Ah, the dual-clutch transmission. There was once a time where upon hearing that a car was to be equipped with one, motorheads would speculate on whether it would deliver incredibly quick shift times made famous by Volkswagen’s DSG, enhancing a car’s ‘fun to drive’ factor. Things have certainly changed since then – now when a car buyer hears that a car is equipped with an advanced dual-clutch transmission, they ask whether it is a wet clutch or dry clutch system and what the failure rate of the ‘box is.
Although we’ve heard of issues with both types of dual-clutch transmissions, it’s the general belief that dry clutch versions of the transmission are far more problematic. Wet clutches run in an oil bath, and are generally used for higher torque applications, where there is more energy and heat to handle and the oil helps cool the gearbox down. However, as a downside, wet clutch gearboxes have higher parasitic losses because the oil has to be pumped through the gearbox to cool the clutches.
As an example, in Malaysia the six-speed DSG in the Volkswagen Golf GTI as well as the seven-speed S-Tronic units in Audis are wet clutch units. The last-generation Ford Focus TDCI, as well as the current Ford Mondeo and Ford S-MAX, also use wet clutch versions of dual-clutch transmission technology. There’s of course the higher performance applications of the form, like Porsche’s PDK and as found on the likes of the BMW M3 and M5, Mercedes-Benz SLS AMG as well as the grey market Nissan GT-R.
Dry clutch transmissions are said to be more efficient because of the reduced volume of oil being pumped through the system. This is because the torque capacity of these models are designed to be lower. Volkswagen’s seven-speed gearbox – paired to its 1.4 litre TSI engines – is a dry clutch variant, and so are the dual-clutch gearboxes found on the current generation Ford Fiesta and Ford Focus.
Let’s make one thing clear – no car company intentionally puts out a product into the market that it doesn’t think will last the lifespan of the car. The definition of lifespan is not forever of course, but an acceptable service lifespan that differs between manufacturers or products, usually at least 10 years onwards. Technology is tested extensively with all kinds of use cases to try to identify and predict issues that might arise. However, sometimes things slip through.
Swapping out additional cooling that was considered not needed in exchange for additional efficiency proved to have issues. Last year, Volkswagen announced that it would be swapping the synthetic oil in its DQ200 seven-speed dry dual-clutch transmission with mineral oil – essentially, the switch was to rectify a problem where electric malfunctions could occur in the gearbox power supply if synthetic gearbox oil was used, particularly if the vehicle is subject to a hot and humid climate, coupled with a high proportion of stop & go driving. Sounds like Malaysian roads for sure. Recently, Fiat recalled 19,500 units of the Fiat 500L because of dry-clutch DCT issues, and Honda recalled over 80,000 cars that were fitted with its DCT gearbox.
Because of these issues, some gearbox manufacturers are already moving away from dry dual-clutch transmissions for entry level cars. Getrag’s latest – the new 6DCT150 dual-clutch transmission – uses wet clutch technology instead of a dry clutch layout despite being an entry-level gearbox designed to handle loads of up to 170 Nm. As a comparison, the older 6DCT250 currently found in the Fiesta and Focus is rated up to between 240 Nm to 280 Nm, and uses dry clutch technology.
According to Getrag CTO Didier Lexa in an interview with DrivelineNEWS.com, these small engines and dry clutch gearboxes are often coupled with relatively heavy vehicles. When a small engine accelerates a large mass, the gearbox needs a longer synchronising time, which generates more heat. And even when installed in small vehicles, the thermal constraints on these small cars (which have smaller engine bays) are even greater than on C-segment or D-segment cars.
So in the end, it looks like a wet clutch is needed not just for powerful sporty cars but small engines in big cars as well. A dry clutch system only works well in an area in the middle of those two extremes; however, with advancements in oil pump technology, the difference in terms of efficiency between wet and dry clutch has been minimised. So is there really a point for dry clutch technology now?
Indeed, Getrag’s recently announced dual-clutch products all use wet clutch technology – no dry clutch in sight. Other than the new 6DCT150, there’s also a new seven-speed 7DCT300 and a hybrid 7HDT300, designed to pair a combustion engine with an electric motor. However, the existing 6DCT250 will live on for another six to eight years before it is retired.
With these new learnings, car manufacturers should pick and choose wisely before deciding on which gearbox to implement. For example, while Ford has opted to pair the Getrag PowerShift 6DCT250 with the 1.0 litre Ecoboost engine in the Ford Fiesta, the same 1.0 litre Ecoboost unit is paired with a Ford-GM torque converter six-speed automatic, the 6F, in the larger Focus in Europe. Volvo, a PowerShift user and which has a model line-up starting from C-segment (Focus-sized V40) onwards, has moved away from the 6DCT250, replacing it with an Aisin eight-speed torque converter auto in the latest generation of its cars.
Others are also trying to innovate dual-clutch transmissions in other ways. Both GM and Honda are experimenting with dual-clutch transmissions that have a torque converter to help smoothen out stop-and-go situations. Could that be the right path to work out the final few kinks from dual-clutch technology?