Graham Wigley

Unthrottled engine operation using variable valve actuation: the impact on the flow field, mixing and combustion

The effect on the intake flow field, air fuel mixing processes, thermodynamic performance and emissions output has been investigated for a range of valve operating profiles. A standard speed load point of 2000 rpm and 2.7 bar IMEP720° has been reached by throttling the intake whilst running standard cam profiles, by early closing of both inlet valves (EIVC) and by early closing of each inlet individually to generate bulk swirl motions within the cylinder. Data has been recorded at stoichiometric air fuel ratios for both direct injection and port fuelled operation. The valve profiles have been applied to two single cylinder homogeneous gasoline direct injection (GDI) spark ignition engines, developed to investigate the potential of controlling engine load by limiting the inducted air mass using fully variable valve timing (FVVT) to reduce pumping losses at part load. The first engine featured a full length optical liner, allowing 2D Particle Image Velocimetry (PIV) measurements of the intake flow fields to be made, along with Mie imaging studies of the liquid fuel fraction. The second was a thermodynamic engine equipped to measure specific fuel consumption and emissions of CO2, CO, NOX and THC. The work shows that fuel economy benefits can be gained by operating the engine with unthrottled EIVC operation. However, the interaction between the intake air and direct injection fuel spray means performance is highly dependant upon which valve is operated and the timing of the direct injection fuel spray. Copyright

An investigation of string cavitation in a true-scale fuel injector flow geometry at high pressure

String cavitation has been studied in an optical automotive size fuel injector with true-scale flow geometry at injection pressures of up to 2050 bar. The multihole nozzle geometry studied allowed observation of the hole-to-hole vortex interaction and, in particular, that of a bridging vortex in the sac region between the holes. A dependency on Reynolds number was observed in the formation of the visible, vapor filled vortex cores. Above a threshold Reynolds number, their formation and appearance during a 2 ms injection event was repeatable and independent of upstream pressure and cavitation number.

A High Power, High Resolution LDA/PDA System Applied to Gasoline Direct Injection Sprays

A new generation LDA/PDA transmitter system has been designed and constructed. The heart of the optical system is a new type of Bragg cell. Advances in laser power handling and symmetrical splitting at high Bragg angles of the shifted and unshifted beams have made it possible to construct a simple yet elegant LDA/PDA transmitter. The optical system integrates the Bragg cell with a laser beam expander to offer variable beam separation and high beam expansion ratios to produce a measurement volume with a high spatial resolution and at high power levels. The transmitter is proving to be a significant contribution to LDA/PDA optical system design and, not only applicable as a research tool for use in flows of a demanding nature but, due to its simplicity, flexibility and cost, an asset to teaching. The LDA/PDA system is being applied to characterise the atomization of fuel by high pressure automotive injectors as found in Gasoline Direct Injection (GDI) engines. Although the PDA system was configured to measure two orthogonal velocity components (2D) and size, its operation was not restricted to this configuration. An analysis of the spray with the PDA system in 1D and size and the LDA system in 2D and 1D configurations indicated the complexity of the atomization and break-up processes occurring in the spray. Single-shot imaging was used to study the spatial structure of the spray as a function of time. Use of the light sheet imaging technique alone could lead to false impressions of the atomization process.

Un-throttling a direct injection gasoline homogeneous mixture engine with variable valve actuation

Abstract Two direct injection spark ignition (DISI) engines with identical combustion chamber geometries and fuel injection systems were used to investigate fuel economy, exhaust emissions, the in-cylinder flow field, the fuel spray behaviour and combustion characteristics with early inlet valve closure (EIVC) strategies aimed at reducing parasitic induction work owing to throttling. One engine had extensive optical access through a transparent piston crown and transparent cylinder liner, while the other all-metal engine allowed continuous running. Engine running focused at low and intermediate engine loads (∼ 3 and ∼ 6 bar indicated mean effective pressure) and two engine speeds (2000 and 3500 r/min). The results show that the indicated specific fuel consumption (ISFC) could be reduced by almost 6 per cent without significant deterioration in gaseous exhaust pollutant emissions. The results also show that the in-cylinder bulk flow and turbulence and the thermodynamic conditions during combustion are affected significantly by EIVC operation.

Comparison between Unthrottled, Single and Two-valve Induction Strategies Utilising Direct Gasoline Injection: Emissions, Heat-release and Fuel Consumption Analysis

For a spark-ignition engine, the parasitic loss suffered as a result of conventional throttling has long been recognised as a major reason for poor part-load fuel efficiency. While lean, stratified charge, operation addresses this issue, exhaust gas aftertreatment is more challenging compared with homogeneous operation and three-way catalyst after-treatment. This paper adopts a different approach: homogeneous charge direct injection (DI) operation with variable valve actuations which reduce throttling losses. In particular, low-lift and early inlet valve closing (EIVC) strategies are investigated. Results from a thermodynamic single cylinder engine are presented that quantify the effect of two low-lift camshafts and one standard high-lift camshaft operating EIVC strategies at four engine running conditions; both, two-and single-inlet valve operation were investigated. Tests were conducted for both port and DI fuelling, under stoichiometric conditions. Measurements of specific fuel consumption and exhaust emissions were carried out, while the combustion was analysed using heat release analysis. These tests were carried out in a thermodynamic single cylinder engine. In parallel, tests were conducted in a second engine having the same combustion chamber geometry but with extensive optical access through a transparent cylinder liner. The results from the thermodynamic engine were correlated with in-cylinder measurements in the optical engine of the fuel spray. The results show that there are worthwhile fuel consumption and exhaust emission benefits to be gained through de-activation of one of the two inlet valves at part-load conditions. The performance characteristics under certain load conditions were dependant on which intake valve was actuated.

Investigation of the Flow Field in the Upper Part of a Cyclone with Laser and Phase Doppler Anemometry

The cyclone is a well known apparatus for separating particles out of a gas stream. With the modern laser diagnostic technologies of laser and phase Doppler anemometry (LDA and PDA), there is the potential to measure the flow and particle field inside the cyclone. The gas phase only measurements used micron-sized oil seeding droplets, whereas the solid phase, chosen for the PDA particle size measurements, was limestone powder. To assess the possibility of measuring milled limestone particles with PDA, the measured size distribution was compared with those obtained by laser diffraction. The measurements inside the cyclone showed that the flow field in the upper part of the cyclone was different to that commonly thought. Therefore, the vertical height of the cyclones vortex finder could be shortened without deterioration of the separation efficiency. The particles found in the hold-up of the cyclone air flow were considerably larger than the average particle size in the feed pipe.

Optical Diagnostics and Direct Injection of Liquid Fuel Sprays

The research described here addresses the problem of a paucity of high quality data on the full field structure of high pressure liquid fuel sprays for gasoline direct injection, GDI, engines. The paper describes the application of phase Doppler anemometry, PDA, and single-shot laser sheet Mie imaging to the study of GDI sprays and discusses the methodologies adopted for the experimental systems and the optimisation of the techniques. Experimental data is presented which defines the spray structure in terms of PDA vector and scalar fields and single-shot CCD digital images. The work demonstrates the essential complementary nature of the single point and planar optical diagnostics for spray studies.

A Comparison of the Flow Fields Generated for Spark and Controlled Auto-ignition

Valve timing strategies aimed at producing internal exhaust gas re-circulation in a conventional spark ignition, SI, engine have recently demonstrated the ability to initiate controlled auto-ignition, CAI. Essentially the exhaust valves close early, to trap a quantity of hot exhaust gases in-cylinder, and the fresh air-fuel charge is induced late into the cylinder and then mixing takes place. As a logical first step to understanding the fluid mechanics, the effects of the standard and modified valve timings on the in-cylinder flow fields under motored conditions were investigated. Laser Doppler anemometry has been applied to an optical engine that replicates the engine geometry and different valve cam timings. The cycle averaged time history mean and RMS velocity profiles for the axial and radial velocity components in three axial planes were measured throughout the inlet and compression stroke. The turbulent mixing for the two cases are described in terms of the flow field maps of the velocity vectors, vorticity and turbulence kinetic energy and the integrated tumble ratio as a function of crankangle.

Numerical and particle image velocimetry characterizations of flow structures under steady state condition

A combined numerical and experimental study has been carried out to examine the structure of the flow field from a 4 valve cylinder head under steady state conditions. In this paper, Particle Image Velocimetry (PIV) and the finite-volume Computational Fluid Dynamic (CFD) package, STAR CCM+ have been used on a single cylinder Lotus engine head with a pent-roof for a number of fixed valve lifts at two pressure drops of 250 mm and 635 mm of H2O, equivalent to engine speeds of 2,500 and 4,000 RPM respectively. The Reynolds Stress Turbulence Model (RSTM) is used to simulate the turbulent flow structure. The generated mesh is a polyhedral type that holds nearly 3.7 million grid points. The computational results were validated through qualitative and quantitative comparisons with the PIV experimental measurements on both horizontal and vertical cross sections. Results show reasonable agreements between numerical and experimental outcomes for the mean and RMS velocity values at different locations inside the engi...

A CFD Model with Optical Validation on In-cylinder Charge Performances of CAI Engines

Over the past few decades, Homogeneous Charge Compression Ignition (HCCI) or Controlled Auto-Ignition (CAI) if it is fuelled with gasoline type of fuels has shown its potential to overcome the limitations and environmental issue concerns of the Spark Ignition (SI) and Compression Ignition (CI) engines. However, controlling the ignition timing of a CAI engine over a wide range of speeds and loads is challenging. Combustion in CAI is affected by a number of factors; the local temperature, the local composition of the air/fuel mixture, time and to a lesser degree the pressure. The in-cylinder engine charge flow fields have significant influences on these factors, especially the local gas properties, which leads to the influences towards the CAI combustion. In this study, such influences were investigated using a Computational Fluid Dynamics (CFD) engine simulation package fitted with a real optical research engine geometry. Applying a Laser Doppler Anemometry (LDA) to the same engine, the cycle averaged time history mean and Root Mean Square (RMS) velocity profiles for the axial and radial velocity components in three axial planes were measured throughout the inlet and compression stroke. The calculated results were compared with the experimental results in terms of the vectors flow fields, averaged integrated tumble ratio as a function of crankangle and the local velocities in this paper. The results from both studies showed good correlations.