GM’s new RWD PHEV system for Cadillac CT6 designed for fun-to-drive high performance as well as efficiency; Volt on steroids
In a preview of three detailed papers to be presented at the SAE World Congress in April, Tim Grewe, GM’s General Director of electrification, and Pete Savagian, GM General Director of electric drives and systems engineering, provided a technical overview of the new rear-wheel drive PHEV propulsion system for the Cadillac CT6 (earlier post) at the recent SAE 2016 Hybrid and Electric Vehicle Technologies Symposium in Anaheim.
The efficient and very fun-to-drive system, with 335 kW (449 hp) combined system power, propels the CT6 from 0-100 km in 5.6 seconds; delivers an all-electric range of more than 60 km (37 miles) and an all-electric top speed of 125 km/h (78 mph); and features combined fuel consumption of less than 2.0 L/100 km (117.7 mpg US).
We have a PHEV system that has incredible efficiency, but it is also one of the highest performing vehicles on the road with extraordinarily connected feel to it.—Tim Grewe
The CT6 PHEV powertrain combines a 198 kW (266 hp) 2.0L turbocharged 4-cylinder direct-injected gasoline engine with a two-motor, 120 kW hybrid transmission; liquid-cooled 18.4 kWh Li-ion battery; and power electronics (APM, auxiliary power module; OBCM, on-board charge module; TPIM, traction power inverter module).
|Top: CT6 PHEV powertrain elements and vehicle integration. Bottom: CT6 high-voltage architecture. Click to enlarge.|
The battery pack is the main distribution unit; it feeds the main traction power inverter module (TPIM), and drives the two-motor EVT. GM eliminated mechanical oil pumps on the system; the TPIM contains a 2 kW electric oil pump driver.
GRE Electrically Variable Transmission. The new electrically variable rear wheel drive transmission comprises 2 electric motors, 3 planetary gears (two simple, one double-pinion) and 5 clutches that deliver four continuously variable transmission modes with 3 fixed gears.
Providing all-electric launch, selectable regeneration, and power blending with the turbocharged engine, the transmission provides smooth and seamless power through the entire driving range.
The electric motors. GM designed the new powersplit transmission specifically to be able to use an induction machine for Motor A without a reduction in overall efficiency. GM did that for robustness and cost, and also as a hedge for what might happen in the rare earth metals business. Compactness, efficiency and excellent NVH were design objectives from the outset.
GM designed and manufactures the two motors in the EVT. Motor A (closest to the engine) in the CT6 PHEV drive unit is the induction motor; Motor B is a permanent magnet motor. Bar-wound stator construction is utilized for both the motors.
GM believes that the rectangular wire construction—the bar winding—has “salient and tangible” benefits for electric driving that come from the improvement in performance of the stator. With bar winding construction, GM can pack in more copper, with about half of the DC resistance of the stator. Although there can be higher AC resistance at higher speeds, Savagian said that those motoring speeds are rarely reached during normal operation. “The bulk of the energy is spent at much lower motor impedance with the rectangular than with the conventional stranded round wire construction,” said Savagian.The bar shape also assists with cooling.
The use of an induction motor for motor A enables a lower overall system cost and a rare-earth-free design while delivering the required performance. Low spin loss design of an induction rotor as compared to a permanent magnet rotor, especially at higher speeds, is the major enabler for the choice of an induction motor for motor A for this application, GM says.
GM optimized Motor A design for torque, power and NVH.
The induction motor is something we are particularly proud of. We’ve done a great deal of work on interior permanent magnet machines, but we haven’t designed many induction machines. Our eAssist motors are induction type, but they are relatively low power. For the full power capability we went through an elaborate optimization activity to optimize the size of the rotor bars. These would be cast aluminum. We’ve optimized the parameters of the keystone for both power efficiency and torque ripple.
We also examined the number of bars we could put into that rotor. We knew generically we would see a trend toward more and finer bars being better. We explored the limits of that in manufacturing but also in design. This is an eight pole machine, and it is reflected through a 91-bar structure.—Pete Savagian
|Stator and rotor for Motor A (left) and Motor B (right). Click to enlarge.|
Motor B, the interior permanent magnet (IPM) motor, is produced at the GM Baltimore plant. The motor is in the same family as the motor for the Chevy Spark EV, but is a shorter version. The CT6 Motor B as a 75 mm active length, compared to the 125 mm of the Chevy Spark machine, noted Savagian.
Motor B’s magnets are arranged in two layers of ‘V’ (see picture above). The rotor has optimized profile stamped on the rotor outer diameter to target the dominant torque ripple orders. A pair of small slot is stamped in the rotor lamination near the rotor outer surface to further minimize torque ripple and stator teeth radial force.
Rotor magnet placements and shapes within each pole are optimized for performance, efficiency, and noise.
Battery pack. The pack has a nominal capacity of 18.4 kWh, maximum power of 120 kW and nominal capacity of 51.8 Ah—like the second-generation Chevy Volt. The CT6, however, packages the batteries in a very different case.
The pack uses 96 cells in series; each cell has two cell pair (96 2P system). Essentially, Grewe said, GM made a tray to hold the modules that are common with the Volt: common electronics, battery state estimator, etc.
|The battery pack. Liquid cooling (water jackets next to each cell), allow battery modules to be packaged on their sides. Click to enlarge.|
Traction power inverter module (TPIM). The new TPIM incorporates the power electronics for the motors, auxiliary pump and hybrid system. The compact inverter (~9 liters, 13 kg) uses a double-sided power module.
Often it is very enticing to say, “Well I’m just going to load the battery pack into an existing structure in the car. But basically we cut out the whole back of the CT6. We use the tray of the battery pack as the main floor structure. With the ability to handle the weight of the battery in the car structure, it really improved the stiffness. To handle that mass on the rear wheel drive, it actually made driveability better, because it is stiffer and engineered to go together.—Tim Grewe
The design enables double-sided cooling of the most expensive part of inverter. The capacitors also use active cooling, enabling high kVA numbers.
Design and operating modes.For the CT6, GM took the basic design of the drive unit in the Volt (two planetary gearsets, two motors, two clutches)—which enables the CT6 to perform well as an all-electric vehicle—and added a third planetary gearset and two more clutches to enhance the “fun-to-drive”.
Basically, that gives you a Volt on steroids. It gives you tremendous launch torque. It’s actually a 6.8 step down in the EVT low. You look at some of the EV effort we have here, you’re almost above 5,000 N·m. You have that EV launch feel.—Tim Grewe
(As a comparison, the 3,543-lb Volt accelerates from 0-60 mpg in 8.4 seconds; the 4,431-lb CT6 accelerates from 0-100 km/h (0-62 mph) in 5.6 seconds.)
(papers to be published in April 2016 in conjunction with SAE World Congress)
Sinisa Jurkovic, Khwaja M. Rahman, Peter Savagian, Robert Dawsey (2016) “Electric Traction Motors for Cadillac CT6 Plugin Hybrid-Electric Vehicle” SAE 2016-01-1220
Alan Holmes, Jinming Liu, David Ames, Vijay Neelakantan, Khwaja Rahman, Timothy Grewe (2016) “General Motors Electric Variable Transmission for Cadillac CT6 Sedan” SAE 2016-01-1150
Amanullah Khan, Timothy Grewe, Jinming Liu, Mohammad Anwar, Alan Holmes, Richard Balsley (2016) “The GM RWD PHEV Propulsion System for the Cadillac CT6 Luxury Sedan” SAE 2016-01-1159