Press release
Rapid Response and Dynamic

Newly developed automatic 6-speed transmission responds and shifts even quicker - Further fuel consumption reductions and an enhanced power-to-weight ratio at the same high level of comfort - New torque converter and highly efficient software revolutioniz

2006/12/06

The use of a new torque converter in combination with optimized transmission control and hydraulics make the multi-ratio automatic 6-speed transmission of the second generation even more efficient than its predecessor; thus, it contributes simultaneously to the generation of significantly increased fuel savings: The gasoline-powered vehicle needs up to three, cars equipped with a diesel engine even up to six percent less fuel. Since September 2006, the new rapid shift transmission is made available in the 3 Series Coupé and the X3 by BMW.

Increased driving dynamics and efficiency
Driving dynamics and fuel consumption play a decisive role in addition to the comfort requirements for a multi-ratio automatic transmission. “The new automatic 6-speed transmission definitely represents a quantum leap in the fields of automatic transmission development.” says Dr. Michael Paul, Executive Vice President Technology and Group Executive of the Car Driveline Technology division. “With the second generation of the automatic 6-speed transmission, we have succeeded in improving driving characteristics while, in parallel, enhancing the driving performance even further. It is the most agile automatic multi-ratio transmission ever.” At the same time, the new automatic 6-speed transmission does generate fuel savings; for the gasoline-powered engine, this corresponds to three, for the diesel engine even up to six percent. Moreover, response times have been reduced by up to 50 percent and, for the very first time, direct double shifts without interruption are possible. This type of power enhancement and fuel savings are primarily enabled by the new torque converter which is combined with the optimized electronic transmission control unit and the hydraulics. The rapid shift transmission is designed for an input torque of up to max. 850 Newton meters, i.e. for automobile drives featuring powerful engines with up to twelve cylinders.

The new torque converter
In order to optimize fuel consumption, ZF has developed a consistent shift strategy for the torque converter lock-up clutch with the objective of closing the clutch completely as soon as possible, namely directly after setting off. In order to master vibrations in the driveline successfully and safely, ZF uses the newly developed torque converter: For the gasoline-powered vehicle, this task is covered by a turbine torsional vibration damper; for diesel engines, a special twin torsional damper (Twin TD) was invented. In addition to fuel consumption, also driving dynamics have been significantly improved thanks to early closing of the converter clutch and the compensation of the inertia moment on the basis of the smaller sized torque converter. The thereby improved connection of the driveline to the engine leads to the fact that the driver experiences vehicle responses to accelerator pedal actuation more actively and directly.

Hydraulics and the electronic transmission control unit
What’s new? Well, the automatic 6-speed transmission performs all dual shifts directly; moreover, it downshifts of up to four stages are effected without any time losses or noticeable deceleration. This has been enabled by the further development of the hydraulic system. The respectively needed clutch is directly controlled by a separate pressure regulator so that no additional time is required for jumping a drive stage. This target gear finding mechanism, as it is called, makes the shift process extremely precise. In order to trigger that function as quickly as possible, the improved control electronics are in charge of the corresponding optimum drive stage. The control software “decides” on the basis of the data received on load conditions, the driving situation, and the speed with which the accelerator pedal is actuated, whether downshifts are effected for just one single drive stage or several. This decision depends upon the swiftness for acceleration commanded by the driver. The stronger the driver accelerates, the more dynamic is the way of vehicle speeding up.
No matter whether one gear step or several gear steps are shifted, the response time remains the same. And the latter is extremely low: If - at an approximate driving speed of 70 km/h in the sixth gear- a kickdown signal is received, then the automatic 6-speed transmission needs just 100 milliseconds in order to trigger the rapid changeover from the sixth into the second gear. All in all, ZF was capable of reducing the shift time by half in relation to its previous transmissions.

Further developed gear set
Just as in the case of the predecessor version, the planetary transmission system on the basis of the Lepelletier gear set is at the center of the new automatic ZF 6-speed transmission. Thus, six gears can be realized with significantly less components: In the past, only three gear sets and six shift elements were needed for the automatic 5-speed transmission; today, a simple planetary gear set suffices as well as a Ravigneaux gear set in combination with five shift elements. The result: Considerable weight savings of a total of approximately eleven kilograms have been attained in relation to the automatic 5-speed transmission. The increased use of plastics contributes to the reduction of weight and consequently fuel consumption. Thus, for example, the oil tray is no longer made from metal.

Prospects
Already in 2001, ZF successfully introduced the first automatic 6-speed transmission on the market. In the mean time, automatic 6-speed transmissions have more or less completely replaced the automatic 5-speed transmissions. With the new 6-speed transmission range, ZF is well equipped in order to successfully defend its benchmark position for multi-ratio automatic transmissions in the upcoming years.