轻型商用车混合动力传动系统

1 动因

虽然近期由于经济危机的影响,燃油价格有所下降,但从长远来看,石油储量的减少会导致燃油价格的上涨。而且严格的二氧化碳排放法规,尤其针对城市交通的规定逐渐完善。笔者认为,未来对油电混合动力车的需求会不断上升。

2 传动系统概念

本章将提出LCV混合动力传动系统的几个概念。之后将介绍关键的评价标准。混合动力传动系统应当是为用于城市运输的LCV设计的。这些运输任务通常由车队完成。对于车队拥有者来说,单位里程成本是决定所用车型的最重要因素。公共法规和环保形象也是考虑因素之一。额外费用和混合动力车节省的燃油是单位里程成本的重要部分。额外费用主要由混合动力车中必须安装的附加零部件和研发代价决定。两者都是LCV混合动力概念的重要评价标准。节省的燃油等同于二氧化碳排放的降低。短时电驱行驶也是评价标准之一,某些LCV生产商注重此项特性。HEV驾驶舒适度是一项限制,不应劣于普通车辆。

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.

总结起来,评价标准有如下几项：

●燃油节省量/二氧化碳排放减少量;

●零部件成本;

●研发代价;

●电驱行驶(短时);

由于燃油节省/二氧化碳排放降低的重要性越来越高,此项标准的权重应大于零部件成本;由于体积的限制,零部件成本标准的权重又大于研发成本。电驱行驶的潜力重要程度与研发成本相似,因为此功能不影响单位里程成本。为了简化决策矩阵,以“3”代表燃油节省/二氧化碳排放降低的权重,“2”代表零部件成本的权重,“1”代表研发代价和电驱行驶能力。

同时,与下列混合动力系统概念进行比较：

●轻度混合动力;

●单离合器混合动力;

●双离合器混合动力;

●串联混合动力;

轻度混合动力系统的动力结构如表1所示,电机功率约20千瓦。在单离合器和双离合器混合动力系统中,电机功率应达40千瓦,以优化节油率及系统成本,见(3)。串联混合动力系统所需的电机功率与内燃机相近。在(1)中讨论了这一情况。

从节油率方面看,轻度混合动力系统劣于其他设计,因为此类系统不具备电驱行驶和电驱起动能力,回馈制动的能力也很有限。其他混合动力系统能以全混合状态运行,见第4节。

从零部件成本看,轻度混合动力系统优于单离合器和双离合器系统,因为HEV不需要电动转向和制动。单离合器系统的成本小于双离合器系统。由于有两套电机驱动器,串联混合动力系统成本较高。

一项对研发代价的估计显示,双离合器混合动力系统与单离合器混合动力系统类似。轻度混合动力系统的研发代价最低。串联混合动力系统的研发代价最高,因为其变速器设计不能仿照常规的动力系统。

表1中的决策矩阵总结了上述各条。系统以“-”“0”“+”和“++”来计分。基于上述评价标准和权重而分析,单离合器混合动力系统最适用于LCV,其次是轻度混合动力系统。

表1 LCV混合动力传动系统的决策矩阵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 系统设计

下面阐述单离合器混合动力系统的实现。系统的设计目标如下：

●单离合器混合动力传动系统

●降低系统成本

●降低研发代价

混合动力传动系统由下列部分组成：

●自动变速器

●电机

●逆变器

这里讨论的系统不包括高压(HV)电池及转向、制动等部分。这些部分由LCV制造商提供。六速自动变速器由手动变速器改造,加装了机电执行器和控制单元。为了降低系统成本,同步环被取消,换挡时的同步由电机完成。实车测试表明,电机同步完成的时间与同步器相当或更快。考虑加工成本,变速器壳体不作改变。虽然可以缩短长度,但这在实际汽车的制造中并不必要。

电机与逆变器已经量产,经少量改动即可使用。电机选用永磁磁阻电机,是永磁同步电机与磁阻电机的组合。由于此种电机质量较大,考虑典型的功率消耗,不需要水冷系统。电机的定子和转子安装在独立的壳体中,置于内燃机和自动变速器之间。壳体中还包含离合器分离系统和电机的高功率接触件。电机由水冷逆变器控制,不使用转子位置传感器。混合动力控制功能块应集成在变速器控制单元内,应同时适应这两个任务的需要。图1所示为混合动力变速器。

图1 混合动力变速器Fig.1 Hybrid transmission

4 系统功能

系统功能的目标是优化燃油消耗和二氧化碳排放。下列功能由混合动力系统实现：

●电驱行驶和启动

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.

●工作区间切换

●回馈制动

●怠速停机

系统具备电驱行驶能力。然而整个系统并非是为长时间电驱行驶而设计。低速时,电机的扭矩与内燃机的扭矩相当。在上坡条件下可以用电机启动。在电驱模式下,换挡操作无需离合器作用。

工作区间切换,指的是通过智能分配内燃机和电机的扭矩来优化整个混合动力传统系统的效率。如果驾驶者需要极低的扭矩,若完全由内燃机提供扭矩,其效率很差。如果使内燃机工作在效率较高的区域,而由电机提供一个反向的扭矩,整体效率将得到优化。发电产生的电流给高压电池充电,以用于电驱行驶。此功能靠电机在混合动力模式下产生的恒值反向扭矩实现。(参见图2)

回馈制动,指利用电机的发电机模式来降低车速。这个功能仅当刹车踏板的释放方式与“A级”制动系统的ECE R13H一致时才触发。虽然应当有一种智能的制动系统,在需要回馈时关闭传动制动方式,但这种系统对于LCV来说过于昂贵。在回馈制动时,换挡操作应被禁止,因为制动力的损失会使驾驶者感到不适。仿真研究还表明,换挡时回馈制动能量的损失会影响混合动力系统的节油效果。

怠速停机功能指车辆停驶时关闭内燃机,当驾驶者需要再次行驶时自动启动。

4.1 电池充电模式下的启动

与双离合器混合动力系统相比,单离合器系统在下面的行驶状况下表现不佳：如果高压电池的荷电量(SOC)下降到某个阈值以下,需要充电。此时电机由内燃机驱动,以发电机模式运行。此时如果车辆静止,变速器必须置于空挡,离合器接合。即使驾驶者并不期望空挡,这些动作也是必须自动完成的。当驾驶者踩下加速踏板时,单离合器混合动力系统要带动车辆行驶。因此,离合器必须分离,电机要与变速器同步,接合齿轮,车辆然后才开始启动。与双离合器系统相比,单离合器系统不可避免地会有约500毫秒的延时。通过声光信息提示驾驶者此种情况,可改进这一缺陷。

5 节油效果

混合动力传动系统的目标是在某种城市运输循环下节油30%,尤其是日本10模式和新欧洲行驶循环(NEDC)的城市部分。过去曾对日本和欧洲生产的一种典型LCV做过仿真,也进行过实车测试。该车净重4.5顿,由3.0 L四缸柴油引擎驱动,最大扭矩375 Nm。带自动变速器的常规车辆油耗与单离合器混合动力系统车辆的油耗比较。

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.

在仿真中考虑了混合动力部件的附加重量。附属系统,如电制动,转向模块或电池冷确系统的损失也计算在内。高压系统用镍铁混合电池的模型来仿真。锂离子电池效率更高,重量更轻,会提高仿真结果。在电驱模式中使用实测的效率特性。在日本10模式的仿真中,节油达27%,NEDC的城市部分仿真中节油28%。在日本10模式的滚动测功机试验中,测得节油量为29.1%。文献[2]中提到了另一种类似系统的节油效果。图2展示了日本10模式的一次实车测试。

图2 日本10模式下的实车测试Fig.2 Vehicle measurements Japan 10 mode

6 展望

在日本10模式以外的行驶循环中,要用此种混合动力系统节省30%或更多的燃油较为困难。为此,必须在混合动力变速器之前的汽车部件上采取措施。比如,本文中的小型化指在混合动力车中使用较小的内燃机。如果需要达到最大扭矩,内燃机需要电机的支持。一个明显的不足是,混合动力车不能长时间维持常规汽车的最高车速。对于用于城市运输的商用车,这个缺点也许是可以忍受的。进一步提高节油性能则需要智能制动系统或低阻轮胎。全混合动力系统的高研发代价至今仍阻碍了此系统的应用。为对应未来上涨的油价和严格的排放法规,针对排放的车辆税会有助于这种混合动力系统的推广。

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.,Hägele,J.：Modulare Nkw-Getriebefamilie von Handschalt- zu Hybridausführung.Technical Congress of VDI,2008.