Numerical studies of gasoline direct injection engine processes

The GDI engine has a number of practical advantages over the more traditional port-fuel injection strategy, however a number of challenges remain the subject of continued research in an attempt to fully exploit the advantages of the GDI engine. These include complex in-cylinder flow fields and fuel-air mixing strategies, and significant temporal variation, both through an engine cycle and on a cycle-by-cycle basis. Despite advances in experimental techniques, the relative difficulty and cost of taking detailed measurements remains high, thus computational techniques are an integral part of research activities. The research work presented in this thesis has focused on the use of detailed 3D-CFD techniques for investigating physical phenomena of the in-cylinder flow field and fuel injection process in a single cylinder GDI engine with early injection event. A detailed validation of the numerical predictions of the in-cylinder flow field using both the RANS RNG k-ε turbulence model and the Smagorinsky LES SGS turbulence model was completed with both models showing good agreement against available experimental results. A detailed validation of the numerical predictions of the fuel injection process using a Lagrangian DDM and both RANS RNG k-ε turbulence model and Smagorinsky LES SGS turbulence model was completed with both models showing excellent agreement against experimental data. The model was then used to investigate the in-cylinder flow field and fuel injection process including research into: the three dimensional nature of the flow field; intake valve jet flapping, characterisation, causality and CCV, and whether it could account for CCV of the mixture field at spark timing; the anisotropic characteristics of the flow field using both the fluctuating velocity and turbulence intensity, including the increase in anisotropy due to the fuel injection event; the use of POD for quantitatively analysing the in-cylinder flow field; investigations into the intake valve, cylinder liner and piston crown spray plume impingement processes, including the use of a multi-component fuel surrogate and CCV of the formed liquid film; characterisation and CCV of the mixture field though the intake and compression strokes up to spark timing. Finally, the predicted turbulence characteristics were used to evaluate the resultant premixed turbulent combustion event using combustion regime diagrams

Impingement characteristics of an early injection gasoline direct injection engine: A numerical study

This paper describes the use of a Lagrangian discrete droplet model to evaluate the liquid fuel impingement characteristics on the internal surfaces of an early injection gasoline direct injection (GDI) engine. The study focuses on fuel impingement on the intake valve and cylinder liner between start of injection (SOI) and 20° after SOI using both a single- and multi-component fuel. The single-component fuel used was iso-octane and the multi-component fuel contained fractions of iso-pentane, iso-octane and n-decane to represent the light, medium and heavy fuel fractions of gasoline, respectively. A detailed description of the impingement and liquid film modelling approach is also provided Fuel properties, wall surface temperature and droplet Weber number and Laplace number were used to quantify the impingement regime for different fuel fractions and correlated well with the predicted onset of liquid film formation. Evidence of film stripping was seen from the liquid film formed on the side of the intake valve head with subsequent ejected droplets being a likely source of unburned hydrocarbons and particulate matter emissions. Differences in impingement location and subsequent location of liquid film formation were also observed between single- and multi-component fuels. A qualitative comparison with experimental cylinder liner impingement data showed the model to well predict the timing and positioning of the liner fuel impingement

Characteristics of GDI engine flow structures

The benefits of the gasoline direct injection engine over the more traditional gasoline port-fuel injection engine are well known and include the ability to operate lean of stoichiometric for fuel efficiency improvements, reduced knock propensity and reduced unburned hydrocarbons during cold start and transients. Nevertheless, a number of key challenges still remain including cyclic variability, abnormal combustion phenomena and increased particulate emissions. Our progress in each of these challenges is intrinsically linked to our understanding of the flow field formed within the cylinder. This paper presents the development, validation and subsequent utilisation of a 3D-CFD gasoline direct injection engine model for making predictions of the in-cylinder flow field through the intake and compression strokes. An extensive validation exercise was completed using experimental data from a single cylinder optical research engine to validate both the intake runner, intake valve jet and in-cylinder flow fields. Validation results showed the model to generally compare well against experimental data including indicating data, intake runner velocities and flow momentum, valve jet and in-cylinder flow structures. Differences were identified in the timing of the detachment of the intake valve jet from the cylinder head and a subsequent reduction in effective flow area was hypothesised as contributing to an over prediction of the valve jet and in-cylinder flow velocities. A comparison of the spatial and temporal development of the in-cylinder flow field identified the model to well predict the flow structures through the intake and compression stroke. The model was then exercised with a view to evaluate the impact of solid boundaries on the spatial and temporal development of the in-cylinder flow structure. An analysis on the impact of using a pent-roof optical access window in research engines on the flow structure is also provided, indicating that significant asymmetry and additional recirculation zones in the corners of the access window should be considered when evaluating experimental results from a research engine of this configuration

Numerical simulation of a GDI engine flow using LES and POD

This paper presents the findings from a numerical study of a gasoline direct injection engine flow using the Large Eddy Simulation (LES) modelling technique. The study is carried out over 30 successive engine cycles. The study illustrates how the more simple but robust Smagorinsky LES sub-grid scale turbulence model can be applied to a complex engine geometry with realistic engineering mesh size and computational expense whilst still meeting the filter width requirements to resolve the majority of large scale turbulent structures. Detailed description is provided here for the computational setup, including the initialisation strategy. The mesh is evaluated using a turbulence resolution parameter and shows the solution to generally resolve upwards of 80% of the turbulence kinetic energy. The calculated mean and fluctuating velocity components have been validated across multiple cutting planes at key crank angles within the intake stroke with good agreement obtained against experimental data and compared with RANS model predictions. A Proper Orthogonal Decomposition (POD) technique is then used to evaluate the in-cylinder flow field with the results focusing around the eigenvalue/energy content and time coefficients associated with each mode. The findings have shown how this technique can be used to assess the amount of small scale turbulence generated at the point of spark timing, the level of flow field cyclic variability and the degree of statistical convergence to be expected from an ensemble average result based on the number of cycles and the level of cyclic variability present in the flow field