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Technical Paper

Air-Fuel Mixing in a Homogeneous Charge DI Gasoline Engine

For optimum efficiency, the direct injection (DI) gasoline engine requires two operating modes to cover the full load/speed map. For lower loads and speeds, stratified charge operation can be used, while homogeneous charge is required for high loads and speeds. This paper has focused its attention on the latter of these modes, where the performance is highly dependent on the quality of the fuel spray, evaporation and the air-fuel mixture preparation. Results of quantitative and qualitative Laser Induced Fluorescence (LIF) measurements are presented, together with shadow-graph spray imaging, made within an optically accessed DI gasoline engine. These are compared with previously acquired air flow measurements, at various injection timings, and with engine performance and emissions data obtained in a fired single cylinder non-optical engine, having an identical cylinder head and piston crown geometry.
Technical Paper

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° 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.
Journal Article

An Assessment of the Bottoming Cycle Operating Conditions for a High EGR Rate Engine at Euro VI NOx Emissions

This paper investigates the application of a Bottoming Cycle (BC) applied to a 10-litre (L) heavy duty Diesel engine for potential improvements in fuel efficiency. With the main thermodynamic irreversibility in the BC due to the temperature difference between the heat source and the working fluid, a proper selection of the working fluid and its operating condition for a given waste heat is the key in achieving high overall conversion efficiency. The paper reviews a fluid selection methodology based on thermodynamic/thermo-physical and environmental/safety properties. Results are presented using seven pure, dry, isentropic and wet working fluids (synthetic, organic and inorganic) operating with expansion starting from the saturated vapour, superheated vapour, supercritical phase, saturated liquid, and two-phase. Efficiency improvements by recovering Charge Air Coolers (CAC) and Exhaust Gas Recirculation (EGR) cooler heat on two engine platforms were calculated.
Technical Paper

Development of a Two-Stroke/Four-Stroke Switching Gasoline Engine - The 2/4SIGHT Concept

The pursuit of flexibility is a recurring theme in engine design and development. Engines that are able to switch between the two-stroke operating cycle and four-stroke operation promise a great leap in flexibility. Such 2S-4S engines could then continuously select the optimum operating mode - including HCCI/CAI combustion - for fuel efficiency, emissions or specific output. With recent developments in valvetrain technology, advanced boosting devices, direct fuel injection and engine control, the 2S-4S engine is an increasingly real prospect. The authors have undertaken a comprehensive feasibility study for 2S-4S gasoline engines. This study has encompassed concept and detailed design, design analysis, one-dimensional gas dynamics simulation, three-dimensional computational fluid dynamics, and vehicle simulation. The resulting 2/4SIGHT concept engine is a 1.04 l in-line three-cylinder engine producing 230 Nm and 85 kW.
Technical Paper

Fuel Droplet Heating and Evaporation: Analysis of Liquid and Gas Phase Models

Recently developed liquid and gas phase models for fuel droplet heating and evaporation, suitable for implementation into computational fluid dynamics (CFD) codes, are reviewed. The analysis is focused on the liquid phase model based on the assumption that the liquid thermal conductivity is infinitely large (infinite thermal conductivity (ITC) model), and the so called effective thermal conductivity (ETC) model. Seven gas phase models are compared. It is pointed out that the gas phase model, taking into account the finite thickness of the thermal boundary layer around the droplet predicts the evaporation time closest to the one based on the approximation of experimental data. In most cases, the droplet evaporation time depends strongly on the choice of the gas phase model. The dependence of this time on the choice of the liquid phase model, however, is weak if the droplet break-up processes are not taken into account.
Technical Paper

Laser-Induced Fluorescence Investigation of Nitric Oxide Formation and Hydroxyl Radicals in a Diesel Rapid Compression Machine

The research presented here aims at providing a deeper understanding of the formation of nitric oxide in diesel combustion. To this end, in-cylinder distributions of nitric oxide (NO) were acquired by laser-induced fluorescence (LIF) in a rapid compression machine at conditions representative of a modern diesel passenger vehicle. In particular, the effects of injection and in-cylinder pressure on NO formation were investigated temporally and spatially to offer new insight into the formation of NO. Excitation and collection strategies were notably fine-tuned to avoid the collection of spurious signal due to oxygen (O₂) fluorescence. NO fluorescence was first recorded slightly after the onset of the diffusion flame and until late in the expansion stroke. The early low levels of NO were located on the lean side of the high density of hydroxyl radicals (OH).
Technical Paper

Predictive CFD Auto-Tuning Approach For In-Cylinder EU6 LDD DI Engine

Tightening emission regulations and accelerating production cycles force engine developers to shift their attention towards virtual engineering tools. When simulating in-cylinder processes in commercial LDD DI engine development, the trade-off between runtime and accuracy is typically tipped towards the former. High-fidelity simulation approaches which require little tuning would be desirable but require excessive computing resources. For this reason, industry still favours low-fidelity simulation approaches and bridges remaining uncertainties with prototyping and testing. The problem with low-fidelity simulations is that simplifications in form of sub-grid-scale (SGS) models introduce multivariable tuning parameter dependencies which, if not understood, impair the predictive nature of CFD simulations. In previous work the authors have successfully developed a boundary condition dependant input parameter table.
Journal Article

Simulation and Measurement of Transient Fluid Phenomena within Diesel Injection

Rail pressures of modern diesel fuel injection systems have increased significantly over recent years, greatly improving atomisation of the main fuel injection event and air utilisation of the combustion process. Continued improvement in controlling the process of introducing fuel into the cylinder has led to focussing on fluid phenomena related to transient response. High-speed microscopy has been employed to visualise the detailed fluid dynamics around the near nozzle region of an automotive diesel fuel injector, during the opening, closing and post injection events. Complementary computational fluid dynamic (CFD) simulations have been undertaken to elucidate the interaction of the liquid and gas phases during these highly transient events, including an assessment of close-coupled injections.
Technical Paper

The Influence of Injector Parameters on the Formation and Break-Up of a Diesel Spray

The influences of injector nozzle geometry, injection pressure and ambient air conditions on a diesel fuel spray were examined using back-lighting techniques. Both stills and high speed imaging techniques were used. Operating conditions representative of a modern turbocharged aftercooled HSDI diesel engine were achieved in an optical rapid compression machine fitted with a common rail fuel injector. Qualitative differences in spray structure were observed between tests performed with short and long injection periods. Changes in the flow structure within the nozzle could be the source of this effect. The temporal liquid penetration lengths were derived from the high-speed images. Comparisons were made between different nozzle geometries and different injection pressures. Differences were observed between VCO (Valve Covers Orifice) and mini-sac nozzles, with the mini-sac nozzles showing a higher rate of penetration under the same conditions.
Technical Paper

The Mixture Response of a Stratified Charge Gasoline Engine with Independent, Twin, Port-Fuel Injector Control

An experimental study of the mixture response performance of novel, port-fuel injection strategies upon combustion stability in a gasoline engine was undertaken at low engine load and speed conditions in the range of 1.0 bar to 1.8 bar GIMEP and 1000 rpm to 1800 rpm. The aim was to improve the thermal efficiency of the engine, by extending the lean limit of combustion stability, through promotion of stable charge stratification. The investigation was carried out using a modified 4-valve single-cylinder head, derived from a 4-cylinder, pent-roof, production, gasoline engine. The cylinder head was modified by dividing the intake tract into two, separate and isolated passages; each incorporating a production fuel injector. The fuel injection timing and duration were controlled independently for each injector.
Journal Article

The Recuperated Split Cycle - Experimental Combustion Data from a Single Cylinder Test Rig

The conventional Diesel cycles engine is now approaching the practical limits of efficiency. The recuperated split cycle engine is an alternative cycle with the potential to achieve higher efficiencies than could be achieved using a conventional engine cycle. In a split cycle engine, the compression and combustion strokes are performed in separate chambers. This enables direct cooling of the compression cylinder reducing compression work, intra cycle heat recovery and low heat rejection expansion. Previously reported analysis has shown that brake efficiencies approaching 60% are attainable, representing a 33% improvement over current advanced heavy duty diesel engine. However, the achievement of complete, stable, compression ignited combustion has remained elusive to date.
Technical Paper

Visual Analyses of End of Injection Liquid Structures and the Behaviour of Nozzle Surface-Bound Fuel in a Direct Injection Diesel Engine

For efficiency, the majority of modern diesel engines implement multiple injection strategies, increasing the frequency of transient injection phases and thus, end of injection (EOI) events. Recent advances in diagnostic techniques have identified several EOI phenomena pertinent to nozzle surface wetting as a precursor for deposit formation and a potential contributor towards pollutant emissions. To investigate the underlying processes, highspeed optical measurements at the microscopic scale were performed inside a motored diesel engine under low load/idling conditions. Visualisation of the injector nozzle surface and near nozzle region permitted an indepth analysis of the post-injection phenomena and the behaviour of fuel films on the nozzle surface when the engine is not fired. Inspection of the high-speed video data enabled an interpretation of the fluid dynamics leading to surface wetting, elucidating the mechanisms of deposition and spreading.