Systems-engineering a New 4x4 Benchmark

Chrysler Powertrain teamed with AAM to create the industry’s most capable, sophisticated — and arguably most fuel-efficient — AWD/4WD driveline. Top engineers talk about their collaboration.

June 1, 2014

Lindsay Brooke

The 2014 Jeep Cherokee’s 4x4 driveline is being benchmarked by competitors due to its blend of capability and operational efficiency. (Lindsay Brooke)

"We set a high target for this program: Create the industry’s most fuel-efficient four-wheel-drive system, with the greatest off-road capability and functionality,” said Mike Kirk, Chrysler’s Director of Powertrain, Axle, Driveline & Manual Transmission Engineering, about the 2014 Jeep Cherokee. “And we knew from the start that our success was dependent on systems engineering — both internally and in close collaboration with our suppliers.”

Transforming a front-wheel-drive crossover (the KL Cherokee is based on Fiat-Chrysler’s modular Compact U.S. Wide car platform) into a benchmark off-roader is no small feat, even without the requirement for a major leap in fuel efficiency — the base Cherokee is EPA-rated at 31-mpg highway. While ZF’s all-new 9-speed transmission, known as the 948 within Chrysler, would provide gearing benefits, Kirk’s team realized the vehicle driveline also was key to significant efficiency gains.

Cherokee’s ‘naked’ driveline shows no traditional transfer case. The axle-disconnect function delivers 2WD efficiency and 4WD performance.

“Our aim was to take at least 80% of the losses out of the [4WD] system when the customer doesn’t need it,” he noted. “So the idea of a disconnecting driveline emerged quickly in our early planning.” Chrysler Powertrain had been working with USCAR as well as SAE International’s Driveline Committee as it investigated solutions for improving AWD/4WD system efficiency, without sacrificing the “Trail Rated” capability that’s vital to the Jeep brand.

Cherokee is the first vehicle in its segment to feature rear-axle disconnect. The technology helps reduce fuel consumption by automatically switching from 4WD to 2WD, thus eliminating the parasitics of the second drive axle when not required by operating conditions. A similar feature is offered in the Chrysler 300 and Dodge Charger.

The driveline project actually started as a “decoupled” development off the critical path of the vehicle program, which began about a year later. “And i can tell you that’s not enough time for something like this!” Kirk asserted. “This was a difficult one because we started it, then we jumped into the serial development of the new vehicle platform. Everything was done on an accelerated timeline.”

Collaborative design

Mike Kirk of Chrysler Powertrain’s driveline engineering group praised his AAM colleagues in co-executing a robust, reliable, and sophisticated 4x4 system.

Chrysler Powertrain engineers Joe Kubina, Pete Jarzyna, Mike Nemeth, and Don Schmanski had been laying the technology foundation for the new driveline with their invention of a 4WD system for transverse-engine/front-drive architectures, which led to a U.S. patent issued earlier this year (and the 2014 Walter P. Chrysler Technology Award for the engineer quartet). The patent covers the power-takeoff unit (PTU) and associated components — a reduction hub controlled by a range selector with high, low, and neutral settings that is used to engage a planetary gear assembly that drives a transaxle’s output shaft.

According to Kirk, a prototype 2-speed (hi-lo) PTU using the planetary gearset resulted, and Chrysler’s Powertrain Synthesis Group evaluated it in a proof-of-concept vehicle. The results were encouraging and showed a useful efficiency gain. Further vehicle-level analysis included a thorough benchmarking of AWD and 4WD systems in competitive vehicles.

“We then did a paper analysis of a total of 14 different supplier-partners from around the world. We looked at everybody with systems expertise who could work closely with us in its development, and also manufacture the system to our high standards,” Kirk recalled. “Of those 14 we picked two partners to build demo vehicles with the 2-speed disconnectable system we envisioned. We felt that if we got that right, we could then develop the AWD system for other vehicles.” He said Fiat-Chrysler planners were already looking at a broad variety of C/D-segment vehicle applications.

The two finalist suppliers were each given a vehicle, and Kirk’s team set a 90-day deadline for completing all analysis and design reviews. When the clock stopped ticking, Chrysler selected American Axle & Manufacturing (AAM) from the pair as its driveline-systems partner for the KL program.

“We’ve been working with AAM since the early 1990s; they supply the drivelines for our HD trucks,” Kirk said. “We already had an outstanding relationship with them. While they were not in the AWD business with us, they fit well for the Cherokee project because of their expertise in core components and subsystems.” And AAM already had development work under way, having anticipated the industry’s need for driveline-disconnect capability with its EcoTrac system.

Kirk defines a true partnership with systems-proficient suppliers as one in which “we support each other’s strengths, cover each other’s weaknesses, and solve problems together.” For the 2014 Cherokee, the solutions arrived at by Chrysler and AAM are best highlighted by reviewing the vehicle and its three available 4x4 systems. The base system, which Jeep calls Active Drive 1, is fulltime 4WD without low range. it uses an electronically-controlled wet clutch housed in the rear drive module that can vary the amount of input torque according to driving conditions.

Active Drive 2 incorporates a reduction gear in the PTU that provides a 2.92:1 low range, effectively giving the Cherokee a Wrangler-beating 56.1:1 crawl ratio.

Finally, there’s Active Drive Lock, standard on the Trailhawk model. it features the gear-reduction PTU and a true mechanical-locking rear differential. Electronic-control flexibility is provided by a Select Terrain knob on the console, which offers the driver a variety of terrain-focused operating modes — Auto, Sport, Sand/Mud, Rock, and Snow mode — along with hill-descent and ascent control, all of which required extensive software and algorithm development by the Chrysler and AAM teams.

All three systems employ the AAM driveline-disconnect technology it calls EcoTrac. There is no traditional transfer case. The rear axle (known as the rear differential module, RDM) is connected by drive-shaft to the PTU packaged tightly on the rear of the transmission. Along with EcoTrac, AAM co-developed and supplies the PTU and the 2-speed rear unit.

The PTU’s (red circle) design was driven by extremely tight package requirements within the Jeep Cherokee’s cast-aluminum front cradle and engine/transmission assembly. (Lindsay Brooke)

“They [AAM] understood that the mode transitions had to be seamless to the end customer,” Kirk said. “We have up to 111-times torque multiplication from the engine through the driveline. So the controls work was a key to making this fully functional and imperceptible for the driver.” He noted that in Snow mode, for example, the Cherokee actually launches in 2nd gear in the 948 trans to give the customer better traction and a more secure ‘feel’ in slippery conditions. This required creation of unique shift schedules between 4-Hi and 4-Lo modes, and even the ‘Rock’ mode, to improve the performance and driveability under extreme conditions.

Disciplined software approach

Software development began at the earliest stage of the program, Kirk said, and was segmented to a certain degree to ensure timing coordinated with the rest of the vehicle program. “For example, our Transmission System folks first focused on on-road functionality, because obviously we need that to test things like vehicle durability and cold/hot performance. From the perspective of controls and systems functional requirements, we weren’t worried about 4-Lo performance in the early phase.”

The ability to do hardware-in-the-loop and software-in-the-loop, and perform virtual analysis on the controls, helped keep the high-pressure program on track.

Cherokee rear module contains planetary gearset for 2-speed (hi/lo) 4x4 functionality. (Lindsay Brooke)

“AAM had a very disciplined but quick method of putting together proof-of-concept software to provide features and also fix issues,” Kirk explained. “Then we’d come back with a cooperative approach to do validation before we introduced the driveline into the fleet of vehicles used for durability and reliability.”

Contemplating his team’s learnings on the KL driveline program, Kirk says the “wish list” for improving future processes is for even greater systems-level analysis and more standardized controls, to help shorten the development cycle, particularly in critical areas such as NvH attenuation.

“As an industry, we can no longer rely on having to test everything to get to the best answer,” he said.