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RDU for BorgWarner's AWD system. High axial clearance in clutch packs reduces spin losses.

Two approaches to AWD show improved fuel economy, reduced system weight

The increasing popularity of all-wheel drive (AWD) is up against the regulatory demands for improved fuel economy, while the buying public also is looking for improved system performance. Nor Hairuddin, a lead engineer at Honda R&D and Thad Kopp, senior staff engineer at BorgWarner TorqTransfer Systems, described their companies' efforts to reconcile the seemingly inherent conflict during a 2015 SAE World Congress session.

The BorgWarner development was intended to determine how much a full-disconnect system could improve fuel economy. Engineers installed it in a 2014 Ford Explorer Sport, for the purpose of comparing it with the vehicle's base AWD system. The OE setup has a permanently engaged power transfer unit (PTU) at the front transaxle, and a rear differential unit (RDU) with an electro-magnetic AWD clutch pack in front of the rear differential pinion.

The system's PTU in front incorporates an electronically-controlled solenoid acting on an engage-disengage synchronizer. The RDU has right- and left-side hydraulic clutches controlled by variable force solenoid valves in an integral valve body fed by an electric oil pump. The clutch packs have high axial clearance (0.33 mm/.0016-in) combined with wave spring clutch plate separators to minimize drag.

A hydraulic accumulator also is built into the valve body to pre-charge the hydraulic system for fast response. Lubrication is on-demand, to reduce the effect on fuel economy, with the flow controlled by a solenoid valve.

Fuel economy improvements

The vehicles were tested on the same dynamometer, using the same driver, on both the Federal Test Procedure 75 (city cycle) and U.S. EPA highway cycle. The BorgWarner-modified vehicle was kept in full-disconnect for all the testing. This enabled a comparison between the factory AWD and as close to a front-wheel-drive system as the disconnected AWD would allow.

On the FTP75 cycle, the BorgWarner delivered 19.59 mpg, which was 3.3% better than the 18.97 mpg for the OE. On the highway cycle it produced 30.71 mpg, a 0.71 mpg improvement vs. the OE's 30.0 mpg (2.4%).

BorgWarner also measured the difference in spin losses, recorded in watts, for the PTU and the RDU. The PTU saving ranged from 140 w at 20 mph/32 kph to more than 450 w at 70 mph/112 kph. The RDU was compared with the Explorer's production unit, and although the Ford RDU had slightly lower spin losses until the road speed was about 33 mph (53 kph), by 50 mph (80 kph) the BorgWarner was more efficient, delivering a saving of close to 200 w.

This disconnect system is relatively close in operating principles to the one in the new Jeep Renegade, which has a motor-driven dog clutch for the PTU and motor-operated multi-disc clutch in the RDU that can disconnect the differential carrier from the side gears and axle shaft. In both systems the only rotating components are for the front-wheel drive; and in the rear, the wheels and axle shafts.

Redesigned Honda system

Honda's approach, which it calls "Super-Handling AWD," was introduced in 2005 when it incorporated an "acceleration device" (clutch-activated planetary gearset in front of the RDU), with torque split in the RDU to power the outside wheel through a turn. In 2009 Honda switched to a speed-increasing drive ratio in the RDU to perform this function. The ratio (vs. the front axle) is fixed; the ratio was 1.7% from 2009 to the 2015 model year, when it was increased to 2.7%. The ratio increase means the cars can be powered through a tighter turn — a radius of 15 m (49.2 ft) vs. 21 m (68.9 ft).

The new RDU is a total redesign. It uses hydraulic actuation of the left and right side clutch packs, employing a single electric motor-driven dual pumping system with a pressure sensor for each clutch pack (and bearing lubrication). The hydraulic circuit at each side is regulated by a linear solenoid valve.

This is a much simpler arrangement than the previous design, which had an electro-magnetic clutch pack and planetary gearset at each side with two-step gear reduction, plus a mechanical oil pump. Eliminating the coils for the electro-magnetic clutches and the planetary gears, and incorporating the new oil pumping system results in an RDU reduction in physical size. Mass has also been reduced, from 41.7 kg (91.9 lb) to 31.2 kg (68.8 lb).

The torque throughput capacity for each axle was maintained at 1200 N·m (888 lb·ft). Mechanical drag was reduced by more than 50%, Hairuddin said, but no fuel economy data was provided.

Solenoid control strategy

Honda's operating principle is straightforward: Some hydraulic pressure is maintained in the lines for fast response. For acceleration through a turn, the system closes a solenoid at the outside wheel circuit, which restricts an orifice. This causes hydraulic pressure to build up in that side's clutch pack, while the solenoid on the other side remains open. With this set-up, almost all rear differential torque can be transferred to that wheel.

Most impressive in the final design was the hydraulic pressure feedback control algorithm that Honda developed. Hairuddin noted that in the early development the difference between the target and actual pressures could be significant. The feedback algorithm turned the curves for target and actual pressures (and torque) into nearly identical lines with an almost indistinguishable space between them. When a pressure rise need is imminent, there would normally be a delay, but early application of add-on pressure raises the actual pressure up to virtually the target line.

Another engineering issue Honda had to overcome was that left and right side clutch pistons did not apply equally. More pressure was going to the left side, as a result of a pressure distribution variation, particularly at low temperatures. Among a number of modifications to equalize, Honda redesigned the case housing.

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