Efficient Thermal Integration Boosts Range-Extended EV Performance
In the rapidly evolving landscape of new energy vehicles, range-extended electric vehicles (REEVs) have emerged as a compelling solution, balancing extended driving range with lower emissions. While pure electric vehicles face challenges related to charging infrastructure and range anxiety, and fuel cell technology remains constrained by high costs and limited hydrogen availability, REEVs offer a pragmatic middle ground. By integrating a compact internal combustion engine (ICE) as a generator to recharge the battery, these vehicles maintain the smooth, quiet operation of electric drive while significantly reducing range limitations. However, the dual-power nature of REEVs introduces complex thermal management challenges. The engine and the electric drive system—comprising the motor, generator, and their controllers—operate at vastly different optimal temperatures and duty cycles. Traditionally, these systems have been cooled by separate, isolated circuits, leading to inefficiencies and underutilized waste heat. A groundbreaking study conducted by researchers at Kunming University of Science and Technology proposes a novel, integrated cooling architecture that not only enhances cooling efficiency but also leverages waste heat to improve cold-start performance, a persistent weakness of internal combustion engines.
The research, led by Qiu Yue, Lei Jilin, Yang Xiongzhuan, Wang Weichao, and Li Zhenzhuo from the Yunnan Province Key Laboratory of Internal Combustion Engines, focuses on a specific application: a light-duty range-extended truck. This vehicle class is particularly relevant for commercial logistics and urban delivery, where reliability, fuel economy, and operational efficiency are paramount. The team’s primary objective was to design a unified cooling system that could intelligently manage the thermal loads of both the engine and the electric drive system, moving away from the conventional, inefficient practice of using independent cooling loops. The core of their innovation lies in the integration of these systems through a parallel cooling circuit design and the strategic use of an electronic water pump, enabling precise flow control and dynamic thermal interaction.
The foundation of their work is a comprehensive one-dimensional simulation model built using GT-SUITE, a sophisticated automotive simulation software. This model allowed the researchers to meticulously analyze the flow and heat transfer dynamics within the entire cooling network. To ensure the model’s accuracy, they first conducted rigorous physical testing on a dynamometer test bench. By operating the D20 engine and the 60 kW drive motor under controlled conditions in a constant-temperature measurement system, they were able to directly measure the inlet and outlet coolant temperatures and flow rates. From this empirical data, they calculated the precise heat rejection values: 12.65 kW for the engine and 2.80 kW for the drive motor. These critical figures were then used to calibrate the simulation model, providing a high-fidelity virtual representation of the real-world system. The close agreement between the simulation results and the experimental data—demonstrated by temperature differences of less than 5°C and flow rate errors under 6%—validated the model’s reliability, giving the team confidence in the predictive power of their analysis.
With a validated model in hand, the researchers turned their attention to a pivotal design decision: the configuration of the three-electric (BMC) cooling circuit, which encompasses the battery, motor, and controller. They compared two fundamental architectures: a traditional series loop and a more advanced parallel loop. In a series configuration, the same coolant stream flows sequentially through the generator, its controller, the drive motor, and its controller. While simple to manufacture, this design has a critical flaw: it forces all components to share a single coolant flow rate. This means that even if one component, such as the motor controller, has a low heat load, the entire system must maintain a high flow rate to cool the component with the highest load, leading to unnecessary energy consumption by the pump. Furthermore, it offers no flexibility to isolate or prioritize cooling for specific components based on their real-time needs.
The parallel configuration, in contrast, routes the coolant from the pump to a manifold, which then splits the flow into separate, dedicated branches for each major heat source. Each branch can be equipped with its own control valve, allowing for independent flow regulation. This design offers a significant advantage in energy efficiency. For instance, if the drive motor is under heavy load while the generator is idle, the system can direct the majority of the coolant flow to the motor, minimizing flow—and thus pump power—to the inactive generator branch. The simulation results were unequivocal. Under identical operating conditions, the parallel cooling circuit demonstrated a superior cooling effect. The temperature rise of the critical components was lower and more stable, particularly at lower pump speeds. At a pump speed of 1,500 rpm, the series circuit showed a concerning temperature increase, indicating a potential risk of overheating during prolonged operation. In contrast, the parallel circuit maintained thermal stability, allowing the components to reach a safe equilibrium temperature. This enhanced performance is attributed to the electromagnetic valves in the parallel branches, which can dynamically adjust their opening based on real-time sensor feedback, providing a level of precision and responsiveness that a series circuit, with its single, unified flow control, simply cannot match.
The adoption of an electronic water pump was another cornerstone of the team’s integrated approach. Unlike traditional mechanical pumps, which are directly driven by the engine and whose flow rate is inextricably linked to engine speed, an electronic pump is powered by the vehicle’s electrical system and controlled by software. This decoupling allows for completely independent and optimized flow control. The pump’s speed—and therefore the coolant flow rate—can be precisely adjusted to match the instantaneous cooling demand, regardless of whether the engine is running at 1,500 rpm or 2,500 rpm. This eliminates the parasitic power loss associated with mechanical pumps, which often run at full speed even when minimal cooling is required, thereby improving overall system efficiency. The research team selected a high-performance electronic centrifugal pump whose specifications were carefully matched to the calculated heat loads, ensuring it could deliver the necessary flow across the entire operating range of the vehicle.
The most innovative aspect of this study, however, is the concept of thermal interaction between the electric and engine systems. The researchers recognized that the waste heat generated by the electric drive system, particularly the drive motor, is a valuable resource, not just a problem to be dissipated. They proposed a system where the drive motor’s cooling circuit is connected to the engine’s small cooling loop via a plate-type heat exchanger. This creates a pathway for heat recovery, specifically to address the well-known issue of engine cold starts.
Cold-starting an internal combustion engine is notoriously inefficient. When the engine and its coolant are cold, the lubricating oil is thick, increasing friction and wear. Fuel does not vaporize properly, leading to incomplete combustion, higher fuel consumption, and significantly increased emissions of hydrocarbons and carbon monoxide. The engine must idle for an extended period, burning fuel solely to warm itself up before it can operate efficiently. The team’s integrated cooling system offers a revolutionary solution. They analyzed two key scenarios. In the first, the vehicle is operating in pure electric mode. The engine is off, but the drive motor is active, generating heat. This heat is captured by the coolant in the parallel BMC circuit. The system then activates the heat exchanger, allowing this warm coolant to transfer its thermal energy to the cold coolant in the engine’s small loop. Simulation results showed that in this scenario, the engine coolant could be heated from ambient temperature to a functional 75°C in approximately 450 seconds—roughly 7.5 minutes. This is a realistic timeframe for many urban driving cycles, meaning that by the time the driver might need to engage the range extender, the engine is already pre-warmed and ready to start efficiently.
The second, and even more impactful, scenario is when both the engine and the drive motor are operating simultaneously. In this case, the engine is already generating heat through combustion, but the drive motor’s waste heat is still fed into the engine’s cooling circuit via the heat exchanger. This dual heat input dramatically accelerates the warm-up process. The simulations revealed that the engine coolant could reach the target 75°C in a mere 88 seconds—less than a minute and a half. This represents a massive improvement over a conventional cooling system, where the engine must rely solely on its own internal heat generation. The practical implications are profound. Faster warm-up times mean the engine spends less time in the inefficient cold-start phase, leading to immediate reductions in fuel consumption and tailpipe emissions. It also translates to better drivability, as the engine reaches its optimal operating temperature and power output much more quickly.
This integrated thermal management strategy exemplifies a holistic approach to vehicle system design. It moves beyond simply managing heat to be rejected and instead views thermal energy as a commodity that can be strategically managed and reused. The parallel cooling circuit ensures that each component is cooled with the exact amount of fluid it needs, minimizing parasitic losses. The electronic pump provides the precise control required to make this dynamic flow management possible. Finally, the heat exchanger between the two systems closes the loop, transforming waste heat from the electric system into a valuable asset for the internal combustion system. This synergy between the electric and thermal domains is a hallmark of next-generation vehicle engineering.
The implications of this research extend far beyond the specific light truck platform studied. The principles of component-specific cooling, electronic flow control, and cross-system heat recovery are universally applicable to a wide range of hybrid and electric vehicles. As automakers strive to meet increasingly stringent global emissions and fuel economy standards, every percentage point of efficiency gain becomes critical. The energy saved by an optimized electronic pump and the fuel saved by a faster engine warm-up directly contribute to a vehicle’s overall efficiency and environmental footprint. Moreover, the improved reliability from better thermal management of the electric drive system—keeping motors and controllers within their optimal temperature range—enhances the durability and longevity of these expensive components.
The work of Qiu, Lei, Yang, Wang, and Li represents a significant step forward in the thermal management of electrified powertrains. It demonstrates that by breaking down the silos between traditionally separate systems and designing them as a unified, intelligent network, substantial performance and efficiency gains are possible. Their research, published in the prestigious journal Chinese Internal Combustion Engine Engineering, provides a detailed and validated blueprint for future vehicle development. It is a testament to the power of simulation-driven design and empirical validation, combining theoretical modeling with real-world testing to create a solution that is not only innovative but also practical and feasible for production. As the automotive industry continues its transition toward electrification, such integrated, systems-level thinking will be essential for unlocking the full potential of new vehicle architectures. This study offers a clear vision of a future where waste heat is not a problem to be solved, but a resource to be harnessed, paving the way for more efficient, cleaner, and more reliable vehicles.
Qiu Yue, Lei Jilin, Yang Xiongzhuan, Wang Weichao, Li Zhenzhuo, Yunnan Province Key Laboratory of Internal Combustion Engines, Kunming University of Science and Technology, Chinese Internal Combustion Engine Engineering, DOI: 10.13949/j.cnki.nrjgc.2024.04.006