综合式电动汽车传动系统的解决方案

Approach to an Integrated Electric Vehicle Drive Solution

1 动因

为在不断增加的燃油价格和必须减少CO2排放努力之间到达平衡，汽车制造商采用了以下不同途径。通过最佳的内燃机、传动系统以及更大范围的使用混合动力技术来满足今天CO2的排放指标。在今后几年内传统技术的改进仍将扮演重要角色［1，2］，而混合动力技术将在中期内对市场产生影响。在一个纯电动汽车的虚拟世界中，其它技术将被放弃，尽管各种混合动力技术的各种部件（电动机、蓄电池）可被重复利用。从这点看，混合动力技术经常被标注成一个“过渡期”的方案。但是，今天有大批专家认为用纯电动汽车完全替代现有车辆将在很久以后出现或者根本不可能实现。如果假设上述所有技术并行存在的话。因而开发包括混合动力在内的高效传统技术和诸如纯电动汽车的新型传动概念必须同时加速进行。

2 定义，电气化等级

对于即将到来的车辆电力传动而言，重要的是在讨论车辆电气化问题时采用同一种语言。在从电力传动中区分混合动力传动或者从串联结构从区分增程器时，给出一个清晰定义是必要的。

通常传动系统电气化的路标被描述成从“大内燃机、小电动机”到“大电动机、无内燃机”等这样一个连续报道。为明确技术方案，必须避免这种连续化描述。因此规范必须进行分类、组合，差别必须被定义而各组别间的技术等级必须建立，图1。

1 Motivation

In an effort to strike a balance between increasing fuel prices and the necessity to reduce CO2emissions，different paths are followed by the automotive industries.The pursuance of optimization measures concerning combustion engines and transmissions as well as the broader use of hybrid technology provide the possibility of achieving today’s CO2targets.

The improvement of conventional technologies will still play a decisive role in the near future［1，2］，while hybrid technology will affect the market in the medium term.In a fictional world of only pure electric drivelines，other technologies would be replaced，even if various components of the hybrid technology（electric motors，batteries）could be reused.On that score，hybrid technology is often labeled as“transitional”solution.However，nowadays lots of experts agree that an entire substitution by electric vehicles will take place very late or maybe never.It is rather assumed that all mentioned technologies will exist in parallel.Consequently the development of both，highly efficient conventional technology including hybrids and new driveline con－cepts such as drivelines for pure electric mobility，has to be pushed forward.

图1 不同功能等级和规范的电气化道路图Fig.1 Electrification road map with functional steps and specifications

混合动力车辆（HEV）

根据联合国组织的定义，混合动力车辆具有两种不同的功率转换设备和两种不同的能量存储装置。通常该类车辆在内燃机（ICE）和一个或多个电动机（EM）之间转换。因此被定义成混合动力车辆（HEV）。HEV的储能装置是向内燃机供油的油箱和向电动机供能的蓄电池（或超级电容）。

HEV的主要潜力是在减速期间可以回收动能。其优点是能够向电力储能装置输送和存储能量。当电动机作为发电机使用时，原来制动时所耗散的能量可转换成电能。ZF公司的经济型并联混合动力方案早已投放市场［1，2］。

纯电动车辆（EV）

纯电动车辆仅由电动机驱动，电能来源于蓄电池。电力驱动能够满足所有规范。EV的一种特殊变型是增程型电动汽车，除了满足上述条件外，这类车辆还可采用增程驱动模式，例如临时使用内燃机，如果内燃机与驱动桥相联的话。从定义上理解，该增程器仅用来扩展行驶里程而并不满足电力驱动的其它行驶规范（诸如最高车速、性能等）。EV车辆的增程器驱动形式类似于混合动力车辆。并联结构将内燃机直接联接到驱动桥上；串联结构则需要一个额外的发电机向蓄电池充电或向电动机供能，详见第五节。

2 Definitions，Steps of electrification

With the upcoming of electric vehicle drives it is important to speak the same language when discussing vehicle electrification issues.Clear definitions are essential to distinguish e.g.hybrid drives from EV drives or serial architectures from range extender solutions.

Often the roadmap of driveline electrification is described as a continuous ramp from “much ICE，little EM”to“much EM，no ICE”.To develop distinct technical solutions such a continuous approach has to be avoided.The specifications rather have to be separated and grouped，differences have to be identified and technological steps between the groups have to be found，Figure1.

Hybrid Vehicles（HEV）

By definition of the UNO a hybrid vehicle has two different power converters and two different storage devices.In comm on hybrid electric vehicles the converters are a conventional internal combustion engine（ICE）and one or more electric motors（EM）.Therefore the term hybrid electric vehicle arose（HEV）.The storage devices of HEVs are a fuel tank which supplies the ICE with energy and a battery（or super－capacitor）.

One of the main potentials of HEVs is the possibility to recover kinetic energy in deceleration phases.It is taken advantage of the ability of the electric storage device to deliver and to collect energy.Thereby energy，that was formerly lost while braking mechanically，can be transformed into electric energy by using the EM as a generator.Economic parallel hybrid solutions from ZF are in the market already［1，2］.

HEV EV的差别

上述两种驱动的差别主要在于为实现传动系统所有行驶规范所采用的功率转换方式。在HEV中100%来源于内燃机，而在EV中则来自于电动机。

3 电动汽车行驶－发展目标

电动汽车常被称为未来车辆。事实上零排放的潜力似乎很具吸引力。对使用者而言，必须了解电力驱动的优缺点。图2归纳了一些重要因素。

图2 最终用户的电力驱动优缺点Fig.2 Drawbacks and benefits of electric mobility for the end customer.

在开发传动系统时，感兴趣的是纯电动汽车的传动和常规传动系统的不同之处。显然装置的特殊化因素发生了变化。先前的标准如换档品质、响应和换档模式等的重要性下降，而传动系统的效率、质量和变速器的声学特征等变得很重要，图3。

将来纯电动汽车传动系统最重要的特性之一是效率。即便在全新紧凑型车身的设计中加热、冷却成为非常重要的议题，或者纯电动汽车关键部件－蓄电池在功率能量密度方面取得突破的话：绝大部分的能量传递仍将出现在传动系统中，该系统包括逆变器、电动机和介于电动机及车轮之间的变速器。传输必须尽可能的高效，尤其是在驱动/制动双向传输工况时。

Electric Vehicle（EV）

An electric vehicle（EV）is driven exclusively by electric power coming from the vehicle battery.All driving specifications can be fulfilled by the electric drive.A special variant of EVs are range extended electric vehicles，for which the last mentioned condition is valid as well，but with the addi－tion that these vehicles might be driven by the range extender device–e.g.an ICE–temporarily，if this device is connected to the drive axle.The range extender by definition does only extend the range and does not fulfill other driving specifications（like top speed，performance...）than the electric drive.The drive structures of range extender EVs are comparable to those of hybrid drives.Parallel structures are offering a direct connection of the above mentioned ICE to the axle；serial structures need an additional generator to re－charge the battery or to supply the motor with energy，see chapter 5.

Differentiation between HEV and EV

Like mentioned above the difference between the two drives should be defined by the power converter which is designed for all driving specifications of the driveline.In an HEV 100per cent of the spec shall be fulfilled by the ICE，in an EV all of by the EM.

3 Electric Vehicle Drives– Development goals

Electric vehicles very often are described as the future mobility solution.And indeed the potential of emission free driving seems to be attractive for the benefit of mankind.The meaning of electric mobility for the user–with all its benefits and drawbacks–has to be mentioned as well.Figure2 summarizes the important factors.

For the development of driveline systems it is of great interest how the characteristics of EV drives differ from the conventional ones.Obviously the set of differentiation factors changes.Former criteria like shift quality，response and shift patterns lose importance，while the efficiency of the driveline，its weight as well as the acoustic behavior，e.g.of the transmission，become very important，Figure3.

One of the most important characteristics of an EV drive of the future will be the driveline efficiency.Even if there will be energetically important issues like heating and cooling with new impacts on the cabin design，and even if battery developers will achieve big successes concerning the power and energy density of the EV drive’s key component：the biggest part of energy transformation will still take place in the driveline system consisting of the inverter，the electric motor and the transmission between the latter and the wheels.This transformation has to be as efficient as possible，especially because it occurs both for driving and braking（regeneration）phases.

从传动系统开发者的观点看，最重要的是研究整个动力传输链即包括“逆变器、电动机、齿轮”的整个系统而不是单一部件。必须加以综合考虑，见图4。

图3 纯电动汽车卖点的变化Fig.3 Change of Unique selling propositions（USP）of electric vehicles

图4 纯电动汽车的集成水平。水平2有望实现在经济性和效率方面的最佳系统Fig.4 Integration levels for electric drives.Level 2promises best results in terms of system optimization towards economics and efficiency

4 ZF公司的纯电动汽车传动系统方案

ZF EVD1（纯电动汽车传动系统）是ZF已经研制成功的第一个产品。在这个高速方案中，所有动力电子设备、电动机和机械（例如传动、轴承）方面的知识和总体设计（处于水平1）的各种可能性都被用来开发一套电动传动系统，使其满足质量轻、紧凑、高功率密度和声学最优。图5。

From the driveline developer’s point of view it is absolutely crucial to focus on the whole chain of power transformation，i.e.the complete system“inverter，motor，gears”instead of the single components.An integration approach has to be considered as shown in Figure4.

4 ZF’s solution for an Electric Vehicle Drive

A first approach has been made by ZF with the ZF EVD1（Electric Vehicle Drive 1）.In this high speed solution all knowledge on power electronics，electric motors and mechanics，like gearings and bearing concepts and all possibilities of integrated design（on level 1）have been used to develop an electric vehicle drive which is light－weight，compact，highly power dense and acoustically optimized，Figure5.

图5 作为集成化设计成果的ZF EVD1 Fig.5 ZF’s Electric Vehicle Drive 1as result of an integrated design

速比

经常讨论的问题是关于EV传动系统的速比。许多专家都同意，在假设电动轿车主要用于通勤目的，也就是短途行驶的话，其最高车速约在130km/h［3－7］左右这样一个较低的水平。同样也假设该紧凑型轿车的爬坡能力一般。因此可以设想驱动桥最大转矩约为1 200Nm。将转矩和车速这两个主要需求与现代化电动机的转矩－车速图相比，便能够清晰地发现无需多级变速，除非最高车速或最大转矩超过了上述限值。而行驶效率则必须加以考虑。图6是该研究的仿真结论。

图6 与单级方案相比，两级速比变速器在EVD1中的效率优势相当小。其优势在于更好地利用电动机的效率图。这些优势（部分优势）被附加部件和质量以及换档操作所抵消Fig.6 The efficiency－wise advantage of a two－speed transmission in an EVD1versus a simple one－speed solution is quite low.The advantages are linked to the better use of the efficiency map of the electric drive.These advantages are（partly over－）compensated by losses originated by additional parts and masses as well as by operations like the synchronization during shifting

高速传动概念

为获得小尺寸、低质量和低成本方面的优势，ZF决定电动机采用高速传动概念，其最高转速达到21 000r/min同时变速器速比约为16。高速传动概念可减轻电动机质量约50%，而采用异步技术（ASM）带来的功率密度劣势也可被消除。这意味着，降低成本和质量的双重目标可通过采用高速传动的ASM实现，图7。

Number of speeds

A discussion often starts when the question of gears for an EV driveline arises.Many authors agree on the assumption that an electric passenger car serves mainly for commuter purposes，i.e.on short trips with relatively low maximum speeds around 130km/h［3to 7］.Also it is commonly assumed that the vehicle’s gradeability is that of a compact car in general.So a good assumption for a maximum axle torque could be around 1.200Nm.Comparing these two main demands–torque and speed–to the torque－speed map of modern electric motors，one can easily find out that multiple transmission ratios are not needed unless top speed or maximum torque exceeds the mentioned limits.At second sight the efficiency in cycles and customer routes has to be taken into account.The result of such an investigation by simulation means is described in Figure6.

High speed concept

To achieve the advantages of small package，low weight and cost advantages in comparison to standard approaches ZF decided to choose a highspeed concept with an electric machine that features a maximum speed of 21，000r/min and a transmission with a mechanical ratio of approximately 16.The high－speed concept reduces the weight of the electric machine by about 50per cent，and even the disadvantage of the asynchronous technology（ASM）in terms of power density can be eliminated.This means，that both targets，cost and weight reduction，are reached by choosing an ASM in a high－speed concept，Figure7.

Overall Efficiency

Besides compactness，efficiency is another target for an electric vehicle drive.With respect to this criterion，the advantages of a PSM over an ASM become negligible when choosing a high－speed concept.Although ZF has chosen the ASM technology，the technology investigation showed that both electric machine technologies（PSM and ASM）are feasible for the high speed concept with comparable performance data.The ASM technology was chosen，but for future developments the PSM technology is still a viable option.

图7 采用高速传动降低质量和成本。电动机转速为21 000r/min变速器速比为16时的最高车速为150km/hFig.7 Weight－and cost reduction due to high－speed concept.The transmission ratio 16gives a top speed of 150km/h at an electric motor rotational speed of 21 000r/min.Mass质量 Ratio速比

总效率

除了小型化，效率是纯电动汽车传动系统追求的另一个目标。就该标准而言，一旦选择高速传动后，PSM（永磁同步电机）相对于ASM（异步电机）的优势可忽略不计。因此ZF采用ASM技术，技术研究表明在上述参考数据前提下两种电机均能采用高速传动概念。目前采用ASM技术，将来PSM仍将是一项可行的选择。

就效率而言必须了解相应的工作模式。因此必须根据其性能需求分析消费者行驶循环与官方行驶循环的不同。分析发现，处于低或中等负荷下即接近车辆零阻力曲线附近时电力驱动车辆的效率是非常重要的。对应于预期最重要的城市使用工况，40－120km/h的车速范围是一个有效的选择，这时就功耗而言，高速远比低速更重要。因此，EVD1的电动机被设计成在该模式下具有最佳的效率，图8。

顺便提及，该发现并不适用于以电动机或发电机模式经常工作在最高可用功率的HEV驱动。

EVD1已造出数台样机，目前正在评估性能指标。试验在台架上进行图9。试验结论表明其载荷/效率和声学特性很有前途。样车上的集成工作正在进行中。EVD1的试验情况即将公布。

5 增程器概念

纯电动汽车的瓶颈在于蓄电池。改进不仅涉及到功率密度、成本，还涉及到充电时间等。由于这些方面的进展很慢，目标定义为减少每车所需的总蓄电池量。为能够使用容量有限的蓄电池，用户要么接受行驶里程受限，或者采用通过车载附加能源扩展里程的装置。通常该能源是燃油，用于驱动一台小型内燃机。发动机的动力可经发电机转化为电能向蓄电池充电，或者直接驱动电动机（串联结构）。另一种选择是机械能直接作用在驱动桥上。这通过一套附加的传动装置（并联结构）实现。上述两者均有其优缺点，这将导致两者共存，正如并联和功率分流混合动力那样，图10。

With respect to efficiency the most relevant operating modes have to be known.Therefore different customer driving cycles and official driving cycles were analyzed with respect to their performance requirements.It could be found out，that especially for vehicles with electric traction the efficiency at small and medium loads，adjacent to the natural vehicle resistance curve，is of utmost importance.With the anticipated predominant urban usage profile，the speed range of 40to 120kph seems to be a viable choice，where，with respect to consumed power，higher speeds are more important than lower speeds.Therefore the EVD1electric machine is designed for optimum efficiency in these operation modes，Figure8.

This finding does–by the way–not apply to electric motors of HEV drives，which are most frequently used at maximum available power，be it in motor or regeneration mode.

The EVD1has been built up in several prototypes and is currently under thorough evaluation on the promised properties.Therefore test runs on load rigs are evaluated，Figure9.The results are– both with respect to load/efficiency and to acoustics–promising.The integration in a demonstrator vehicle is currently running.Experiences with driving the EVD1will be reported in presentations yet to come.

5 Range－Extender Concepts

The main bottleneck for pure electric mobility is the battery.Improvements have to be made particularly with regard to power density and costs，but also in terms of duration of recharge.Since these successes will be achieved slowly，efforts aim at reducing the required amount of stored electric energy per car.To enable the use of batteries with restricted capacity，either customers need to accept the limited range or devices are required that are able to extend the range by carrying extra energy supply onboard.Generally this energy is fuel which powers a small combustion engine.The engine power may then be transformed by agenerator into electric energy in order to charge the battery or to directly drive the motor（serial architecture）.As an alternative the mechanical power may also be directed to the axle drive.This can be managed by an additional transmission module （parallel architecture）.Both concepts have their pros and cons，which may result in a coexistence of solutions similar to those of parallel and power split hybrid drives，Figure10.

图8 工况分析Fig.8 Analysis of relevant operation modes

图9 试验台架上的EVD1样机Fig.9 EVD1prototypes on the test rig

图10 EV传动系统增程器构架Fig.10 Range Extender architectures as derived from an EV driveline

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