New Co-Simulation Framework Brings PEM Fuel Cell Systems Closer to Real-Time Virtual Testing

Hydrogen fuel cells have long promised clean mobility, but turning that promise into a production-ready automotive powertrain requires an extraordinary level of engineering detail. A proton exchange membrane (PEM) fuel cell stack is not an isolated device; it sits at the center of a web of pumps, compressors, humidifiers, valves and cooling circuits that together determine how much useful electricity actually reaches the wheels. Understanding how the electrochemical heart of the system interacts with these auxiliary components, known as the balance of plant, has been one of the persistent modeling challenges in fuel cell development. Now, researchers at Graz University of Technology and the Technical University of Darmstadt have unveiled a modular co-simulation framework that couples a physics-based stack model with a dynamic system-level balance-of-plant model, allowing engineers to analyze the entire fuel cell system as one coherent, transient entity rather than as fragmented pieces.

The study, published in the journal Automotive and Engine Technology, addresses a long-standing trade-off in fuel cell simulation. Highly detailed stack models, which resolve the electrochemistry, heat transfer and fluid dynamics inside individual cells, are computationally expensive, making them impractical for full-vehicle or system-level investigations where thousands of operating seconds must be simulated. On the other end of the spectrum, reduced-order system models run quickly but often strip away the physical fidelity needed to capture how electrochemical processes respond to rapid changes in load, air supply and temperature. The new framework bridges this gap by running a physics-based stack model and a dynamic balance-of-plant model in parallel, exchanging data at every time step so that electrochemical, thermal and auxiliary losses are all accounted for within a single simulation environment.

Technically, the framework is implemented in AVL CRUISE™ M, a system simulation platform widely used in powertrain engineering, and MATLAB®, with the two tools synchronized during the simulation. The researchers evaluated their approach under representative transient operating conditions, the kind of rapidly fluctuating duty cycles that mirror real-world driving, including scenarios aligned with standard vehicle test cycles such as the Worldwide Harmonized Light Vehicles Test Cycle. A central contribution of the work is its systematic assessment of solver synchronization and coupling strategies, examining how the choice of data exchange scheme affects numerical stability, simulation accuracy and computational performance. This matters because co-simulation introduces a fundamental risk: if two solvers exchange information too infrequently or inconsistently, the coupled system can become unstable or produce physically meaningless results, particularly during sharp transients such as sudden acceleration demands.

The headline result is striking: the coupled framework achieved a real-time factor of 1.74, meaning the simulation ran faster than the physical process it represents. For automotive engineers, this figure is significant beyond mere convenience. Real-time capability is the entry ticket to hardware-in-the-loop testing, where a physical controller, such as an actual fuel cell management electronic control unit, is connected to the virtual plant and must respond to simulated signals within strict timing limits. It also opens the door to digital twin implementations, in which a running fuel cell system in a vehicle or test bench is mirrored by a live simulation model that can predict behavior, flag anomalies and support predictive maintenance. A framework that is both physically grounded and fast enough for real-time operation therefore has immediate practical value.

Equally important is what the framework reveals about where the energy actually goes. By integrating the stack model with the balance-of-plant model, the researchers performed an integrated loss analysis that links electrochemical stack losses with system-level inefficiencies. The results show that overall system efficiency declines from 49.6 percent at low load to 38.8 percent at high load, and that the relative contribution of individual loss mechanisms shifts dramatically with operating conditions. At low loads, activation losses associated with the sluggish kinetics of the oxygen reduction reaction at the cathode dominate, while at higher current densities ohmic losses from membrane resistance and concentration losses caused by mass transport limitations grow in importance. Simultaneously, the auxiliary components consume an increasing share of the produced power, since the air compressor must work harder to deliver the oxygen the stack demands at high current.

This load-dependent loss structure has direct implications for how fuel cell vehicles are designed and operated. A fuel cell powertrain that is efficient during gentle city driving may shed several percentage points of efficiency during highway cruising or hill climbing, not because the electrochemistry itself becomes dramatically worse, but because the balance of plant consumes more and the stack crosses into loss-dominated regimes. With a validated co-simulation environment, engineers can quantify these trade-offs before a single physical prototype is built. They can explore, for example, how a more efficient compressor, a revised humidification strategy or an altered operating temperature might shift the efficiency map, and how those choices interact with transient behavior during real driving profiles rather than idealized steady-state points.

The modular architecture of the framework is another distinguishing feature. Unlike existing co-simulation approaches that focus primarily on stack-internal behavior, the new design treats the stack and the balance of plant as interchangeable modules with well-defined interfaces. This means individual components can be swapped, refined or replaced without rebuilding the entire simulation. A researcher could substitute a more detailed compressor model, test a different membrane-electrode-assembly characterization, or plug in an alternative cooling concept while keeping the rest of the environment intact. Such flexibility is essential in industrial practice, where simulation tools and model detail levels evolve continuously across development programs and where different teams may be responsible for different subsystems.

The broader context makes this work timely. As automakers ramp up hydrogen programs for heavy-duty trucks, buses and passenger vehicles, the pressure to compress development cycles while meeting ever-stricter efficiency and durability targets has intensified. Fuel cell durability is highly sensitive to operating conditions: water management, membrane humidity, temperature gradients and load transients all influence degradation mechanisms. A system-level simulation environment that faithfully represents the interplay between the stack and its auxiliaries under realistic transient loads provides a virtual proving ground for control strategies that balance performance against long-term reliability. The authors specifically point to control strategy development, hardware-in-the-loop testing and digital twin implementations as the natural next applications of their framework.

In an era when the hydrogen economy is alternately hyped and dismissed, studies like this one do the unglamorous but essential work of turning chemistry into drivable engineering. By demonstrating that a physics-based stack model and a dynamic balance-of-plant model can be coupled stably, accurately and fast enough for real-time use, the Graz and Darmstadt researchers have given the fuel cell community a practical tool for answering one of its most consequential questions: not just how efficiently a membrane can convert hydrogen into electricity in isolation, but how an entire system behaves when the road demands everything at once. With system efficiency spanning nearly eleven percentage points between low and high load, and with loss mechanisms shifting beneath every change of the accelerator pedal, that system-level view may prove to be exactly what the next generation of hydrogen vehicles needs.

Beyond the headline performance figures, the publication carries several details that will interest specialists in simulation methodology. The framework’s modular design is consistent with the Functional Mock-Up Unit (FMU) standard, an industry convention that allows models developed in different tools to be packaged with standardized interfaces and exchanged between simulation environments. Adherence to such conventions is what makes a co-simulation genuinely cross-tool rather than a bespoke pairing of two specific software packages, and it explains how a physics-based stack model developed in MATLAB can coexist with a commercial powertrain platform like AVL CRUISE M without forcing either side to abandon its native solver.

The abbreviation list of the paper also hints at the physical depth retained inside the stack module. Terms such as area-specific resistance, gas diffusion layer, bipolar plate, and membrane-electrode-assembly indicate that the model resolves the layered structure of an individual cell rather than treating the stack as a black box. Likewise, the explicit distinction between the hydrogen oxidation reaction at the anode and the oxygen reduction reaction at the cathode, alongside side reactions such as hydrogen evolution, suggests that the electrochemical submodel distinguishes loss pathways that simplified models typically lump together.

Another noteworthy aspect is the choice of reference for efficiency. The reported values are given on the basis of the higher heating value, the more conservative convention that accounts for the latent heat of water vaporization. Efficiencies quoted on a higher heating value basis are systematically lower than those quoted on the lower heating value basis often used in combustion engineering, which is worth remembering when comparing the reported 49.6 percent peak against figures from other fuel cell studies.

The institutional collaboration itself is instructive. The work emerged from the Institute of Thermodynamics and Sustainable Propulsion Systems in Graz and the Institute for Internal Combustion Engines and Powertrain Systems in Darmstadt, both groups with deep roots in conventional powertrain simulation now applying that heritage to hydrogen systems. Published open access with funding from Graz University of Technology, and having passed through peer review between May and August 2026, the study is freely available to the development community it aims to serve.

Subject of Research: Integrated co-simulation of PEM fuel cell systems coupling electrochemical stack modeling with balance-of-plant dynamics for automotive applications

Article Title: Integrated co-simulation of PEM fuel cell systems for automotive applications: modeling, coupling, and system behavior

Article References: Strametz, F., & Endres, J. (2026). Integrated co-simulation of PEM fuel cell systems for automotive applications: modeling, coupling, and system behavior. Automotive and Engine Technology. https://doi.org/10.1007/s41104-026-00177-x

Image Credits: AI Generated

DOI: 10.1007/s41104-026-00177-x

Keywords: PEM fuel cell, co-simulation, balance of plant, fuel cell stack modeling, system-level simulation, real-time simulation, loss analysis, AVL CRUISE M, MATLAB, automotive powertrain, digital twin, hardware-in-the-loop