In designing the second-generation system, GM engineers built on the knowledge and data gained from the first-generation Volt. A GM analysis of vehicle data collected from first generation found that Volt vehicles were driven more than half a billion miles in North America from October 2013 through September 2014, and that 74% of the miles were all-electric. (A fifth paper presented at the World Congress assessed one year of in-use operating data from first-generation Chevrolet Volt and projected the performance of the second generation.)

For the second-generation, GM wanted to enhance the electric performance of the Volt. Key design goals included:

1. Increased all-electric range
2. Increased charge sustaining fuel economy
3. Improved performance
4. Smooth, unobtrusive, power on demand regardless of whether the vehicle is operating in EV or range extended mode.

The major components of the propulsion system are the battery pack, the drive unit, and the engine. The drive unit contains two newly developed electric motor-generators for propelling the vehicle and other duties; power electronics to supply and control them; a system of gears, clutches, and hydraulic controls to combine and deliver the mechanical power from the motor-generators and the engine; and an electric oil pump.

The range-extending engine is a 1.5-liter four-cylinder, naturally-aspirated direct-injection spark-ignition engine with sufficient power to continue normal driving after most of the battery energy has been depleted by EV driving or during conditions when available battery power is low because of extreme temperatures.

Transaxle configuration and operating modes. GM took a blank-sheet approach to designing the second-generation system. A team from GM Research and Development (GMR&D), Advanced Engineering, and Product Engineering examined more than 50 types of electric and hybrid propulsion systems to find the best type for the second-generation Volt. About half of these were chosen for energy modeling in both EV and extended range operation, using a simulation tool developed by GMR&D.

Out of this, the GM team selected several systems for more intensive study and comparison with two benchmark systems from other OEMs.

The resulting powerflow concept selected contained several features especially well-suited to electric vehicle propulsion:

• The planetary gear ratios required for efficient operation with engine on also resulted in gear ratios that were suitable for electric motor traction drives, with similar speed ratios between the two motors and the transmission output.

• The powerflow was capable of operating at high vehicle speeds both as an EV and with engine on.

Drive unit power flow. Schematic lever diagram showing the gearing of each of the two propulsion motor-generators to a common main transmission shaft using an individual planetary gear set. Clutches are OWC1, C1 and B1. Click to enlarge.

A key decision was to split EV propulsion between two motor-generators (motors A and B). While the first-generation Volt also featured two motors, the roles were split between the 111 kW main traction motor and 63 kW generator motor. In Gen2, the two motors share both roles.

Each of the two propulsion motor-generators is geared to a common main transmission shaft using an individual planetary gear set. Each motor-generator is connected with a sun gear, and a clutching device can provide reaction torque by holding the ring gear of that planetary gear set.

When both of these clutching devices are active, the two motor-generators are connected in parallel through similar planetary gear sets and a single common final drive and differential to drive the front wheels of the Volt.

The split of EV propulsion between the motors in Gen2 allows engine starting for extended-range driving using torque from an electronically-controlled motor, which makes the starts smoother, quieter, and more efficient than using a slipping clutch or a conventional starter. The split also helps to eliminate the mass of hardware that is not useful for EV driving.

The use of the two motors allows enables two EV driving modes (one-motor and two-motor) to increase efficiency. In two-motor EV mode, the task of providing peak torque at low speed for crisp launch is split between the two electric machines; nearly every part of the drive unit is in use to provide maximum output torque and EV acceleration. The one-motor EV mode of operation reduces losses under conditions of relatively low torque, such as cruising on a street or highway.

During extended-range operation, the Volt operates with engine power under the following 3 modes:

• Low Extended Range: Efficient operation at high tractive efforts or low vehicle speeds.

• Fixed Ratio Extended Range: Very efficient operation during typical driving conditions when the vehicle is accelerating, and also allows efficient charging of the battery pack under light vehicle load conditions.

• High Extended Range: Efficient operation at higher speeds for highway cruising.

Typical Mode selection during EV operation. Click to enlarge.		Typical mode selection during extended range operation. Click to enlarge.

Voltec drive unit. Because the Gen2 Voltec system can draw on both motor A and B for propulsion, GM engineers were able to reduce the total torque and power requirements for each of the motors. This in turn enabled a reduction in overall motor volume and also allowed the bearing size to be substantially reduced. Gen 2 system motor volume was reduced by 20% vs. GEN 1 and motor mass was reduced by 40%.

Top:The integrated motor/TPIM drive unit. The drive unit is similar to a conventional front-wheel drive transaxle in location and form. On top it has an angled rectangular housing receiving high voltage direct current (DC) cables connected to the battery pack and containing the inverter. Bottom: Cross-section of the drive unit. Click to enlarge.

The GEN 2 drive unit integrates the power inverter with the transaxle, providing a significant reduction in volume through elimination of the 3-phase high voltage cables. The volume of the power inverter was reduced from 13.1L to 10.4L.

GM engineers sought to minimize the hydraulic power required for transmission operation. (The three clutching elements are implemented as hydraulically actuated plate clutch and brake, and mechanical diode.) The hydraulic system serves three functions: clutch/brake actuation; lubrication; and motor cooling.

The designers optimized the clutches and hydraulic circuits to balance these requirements and to reduce the overall hydraulic power. In addition, the oil pump and motor were sized to accommodate the full temperature range of operation, eliminating the need for an engine-driven pump.

Electric machines. In the first-generation Voltec system, both motors used NdFeB magnets. In the second generation, to optimize the EV range of the system, motor A was designed with a Ferrite magnet rotor while motor B was designed with an NdFeB magnet rotor. The Gen 2 system transmits most power through motor B under typical driving conditions, while motor A is used to augment power at high loads. Each motor design was optimized to match its distribution of operating points.

Since motor A rotates during most operating conditions but is mostly at zero torque, it was designed with weaker magnet flux to minimize speed related losses, while motor B was designed as a stronger magnet flux machine to minimize losses at its operating points which are typically under load.

Although using ferrite magnets in a motor is not a new idea, most of the machines designed with these types of magnets are intended for use in industrial applications where requirements in terms of size, torque density and wide operating temperature range are not as extreme as in automotive traction application.

GM engineers developed other ways to increase the torque capability of the machine using a topology referred to as Permanent Magnet Assisted Synchronous Reluctance Machine (PMASRM).

Although Nd-FeB magnets are well-established and have outstanding magnetic properties and high energy, the coercivity of the magnets at higher operating temperature is a challenge. The mainstream approach to increasing the coercivity of Nd-FeB magnets is to add heavy rare earth such as Dysprosium to increase its intrinsic coercivity.

While this approach is demonstrated to work and widely accepted in the industry with mature processes in place, it also brings along a significant price tag associated with the cost and amount of required heavy rare earth material. In addition it also reduces the remanent flux (Br) of the resulting magnets.

Over the last several years a new process, Grain Boundary Dysprosium Diffusion (GBD), has been developed toward enhancing the coercivity of NdFeB magnets while minimizing the use of heavy rare earths and impact on Br. GBD focuses on concentrating dysprosium near the grain boundaries of the Nd-Fe-B sintered magnet...and around the surface of the finished magnet. Using this type of magnets allows for significant reduction in machine size and magnet mass as well as rare earth reduction. It also offers demagnetization protection around the magnet corners where they are typically most vulnerable since the finished magnet will now have the highest Dy concentration in those areas.

—Jurkovic et al.

The resulting electric machines reduce rare earth and heavy rare earth content by more than 80% and 50% respectively while maintaining and improving the drive unit and vehicle performance.

Motor A (the ferrite machine) exhibits a slightly lower peak efficiency compared to Motor B (the sintered NdFeB machine): 95% versus 96%. This is in part due to use of weaker ferrite magnets, but also due to the overall shorter active length of the A machine, 31.5mm versus 51.5mm for the B machine.

Measured efficiency map of Motor A. Click to enlarge.		Measured efficiency map of Motor B. Click to enlarge.

Traction Power Inverter Module (TPIM). The two motors that are commuted by two traction inverters: PIM-A (power inverter-A), PIM-B (power inverter-B) and an oil pump motor run by a third inverter. All these three inverters are contained in one package named the Traction Power Inverter Module (TPIM), integrated with the drive unit.

TPIM is a single component that is bolted into the transmission in a single operation. The motor interface is done with ridged bus bars that pass through a cast wall in the transmission case. The coolant interface is achieved through a gasketed joint. Coolant is routed through the transmission case to an external cooling pipe assembly. The coolant pipe assembly can be unique for each vehicle application to keep the plant interface common for all versions of the transmission.

The Gen 2 drive unit integrates the power inverter with the transaxle, providing a significant reduction in volume through elimination of the 3-phase high voltage cables. Volume of the power inverter was reduced from 13.1L to 10.4L. Click to enlarge.

The main components of the TPIM are the power board, DC bus capacitor, EMI filters, control and gate drive boards, sensors and busbar. The motor controls algorithms are programmed into the control boards.

In the second-generation unit, better power flow between the inverters, better efficiency and thermal robustness enabled an average electric drive system FTP city efficiency improvement of 6;, a projected charge sustaining (CS) label fuel economy increase of 10%; and both low end torque and high speed power improvements.

GM selected Delphi’s novel dual-side cooled Viper as the power device for the TPIM. Viper contains silicon IGBT and diode in an electrically isolated package that is thermally conductive on both sides and provides a low profile compact solution for inverter. Unlike a conventional power module, the better silicon and dual-side cooled Viper enabled the reduced of the silicon footprint, allowing for greater layout flexibility and reduced cost.

Design of the heatsink was critical. To handle the large phase currents, GM engineers used a copper metal injected molded (MIM) heatsink. This unique MIM device provides a low pressure drop with a high heat convection to handle the losses for each specific IGBT and diode. The heatsink and housing loop also passively cools the 3-phase AC busbar and capacitor assembly.

High-power electronics for electrified vehicles need liquid-cooling for heat dissipation. Coolant passages and chambers with internal fins are usually formed from multiple pieces and joints to keep costs down; these joints need to be strong to handle the pressures and vibrations in a vehicle, and to be leak-proof to eliminate the risk of leakage inside a high-power electrical enclosure.

Conventional design uses pressure seals with rubber gaskets and many screws, typically with a thick die-cast cover. However the Gen 3 TPIM housing and liquid cooled chamber uses friction stir welding (FSW) technology adapted by Delphi. This design was validated to several thousand pressure cycles without failure.

The process integrates a liquid heat exchanger to the body of the electronic module with robust leak-free hermetic joints and welds aluminum covers to cast aluminum cases to enclose the product replacing glue-and-screw processes. This process outperforms the industry-standard with a 45% smaller footprint, fast change tooling, 3-D force monitoring, and a 50% shorter process time that results in reduced carbon footprint and 40% power savings.

The TPIM housing and liquid cooled chamber uses friction stir welding (FSW) technology adapted by Delphi. This design was validated to several thousand pressure cycles without failure.

To enable the smaller packaging size required for on-transaxle mounting, inverter current requirements for both motors were limited to 325 Arms, primarily as a result of reduced motor torque and power requirements. The lower current requirements, in conjunction with silicon and power module cooling improvements, enabled inverter silicon die area as well as inverter switching loss to be reduced by more than 50%.

Comparison of Gen 1 and Gen 2 Voltec drive units
	Gen 1	Gen 2
Gear ratio, EV mode	A: 1.45 B: 3.24	A: 2.87 B: 3.08
Modes, engine on	2	3
Final drive arrangement	Parallel axis gear reduction	Chain transfer and planetary gear reduction
Motor A type	Concentrated winding NdFeB magnet	Distributed bar wound Ferrite magnet
Motor A peak torque/power	186 N·m/55 kW	118 N·m/48 kW
Motor B type	Distributed bar wound NdFeB magnet	Distributed bar wound NdFeB magnet
Motor B peak torque/power	370 N·m/111 kW	280 N·m/87 kW
Final effective drive ratio	2.16	2.64
Power electronics	Separately mounted	Integrated
Pumps	1 engine-driven 1 motor-driven	1 motor-driven
Transmission fluid	Dexron VI	Dexron VI
Total system mass	164 kg	119 kg

Engine. GM developed a hybrid-optimized engine based on the new Ecotec family of engines. GM sized the engine based on three key factors: maximizing efficiency given the operation typical of an EREV vehicle application; ensuring that the engine noise would not be obtrusive during CS operation, thereby maintaining the EV drive character; and providing sufficient performance in extended range mode to minimize the battery pack energy required for CS operation.

Key technologies and features of the Gen 2 engine, a hybrid-optimized version based on the new Ecotec family. Features outlined with a border are specific to the range-extender engine, while the others are common to the entire engine family. Click to enlarge.

Operation of a range extender differs form that of a typical hybrid or conventional engine in that the level of available battery power allows considerably more flexibility in engine speed and load operating points.

• At the low end of the power band, the engine remains off during many of the low speed and power conditions.

• At the high end, the ample supply of on-demand electric power allows the system to minimize the demand for maximum engine power (which in general occurs at less efficient operating points).

• The available electric power ideally allows the system to operate the engine closer to peak efficiency points.

These factors tend to result in the operation in a fairly narrow band of power away from the power extremes that would occur in other applications.

While it may seem at first glance that the ideal choice for engine efficiency would be to minimize displacement due to the large amount of electric power available, it was actually found that a larger normally aspirated engine provided advantages in both efficiency and EV drive character.

—Jocsak et al.

GM initially assessed several 4-cylinder and 3-cylinder options. The engineers found that while the 3-cylinder normally aspirated engine showed an efficiency advantage at powers below 10 kW, these points were not prevalent in usage for the Volt. The lower torque and power of the 3-cylinder increased the speed required to run at the higher end of the power distribution.

The 3-cylinder turbocharged engine had lower peak efficiency than the normally aspirated engines. Further, since the peak efficiency of the 3-cylinder turbocharged engine occurred at a relatively low torque as compared to the peak torque, the system was not able to fully utilize the available torque to achieve desired low engine speeds without compromising efficiency. On the other hand, the 4-cylinder normally aspirated engines were more efficient in the range of speeds and powers suited to the Volt application. High efficiency near the peak torque allowed operation at the lower speeds needed to minimize engine noise.

In the final analysis, the largest displacement normally aspirated engine available within the Ecotec family, 1.5L, was selected, as it achieved the best efficiency under the relevant operating conditions, allowed the desired low speed/high torque operation, and minimized the need for CS buffer size. In the GEN 2 Volt, the improved engine and drive system are able to deliver more power to the wheels for hill climbing, so the need for the use of mountain mode to extend CS operation is reduced.

—Jocsak et al.

GM improved the efficiency of the engine through the use of a higher geometric compression ratio (12.5:1) and external cooled exhaust gas recirculation (EGR). Camshaft park positions were optimized to reduce effective compression ratio during engine starts, reducing oscillating cranking torque and improving the smoothness of engine starts.

The piston bowl was developed to enable spray-guided direct injection to improve combustion system performance and robustness for catalytic converter light-off.

The cooled external EGR system extracts exhaust gas upstream of the catalytic converter and downstream of the integrated exhaust manifold (IEM) outlet, passes the gas through a butterfly style EGR valve, then through an external EGR cooler, and into the intake manifold downstream of the throttle. The EGR cooler utilizes engine coolant as the heat exchange fluid.

A key goal in designing the system was to minimize pressure losses in order to maximize the amount of EGR available for introduction into the intake manifold under high load when manifold air pressure (MAP) is high. As a result, GM subsequently optimized the intake manifold design and EGR introduction location.

EGR system. Click to enlarge.		EGR system on the engine. Click to enlarge.

Selection of the compression ratio was made by analyzing its effect on engine fuel consumption at high-usage operating points, and on wide open throttle (WOT) torque.

BSFC impact of varying engine CR at key engine operating points. Click to enlarge.		Torque impact of varying engine CR from 11.5 to 13. Click to enlarge.

GM optimized the engine control strategy to use dual cam phasers with cooled external EGR to improve efficiency across the range of engine operation, particularly within the power band of 10kW to 26 kW, which comprises the majority of operation in the Volt vehicle. The engine was designed to operate at stoichiometric during full load operation below 4000 rpm.

Intake cam park timing was chosen to balance conflicting engine requirements. Electric system restart of the warm engine dictates late park of the intake camshaft, retarding the closing timing of the intake valve, and reducing the effective compression ratio of the engine. This in turn reduces the oscillating cranking torque of the engine, minimizing the noise, vibration, and harshness (NVH) impact of engine restart. Conversely, engine cold-start at high altitudes dictates an early park position of the intake camshaft, increasing the effective compression ratio of the engine.

GM chose exhaust cam park timing to optimize cold-start emissions and engine efficiency based on the chosen intake cam park timing selection. The result was more retarded intake and exhaust park timing than employed on the conventional version of the 1.5 L Ecotec engine.

The combination of LIVC operation at lower loads, enabled by the dual high-authority cam phasers, and external EGR at higher loads combined with direct injection for engine knock mitigation resulted in specific power comparable to that of a conventional non-hybrid optimized engine, minimizing engine size and mass, while meeting vehicle performance requirements.

Gen 2 Volt system results. GM projects the Gen 2 Volt will achieve a 30% increase in EV range, an 11% improvement in charge sustaining label fuel economy, and improved vehicle performance both as an electric vehicle, and in extended range mode.

Resources

• Conlon, B., Blohm, T., Harpster, M., Holmes, A. et al. (2015) “The Next Generation “Voltec” Extended Range EV Propulsion System,” SAE Int. J. Alt. Power 4(2) doi: 10.4271/2015-01-1152

• Anwar, M., Hayes, M., Tata, A., Teimorzadeh, M. et al. (2015) “Power Dense and Robust Traction Power Inverter for the Second-Generation Chevrolet Volt Extended-Range EV,” SAE Int. J. Alt. Power 4(1) doi: 10.4271/2015-01-1201

• Jurkovic, S., Rahman, K., Patel, N., and Savagian, P. (2015) “Next Generation Voltec Electric Machines; Design and Optimization for Performance and Rare-Earth Mitigation,” SAE Int. J. Alt. Power 4(2) doi: 10.4271/2015-01-1208

• Jocsak, J., White, D., Armand, C., and Davis, R. (2015) “Development of the Combustion System for General Motors’ High- Efficiency Range Extender Ecotec Small Gas Engine,” SAE Int. J. Engines 8(4) doi: 10.4271/2015-01-1272

• Duhon, A., Sevel, K., Tarnowsky, S., and Savagian, P. (2015) “Chevrolet Volt Electric Utilization,” SAE Int. J. Alt. Power 4(2) doi: 10.4271/2015-01-1164