工程機械用混合型動力傳動系統(tǒng)

1 前言

對于工程機械而言,不斷增加的燃油價格和環(huán)保要求使得降低油耗日趨重要。此外,排放法規(guī)Tier 4的引入將導致車輛做出重大改進,因為需進行廢氣后處理。車輛的變動在部分程度上會對油耗起負面影響。目前正在開發(fā)的變速器和驅(qū)動橋?qū)⑹墙窈髾C動車傳動系統(tǒng)的重要基礎(chǔ)。這些系統(tǒng)通過不斷提高效率和優(yōu)化與內(nèi)燃機、變速器和液壓系統(tǒng)的協(xié)同工作可明顯地降低油耗。在提高性能的同時進一步降低油耗和排放,則必須采用其他途徑。這就可以看出電力驅(qū)動和混合動力在機動車方面應用的巨大潛力。但是,從這些技術(shù)的應用經(jīng)濟性考慮,必須抓住在機動車各個分支方面的應用可能性,例如商用車輛。

2 ZF的混合動力系統(tǒng)

在ZF公司,各種形式的商用車輛混合動力系統(tǒng)正處于產(chǎn)品開發(fā)階段。它們包括適用于城市公交的EcoLife-Hybrid和適用于貨車的Hy-Tronic。上述兩系統(tǒng)使用的電動機為120和60 kW,并具有強混合動力系統(tǒng)的功能。

1 Introduction

Increasing fuel prices and increased environmental requirements lead,for construction machinery,to an increased importance of measures to reduce fuel consumption.Also,the introduction of the exhaust regulation Tier 4 leads,for meeting the regulation,to drastic changes on the vehicle because of the required exhaust gas after-treatment.These changes on the vehicle partly affect the fuel consumption in a negative w ay.An important basis for future drivetrains of mobile machinery is provided by the currently developed transmissions and axle systems.These systems lead,through their continuous optimisation with regards to efficiency and an optimalcooperation ofcombustion engine,transmission and hydraulics,to clear fuel savings.To further reduce fuel consumption and exhaust emissions,whilst increasing the performance,additional degrees of freedom have to be created.This is where an electrification and hybridisation can show a great potential for application to mobile machinery.However,for an economic application of these technologies,despite the relatively low production volumes,one has to seize all possible synergies with other vehicle branches,e.g.commercial vehicles.

ZF所提供的產(chǎn)品除了變速器集成和電動馬達之外,還包括鋰離子蓄電池、動力逆變器和混合動力控制單元等必要的混合動力組件。系統(tǒng)還能配備用于隨車供能DC/DC轉(zhuǎn)換器以取代交流發(fā)電機。由ZF提供的混合動力控制軟件具有部件控制、混合動力功能運轉(zhuǎn)和車輛功能策略等功能。

圖1是為轎車和商用車輛開發(fā)的ZF模塊化軟件系統(tǒng)中關(guān)于混合動力主要功能的一覽表。它包括典型的混合動力功能例如發(fā)動機起停系統(tǒng)、制動能量回饋、發(fā)動機工作點的換檔、電力驅(qū)動以及在內(nèi)燃機和電動汽車之間動力管理的混合動力策略。

這些模塊化的硬件和軟件系統(tǒng)作為機動車混合動力系統(tǒng)的基礎(chǔ)。附加功能例如改善排放的換檔工作點、發(fā)動機穩(wěn)定性和輔助動力需求等主要集中在工程機械和農(nóng)業(yè)機械方面。

ZF-部件

ZF公司在開發(fā)電力驅(qū)動方面已有超過15年的經(jīng)驗。在2008年5月ZF向奔馳公司S級轎車提供混合動力模塊的批量化產(chǎn)品。該混合動力系統(tǒng)的電動馬達是帶單個線圈的永磁同步電機。其優(yōu)點的無需水冷、耐用、高轉(zhuǎn)矩和功率密度和高效率。

用于工程機械和農(nóng)業(yè)機械的Hy-tronic的電機功率提高到85 kW,此外EColife的電機功率為120 kW;而且采用了較小的法蘭直徑(SAE2/3).持續(xù)功率依據(jù)電機尺寸不同在50 kW到70 kW之間。

2 Hybrid Systems at ZF

At ZF,various hybrid systems for commercial vehicles are in production development.These include amongst others,the EcoLife-Hybrid for city buses and the Hy-Tronic for trucks.The electric machines,used on these tw o systems have a power of 120 and 60 kW respectively and offer the functionalities of a full-hybrid system[1].

The scope of supply of ZF includes not only the,in the transmission integrated,electric machine,but also all other necessary hybrid components ranging from the Li-Ion battery over to the power inverter and a hybrid control unit.The system can be furthermore equipped with a dc/dc con verter for onboard power supply to replace the alternator.The software on the hybrid control unit provided by ZF can include functions for component control,operative hybrid functions and strategic vehicle functions.

圖1 各種用途的混合動力系統(tǒng)功能Fig.1 Functions of hybrid systems for different applications

產(chǎn)生旋轉(zhuǎn)磁場和控制電機工作的功率逆變器采用彈簧接觸型IGBT(絕緣柵場效應晶體管)和二極管以獲得所需的耐用性和和高功率密度。經(jīng)優(yōu)化的動力電子設備用來控制電機工作和提供高效率。這是ZF公司專門為商用車輛開發(fā)的,并且結(jié)構(gòu)堅固、使用壽命長。

混合動力蓄電池與大陸公司聯(lián)合開發(fā),特別針對商用車輛的高耐久性要求,該電池是水冷鋰電池,帶電池集成管理系統(tǒng)。電池設計模塊化,因而能夠組成兩個不同能量的電池尺寸。

圖2所示的部件構(gòu)成了一個并聯(lián)式混合動力系統(tǒng)的基礎(chǔ),也適用于各種機動車。特別是那需要工作在高車速和高牽引力(例如商用車輛或裝載機、物料輸送機)的混合型動力傳動系統(tǒng),該系統(tǒng)的布局優(yōu)點明顯。同時,這些部件還能應用于一系列的混合動力系統(tǒng),例如回轉(zhuǎn)驅(qū)動式挖掘機或小型叉車等。

圖2 ZF開發(fā)的并聯(lián)式混合動力產(chǎn)品Fig.2 ZF-Components in parallel hybrid architecture

ZF-系統(tǒng)功能

圖3是未來機動車動力傳動系統(tǒng)概念中最重要的需求。電力混合動力能夠滿足每項需求。

特殊需求的輔助設備,例如發(fā)動機停機時的空調(diào)壓縮機,對機動車而言是非常重要的。采用電力混合動力后發(fā)動機的啟動可很方便地通過電機實現(xiàn)。

圖4顯示了當輔助動力為5 kW電動機、車輛處于靜止狀態(tài)時內(nèi)燃機的間隙工作狀況。這里,輔助動力由蓄電池供電,一旦SOC(儲電量狀態(tài))過低則繼續(xù)充電。隨后,內(nèi)燃機必須工作一小段時間用于向蓄電池充電。在短時充電期間如果伴隨高負載,內(nèi)燃機將在燃油消耗率較低的點上工作。這樣可平均節(jié)省22%的油耗。靜止狀態(tài)時性能規(guī)格越低,節(jié)能潛力越高。由于儲存在蓄電池中的能量可在較長的時間內(nèi)向輔助動力供能,因此內(nèi)燃機的起動頻率降低。根據(jù)文獻[2],在一輛輪式裝載機的負載周期內(nèi),怠速工況可達14%。

Figure 1 shows an overview over theimportant hybrid functions out of the ZF modular software system for different production projects for passenger cars and commercial vehicles.This includes typical hybrid functions like stop/start of the engine,recovery of brake energy,shift of engine operating point,electric driving and a hybrid strategy for a power management between combustion engine and electric machine.

This modular system of hardware-and software components serves as a basis for the hybrid system for mobile machinery.Additional functions like shift of operation point for improved exhaust emissions,stabilization of the engine and power on demand operation of vehicle auxiliaries are developed focussing on the requirements of construction and agricultural machinery.

ZF-Components

ZF has experience with the development of electric drives for over 15 years.In May 2008 the ZF production facility for hybrid modules for supply of the Mercedes S-class hybrid was opened.The electricmachines used in the hybrid systems are of permanent magnet synchronous type with single coils.These excel through their no-wear behaviour,robustness,high torque and powerdensity and through their high efficiencies.

The power of the electric machine from the Hy-tronic was increased to 85 kW for use in construction and agricultural machinery,thus represents,besides the bigger electric machine from the EColife with 120 kW,a powerful variant for smaller flange diameters(SAE2/3).The permanent power depends on the size of the electric machine and amounts to either 50 kW or 70 kW.

The power inverter employed to produce the rotating magnetic field and to control the electric machine is build up with spring contacted IGBT's(insulated gate bipolar transistor)and diodes in order to achieve the required durability and has a high power density.The power electronics are optimised for the control of the electricmachine and offer a high efficiency.It was developed by ZF specifically for the mechanical and electric requirements of commercial vehicles and thus features a rugged construction and high durability.

圖3 需求和功能Fig.3 Requirements and functions

圖4 帶起停系統(tǒng)和不帶起停系統(tǒng)的對比Fig.4 Comparison of a system with and without stop/start

The hybrid battery was developed together with Continental specifically for the high durability demands of commercial vehicles and is of water cooled Li-Ion type with integrated battery management system.The design of the battery is modular and can hence be built up in tw o different sizes with different energy contents.

The components depicted in Figure 2 form the basis for a parallel hybrid architecture,which is also favoured for most applications for mobile machinery.Specifically,for the hybridisation of the drivetrain,which requires operation at relatively high vehicle speeds and tractive efforts(e.g.commercial vehicles or wheel loader,material handling machinery,etc.),this system layout provides clear advantages.However,the available components can also beemployed for a serial hybrid system,for application in,e.g.an excavator swing drive or for small fork lifts.

在工程機械和農(nóng)業(yè)機械在道路上行駛時,也就是在運輸作業(yè)或進行穩(wěn)定的重復工作循環(huán)并使用制動工況時,對制動能量的回收有興趣。這些被回收的能量能夠儲存在蓄電池中用來驅(qū)動輔助動力裝置或通過電動機輔助內(nèi)燃機驅(qū)動車輛。這附加的動力能夠提升總工作性能。而且總工作性能的提升還可通過電氣化輔助裝置的能量管理來實現(xiàn)。電氣化輔助裝置響應迅速,因而優(yōu)于液壓或機械輔助裝置。當動力傳動系統(tǒng)和液壓工作系統(tǒng)需要的動力多于內(nèi)燃機能夠提供給輔助裝置時,通過短時間關(guān)閉輔助裝置,可使被動推進變成可能。

起停系統(tǒng)和內(nèi)燃機穩(wěn)定工作可使發(fā)動機怠速和瞬變工作狀態(tài)時的排放降低。在圖中,顆粒物的增加是由于轉(zhuǎn)矩的突然增加所致。通過平穩(wěn)增加內(nèi)燃機轉(zhuǎn)矩(穩(wěn)定性)避免轉(zhuǎn)矩突然增加,可消除顆粒物的峰值?；旌闲蛣恿鲃酉到y(tǒng)能夠在動轉(zhuǎn)矩階段補償發(fā)動機動態(tài)性能的降低和維持車輛動力特性。

除了有益于發(fā)動機穩(wěn)定工作之外,內(nèi)燃機工作點的切換也成為可能,因而影響顆粒物過濾器的再生性能(主動和被動)和利用所需的自由燃燒功率使顆粒物過濾器以一種積極的方式再生。

另一種降低油耗和減少排放的途徑是通過縮小發(fā)動機尺寸以降低發(fā)動機功率。較小的內(nèi)燃機所減少的功率由電動機補償,推動功率來源于存儲在蓄電池中的能量。這僅適用于那些短時全功率或經(jīng)常處于低功率的場合。較小內(nèi)燃機減少的成本可部分補償混合系統(tǒng)所帶來的較高成本。由于減少了排放廢氣后處理系統(tǒng)變得簡單,這將有益于降低總成本。另外這些簡化有益于整個廢氣后處理系統(tǒng)。

粗看的話,電力驅(qū)動對于工程機械興趣不大。但是零排放在某些場合(貨物搬運)尤為重要,特別是那些對排放嚴格限制或零排放的場所(港口、機場等)。部分電力驅(qū)動也可用于短距離機動、輔助驅(qū)動、回轉(zhuǎn)(場地盡頭)、建筑物內(nèi)的短時工作等。

液力與電力混合驅(qū)動的對比

液力混合驅(qū)動能夠覆蓋高功率并且在加大推力時能夠快速驅(qū)動傳動系統(tǒng)。但是電力混合驅(qū)動的優(yōu)勢在于效率、總存儲能量、能量存儲長時間穩(wěn)定性和系統(tǒng)動力性和可控性等。因此,發(fā)動機工作點的切換、發(fā)動機穩(wěn)定工作和能量回收等能夠被理想地利用。圖6給出了20km/h車速時液力和電力系統(tǒng)能量回收和助力的差異。在制動能量回收的仿真中,液力混合驅(qū)動的效率為59%而電力混合驅(qū)動為73%。因此電力系統(tǒng)具有更高的助力特性。即便加上可忽略不計的牽引損失,電力混合系統(tǒng)由于能量回收和助力性能優(yōu)越因而效率更高、燃油經(jīng)濟性更好。

ZF-System Functions

Figure 3 shows the most important requirements for future powertrain concepts for mobile machinery.Every one of these requirements can be covered by a function of an electric hybrid.

Specifically the supply of auxiliaries,like e.g.the climate(a/c-)compressor,during engine stop,has a high importance for mobile machinery.With electric hybrids a fast cranking of the engine can be carried out comfortably through the electric machine.

Figure 4 shows the intermittent operation of the internal combustion engine when the auxiliaries must be supplied with 5 kW electrical power during standstill.Here,the auxiliaries are fed from the battery until the lower SOC(state-of-charge)-is reached.Subsequently,the internal combustion engine must be operated again for a short period of time in order to charge the battery.During this short recharge period with high load,the operating point of the combustion engine can be raised to a point with lower specific fuel consumption.Doing this,an advantage of 22%can result in fuel consumption reduction.The lower the performance requirement is during standstill,the higher the reduction potential will be.The frequency of combustion engine starts decreases,because the energy stored in the battery can supply the auxiliaries for a longer time period.According to[2],the timeshare in idling lies at 14%for the load cycle of a wheel loader.

圖5 轉(zhuǎn)矩突變時顆粒物排放峰值(左圖)和發(fā)動機穩(wěn)定工作的顆粒物排放(右圖)Fig.5 Particulate matter emission peak during sudden torque step(left)and principle of engine stabilization(right)

In roading conditions of construction and agricultural applications,i.e.during transport w ork or steadily recurring work cycles with braking opera-tions,the recovery of brake energy is interesting.Such recovered energy can be stored in the battery in order to operate the electrical auxiliaries or to support the combustion engine(boost)with the electric machine.An increase of overall performance can be achieved by this additional active boost.Furthermore,the overall performance can be increased by smart power management of the electrified auxiliaries.Electrified auxiliaries have an advantage over hydraulically or mechanically fixed ones in the ability to be operated on-demand.By switching the auxiliaries off for a short period of time,a so-called passive boost becomes possible,when drivetrain and working hydraulics need more power than the combustion engine could supply in auxiliary loaded condition.

圖6 液力和電力系統(tǒng)效率對比Fig.6 Comparison of Efficiencies of the hydraulic and electric system

同樣,電氣化輔助裝置額外的潛能也是液力混合系統(tǒng)所不具備的。理論上,起停功能也能在液力系統(tǒng)中實現(xiàn)。但是,當發(fā)動機停機時需要輔助裝置(例如空調(diào)壓縮機)工作時,液力系統(tǒng)很難以最優(yōu)的方式實現(xiàn),因為液力傳動損失和液壓蓄能器較低的長期儲存性。而且在使用高動力性功能時(例如發(fā)動機穩(wěn)定),不管最大功率是否較小,電力混合系統(tǒng)在轉(zhuǎn)矩突然變化時在動力性和可控性方面具有優(yōu)勢?；谀軌虼蠓冉档陀秃暮兔黠@增強功能,ZF公司首先開展了電力曲軸起動機系統(tǒng)的研發(fā)。在建筑機械和農(nóng)業(yè)機械領(lǐng)域,ZF公司預見到電力混合系統(tǒng)在中長期應用方面的明顯優(yōu)點。

混合系統(tǒng)在工程機械上的應用

電力混合系統(tǒng)在工程機械方面的應用見圖7。請注意應用于動力傳動系統(tǒng)的混合系統(tǒng),ZF關(guān)注的是并聯(lián)結(jié)構(gòu),這里電動機安裝在發(fā)動機和變速器之間,向發(fā)動機提供附加動力(并聯(lián))或單獨驅(qū)動。系統(tǒng)以兩種方式構(gòu)成。

1、作為完整系統(tǒng)的一部分,電動機被集成在變速器的殼體上(見圖8)。

With the stop-start function and by stabilizing the combustion engine,exhaust emissions can be reduced during engine idle and transient operating conditions.In Figure 5,the increase of raw emissions of particulate matter can be seen,as caused by a sudden torque rise.Avoiding sudden torque steps by slowly guiding the combustion engine torque(stabilization),eliminates emission peaks of particulate matter.The hybrid drivetrain compensates for the reduced engine dynamics and keeps the overall vehicle dynamics unaffected during dynamic torque steps.

圖7 工程機械用并聯(lián)式混合系統(tǒng)Fig.7 Parallel hybrid system for construction machinery

圖8 ZF ERGOPOWER混合動力系統(tǒng)Fig.8 ZF ERGOPOWER Hybrid

Besides the advantages of engine stabilization,a shift of the operating point of the combustion engine is also possible,thus influencing the regeneration behaviour(active and passive)of the particle filter and the required free-burning power to regenerate the particle filter in a positive way.

An additional approach to reduce fuel consumption and exhaust emissions is to reduce combustion engine power by downsizing the engine.The power reduction of such a smaller combustion engine will be compensated by the electric machine and boost pow er from stored energy in the battery.This is a specifically viable option in applications that need full engine power for only short periods of time or be operated in low load conditions frequently.The usage of smaller combustion engines reduces cost and partially compensates for the higher cost of a hybrid system.Due to reduction of emissions,exhaust gas after-treatment can be simplified,w hich will in turn again positively affect overall cost.Additionally these simplifications can lead to advantages in the packaging of complex exhaust gas after-treatment systems.

Electric driving appears to be less interesting for mobile construction machinery at first glance.

2、作為一個模塊,電動機安裝在自帶的殼體上,與內(nèi)燃機或變速器聯(lián)接。

電動機(85或120 kW)通過功率逆變器和650V直流連接器與鋰電池相連接。ZF公司的混合系統(tǒng)不僅包括硬件,也包括用于控制混合系統(tǒng)組件(起停系統(tǒng)、能量回收等重要功能)的軟件和混合驅(qū)動策略軟件(動力管理)。并聯(lián)式混合系統(tǒng)具有多種應用場合,例如輪式裝載機、集裝箱起重機、伸縮臂叉車、升降式叉車、自卸貨車和拖拉機等。

圖8是一套混合系統(tǒng),包括Ergopower變速器以及集成在混合驅(qū)動模塊殼體上的電動機、分離軸承和位于液力變矩器(包括鎖止離合器)前方的扭轉(zhuǎn)減振器。這樣布置非常緊湊。在可以利用變速器潤滑油回路時,行星齒輪從電動機處獲得更大動力或通過離合器切斷內(nèi)燃機是可行的。混合驅(qū)動模塊也能和ZF公司的其它變速器相連接,例如最新開發(fā)的液力機械動力換檔無級變速器cPOWER。

圖9 開鑿機混合驅(qū)動系統(tǒng)Fig.9 Hybrid system for excavators

圖9是開鑿機混合系統(tǒng)的設計圖。系統(tǒng)由動力傳動系統(tǒng)和工作液壓系統(tǒng)組成的并聯(lián)式混合驅(qū)動,為回轉(zhuǎn)驅(qū)動提供連續(xù)的混合驅(qū)動。在這里,相同的混合驅(qū)動模塊可被應用于上述提及的并聯(lián)式混合系統(tǒng),例如輪式裝載機等。將混合驅(qū)動模塊安裝在內(nèi)燃機和液壓馬達之間,并聯(lián)式混合驅(qū)動的典型功能(例如起停、助力)可由液力馬達和動力傳動系統(tǒng)實現(xiàn)。安裝在混合驅(qū)動模塊中的電動機和逆變器也能用于回轉(zhuǎn)作業(yè)。與鋰電池相聯(lián)后,回轉(zhuǎn)運動的能量能夠被回收和再次用于助力。圖9是開鑿機的混合驅(qū)動系統(tǒng),可安裝在軌道和輪式開鑿機上。

However,the possibility of zero-emission operation can be of particular importance for some applications(material handling),particularly on sites with highly restricted emission limits or even zero-emission requirements(ports,airports or similar).Partial electric driving could also be useful e.g.over short distances,launch assist drive,turning around(field end),short-term operation in buildings,etc.

Comparison of hydraulic and electric hybrid

Hydraulic hybrids are able to recover high power and quickly supply it back into the drivetrain when boosting.However,electric hybrids have advantages over the hydraulic hybrids,such as efficiency,amount of storable energy,suitability for long-term storage as well as system dynamics and controllability.Therefore,functions like shift of engine operation point,stabilization and energy recovery can be utilized ideally.Figure 6 shows the recovery of energy and boosting from 20 km/h and the difference in efficiency between the hydraulic and electric system.In the simulation of this recovery of braking energy the hydraulic hybrid showed an efficiency of only 59%,whereas theelectric system showed an efficiency of 73%.Therefore more boosting energy is available in the electric system.This efficiency advantage of the electric hybrid system,together with the negligible drag losses of the electric hybrid,leads to significantly higher fuel consumption advantages for functions like energy recovery and boosting.

Also,the additional potentials through an electrification of auxiliaries and implements cannot be obtained with a hydraulic hybrid system.Theoretically,a stop-start function can be realized with a hybrid system as well.However,when the supply of auxiliaries(like an air conditioning compressor)is required during engine standstill,this can not be achieved in an optimal way with a hydraulic system,because of losses in the drives and the low-er long-term storability of a hydraulic accumulator.Furthermore,when using highly dynamic functions(e.g.stabilization)the electric hybrid system has advantages regarding dynamics and controllability during sudden torque step demands,despite the lower max.power capability.Based on the significant higher reduction of fuel consumption and because of significantly increased functional possibilities,ZF pursues simple electric crankshaft-starter systems with and without additional battery already in the first step.In all construction and agricultural applications ZF sees significant advantages for the electric hybrid system on a mid-and long-term basis.

Hybrid System for Construction Machinery

The layout of an electric hybrid system for construction machinery is show n in Figure 7.Looking at hybrid systems for the drivetrain,ZF focuses on the parallel architecture,in which the electric machine sits betw een combustion engine and transmission,supplying the power additional(parallel)to the combustion engine or alone by itself.This system can be implemented in two ways：

1. As an integrated layout,which means that the electric machine is integrated in the transmission housing(see Figure 8)

2. As a modular layout,which means that the electric machine is located in its own housing,being mounted either on the combustion engine or the transmission?

The electric machine(85 or 120 kW)is connected to the Li-Ion battery through a power inverter and a 650 V DC linkage.TheZF hybrid system comprises not only the hardware,but also the software for the control of the hybrid components(operative functions like stop-start,energy recovery,etc.)and the hybrid strategy software(power management).

This parallel hybrid system can be utilized in multiple applications,such as wheel loaders,reachstacker,tele-handler,forklift trucks,dump trucks and tractors.

Figure 8 show s the hybrid system with the Ergopower transmission in an integrated setup with a hybrid module housing the electric machine,individual rotor bearings and torsional vibration damper being situated in front of the hydraulic torque converter(including lock-up clutch).This layout is very compact.By possible usage of the transmission oil circuit,a simple enhancement with a planetary gearset to gain higher power from the electric machine or with a clutch to disconnect the combustion engine is feasible.The hybrid module can indeed also be w ell combined with different ZF transmission systems,like the newly developed hydrostatic mechanicalpowersplitCVT transmission cPOWER.

Figure 9 show s the layout of a hybrid system for excavators.The depicted system combines a parallel hybrid for the drivetrain and w orking hydraulics with a serial hybrid for the swing drive.In this layout the same hybrid module can be utilized like in the above mentioned parallel hybrid for e.g.wheel loaders.With the hybrid module being situated between combustion engine and pump drive,also typical functions of a parallel hybrid(i.e.stop-start,boost)can be realized for pump drive and drivetrain.The electric machines and inverters,which are installed in the hybrid module,can also be utilized for the electrified swing drive.Combined e.g.with the Li-Ion battery,the energy of the swing movement can be recovered and can be boosted back into the system.Figure 9 show s the excavator hybrid system,which can be installed in tracked and w heeled excavators.

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[3]G? tz;M.;Gruhle,W.-D.;Mohr,M.;Grad,K.：E-lectrically Assisted Powertrains for Agricultural Machinery,in particularTractors.VDI-Berichte Nr.2060,2009.