In the 2013 Ram 1500, Chrysler has what is arguably the light-truck segment’s most sophisticated and efficient powertrain/driveline—V6 with 8-speed automatic and stop-start, rotary shift controller, transmission fluid heater, plus an active transfer case and automatic-disconnect front drive axle on 4WD models. Mircea Gradu, Chrysler’s Vice President of Transmission, Powertrain and Driveline Engineering, and SAE International’s Automotive VP, spoke with Senior Editor Lindsay Brooke about its development.
Chrysler executed the 2013 Ram 1500 program in lightning speed. What was your greatest learning from its powertrain development—how quickly your engineers can get the job done?
(Laughs) That’s certainly one of them! I think the value of planning was a big learning. Through the ‘dark’ period of 2008-09 we dreamed of the 8-speed being in our trucks. We’ve actually been on board with this development since 2007, when ZF started to materialize the 8-speed into a viable product.
How did your team and ZF collaborate on the 8HP45 and 8HP70?
ZF presented to us the basic hardware solution, which we then analyzed in terms of its performance in the vehicle. We handled integration of the transmissions into our drivelines and vehicle—a significant task. We have the NVH and torque converter experts in-house; selecting an optimum K-factor of the converter from multiple values is our job.
What makes the 8-speed transmissions an ideal choice for Ram?
Their wide ratio spread. The 1st gear ratio is in the 4.7:1 range; it allows torque-multiplication for crawling and towing on a high grade. It also allows for a very efficient torque converter architecture, and very good NVH isolation. And the 8-speed’s ratio steps are smaller than 1.5, which gives us excellent shift quality and driveability. All of that really helped us optimize the entire driveline in terms of vibration modes. The transmission features an off-axis pump, lower viscosity ATF, and a clutch package in which there are only two open clutches in every gear, which minimizes drag.
The basic 8-speed’s reliability, durability, and efficiency have proven tremendous in the Chrysler 300 and Dodge Charger applications. The 8HP fits within the same package envelope as the ZF 6-speed with equivalent input-torque capability.
What role does the new active transmission warm-up system play in this program? (see http://www.sae.org/mags/sve/11310/)
Through optimum temperature management of the transmission and ATF itself, we can extract up to a 2% fuel economy improvement. We are essentially using some of the heat generated by the engine in the early warm-up phase of the transmission. After roughly five minutes we bring the transmission up to the ideal operating temperature of 80°C, vs. running continuously at 55-60°C, which creates a lot of inefficiencies and reduces the durability of the transmission.
Was this system a Chrysler in-house development?
We have a lot of IP related to it. The concept is pretty widely embraced by the industry. It’s a first application for us, and a first for the industry at this level of integration.
Another first-in-segment application is Ram’s stop-start system.
It’s enabled by the so-called HIS unit, which operates on the principal of hydraulic-impulse storage. It’s an elegant solution—rather than running on an electric pump during the engine stop period, we use an hydraulic cylinder to accumulate pressure and release it during that phase, to ensure transmission lubrication.
Was the stop-start system development off the Ram program’s critical path?
It was integrated into the transmission program. It was a development by ZF that fits with our overall transmission strategy. They protected for this within the transmission development and included it in the transmission-assembly process. We got it first and are the first to apply it at this level. There are many places over the U.S. FTP and European NEDC drive cycles that provide opportunities to utilize the start-stop. From the customer perspective it allows us to achieve up to 3.5% fuel economy improvement.
Integration of stop-start with the vehicle can be time-consuming and tricky to get right, which is critical to customer satisfaction.
That’s correct. The integration was at least a year and a half. You have to avoid ‘bumps’ during restart, and any kind of shudder conditions. There are a lot of NVH measures that have to be applied to make stop-start seamless. And there are safety aspects you have to meet.
What other changes within the powertrain and driveline have been made to improve efficiency?
We looked at every area. On the differentials, for example, there’s a very notable change to the axle design and bearing design. We’ve replaced the traditional tapered-roller bearing that supports the head of the pinion with an angular-contact ball bearing. It provides a 0.8 to 1% gain in fuel efficiency, by our measurements.
Have you had to beef up your transmission and driveline engineering teams in order to implement these and other significant product upgrades?
Yes, we have. We combined the former front-wheel-drive and rear-wheel-drive groups into a single team, which was reduced by almost half during the so-called ‘dark’ period of 2008-09. Then we rebuilt, so now we’re at very decent internal resource levels and have strong relationships with our suppliers because we’re back to being a very credible company. ZF, for example, is a very conservative company that uses its own capital for engineering developments. They only put their money into reliable projects. We’re attracting great talent in both individual engineering hires as well as the best global suppliers.
(AEI-captured video of Mike Cairns, Head of Ram Truck Engineering, explaining weight-reduction measures and other key changes to the 2013 model, including an air suspension system adapted from the Jeep Grand Cherokee, can be viewed on the SAE Video site, or on YouTube at http://youtu.be/xb1Wie5KpY4.)