The Role of the VCU (Vehicle Control Unit) in Electric Vehicles
The Role of the VCU in Electric Vehicles In traditional internal combustion engine (ICE) vehicles, control systems are relatively segmented, with components like the ECU, TCU, ABS, BCM, PEPS, and IPC. In electric vehicles (EVs), the engine and transmission are replaced by the electric motor and battery system. This shift introduces new key systems like the BMS and MCU. Beyond this fundamental shift, EVs also integrate a broad array of high- and low-voltage systems, such as DC/DC converters, onboard chargers (OBC), PTC heaters, and electronic braking systems (EBS), alongside intelligent controllers including the In-Vehicle Infotainment (IVI) system, Thermal Management System (TMS), Telematics Control Unit (T-Box), and Integrated Power Brake (IPB), etc. Why is a VCU Essential for Electric Vehicles? As vehicles evolve, so does the complexity of electronic controls. The number of controllers on board has significantly increased, especially in hybrid electric vehicles, where coordination between the traditional engine system and the electric drivetrain is critical. For example, when the ECU and MCU issue conflicting commands, which system should take priority? This type of decision-making requires a central coordinator – the VCU – to act as the vehicle’s brain. Beyond conflict resolution, EVs also have higher demands for drivability, energy efficiency, and real-time coordination across multiple subsystems. A VCU is necessary to optimize energy use, balance power delivery, improve safety, and ensure consistent performance across various scenarios. VCU Functional Overview The functions of the VCU vary depending on the overall vehicle system architecture. They can be categorized into several functional domains, including vehicle system control, powertrain management, electric power systems, thermal management, diagnostics, communication, and safety monitoring. Key functions include torque control and management, overall energy management, charging and thermal management, fault diagnosis and handling, as well as vehicle status monitoring. 1. Torque Management Torque management governs a vehicle’s acceleration and braking performance – both of which are determined by torque output from the electric motor or engine. The VCU interprets signals from the accelerator and brake pedals (e.g., depth and speed of press), determines the required torque, and coordinates the engine, generator, and coordinates the engine, generator, front and rear drive motors to respond accordingly based on the vehicle’s current operating mode. Case Example: Torque Distribution in AWD EVs All-wheel-drive EVs (as opposed to two-wheel-drive models) feature both front and rear motors. The VCU is responsible for intelligently distributing torque between the two, depending on the efficiency (economy), performance, and stability. The goal of economic torque distribution is to achieve optimal overall efficiency under the current torque demand. This involves intelligently coordinating dual-, triple, or quad-motor systems to ensure the most efficient power distribution, reducing energy consumption and extending battery range. This strategy is typically applied during steady-speed driving scenarios, such as cruising on highways. In the performance-oriented torque distribution, the load distribution function calculates the optimal torque ratio between the front and rear axles by recognizing current road gradients and the vehicle’s acceleration or deceleration status. By building a dynamic load model, the system automatically adjusts torque distribution during load transfers to make full use of the maximum available tire grip. This reduces wheel slip and enhances the vehicle’s acceleration performance. Performance-oriented torque distribution must especially account for scenarios where a wheel becomes stuck and starts to slip, such as in mud or loose terrain. The basic principle is to actively adjust the torque distribution between the front and rear axles, transferring power to the axle that still has traction when one is slipping. This helps reduce power loss. When alternating slip between the front and rear axles is detected, the system dynamically adjusts torque distribution to maximize available grip, enhancing the vehicle’s ability to escape low-speed traction challenges. Stability-oriented torque distribution focuses on maintaining vehicle stability during steering maneuvers. While systems like ESP are designed to ensure body stability, frequent ESP interventions can lead to an uncomfortable driving experience. By monitoring steering behavior and controlling steering torque, the VCU can proactively adjust front and rear axle torque distribution in real time – before ESP activation – to correct vehicle dynamics. This helps suppress understeer (US) and oversteer (OS), reducing the need for ESP intervention during acceleration and cornering. As a result, it minimizes braking jolts and yaw disturbances, enhancing overall driving comfort and control. When calculating the drive torque, it’s also necessary to consider the vehicle’s driving mode. Under different modes, the accelerator pedal position and vehicle speed are used to first determine the base drive torque. This base drive torque typically corresponds to the ECO mode. If the vehicle is in Normal mode, an additional compensation torque is applied. In Sport mode, a larger compensation value is added to enhance performance. 2. Mode & Energy Management 2.1 Operating Modes In addition to driving modes, vehicles – especially hybrid models – also operate under different operation modes. These include pure electric mode, series (range extender) mode, and parallel mode. While both driving modes and operation modes aim to optimize energy efficiency and power distribution, they differ in how they are set: Driving modes (such as ECO, Normal, and Sport) are manually selected by the driver. Operation modes are automatically determined by the vehicle. In pure electric mode, the vehicle is powered solely by the electric motor using energy from the battery. The engine remains off. In series mode (range extender mode), the engine, generator, and battery are connected in series. When the battery’s state of charge is low, the engine activates the generator to produce electricity, which recharges the battery – effectively extending the vehicle’s range. In parallel mode (direct drive mode), both the engine and the battery provide propulsion simultaneously. Typically, the engine drives one axle while the electric motor powers the other. Since the engine directly contributes to wheel drive in this mode, it is also referred to as direct drive mode. So how does the vehicle decide whether to use fuel or electricity? The decision of whether to use fuel or electricity in different driving scenarios is made based on energy efficiency.
