The all-wheel-drive side is exciting because we get to solve a lot of unique problems with integration. But the e-technology is really exciting. It's an exciting time to be an engineer.”
 
Author: Paul Seredynski
Source: SAE Automotive Engineering Magazine