CTI Mag: P3 Hybridization of Manual Transmissions
In terms of cost and fuel efficiency, manual transmissions still take a leading position. However, solutions are needed to make them compatible with future ADAS and powertrain hybridization. The GETRAG modular upgrade concept offers a scalable approach, adding automation and electrification on a common MT platform.
Dr Ing. Frank Casimir, Director, Product Segment Manual Transmissions, GETRAG Ford Transmissions GmbH, Magna Powertrain
Why invest into manual transmissions?
Governing bodies of all major markets are planning to continuously tighten CO limits. Between 2020 and 2025, and depending on market specific limits and vehicle sizes, more and more powertrain electrification will be applied to compensate for the limits of pure internal combustion engines. At the same time, advanced driver assistance technologies will increasingly enter the market. Existing systems like pre-collision assist, adaptive cruise control and Active Park Assist will be extended to systems like Traffic Jam Assist, ACC Stop&Go and Fully Assisted Parking. In the longer run, fully autonomous systems will enter the market.
However, these advanced functions cannot be supported with a manual transmission. Therefore, GETRAG has developed a concept to upgrade manual transmissions to make them compatible with future ADAS system, while further reducing CO emissions through hybridization.
The GETRAG Modular Hybrid Concept
GETRAG has long-term experience in developing hybrid designs for layshaft transmissions, which have been implemented into several market-ready products. The existing designs were initially developed for dual clutch transmissions. The transfer to manual transmission is done based on the same principles:
- Off-the-shelf components for specific hybridization and automation elements (like e-motor, inverter, e-clutch actuator, etc.), Figure 1. This will generate economies of scale and further cost advantages.
- Portability of concepts between transmission base architectures, e.g. from DCT to hybrid DCT (HDT) and from manual transmission type to hybrid manual transmission (HMT). This will minimize technical risks, complexity and application development cost.
- Integration of the hybrid function into the transmission. This allows for “turnkey” solutions for the affected vehicle application development, reduce the complexity of control interfaces and, simplify vehicle assembly and EOL testing by providing a fully tested unit to the line.
The upgrade of a base manual transmission based on these principles can be done in three steps. Each of them provides a functional enhancement of the base transmission. The technology upgrade options can be used selectively over time to respond to specific market needs without the risk for dead-end solutions.
E-Clutch, HMT and AHT
Step 1 is the integration of an independently controllable clutch actuator (e-clutch), either through a clutch pedal override system, a clutch-by-wire system (both 3-pedal solutions) or a fully autonomous clutch actuation system (2-pedal). This step can generate moderate CO benefits by enabling idle and stop/start coasting (“sailing”), and support basic parking, vehicle launch/stall prevention and emergency braking assistance function. A secondary but valuable benefit is the prevention of unintended clutch misuse, thus improving durability and reliability.
Step 2 is the integration of an e-motor and inverter into the transmission, creating a hybrid manual transmission (HMT). GETRAG went for a P3 concept, connecting the e-motor to the final drive of the transmission, Figure 2. The 48 V motor/inverter unit is carried over from the GETRAG HDT family (6HDT200 and 7HDT300). The e-motor integration substantially reduces CO emissions. Moreover, the P3 design enables fully autonomous parking using the e-motor.
Step 3 includes the integration of an automatic gearshift unit (e.g. shift drum), thus transferring the hybrid MT into an automated hybrid transmissions (AHT), Figure 3. In this configuration, the P3 arrangement of the e-motor helps to fill the torque interrupts that are typical for AMTs, thus significantly improving shift comfort.
The GETRAG HMT concept
The first HMT development is based on the new 6MTT215 6-speed manual transmission, which was launched end 2016. Within its torque segment, this new transmission is benchmark in terms of torque-to-weight ratio and is intended for the B- and C-car segment.
While the hybrid versions of the GETRAG dual clutch transmission portfolio are set up in P2 configuration, for hybrid versions of manual transmissions a P3 configuration was selected. The main drivers for this decision were Driver-independent low speed electric manoeuvring including autonomous parking
The ability to provide pulling force and fill torque interrupts via the e-motor during shifts.
The hybrid components are integrated as shown in Figure 4. The e-motor is connected with the transmission final drive ring gear by a two-ratio cascade of auxiliary gears with a three way dog clutch (ratio 1 – open – ratio 2) to connect to either of the two ratios or to disconnect. The low ratio step provides sufficient torque to the wheels for low speed electric driving and parking, even on grades. The high ratio step allows to operate the e-motor connected at higher vehicle speeds, offering usable torque for coasting primarily.
Next evolutionary Step: AHT
By replacing the internal manual shift mechanism with an automated actuation (adopted from the DCT150 shift drum actuator), the HMT is converted to an AHT following the modular logic described above.
Other than with dual-clutch transmissions, torque interrupts of AMTs cannot be eliminated by means of an overlapping clutch operation. This was one reason, why GETRAG chose a P3 configuration that in¬tegrates the e-motor at the final drive. This is regarded to be a major feature to gain market acceptance. The ability to fill torque interrupts with the e-motor obviously depends on the available torque of the e-motor. However, even in a “mild” 48 V configuration, shift comfort can be significantly improved. Although an 48 V e-motor generates only moderate torque output, the ratio of the connecting gear set enables torque fill levels of 20 to 40 percent (vs. the corresponding maximum engine torque). Figure 5 shows the available electric torque at each vehicle speed in comparison to the maximum tractive effort provided by the combustion engine.
The resulting vehicle acceleration during shifting with and without electric torque fill is shown in Figure 6. The objective assessment of the shift quality with the AVL Drive tool leads to the conclusion that the achievable shift quality rating is comparable to a good automatic transmission with the exception of the 1–2 WOT upshift which achieves a rating of 7.5 on a 1–10 scale. Therefore an AHT can be considered as a viable upgrade option to the hybrid manual transmission to make it long-term compatible to the requirements of fully autonomous driving.