Among the car manufacturers of the world, the Hyundai Motor Group is the producer of the most diverse lineup of transmissions. In the Hyundai/Kia Powertrain Conference held in Oct. 2019, the group unveiled the cutting-edge Smartstream Wet 8DCT (Dual Clutch Transmission), continuing to solidify its leading role in the transmission industry. At the time, two keywords had headlined the unveiling: efficiency in power transmission and responsiveness. And indeed, the Smartstream Wet 8DCT has since come to occupy an important place in the Smartstream brand. But it has recently evolved further in marketability—by optimizing gear arrangements, installing a new electronic oil pump system, and applying a DCT-tailored control logic algorithm, the Smartstream Wet 8DCT has achieved a multidimensional evolution that will appeal to a larger segment of consumers.
DCT Expands its Market Share
The DCT is often dubbed as the final evolved form of the automatic transmission (AT), and its performance in nearly all aspects lives up to the dubbing. Its fast shifting and power transmission efficiency supports the vehicle’s dynamism in driving, and its fuel economy can come close to even that of a manual transmission (MT). Simply put, the DCT combines the MT’s fuel economy and engaging shifting feel with the AT’s convenience. This ideal combination, of course, has led to the DCT’s mainstream appeal—since its advent, the DCT has consistently been increasing its market share.
Long aware of the DCT’s inherent excellence, the Hyundai Motor Group independently developed its first DCT unit in 2011 and has continued to utilize it ever since. For example, beyond the standard DCT that prioritizes driving performance, it has also developed an efficiency-minded DCT solely for hybrid vehicles. More recently, the new Smartstream Wet 8DCT was mounted on Kia’s 4th-generation Sorento, making it the first midsize SUV in the brand to have a DCT installed.
The New Smartstream Wet 8DCT’s Powerful Clutch Cooling
The reason why the new Smartstream DCT could be mounted on a mid-size SUV is mainly in the changed clutch cooling system. Depending on the way in which cooling is done, the DCT is divided into two categories: the dry-type, which uses only air, and the wet-type, which uses an oil pump to cycle the coolant oil.
The dry DCT is structurally simple and therefore is lightweight, which underlies its good power transmission efficiency and fuel economy. But the simple structure presents limits to the cooling performance, so it cannot be used with engines over a certain torque threshold. However, because the wet DCT uses a separate electric oil pump for this cooling purpose, it is free from such limitations. The higher the torque in the engine, the greater the burden on the clutch—but the wet DCT can handle high torques with ease.
But for long, the oil pump was considered a minor weakness of the wet DCT as well; mechanical oil pumps that wet DCTs typically came with functioned in sync with the engine RPM, which led them to pump oil even when cooling wasn’t necessary. The resulting efficiency loss offset the advantage of the wet DCT to a considerable degree. To resolve this issue, however, the Hyundai Motor Group has replaced the mechanical pump system with an electric one that functions only when the need for cooling arises.
This electric pump system is composed of the High Flow Electric Oil Pump (HF-EOP), which is responsible for gear lubrication and clutch cooling, and the High Pressure Electric Oil Pump (HP-EOP), which supplies oil to the accumulator and maintains the pressure needed to control the gear shifting. Together, they ensure that the Wet DCT can function without losses in power and fuel economy.
Composed of a motor, a pump, and an inverter, the HF-EOP supplies the coolant oil through the clutches heated by repeated shifting. More than just cooling, though, the oil serves to lubricate the gears and ensures its smooth mechanical operation. Notably, the HF-EOP functions independently from the engine RPM and is associated instead with the perceived need for coolant oil, which amounts to an advantage in the vehicle’s fuel economy.
Meanwhile, the HP-EOP is responsible for maintaining the oil pressure in the accumulator needed to control the shifting. It functions on an on-demand scheme, meaning that it turns on when the need for the pressure maintenance arises and turns off when the need has been sufficiently addressed. Thanks to these innovative pumps, the Smartstream Wet 8DCT has drastically reduced needless oil pump activities, thus leading to a significant improvement in fuel economy.
But the pros in the division of labor between these two fuels do not end with just fuel economy. When one stream of oil must perform various roles, it cannot do its work at the highest level. But with the Smartstream Wet 8DCT, there are two completely separate streams of oil responsible for different tasks. This arrangement gives the engineers an ability, for example, to select the oil that is most suitable for cooling in one stream and one that is suitable for lubricating in another. And because the oil for pressure level control is not used for cooling/lubricating, it stays clean for far longer, giving it a long-term maintenance advantage.
Three Birds with One Stone: Performance, Fuel Economy, and Driving Fun
As shown by the figure above, the new Wet 8DCT can handle torques up to 53kgf.m—an advancement that finally allows a DCT to be installed with high-performance diesel engines. As a transmission that embraces the MT’s shifting mechanism, it also shows meaningful improvements in such performance standards as power transmission efficiency and acceleration performance.
The Wet 8DCT’s power transmission efficiency stands at 93.8%, 8.7% higher than that of the existing 8AT. This higher efficiency essentially means that the power from the engine won’t be wasted. Moreover, Hyundai has also designed a part for the Gear Shift Cylinder (GSC) to work independently, an arrangement that led to an appreciable increase in shifting performance.
The Wet 8DCT’s application has not resulted in mere increases in performance indices; in very real terms, the driver will enjoy tangible benefits on the road. Dynamic yet smooth shifting from the DCT has generally improved rider comfort; the coolant oil consistently keeps the clutch cool, so the typical overheating from overexerting the vehicle has nearly disappeared. Even on congested roads or steep inclines where the transmission faces a massive burden, the Wet 8DCT will guarantee a smooth and stable journey.
Finally, Hyundai has applied a precision solenoid valve to the Wet 8DCT to reduce the extent of the transmission’s oil pressure loss, and it has also incorporated a CSC (Concentric Slave Cylinder) with a structure similar to that of MT’s shifting mechanism into the wet multi-plate clutches. These structural flexibilities allow the Wet 8DCT to be tailored to fit the smaller car segments despite its concept design as a high-capacity transmission for high-performance engines.
The DCT is the spearhead of the technological movement to optimize powertrain efficiency. It gained its initial appeal by increasing gear speeds and offering MT-like efficiency and shifting feel, but the development of the Wet DCT has expanded its horizons. The Hyundai Motor Group has decided to apply the new Wet 8DCT on many upcoming model releases, starting with the new Santa Fe, which will be released in the first half of 2020. The decision will surely satisfy the most demanding of the consumers, who are willing to sacrifice neither high performance nor fuel economy.