Dual Clutch Transmissions. What's New?
When I was a kid, my brother and I used to watch the television series “Combat,” which aired from 1962 to 1967. The show was about a U.S. army squadron in France during World War II. When my brother and I played army, he would always be Sarge (Vic Morrow), and I would be Lt. Hanley (Rick Jason). We also had plastic army soldiers like the ones in “Toy Story,” and one of these we called Sarge, too, because Vic Morrow was so cool. During this time, I would save allowance money to buy model kits of battleships, submarines, fighter planes and various war equipment. I was the oldest of four boys, and we fit our gender stereotype pretty much dead center.
One of my favorite model vehicles, which we saw on the TV show, was a half-track. The half-track was a hybrid between a tank and a transport truck. That is, the back half had the continuous tread of a tank, but the front wheels provided the vehicle better handling. In other words, instead of a vehicle with wheels, it was half-track. I later learned that the inventor of the half-track was a French military engineer named Adolphe Kégresse, the same man who invented the dual-clutch transmission.
The guy must have lain awake nights thinking of ways to combine disparate items in order to create something better. The half-track is a cross between a tank and a truck. The dual-clutch transmission is a cross between a manual and an automatic transmission. Though Kégresse developed this new, dual-clutch gearbox in 1939, the war intervened and the new technology was never really adopted in a broad manner.
The dual-clutch transmission (DCT) is relatively simple to explain. Essentially it has two clutches, one of which works for the odd-numbered gears (1, 3 and 5), while the other takes care of the even-numbered gears. Today’s dual-clutch transmissions are smart, too. They anticipate when the driver is going to shift gears, whether up or down, and do so accordingly.
While you’re accelerating in first gear, for example, the computer selects second gear on the other gear shaft. When it’s time to up-shift, the clutch that controls the even gears disengages and the clutch that controls the odd gears engages. Compared to a traditional automatic transmission, gears shift much more quickly and smoothly in a DCT — the perfect complement to a powerful, high-performance engine.
The advantage of dual-clutch technology is chiefly this: DCTs use the two clutches to minimize the amount of time it takes to shift gears, thereby delivering power to the ground more continuously and improving efficiency while being able to handle high torque applications.
After years of sitting on the sidelines, dual-clutch technology was eventually adopted in several 1980s-era race cars. It wasn’t until 2003 that Volkswagen’s Golf Mk4 R32 became the first production vehicle to feature the technology. Nowadays they can be found in a variety of cars, from the relatively tame Hyundai Sonata to the Nissan GT-R supercar.
The Special Fluid Needs of DCTs
While DCTs are capable of seamless shifts, they can suffer from shudder or lurching at slow speeds. As a result, DCT lubes have special requirements that DCT fluids must satisfy. According to Amsoil engineer Matt Erickson, “The main differentiator for DCT fluids is the frictional properties. The demands are unlike traditional manual or automatic transmission fluids, especially at low speeds. Managing shudder at low speeds has been a challenge for DCTs because the clutch needs to slip in a controlled fashion. In order to do so, the frictional characteristics need to be predictable, and they need to stay consistent in all conditions throughout the drain interval. This is a key area where the quality of the fluid has significant influence.”
According to the SAE paper “Lubricant Technology for Dual Clutch Transmissions,” the new dual-clutch transmission lubricant “requires a new, specialized additive technology to meet the unique, often competitive, requirements of the various collars and gears of the hardware system: specific clutch performance requirements related to the wet clutches along with high load-carrying and thermal stability associated with the manual transmission. DCT fluid must also maintain the proper viscosity to provide protection during the high-heat operation native to high-performance sports sedans and supercars.”
There’s another reason these transmissions are getting some recognition. They provide better fuel efficiency than either automatic or manual trannies alone. As anyone in this industry for any length of time has observed, since the early ‘70s everything that improves fuel economy or reduces emissions is going to be given consideration and welcomed.
DCTs offer the convenience of an automatic while providing the performance of a manual, with additional benefits that will lead to wider use in the future. They also have specialized lubricant requirements. Be sure you and your team are trained to pay attention to specs in this increasingly complex arena. Not all fluids are created equal. As always, I propose a synthetic solution.