Integrated 1D-MultiD Fluid Dynamic Models for the Simulation of I.C.E. Intake and Exhaust Systems

Transcription

1 Integrated -MultiD Fluid Dynamic Models for the Simulation of I.C.E. Intake and Exhaust Systems G. Montenegro, A. Onorati, F. Piscaglia, G. D Errico Politecnico di Milano, Dipartimento di Energetica, Italy Abstract In the last decade fluid dynamic simulation tools have encountered an increased demand to assist the design phase. As a matter of fact, the aid of CFD tools can remarkably reduce the duration and the costs of this stage. However, the numerical simulation, if not correctly handled, can produce the opposite effect. For instance, a fully multidimensional analysis of a multi cylinder engine coupled to its own intake and exhaust system might become a challenge even using a PC cluster. A factor that has to be considered is the possibility of relying on simpler and lighter calculation tools. Large systems, in fact, are often analyzed using one dimensional simulation tools to save time and simultaneously conserve computational resources. Certain components, however, such as the intake and exhaust manifolds, exhibit an high degree of geometric complexity which can not be accurately modeled by codes. Those models suffer mainly from the limitation of the plane wave assumption, which is not appropriate for the simulation of complex shape components (air boxes, resonators, asymmetric expansion chambers, multipipe junctions, etc.), characterized by a significant non-planar wave motion. Therefore, the application of multid codes might be exploited to model only complex parts of an intake or exhaust system whereas the approach is applied elsewhere, suitably coupling the two models at their interfaces, balancing the requirements of short runtime and result accuracy. To combine the benefits of the two approaches, an integration of the two models was realized, coupling a code with a CFD code. The HLLC Godunov type Riemann solver has been implemented both in the CFD and the codes to solve the Euler system of equations, in order to limit eventual instabilities which might be generated when using different numerical schemes. Moreover, the HLLC solver has been applied to treat the boundary conditions at the interface between the two domains, in such a way to allow the propagation of flow non uniformities through the domain interface, without affecting the 1

2 solution accuracy. Hence, during the simulation the informations are continually passed back and forth between the two codes, resulting in a complete integration time step by time step. Numerous tests, involving the shock tube case, have been carried out to prove the stability and accuracy of the integrated code. The validation was carried out on two different cases. The first case consisted in the simulation of a V10 Lamborghini engine in which the 5 into 1 junction (Figure 1) has been modeled by the multidimensional approach. The measured pressure pulses downstream of one cylinder have been compared to the calculated ones showing an interesting agreement (Figure 2). The improved resolution of the wave motion by means of the integrated code has allowed also a better estimation of the engine volumetric efficiency. The second case was the optimization of a Formula SAE single cylinder engine, in which an Helmholtz resonator was inserted in the intake system, downstream of a Venturi tube (Figure 4), to improve the performance at high engine revolution speeds. In this presentation it will be shown the development of the and its application to the following cases: Simulation of a 5 into 1 junction of a V10 Lamborghini Engine ; Simulation of a Helmholtz resonator for the improvement of the engine volumetric efficiency; Simulation of a motorbike air box system; Non linear acoustic analysis of silencing devices (Figure 5). Figure 1: Lamborghini V10 engine schematic and the 5 into 1 junction mesh. 2

3 Pressure [bar] rpm Pressure [bar] rpm Crank angle [deg] Crank angle [deg] Figure 2: Pressure versus crank angle at 6000 rpm full load and 7500 rpm full load: measured, calculated by means of and fully calculation. Figure 3: Velocity field inside the 5 into 1 junction. 3

4 Brake torque [Nm] D baseline Engine speed [rpm] Figure 4: Brake torque curve and velocity field inside the Helmholtz resonator. 4

5 Figure 5: Velocity field and pressure iso-surfaces inside a series chamber muffler. 5