Do AWD EVs need two motors? GKN does the math.

Two-motor AWD EVs may meet some OEM criteria, but results from GKN Automotive’s new study suggest the temptation should be resisted.

One electric motor for AWD EVs is better than two. That’s the conclusion reached by GKN Automotive after carrying out extensive research into facts, figures and costs of both. “The benefits of disconnect technology initially introduced for use in ICE powered vehicles are just as relevant in the next generation of EVs,” said Michael Höck, GKN Automotive’s manager of AWD systems. “Simulations conducted at our Technical Center in Lohmar, Germany, show that the most efficient way of transmitting power to all wheels is to use a conventional AWD system with disconnect.”

Although most AWD equipped EVs and HEVs send power to the wheels via the electrification of the second axle – with a “hang-on” electric motor that supplements a primary electric motor at the front or rear – it is an expensive configuration, also demanding two inverters and transmissions. “And it results in reduced efficiency, the extra inverters and transmissions causing power losses,” Höck stated. “As everyone connected to EVs knows, reduced efficiency means reduced range.” Also, the two-motor configuration provided no safety or traction benefits for the majority of the drive cycle.

With increasing numbers of consumers requiring EVs with AWD, GKN decided it needed to carry out a detailed study of options and opportunities. Simulations focused particularly on the fact that most dual-motor EV systems run permanently in AWD. “Some systems allow one of the two motors to disconnect when AWD is not required, thereby providing some efficiency gain, but this is purely at the motor coupling; the rest of the driveline still runs,” Höck explained.

“Not only does this create inevitable parasitic losses, it also makes it impossible to optimize gear ratios for both motors,” he added. Gearing is either optimized for a single motor or dual motors but cannot be tuned to deliver maximum efficiency as the drivetrain switches between two-wheel drive and AWD. “The benefit of a mechanical AWD system is that no compromise is required,” Höck said.

GKN’s study commenced with research simulating a vehicle with a “hang-on” AWD system to determine how often it would be needed. “The tests show that even in low-grip conditions such as ice and snow, the AWD system is only required 25% of the time,” noted Christoph Schmahl, GKN’s AWD systems test coordinator. “In high grip urban and rural driving this figure varies between 9% and 14%, while on the main highway, it drops to 1%. The findings indicate that powering all wheels at all times is unnecessary, and significantly decreases overall drive cycle efficiency.”

Simpler, lower cost
A simpler, lower-cost and more-efficient strategy for automakers to deliver AWD in an EV is one which appears, initially, more traditional. Employ a single electric motor with a mechanical AWD layout including a Power Take-off Unit (PTU) propeller shaft, and a Rear Drive Unit (RDU) to transmit power to all four wheels.

The key to efficiency gains for AWD EVs, Höck suggests, is the application of clutches capable of disconnecting the PTU, the propshaft and the RDU in instances where AWD is not required. The significant efficiency optimization that is so critical for EVs is then achievable, while still providing the stability and safety of AWD when needed. Compound this with the weight saved from not requiring a second motor or inverter, and further efficiency gains are also achieved.

GKN’s “ActiveConnect” system utilizes a PTU with a dog clutch allowing it to disconnect at the input shaft. At the rear, depending on the application and the level of sophistication required, GKN offers two modular RDUs designed for ease of integration. The Booster RDU has a single, side-mounted clutch to provide the benefits of a conventional AWD system in a more compact, lightweight package.

GKN’s “Twinster” RDU replaces a conventional differential with twin disc clutches to enhance driving dynamics yet further “With torque control to both rear wheels, and longitudinally along the propshaft, traction is improved both on and off road,” Höck explained. “Both systems incorporate an identical disconnect capability, and therefore the efficiency gains so crucial to EVs.”

Höck said that ActiveConnect had been used in conventional ICE vehicles since 2014, enabling “significant” fuel savings to be made. In terms of suitability for EV applications, this “conventional” AWD system would make a substantial impact in controlling the initial torque of the electric motor, and could be disconnected at relatively low speeds, allowing for maximum efficiency gains under normal driving conditions.

Single-motor solutions
With a single motor ActiveConnect system, torque could be distributed almost instantly to where it was needed. “For example, during off-road driving the system can deliver all of the torque to the front wheels if they have better traction," Höck explained. "A dual-motor setup can only ever transmit to one axle the total torque of one of the two motors. Equipped with Twinster, the ActiveConnect system also allows optimized traction between the rear wheels.”

If the system detected a loss of traction, it could, again, “almost instantly” transmit up to 100% of the torque being transferred to the rear, delivering it to the wheel with greater traction. And torque can be distributed to help correct a yaw moment if the vehicle loses traction mid-corner.

During the test program, ActiveConnect simulations within an EV driveline demonstrated the system to be 9% more efficient than a dual-motor permanent AWD system. And it was also 6% more efficient than a dual-motor setup with an e-motor disconnect function. Simulations were performed using the WLTC (Worldwide Harmonized Light Duty Test Cycle) and a low disconnect speed, indicative of normal driving conditions.

Höck summed up the test findings and the tempting technology of two motors: “A single-motor AWD setup with disconnect also provides a large cost saving to OEMs when compared to a dual-motor disconnect, greatly reducing the number of drivetrain components required. Without a second motor and inverter, and additional gears, a cost saving of up to 13% can be made in driveline componentry alone.”

Höck added that in the future, certain EV applications might also warrant the use of multi-speed e-transmissions, and that an EV with a single-motor AWD ActiveConnect system could “easily” integrate this, increasing its potential range by decreasing energy consumption, or boosting its performance.

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