AWD comparisons

[asmventure]

A newbie's first post...

This is GCZ. Why is the MB G-Series not being raved or ranted about here?

[martinbo]

I've been surprised by the lack of commentary about Audi's new AWD configuration in the A5 / S5. Ok the layout has been seen before in the A8 but its significance here in the A5 Coupe is greater due to it's application in a compact platform.

So the Audi engineers sprung one on me... they've managed to mount the front differential ahead of the clutch - and thus closer to the engine - in the longitudinally laid out engine / drivetrain configuration. This allows the A5 a shorter front overhang thus - in theory at least - improving steering feel and turn-in response.

This must've come at some cost I surmise, particularly in the area of mechanical complexity. Check out this pic:



Notice the shape of the transmission casing: there's a distinct 'bulge' running along the length of the casing from the area around the centre differential toward the front axle. This appears to house the propshaft directing drive to the front diff. Quite the departure for a quattro system so typically admired for its compact layout previously...

Does anyone have detailed technical diagrams of this new transmission / drivetrain? Yannis?

[Imhotep Evil]

Basically Audi will dump the 50/50 with off-road WRC racer capability orientated (compact) aproach(not Hilux/Defender style) with RWD emulation on-road approach to go after BMW.

It makes sense, since this types of people don't really go off road and really want that RWD feeling.

[martinbo]

Originally Posted by Imhotep Evil

Basically Audi will dump the 50/50 with off-road WRC racer capability orientated (compact) aproach(not Hilux/Defender style) with RWD emulation on-road approach to go after BMW.

It makes sense, since this types of people don't really go off road and really want that RWD feeling.

Looks to be that way to a certain extent doesn't it?

[Imhotep Evil]

I should have said 1980s WRC racer.

On a similar tread I wrote:

Both next gen. BMW and Audi AWD systems seem to copy somewhat the general idea of the recently banned WRC tri-active-diferential system. Except they lack the ability to split torque between the front wheels.
I guess that's left for the next next gen. AWD.
Honda, I belive, beat them to it.

Audi seems to go the BMW path since they realise that people won't buy medium to big premium products with FWD.
But where does this leave the "Never follow" motto ?!

To the article paragraph:

Audi works intensively on the improved quattro drive, which for the introduction on the market of the A5 Sportback should be ready for production. The new all wheel system is simpler , has less internal friction and still more versatile. After the rear biased torque distribution the next driving dynamics system called Quattro Torque Vectoring (QTV) is now on the works. QTV regulates the torque distribution between the axles and between the rear wheels. A goal is more agility, more stability and an even better traction.

[Yannis]

Originally Posted by martinbo

Does anyone have detailed technical diagrams of this new transmission / drivetrain? Yannis?

Μartin i don't think that we can find anything more detailed than that.

These are the chassis pics from the press release:

Fourtitude.com The Audi enthuisast web site

[EnI]

BMW Dynamic xDrive (xDrive + DPC + SCI):



BMW has developed two new drive and chassis systems which increase agility, traction and active safety without compromising dynamic driving and efficiency: Dynamic Performance Control and a new function of Dynamic Stability Control, called Selective Control Intervention.


Dynamic Performance Control

Dynamic Performance Control is based on a mechatronic system that takes over variable distribution of drive torque to the rear wheels independently of engine power. The difference in torque can be as much as 1800Nm and this enhances handling, steering precision, directional stability and traction. At the same time, the steering responds more directly, needing fewer steering corrections and stabilising interventions from the electronic control systems, such as Dynamic Stability Control (DSC).
The rear axle differential is combined with two superimposed differential drives and two electronically controlled lamella brakes in order to continuously vary the distribution of the drive torque between the rear wheels.

BMW has developed a system that operates under load and, for the first time, also actively distributes torque during thrust operation and when the clutch is engaged.
The combination of Dynamic Performance Control and the variable four-wheel drive BMW xDrive allows longitudinal torque to be variably distributed between the front and rear axle. Also, it distributes drive torque to the rear axle in response to driving conditions, and neutralizes the inherent tendency of four-wheel vehicles to understeer.


Selective Control Intervention

The xDrive four-wheel drive system, DSC electronic control system and engine control through integrated chassis management are combined for fast distribution of the drive torque in the longitudinal direction.
Meanwhile, further development of Dynamic Stability Control (DSC) has seen the introduction of Selective Control Intervention. During tight cornering it actively counteracts the tendency to understeer. This control intervention, which is also active when DSC is switched off, will initially be introduced from March 2007 in the four-wheel drive models of the new BMW 5-Series. Other models and model series from the BMW brand will follow.

If a vehicle tends to understeer when cornering, the current DSC system increases the braking pressure on the nearside rear wheel in order to generate a stabilizing yaw moment. That action remains identical on the new system, but now engine torque can now increased independently of the position of the accelerator. This can generate an additional stabilizing torque of up to 500Nm.

The torque compensation when the engine is running at partial load contributes to increasing the precision of the steering manoeuvres, tracking stability and traction when cornering. This is because the system enables the car to respond faster to the course determined by the driver, and steering corrections are less frequently required. The vehicle follows the path of the curve at constant speed despite the stabilizing braking intervention.


In detail:

BMW Dynamic Peformance Control in Detail


Original GCZ Dynamic xDrive thread:

http://www.germancarzone.com/bmw-lou...ic-xdrive.html

[martinbo]

Well, I thought it was time to resurrect this old thread given some rather neat developments in AWD drivetrain packaging in some new(ish) models and in anticipation of the xDrive system in the new X1.

Those of you who are interested in AWD and the challenges facing engineers when it comes to packaging and locating the drive components will know that engineering an AWD layout is an exercise in compromise. Some manufacturers like Subaru will engineer themselves into a corner with their dogmatic, persistent pursuit of "Symmetrical AWD" and others like Audi will employ vastly differing AWD systems yet branded consistently as "quattro".

One of the big challenges in front-engined, longitudinally-oriented AWD drivetrain layouts is how to get drive to the front differential and where to locate this diff relative to the engine. The key consideration is where you want the engine mass located relative to the diff. In front of the diff (Audi A4) means a nose heavy design; underneath the engine (Cayenne, GT-R) makes for a high centre of gravity and ahead of the engine (xDrive) presents problems in routing propshafts and off-setting of the diff.

With modern design and manufacturing processes car makers are able to package AWD drivetrains in ever more compact and elegant solutions without incurring nearly as much of the penalties associated with nose-heaviness or high centre-of-gravity.

Two superb examples are that of the Panamera and Mercedes Benz GLK. These two cars show how the front axle, differential and engine are much more closely integrated for a more complete front-end AWD solution. Sure, they're a little more complex but the philosophy behind the execution is appreciable. What we're seeing is that the front diff is now placed much closer (and even mounted) to the engine block, with the one driveshaft actually running through the engine oil sump. This means that:
a) The front axle can be moved closer to the front of the car - thus reducing nose heaviness
b) The front drivetrain components needn't be mounted beneath the who engine - thus lowering the centre of gravity.

First up, Porsche's engineering tour-de-force (but still fat-arsed and chocolate brown) Panamera:



Notice the electro-mechanical centre differential with multi-clutch pack, offset propshaft, front diff directly adjacent to engine block and left driveshaft running through the sump.

Another very elegant design is, somewhat surprisingly found in the Merc GLK. Mercedes are lovely - never ones to make a song-and-dance over their all-wheel drive systems - they nevertheless have managed to sneak this impressively compact and complete design under the radar.



Here we can see the close proximity of the front diff to the engine, the path of the driveshaft through the sump and we can appreciate how much farther forward the front drive components have been located as well as how much lower the engine is mounted relative to these components. Just and inch or two from the crankshaft! All of this is a good thing.



In the picture above we're shown the centre differential and the coupling of the front axle propshaft. Unlike the Porsche the GLK makes use of a "Torque Sensing" mechanical centre diff employing a helical, planetary gearset. This then infinitely adjusts the amount of torque being distributed between the front and rear axles from a static 45:55 front-to-rear split.



This picture very nicely shows the tight integration between the front diff, driveshafts and engine.



And, to wrap it up, this picture shows the actual mechanical arrangement of the ring, sun and planetary gearsets and how this connects drive to the front & rear propshafts.

So in closing, AWD is coming along in leaps and bounds and even diverse cars like the beautifully engineered Panamera and GLK represent the leading edge in AWD thinking, engineering and execution for front-engined cars with the powertrain running lengthways.

[Giannis]

Great post Martin! Too bad, the system doesn't let me give you some karma!

[martinbo]

Thanks is more than enough mate. Y'know me - I just love this topic.