輕型商用車混合動力傳動系統

1 動因

雖然近期由于經濟危機的影響,燃油價格有所下降,但從長遠來看,石油儲量的減少會導致燃油價格的上漲。而且嚴格的二氧化碳排放法規,尤其針對城市交通的規定逐漸完善。筆者認為,未來對油電混合動力車的需求會不斷上升。

2 傳動系統概念

本章將提出LCV混合動力傳動系統的幾個概念。之后將介紹關鍵的評價標準。混合動力傳動系統應當是為用于城市運輸的LCV設計的。這些運輸任務通常由車隊完成。對于車隊擁有者來說,單位里程成本是決定所用車型的最重要因素。公共法規和環保形象也是考慮因素之一。額外費用和混合動力車節省的燃油是單位里程成本的重要部分。額外費用主要由混合動力車中必須安裝的附加零部件和研發代價決定。兩者都是LCV混合動力概念的重要評價標準。節省的燃油等同于二氧化碳排放的降低。短時電驅行駛也是評價標準之一,某些LCV生產商注重此項特性。HEV駕駛舒適度是一項限制,不應劣于普通車輛。

1 Motivation

In the long term reducing oil stocks will increase the fuel price,although at present the fuel price drops due to the w orldwide economical crisis.Furthermore rigorous CO2emission regulations especially for urban traffic are expected.Therefore the author expects an increasing demand for hybrid electric vehicles(HEV)for the future.

2 Powertrain Concept

A hybrid powertrain concept for LCV will be proposed in this chapter.Several concepts are discussed.In the following the deciding criteria are introduced.

The hybrid transmission system should be designed for LCV,which are used in urban delivery missions.These missions are mainly carried out by fleets.For the owners of these fleets the cost per mileage is most important for the decision,which kind of vehicle they use.Public regulations as well as a green image have also an impact on the product decision.

總結起來,評價標準有如下幾項：

●燃油節省量/二氧化碳排放減少量;

●零部件成本;

●研發代價;

●電驅行駛(短時);

由于燃油節省/二氧化碳排放降低的重要性越來越高,此項標準的權重應大于零部件成本;由于體積的限制,零部件成本標準的權重又大于研發成本。電驅行駛的潛力重要程度與研發成本相似,因為此功能不影響單位里程成本。為了簡化決策矩陣,以“3”代表燃油節省/二氧化碳排放降低的權重,“2”代表零部件成本的權重,“1”代表研發代價和電驅行駛能力。

同時,與下列混合動力系統概念進行比較：

●輕度混合動力;

●單離合器混合動力;

●雙離合器混合動力;

●串聯混合動力;

輕度混合動力系統的動力結構如表1所示,電機功率約20千瓦。在單離合器和雙離合器混合動力系統中,電機功率應達40千瓦,以優化節油率及系統成本,見(3)。串聯混合動力系統所需的電機功率與內燃機相近。在(1)中討論了這一情況。

從節油率方面看,輕度混合動力系統劣于其他設計,因為此類系統不具備電驅行駛和電驅起動能力,回饋制動的能力也很有限。其他混合動力系統能以全混合狀態運行,見第4節。

從零部件成本看,輕度混合動力系統優于單離合器和雙離合器系統,因為HEV不需要電動轉向和制動。單離合器系統的成本小于雙離合器系統。由于有兩套電機驅動器,串聯混合動力系統成本較高。

一項對研發代價的估計顯示,雙離合器混合動力系統與單離合器混合動力系統類似。輕度混合動力系統的研發代價最低。串聯混合動力系統的研發代價最高,因為其變速器設計不能仿照常規的動力系統。

表1中的決策矩陣總結了上述各條。系統以“-”“0”“+”和“++”來計分。基于上述評價標準和權重而分析,單離合器混合動力系統最適用于LCV,其次是輕度混合動力系統。

表1 LCV混合動力傳動系統的決策矩陣Table 1 decision matrix hybrid transmission system for LCV

The extra charge and the fuel savings of the hybrid vehicle play a decisive role for the cost per mileage.The extra charge is essentially affected by the cost of the additional components,which have to be installed in the hybrid vehicle,and the R&D effort.Both have to be considered as deciding criteria for the hybrid concept for LCV.The fuel savings are equivalent with CO2emission reduction.

Furthermore the possibility of short-time electric driving is introduced as deciding criteria,because this feature is requested by some LCV manufacturer.

The driving comfort of the HEV is recognized as a constraint.It should not be worse than the basic vehicle.In summary the following criteria are taken into account：

fuel savings/CO2emissions,

component cost,

R&D effort

electric driving(short-time).

Because of the increasing importance of fuel savings/CO2emissions this criteria is weighted higher than component cost and this criteria is weighted higher than R&D effort due to the expected volume.The potential to drive electrically is weighted in the same manner as R&D effort,because this feature does not improve the cost per mileage.In order to keep the resulting decision matrix simple‘3'is chosen as the weight for fuel savings/CO2emissions,‘2'for component cost,and‘1'for R&D effort and electric driving.

3 系統設計

下面闡述單離合器混合動力系統的實現。系統的設計目標如下：

●單離合器混合動力傳動系統

●降低系統成本

●降低研發代價

混合動力傳動系統由下列部分組成：

●自動變速器

●電機

●逆變器

這里討論的系統不包括高壓(HV)電池及轉向、制動等部分。這些部分由LCV制造商提供。六速自動變速器由手動變速器改造,加裝了機電執行器和控制單元。為了降低系統成本,同步環被取消,換擋時的同步由電機完成。實車測試表明,電機同步完成的時間與同步器相當或更快。考慮加工成本,變速器殼體不作改變。雖然可以縮短長度,但這在實際汽車的制造中并不必要。

電機與逆變器已經量產,經少量改動即可使用。電機選用永磁磁阻電機,是永磁同步電機與磁阻電機的組合。由于此種電機質量較大,考慮典型的功率消耗,不需要水冷系統。電機的定子和轉子安裝在獨立的殼體中,置于內燃機和自動變速器之間。殼體中還包含離合器分離系統和電機的高功率接觸件。電機由水冷逆變器控制,不使用轉子位置傳感器。混合動力控制功能塊應集成在變速器控制單元內,應同時適應這兩個任務的需要。圖1所示為混合動力變速器。

圖1 混合動力變速器Fig.1 Hybrid transmission

4 系統功能

系統功能的目標是優化燃油消耗和二氧化碳排放。下列功能由混合動力系統實現：

●電驅行駛和啟動

The following hybrid concepts are compared：

●Mild hybrid,

●Single clutch hybrid,

●Two clutch hybrid,

●Serial hybrid.

The mild hybrid is here defined as a hybrid system with the powertrain structure as show n in table 1 and an electric motor with a power of approximately 20 kW.For the single and the two clutch hybrid the power of the electricmotor should be approximately 40 kW in order to optimize fuel savings and system cost as examined in(3).The serial hybrid needs an electric power comparable to the combustion engine.This concept is also discussed in(1).

Regarding fuel savings the mild hybrid is inferior to all other concepts,because this hybrid system does not have the possibility of electric driving and setting-off and only a very limited potential for regenerative braking.All other hybrid systems can be used as a full hybrid with functionality as characterized in chapter 4.

Regarding component cost the mild hybrid is superior to single and two clutch hybrid,because the HEV does not need electric steering and braking.The single clutch hybrid generates lower component cost than the two clutch hybrid,because it needs only one clutch system.The serial hybrid is expensive due to the two electric drives.

An estimation of the R&D effort shows,that the tw o clutch hybrid is comparable to the single clutch hybrid.The lowest R&D effort can be expected with the mild hybrid.The serial hybrid demands the highest R&D effort,because the transmission cannot be carried-over from a conventional powertrain.

The decision matrix in table 1 summarizes the discussion above.All systems are evaluated with‘-',‘0',‘+',and‘++'.An overall analysis based on the chosen criteria and weights leads to the result,that the single clutch hybrid is the best choice for LCV followed by the mild hybrid.

●工作區間切換

●回饋制動

●怠速停機

系統具備電驅行駛能力。然而整個系統并非是為長時間電驅行駛而設計。低速時,電機的扭矩與內燃機的扭矩相當。在上坡條件下可以用電機啟動。在電驅模式下,換擋操作無需離合器作用。

工作區間切換,指的是通過智能分配內燃機和電機的扭矩來優化整個混合動力傳統系統的效率。如果駕駛者需要極低的扭矩,若完全由內燃機提供扭矩,其效率很差。如果使內燃機工作在效率較高的區域,而由電機提供一個反向的扭矩,整體效率將得到優化。發電產生的電流給高壓電池充電,以用于電驅行駛。此功能靠電機在混合動力模式下產生的恒值反向扭矩實現。(參見圖2)

回饋制動,指利用電機的發電機模式來降低車速。這個功能僅當剎車踏板的釋放方式與“A級”制動系統的ECE R13H一致時才觸發。雖然應當有一種智能的制動系統,在需要回饋時關閉傳動制動方式,但這種系統對于LCV來說過于昂貴。在回饋制動時,換擋操作應被禁止,因為制動力的損失會使駕駛者感到不適。仿真研究還表明,換擋時回饋制動能量的損失會影響混合動力系統的節油效果。

怠速停機功能指車輛停駛時關閉內燃機,當駕駛者需要再次行駛時自動啟動。

4.1 電池充電模式下的啟動

與雙離合器混合動力系統相比,單離合器系統在下面的行駛狀況下表現不佳：如果高壓電池的荷電量(SOC)下降到某個閾值以下,需要充電。此時電機由內燃機驅動,以發電機模式運行。此時如果車輛靜止,變速器必須置于空擋,離合器接合。即使駕駛者并不期望空擋,這些動作也是必須自動完成的。當駕駛者踩下加速踏板時,單離合器混合動力系統要帶動車輛行駛。因此,離合器必須分離,電機要與變速器同步,接合齒輪,車輛然后才開始啟動。與雙離合器系統相比,單離合器系統不可避免地會有約500毫秒的延時。通過聲光信息提示駕駛者此種情況,可改進這一缺陷。

5 節油效果

混合動力傳動系統的目標是在某種城市運輸循環下節油30%,尤其是日本10模式和新歐洲行駛循環(NEDC)的城市部分。過去曾對日本和歐洲生產的一種典型LCV做過仿真,也進行過實車測試。該車凈重4.5頓,由3.0 L四缸柴油引擎驅動,最大扭矩375 Nm。帶自動變速器的常規車輛油耗與單離合器混合動力系統車輛的油耗比較。

3 System Design

Below the implementation of the single clutch hybrid is explained.The targets of the system design are：

●single clutch hybrid transmission system,

●reduce system cost,

●reduce R&D effort,

The hybrid transmission system consists of the following components：

●automated transmission,

●electric motor,

●inverter.

The considered system does not include the high voltage(HV)battery and any steering and braking components.These components should be supplied by the LCV manufacturer.

The 6-speed automated transmission is based on a manual one,which is automated by electromechanical add-on actuators and a control unit.

In order to reduce the system cost,the synchronizer rings are left out.Transmission synchronization during shiftings is carried out by the electric motor.Vehicle tests show,that the synchronization time with an electric motor is faster or equal to the time,which is achievable with synchronizers.

The transmission housing remains unchanged due to tooling cost,although the system could be shortened,because the actual vehicle packaging studies does not show the need for this.

Electric motor and inverter are already in volume production and can be carried over with minimal modifications.The electric motor is a permanent magnet reluctance motor,which is a composition of a permanent magnet synchronous motor and a reluctance motor.Because of the high mass of this motor and the typical electrical power consumption,there is no need for w ater cooling of the electric motor.

在仿真中考慮了混合動力部件的附加重量。附屬系統,如電制動,轉向模塊或電池冷確系統的損失也計算在內。高壓系統用鎳鐵混合電池的模型來仿真。鋰離子電池效率更高,重量更輕,會提高仿真結果。在電驅模式中使用實測的效率特性。在日本10模式的仿真中,節油達27%,NEDC的城市部分仿真中節油28%。在日本10模式的滾動測功機試驗中,測得節油量為29.1%。文獻[2]中提到了另一種類似系統的節油效果。圖2展示了日本10模式的一次實車測試。

圖2 日本10模式下的實車測試Fig.2 Vehicle measurements Japan 10 mode

6 展望

在日本10模式以外的行駛循環中,要用此種混合動力系統節省30%或更多的燃油較為困難。為此,必須在混合動力變速器之前的汽車部件上采取措施。比如,本文中的小型化指在混合動力車中使用較小的內燃機。如果需要達到最大扭矩,內燃機需要電機的支持。一個明顯的不足是,混合動力車不能長時間維持常規汽車的最高車速。對于用于城市運輸的商用車,這個缺點也許是可以忍受的。進一步提高節油性能則需要智能制動系統或低阻輪胎。全混合動力系統的高研發代價至今仍阻礙了此系統的應用。為對應未來上漲的油價和嚴格的排放法規,針對排放的車輛稅會有助于這種混合動力系統的推廣。

The stator and rotor of the electric motor are installed in a separate housing,w hich is arranged between combustion engine and automated transmission.This housing contains also the clutch release system and the high power contacts of the electric motor.

The electric motor is controlled by a watercooled inverter without the use of a rotor position sensor.

The hybrid control functionality should be integrated into the transmission control unit,which performance must be suitable for both tasks.Figure 1 shows the hybrid transmission：

4 System Functionality

The target of the system functionality is to optimize fuel savings and by this CO2emissions.

The following functions are realized by the hybrid transmission system：

●electrical driving and setting-off,

●transition of operation range,

●regenerative braking,

●idle stop.

The system is able to drive electrically.However the overall system is not designed for long term electricdriving.The torque of the electricmotor is comparable to the torque of the combustion engine at low speed.It allow s electrical setting-off even in uphill condition.In electric driving mode the shiftings are carried out without clutch operation.

Transition of operation range means to optimize the overall hybrid powertrain efficiency by intelligent torque distribution between combustion engine and electric motor.If e.g.the driver is requesting a very low torque,the combustion engine would be operated at a very bad efficiency,if it supplies the torque exclusively.The overall efficiency can be improved,if the combustion engines torque is increased to an optimal operation range and if the surplus of torque is balanced by a negative electric motor torque.The generated electric current can be used to store energy in the HV battery,which is available for electric driving later.This function is realized by a constant negative torque of the electric motor in hybrid driving mode as a first step(refer to figure 2).

Regenerative braking means to use the electric motor in generator mode to reduce the vehicle speed.This functionality should be activated by releasing the accelerator pedal only in compliance with ECE R13 H for“class A” braking systems.An intelligent braking system,which disables the conventional braking system during regenerative braking,is desirable,but usually such systems are judged as too expensive for LCV applications.During regenerative braking shiftings should be inhibited,because the loss of braking force is an uncomfortable feeling for the driver and simulation studies show,that the loss of energy regeneration during shiftings reduces the fuel savings of the hybrid system.

The idle stop functionality means to stop the combustion engine in vehicle standstill condition and to restarts it automatically,if the driver wants to continue his mission.

4.1 Setting-Off in battery charge mode

In comparison with the tw o clutch hybrid the single clutch hybrid is sometimes criticized because of the following driving situation：

If the state of charge(SOC)of the HV battery falls below a certain threshold,it is necessary to charge it.This can be done by the electric motor in generator mode,driven by the combustion engine.To do this in standstill condition the transmission has to be in N and the clutch has to be closed.This condition has to be adopted automatically even if the driver does not request N.If the driver presses the accelerator pedal in this situation,the single clutch hybrid system has set-off the vehicle.For this,the clutch has to open,the electric motor has to synchronize the transmission,the gear has to be engaged and after this the setting-off maneuver starts.

An inevitable delay of approximately 500 ms happens,if the single clutch hybrid is compared to the conventional vehicle or the two clutch hybrid system.

This disadvantage could be reduced by display indications or acoustical indications,which makes the situation aware for the driver.

5 Fuel Saving Results

The target of the hybrid transmission system is to save up to 30%fuel in certain urban delivery cycles.Especially the Japan 10 mode and the city share of the New European Driving Cycle(NEDC)are considered.

Vehicle simulations for a typical LCV produced in Japan and Europe were done in the past.Actually also vehicle measurements are available.

The gross vehicle weight is 4.5 t.It is driven by a 3.0l diesel engine with four cylinders and a maximal torque of 375 Nm.

The fuel consumption of the conventional vehicle with an automated transmission is compared to the fuel consumption of the vehicle fitted with the single clutch hybrid transmission system.

During the simulations the additional weight of the hybrid components is considered.The losses of auxiliaries like electric braking and steering units or battery cooling systems are taken into account.For the HV battery a simulation model of a nickel metal hybrid battery is used.A lithium ion battery with a higher efficiency and less weight would improve the simulation results.For the electric drive measured efficiency characteristics are used.

Fuel savings of 27%are simulated for the Japan 10 mode and 28%for the city share of the NEDC.On a roller dynamometer 29.1%fuel savings were measured for the Japan 10 mode.

The correspondence between simulation and measurements seems to be very good.

Fuel saving results with a similar system are also published in[2].Figure 2 show s an example of a vehicle measurement of the Japan 10 mode.

6 Outlook

Fuel savings of 30%or more are difficult to reach with the proposed hybrid system in other driving cycles than the Japan 10 mode.For this purpose measures on vehicle side beyond the hybrid transmission system are necessary to perform.

E.g.downsizing means in this context to use a smaller combustion engine for the hybrid vehicle as for the conventional vehicle.The combustion engine has always to be supported by the electrical motor,if the maximum torque is requested.The inevitable disadvantage is that the maximum speed of the conventional vehicle cannot be hold for a longer time with the hybrid vehicle.For a commercial vehicle,used in urban delivery missions,this disadvantage might be acceptable.

Further potential to improve fuel savings are e.g.intelligent braking systems or?-low tires.

High R&D effort and the component cost of a full hybrid system are slowing till now a comprehensive application of this powertrain concept.For the future an increasing fuel price,rigorous emission regulations,and a more emission oriented vehicle taxation will support such hybrid systems.

[1]Vahlensieck,B,Speck,F.-D.,and Kubalzyck,R：Economic Concepts and Solutions for Hybrid Drivelines.Technical Congress of JSAE,Yokohama,May 2008.

[2]Griesmeier,U.,Blattner,S.,Mittelberger,C.：Parallel-hybrid mit automatisiertem Schalt-getriebe in einer Trans-porteranwendung,Technical Congress of VDI,Dresden,November 2008.

[3]Lamke,M.,Speck,F.-D.,H?gele,J.：Modulare Nkw-Getriebefamilie von Handschalt- zu Hybridausführung.Technical Congress of VDI,2008.