Design and development of SAIC dual clutch transmission (DCT)

This article introduces the development process of DCT based on the development process of SAIC dual-clutch transmission (DCT).

1 Design and development of DCT

In the early stage of project development, we investigated the development trend of gearboxes in the market, customer needs and combined with internal matching needs, focused on low fuel consumption and low cost, and took into account the platformization and co-production of parts to set development goals. Since its development involves collaboration between multiple development departments, in order to better manage the project development progress and avoid risks, SAIC adopted the Global Powertrain Development Process (GPDP) and established 9 project nodes from PG9 to PG1 to ensure the development status of each stage.

1.1 Hardware Design and Development

In terms of DCT hardware design, firstly, according to the design input, the design of each small assembly scheme, the 3D digital model of the assembly, and the preparation of the overall layout are completed, and the development, design and verification of the gear shaft, housing, differential, and lubrication system are completed using computer-aided engineering (CAE). Secondly, the selection and positioning of purchased parts should be done well, such as dual clutch, valve body, synchronizer, parking mechanism, bearing, etc. The layout of several important parts will be introduced below.

1.1.1 Determination of gear shaft arrangement and overall arrangement

The gear shaft arrangement design of the gearbox not only has an important impact on the mass and length of the gearbox, but is also related to the control strategy of the gearbox. Because the two clutches of the dual-clutch gearbox control different gears respectively, the position of the shift fork pre-engagement after the vehicle stops will affect the control strategy of starting. Figure 1 shows the gear shaft arrangement and the corresponding shift principle matrix table of a gearbox of SAIC. After the vehicle stops, the 1st gear and the R gear of this gearbox are controlled by two different clutches respectively, avoiding the risk of jamming caused by the gear shift operation when starting a cold car.

Figure 1 Gear shaft arrangement and shifting principle matrix

After the gear shaft arrangement is locked, the layout inspection of the gearbox in the whole vehicle is also essential. The hardware related to the gearbox mainly includes: left longitudinal beam, subframe, battery tray, transmission shaft, steering gear and other parts. These vehicle parts are easy to interfere with the gearbox. It is necessary to check the layout of the gearbox in the whole vehicle according to input conditions, such as suspension stiffness, mass, inertia and center of gravity of the powertrain, DCT gear ratio, maximum engine torque, suspension position and angle, center position of the drive half-shaft moving section, etc. Figure 2 shows the overall layout of a DCT project in the whole vehicle.

Figure 2: Transmission overall layout and partial dynamic envelope diagram

1.1.2 Speed ​​ratio design

There are many evaluation indicators for speed ratio selection. When designing DCT speed ratio, it is generally necessary to consider constant speed fuel consumption, New European Driving Cycle (NEDC) fuel consumption, engine effective power, starting ability, emissions, maximum speed, climbing performance, the lowest stable speed of the highest gear, the stable speed of the lowest gear, and calculation of the power factor of each gear. Figure 3 is a partial process diagram of speed ratio design.

Figure 3 Speed ​​ratio design process diagram

1.1.3 Finite element analysis of shell

The forward modeling process of the shell is shown in Figure 4. The finite element analysis results of the shell are as follows:

(1) Vibration characteristics analysis: transient response analysis of the box and structural noise analysis;

(2) Gearbox structural strength analysis: static strength analysis, dynamic strength analysis, fatigue analysis, etc.;

(3) Testing: baseline test, calculation boundary condition test, and final test evaluation and analysis.

The analysis conditions mainly include: 100% torque transmission in first gear, steering condition, maximum vehicle acceleration condition, maximum vehicle deceleration condition and maximum gearbox jump condition.

Figure 4 Forward modeling analysis

FIG5 is a partial process diagram of the finite element analysis of the shell, showing the stress conditions of the shell under different impacts.

Figure 5 Shell stress cloud computing

1.1.4 Lubrication system design

The design of lubrication strategy is crucial for dual-clutch transmissions. The formulation of this strategy will affect the total amount of oil added to the transmission and the oil stirring power loss of the system. In particular, wet dual-clutch transmissions are more sensitive to lubrication strategies. Currently, there are three main lubrication methods: forced lubrication, guided lubrication, and splash lubrication. The formulation of lubrication strategies for each part, the source of lubricating oil, and the driving source requires an overall design plan. Table 1 shows the lubrication system strategy of a certain transmission.

Table 1 Lubrication strategy for gearbox

1.1.5 Hydraulic system and clutch design

The design of the hydraulic system affects the speed of the gearbox shift response and the gear shift noise. Therefore, when designing the hydraulic system, the control strategy of pressure and flow, the design and selection of low-leakage or zero-leakage valve bodies and high-precision solenoid valves are crucial. Taking the pressure analysis of the fork piston cylinder as an example, the working process of the fork QVFS valve is first analyzed, as shown in Figure 6.

Figure 6 QVFS valve working process

At present, there are three main oil supply methods for hydraulic systems: pure mechanical pump, electronic pump + mechanical pump, and pure electronic pump. The latter two are the mainstream. Dual clutches are divided into dry and wet types, and there are many differences in design and drive type, including torque, capacity, spatial layout, mass-level moment of inertia, etc. [1]. At present, dual clutch technology is still in the hands of foreign suppliers, so the clutch should be selected as a purchased part according to design requirements.

1.2 Software and Calibration Development

In addition to the advantages in hardware, an excellent gearbox also has high requirements in software, which are mainly reflected in fast gear shift response, smooth gear shift and high safety. The software development of DCT generally refers to the Standard for Software Process Improvement and Capability Measurement (ASPICE) or Capability Maturity Integration Model (CMMI) process. Taking ASPICE as an example, it mainly goes through the stages of requirement definition, software design, software testing and system testing, as shown in Figure 7. In the early software development process, according to the requirements of safety regulations, vehicle functions, basic functions and other controller interaction functions, the system requirements and system architecture are formed, and then the software requirements are decomposed and the software architecture and detailed design are carried out. Then, with the help of Matlab/Simulink software tools, the control model is established and unit testing and loop model (MIL) testing are carried out. After the controller prototype is produced, the initial software code is generated and refreshed into the controller for hardware loop (HIL) testing, and the requirements defined in the early stage are tested. After passing, it enters the calibration stage.

Based on the software that has passed the initial test, it has been continuously optimized to form 50% of the calibration data released to support bench testing, mainly to achieve the basic functions of the gearbox and the performance of the hydraulic layer to achieve the phased goals. After the engine software and vehicle development reach the matching stage, performance calibration begins on the vehicle. During the calibration process, the external environment (temperature, road conditions), driver habits (throttle opening, braking), engine and peripheral controller status, and gearbox status and other parameters must be considered comprehensively, and continuous optimization is performed, and finally the phased software is released for vehicle testing. During the vehicle test process, in addition to normal durability tests, it is also necessary to undergo environmental tests (high cold, high heat, plateau) adaptability tests, and continuously release data at the 65%, 80%, and 99% stages, and finally form a 100% calibration release.

1.3 Design Verification and Manufacturing

1.3.1 Experimental design

After the hardware and software design is initially completed, in addition to problem checking and verification and simulation, it is also necessary to verify through experiments. The experimental verification is divided into two parts: bench and vehicle. The working conditions of the bench test include input speed, torque, time and other factors. The working condition design requires the transformation of the vehicle load spectrum, as shown in Figure 8.

Figure 7 Software development process

Figure 8. Road load spectrum raw data

The gearbox load spectrum is a statistical collection of load data collected from typical roads and load conditions, taking into account the usage of different customers. The collected data is calculated through comprehensive calculations to be equivalent to the full life mileage of the vehicle, and is ultimately used for the design analysis and bench testing of the gearbox [2]. The load spectrum is generally converted into test conditions with reference to the Miner fatigue accumulation calculation formula.