Steven Begg

Multiple injection strategies for improved combustion stability under stratified part load conditions in a Spray Guided Gasoline Direct Injection (SGDI) engine

Compared to conventional homogeneous direct injection or port-fuel injected engines, the second generation, spray guided, direct injection engine (SGDI) has the potential for significantly improved fuel economy during part load stratified charge operation. Multiple fuel injection strategies can be utilised to increase the unthrottled operating range, leading to further improvements in fuel economy. However, careful optimisation of these strategies is essential to ensure that benefits are maintained whilst further minimising emissions within combustion stability limits and consumer driveability demands.

Vortex ring-like structures in gasoline fuel sprays under cold-start conditions

Abstract A phenomenological study of vortex ring-like structures in gasoline fuel sprays is presented for two types of production fuel injectors: a low-pressure, port fuel injector (PFI) and a high-pressure atomizer that injects fuel directly into an engine combustion chamber (G-DI). High-speed photography and phase Doppler anemometry (PDA) were used to study the fuel sprays. In general, each spray was seen to comprise three distinct periods: an initial, unsteady phase; a quasi-steady injection phase; and an exponential trailing phase. For both injectors, vortex ring-like structures could be clearly traced in the tail of the sprays. The location of the region of maximal vorticity of the droplet and gas mixture was used to calculate the temporal evolution of the radial and axial components of the translational velocity of the vortex ring-like structures. The radial components of this velocity remained close to zero in both cases. The experimental results were used to evaluate the robustness of previously developed models of laminar and turbulent vortex rings. The normalized time, , and normalized axial velocity, , were introduced, where t init is the time of initial observation of vortex ring-like structures. The time dependence of on was approximated as and for the PFI and G-DI sprays respectively. The G-DI spray compared favourably with the analytical vortex ring model, predicting , in the limit of long times, where α = 3/2 in the laminar case and α = 3/4 when the effects of turbulence are taken into account. The results for the PFI spray do not seem to be compatible with the predictions of the available theoretical models.

Continuous–Discrete Time-Observer Design for State and Disturbance Estimation of Electro-Hydraulic Actuator Systems

In this paper, a continuous-discrete time observer which simultaneously estimates the unmeasurable states and the uncertainties for the electro-hydraulic actuator (EHA) system is presented. The main feature of the proposed observer is the use of an intersample output predictor which allows the users to increase the frequency acquisition of the piston position sensor without affecting the convergence performance. The stability analysis of the proposed observer is proved using Lyapunov function adapted to hybrid systems. To show the efficiency of our proposed observer, numerical simulations and experimental validation involving a control application, which combines the designed observer and a proportional-integral (PI) controller for the purpose of piston position tracking problem, are presented.

Tomographic imaging of the liquid and vapour fuel distributions in a single-cylinder direct-injection gasoline engine

This article reports the application of optical tomography and chemical species tomography to the characterisation of the in-cylinder mixture preparation process in a gasoline, direct-injection, single-cylinder, motored research engine. An array of 32 near-infrared beams is transmitted in a horizontal plane across the cylinder bore near the top of the cylinder, through a circular quartz annulus. A novel approach to enable the optical alignment of the transmitting and receiving optics is utilised. The engine is operated at a stoichiometric condition at 1200 r/min, with negative valve overlap timing. Two tomographic measurement schemes (optical attenuation and chemically specific absorption) were used to acquire data on the spatial and temporal distribution of fuel throughout the engine cycle. Optimised data pre-processing methods are described for maximal beam count and data reliability. The presence of fuel during the intake stroke was detected by the optical beam attenuation due to scattering from the liquid gasoline droplets. Optical tomographic reconstruction of the spatial distribution of these droplets was achieved at an imaging rate of 7200 frames per second, revealing rapid intra-cycle spatial variations that were consistent between consecutive cycles. During the compression stroke, chemical species tomography images of fuel vapour were reconstructed from data acquired using chemically selective spectral absorption by the hydrocarbon molecules, at an imaging rate of 2400 frames per second. Later in the compression stroke, the temporal evolution of the fuel vapour distribution in the plane of observation is relatively slow and displays inhomogeneities that are consistent between consecutive cycles. This is the first report of the use of tomography to image, within individual engine cycles, the in-cylinder evolution of both fuel spray droplet distribution and fuel vapour distribution.

Approximate Analysis of Thermal Radiation Absorption in Fuel Droplets

The values of absorption coefficients of gasoline fuel (BP Pump Grade 95 RON ULG (research octane number unleaded gasoline)), 2,2,4-trimethylpentane (CH 3 ) 2 CHCH 2 C(CH 3 ) 3 (iso-octane) and 3-pentanone CH 3 CH 2 COCH 2 CH 3 have been measured experimentally in the range of wavelengths between 0.2 μm and 4 μm. The values of the indices of absorption, calculated based on these coefficients, have been compared with those previously obtained for low sulphur ESSO AF1313 diesel fuel. These values are generally lower for pure substances (e.g., iso-octane and 3-pentanone) than for diesel and gasoline fuels. The values of the average absorption efficiency factor for all fuels are approximated by a power function aR b d , where R d is the droplet radius a and b in turn are approximated by piecewise quadratic functions of the radiation temperature, with the coefficients calculated separately in the ranges of droplet radii 2-5 μm, 5-50 μm, 50-100 μm, and 100-200 μm for all fuels. This new approximation is shown to be more accurate compared with the case when a and b are approximated by quadratic functions or fourth power polynomials of the radiation temperature, with the coefficients calculated in the whole range 2-200 μm. This difference in the approximations of a and b, however, is shown to have little effect on modeling of fuel droplet heating and evaporation in conditions typical for internal combustion engines, especially in the case of diesel fuel and 3-pentanone.

On the use of laser-induced fluorescence for the measurement of in-cylinder air–fuel ratios

Abstract This paper presents the development of a new strategy for the calibration of air-fuel ratio measurements in engines by laser-induced fluorescence (LIF). After a brief introduction to the LIF technique, the paper highlights the structured approach undertaken to ensure that accurate quantitative measurements were produced. In particular, the new approach to coping with the fluorescence dependency on pressure and temperature, the issues related to the choice of a fluorescence tracer, the careful determination of the optimum tracer concentration and the complete calibration methodology are described, together with the resolution of some of the obstacles encountered. The paper concludes with some examples of calibrated measurements accompanied by a comparison of the results with combustion and emission performances. These results show a very good correlation.

Airflow and Fuel Spray Interaction in a Gasoline DI Engine

Two optical techniques together with a CFD simulation have been used to study the interaction of intake airflow with the injected fuel spray in a motored direct injection gasoline engine. The combustion chamber was of a pent-roof construction with the side-mounted injector located low down between the inlet valves injecting at a 54 o angle to the cylinder axis. The two-dimensional piston bowl shape allowed optical access for the Mie scatter technique to be used to investigate the liquid fuel behaviour in the central axial plane of the cylinder lying midway between the two inlet valves and passing through the centre line of the injector nozzle. A second set of images was obtained using backlighting, this time looking through the glass cylinder liner directly towards the injector. The in-cylinder simulation was run using the VECTIS software. Measurements and simulations were conducted for a range of early SOI timings between 20° and 80° ATDC. The results demonstrated clearly that the incoming airflow tended to flatten the jet and constrain it towards its centreline.

Validation of a CFD Model of a Hollow-Cone Spray with Gasoline Fuel Blends

This paper presents the summary of the development of a two-phase spray model of a hollow-cone fuel injector commonly applied to spray-guided, gasoline direct injection, (SGDI) engines. The model was simulated using the Ricardo VECTIS CFD code and takes into account the physical and chemical effects of oxygenated fuel blends (flexfuels). The characteristics of the fuel sprays at typical gasoline part-load conditions, identified in a parallel study, were of particular interest. An injection duration of 0.3 ms was chosen which represented a stratified charge, unthrottled, part-load operating condition in a spray guided GDI engine with a piezoelectric fuel injector and a fuel injection pressure of 200 bar gauge. In the first instance, the spray model was validated against data recorded in a constant volume spray chamber. Secondly, the robustness of the model was tested against data measured in an optically-accessed engine. The Ricardo WAVE 1-D gas dynamics code was used to determine the gas phase boundary conditions in the engine. Initial spray input data for the model were obtained using an injection rate tube and the high-pressure and temperature (HP-HT) spray chamber. The quiescent gas pressure and temperatures in the chamber were varied in the range of between 1 and 7 bar absolute and 293 and 423 K respectively. The fuels used were pump grade, 95 RON gasoline, a blend of gasoline and ethanol (E85) and a blend of gasoline and methanol (M30) mixed by volume. In each case, the fuel injection processes (geometry and penetration rate characteristics) were visualised using Mie imaging, illuminated with a LASER sheet, as well as high-speed shadowgraphy. The camera frame rates were 10 Hz and 4 kHz respectively. The droplet size and velocity distributions in a plane coincident with the injector nozzle that bisected the axis of symmetry of the injector were simultaneously measured using Phase Doppler Anemometry (PDA). Analysis of the data sets were used to define the boundary conditions and to optimise the parameters of the spray model in a given time step. In addition, the data from the experimental HP-HT chamber and the spray model were compared with complementary data obtained in a new Ricardo single cylinder, spray-guided, optically-accessed, Hydra engine. High-speed photography, performed at frame rates in the range of 10 to 200 kHz was carried out, in the motored engine, using a glass piston bowl and a specialist cylinder liner that protruded into the pent-roof. The optimised model was implemented into a 3D CFD simulation of the optical engine, incorporating models for spray and mixture preparation. The distribution of the local air to fuel ratio, predicted by the VECTIS simulation, showed good agreement with planar laser-induced fluorescence (PLIF) measurements of the liquid and vapour fuel distributions in the optical engine. The applicability of the validated spray model, with respect to the accurate prediction of mixture preparation was discussed in the context of multiple fuel injection and flexfuel strategies.

Spray dynamics as a multi-scale process

The analysis of the processes in sprays, taking into account the contribution of all spatial and temporal scales, is not feasible in most cases due to its complexity. The approach used in most applications is based on separate analysis of the processes at various scales, and the analysis of the link between these processes. This approach is demonstrated for the analysis of spray break-up and penetration in Diesel engine-like conditions, and vortex ring-like structures in gasoline engine-like conditions. The conventional WAVE, TAB, stochastic and modified WAVE (taking into account transient effects) models are reviewed. It is pointed out that the latter model leads to the prediction of spray penetration in Diesel engine-like conditions closest to the one observed experimentally. In gasoline engine-like conditions, spray penetration is often accompanied by the formation of vortex ring-like structures, the spatial scale of which is comparable with the scale of spray penetration. The general expression of the velocity of the vortex ring centroid can be simplified for short and long times, the latter simplification being particularly simple and useful for engineering applications. The thickness of the vortex ring is expressed as l = atb, where a is an arbitrary constant and 1/4 ≤ b ≤ 1/2. The cases when b = 1/2 and b = 1/4 refer to laminar and turbulent vortex rings respectively. The model is compatible with the observation of vortex ring-like structures in gasoline engine-like conditions.

An efficient Adaptive Mesh Refinement (AMR) algorithm for the Discontinuous Galerkin method: applications for the computation of compressible two-phase flows

We present an Adaptive Mesh Refinement (AMR) method suitable for hybrid unstructured meshes that allows for local refinement and de-refinement of the computational grid during the evolution of the flow. The adaptive implementation of the Discontinuous Galerkin (DG) method introduced in this work (ForestDG) is based on a topological representation of the computational mesh by a hierarchical structure consisting of oct- quad- and binary trees. Adaptive mesh refinement (h-refinement) enables us to increase the spatial resolution of the computational mesh in the vicinity of the points of interest such as interfaces, geometrical features, or flow discontinuities. The local increase in the expansion order (p-refinement) at areas of high strain rates or vorticity magnitude results in an increase of the order of accuracy in the region of shear layers and vortices. A graph of unitarian-trees, representing hexahedral, prismatic and tetrahedral elements is used for the representation of the initial domain. The ancestral elements of the mesh can be split into self-similar elements allowing each tree to grow branches to an arbitrary level of refinement. The connectivity of the elements, their genealogy and their partitioning are described by linked lists of pointers. An explicit calculation of these relations, presented in this paper, facilitates the on-the-fly splitting, merging and repartitioning of the computational mesh by rearranging the links of each node of the tree with a minimal computational overhead. The modal basis used in the DG implementation facilitates the mapping of the fluxes across the non conformal faces. The AMR methodology is presented and assessed using a series of inviscid and viscous test cases. Also, the AMR methodology is used for the modelling of the interaction between droplets and the carrier phase in a two-phase flow. This approach is applied to the analysis of a spray injected into a chamber of quiescent air, using the Eulerian–Lagrangian approach. This enables us to refine the computational mesh in the vicinity of the droplet parcels and accurately resolve the coupling between the two phases.