X***9
发帖数: 7385 | 1 在这篇文章中，我们将分析SH-AWD的细节，专注于这三个问题：
1.如何的SH-AWD的工作
？2。为什么SH-AWD可以大量的电能传送到后轮不断，而不必担心磨损和过热问题
？3。讴歌SH-AWD和本田的VTM-4系统之间的区别是什么？
概述：SH-AWD系统
目前有4种不同的SH-AWD的：
1. SH-AWD用在2005-2012讴歌RL
2。在SH-AWD在2007-2012讴歌RDX用2007-2013年MDX（第2根），2014至2015年MDX（第3
代）2010 - 2013年ZDX和二零零九年至2014年TL
3。用在2015年讴歌TLX和2016年MDX的SH-AWD
4。在2014年讴歌RLX运动混合使用的SH-AWD
请注意：虽然2016年MDX是同一代的二千○十五分之二千○十四车型年，他们的SH-AWD
系统，彼此明显不同。总之：在TLX和MDX最新的SH-AWD是不相同的SH-AWD你可能会想-
事实上，你可以认为这是一个VTM-4 Plus版本（我将在后面的章节解释） 。
类型4（2014讴歌RLX运动杂交的SH-AWD）比其余类型完全不同：它的前轮由气体发动机
驱动，后轮由电机驱动。这SH-AWD是不是讨论了本文的范围之内。
下面的图片是讴歌RL采用了SH-AWD系统。
首先，让我们关注的SH-AWD型＃2上面提到的。
SH-AWD的讴歌MDX（前2016年），ZDX，RDX（第1根）和TL
你可能已经知道了SH-AWD系统用的是一端连接于前变速器，以及安装在后驱动单元的另
一端的中心驱动轴。内后驱动单元，有一个准双曲面齿轮转接该功率到每个后轴。此时
，对SH-AWD仍然是相同的VTM-4系统。然而，事情成为这一步后更有趣。
为SH-AWD，它的每一个后轮的被连接到行星齿轮组和一个电磁多片离合器组件，其接收
功率从上述双曲线齿轮。一个常见的误解是：电磁多片离合器组件充当
功率耦合，并且它是在电荷传送功率从发动机到后轮。这种观念是错误的。
事实上，对SH-AWD是类似于AWD系统，其使用与多片离合器充当限滑功能的行星齿轮中
央差速器; 所不同的是，SH-AWD使用两个这样的差速器，一个用于每个车轮。请看下图
，显示每个后轮的潮流。
对于简要介绍了什么是行星齿轮组，请参考附录1在这篇文章的末尾。为SH-AWD，有一
个行星齿轮为每个后轮设置的。的环形齿轮连接于中心驱动轴（经由双曲线齿轮）; 的
行星齿轮架连接到半后轴（其驱动后轮）; 一组离合器片被连接到所述太阳齿轮，以及
另一组离合器片被固定在后动力单元壳体且完全静止。下面的图片清楚地显示了离合器
组件是在后驱动单元构成。
上图：切开的行星齿轮组，并且还电磁离合器部分
如果离合器组件被完全锁定（这意味着该太阳轮是静止的），所述环形
齿轮与行星齿轮之间的不同齿轮号码将引起环形齿轮（连接到前轮最终）和行星齿轮架
（连接到后轮）旋转速度不同，因此，后轮转速加快1.7％（过驱动）比前轮。
然而，如果车是在一条直线上，并在道路表面上行驶具有优异的牵引（例如，行驶在一
条直线州际公路），这是不可能的后轮旋转比前轮速度快1.7％。这是因为：如果可能
发生，前轮被有效地制动车，这是没有意义的。因此这种情况下，无论是前轮和后轮有
旋转在一个类似的速度，离合器组件也没有锁定，因此太阳齿轮会慢慢旋转，滑动摩擦
离合器片内的存在。对SH-AWD控制单元调整施加在摩擦片的压力。由太阳齿轮和环形齿
轮的摩擦力产生将利用通过，并放大转矩，然后提供给行星齿轮架，最后到达后轮。这
是如何在后轮从发动机获得转矩的过程。
当离合器组件完全脱开，也就没有传送到后轮处的所有扭矩。取的MDX例如，对SH-AWD
可传送多达总转矩的70％到后轮。只能达到该扭矩分流比当离合器组完全锁定，这当量
到后轮纺丝比前轮速度快1.7％的情况下。从上面的讨论，我们知道这只能在急转弯发
生的，所以这也是为什么在官方的产品介绍，讴歌说：“在一个原因曲线下急加速，有
效扭矩的70％，可向后方” -要注意措辞，讴歌提到的“曲线”的原因。
从下面的图像就可以看到从空气间隙被传递到活塞上的电枢，这将压缩该被固定在壳体
离合器板中产生的电磁力。
上图：切开的行星齿轮组，并且还电磁离合器部分
这里有一个问题，它可以测试你的SH-AWD系统的了解。
问：W 帽子是用行星齿轮组的中央差速器+多片离合器的限滑SH-AWD，和全轮驱动系统
之间的区别？
答：使用行星齿轮组的中央差速器+多片离合器的限滑四驱系统，有一个输入：发动机
功率; 两个输出：一个前轮和另一至后轮。离合器组件是用来改变基于所述缺省比值的
扭矩分配比例。为了更深入的分析，行星齿轮中央差速器，请参阅附录2，在这篇文章
的末尾。
为SH-AWD中，每个行星齿轮组+离合器组件：输入是发动机功率，输出：一个用于后轮
（行星齿轮架），另一种是在离合器（太阳齿轮） - 其中“模拟“虚拟轮。如果一“
轮”（由离合器模拟）获得牵引力（由接合离合器来实现），另一个车轮（后轮）将得
到力量了。离合器被用于确定多少扭矩将交付。
因为每个后轮具有其专用的行星齿轮组和离合器，施加在他们每个人的扭矩可分别调节
，这实现了转矩引导目标。
下面是行星齿轮中央差速器的说明，以多片离合器作为限滑功能，在大众途锐。它显示
了SH-AWD和行星齿轮中央差速器之间的相似性/差异。你可以看到离合器组件，其中调
制该前/后车轮之间的扭矩分配。
为什么SH-AWD能够不断地处理大扭矩？
从我们之前的文章描述了如何在福克斯RS的Twinster全轮驱动系统的工作，我们知道
GKN Twinster离合器组件完全负责动力传递到后轮。这意味着在福克斯RS，如果每个后
轮得到50HP，那么这50马力的全资传送通过离合器内的滑动摩擦。因此，会出现严重的
磨损和过热问题，如果离合器接合很长一段时间。
在另一方面，如果你驾驶过款讴歌TL / MDX，你会看到后轮经常得到大量的权力; 在定
速巡航，后轮仍然得到的扭矩分配至少10％。为讴歌RL，后轮甚至可以得到发动机输出
的30％不断。为什么SH-AWD不必担心磨损和过热？
要回答这个问题，我们需要记住：在SH-AWD离合器组件不是主要负责发射功率，相反，
它是用来“调制”（齿圈（输入）和行星轮架之间的扭矩传递关系输出），则施加在离
合器片要少得多的压力。
里面的SH-AWD的每一个行星齿轮组，齿圈有78个齿，太阳轮有30个，每个行星齿轮有24
个齿。简单的代数运算告诉我们：在环形齿轮的扭矩是太阳轮（三十零分之七十八= 2.
6）的2.6倍。因此，行星齿轮架接收的总扭矩是施加于离合器组件（其直接连接到太阳
齿轮）的转矩的3.6倍（2.6 + 1 = 3.6）。换句话说，为了传输1HP，在离合器板上的
负荷只有0.278hp; 其他机械传动部件，负责传递0.722hp的其余部分。从另一个方面，
离合器组件也可看作是一种能量吸收器，它接受由1.7％前/后车轮速度差所产生的过量
的能量的照顾。
因此，由滑动摩擦所产生的磨损和热是在对SH-AWD要少得多。相对于其他的AWD系统，
如GKN Twinster和瀚德，在离合器组件的SH-AWD只需要处理27.8％（1 /工作量的
Twinster /瀚德系统需要相同的输出条件下处理的3.6节）。这也就是为什么更高的负
载可在后轮上更经常地对SH-AWD系统施加，没有磨损和过热关注的原因。
当SH-AWD会不工作？
在最后讴歌RL对SH-AWD是对SH-AWD对MDX和TL使用的升级版本。这意味着它必须解决一
些情况，关于MDX / ZDX / TL了SH-AWD系统不工作。在讨论用在2005-2012讴歌RL的SH-
AWD，让我们先问一个问题：
为什么讴歌构建前后轮之间的1.7％超速传动比，在MDX / ZDX / RDX / TL？
如果没有极品的前/后转速差不建？如果是这样的话，当你右转，你的左后轮将旋转比
前轮要快得多。每当离合器的接合，行星齿轮组会落入该输入试图驱动某物（输出），
其旋转速度更快的情况。这意味着发动机提供的制动效果的后轮，这将引起失控而不是
我们所希望的。所以，如果没有堆积在旋转差，后轮是无法在转向期间获得的任何功率
，更不用说转矩引导。
这也自然带来了另一个问题：
是否有后轮旋转比前轮快超过1.7％，可能吗？
答案是肯定的。假设你正在转弯及回转直径大到足以承担两个前/后轮转向与此相同的
直径。取的MDX为例，它的后履带宽度为1.684米，因此，如果转弯半径49.5米，外后轮
会旋转（1.684 / 2）/49.5 =比前轮速度快1.7％。因此，如果转弯半径小于49.5米（
162.4英尺），从理论上讲MDX的后轮将纺丝速度更快，并使用了所有的1.7％转速差津
贴。当这种情况发生时，输出离合器片（连接到太阳齿轮）将开始在相反的方向旋转，
和离合器必须完全脱离，以避免前面提到的“制动效果”; 和的过程中，后轮不会在这
段时间内得到任何功率。
有些读者可能会问：
很多人都推动了MDX / ZDX / TL在各种条件下，为什么从来没有人注意到了上述问题？
首先，将上述只是理论上计算，有两个因素，以帮助缓解上述问题。
1.在现实世界中，由于对MDX / ZDX / TL是基于前轮驱动平台，同时使一转，鼻重特性
将导致更高的滑移率就比后轮前轮，这有效地加在1.7％津贴顶“缓冲”的另一层;
2.当转弯半径变足够小，事实上，前轮具有较大转弯半径比后轮将开始生效，这是另一
种类型的“缓冲”，可在1.7％的过驱动津贴被触摸之前被消耗的。
因此，转弯半径，使得SH-AWD无效可能并不49.5米（162.4英尺）如上计算，但它会存
在于一个范围在0和49.5米。
下图向我们展示了，在一个急转弯车子的前轮将有一个相对较大的转弯半径比后轮。
对讴歌RL的SH-AWD
于RL和MDX / ZDX / TL了SH-AWD之间的主要区别是，它在后驱动装置的前面安装了一个
加速装置。这种“加速装置”实际上是一个2档变速：一档为1.7％，前/后轮超速传动
比; 另一个齿轮提供了更高的5.7％的比例。下图显示了安装在后驱动单元前面的加速
装置。
上图：切掉了SH-AWD与加速装置
为什么它需要的5.7％的比例的原因是为了解决上述问题的方法：小转弯半径会使SH-
AWD无效的。通过提供更高超速传动比，它是不太可能的是，后轮将使用了由超速传动
比提供的津贴。这意味着，在讴歌RL的SH-AWD可以覆盖更广泛的转弯半径范围比SH-AWD
其他讴歌车型。
然而，虽然不完全占据工作量，后轮离合器仍需要采取的能量的一部分的照顾。因此，
一个更大的超速传动比是指在离合器组件较高的磨损和发热。所以正常驾驶条件下，在
加速装置将转向使用较低超速传动比以减小磨损和发热另一个齿轮。
这种设计的唯一缺点是：有一个时间滞后的加速装置的两个齿轮之间移动，所以它可能
无法正常工作，希望在某些特殊驾驶情形一样快。
讴歌SH-AWD VS本田的VTM-4
在SH-AWD系统，每个后轮，如果你更换行星齿轮组以及离合器用一个湿式离合器组件，
您将得到VTM-4系统（当然某些组件，也是ECU是不同的，然而，机械架构将是这样的）
。参见下图。
这改变如何后轮驱动装置的工作根本的方式。现在离合器组件是100％负责将动力传递
到后轮。它的工作原理相同的方式作为GKN Twinster。穿和从摩擦产生的热成为一个大
问题，因此，VTM-4系统不能功率传递到后轮长时间。事实上，在车辆到达18英里每小
时，并在恒速状态的速度，在VTM-4的离合器将完全脱离，这意味着车辆有效地在纯FWD
方式在此条件下（加速，VTM-4中仍然可以踢在和提供临时的扭矩至后轮）。
SH-AWD在2015年讴歌TLX和2016 MDX
在最新的TLX和MDX，讴歌“暗中”改变了SH-AWD系统的设计。下面是一个切去图像示出
其机械结构，可以看到，可以认为是本田VTM-4的变体。
这个新的SH-AWD和VTM-4之间的主要功能区别是：
1，后轮将获得动力;
2。在后轮扭矩引导功能（VTM-4的硬件可以做到这一点，但本田选择不实现它）
相比之前的SH-AWD，这种新的SH-AWD有这些不同的特点：
1.高速率提高至2.7％（从1.7％） -这将降低的可能性的SH-AWD变得无效，如在上述“
当SH-AWD无法运行”一节中讨论;
2。液压控制离合器（老SH-AWD：电磁离合器
）3。更小，更轻
简单地说，在新的SH-AWD系统离合器组件是负责的工作量100％，与增加的2.7％超速传
动比，磨损和过热问题，应该是比以前更严重。为什么讴歌仍然可以让这种系统游弋在
一条直线上时传递转矩的至少10％的原因是：在材料技术的进步使得有可能制造离合器
片具有优良的抗磨损性能，所以虽然有许多正常工作条件下的摩擦磨损和发热，新的SH
-AWD组件仍能保持良好的可靠性。
有关这个新的SH-AWD更多的技术细节，请参见本SAE论文（这将在二零一五年四月十四
日出版）：
高效率的下一代SH-AWD后驱动单元的研制
附录
1.行星齿轮组
如图所示，在上面的图片，行星齿轮组由三个主要部分组成：一个环形齿轮，行星齿轮
组（包括3个行星齿轮和一个行星齿轮载体 - 上图中的“手臂”）和太阳齿轮。对于这
样的3成分，它可以被配置为2输入+ 1输出，或1为输入+ 2进行输出。或通过控制在部
件中的一个的负荷，在其余的2分量，将输入/输出关系是固定的。
2.如何一行星齿轮组充当与限滑功能的中央差速器
如果您已经阅读我们以前的文章中涉及到全轮驱动系统（例如奥迪的Quattro /大众的
4Motion），您可以使用行星齿轮组中央差速器的多片式离合器充当限滑功能认识的一
些全轮驱动系统。这也正是这样的中央差速器可以如何改变前后轴之间的扭矩传递的百
分比：它利用离合器设置为控制在行星齿轮组的部件的负荷（通过改变施加到所述摩擦
片的压力），则功率投递率可以为两个输出组件（一个连接到前和另一个连接至后）被
确定。当离合器被完全打开（释放），所述扭矩分配比为缺省比值。例如，38:62装置
保时捷卡宴的默认前/后功率比，当限滑离合器片被完全释放时，在行星齿轮的其他两
个组件设置中央差动接收38％，总输入的62％分别权力。 | a********c
发帖数: 3657 | 2 这种ds太公四区是给mower设计的，别瞎咋呼了 | s******s
发帖数: 2721 | 3 本田哥操鸡不错
【在 a********c 的大作中提到】
: 这种ds太公四区是给mower设计的，别瞎咋呼了
| b******s
发帖数: 5329 | | l****1
发帖数: 308 | 5 技术在不断发展，请不要说“地球最先进”之类的词，否则有王婆卖瓜之嫌…… | X***9
发帖数: 7385 | 6
本田10年前就研发了sh-awd4驱技术，当时业界就普遍认为是地球上最先进的4驱系统，
至今
各大媒体对4驱系统的评测层出不穷，但没有一个人敢说哪家的系统是比sh-awd还更为
先进。就拿左右自由动力分配来说，“大名鼎鼎”的斯巴鲁四驱系统这个功能以前完全
不存在，直到2015最新款的 wrx sti 才有所谓的torque vectoring能力,不过和本田主
动自由分配扭力大为不同，斯巴鲁是通过被动的给单侧的轮子施加刹车力道来实现的，
可见其技术之落后。
现在，本田已经有了更为先进的4驱系统，即sport hybrid sh-awd，目前限于nsx和rlx
车型
当然对拥有世界上最先进的机器人技术储备的本田来说，搞个简单的多的4驱系统的
确不算什么。
【在 l****1 的大作中提到】
: 技术在不断发展，请不要说“地球最先进”之类的词，否则有王婆卖瓜之嫌……
| d***a
发帖数: 13752 | 7 从技术上说，总结得不错。去掉“最先进”比较好，不过，加上这样的字眼可以增加流
量。:)
如果把sh-awd搞懂，对了解四驱的原题有很大帮助。 | b******7
发帖数: 8200 | 8 丫的懂个屁，拿翻译软件翻译的人神共愤的文章，你们也看。
【在 d***a 的大作中提到】
: 从技术上说，总结得不错。去掉“最先进”比较好，不过，加上这样的字眼可以增加流
: 量。:)
: 如果把sh-awd搞懂，对了解四驱的原题有很大帮助。
| z**x
发帖数: 3629 | 9 http://blogs.youwheel.com/2015/03/31/acura-sh-awd-comprehensive
Acura SH-AWD: A Comprehensive Analysis
Written By Editor on 2015-03-31, Posted in Car Chat, Technical Talk | 2
comments
The Acura SH-AWD system (short for: Super Handling-All Wheel Drive) is often
described ambiguously in most automobile literature. Some articles even
said it is an enhanced version of the Honda VTM-4, which is incorrect.
2012_Acura_TL_Rear
In this article, we will analyze the SH-AWD in detail, focusing on these
three questions:
1. How does the SH-AWD work?
2. Why the SH-AWD can transfer large amount of power to rear wheels
constantly, without worrying about the wear and overheating issues?
3. What is the difference between the Acura SH-AWD and the Honda VTM-4
system?
Overview: The SH-AWD Systems
Currently there are 4 different types of SH-AWD:
1. The SH-AWD used on the 2005-2012 Acura RL
2. The SH-AWD used on the 2007-2012 Acura RDX, 2007-2013 MDX (2nd-gen),
2014-2015 MDX (3rd-gen), 2010-2013 ZDX, and 2009-2014 TL
3. The SH-AWD used on the 2015 Acura TLX and the 2016 MDX
4. The SH-AWD used on the 2014 Acura RLX Sport Hybrid
Please pay attention: although the 2016 MDX is the same generation as the
2014/2015 model year, their SH-AWD system are dramatically different from
each other. In short: the latest SH-AWD on the TLX and MDX is NOT the same
SH-AWD you might be thinking of – in fact you can consider it to be a VTM-4
PLUS version (I will explain why in the later sections).
The type 4 (2014 Acura RLX Sport Hybrid SH-AWD) is totally different than
the rest types: its front wheels are driven by the gas engine, and the rear
wheels are driven by an electric motor. This SH-AWD is not within this
article’s scope of discussion.
Below image is the SH-AWD system used on the Acura RL.
Acura SH-AWD All-Wheel-Drive System Rear Differential
Firstly, let’s focus on the SH-AWD type #2 mentioned above.
SH-AWD on the Acura MDX (pre-2016), ZDX, RDX (1st-Gen) and TL
You may already know that the SH-AWD system is using a center drive shaft
with one end connected to the front transaxle, and the other end attached to
the rear drive unit. Within the rear drive unit, there is a hypoid gear
that diverts the power to each rear axle. At this moment, the SH-AWD is
still the same as the VTM-4 system. However, things become more interesting
after this step.
2010_Acura_ZDX
Above image: the 2010 Acura ZDX with SH-AWD
For SH-AWD, each of its rear wheel is connected to a planetary gear set and
an electromagnetic multi-plate clutch pack, which receives power from the
above mentioned hypoid gear. One common misconception is: the
electromagnetic multi-plate clutch pack is acting as a power coupling, and
it is in charge of delivering power from the engine to the rear wheel. This
conception is wrong.
In fact, the SH-AWD is similar to an AWD system which uses a planetary gear
central differential with multi-plate clutch acting as limited slip feature;
the difference is, SH-AWD uses two of such differentials, one for each
wheel. Please see the diagram below, showing the power flow for each rear
wheel.
SH-AWD_Structure
For a brief introduction of what is a planetary gear set, please refer to
the Appendix 1 at the end of this article. For SH-AWD, there is a planetary
gear set for each rear wheel. The Ring Gear is connected to the center drive
shaft (via the hypoid gear); the Planet Gear Carrier is connected to the
half rear axle (which drives the rear wheel); One group of the clutch plate
is connected to the Sun Gear, and the other group of the clutch plates is
fixed to the rear power unit casing and is completely stationary. The below
image clearly shows how the clutch pack is constructed in the rear drive
unit.
SH-AWD_Component_3
Above image: cut-away of the planetary gear set and also the electromagnetic
clutch section
If the clutch pack is fully locked up (which means the Sun Gear is
stationary), the different gear numbers between the Ring Gear and the Planet
Gear will cause the Ring Gear (connected to the front wheel ultimately) and
the Planet Gear Carrier (connected to the rear wheel) to spin at different
speeds, thus the rear wheel will spin 1.7% faster (overdriven) than the
front wheel.
However, if the car is running on a straight line and the road surface has
excellent traction (for example, driving on a straight interstate highway),
it is impossible that the rear wheels are spinning 1.7% faster than the
front wheels. This is because: if that could happen, the front wheels are
effectively braking the car, which does not make sense at all. Therefore
under that situation, both the front and rear wheels have to spin at a
similar speed, the clutch pack also does not lock up, therefore the Sun Gear
will rotate slowly, and sliding friction exists within the clutch plates.
The SH-AWD control unit adjusts the pressure applied on the friction plates.
The torque generated from the friction force will leverage through, and
magnified by the Sun Gear and the Ring Gear, then supply to the Planet Gear
Carrier, and finally reach the rear wheel. This is the process of how the
rear wheel obtains torque from the engine.
When the clutch pack is fully disengaged, there will be no torque
transferred to the rear wheel at all. Take MDX for example, the SH-AWD can
transfer up to 70% of the total torque to the rear wheels. This torque split
ratio can only be achieved when the clutch pack is fully locked up, which
equivalents to the case that rear wheels are spinning 1.7% faster than the
front wheels. From the above discussion we know this can only happen during
hard cornering, so this is also the reason why in the official product
description, Acura says “On a curve under hard acceleration, 70% of
effective torque can be directed to the rear” – pay attention to the
wording, Acura mentioned “curve” for reasons.
From the below image you can see the solenoid force generated from the
armature air gap is passed on to the piston, which will compress the clutch
plates that are fixed to the casing.
SH-AWD_Component_4
Above image: cut-away of the planetary gear set and also the electromagnetic
clutch section
Here is a question that can test your understanding of the SH-AWD system.
Question: What is the difference between SH-AWD, and AWD system using a
planetary gear set center differential + multi-plate clutch as limited-slip?
Answer: for AWD system using a planetary gear set center differential +
multi-plate clutch as limited-slip, there is one input: engine power; two
outputs: one to the front wheels and the other to the rear wheels. Clutch
pack is used to vary the torque split percentage based on the default ratio.
For more in-depth analysis of the planetary gear center differential,
please see Appendix 2 at the end of this article.
For SH-AWD, each of the planetary gear set + clutch pack: input is the
engine power, output: one to the rear wheel (Planet Gear Carrier), and the
other one is to the clutch (Sun Gear) – which “simulates” a virtual wheel
. If one “wheel” (simulated by the clutch) gets traction (implemented by
engaging the clutch), another wheel (the rear wheel) will get power too. The
clutch is used to determine how many torque will be delivered.
Because each rear wheel has its dedicated planetary gear set and clutch, the
torque applied on each of them can be adjusted separately, which achieves
the torque vectoring goal.
Below is an illustration of the planetary gear center differential, with
multi-plate clutch acting as limited-slip feature, on the Volkswagen Touareg
. It shows the similarity/difference between the SH-AWD and a planetary gear
center differential. You can see clutch pack, which modulates the torque
distribution between the front/rear wheels.
Touareg_Center_Differential
Want to know more about AWD systems? You may like our other articles:
2016 Ford Focus RS AWD System Analysis
AWD System Dissected: Audi Quattro vs. Volkswagen 4Motion
Luxury Sedan AWD System Review
Why SH-AWD Can Constantly Handle Large Torque?
From our previous article describing how the Twinster AWD system work in the
Focus RS, we know the GKN Twinster clutch pack is fully responsible to
transfer power to the rear wheels. This means in the Focus RS, if each rear
wheel gets 50hp, then this 50hp is transmitted wholly through sliding
friction within the clutch. Hence, there will be serious wear and
overheating issue if the clutch is engaged for a long time.
On the other hand, if you have driven the Acura TL/MDX, you will see the
rear wheels frequently get lots of power; in constant speed cruising, the
rear wheels still get at least 10% of torque split. For the Acura RL, the
rear wheels can even get 30% of the engine output constantly. Why the SH-AWD
does not need to worry about the wear and overheating?
To answer this question, we need to remember: the clutch pack in the SH-AWD
is NOT mainly responsible for transmitting power, instead it is used to “
modulate” the torque transfer relationship between the Ring Gear (input)
and Planet Gear Carrier (output), there is much less stress applied on the
clutch plates.
Inside SH-AWD’s each planetary gear set, the Ring Gear has 78 teeth, the
Sun Gear has 30 and each Planet Gear has 24 teeth. Simple algebraic
computation tells us: the torque on the Ring Gear is 2.6 times of the Sun
Gear (78/30 = 2.6). Therefore, the total torque the Planet Gear Carrier
receives is 3.6 times (2.6 + 1 = 3.6) of the torque applied on the clutch
pack (which is connected to the Sun Gear directly). In other words, to
transmit 1hp, the load on the clutch plates is only 0.278hp; Other
mechanical gear components is responsible for transmitting the rest of 0.
722hp. From another aspect, the clutch pack can also be viewed as an energy
absorber, which takes care of the excessive energy generated by the 1.7%
front/rear wheel speed difference.
Therefore, the wear and heat generated by the sliding friction is much less
in the SH-AWD. Compared to other AWD systems such as the GKN Twinster and
the Haldex, the clutch pack in SH-AWD only need to handle 27.8% (1/3.6) of
the workload that the Twinster/Haldex systems need to handle under the same
output condition. This is the reason why higher load can be applied on the
rear wheels more frequently in the SH-AWD system, without the wear and
overheating concern.
2014_Acura_TL
Above image: the 2014 Acura TL with SH-AWD
When SH-AWD Will NOT Work?
The SH-AWD on the last Acura RL is an upgraded version of the SH-AWD used on
the MDX and TL. This means it must be addressing some situations that the
SH-AWD systems on the MDX/ZDX/TL do not work. Before discussing the SH-AWD
used on the 2005-2012 Acura RL, let’s first ask a question:
Why does Acura build a 1.7% overdrive ratio between the front and rear
wheels, in MDX/ZDX/RDX/TL?
What if Acura did NOT build in the front/rear rotation speed difference? If
that was the case, when you are making a right turn, your left rear wheel
will spin much faster than the front wheels. Whenever the clutch is engaged,
the planetary gear set will fall into the case that the input is trying to
drive something (the output) which is spinning faster. This means the engine
is providing a braking effect to the rear wheel, which will cause loss of
control and is not what we want. So if there is no build-in rotation
difference, the rear wheel is unable to get any power during cornering, not
to mention the torque vectoring.
This also introduces another question naturally:
Is there any chance that the rear wheel spins more than 1.7% faster than the
front wheel?
The answer is yes. Suppose you are making a turn and the turning diameter is
large enough to assume both the front/rear wheels are turning with this
same diameter. Take the MDX as an example, its rear track width is 1.684
meters, therefore if the turning radius is 49.5m, the outer rear wheel will
spin (1.684/2)/49.5 = 1.7% faster than the front wheels. Therefore, if your
turning radius is less than 49.5 meters (162.4 ft), theoretically speaking
the MDX’s rear wheel will be spinning faster, and using up all of the 1.7%
rotation speed difference allowance. When that happens, the output clutch
plates (connected to the Sun Gear) will start to spin in the opposite
direction, and the clutch has to be completely disengaged to avoid the
previously mentioned “braking effect”; and of course, the rear wheel does
not get any power during that time period.
Some readers may ask:
Many people have driven the MDX/ZDX/TL under various conditions, why no one
has ever noticed the above issue?
First, the above is just a theoretical computation, there are two factors
that help to alleviate the above issue.
1. In real world, since the MDX/ZDX/TL are based on a front-wheel-drive
platform, while making a turn, the nose-heavy characteristic will cause a
higher slip ratio on the front wheels than the rear wheels, this effectively
added another layer of “cushion” on top of the 1.7% allowance;
2. when the turning radius becomes small enough, the fact that the front
wheels have larger turning radius than the rear wheels will start to take
effect, this is another type of “cushion” that can be consumed before the
1.7% overdrive allowance is touched.
Therefore, the cornering radius that makes SH-AWD ineffective may not be 49.
5 meters (162.4 ft) as calculated above, but it will exist in a range
between 0 and 49.5m.
Below illustration shows us, during a tight turn the car’s front wheels
will have a relatively larger turning radius than the rear wheels.
Turning_Radius
SH-AWD on the Acura RL
The major difference between the SH-AWD on the RL and MDX/ZDX/TL is, it
installed an Acceleration Device in front of the rear drive unit. That “
Acceleration Device” is in fact a 2-speed transmission: one gear provides
the 1.7% front/rear wheel overdrive ratio; the other gear provides a higher
5.7% ratio. Below illustration shows the Acceleration Device installed in
front of the rear drive unit.
SH-AWD_Component_1
Above image: cut-away of the SH-AWD with Acceleration Device
The reason why it needs the 5.7% ratio is to address the above mentioned
issue: small turning radius will make the SH-AWD ineffective. By providing a
higher overdrive ratio, it is less possible that the rear wheel will use up
the allowance provided by the overdrive ratio. This means, the SH-AWD on
the Acura RL can cover a broader turning radius range than the SH-AWD on
other Acura models.
However, although not taking the full workload, the rear wheel clutch still
need to take care of a portion of the energy. Therefore a larger overdrive
ratio means higher wear and heat in the clutch pack. So under normal driving
conditions, the Acceleration Device will shift to another gear that uses a
lower overdrive ratio to decrease the wear and heat.
2009_Acura_RL
Above image: the 2009 Acura RL SH-AWD
The only disadvantage of this design is: there is a time lag for the
Acceleration Device to shift between the two gears, so it may not work as
fast as you wish under some special driving situations.
Acura SH-AWD v.s Honda VTM-4
In the SH-AWD system, for each rear wheel if you replace the planetary gear
set plus the clutch with a single wet-type clutch pack, you will get the VTM
-4 system (of course some components and also the ECU are different, however
the mechanical schema will be like this). See the below illustration.
Honda_VTM-4
This changes the way how the rear wheel drive unit work fundamentally. Now
the clutch pack is 100% responsible for transmitting power to the rear wheel
. It works in the same way as the GKN Twinster. Wear and heat generated from
friction become a big concern, therefore the VTM-4 system cannot deliver
power to rear wheels for a long period. In fact, after the vehicle reaches
the speed of 18mph and in constant-speed status, the clutches in the VTM-4
will completely disengaged, which means the vehicle is effectively in pure
FWD mode under this condition (during acceleration, the VTM-4 can still kick
in and provide temporary torque to the rear wheels).
SH-AWD on the 2015 Acura TLX and the 2016 MDX
On the latest TLX and MDX, Acura “secretly” changed the SH-AWD system
design. Below is a cut-away image showing its mechanical structure, you can
see it can be considered to be a variant of the Honda VTM-4.
Acura_TLX_SH-AWD
The major functionality differences between this new SH-AWD and the VTM-4
are:
1. Rear wheels will always get power;
2. Torque vectoring feature in the rear wheels (the hardware of VTM-4 can
do this too, however Honda chose to not implement it)
Compared to the previous SH-AWD, this new SH-AWD has these different
features:
1. Overdrive ratio raised to 2.7% (from 1.7%) – this will decrease the
possibility that SH-AWD becomes ineffective, as discussed in the above “
When SH-AWD Will Not Work” section;
2. Hydraulically-controlled clutch (old SH-AWD: Electromagnetic clutch)
3. Smaller and lighter
Simply speaking, clutch packs in the new SH-AWD system are responsible for
100% of the workload, with the increased 2.7% overdrive ratio, the wear and
overheating issue should be more serious than before. The reason why Acura
can still let such system pass at least 10% of the torque during cruising in
a straight line is: progress in the material technology make it possible to
manufacture clutch plates with excellent anti-wear properties, so although
there are lots of friction wear and heating under normal operating
conditions, the new SH-AWD component can still maintain good reliability.
For more technical details about this new SH-AWD, please see this SAE paper
(it will be published on 04/14/2015):
Development of High Efficiency Next-Generation SH-AWD Rear Drive Unit
In a summary:
1. The “old” SH-AWD can be thought as working with a similar principle as
other AWD vehicles who are using planetary gear center differential + multi-
plate clutch as limited slip; their difference is just the layout and also
the limited-slip ratio settings.
2. The new SH-AWD and the GKN Twinster (used on the 2016 Ford Focus RS) are
intrinsically the same.
Appendix
1. Planetary Gear Set
Planetary_Gear_Set
As shown in the above image, a planetary gear set is composed of THREE
major components: a Ring Gear, a Planet Gear Set (including 3 planet gears,
and one planet gear carrier – the “Arm” in the above image) and a Sun
Gear. For such 3 components, it can be configured as 2 for input + 1 for
output, or 1 for input + 2 for output. Or by controlling the load on one of
the component, the input/output relationship in the rest 2 components is
fixed.
2. How a planetary gear set act as a central differential with limited-slip
feature
If you have read our previous articles related to AWD systems (for example
the Audi Quattro/VW 4Motion), you may know some of the AWD systems are using
a planetary gear set central differential with multi-plate clutch acting as
the limited-slip functionality. This is exactly how such central
differential can vary torque delivery percentages between the front and rear
axles: it use the clutch set to control the load on components of the
planetary gear set (by varying the pressure applied to the friction plates),
then the power delivery ratio can be determined for the two output
components (one connected to the front and another one connected to the rear
). When the clutch is completely opened (released), the torque split ratio
is the default ratio. For example, the Porsche Cayenne’s default front/rear
power ratio of 38:62 means, when the limited-slip clutch plates are
completely released, the other two components in the planetary gear set
central differential receive 38% and 62% of the total input power
respectively.
【在 b******7 的大作中提到】
: 丫的懂个屁，拿翻译软件翻译的人神共愤的文章，你们也看。
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