Powertrain & Energy - September 26, 2012 - 11

Engineering Ram’s leading-edge driveline
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 AEI 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
SAE Powertrain & Energy

helped us optimize the entire driveline in terms of vibration modes. The transmission features an offaxis 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? 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.

September 26, 2012

Powertrain & Energy - September 26, 2012

Table of Contents for the Digital Edition of Powertrain & Energy - September 26, 2012