In the rapidly evolving landscape of new energy vehicles, the powertrain system stands as the core component, and its functional strategies are paramount to the overall vehicle development. From the perspective of an engineer deeply involved in the development process, I will elaborate on the methodology for designing and implementing these strategies, focusing on the electric car. This review begins with the architectural design of the powertrain, proceeds to compile a functional development list based on user needs, and then delves into the detailed design of key functions, including high-voltage power-up/power-down control, torque control, charging control, thermal management control, low-voltage power management, and fault diagnosis. The proposed functional development methodology provides theoretical support and engineering practice references for the functional strategy development of pure electric car powertrains. Relevant results have been validated through real vehicle testing, contributing to the technological advancement of the new energy vehicle industry.
The fundamental distinction between a pure electric car and a traditional internal combustion engine vehicle lies in the composition of the powertrain. The driving principle of an electric car primarily involves converting electrical energy from the battery into mechanical energy, where the electric motor drives the wheels to rotate, enabling vehicle movement. This process is realized through the coordinated operation of a series of functional strategies within the powertrain system. For an electric car, the powertrain encompasses several key subsystems: the power battery serves as the energy storage device, providing the necessary electrical energy for vehicle operation; the electric drive system acts as the propulsion unit, converting the battery’s electrical energy into mechanical energy to drive the vehicle; the electronic control system manages and controls the battery and drive systems, encompassing functions such as high-voltage power control, drive torque control, charging control, energy management, and fault diagnosis monitoring. Additionally, the On-Board Charger (OBC) is responsible for converting AC power to DC power to charge the power battery, and the DC-DC converter transforms high-voltage DC power into low-voltage 12V DC power for auxiliary systems. Understanding these components is crucial for developing robust functional strategies for the electric car.
The starting point for functional development is the vehicle function list, which defines the essential functions the vehicle must achieve. This list serves as the directory and纲领 for software function development. The definition of the vehicle function list typically involves several stages. First, we collect requirements from market research, sales configurations, user needs, regulatory requirements, and competitor benchmarking. Next, we translate these collected requirements into specific functional items, clearly defining the function name, description, application scenarios, and implementation methods. The functions are then hierarchically classified into main functions, sub-functions, and use cases (UC). Main functions are based on the user’s primary scenario actions, such as seat adjustment. Sub-functions address细分场景 needs or associated component functions, like seat forward/backward adjustment and backrest adjustment. A UC describes the specific scenario for implementing a sub-function, including steps like input, judgment, and output. In function descriptions, the function name should express the relationship between the vehicle, system, or components, rather than directly naming a part or system. The description must cover the function’s application scenario, user operation, system response, and exception handling. Finally, the list is refined through multiple rounds of review to ensure completeness and accuracy. Building the vehicle function list is a dynamic process that requires continuous iteration to adapt to technological developments and changing user needs. Based on the inputs from this list definition, we can proceed with the functional development of the powertrain system. For instance, Table 1 provides examples of parts of the function list related to the powertrain system for an electric car.
| Main Function | Sub-function | Function Description | Use Case Name |
|---|---|---|---|
| Charging Management | Charging Limit Setting | The user can set the charging cut-off State of Charge (SOC) to reduce the number of full charges. | Set full-charge SOC limit via mobile app; Set full-charge SOC limit via vehicle infotainment interface. |
| Charging Management | AC Charging Gun Unlock | The user can unlock the AC charging gun electronic lock through multiple methods. | Unlock AC charging gun via mobile app; Automatic unlock after charging stop; Automatic unlock when key approaches; Unlock via infotainment interface button. |
| Driving Experience | Switch Vehicle Driving Mode | Switch driving modes according to different driving needs to experience different driving styles. | Switch to Eco mode; Switch to Custom mode; Switch to Sport mode; Switch to Comfort mode; Switch to Super Sport mode. |
| High-voltage Safety | Collision Damage Prevention Control | When a collision occurs, the high-voltage power should be cut off urgently, and hazard warning lights should be activated as an alert. | Automatic high-voltage power cut-off during collision; Automatic activation of hazard warning lights during collision. |
Delving into the detailed development of powertrain functions, we first consider high-voltage power-up and power-down control. This refers to the process of connecting and disconnecting the high-voltage battery to the high-voltage system, which is crucial for ensuring safe operation and protecting high-voltage components in an electric car. High-voltage power-up involves supplying power from the high-voltage battery to high-voltage components like the drive motor, electric air conditioning, and DC-DC converter. High-voltage power-down cuts this connection, placing the vehicle in a non-operational state. The Vehicle Control Unit (VCU) is responsible for arbitrating various high-voltage power-up conditions, checking the self-test completion status of high-voltage system components, and verifying the normalcy of high-voltage interlock, system faults, network communication, etc. Upon meeting all conditions, the VCU allows the high-voltage system to power up and requests the Battery Management System (BMS) to enter the corresponding power-up state. Scenarios for high-voltage power-up include key start, DC/AC charging, vehicle-to-load discharge, remote air conditioning start, remote battery heating, sentry mode, and low-voltage battery automatic replenishment. For power-down, the VCU arbitrates conditions and requests the BMS to enter the corresponding power-down state. After the BMS complies, the VCU simultaneously requests the Drive Control Unit (DCU) to discharge any residual electrical energy on the high-voltage DC bus.
The BMS is tasked with monitoring battery status and controlling high-voltage relays. Before high-voltage power-up, the BMS performs a self-check, detecting battery voltage, temperature, insulation status, etc., to ensure the battery system is safe and fault-free. Upon receiving the power-up command from the VCU, the BMS controls the closure of the main negative relay and the pre-charge relay. The pre-charge circuit charges the bus capacitor in the motor controller. When the bus voltage reaches a certain percentage (e.g., 80% to 95%) of the total battery voltage, pre-charge is deemed complete. The BMS then closes the main positive relay and opens the pre-charge relay, completing high-voltage power-up. For power-down, upon receiving the VCU’s command, the BMS opens the main positive and negative relays. In case of severe faults (e.g., severe overtemperature, overvoltage, undervoltage, thermal runaway), the BMS proactively cuts off the high-voltage circuit for safety.
The DCU is responsible for monitoring the electric drive system status and assisting in pre-charge during power-up and energy discharge during power-down. The high-voltage system contains substantial capacitive loads; direct connection would cause inrush current. The DCU has internal bus capacitors. During power-up, pre-charging these capacitors protects against large voltage transients. During power-down, upon receiving the discharge command from the VCU, the DCU quickly discharges residual energy from the bus capacitors via an internal discharge circuit, ensuring the bus voltage drops to a safe level within a specified time (e.g., 2 seconds). If active discharge fails, passive discharge occurs naturally through the motor controller’s electronic components, though this takes longer, typically within 5 minutes.
Next, driving torque control is fundamental for an electric car. The core principle is converting electrical energy from the battery into mechanical energy via the motor to drive the wheels. This requires coordinated action from the battery system, electric drive system, and VCU. The power battery system stores electrical energy, providing the power source. The BMS monitors the battery system and sends the available discharge power to the VCU based on real-time parameters like temperature, SOC, and voltage, which is used to compute the power available for propulsion. The drive motor operates on electromagnetic induction principles, converting electrical energy to mechanical energy. When the motor controller supplies alternating current to the motor windings, a rotating magnetic field is generated, causing the rotor to spin. This torque is transmitted through the drivetrain to the wheels. The inverter within the DCU is key; based on torque requests from the VCU, it uses Pulse Width Modulation (PWM) signals to control power semiconductor switches (e.g., IGBTs or MOSFETs), regulating the motor’s voltage, frequency, and current to precisely control speed and torque. The DCU also includes protection functions (overload, overtemperature, short circuit, undervoltage, overvoltage) and monitors motor status (current, voltage, temperature), performing fault diagnosis and reporting to the VCU. Furthermore, by reversing current direction, the motor controller enables motor reversal for vehicle reverse gear and regenerative braking. We can summarize motor operating modes:
- Motor speed positive, torque positive: Motor converts electrical energy to mechanical energy, driving wheels forward.
- Motor speed negative, torque negative: Motor reverses, vehicle moves backward.
- Motor speed positive, torque negative: Motor enters generator mode, converting wheel mechanical energy into electrical energy. The motor controller rectifies AC to DC, feeding energy back to the battery. This regenerative braking significantly improves the driving range of an electric car.
The VCU acts as the “brain” for torque control, coordinating all systems. Based on driver inputs (gear selection, accelerator pedal, brake pedal) and vehicle state (speed, battery SOC), it sends commands to the motor controller, BMS, etc., to manage driving torque and energy recovery. The torque transmission path can be represented as a control diagram. Driving gear control is primarily handled by the VCU and gear selector. The gear selector sends signals via CAN bus to the VCU indicating driver intent (D, N, R, P). The VCU synthesizes these with conditions like vehicle speed, door status, brake pedal status, and anti-theft status. Particularly, releasing P must comply with safety standards like GB 15740, and D/R shifts must meet standards like GB/T 18384.2. Once conditions are met, the VCU enables the actual gear and illuminates the corresponding indicator on the instrument cluster. For D gear, the VCU requests positive torque from the DCU; for R gear, negative torque.
The VCU calculates driver demand torque by reading accelerator pedal position and combining it with vehicle state (speed, gear). This is implemented through PedalMAP software calibration, which maps accelerator pedal opening and vehicle speed to specific demand torque. To prevent abrupt torque changes causing jerks, the VCU filters the demand torque, especially near zero torque, using parabolic smoothing. When multiple systems (e.g., chassis safety, cruise control) have conflicting torque requests, the VCU arbitrates based on priority. For energy recovery, during coasting or braking, the VCU requests the DCU to enter regeneration mode based on vehicle state and motor/battery capabilities, enhancing the electric car’s range. The VCU also imposes torque limits for energy management, preventing motor or battery overload. For instance, based on battery max charge/discharge power, motor temperature, and speed, it limits output torque to protect components.
Modern electric cars often offer multiple driving modes. Upon receiving signals from mode selection switches, the VCU alters the PedalMAP relationship to deliver different power outputs and regeneration characteristics. Common modes include:
- Eco Mode: Gentle torque output at initial pedal travel, reduced overall power, enhanced regeneration for lower energy consumption and extended range, suitable for city driving.
- Normal Mode: Linear torque response, moderate regeneration, balanced for daily use.
- Sport Mode: Aggressive torque response at initial pedal travel,接近 maximum torque, reduced regeneration for spirited driving.
The relationship can be summarized with a formula for demand torque \( T_{req} \) as a function of pedal position \( \alpha \) (normalized 0 to 1) and vehicle speed \( v \), with mode-specific coefficients \( k_{mode} \):
$$ T_{req}(\alpha, v) = k_{mode} \cdot f(\alpha, v) $$
where \( f(\alpha, v) \) is the base PedalMAP function. A comparative plot of torque versus pedal position for different modes illustrates these characteristics.
Charging control is essential for replenishing the energy consumed during electric car operation. The principle involves establishing communication between the charging station and the vehicle, performing a handshake, and transferring external electrical energy to the vehicle’s power battery. The charging system comprises external power equipment (AC charging pile, DC charging pile, portable charger), charging interface, OBC (for AC charging), power battery, and BMS. Charging methods are primarily AC (slow) and DC (fast). The process stages are:
- Connection and Communication: The charging gun is inserted; communication protocol is established. For AC, the OBC uses the Control Pilot (CP) signal for handshake; for DC, the BMS uses CAN communication via dedicated pins.
- Charging Preparation: Charging parameters are negotiated; BMS checks battery status and sends allowable charging current/voltage ranges; VCU checks vehicle readiness and requests BMS/OBC to enter charging standby.
- Charging Phase: The charger outputs power based on requested parameters; BMS monitors battery status dynamically, adjusting parameters for safety and efficiency.
- Charging Completion: Upon reaching full SOC or set limit, BMS stops charging; charger cuts power.
Key control strategies involve the VCU integrating signals from charging connectors, BMS, and OBC to manage the sequence. For instance, during DC charging, the BMS communicates directly with the charger to control voltage and current. The charging power \( P_{charge} \) can be expressed as:
$$ P_{charge} = V_{bat} \cdot I_{charge} $$
where \( V_{bat} \) is the battery voltage and \( I_{charge} \) is the charging current, which is modulated based on battery temperature \( T_{bat} \) and SOC, often following a curve like:
$$ I_{charge} = I_{max} \cdot g(SOC, T_{bat}) $$
with \( g(SOC, T_{bat}) \) being a derating function.
Thermal management control is critical for an electric car, as excessive heat reduces motor efficiency and battery performance. The system ensures batteries, motors, and electronics operate within optimal temperature ranges, directly impacting performance, range, and lifespan. The principle involves circulating coolant or refrigerant, combined with sensors and controllers for intelligent heat distribution. A typical thermal management system for an electric car includes cooling circuits (coolant pumps, radiators, fans), heating circuits (PTC heaters, heat pumps), control units (Thermal Management Controller – TMC), sensors (temperature, flow, pressure), and valves/heat exchangers (multi-way valves, electronic expansion valves, chiller). Figure 5 (referenced in original) shows an architecture; we describe key functions:
- E-drive Cooling: When the e-drive generates heat, the DCU requests coolant flow based on its temperatures. The TMC drives the pump and requests radiator fan operation based on coolant temperature to dissipate heat.
- Battery Cooling: Two modes: slow cooling using the radiator (for mild conditions) and fast cooling using the air conditioning system’s chiller (for high heat). The BMS requests cooling mode and flow based on cell temperature. The TMC controls valves to switch circuits and coordinates with the AC system.
- Battery Heating: At low temperatures, the TMC, per BMS request, switches valves to the heating circuit and activates a PTC heater or heat pump to warm the coolant, which then heats the battery via cold plates.
Advanced technologies enhance electric car thermal management:
- Heat Pump Systems: Use refrigeration cycles to move heat from outside to inside for heating, offering higher efficiency than resistive heaters, crucial for winter range.
- Waste Heat Recovery: Recovers heat from the motor or battery for cabin heating or battery warming, improving overall energy efficiency.
- Multi-way Valves: Enable flexible circuit switching for integrated thermal management across battery, motor, and cabin.
- Integration: Combining components (pump, heat exchanger, sensors, controller) reduces parts count and optimizes layout and energy use.
The thermal dynamics can be modeled with a heat balance equation for a component (e.g., battery):
$$ m c_p \frac{dT}{dt} = Q_{gen} – Q_{diss} $$
where \( m \) is mass, \( c_p \) specific heat, \( T \) temperature, \( Q_{gen} \) heat generation (from current \( I \)), and \( Q_{diss} \) heat dissipation to coolant or ambient. The control strategy aims to maintain \( T \) within bounds.
Low-voltage power management is vital since an electric car lacks an alternator. The DC-DC converter supplies 12V power from the high-voltage battery. After high-voltage power-up, the VCU requests the DC-DC to enter generation mode. It can adjust output voltage based on the low-voltage battery’s SOC for optimal charging. During prolonged parking, if the VCU detects low 12V battery SOC, it triggers automatic high-voltage power-up to activate the DC-DC for replenishment, ensuring the electric car remains operable.
Fault handling strategies protect components and ensure safety in an electric car. Common faults in battery systems include over-temperature, over-current, over-voltage, under-voltage, over-charge, relay adhesion, pre-charge failure, cell sampling line fault, and insulation faults. In the e-drive system, faults include over-temperature, under-voltage, IGBT fault, short circuit, over-speed, and bus communication failure. These are categorized by severity with corresponding actions, as summarized in Table 2.
| Component | Fault Level | Vehicle Manifestation |
|---|---|---|
| VCU, BMS, DCU | Minor Fault | Essentially no impact. |
| VCU, BMS, DCU | Moderate Fault | Power reduction/speed limiting; affects driving experience. |
| VCU, BMS, DCU | Severe Fault | Loss of propulsion, emergency power-down; vehicle inoperable. |
| DC-DC Converter | Minor Fault | Temporary power output limitation. |
| DC-DC Converter | Severe Fault | Stops working; prolonged operation leads to loss of auxiliary power, vehicle may become inoperable. |
The strategy involves continuous monitoring by each controller. Upon fault detection, a Diagnostic Trouble Code (DTC) is set, and appropriate actions are taken, such as torque derating or system shutdown. For example, if the BMS detects a cell over-temperature, it may calculate a reduced allowable power \( P_{allow} \):
$$ P_{allow} = P_{max} \cdot \min\left(1, \frac{T_{max} – T_{cell}}{T_{max} – T_{nom}}\right) $$
where \( T_{max} \) is the maximum safe temperature, \( T_{cell} \) the measured temperature, and \( T_{nom} \) a nominal reference. The VCU then uses this limit in torque arbitration.
Functional development verification for an electric car powertrain is essential to ensure performance, safety, and reliability. Based on the function list, we compile a test list covering all functions. Each test item includes steps, design requirements, and results. Table 3 shows a partial example.
| Test Item | Primary Sub-item | Secondary Sub-item | Test Steps | Design Requirements | Test Result |
|---|---|---|---|---|---|
| Power-up/Down State Transition Test | Power-up (On) Test | Normal no-load (sleep) power-up test | Loads off, vehicle in sleep, power-up without brake pedal. | 1. VCU requests power-up start; all nodes complete within 1.2s. 2. With key On, VCU requests DC-DC output 13.75V. | OK |
| Rapid power-up during On-to-power-down | Key On, loads off, during power-down wait states (1/2/3/5/10s or sleep), initiate power-up without brake; repeat 3 times. | If VCU receives power-up request before disconnecting high-voltage, it should abort power-down and maintain current high-voltage state. | OK | ||
| DC Charging Function Test | Physical Connection Phase Test | A+ auxiliary power wake-up test | After vehicle sleep, apply +12V between A+ and A-. | BMS shall be awakened by A+ and awaken other ECAN nodes via network management messages. | OK |
| CC2 resistance wake-up test | After vehicle sleep, connect 1030Ω resistor between CC2 and PE. | BMS shall be awakened by CC2 rising edge, recognize as charging gun half-insertion, and awaken other ECAN nodes. | OK |
Verification involves bench testing, Hardware-in-the-Loop (HIL) simulation, and real vehicle testing under various environmental conditions to validate all functions and fault responses. For instance, torque control is tested on dynos to verify PedalMAP accuracy and mode transitions, while thermal management is validated in climate chambers.
In conclusion, the development of functional strategies for a pure electric car powertrain is a complex, iterative process. Guided by user needs and rigorous engineering, we define comprehensive function lists and implement detailed control strategies for high-voltage management, torque delivery, charging, thermal regulation, low-voltage supply, and fault handling. These strategies are verified through extensive testing to ensure safety, performance, and reliability. Looking ahead, the integration of artificial intelligence, big data, and 5G communication will further propel the intelligence and personalization of electric car powertrain systems. Predictive thermal management, adaptive torque control based on driving style learning, and enhanced fault prediction using machine learning are promising directions. These advancements will deliver safer, more convenient, and efficient mobility experiences, solidifying the role of the electric car in sustainable transportation. The continuous refinement of powertrain functional strategies remains a cornerstone for the evolution of electric vehicles.