Reed Valve, Crankcase and Exhaust Models Coupled to 3D Fluid Domains for the Predictive CFD Simulation 2007-32-0030

The development and optimization of two stroke engines, especially the development of internal mixture preparation and the combustion process, require effective and reliable simulation in order to shorten the development time and to reduce prototype and test bench costs. CFD (Computational Fluid Dynamics) is a state of the art tool to optimize and visualize the fluid processes, e.g. scavenging, in-cylinder charge motion, spray formation, mixture preparation or combustion. The drawback of full 3D CFD simulation is the required time for grid generation and calculation of the model, especially for the simulations in the early development phase or in the concept phase as the available time for simulation is limited. Additionally, two stroke specific models e.g. for the reed valve, are not available in commercial 3D CFD codes.

In previous investigations [SAE 2005-32-0099] the strategies and the requirements for a predictive simulation have been discussed. Consequently, a new methodology which bases on the combination of 3-dimensional (3D) and 0/1-dimensional (0/1D) CFD simulation has been presented. This methodology uses a new multidimensional interface technology which has been discussed in [SAE 2006-32-0059]. The new interfacing technology is able to handle 3D/3D, 3D/1D or 3D/0D connections. This allows the replacement of regions with typically high numbers of cells by fast-calculating 0D or 1D models in order to reduce the number of 3D cells and therewith computational time.

In the present paper these new models and their formulation and validation are presented in detail:

Reed valve model: Two different approaches for the simulation of the behavior of the reed petal are developed and compared. The first one describes the reed motion based on a spring - damping system for a single oscillating mass. The second model is based on a bending beam model using the superposition of Eigenmodes. These models are validated with measurements using a high-speed camera to determine the reed motion shape and lift. Accompanying 3D CFD simulations are used for the visualization of the pressure and mass flow distribution.

Exhaust model: For the simulation of the gas dynamics and the heat transfer in the exhaust, a 1D simulation tool using a 2^nd order Total Variation Diminishing (TVD) solver is developed.

Crank case model: The crank case is described by a 0D model of a variable volume with wall heat flux.

The exhaust and the crank case models are validated by comparison to commercial 1D CFD codes.

The reed valve, the crank case and the exhaust model form a tool box for predictive two stroke engine simulation. This methodology allows adjusting the model in the development process according to the required flow details, accuracy and available simulation time.

The discussion about the impact of the tool box on the development work and the comparison to conventional methodology concludes the paper.