Development and calibration of an original 1D scavenging model for opposed-piston two-stroke engines

Abstract

The use of hydrogen in internal combustion engines may pose several issues and concerns, above all if, with the aim to reach near zero emissions, the adoption of lean mixtures is considered: in this condition, the engine power density may reveal too low and seriously limit the implementation in sport cars. A possible solution to this problem may be offered by the 2 stroke engines, above all in the opposed piston configuration, whose characteristic of high-power density may allow sport car requirements to comply with hydrogen fuel. As widely known, the first step for performance verification and optimization is represented by thermodynamic simulations, usually performed with an in-cylinder zero-dimensional (0D) approach coupled with a two-zones combustion model. Opposed-piston two-stroke (OP2S) engines feature a longitudinal scavenging process that strongly influences overall performance and emissions. Conventional scavenging modelling approaches rely on 0D formulations fundamentally based on blending the perfect mixing and the perfect displacement models, or on the “scavenging curve” (also called “scavenging profile”) obtained by means of extensive three-dimensional computational fluid dynamics (3D CFD) simulations. These approaches however cannot capture the axial evolution of the longitudinal gas exchange typical of OP2S. This paper presents an innovative one-dimensional (1D) scavenging model developed to introduce axial resolution in the scavenging process of OP2S engine while remaining compatible with system-level simulations. The proposed model is fully specified through explicit governing equations and introduces axial resolution through a continuous dilution-front formulation, avoiding the need for proprietary 1D toolchains and CFD-derived closure quantities. The model couples a full-cycle thermodynamic solver, which provides the in-cylinder state at port opening, with a 1D two-zone dilution formulation describing the interaction between fresh charge and residual gases. The model proposed only requires the identification of two scalar parameters to correctly predict scavenging parameters. The model calibration was performed using experimental scavenging and trapping-efficiency data from a reference OP2S engine, by minimizing the mean-square error across multiple intake-pressure levels. As an overall result, the model demonstrated accurate reproduction of measured scavenging and trapping efficiency.