HEV P2 Module Concepts for Different Transmission Architectures
• Brad Chamberlin, Technical Specialist, Innovation and New Concepts, BorgWarner
• Eckart Gold, Senior Manager, Advanced Engineering, BorgWarner
• Jörg Nitsche, Director, Business Programs, BorgWarner
Driven by tough regulations and consumer demands for CO reduction, efficiency and performance, automakers are accelerating the introduction of electrified and hybrid vehicle propulsion systems. Considerations such as system architecture, packaging, performance and cost are all critical aspects during development. By carefully evaluating the system trade-offs to maximize the value to the consumer, BorgWarner has developed P2 systems with an integrated clutch and E-motor.
Hybrid concepts combine an electric motor (or E-motor) with an internal combustion engine (ICE). Common nomenclature defines the possible electric motor positions as P0 to P4, as shown in Figure 1. Depending on requirements for fuel savings, performance, cost, packaging space and other targets, automakers can choose between several technical solutions and arrangements, each with specific advantages and disadvantages.
A currently favored hybrid concept is the P2 arrangement, where the E-motor is placed between the ICE and the transmission. The cost-effective P2 concept allows producers of a step automatic transmission (AT), dual-clutch transmission (DCT), continuous variable transmission (CVT) and automated manual transmission (AMT) to transform an existing vehicle into a hybrid most easily. The main P2 elements are an E-motor, disconnect clutch and related actuation system elements.
P2 Module On-axis Arrangement
The P2 module is completely located between the ICE and transmission. The ICE crankshaft, damper element, P2 module (with disconnect clutch and E-motor) and transmission input shaft(s) are all on the same axis. The disconnect clutch is nested in the E-motor. On the input or ICE side, the disconnect clutch is connected to the damper element. The output side of the disconnect clutch is coupled directly to the rotor of the E-motor and the input of the launch and/or shift element. The stator of the E-motor is fixed in the module housing.
P2 Module Detail Configurations
When the strategy is to upgrade an existing transmission with P2 functionality, the design goal is to maximize the use of existing components. In this case, the disconnect clutch and E-motor are simply added in front of the existing transmission. (For a step AT, the torque converter might be omitted.) However, the transmission control system must be adjusted for control and lubrication of the disconnect clutch. This configuration is very popular due to its short development time and cost benefits.
The development of a completely new transmission architecture opens further opportunities to optimize packaging for the P2 module, especially in combination with a DCT. In this case, the two transmission clutches and the disconnect clutch can be integrated into a triple clutch module completely nested within the E-motor. Table 1 identifies the typical P2 configurations and the corresponding transmissions architectures that they can support.
P2 Module Clutch Design Considerations
Depending on the OEM’s preferences for efficiency, performance, robustness, technical maturity and cost, the clutch actuation concept can vary significantly. The most popular variants are hydraulically (high pressure or low pressure) or electro-mechanically actuated.
The required torque capacity of the transmission clutches depends on the maximum torque of the ICE and E-motor, as well as their interaction. As ICE and E-motor can be operated simultaneously, the torque capacity of the transmission clutches could meet the summation of both maximum drive torques. If the maximum torque of the ICE and the maximum torque of the E-motor do not occur simultaneously, downsizing the clutches may be an option to save weight, inertia and cost. The torque capacity of the disconnect clutch must be adapted to support the maximum torque of the ICE to ensure that there is not any unintended slipping during operation.
The clutch friction technology used depends on the launch, shift and drive cycle strategy. If the disconnect clutch needs to provide full launch capability or must be used in a continuous slip mode, adequate friction materials must be selected to maintain the performance over the complete product life.
Due to the fact that the clutch(es) are nested inside the E-motor in an on-axis configuration, the lube oil, which is applied to cool the clutches to maintain high functionality and performance, can be guided to the E-motor for effective cooling of the rotor.
Depending on the pump concept itself (e-pump and possibly an additional conventional ICE-driven pump), the P2 module can also provide a clutch-integrated pump drive gear.
All of these design aspects are considered in the BorgWarner P2 module. Several products are available to best suit the OEM’s specific demand. As an example, the on-axis triple clutch P2 module with an overall torque capacity of 500Nm and disconnect clutch torque capacity of about 350Nm can be packaged within an axial length of significantly less than 170mm.
P2 Module E-motor Design Considerations
A variety of design considerations need to be evaluated when designing the E-motor, both from the stator and rotor perspectives. Considerations such as torque or power density, thermal management, packaging space, and noise, vibration and harshness (NVH) related aspects (such as torque ripple) are evaluated to determine the best configura-tion.
Regarding the stator, the most common winding methodology is either a concentrated-wound or distributed-wound stator. These winding types can use either round or rectangular wire. Distributed-wound stators can also be bar wound, or concentric-wound.
With respect to the rotor assembly, common designs include induction and permanent magnet variants. Permanent magnet options can be either an interior permanent magnet (IPM) or a surface permanent magnet (SPM).
The chosen architecture of the BorgWarner P2 E-motor consists of a rectangular wire distributed-wound stator assembly with an IPM rotor for both the on-axis and off-axis solutions.
The distributed-wound stator assembly provides for extremely low (less than 5% in the on-axis configuration) torque ripple and cogging
torque. This equates to lower NVH from the E-motor and reduces the overall system NVH by eliminating a significant portion of the pulsations from the E-motor. In comparison, an alternative concentrated-wound E-motor may have as much as 20% torque ripple, requiring additional controls and dampening from the system level to minimize the adverse impact of NVH.
The distributed-wound winding utilizes a rectangular cross-sectioned conductor to maximize current density, allowing the on-axis E-motor with an outside diameter of 270 mm to deliver peak performance in excess of 345 Nm of torque and 110 kW of electrical power as well as continuous performance of 170 Nm of torque and 60 kW of power. Increasing the surface area of the conductor in contact with the stator laminations improves the ability to transfer heat out of the conductor in a more efficient manner. Both analysis and testing have shown that an E-motor with a properly designed rectangular conductor can achieve far higher continuous performance levels compared with a round wire electric machine, all while maintaining the same packaging envelope and cooling methodology.
The optimal method for cooling depends upon the available medias and their corresponding temperatures. In general, a cooling system utilizing oil is the most effective for removing heat from the E-motor because it allows the coolant to be placed in direct contact with key thermal components of the electric machine, including the stator lam-inations and conductors, as well as the rotor laminations and the magnets. By allowing the oil to make direct contact with these components, the heat is efficiently removed by minimizing the conductive path that exists with other cooling methods.
In systems that have particularly high oil temperatures, high performance can be achieved by using a water ethylene glycol (WEG) system in combination with internally sprayed oil, if the WEG can be provided at a reduced temperature (typically 65 °C vs. 90 °C). In this case, optimal performance is achieved by combining cool WEG around the stator assembly with oil flow onto the rotor assembly. This combination allows for direct contact between the oil and the internal components of the E-motor assembly as well as a large temperature delta between the E-motor and the WEG coolant. Both cooling options are available with the BorgWarner P2 E-motor.
The IPM rotor balances the trade-offs between magnet retention, demagnetization resistance, magnet weight (and associated cost) and E-motor performance. By embedding the magnet into the lamination stack, the inertia of the system and the cogging torque can be minimized while improving the structural robustness and balance of the rotor assembly.
The BorgWarner P2 E-motor was designed specifically to be a compact, best-in-class electric machine providing high torque and power densities. The on-axis design accepts either single- or triple-clutch modules and easily packages in-line with the transmission.
BorgWarner’s P2 Module Design Family Approach
Transmission manufacturers have different strategies for the system integration of their P2 modules, from their own in-house integration to purchasing complete pre-assembled and pre-tested P2 modules including the aluminum housing.
The challenge for component suppliers is to provide flexible technical solutions on one hand, and, on the other hand, keep a component family approach for cost efficiency.
From a value perspective, the E-motor is the most important element in the P2 module. Therefore, a standardized family of E-motors provides significant benefits. The dimensions of the E-motors define the design of other adjoining components in the P2 module, such as clutch housings and friction packages. Key E-motor characteristics are the motor performance data, rotor inner diameter, stator outer diameter and the axial length.020 40 60 80 100 120 140 160 0 50 100 150 200 250 300 350 400 0 1000 2000 3000 4000 5000 6000 7000 Generating Power (kW) Motoring Torque (Nm) Machine Shaft Speed (RPM) BorgWarner On-Axis P2 E-motor Peak Performance.
Clutch design is mainly defined by the transmission concept, actuation concept, rotor inner diameter and targeted axial length. Standardized friction package dimensions can provide solutions for different torque levels by adjusting the number of plates.
Another aspect of the family approach is the possibility to integrate different kind of actuation systems, such as low-pressure conventional or high-pressure CSC type actuation.
Considering the variety of potential technical solutions for the specific applications, close collaboration is required to develop a tailored solution with optimized value.
BorgWarner is actively developing a family of E-motors and clutches for use in P2 modules. These products are configured specifically to maximize performance and benefits to the customer, at the same time being compact in size and allowing for flexible customer integration.