Simulation is one of the first steps in almost all development activities and this is especially true in the automotive industry. As computing power increases and more sophisticated and accurate models are developed, simulation activities become even more important in concept definition, design, pre-calibration and even vehicle validation. Depending on the targeted application and available resources, simulation activities can be performed with different levels of detail and accuracy. FEV often proposes a multi-level simulation approach, covering the range from the pre-concept phase to function development for electrified vehicles. Using a very detailed simulation model in the pre-concept or concept phase is not only impractical due to limited time and lack of input data, but it is also unnecessary, considering the purpose of the simulation activity. Highly detailed models are, on the other hand, typically needed in plant models for hardware-in-the-loop (HiL) or model-in-the-loop (MiL) applications. The benefits of the FEV multi-level system simulation approach for hybrid and electric vehicles are twofold: First, it is flexible in adapting to customer needs and project boundary conditions (e.g. budget, availability of data and time constraints), and secondly, its capability to covering all of the simulation steps, from the pre-concept phase through to the start of production. Since each simulation level compliments the previous one, input data and models can be re-used in the next level in order to save time and maintain consistency. Furthermore, simulation results can be easily verified using the outcomes obtained in the previous step.
Level 0 System Simulation:
Level 0 system simulation is the highest level in FEV’s system simulation toolchain for simulating hybrid and electric vehicles. This level is based on an open-loop simulation approach with user interfaces in Microsoft Excel® and focuses on the key powertrain components that have the highest impact on energy consumption and performance. For example, to evaluate fuel consumption over a cycle, an engine fuel flow map with a correction factor accounting for engine warm-up—and, if applicable, catalyst heating—is included. However, for the sake of simplicity, transmission efficiency is assumed to be constant and independent of temperature or engine speed. “Using this lean method allows us to simulate a hybrid powertrain with minimum input data, in a minimum amount of time, and with acceptable accuracy”, explains Dr. Michael Stapelbroek, Department Manager Hybrid and E-Mobility at FEV. “However, one drawback to this lean approach is obviously lower accuracy. By finding a good compromise between model simplification and accuracy of the simulation in the Level 0 model, we are able to perform the complete simulation without use of other commercially available software tools and yet achieve about 90% accuracy versus measurements.”
Extensive Library of Hybrid Topologies
FEV has developed a library of different hybrid topologies with an extensive list of components that are available on the market. One main outcome of the ‘Level 0’ tool is a comparison between the proposed concept and other main competitors to give our customers the best possible benefit. FEV is also able to perform an initial sensitivity analysis of some decision-making parameters such as battery size, electric motor power, and engine downsizing, among others, and can also estimate the additional hybridization costs in each case to provide the best trade-off between CO2 reduction and hybridization costs, both currently and for the foreseeable future.
The main motivation to develop such a lean approach is to provide customers with fast, yet reliable, consulting as the first hybridization step of their conventional vehicles. The input data is directly collected via the Excel interface and a PowerPoint report which is linked to the Level 0 model, automatically reads the simulation results from the Excel tool and presents them elegantly in the report automatically.
Level 1 System Simulation:
Although ‘Level 0’ simulation is a very useful tool in the pre-concept phase to decide if hybridization is a viable option, it is not accurate enough for the concept phase. At this stage, a better understanding of powertrain behavior is required in order to determine the sizing of components such as the electric motor and battery and to develop a suitable hybrid strategy. To do this, FEV employs its Level 1 simulation platform.
This tool is a zero-dimensional, closed loop simulation model in Matlab Simulink®. All powertrain components are modeled with a higher level of detail compared to the Level 0 simulation tool. The transmission efficiency, for example, is now map-based and is a function of input speed and torque, and gear number, and includes a semi-empirical, thermal model. The Level 1 simulation approach is commonly used for powertrain component sizing, hybrid topology selection, hybrid strategy optimization and vehicle performance, and fuel economy estimation.
Tailored Detailing Levels
The ‘Level 1’ simulation approach includes the modular separation of physical components from the controllers that operate them – this offers a high degree of flexibility in the calibration of the controllers without changing the physical models. Another benefit of using Matlab Simulink® over other commercial software is the freedom to make changes to the sub-models when required. Since the sub-models are all FEV in-house models, the detailing level can be completely tailored to meet the needs of the customer and/or activity. For example, if the simulation activity focuses on the estimate for required battery size, the battery sub-model can be modeled with a higher level of detail whereas the other powertrain sub-models – such as the engine – remain untouched.
This simulation approach also guarantees a very short execution time, which is approximately 1,000 times faster than real time (e.g. hybrid vehicle over NEDC on a laptop with Core i7 CPU and 8 GB of memory runs in about one second). This rapid execution makes such a simulation approach very useful for sensitivity analysis, component sizing and optimization of the operating strategy.
Level 2 System Simulation: Powertrain Level
FEV’s Level 2 system simulation approach ensures a sufficient level of detail if simulations more sophisticated than cycle simulation and performance test cases are required. The Level 2 simulation tool is a multi-dimensional simulation platform that is structurally similar to Level 1, but with more accurate physical component behavior.
The main difference between Level 1 and Level 2 is the amount of detail in the component simulation. For example, when considering a gear change in a full hybrid vehicle, both propulsion sources, (the engine and the motor) as well as the clutch capacity, must be controlled in such a way that component limits are not exceeded and driving comfort is within acceptable boundaries. The Level 2 simulation features more accurate physical behavior – such as torsional vibration and the damping of the different shafts together with accurate clutch actuation – to allow a thorough analysis of the shift sequence process. Having a model with such a level of detail enables offline calibration of gear shifting prior to calibration on either a test bench or a complete vehicle, with resulting benefits in terms of a reduction in both in-vehicle testing and the risk of component damage.
The Level 2 model is based on the same modular structure as the Level 1 model and the majority of the sub-models are interchangeable between Levels 1 and 2. As mentioned earlier, the modular model structure, coupled with the sub-models built in-house at FEV, enable the user to tailor the detailing level of specific sub-models to activity needs. This allows the user to simulate very specific test cases; for example, a vehicle accelerating in reverse on an incline or simulation of an engine stop phase while driving a full hybrid vehicle.
To support effective delivery of this multi-level simulation platform, FEV developed easy-to-use model documentation to explain structure and contents of each level. All simulation models come with their own user guides that describe the overall simulation tool and each sub-model.