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ZF Dual Clutch System Blends High Performance with Fuel Economy

Wayne McCluskey, technical training manager, ZF Services UK Ltd (Supplier of SACHS clutches and dual-mass flywheels to the aftermarket)

ZF’s development work on the passenger car driveline aims to enhance power transmission and to reduce energy loss as much as possible. Lower fuel consumption with improved comfort and driving dynamics are particularly important considerations, all of which can be accomplished with a dual clutch transmission (DT) system incorporating two interlocking transmission shafts, each linked to a separate clutch.

A wet dual clutch (Fig. 1) and dual-mass flywheel (DMF) combination from ZF is employed in the 7DT seven-speed dual clutch transmission (Fig. 2) from ZF. The 7DT offers the comfort and convenience of an automatic transmission, with the enhanced dynamic response of a manual gearbox. The internal layshaft architecture effectively resembles two manual transmissions in one casing and requires two clutches. By using separate input shafts, two gears can be engaged at the same time, with engine power being transmitted through one part of the gearbox and one clutch, while the next gear is preselected in the other part of the gearbox.

One of the clutches serves the odd-numbered gears via a solid input shaft, while the other clutch caters for starting from rest and engaging the even-numbered gears via a second, concentric hollow input shaft. This has advantages for both efficiency and the driving experience; during a gear change, one clutch opens and the other closes within milliseconds, enabling gear changes to take place imperceptibly under load without interrupting torque or traction. The result is extremely fast gear changes with no interruption of power delivery (Fig. 3), providing improved acceleration over a manual transmission with the added advantage of reduced fuel consumption.

The shift strategy in the dual clutch system can be varied from sporty to comfortable by controlling the shift speed within the 100ms to 300ms range. The incorporated electronic control unit regulates the dual clutch system and the transmission actuation, monitoring all relevant parameters such as speed, rpm and transmission ratios, and adjusts shifting depending on the driving situation.

The DMF is a vital component of the wet dual clutch system, providing protection from vibration for the complex internal components of the transmission and its sophisticated electronic controls. The root cause of any unusual sound or vibration symptoms should therefore be investigated promptly, employing a structured diagnostic routine (see ‘DMF fault diagnosis’ section below) to check the vehicle systems and if necessary the DMF itself if no other fault is found.

The wet dual clutch uses a combined sump for the transmission’s gears and clutches, and ZF has performed extensive testing with lubricant manufacturers and additive suppliers to develop a dedicated transmission fluid that meets the needs of both applications. It is essential to use the correct DT fluid if refilling is necessary. Operating at pressures of up to 15 bars and with a flow rate of up to 35 litres per minute for cooling purposes, it must meet the diverse needs of lubrication for clutches, gears, shafts and bearings, the hydraulic actuation of clutches and gear changes, and provide wear and corrosion protection. A reference list of lubricants is constantly updated at:

http://www.zf.com/global/media/en_zf/lubricantslists/TE-ML_11.pdf.

DMF fault diagnosis

When the workshop is presented with a noisy DMF, the tendency is to assume that it is faulty and to replace it with a new one. The initial perception is that this has cured the problem, but in most cases it merely masks the symptoms for a few weeks until the same noise returns. To avoid the time and cost of replacing incorrectly diagnosed parts, and prevent repeated failures of the same part within a short timeframe, it is therefore vital to look beyond the symptom to isolate and correct the root cause.

As an example, one case required the interpretation of live diagnostic data (Fig. 4) to identify slow starter motor rotation during engine starting as the underlying cause of DMF failure.

Although DMF function can only be reliably checked on a special test rig, there are some indications of failure that can be provisionally assessed in the workshop. If there is heat discolouration around the centre part of the DMF, it will certainly require changing. More subjectively, if rotating the primary flywheel against the secondary flywheel of a Sachs DMF by hand has a markedly different ‘feel’ to that of a new unit, it is possible that the inner torsion damper springs have fractured. Sometimes this can only be determined by stripping the DMF down, but it would be wise to replace the unit as a precautionary measure to prevent having to do the job soon after.

Relevant vehicle systems should also be checked for correct operation before a new DMF is installed. The following checklist will help to isolate items that could be responsible for causing noise or vibration whose source could be wrongly attributed to the DMF:

 Engine code: confirm correct DMF for vehicle

 Engine fault or flash codes

 Cylinder compression pressures correct and equal, within the vehicle manufacturer’s tolerances

 Camshaft wear

 Correct operation of fuel injection components – check injection rate at idle

 Correct operation of ignition components

 Excessive crankshaft end float or lift

 Crankshaft damper damage

 EGR valve operation

 Swirl valve and actuator operation

 Operation of torque limiting valves and frequency modulators

 Condition of turbo boost pipework and hoses

 Chip tuning or other performance enhancements (can increase vibration)

 Wear on gearbox input shaft and associated bearings.

In addition to checking that all vehicle systems are performing correctly, observing a few precautions during installation is also beneficial:

 On a Sachs DMF, use a flywheel locking tool to prevent the primary and secondary flywheel from rotating when removing the DMF from the crankshaft, otherwise the secondary flywheel could be damaged by a bolt head

 Ensure that any recommended specialist tools for the vehicle application are used to install the clutch and DMF to avoid causing damage to the components

 Always renew fasteners and observe the correct tightening sequence and torque settings for them, using only hand tools, not air wrenches

 Bolts supplied with a Sachs clutch, DMF or ZMS Modul (clutch and DMF) kit should always be used, as the length differs from one application to another. Too long a bolt may bottom out on the crank or DMF before clamping the DMF or clutch cover correctly, leading to damage once the vehicle is started.

 Avoid use of high pressure cleaners, steam jets, cleaning sprays or compressed air when cleaning the DMF, as these could cause dirt or cleaning agents to enter the DMF and accelerate wear. Use only clean shop rags free of oil and grease

 When a vehicle’s clutch is being replaced for the second time, always replace the DMF as well; the torsion damper in the DMF is a wear component in the same way as that of a conventional clutch.

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