New DCT from ZF gains eighth speed and hybrid module but keeps same package size
Extreme sports cars from elite brands such as Ferrari and McLaren have almost all switched over to dual clutch gearboxes as their preferred transmission type. Only the Chevrolet Corvette stands as an exception, though with its rumored next-generation shift to a mid-engined architecture there may yet be reason for the premier U.S. sports car to join the DCT club, too.
Premium and luxury sports sedans, however, are an entirely different proposition and a separate set of rules applies. On occasion, luxury car drivers may fancy a short spell of extroversion at the wheel, with the rev counter redlining and the gear shifts snapping in sharply with a satisfyingly sporty jolt. But that’s no more than the luxury car paying a token visit to the gym; for the vast majority of its workaday life the car is doing the day job it was designed for – whisking important people smoothly and silently from place to place. To succeed in this there must be no fuss, no temperament and certainly none of the histrionics often displayed by highly-strung supercars. In particular, launches from rest must be totally smooth and all subsequent gear shifts must remain imperceptible to everyone concerned.
And that is the main reason why DCTs have not yet made real inroads into top luxury markets. The DCT is still seen as being slightly on the sportier side of the spectrum and total refinement in operation is harder to guarantee than with the familiar torque converter planetary automatics. To make the DCT’s task harder still, the very latest planetary transmissions with up to ten speeds now set extraordinarily high levels of refinement, and, if so required, their sport-mode behavior can easily be programmed to emulate that of a responsive DCT.
Porsche: the one exception
Only one automaker has been brave enough to apply DCT thinking to a prestige sports sedan. Back in 2009, Porsche entered the sports sedan segment with its Panamera. While the market move and the 911-like styling were no surprise, the choice of a ZF co-developed DCT transmission certainly was. Unfortunately, that choice initially did the DCT’s luxury case little good, for though they were satisfying for sporty driving, early Panameras proved jerky and unrefined in city driving. The issues were soon resolved, but it was far from clear whether the Panamera would stick with DCT for its second generation – especially as the Cayenne luxury SUV produced in parallel and with many of the same engines had always featured planetary automatic transmissions.
In any event, Porsche did choose to keep faith with the DCT solution when it launched the second-generation Panamera for the 2017 model year. ZF has again been the design and manufacturing partner, but the transmission has been revised to incorporate eight speeds instead of seven, and to allow for both plug-in hybrid operation and the option of AWD to supplement the standard rear-wheel drive.
The core six-cylinder E-hybrid version of the new Panamera, incorporating the new 8DT transmission and e-motor unit, was first unveiled in the second half of 2016. But it was only with the announcement of a further plug-in hybrid derivative in February 2017 that closer attention began to focus on the driveline – for the four-liter, twin-turbo V8-powered Turbo S E-hybrid is not only the flagship model of the range but also, at 680 hp, the most powerful volume-production Porsche to date.
Hybrid module packages within clutch bellhousing and incorporates disconnector clutch
Impressive claims
Both ZF and Porsche make bold claims for the top-of-the-line plug-in hybrid. With a combined output of close to 700 hp from its gasoline and electric power sources, along with an aggregate torque value of nearly 850 Nm, the 2.3-ton limousine rockets to 100 km/h in just 3.4 seconds and continues to a top speed of 310 km/h. More remarkably still, its hybrid configuration allows fuel consumption of just 2.9 liters per 100 km on the European cycle, giving a zero-emission operating radius of up to 50 km.
ZF, for its part, makes an impressive case for its new transmission, claiming “lightning-speed shifts, outstanding efficiency and the highest levels of comfort” – on the face of it an ideal set of qualities for a car such as a luxury Porsche which needs to perform such a broad spectrum of roles.
Impressively, ZF has succeeded in packaging eight speeds and the substantial hybrid module within the same space occupied by the outgoing seven-speed system. This has been enabled by a newly devised gear set architecture which ZF describes as having two countershafts and a single “summation” shaft. This, says the company, allows the fixed gears, which are all located on the transmission input shafts, to be used several times, resulting in “less wheel levels” and in turn making the basic transmission significantly shorter in length.
ZF adds, intriguingly, that the new gear set architecture offers another advantage: “Based on the modular concept, it is not only suitable for front-longitudinal applications but will also be able to cater for other driveline configurations.”
Neither Porsche nor the transmission maker gave any further explanation, but possible additional driveline configurations might be the mid-rear engine layout of the 718 Boxster and Cayman, or the rear-engine set-up of the 911 series pure sports cars.
As of now, however, the ZF8DT is offered in a single basic format but with four different modular permutations to suit front engined models: conventional and hybrid powertrains, and rear- or all-wheel drive. As with the predecessor transmission, the drive to the front axle is taken from a hang-on wet multiplate clutch at the tail of the casing. The clutch operates under continuous variable slip, with the clutch application pressure varying in real time so as to allow the vehicle to modulate drive to the front axle in rapid response to changes in driving conditions.
The hybrid module, which includes a torsion damper at the engine interface and an electromechanically operated decoupler, packages entirely within the clutch bellhousing. It is rated at 136 hp in the Turbo S E-hybrid application and is capable of propelling the vehicle at up to 140 km/h with the combustion engine switched off. Stop-start operation is enabled below the 10 km/h threshold when slowing to a halt, and at high speeds engine-off coasting is also possible – as are familiar hybrid functions such as boost mode and energy recuperation on deceleration.
Improved efficiency
Power losses in the new unit have been reduced by up to 28 percent, claims ZF. Without quantifying the relative benefits of individual features, ZF lists the principal improvements as the move to a very wide ratio spread of 11.17, with both seventh and eighth speeds strongly overdriven; a demand-driven lubrication system; and an optimized Transmission Control Unit (TCU) designed in-house and positioned remote from the transmission so as to allow the lubricant the freedom to exceed 150˚C without causing damage to the electronics.
The latter two features could be of considerable interest. The demand-driven lubrication system is controlled by Porsche-designed Cool Oil Flow Management software built into the TCU. Though full details have yet to be revealed, it would appear that lubricant flow to each subsection of the transmission can be controlled individually and in real time. Twin pumps, coupled via a hydraulic control unit, coordinate their outputs to set the oil flow to precisely the required value for each set of gears, depending on the load each set is under at any given moment. For example, when second gear is in use for hard acceleration, that set of gears would receive maximum flow, while under cruise conditions the flow to all gear pairs might be reduced.
ZF does not state whether the new unit follows its predecessor’s practice of separating the compartments and using different lubricant types for its dual clutches and its gear enclosure, but initial study of the cutaway would suggest this is the case.
The 8DT architecture will be available in three different torque classes, covering values up to 1,000 Nm to take in all but the most powerful engine/motor combinations currently on the market. Whether the ZF system in its latest incarnation will be sufficiently sophisticated to persuade the ultra-choosy luxury car buyer away from the latest and smoothest torque converter automatics is another issue. The answer will have to wait until a test drive of the Turbo S E-hybrid but, with Porsche’s massive engineering expertise and the added element of hybrid versatility now in the mix, this will undoubtedly be DCT’s best shot to date at challenging the dominance of the planetary in the luxury sector.
