Combustion for Gasoline Direct Injection Engines

Background:

Numerous car manufacturers are currently promoting Gasoline Direct Injection (GDI) engines in order to improve the fuel efficiency of gasoline-powered vehicles.

Numerical modeling of the fuel injection and combustion processes can assist in improving the combustion chamber design and combustion parameters in order to achieve more efficient combustion and eliminate
emissions formation.

In this work the computational code KIVA-3V has been used as the modeling platform for GDI engine simulations. Improved models have been developed for fuel injection, spray breakup, wall impingement and stratified combustion.

Combustion for Gasoline Direct Injection Engines

Researchers
Bruno Vanzieleghem
Dennis Assanis
Hong G. Im

Abstract:

An extended coherent flamelet model is developed based on the KIVA-3V platform in order to achieve high fidelity simulation of gasoline direct injection (GDI) combustion, allowing the identification of fuel economy improvements and emissions implications for this technology in addition to the investigation of new operating strategies.

In the present coherent flame model, the flame is represented by a transport equation for flame density. Stratification of the engine charge is incorporated by the diagnostic equations for the unburned fuel, oxygen, and enthalpy. This allows the characterization of the cell properties on a conditionally-averaged basis, by separately averaging over the burned or unburned fraction. A near-wall flame treatment was also implemented to
represent the realistic behavior of the flame near the walls more accurately.

As a further extension, a diagnostic equation for carbon dioxide present in the exhaust gas recirculation (EGR), as opposed to a product of combustion, was implemented in the model. This addition allows the evaluation of the effects of the mixture stratification by EGR on the combustion and pollutant formation processes. This model is useful to examine the difference between internal and external EGR operation. The formation of nitric oxides is calculated based on the conditionally-averaged burned gas temperature in each computational cell.

To validate the different components of the simulation tool, the combustion model was integrated with updated models for spray breakup and spray wall impingement [1-3]. The complete model was applied to simulate the engine cycle of an optical 4-valve GDI engine [4]. The experimental setup is a single-cylinder, four stroke GDI engine with a four valve head, featuring full optical access, and variable swirl, allowing us to thoroughly validate the different aspects of the simulation. Laser-induced fluorescence (LIF) experiments performed by Frieden et al. [5] were used for validation of the different aspects of the model: fuel-injection, mixing and the combustion process.