Search Results

Aerodynamic had played a primary role in high performance car since the late 1960s, when introduction of the first inverted wings appeared in some formulas. Race car aerodynamic optimisation is one of the most important reason behind the car performance. Moreover, for high performance car using naturally aspired engine, car aerodynamic has a strong influence also on engine performance by its influence on the engine airbox. To improve engine performance, a detailed fluid dynamic analysis of the car/airbox interaction is highly recommended. To design an airbox geometry, a wide range of aspects must be considered because its geometry influences both car chassis design and whole car aerodynamic efficiency. To study the unsteady fluid dynamic phenomena inside an airbox, numerical approach could be considered as the best way to reach a complete integration between chassis, car aerodynamic design, and airbox design.

This paper deals with the assessment of the use of LES simulation technique on a real airbox geometry designed for a high-performance engine. Large Eddy Simulation is a promising technique to yield a CFD tool able to predict flow unsteadiness: in LES modeling only a small part of the energy spectrum is modeled while the large scales of motion (correlated with the energy transport phenomena) are directly resolved. Given this observation, LES model becomes a very attractive tool for the fluid dynamic analysis of components characterized by a strong dynamic flow behavior like an airbox geometry. The airbox simulations were performed by Fluent v6.3 CFD code and the Wall Adaptive Local Eddy-Viscosity (WALE) sub-grid (sgs) stress model was used. A bounded second order central differencing scheme (BCD) was adopted and a discussion of the kinetic energy conservation attitude of this scheme was performed.

The short time available for injection and mixing in high-speed engines requires an accurate modeling of the fuel related processes to obtain a valuable in-cylinder charge description, and then a good combustion performance prediction. An advanced version of the KMB code of IFP has been used to compute a racing engine. It includes a fitted on experiments spray model, a comprehensive wall-film model, the AKTIM ignition and ECFM combustion models. A major difficulty was the necessity to compute numerous cycles before reaching a cycle-independent solution. A procedure has been defined to minimize calculation time. Another difficulty was the high concentration of liquid in some zones, which requested a careful meshing. Effects such as the influence of the strong acoustic waves on the spray dynamic, the wall wetting effects on the engine time response, injector position on fuel distribution in the cylinder, charge homogeneity on the combustion process have been investigated.

Modelling small and large scale fluctuations of fuel distribution is of high interest for stratified direct injection spark ignition (DISI) engines. Homogeneous combustion models need to be extended or replaced in order to account for these fluctuations. They are presently neglected in most engine simulations. Effects of mean fuel/air equivalence ratio gradient have been recently included in previous homogeneous mixture approaches. To account for local fluctuations of mixture composition, the new model ECFM-Z has been developed on the basis of recent Direct Numerical Simulation results and Coherent Flame Surface modelling. The model has been implemented in a CFD code (KMB) The influence of mixture fraction is integrated in the Extended Coherent Flame Surface combustion model. The model is based on a conditional approach. Unburnt hydrocarbons produced by lean flame local extinctions are taken into account.

The Lagrangian-Eulerian approach is commonly used to simulate engine sprays. However typical spray computations are strongly mesh dependent. This is explained by an inadequate space resolution of the strong velocity and vapor concentration gradients. In Diesel sprays for instance, the Eulerian field is not properly computed close to the nozzle exit in the vicinity of the liquid phase. This causes an overestimated diffusion that leads to inaccuracies in the modeling of fuel-air mixing. By now it is not possible to enhance grid resolution since it would violate requested assumptions for the Lagrangian liquid phase description. Besides, a full Eulerian approach with an adapted mesh is not practical at the moment mainly because of prohibitive computer requirements. Keeping the Lagrangian-Eulerian approach, a new methodology is introduced: the full Lagrangian-Eulerian Coupling (CLE).

The high pressure injectors used in direct injection Diesel engines introduce major perturbations in the air flow field inside the combustion chamber leading to strongly strained and turbulent flow. This fuel/air mixing process plays a critical role in enhancing self-ignition. However, in most Diesel combustion models, the interaction between turbulent mixing and self-ignition is not directly taken into account. Typically, the calculated average self-ignition combustion rates are pseudo laminar reaction rates based on simplified kinetic mechanisms. The mean values of the reaction rate are determined as a function of the mean values of the reactant concentrations and temperature. But due to the high non linearity of the reaction rate during self-ignition, this assumption is not valid. A turbulent self-ignition model developed from direct numerical simulations is presented.

The flow-field of an automotive DI Diesel engine is characterized by experiments in a motored engine using Laser Doppler Velocimetry and by CFD simulations. Only one cylinder is active and a specific swirling intake duct is used. Various bowls with different shapes are investigated: fiat or W-shaped bowls, with or without re-entrant. The influence of engine speed is also studied. The mean velocity and turbulence evolutions are measured with back-scatter LDV experiments using an optical access in an extended piston. The simulations are performed using the KMB code, a modified version of KIVA-II. Along with the detailed flow-field description, integral quantities characterizing the flow are derived. The comparison between LDV data and CFD results is shown to be satisfactory. The effects of geometry and engine speed on spatial profiles and temporal evolution of mean and turbulent velocities are correctly reproduced.

To simulate the air-fuel mixing in the intake ports and cylinders of internal combustion engines, a fuel liquid film model is developed for integration in 3D CFD codes. Phenomena taken into account include wall film formation by an impinging spray, film transport such as governed by mass and momentum equations with wall and air flow interactions and evaporation considering energy and convection mass transfer equations. A continuous-fluid method is used to describe the wall film over a three dimensional complex surface. The basic approximation is that of a laminar incompressible boundary layer; the liquid film equations are written in an integral form and solved by a first-order ALE finite volume scheme; the equation system is closed without coefficient fitting requirements. The model has been implemented in a Multi-Block version of KIVA 2 (KMB) and tested against problems having theoretical solutions.

Pollutant emissions from D.I. Diesel engines strongly depend on injection system characteristics and mainly on injection pressure and timing. In the latest years some solutions have been proposed based on very high fuel pressure values (up to 150 MPa). Among them, the so called “Common rail” system configuration, being able to electronically control needle lift and injection pressure, seems to be particularly promising. Much experimental and theoretical work has been done to improve system performance for automotive applications. With the aim of investigating the influence of some details of geometrical configuration on the injector operating mode, a mathematical model able to describe the pressure-time history in any section of the delivery pipe and the fuel injection rate through the nozzle has been developed, based on a semi-implicit finite volumes approach. The computed results have been compared with experimental data provided by the Institut Français du Pétrole.

Computation of high pressure Diesel injection requires improvement of present spray atomization and droplet breakup models. The surface wave instability atomization (Wave) model of Reitz [2] has been coupled to a new breakup model (FIPA) which is based on the experimental correlations of Pilch et al.[3]. It has been integrated in the 3D KMB code [1] derived from the Kiva 2 code [4] of Los Alamos already including a stochastic Lagrangian description of sprays. The droplet breakup FIPA model was first fitted and validated using the monodisperse drop breakup experiments of Liu and Reitz [5]. The response of the modified spray model including the global Wave-FIPA breakup model is compared to well characterized data obtained in a high pressure and temperature vessel. This vessel is fitted with a common-rail injection system with a single hole injector tip.