Following the introduction of Dual-Clutch Transmissions (DCTs) to the market by Volkswagen in 2003, Porsche has now added its name to the ever-growing list of OEMs that are using this technology. Chris Guile, powertrain analyst with CSM Worldwide, reports.
In Porsche’s case, it contracted transmission specialist ZF to both develop and supply the transmissions, these being the first ZF DCTs to go into series production. This is against a backdrop of mixed fortunes for the DCT, which is currently being embraced by some OEMs but treated with caution by others. Despite OEMs such as the VW Group and Ford (with Getrag) forging ahead with multiple DCT programs, others are pulling back from earlier plans. For example, BMW has recently decided not to install DCTs into its mainstream models, and is now limiting this technology to a few performance models in the 3-series range and the Z4.
Contrary to earlier suggestions in the press, the ZF DCT for Porsche was completed on time after only three years in development. This is significantly faster than the four-and-a-half years that would normally be expected for such a project. Some of this confusion may have arisen from earlier plans by ZF to manufacture DCTs for BMW – a plan that was later dropped. That particular version was then revived for use in a future Japanese application, but that vehicle was also dropped in light of the current financial situation.
Approximately 300 engineers from across the ZF Group were involved in the development of these 7-speed ‘7DT’ DCT programs. While this might seem high, it is not so surprising when you consider that most of the components are manufactured in-house by the ZF Group. These include the main clutch pack, the gear-sets, the hydraulic control unit and the bevel gears/differential. It is only the more specialised components, such as bearings and the Transmission Control Unit (TCU), which are sourced externally.
The claimed benefits of DCT technology have already been covered extensively in other articles. In Porsche’s case, it needed something to replace the now quite elderly W5A (NAG1) 5-speed automatic, from Daimler, with something that was more appropriate for the car and, which also offered reduced fuel consumption. What it got was a fast-shifting transmission which, in combination with the new direct injection gasoline engines, gives Porsche a claimed CO2 improvement of approximately 15% (18% for the 911 Turbo), of which roughly half was attributable to the transmission. What is really interesting is that for the Panamera S, where a direct comparison of the manual and the DCT can be made, the CO2 value for the manual version is 16% higher than the DCT version, over the New European Drive-Cycle (NEDC).
In fact, two separate DCT ranges or ‘platforms’ have been developed by ZF, both fitted with wet clutches, for use in Porsche’s various longitudinal applications. The first is for use in the mid- and rear-engine sports cars (the 911, the Cayman and the Boxster), while a completely different platform has been developed for use in the larger Panamera. For each platform, two different torque options are available, with the 500N.m versions using an ‘ND2015’ clutch pack, and the 780N.m versions using an ‘ND2216’ clutch pack, both supplied by ZF Sachs.
To make things slightly confusing, ZF is using the same 7DT prefix for all variants, despite there being two separate platforms. The various programs that are built off each platform are detailed in the following table:
Porsche 911 4-Wheel-Drive
Porsche Boxster and Cayman
Porsche 911 Turbo
Porsche Panamera 4-Wheel-Drive
One key benefit of the DCT is that any gear ratios can be specified to exactly match the requirements of the application/engine, unlike the automatic, where the ratios are largely fixed. In addition to allowing the desired ratio progression, this also allows the designers to increase the ratio spread, which reduces the engine speed when cruising and hence reduces CO2. For the first platform, for the sports car applications, ZF has several different ratio sets, with ratio spreads extending up to a very creditable 6.4. This has reduced the engine speed at 100km/h from 2450rpm to 1750rpm in the 911 Carrera, compared to the previous version. However, on the second platform for the Panamera, ZF and Porsche have decided on an even wider ratio spread of 10.1, which exceeds the current highest example (the VW DL501), which has a ratio spread of 8.0.
On the 7DT75 for the Panamera, ZF has included a stop-start system, which allows the transmission to be ready as soon as the engine is restarted. As is common with many other stop-start systems on the market, the driver has the option to turn this function off. Unlike other similar systems, it does not rely on an electric oil pump to maintain the transmission oil pressure. Instead, it uses a form of non-return valve, although ZF declined to go into any more detail. One clue lies in the claim that the transmission needs no oil pressure once the gear is selected, which also gives ZF the ability to offer a limp-home feature, should the electronic controller fail. Curiously, Porsche did not specify stop-start for the versions in the sports cars, although ZF claimed that there was no technical reason why this could not have been included.
All of the 7DT transmissions were developed (and successfully tested) to use a lifetime oil fill, but Porsche decided to err on the side of caution and opted for a 90,000km oil change interval. The hydraulic controller features pre-controlled or ‘piloted’ valves, which are supposedly less susceptible to oil contamination; some observers believe that these are somewhat old-fashioned now, having given way to direct-acting valves in many modern applications.
In terms of the oil circuit itself, two completely different approaches have been employed for the two platforms. The 7DT45 and 7DT70 have two oil circuits, and hence two different oils; the first is Pentosin FFL-3 for the clutch and hydraulics, and the second is ExxonMobil Mobilube PTX 75W-90 for the gear-set and bevel gear. The oil levels have been kept as low as possible, to reduce churning losses for those moving parts that are immersed in oil. Conversely, the 7DT75 has a single oil circuit and a dry sump (to minimise churning losses), with an ‘active lubrication system’ to feed oil to each gear-set and clutch. This version uses only the Pentosin FFL-3 lubricant, which was developed exclusively for the ZF DCTs. One of the main reasons for using a single oil circuit is that clutch cooling is required at both ends of the transmission, for the main dual-clutch module and for the hang-on clutch used in the four-wheel-drive variant. This would have presented significant sealing complications had multiple circuits been chosen.
In addition, the 7DT75 is unusual in that the layshaft is mounted above the input shaft rather than below it. This was done to meet Porsche’s requirement that the package should be ‘narrow with a high tunnel’, presumably so that the engine/transmission could be mounted lower in the chassis to give a lower centre of gravity for the vehicle.
All of the 7DT programs feature software with an adaptive shift strategy, which not only adjusts the shifting patterns according to the physical vehicle and road conditions (including mountain and road-surface sensing), but also monitors the way in which the car is being driven. So, in effect, they are monitoring the mood of the driver and will adjust the shift strategy accordingly. This kind of fuzzy logic has been used in the past, with mixed results, so it will be interesting to see how owners react to this latest implementation of the technology.
As can be seen above, each platform consists of several different variants, depending on the specific application/layout, and depending on whether it is a 2-wheel-drive or a 4-wheel-drive version. For each variant, there is considerable modularity of design to keep the cost and overall part count down. In addition to this, there are also some components that are common or similar across both platforms, despite the significant differences, such as the Transmission Control Unit (TCU), the hydraulic controller and the software.
In terms of driving, the experience is very much like driving a standard automatic transmission, the main difference being that the shifts are considerably faster. Having said this, there is a noticeable delay between pressing one of the steering-wheel mounted switches and the shift being performed. The actual shift times are open to interpretation, as different people define shift time in different ways, but ZF is quoting times of about 400-500ms. In most cases, you only know that a shift has occurred because the engine revs drop and the dashboard indicator changes. This compares to shift times of about 750ms in the previous versions using the Daimler 5-speed automatic.
In practice, in Comfort mode, the transmission changes gear almost imperceptibly, and in a way that does not distract the driver, so in this respect it achieves its goal. In the Sport mode, the shifts occur much later, allowing the engine to rev higher, for a more interactive and sporty feel. And in those vehicles in which the Sport Plus option is available, the driver has the ability to reduce shift quality in favour of performance, which results in a shift strategy that is probably only appropriate for track use. The Sport Plus mode also includes a launch control function, which allows the driver to build the engine speed to maximum before releasing the brake for an exceptionally quick getaway. When this happens, the transmission oil is circulated over the clutch much more quickly than normal, to prevent it from overheating.
So, will ZF start to supply more DCTs instead of planetary automatics in the future? Well, the short answer is no. ZF is still very much committed to automatics, as can be seen from its plans to replace the existing 6-speed 6HP with the new 8-speed 8HP, with combined volumes of well over 1 million units per year. Most of these will feature increasingly efficient torque converters, although ZF, along with many of the other major automatic suppliers, is also investigating the use of clutches as launch devices instead.
For now at least, CSM Worldwide is forecasting that DCTs will remain a niche product for ZF, with annual volumes rising to about 60,000 units. Despite this, ZF is already working on the second-generation 7DTs, concentrating on cost-down engineering and any further modifications that can improve the efficiency. ZF says that weight has already been optimised, thanks to some very thorough analysis using ‘topology optimisation’, in which each part is analysed under the worst conditions (e.g., first, second and reverse gears), to build up a minimum physical shape to meet all conditions.
CSM has also learned that ZF is developing a new technology that allows manual transmissions to be built on the same platform as DCTs, to leverage economies of scale for the common parts. This has not been possible until now in any significant way, but ZF’s new patented system should allow true modularity between manuals and DCTs. This begs the question ‘Would such a manual have 7 forward gears rather than 6, if it were based on one of the 7DT programs?’ Well, technically there is no reason why not, but it is most likely that ZF would still limit the number of forward gears to 6.
For those OEMs and suppliers involved with DCT programs, the future is still looking good. Despite changes in strategic direction at some OEMs, and despite other DCT programs being cancelled altogether, there is still a growing trend to offer DCTs to customers. According to the latest CSM forecasts, at least 11% of cars and light commercial vehicles manufactured in Central/Western Europe will be fitted with DCTs by 2015. In reality, the penetration rate is likely to be even higher, at about 16%, given that some OEMs will decide to commit to DCTs between now and then.
Chris Guile is a Powertrain Analyst with CSM Worldwide.