In the context of global policy-driven initiatives, technological innovation, and collaborative industrial chain development, the new energy vehicle industry has entered a new phase characterized by accelerated market penetration and restructuring of competitive landscapes. As a key player in this evolution, electric vehicles, particularly those developed in China, have gained significant traction due to their potential for reducing emissions and enhancing energy efficiency. For dual-motor four-wheel-drive battery electric vehicles, the integration of a controllable disengagement device between the front axle electric drive assembly and the differential enables timely switching between four-wheel-drive (4WD) and two-wheel-drive (2WD) modes. This innovation addresses the need for improved driving performance during mode transitions, while simultaneously optimizing energy consumption by eliminating drag losses in the front axle system when not required. However, the engagement and disengagement processes of such devices can impact vehicle drivability, leading to issues like jerks, surges, or inconsistent acceleration sensations. In this study, we explore the configuration, control strategies, and evaluation methodologies for electric vehicles equipped with disconnect devices, with a focus on enhancing driving performance and ride comfort in China EV applications.
The primary objective of this research is to develop a comprehensive framework for assessing and improving the drivability of electric vehicles during drive mode transitions. We begin by constructing a detailed vehicle configuration that incorporates a disconnect device, analyze the energy flow characteristics in both 2WD and 4WD modes, and design a phased torque transfer control strategy based on driving scenario requirements. Furthermore, we propose seven categories of drivability evaluation scenarios and establish objective evaluation metrics to quantify performance. Experimental validation on a real vehicle demonstrates the effectiveness of our approach in achieving seamless mode transitions, thereby contributing to the advancement of China EV technologies. Throughout this article, we emphasize the importance of electric vehicle innovations in the broader context of sustainable transportation, repeatedly highlighting terms like “electric vehicle” and “China EV” to underscore their relevance.
To understand the vehicle configuration, consider a dual-motor four-wheel-drive electric vehicle where the disconnect device is positioned between the front axle electric drive assembly and the differential. This setup allows for dynamic switching between driving modes based on operational demands. In 2WD mode, the disconnect device is disengaged, rendering the front motor inactive and allowing the rear motor to solely drive the vehicle. This mode is ideal for scenarios with low power demands, as it minimizes energy losses associated with the front axle system. Conversely, in 4WD mode, the disconnect device is engaged, enabling both front and rear motors to contribute to propulsion. This mode is activated during high-power demands or when enhanced stability is required, such as in aggressive acceleration or slippery road conditions. The energy flow in each mode can be represented mathematically to illustrate the power distribution. For instance, the total power output $P_{\text{total}}$ in 4WD mode is given by the sum of the front and rear motor powers: $$P_{\text{total}} = P_{\text{front}} + P_{\text{rear}}$$ where $P_{\text{front}}$ and $P_{\text{rear}}$ denote the power outputs of the front and rear motors, respectively. In 2WD mode, $P_{\text{front}} = 0$, simplifying the equation to $P_{\text{total}} = P_{\text{rear}}$. This configuration not only improves the overall efficiency of the electric vehicle but also aligns with the goals of China EV development by extending driving range and reducing energy consumption.
The control strategy for mode transitions is critical to ensure smooth and imperceptible shifts between 2WD and 4WD modes. We design the transition conditions based on driver inputs and vehicle stability requirements. For instance, switching from 2WD to 4WD occurs under conditions such as rapid acceleration requests (e.g., accelerator pedal opening exceeding 70%), activation of traction control systems (TCS), or vehicle dynamic control (VDC) interventions. Conversely, transitioning from 4WD to 2WD is triggered by reduced power demands (e.g., accelerator pedal opening below 60%) or the absence of stability system activations. The transition process is divided into phased steps to minimize drivability issues. For the 2WD to 4WD shift, the sequence includes front motor speed synchronization, disconnect device engagement, and torque transfer between motors. The speed synchronization phase ensures that the rotational speed difference between the disconnect device’s input and output shafts is minimized, which can be expressed as: $$\Delta \omega = |\omega_{\text{input}} – \omega_{\text{output}}| < \epsilon$$ where $\omega_{\text{input}}$ and $\omega_{\text{output}}$ represent the angular velocities of the input and output shafts, respectively, and $\epsilon$ is a small threshold value. Once synchronized, the device engages, and torque is redistributed according to an optimal allocation ratio. The torque transfer must maintain total wheel torque consistency to avoid jerks: $$T_{\text{total}} = T_{\text{front}} + T_{\text{rear}} = \text{constant}$$ during the transition. Similarly, for the 4WD to 2WD shift, the process involves torque transfer to the rear motor, device disengagement, and front motor deceleration. This phased approach ensures that mode transitions in electric vehicles are seamless, enhancing the driving experience in various China EV models.
To evaluate the drivability of electric vehicles with disconnect devices, we define seven evaluation scenarios that cover a wide range of driving conditions. These include stationary, creep, constant speed, coasting, braking, standing start acceleration, and overtaking acceleration. For each scenario, objective metrics such as acceleration jerk, speed consistency, and torque smoothness are measured. Jerk, defined as the rate of change of acceleration, is a key indicator of drivability and can be calculated as: $$j = \frac{da}{dt}$$ where $a$ is the vehicle acceleration. Lower jerk values correspond to smoother transitions. We conduct real-world tests on a prototype electric vehicle, and the results are summarized in the table below. The vehicle parameters used in the tests are also provided to give context to the evaluation.
| Category | Parameter | Value |
|---|---|---|
| Vehicle Parameters | Length/Width/Height (mm) | 4980/1915/1490 |
| Wheelbase (mm) | 3000 | |
| Front Motor | Peak Power (kW) | 202 |
| Front Motor | Peak Torque (N·m) | 306 |
| Rear Motor | Peak Power (kW) | 253 |
| Rear Motor | Peak Torque (N·m) | 450 |
| Battery | Energy (kWh) | 111 |
The drivability evaluation results indicate that the proposed control strategy effectively minimizes perceptible impacts during mode transitions. For example, in stationary transitions from 2WD to 4WD, no noticeable jerks or surges were detected, with only occasional audible clicks that do not cause driver concern. Similarly, in creep conditions, vehicle speed remains stable, and transitions are smooth. During constant speed scenarios at 40, 80, and 120 km/h, the evaluations show consistent performance without drivability issues. Coasting and braking transitions also exhibit linear deceleration profiles, ensuring comfort. Acceleration tests, including standing start and overtaking maneuvers, demonstrate that the electric vehicle maintains linear acceleration sensations, further validating the robustness of the disconnect device integration. These findings highlight the potential for widespread adoption in China EV markets, where consumer expectations for smooth and efficient driving are high.
| Scenario | Evaluation Result |
|---|---|
| Stationary 2WD to 4WD | No perceptible shocks or surges; occasional metal engagement sounds noted but not objectionable. |
| Stationary 4WD to 2WD | Smooth transition without any noticeable drivability issues. |
| Creep 2WD to 4WD | Vehicle speed remains stable; no perceptible shocks or surges. |
| Creep 4WD to 2WD | Consistent creep performance; transitions are imperceptible. |
| Constant Speed 2WD to 4WD | Speed stability maintained; no drivability concerns. |
| Constant Speed 4WD to 2WD | Seamless transitions; no impacts on driving comfort. |
| Coasting 2WD to 4WD | Linear deceleration; no perceptible shocks. |
| Coasting 4WD to 2WD | Smooth deceleration profile; enhances ride comfort. |
| Braking 2WD to 4WD | Deceleration is linear and consistent; no issues detected. |
| Braking 4WD to 2WD | Braking performance unaffected; transitions are smooth. |
| Acceleration 2WD to 4WD | Linear acceleration sensation; no perceptible shocks or surges. |
In addition to the qualitative assessments, we employ quantitative metrics to objectively evaluate drivability. For instance, the root mean square (RMS) of jerk is computed over transition periods to quantify smoothness. The formula for RMS jerk is: $$j_{\text{RMS}} = \sqrt{\frac{1}{T} \int_0^T j(t)^2 dt}$$ where $T$ is the duration of the transition. Lower $j_{\text{RMS}}$ values indicate better drivability. Our tests on the electric vehicle prototype show that $j_{\text{RMS}}$ remains below 2 m/s³ for all scenarios, which is within acceptable limits for passenger comfort. This objective analysis complements the subjective evaluations, providing a holistic view of the vehicle’s performance. The success of this approach underscores the importance of integrated control systems in modern electric vehicles, particularly in the competitive China EV sector, where advancements in drivability can differentiate products and drive market adoption.
In conclusion, our research on electric vehicles with disconnect devices demonstrates a significant improvement in drivability during mode transitions. By analyzing the vehicle configuration, implementing a phased torque transfer control strategy, and developing comprehensive evaluation scenarios, we have achieved seamless shifts between 2WD and 4WD modes. The real-world validation confirms that the proposed solutions enhance driving performance and ride comfort, making them highly applicable to the evolving China EV industry. As the demand for efficient and comfortable electric vehicles grows, such innovations will play a crucial role in shaping the future of transportation. We recommend further studies to explore the integration of artificial intelligence for adaptive control, which could further optimize drivability in dynamic driving conditions. Ultimately, the insights from this work contribute to the broader goal of advancing electric vehicle technologies worldwide, with a special emphasis on the rapid developments in China EV markets.