At two separate events recently, ZF publicly confirmed rumours that it is developing a new 8-speed automatic transmission, for launch in 2010.  This follows the introduction by Mercedes-Benz of its 7-speed W7A-700 in 2003, and more recently the Aisin TL-80SC (=Toyota/Lexus AA80E) 8-speed automatic, which is fitted in the Lexus LS460 [250L] writes Chris Guile, powertrain analyst at CSM Worldwide.

The new ZF ‘8HP’ transmission will, like its 5HP and 6HP predecessors, be used in passenger car applications with longitudinally mounted engines, and rear-wheel or all-wheel drive configurations.  It will, therefore, be no surprise to learn that the 1st application for the 8HP will be the new BMW 7-Series [F01], which CSM Worldwide is currently forecasting to launch in late 2008, rather than 2010.

With subsequent applications likely to include several other BMW cars, plus those from the VW/Audi Group and the Ford Group, ZF has ambitious plans for the 8HP, expecting to supply 1.1 million units annually by 2013.  However, this may be a best-case scenario, in which GM decides not to follow ZF with an 8-speed, and loses its business with BMW (currently both ZF and GM supply automatics to BMW).  This would be great news for ZF, but not so good for BMW, which has been able to play the two suppliers off against each other, in terms of price and technical performance.  But even assuming this scenario plays out, ZF will find that it has a new competitor to contend with, in the form of GETRAG, which will by then have been supplying BMW with the 436 (7DCI600) Dual Clutch Transmission (DCT) for a couple of years.  So, maybe BMW won’t be in such a bad position after all.

In the past there has only been limited competition between these two independent transmission suppliers, with ZF concentrating on automatics, and Getrag on the manuals.  But the new DCTs will change all of that, with both of the companies trying to get a foothold in this new market where, so far, Volkswagen has had a monopoly with its wet-clutch DQ250 and the Ricardo manufactured DL1300 which is fitted to the Bugatti Veyron [BG824].

As mentioned above, BMW will not be the only customer for the new 8HP, with all consumers of the current 6HP being potential targets.  This is particularly true as the 8HP is said to have a similar package size and weight (and possibly even cost) to the 6HP, making them essentially interchangeable.  But these transmission changeovers usually only occur at certain times, for a given vehicle, such as a new vehicle launch, a mid life-cycle refresh, or when a new or re-engineered engine is added to the existing line-up.  For this reason there is a limit to how quickly ZF could phase out the 6HP, and ramp up volumes of the 8HP.  Based on our current assumptions, CSM Worldwide is forecasting that demand for the 8HP will rise to about 900,000 units by 2013, compared to the 1.1 million expected by ZF.

Despite now concentrating on its 1st and 2nd generation 6HP and new 8HP transmissions, it might seem surprising to learn that ZF still manufactures a small number of its old 4-speed 4HP programs, in Germany, for use in the Peugeot 407 [D2], whilst the 5-speed 5HP is still supplied to Porsche, Skoda and Volkswagen.  One production line for the 5-speed 5HP has already been shipped to China, where it supplies the Volkswagen Passat/Lingyu [VW451/1].  And in the next few years, CSM also expects to see a 6HP line being transferred from the German Saarbrüken plant to China, as demand falls in Europe in favour of the 8HP.

In terms of technology, the 8HP does away with the Lepelletier design of the current 6HP, opting instead for 4 simple planetary gear-sets, with 5 shifting elements, compared to the 3 gear-sets and 5 shifting elements for the 6HP.  By minimising the number of open clutches/brakes to just two, in any gear, ZF believes it has improved the efficiency of the transmission, based on computer simulations.  The reason that the open clutches and brakes are important is because they lose energy in the form of friction, to the fluid that surrounds them. 

Additional efficiency gains come from the gears themselves, with the 6HP gears having between 96% and 99.5% efficiency, while the 8HP has between 98% and 100% efficiency.  This 100% figure is achieved only in 6th gear, which has a direct or 1:1 ratio between input and output.  These differences, plus some other contributing design factors, leads ZF to believe that the 8HP will be up to 6% more efficient than the recently launched 2nd generation 6HP.  This now puts the automatic squarely up against the manuals and newer DCTs in terms of efficiency, an issue which has been worrying automatic manufacturers for some time.

The ratio spread (the difference between first and top gear) is being increased from 6.04 on the 6HP, to 7.01 on the 8HP.  This allows both a higher ratio in first gear, for better launch performance, and a lower top gear ratio, for reduced engine speed when the vehicle is at cruising speeds – this being particularly important for diesel engines.  It also enables the steps between gears to be smaller, which results in better acceleration and higher shifting quality.

ZF says that the 8HP is capable of using different types of start-up or launch devices, which suggests that it could replace the torque converter with a clutch, much like it did on the 7-speed concept automatic which was shown in 2003.  In addition, ZF also say that it is capable of being used in a mild hybrid configuration, or a full hybrid configuration with the torque converter replaced by a motor, resulting in an identical package size.

So what of the competitors – will they follow ZF to 8 speeds or above?  Well, Mercedes-Benz is still carrying out further developments to its 7-speed, having recently launched a new version which combines the transmission and transfer case, for use in its 4 wheel drive ‘4MATIC’ applications.  This W7A-700 (and W7X-550 variant) has a ratio spread of 6.0, which Mercedes-Benz will almost certainly want to increase with the next design iteration, possibly with a clutch instead of a torque converter, although time will tell if this is a 7, 8 or even 9-speed.

And as for GM, it has only recently launched its new 6-speed automatics (6L45, 6L50, 6L80, 6L90), so may not have the time to develop an 8 or 9-speed to compete with ZF.  Perhaps GM could make better use of its time and investment in developing DCT technology instead.

ZF claims that 8 forward speeds is an optimum, resulting in lower fuel consumption and a higher ratio spread, calculated following extensive simulations of many different configurations.  But it also happens to be one more gear than the Mercedes-Benz W7A-700, which is probably its most direct competitor.

One argument that has been used to counter the proliferation of gears is that, in many cases, the drivers of premium cars with 6 or 7 forward gears frequently don’t even realise it, so what is the point of giving them more?  Well, the usual justification is fuel consumption, and ZF is no exception, but there has to come a point where the efficiency gains of the extra gears is outweighed by the additional gear-set losses.  Plus, the drivers of those same vehicles often don’t care about the fuel costs, so perhaps this really is about one-upmanship between the brands……

Chris Guile is a Powertrain Analyst with CSM Worldwide.

ZF 8HP Preliminary Specifications
Torque Capacities: Two versions with c.500/550N.m and c.750/800N.m
Configurations: Longitudinal, RWD and AWD (including transaxle)
Weight: c.90kg Gear Ratios:

1st 4.70
2nd 3.13
3rd 2.10
4th 1.67
5th 1.29
6th 1.00
7th 0.84
8th 0.67

click table to enlarge