Well, winter's round the corner and interest in AWD vehicles seems to be gathering. Since I've been babbling quite a bit about AWD, I'm putting together a short writeup about what I know about AWD systems and also provide some subjective comments about what I think.
This article, for what it's worth is:
Copyright 1989 by Eliot Lim, all rights reserved
Permission is granted to distribute this article free of charge, provided that the entire article and this header is distributed.
(phew!) ok, let's go.
First, some definitions:
AWD stands for All-wheel drive, 4WD for 4-wheel drive, the difference being that AWD implies permanently engaged four-wheel drive and 4WD for part-time manually selected four-wheel drive. The car industry usually uses these definitions but not in all cases. For example the part-time 4WD Ford Tempo is advertised as "All-wheel-drive", which is true, but goes against popular convention. For this article, I will use AWD and 4WD loosely, being more specific when it matters.
A differential is basically a mechanical device which takes input torque and splits it evenly (usually) to two axles, allowing them to rotate at different speeds. (Arrgggg!!!! Someone provide a better definition). A differential in a FWD or RWD car allows both wheels to apply power to the road and yet be able to turn at different speeds so that the car can turn without resistance from its transmission. A permanently engaged 4WD system needs 3 differentials to enable it to apply power to 4 wheels and be able to turn without resistance: The front, rear and center diffs. (diff = short for differential) This is because the distance travelled by the turning front wheels are not the same as the distance travelled by the stationary rear wheels. (Draw a diagram to see for yourself) Power leaving the gearbox first goes to the center diff, which then splits it to the front and rear diffs. A part time 4WD system in almost all cases do not have a center diff, so they cannot be used in the dry. When a part time 4WD engages 4WD, the front and rear axles are locked together and will rotate at exactly the same speeds, which will cause resistance in turning if the speed difference between front and rear wheels are not lost to the road/mud/snow.
This is a very new concept in AWD technology these days because they have a profound effect on the cars' road behavior. Differential locking is important in full-time AWD because in the simplest AWD with 3 free wheeling diffs, the car can be rendered immobile if any one of 4 wheels loses traction. Basic differentials send power to the axle with the LEAST grip. So if one wheel loses grip, all the power is sent there, leaving nothing for the remaining 3. Remember that a 4WD has twice as many wheels as a 2WD to lose grip and mobility on. And since 4WD vehicles would be used more in bad conditions, it is prudent to have some form of differential locking. Every full-time 4WD car that I know of have some form of diff locking. Audi was the first manufacturer to market high-performance 4WD with the quattro, released in Europe in 1981 and in the US in 1983. The cars won big in rallying and Audi decided to spin off 4WD and the quattro name to its entire range of cars. The first generation quattros had simple locks for the center and rear diffs, which locked them solid (no speed difference) to dig you out of deep trouble. When the center diff was locked, it meant that you had to lose grip on ONE rear and ONE front wheel to become immobile. When both the center and rear diffs were locked, you had to lose BOTH rear wheels and ONE front wheel to get stuck. The locks on these Audis were manually engaged and were quite cumbersome since the driver already has to worry about shifting and steering etc. Thus development went in the direction of automatically locking differentials. First on the scene was the viscous coupling (VC for short) which used a special kind of liquid in a casing designed so that minor speed differences were allowed between the 2 axles but the viscosity of the fluid would prevent further "slip" as in a wheelspin situation. The viscous coupling was used quite differently by different manufacturers. Some manufacturers used differentials in conjuction with the VC both in the center and rear, to lock the diffs progressively and automatically, unlike the Audi. Examples are Mitsubishi, Toyota, BMW, Ford Germany. Others like VW and Honda used the VC as THE center diff, resulting in a part-time, automatically engaging 4WD. In this case, the car is basically FWD, with the rear wheels following along and with minor speed differences absorbed by the VC when turning. However when front wheelspin occured, the speed difference increased to the point where the VC with its viscous liquid churning, would start transferring some of the energy from the front to the rear wheels. Note the difference between this system and the former. The latter is auto-engaging part time 4WD, while the former is full-time auto-diff-locking 4WD.
Next came the torsen (stands for TORque SENsing) differential, which was embraced by Audi in its second generation quattro system. Audi was approached by FF development (owners of the VC patent) during development of the original quattro back in the late 1970's but was rejected because of the complicity of its problems. The torsen diff was invented by an American company (Gleason corp. I think) and had all the advantages of the VC and more and none of its disadvantages. It was a fully mechanical device of worm gears (whose workings I cannot picture). The torsen differential would split torque 50:50 in a no-slip condition. However, when one axle slips, the torsen diff starts sending more torque to the axle with MORE grip, in other words, it works in exact opposite to a conventional diff. Torque splits of up to 78:22 is available with the current systems. And since it was a completely mechanical device, the torque split ratio was changed instantaneosly as opposed to the VC, which had a very slight lag for the viscous fluid to "catch up".
More importantly, the torsen did not lock or inhibit speed differences when braking, thus allowing all 4 wheels to rotate at their own speeds. The torsen diff only worked in a power application situation while the VC worked both in accelerating and braking, which caused lots of problems since it added its own resistance to the wheels. ABS systems rely very much on speed differences between the 4 wheels to decide if a wheel was locking. Needless to say, when the transmission tries to make all 4 wheels rotate at the same speed, it messes up ABS. Therefore the engineers had to use a variety of hacks to get around this problem. VW simply disconnected 4WD the moment the brake pedal was stepped on via a secondary clutch. Lancia used a vast array of sensors to assist ABS. They even went as far as to apply some power (via the engine computer) to reduce the drag of the VC when the brakes were applied! (60 minutes will be stalking them, for sure!!!) The easiest hack was to reduce the viscosity of the fluid in the coupling, so that the drag was reduced. This also meant that the VC's locking effectiveness was reduced, which affected handling in poor conditions. The BMW 325ix is widely regarded by experts to be in this category of an unhappy marriage between AWD transmission and brakes. Performance in snow/ice was found to be poor, while the brakes were found to be much worse than the RWD 325i. (One of the experts being Mikael Sundstrom, Finnish rally ace).
Porsche and Mercedes were threading slowly and had entries that came out much later than the pioneers. Mercedes devised an extremely complex array of sensors on each wheel to determine wheelspin. In the dry, the Benz was a RWD car. When the wheel sensors determine that the rear wheels were spinning, a signal was sent to the central computer to start engaging a multi plate clutch to send power to the front wheels. Clutch engagement was progressively altered by the computer. When the computer determined that even more traction was needed, a second clutch would start locking the rear diff. When the brake pedal was pushed, both clutches disengaged to allow ABS to work without interference.
The Mercedes 4-Matic is a part time 4WD system. Mercedes went to great pains to design a part-time 4WD because they did not want to upset their loyal clientele with a full-time 4WD, which because of the driven front wheels, would "change the traditional feel of a Mercedes". In practice, the 4Matic system works no better and no worse than the other crop of 4wds. It's enormous cost makes it look bad.
Porsche used a similar system of locking clutches (though they are implemented quite differently) as the Mercedes in the 959 and 964 (Carrera 4), but the center diff (clutch too) was engaged at all times except when parking so that the steering would be easier to turn. (I'd be interested to know how the 959 computes "parking" mode). Torque split in the 959 varied with load and conditions. Unlike most AWD's which split torque 50:50, the 959 split it front 40 rear 60 at normal speeds, 20:80 at full acceleration, 50:50 for snow etc. This is the most advanced AWD system to date.
The Japanese seem to be lagging behind in AWD technology. I don't know of a Japanese AWD that uses a Torsen diff; Subaru is the only Japanese maker that I know of that uses an electronically controlled multiplate clutch such as Porsche & Mercedes. Mazda uses the "ancient" manual lock in the 323 GTX. Honda does not even have a high performance AWD to offer. Perhaps they are waiting for the market to take off, then do furious development and zap the competition. This, sadly is not happening. All around the world, AWD's are not causing tidal waves in the showrooms. Audi sold no more than 20% AWDs during the peak of the quattro's popularity. Nowadays it is about 10%, and even less for the others.
From: firstname.lastname@example.org (Eliot Lim)
Subject: All you wanted to know about AWD (Part 2)
Keywords: Torque splits, torsen, Porsche, BMW, Audi
Date: 22 Nov 89 21:59:04 GMT
Organization: Univ of Washington, Seattle
Copyright (C) 1989 by Eliot Lim, all rights reserved
Permission is granted to distribute this article free of charge, provided that the entire article and this header is distributed.
Torque splits and advantages of AWD in the dry.
After describing the issues surrounding differential locking, next comes the issue of torque splits. A designer of a AWD car with a center differential has to decide the ratio of torque going to the front versus to the rear. A simple conventional differential will split the torque 50:50. Most of the AWD vehicles use a 50:50 torque split but there are exceptions: The BMW uses a 34:66 (or was it 33:67) split, the Ford Sierra XR4X4 (4wd version of the Merkur XR4ti) is similar, Lancia used 56:44 in the 8V versions of the Integrale and now use 44:56. These vehicles use a planetary gear system to achieve the unequal torque splits. The marketing men for these companies claim that sending more torque to the rear wheels improves handling, since one could get the tail out at the limit, thus appealing to the enthusiast more (and the rally drivers). Indeed, the first generation Audi quattros would understeer at the limit, rally drivers would lock the center diff to get the tail out. However, the main motivation for the uneven torque split was that since the chassis of these RWD cars were never designed with 4wd in mind, adapting it for 4wd was difficult. This difficulty could be reduced somewhat if one used smaller and weaker components for front drive, while retaining existing parts for rear drive. If one looked at the bottom of these cars, one would see that the drive shaft to the front wheels was much narrower in diameter. If memory serves me right, the Ford does not even use a drive shaft for the front wheels. It uses a chain (like a motorbike). This is not to imply that the chain is inadequate. It merely illustrates how sending less torque to the front wheels made life so much easier for the engineers. Also BMW had never made a front drive car before in the company's history, so it had no experience with front drive suspension layouts. Sending less torque to the front wheels made the potential problems less.
Whatever the difficulties were, the engineers at Ford and BMW did a relatively good job with their unequal torque split layouts, both cars being oversteerers at their (high) limits. The 325ix took care of the 3-series tail-happiness in the wet. 4wd enabled the oversteer to be far more controllable than the RWD counterparts. (More about this later)
Lancia, in contrast wanted the Delta Integrale to be rally king at all costs and went on to design the ultimate (for its time) 4wd car. When the Integrale came out, it boasted a 56:44 torque split with a viscous coupled center diff and a torsen rear diff. The torque split was chosen based on the car's static weight distribution according to Lancia. Later on they changed it to a rear-biased split (like 46:54) because they said a front-bias split produced too much understeer. The 2 liter turbo engine had balance shafts and a overboost feature, which enabled it to pump out lots of power and torque. This car swept away all the rally victories in the same way Audi did earlier in the decade. Never mind that the tail would come out under heavy braking. To the rally driver that would be an advantage. This car is still the rally king today.
Audi and others believed in the 50:50 split saying that it is the best all round compromise. For example, when accelerating, the weight transfer altered the nose heavy cars' distribution to about 50:50, thus optimizing the traction. The Audi's secret weapon was, of course the torsen diff. When the car reached its very high limits of adhesion it would understeer. However, if the driver persisted and applied more power, the torsen diff would sense that the front wheels had no more grip and would start sending more power to the rear wheels. In this way, the Audis could be power-oversteered in the way that enthusiasts love. One could not overcook it either because the torsen diff would send the power back to the front wheels if the rear end was sliding too much. This meant that the front end would pull you out of an excessive oversteer situation. This is what Hans Stuck and Hurley Haywood means when they say you have to keep your foot on the gas through the corners.. the car would take care of itself. The way the Audi divided power back and forth between the front and the rear gave it superb balance. Ordinary drivers with average skills (such as me) could drive the car right up to its limits with total confidence. This, I feel is the biggest reason to buy a AWD. Sure, the extra traction one gets from 4 driven wheels in the winter is a bonus, but many people are missing its chief benefit. The other cars, with a fixed torque split and a variety of locking mechanisms would do almost as well too, certainly better than 2wd cars. (Remember that the torsen diff could split torque from 50:50 to 78:22 to 22:78 depending on conditions).
The next "logical" step in exploring variable torque splits is to use computer control, of course. (I know some of you are puking, but I'm talking from an engineer's point of view). This, Porsche did to perfection in the 959. The 959 was Porsche's statement to the motoring world of who they are and what they are all about. The 959 used sensors to control everything from ABS braking, differential locking torque splitting, even damper height. For example, under full acceleration, it would send 80% of the torque to the rear wheels, while in snow it would split it 50:50. Porsche also used the diff locks to tame the tail happiness of the 911. I haven't come to grips with how this works yet, but they used the technology they learnt from the 959 to tame the new rear engined Carerra 4 and Carrera 2's. Road tests of the new Carrera 4 and the torsen equipped quattro showed that the Porsche had the upper hand, which I don't doubt.