Q&A with ZF Sachs

By Matthew Beecham | 19 June 2007

The dual-clutch transmission (DCT) combines the advantages of a conventional manual shift with the qualities of a modern automatic transmission. ZF is among the front-runners developing DCT systems. Matthew Beecham talked to Dr Wolf-Ekkehard Krieg, Executive Vice President ZF Sachs AG, Powertrain Division, about the company's DCT.

just-auto: How do you think the clutch-less transmissions market will develop in Europe?

Dr Krieg: If you consider the fact that a torque converter is also a kind of clutch, in this case a hydrodynamic one, we think that the range of starting elements will still be incorporated in most successful transmission types on the market in the mid-term perspective.

just-auto: Could you talk us through the background to ZF´s research into its DCT concept?

Dr Krieg: In general we try to optimize all our starting elements concerning reduced fuel consumption and driving comfort. The efforts to reduce fuel consumption and to improve efficiency based on governmental requirements and an increasing environmental consciousness of society are leading today to further developments of modern transmission concepts. Apart from an increase in driving comfort, progressive automation has the overall goal of allowing the vehicle to operate in all driving situations in the optimum engine characteristic range, thereby economically with low emissions while taking account of the driver's wishes. DCTs offer the potential to reduce fuel consumption, enhance driver comfort as well as offer excellent driving dynamics.

just-auto: How does your advanced DCT work?

Dr Krieg: The fundamental principle of the new DCT resides in a combination of a standard countershaft transmission with two power shift clutches. The dual clutch transmission, designed for torques up to 500 Nm, consists of two sub-drives, which are connected to the engine via a dual clutch module with two parallel power shift clutches. Here, one sub-drive integrates the even gears, the other one the odd gears.

One of the advantages of this type of transmission architecture is that while the vehicle runs with one sub-drive, the next gear is already pre-selected in the other. This pre-selection takes place via an electro-hydraulic control unit, the structure of which is based on the pre-controlled valves and pressure regulators of the well-proven ZF automatic transmissions. Based on the pre-selection principle, shifting itself is done by the change between the two power shift clutches only and takes place spontaneously within a few one hundredths of a second without interrupting the tractive force at all. The two wet power shift clutches are integrated in a so-called dual clutch module. In a dual clutch transmission, a converter is entirely dispensed with. Thereby, power losses through additional power transmission elements can be avoided.

just-auto: Where can it be applied?

Dr Krieg: Comparisons of different transmissions concepts have shown that dual clutch transmissions have the potential to be widely accepted besides the current concepts of manual and automatic transmissions. The dual dry system could be an acceptable design for vehicles with an engine torque below 250 (300) Nm in order to reduce fuel consumption and system costs.

An electromechanical actuation of the dual dry clutch and dual clutch transmission would be preferred dependent on the available packaging. Above 250 (300) Nm a dual wet system would be the preferred system principle based on better driving dynamics, achievable comfort level and torque capacity. With this system also, fuel savings can be generated when compared to the traditional manual and automatic transmissions. Peak values of 9,000 revolutions per minute further underline that it is particularly suitable for use in sports vehicles.

just-auto: In what ways does your DCT concept address the technical constraints of competing concepts?

Dr Krieg: Our concept is a radial one with an inner and an outer clutch while some competitors use an axial design with sequential clutches. The main advantage of our concept is that we need less installation space, which is especially important for front-transverse applications.

See also: Global market review of OE gearbox clutches - forecasts to 2013