|Plug-in hybrid operating modes with the 2MH4EV architecture. From Yang et al. Click to enlarge.|
GM has developed a prototype of its two-mode front-wheel drive (FWD) hybrid transmission (earlier post) with enhanced electric vehicle capability (2MH4EV) for application in a plug-in hybrid; GM presented a paper detailing the work at the SAE 2011 World Congress in Detroit.
The prototype drive unit includes an additional input-brake clutch to the existing FWD 2-mode system, enabling two new electric-only modes in addition to the existing EVT (electrically-variable transmission) modes and fixed gears. The two new electric-only modes have the capability to cover the entire FTP urban cycle with a properly sized battery pack, GM says.
GM’s base FWD 2-mode system incorporates two electric motors, two planetary gear sets and a system of four hydraulically-applied wet-plate range clutches to deliver two continuously variable speed-ratio ranges (modes) and four forward fixed gear ratios. The first continuously variable mode (EVT1 or M1) covers the higher speed ratio range, while the second continuously variable mode cover the lower ratio range. The second fixed gear (FG2) is located at the ratio of the synchronous transmission point between EVT1 and EVT2. Synchronous mode transition can be smooth because the shifting clutches can be pre-synchronized in speeds by using the electric motors.
The basic 2-mode transmission has two electric drive modes (i.e., zero engine speed) EVT1 and EVT2. Due to system constraints, only the EVT1 electric mode is available at lower vehicle speeds; this low-speed electric-only mode is called EV0. EV0 has limited speed range and acceleration capability.
The 2MH4EV architecture adds an additional input-brake clutch (CB5); the damper by-pass clutch in the base transmission is removed, and its hydraulic control circuit is reused to control CB5. With this, there are two new electric-only modes: EV1 and EV2.
In EV1, the transmission is in the first continuously-variable mode EVT1 with both brake-clutch CB12R and CB5 applied. With CB5 applied and providing reaction torque at the first planetary gear set, both motors can provide positive propulsion or negative regenerative braking torque, thus enhancing electric drive capability.
In EV2, the transmission is in the second continuously-variable mode EVT2 with both rotating clutch C234 and CB5 applied, again enabling both motors to provide positive propulsion or negative regenerative braking torque.
Overall, EV1 has a higher output torque capability, with better efficiency in the lower speed, higher torque region. It will be used as a low-speed, electric-only mode. EV2 has better efficiency in the higher speed, lower torque region, and will be used as a high-speed electric-only mode.
In addition to the existing engine-on operating modes EVT1, EVT2 and fixed gears FG1, FG2, FG3 and FG4 as well as the low-speed electric-only mode EV0, the two new electric-only modes EV1 and EV2 cover the entire low-torque, low-speed region assuming a sufficient amount of battery power.—Yang et al.
GM developed the controls for the enhanced system in a dynamic simulation model, then implemented them in the production hybrid software environment and tested them on the hardware in the loop bench, and ultimately tested and refined them in the vehicle.
Hong Yang, Anthony Smith, Shawn Swales, Joel Maguire (2011) Development of Two-Mode Hybrid Powertrain with Enhanced EV Capability. (SAE 2011-01-0883)