GKN's new disconnecting AWD system offers 2-4% fuel economy gain

  • 12-Jul-2011 01:15 EDT

Front portion of GKN's AWD system showing the power transfer unit with the propshaft and hypoid gear set disconnected and not rotating. The yellow-colored components are rotating; the gray ones are stationary.

Improved fuel consumption is the primary intent of a new, production-ready all-wheel-drive system that decouples the vehicle's rear driveline when not required and reconnects when needed.

"AWD disconnect is more of a methodology than a specific system. Most of the technology is not new but rather a repurposing of existing building blocks in the GKN Driveline portfolio from the company's Transaxle, AWD, and eDrive product segments," Robert Genway-Haden, Product Technology Director for GKN Driveline Americas, told AEI.

By stopping as much of the rotating driveline as possible when not needed, a 2-4% fuel economy gain is possible when compared to the same AWD platform without disconnect capability, according to GKN data.

Several key components make AWD disconnect possible.

In a conventional AWD, the power transfer unit contains a dog clutch-type connection that can be synchronized or not synchronized, and the disconnect is wanted as close to the transmission differential output as possible to achieve the best efficiency, noted Genway-Haden.

"Actuation of the dog clutch can be achieved through adaptation of GKN Driveline's proprietary electro-magnetic or electro-mechanical actuators, or by using a more conventional shift actuator," said Genway-Haden.

Although the AWD coupling is upstream of the final drive unit's hypoid gear and differential in a conventional design, the coupling needs to be moved downstream of the hypoid gear in the final drive unit, in order to isolate the hypoid gear train's frictional and churning losses in an AWD disconnect.

According to Genway-Haden, a very low-drag wet clutch is used to disconnect the torque flow between the hypoid ring gear and differential (referred to as differential disconnect), or a similar wet clutch is inserted at one of the differential outputs (referred to as a side-shaft disconnect).

"While both of these scenarios have benefits and drawbacks, the most dramatic difference is that with a side-shaft disconnect, the differential bevel gears constantly tumble when in disconnect mode. This is less efficient than the differential disconnect in which bevel gear tumbling does not occur," said Genway-Haden.

A dedicated ECU orchestrates the reconnection of the AWD system when needed. The process starts when the wet clutch accelerates the stationary final drive unit's hypoid gears, propshaft, and power transfer unit (PTU) components to road speed, according to Genway-Haden.

"As the rear driveline comes up to speed, the PTU is engaged, resulting in a reconnection that is completely transparent to vehicle occupants and does not unbalance the vehicle's equilibrium," he explained. "This process can take as little as 0.25 seconds to be AWD-ready, and then the electro-mechanical actuator can apply full torque transfer in another 0.10 seconds."

The conditions that would trigger a reconnection would be determined by the automaker depending on vehicle application.

"The initiation to reconnect can be driver intervention, such as flipping a switch. It can be a response to environmental conditions, such as temperature, the presence of rain, or a speed threshold. Or the reconnection can result from the immediate need for AWD traction," Genway-Haden said.

AWD disconnect temporarily transforms an AWD vehicle into a FWD vehicle.

"In a conventional on-demand system, the AWD components accelerate and decelerate in line with the vehicle even if no torque is being transmitted to the secondary axle," he said. "Accelerating these inertias absorbs power and therefore blunts the responsiveness of the vehicle. An AWD disconnect system can provide the performance of a FWD vehicle when road conditions are good with the safety and security of AWD when needed."

GKN's AWD disconnect has been laboratory-tested and has undergone vehicle testing by the company's engineers. "Several automakers also have performed tests, but GKN is not at liberty to discuss implementation of this technology in the marketplace," Genway-Haden noted.

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