Bodo Durst

An experimental and numerical investigation of turbulent flame propagation and flame structure in a turbo-charged direct injection gasoline engine

Turbulent flame propagation in a modern, turbo-charged, direct-injection gasoline engine is investigated in detail for the whole engine operating map. Different development tools are used in a combined fashion. Based on experimental results with high and low-pressure indication, a global heat release analysis coupled with a 1D gas dynamics simulation is performed. Using the data from the 1D gas dynamics simulation as initial and boundary conditions, 3D CFD calculations are carried out. From these calculations the characteristic velocity and length scales of the turbulent flow as well as the relevant scales of premixed flame propagation, flame speed and flame thickness, respectively, are analysed for all relevant operating conditions of the engine. This leads to a classification of combustion regimes and flame structures based on the Peters–Borghi diagram. The effects of engine speed and load variations are discussed in detail based on scale analysis, especially the effect on the turbulence-premixed flame interaction. Based on these results and an analysis of the global heat release profiles, a simplified model for the burn duration is derived taking into account the turbulent flow scales obtained from a 3D CFD analysis and the thermochemical properties such as gas composition including exhaust gases, pressure and temperature. The results are compared to experimental data.

Die neuen Vier-und Sechszylinder-Ottomotoren von BMW mit Schichtbrennverfahren

Nach dem Sechszylinder-Twin-Turbo-Motor mit Homogenbetrieb — vorgestellt in der Februar-Ausgabe der MTZ — fuhrt BMW mit den neuen Sechs- und Vierzylinder-Saugmotoren mit Schichtbrennverfahren zwei weitere Motoren mit einer Direkteinspritzung der zweiten Generation (High Precision Injection) ein. Das strahlgefuhrte Brennverfahren eroffnet neue Moglichkeiten zu einer signifikanten Verbrauchsreduzierung sowohl im europaischen Fahrzyklus als auch im realen Kundenbetrieb. BMW verfolgt damit konsequent eine Strategie zur Umsetzung der „effizienten Dynamik“.

Implementation and Validation of the G-equation Model Coupled with Flamelet Libraries for Simulating Premixed Combustion in I.C. Engines

The G-equation model was implemented in the commercial code ANSYS CFX and validated against experimental data in order to successfully simulate turbulent premixed combustion in internal combustion engines. The model is based on the level-set approach. Two transport equations are solved respectively for the G-scalar mean value, representing the local distance function from the time-averaged mean flame front, and its variance, correlated to the turbulent flame brush thickness. The model closure for tracking the flame front is based on an algebraic expression for the turbulent burning velocity. The composition of the reacted mixture is evaluated by coupling the code with flamelet libraries generated with the ANSYS CFX-RIF package by means of a reaction progress variable computed as a function of the G-related quantities. An innovative technique for periodically re-initializing the G-scalar field, in order to enforce geometrical consistency and avoid numerical instabilities, was developed, consisting of a least-square-based interface reconstruction and a minimization of the distance from the discretized flame front. In order to make the code suitable for spark-ignition engine applications the combustion model was coupled with a spark kernel model that simulates the early stages of the ignition process at sub-grid scales. The robustness and accuracy of the level-set approach with moving structured and unstructured meshes were assessed and the parallel performance of the combustion model was optimized in order to deal with large meshes in industrial applications. Validation was carried out by comparing simulations against experimental data for three different test-cases: a steady flame in a slot-burner, a transient spark-ignited premixed combustion in a cylinder with fixed walls and optical access and a spark-ignition research engine with flat head and flat piston. The results show good numerical properties and other attractive features in terms of geometrical description of the flame front and coupling with sub-models for detailed chemistry and spark-ignition. 2009 SAE International.

The New BMW four and six-cylinder spark-ignition engines with stratified combustion

Following the introduction of the six-cylinder twin-turbo engine with homogeneous combustion — presented in the February issue of MTZ —, with the new six-cylinder and four-cylinder naturally aspirated engines with stratified combustion, BMW now presents two further engines featuring a 2nd generation direct injection system (high-precision injection). The spray-guided combustion process opens up new dimensions for achieving significant benefits in fuel consumption both in the European driving cycle as well as in real customer-related operation. BMW is therefore pursuing a consistent strategy aimed at realising “efficient dynamics”.

Fuel-stratification in automotive engines with vortical flow structures: An engineering approach with numerical and experimental methods

Publisher Summary This chapter investigates the numerical and experimental methods for direct injection of liquid fuel into the cylinder of an internal combustion engine. The flow field, fuel injection, and mixture formation of direct injection gasoline engines is simulated with a finite volume method for the complete intake and compression stroke. The injection of a conical liquid fuel jet directly into the combustion chamber creates vortical flow structures that interact with the in-cylinder flow. These induced vortical flow structures are “designed” as vortex rings to enhance the fuel-air mixing under full-load conditions and to expand the part-load range with a stratified charge near the spark plug. As an engineering approach, investigations are carried through for such a “spray-guided” combustion system with a two-equation model based on Reynolds-averaged conservation equations. Simulations are performed for simple spray-chamber geometry to study the vortex ring formation but also for realistic production engines. The results are compared with measurements from a spray-chamber and an engine with optical access using the method of laser-induced fluorescence of evaporated fuel.

Leistungssteigerung von Ottomotoren durch verschiedene Wassereinspritzungskonzepte

Der Trend von immer hoheren Leistungsdichten und hoheren Anforderungen hinsichtlich Verbrauchsreduktion bei Ottomotoren werden aktuell immer ofter durch die Klopfgrenze und die thermischen Bauteilgrenzen limitiert. Eine Moglichkeit, diese Grenzen zu verschieben, ist dabei der Einsatz eines Wassereinspritzungssystems.

3D CFD simulation of the spark ignition process under the consideration of spark channel deflection, diffusion effects, curvature and detailed chemical kinetics

The ignition process in turbocharged direct injection spark ignition engines is an important phase during the overall combustion process.