Abstract
Keywords - Gas exchange processes simulation, Coupling of 1D and 3D compressible flows, Computing time saving method, Manifold design, ICE
Abstract - The design of intake and exhaust manifolds for ICE has been an important application of unsteady fluid dynamics for many years. A first purpose of these simulations is the optimizing of the filling, the accordance with the turbocharger etc. of ICE about wide load and engine speed intervals. A second purpose is the estimating of the required initial conditions for the simulation of the in cylinder air/fuel mixture formation and burning processes, which must have knowledge about the quantity, the composition and the flow-field of the in-take charge.
Since the computation costs are high for the implementation of unsteady fully three-dimensional
(3D) simulations of the gas exchange processes usual one-dimensional (1D) simulations are used instead. In this case, since the 1D-simulation of the gas flow processes cannot describe the whole reality (i.e. the three-dimensionality) of the gas flow correctly, curvatures, asymmetry of the pipes, and channels in the simulation are disregarded. In order to find a compromise procedure for this situation, the quasi-3D method is presented, that improves the quality of the 1D-simulation results noticeably without increasing the cost of computation proportionally. This method is based on the 1D partial differential equations (PDEs), which model the unsteady compressible flow process of a viscous fluid.
The quasi-3D method is introduced now in the following steps:
1. The 3D flow equations are deduced appropriately, as to consider the distortion of the ve-locity distribution (size and direction) in each pipe cross-section. Their integration over the pipe cross-section results in the 1D flow equations, the terms of which still contain integrals of the velocity distribution. In the following, these integrals are designated as adjustment coefficients of the 1D-flow. The 1D-flow equations together with the adjustment coefficients form the quasi-3D PDEs.
2. The adjustment coefficients are further treated as temporally independent parameters in each pipe cross-section. For integration of the quasi-3D PDEs the total variation diminishing finite difference method (as example) is used.
3. The determination of the adjustment coefficients take place with the help of a steady 3D-simulation
(here FIRE is used), which is only done for steady forward and reverse flow through the pipes. The resulted 3D flow velocity fields are appropriately processed to produce the requested adjustment coefficients.
4. The quasi-3D method can be used now for the accurate simulation of manifold flow processes throughout the full engine cycles.
5. The flow velocity CA-variations together with the adjustment coefficients enable determination of the 3D flow velocity distributions related to CA in the cylinder interfaces.
Apart from the theoretical basics, the quasi-3D method is applied to a one-cylinder research diesel engine. The comparison between simulation results and pressure measurements in multi-point intake pipe at several engine speeds and loads is shown and commented in detail.