DCTs Need Special Lubricants

Dual Clutch Transmissions (DCTs) are set to become the next big thing in vehicle design and expect to notch up multi-million annual sales within the next five years. For car drivers and the environment that's a double benefit: cars become more fun to drive and emissions are lowered.

But as with any major move to a next-generation technology, the rise of this new transmission type brings with it new demands at other points in the automotive supply chain-most notably, in the lubricants which protect the inner workings of the transmissions and keep them running at peak efficiency for extended periods.

In developing fluids for the new generation of DCTs, lubricant engineers are faced with a whole new set of demands, several of which appear contradictory. In the case of wet-clutch DCTs the major difference is of course that the lubricant now has to cope with a set of clutches, as well as the normal gears, bearings, selectors and shafts. Thus the specific DCT fluid needs to provide:

  • Lubrication of the clutches, gears, shafts, bearings and synchronizers
  • Heat dispersal for the entire system
  • Hydraulic actuation of the clutches and gear changes
  • Protection of the transmission against wear and corrosion<

From these requirements it is evident that neither a conventional automatic transmission fluid (ATF), nor a standard manual transmission fluid (MTF) would be up to the task of protecting a DCT. It is clear that the new generation of DCT-specific lubricants (DCTFs) will require a specialized additive technology if these oils are to guarantee the DCT's technical advantages and its durability in service.

Viscosity and friction<
Running in a permanent lubricant bath, the wet clutches are the most critical components in the system. They act as the launch mechanism as well the gear shifting system, and the precise control of their engagement and disengagement is central to the transmission's smoothness and refinement. The additive technology must be compatible with the clutches' friction material, and the lubricant itself needs a carefully calculated viscosity profile to ensure smooth disengagement in low-temperature conditions as well as adequate protection of system components at high temperatures and under high load conditions.

The DCT fluid's formulation must protect the clutches and sustain consistent launch and anti-shudder durability for the lifetime of the transmission, estimated at two million shifts. The DCT fluid plays a major part in controlling friction response, itself the key to smooth and consistent shift quality - the raison d'etre of the DCT principle. In laboratory testing, a conventional ATF is unable to withstand the stresses that build up over the equivalent of 100,000 to 150,000km; new-generation DCT fluids, on the other hand, retain excellent anti-shudder durability throughout the laboratory tests.

Full-scale high-energy start clutch tests are another key aspect of durability performance: here again the performance of dedicated DCT fluids is superior to that of a typical manual transmission fluid (MTF). While the DCT fluid retains excellent friction and anti-shudder performance throughout the grueling 20,000-cycle laboratory test, the MTF-lubricated clutch displays a variety of problems that would have resulted in poor shift quality and shudder.

The effect clearly shows the need for a new and dedicated additive technology system for fluids used in dual clutch transmissions: in addition to providing correct friction control, the lubricant additive technology must promote cleansing of the lining material to prolong the life of the clutch.

Synchromesh, gears, thermal stability<
Synchronizer components, too, depend on the correct frictional performance of the transmission fluid for their smooth operation: to qualify for top-level status the fluid must maintain a specific frictional coefficient throughout a grueling 5000-cycle test, with no evidence of wear that can cause synchronizer clash or baulk. Again, new DCT fluids give excellent performance.

Further considerations are thermal and oxidative stability - surface temperatures within the clutch system can reach several hundred degrees Celsius. Acidic by-products of thermal breakdown can generate lacquers and sludge, which can in turn impair hydraulic and actuator function. Once more, the new DCT fluids outperform the other fluids.

They perform well, too, in protecting the helical gears of the transmission, where the stress on individual gear teeth is much higher than in an automatic transmission whose epicyclic gear arrangement ensures the load is shared between several sets of gears. The improved wear protection of the new-generation DCT fluids distinguishes them from the other first-generation fluids, say the leading lubricant engineers, particularly as the all-important friction performance and compatibility are not compromised.

DCT Technician Series MV-FS