Wet Clutch or Dry Clutch?
September 8, 2010
The starting point for any transmission designer is knowing the amount of torque the system needs to handle. Nowhere does this apply more clearly than in the design of dual clutch gearboxes, where applications for lower-cost vehicles are already showing a different clutch-design approach to those for higher-powered models. As technologies at both ends of the spectrum improve, the differences can be expected to become even more pronounced.
Wet clutches running in an oil bath or mist are used for higher torque applications where there is more energy to handle and more heat to dissipate. All but a few of the DCT systems on the market today are wet-clutch units, but their relatively high cost has until now limited their application to medium-sized vehicles with torque outputs of 250Nm and upwards.
Wet clutch DCTs also face higher parasitic energy losses as an energy-absorbing pump is needed to deliver cooling oil to the clutch plates.
BorgWarner, key developer of roadgoing DCTs with its wet-clutch DualTronic system in the Volkswagen Golf and other VW group models, has shown how considerable energy can be saved by moving to a clutch cooling system that adjusts the flow according to demand. These developments can be expected to appear in future iterations of the VW transmissions.
Volkswagen’s seven-speed DQ200 DCT is a good example of an efficient dry clutch DCT aimed at the lower end of the market where torque outputs are below 250 Nm. Fiat’s C635 in-house DDCT has raised the bar for dry clutch units: its torque handling capacity is 350 Nm, the highest the industry has yet seen.
Getrag, which manufactures units of both types, is steadily raising the torque capacity of its dry clutch transmissions such as that fitted to the Renault Mégane and, under the Getrag-Ford label, to the North American Ford Focus and Fiesta.
The differentiation between wet and dry clutches has clear consequences not just for the car designer but for the companies which develop transmission lubricants, too. Incorporating clutches into the system means the oil has to handle an additional set of demands compared with a standard gear oil.
Only recently it was generally thought within the industry that dry clutch DCTs would soon win out over their wet clutch equivalents for all but the biggest and most powerful cars. Yet intensive work with wet clutch systems has improved their efficiency and enabled suppliers to highlight their existing advantages in terms of packaging, rotational inertia, durability and smoothness in operation. Once again, competition between the types is leading to improved performance all round.
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