The e-LSD alternative to AWD

May 1, 2016

Lindsay Brooke

Is engineering a new vehicle for all-wheel drive worth the cost, weight, and fuel consumption penalties for perhaps 10% of volume on some platforms?

For mass-market C- and D-segment sedans, forcing the packaging penalty of a propshaft tunnel on 100% of production in order to accommodate a small portion of AWD models may not be a worthwhile tradeoff given the pressures of meeting 2025 CAFE and European CO₂ laws. Some vehicle planners and chief engineers who have weighed these options say a high-performing electronic limited-slip differential (e-LSD) coupled with either front- or rear-wheel drive can satisfy most of the AWD performance expectation.

Vehicles without off-road intentions, including hybrids and plug-ins electrics, are ideal candidates for e-LSDs, which deliver significant mass and fuel-consumption benefits over AWD, versus a base 2-wheel drive setup. Besides providing tractive benefits, they also can enhance vehicle dynamics by retaining some on-throttle stability control rather than scrubbing off momentum through brake and throttle interventions.

A typical AWD system suffers about a 200-lb (91-kg) weight penalty versus 2WD. A new e-LSD system developed by Eaton Corp. that will enter production in 2017 weighs about 20 lb (9.1 lb) and avoids the 10% fuel consumption penalty of typical AWD. Claimed system cost is between traditional 2WD and AWD systems.

“It’s an order-of-magnitude shift,” noted Rick Kukucka, Eaton’s Product Director for Powertrain Controls. He told Automotive Engineering the new system will debut on a North American customer’s MY2018 product. “It’s going to really advance the state of the art for the segment,” he promised, without providing details.

Faster time to peak pressure

A typical AWD system does not control side-to-side torque distribution. So if a wheel or wheels on one side are on a low-mu surface, the driveline (either 2WD or AWD) will send 100% of torque to those wheels that can spin faster. The side with better grip gets zero torque.

An e-LSD will sense this and limit slip on the low-mu side while vectoring torque to the wheels that have better grip. “That’s a major benefit dynamically,” Kukucka explained. In an understeer condition for front-wheel drive, the e-LSD “will force the torque from the unloaded inside wheel that may be starting to slip back toward the outside wheel,” he said.

“And in an oversteer-yaw condition we can intervene to have some drag torque that will force torque across the axle and correct the oversteer.”

Some clutch-based electronic diffs can provide a degree of on-throttle control. But often when the driver is in an event that requires stability control, the first reaction is lift off the throttle. Eaton’s e-LSD reacts to such a situation by forcing torque to the wheel that requires it, even during drop-throttle situations.

Claimed time to peak pressure (defined by SAE as 90% of peak torque capability) is 100 ms. That compares to 250 ms for Eaton’s previous e-Gerotor system (see sidebar) which relied on less than one full wheel rotation of slip before it built that line pressure, Kukucka explained. The reaction-time delta “makes all the difference in how much we assist stability-control events before any brake or throttle interactions come into play,” he said.

The system, which is compatible for front- and rear-drive drivelines as well as AWD, does not do away with brake intervention, however. In the most extreme conditions where the e-LSD steps in to help vector torque if the driver is going too fast, the vehicle’s brake and throttle interventions still will come into play, Kukucka noted. But what the e-LSD is able to do is “drastically” reduce the degree and level of intervention from the brakes and throttle.

“When customers drive our demonstration vehicles, they say they can’t tell when the stability control or our diff are intervening, because the engagement is so seamless and smooth,” he said. “They can’t perceive the dramatic throttle and brake interventions typical of a vehicle that only has stability control.”

Scalable system architecture

The new e-LSD system architecture is compact. The differential unit is fitted with a DC electric motor which is controlled by an ECU. Kukucka notes that a brushless type motor is able to build line pressure slightly faster than a brushed motor, but at higher cost. The motor in turn drives a piston pump that can work to peak performance across all temperature extremes, the company said.

The pump builds line pressure either directly or to an accumulator when it’s not needed. An accumulator is used because when there is a sudden need for intervention, “the stored energy is what gives us that 100-ms response time, guaranteed,” Kukucka said. There is a differential clutch for controlling torque slip, as well as temperature and pressure sensors in the HCU and a feedback loop to ensure precise pressure and intervention control. A plenum located downstream of the HCU actuates the clutch discs. Operating pressures are 750 psi (5171 kPa) at the HCU, and 400 psi (2757 kPa) at the clutch.

Eaton’s new e-LSD is compatible with any powertrain layout and is scalable according to vehicle segment and power.

“You could add clutch discs, or increase/decrease diameter of the clutch discs, or do both,” Kukucka explained. “That’s one of the advantages of the electro-hydraulic approach: we can go from very low-torque demands, say 1200 N·m, to very high, like 4000 N·m and towing a 9000-lb trailer.” Eaton’s electronic controls engineers can supply either an entire ECU with calibration tuned internally or support integrating the software into a powertrain controller ECU.

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