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Virtual Full Engine Development: 3D-CFD Simulations of Turbocharged Engines under Transient Load Conditions
Abstract:
The simulation of transient engine behavior has gained importance mainly due to stringent emission limits, measured under real driving conditions and the concurrently demanded vehicle performance. This is especially true for turbocharged engines, as the coupling of the combustion engine and the turbocharger forms a complex system in which the components influence each other remarkably causing, for example, the well-known turbo lag. Because of this strong interaction, during a transient load case, the components should not be analyzed separately since they mutually determine their boundary conditions.
Three-dimensional computational fluid dynamics (3D-CFD) simulations of full engines in stationary operating points have become practicable several years ago and will remain a valuable tool in virtual engine development; however, the next logical step is to extend this approach into the transient domain. This article evaluates the potential of different turbocharger models being included in 3D-CFD engine models, starting from a map-based approach ranging up to a fully discretized 3D-CFD turbocharger.
The approach presented in this article combines a three-dimensional representation of the entire engine, including the turbine and compressor housing, but replacing the turbocharger rotors with performance maps. Several numeric investigations were conducted to reduce calculation time and improve the system stability.
A virtual hot gas test bench is set up in three different simulation environments to compare the map-based turbocharger against a conventional zero-dimensional (0D)/one-dimensional (1D) modeling and a 3D-CFD turbine. The analysis focuses on differences in stationary behavior and the response to pressure pulses.
The modeling approach, validated on the virtual hot gas test bench, is used to model a two-cylinder engine. It is analyzed under stationary load conditions showing the interaction between turbocharger and engine within a working cycle. In a second step, transient load conditions over multiple working cycles are applied to the engine model, and the results are compared to the respective measurements obtained at the test bench.