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Journal Article

Effect of Injection Pressure on Transient Behaviour of Wall-Interacting Jet Flame Base in an Automotive-Size Diesel Engine

2013-10-14
2013-01-2536
Influence of the injection pressure on the temporal evolution of lifted jet flame base upon the bowl wall impingement has been studied in a small-bore optical diesel engine. Previous studies suggest that the jet-wall interaction causes re-entrainment of combustion products into the incoming jet, which shortens the lift-off length during the injection and thereby increasing downstream soot. After the end of injection, the flame base slowly moves downstream as the diminishing jet momentum results in reduced re-entrainment. How the injection pressure impacts this transient behaviour of the flame base is a main focus of the present study. Common-rail pressure was varied from 70 to 160 MPa at a fixed injection mass (10 mg per hole) and timing (7°CA bTDC).
Journal Article

In-Flame Soot Sampling and Particle Analysis in a Diesel Engine

2013-04-08
2013-01-0912
In-flame soot sampling based on the thermophoresis of particles and subsequent transmission electron microscope (TEM) imaging has been conducted in a diesel engine to study size, shape and structure of soot particles within the reacting diesel jet. A direct TEM sampling is pursued, as opposed to exhaust sampling, to gain fundamental insight about the structure of soot during key formation and oxidation stages. The size and shape of soot particles aggregate structure with stretched chains of spherical-like primary particles is currently an unknown for engine soot modelling approaches. However, the in-flame sampling of soot particles in the engine poses significant challenges in order to extract meaningful data. In this paper, the engine modification to address the challenges of high-pressure sealing and avoiding interference with moving valves and piston are discussed in detail.
Journal Article

Effect of Fuel Volatility and Ignition Quality on Combustion and Soot Formation at Fixed Premixing Conditions

2009-11-02
2009-01-2643
This paper presents experimental results for two fuel-related topics in a diesel engine: (1) how fuel volatility affects the premixed burn and heat release rate, and (2) how ignition quality influences the soot formation. Fast evaporation of fuel may lead to more intense heat release if a higher percentage of the fuel is mixed with air to form a combustible mixture. However, if the evaporation of fuel is driven by mixing with high-temperature gases from the ambient, a high-volatility fuel will require less oxygen entrainment and mixing for complete vaporization and, consequently, may not have potential for significant heat release simply because it has vaporized. Fuel cetane number changes also cause uncertainty regarding soot formation because variable ignition delay will change levels of fuel-air mixing prior to combustion.
Journal Article

Influence of Diesel Injection Parameters on End-of-Injection Liquid Length Recession

2009-04-20
2009-01-1356
Diesel injection parameters effect on liquid-phase diesel spray penetration after the end-of-injection (EOI) is investigated in a constant-volume chamber over a range of ambient and injector conditions typical of a diesel engine. Our past work showed that the maximum liquid penetration length of a diesel spray may recede towards the injector after EOI at some conditions. Analysis employing a transient jet entrainment model showed that increased fuel-ambient mixing occurs during the fuel-injection-rate ramp-down as increased ambient-entrainment rates progress downstream (i.e. the entrainment wave), permitting complete fuel vaporization at distances closer to the injector than the quasi-steady liquid length. To clarify the liquid-length recession process, in this study we report Mie-scatter imaging results near EOI over a range of injection pressure, nozzle size, fuel type, and rate-of-injection shape. We then use a transient jet entrainment model for detailed analysis.
Journal Article

Transient Liquid Penetration of Early-Injection Diesel Sprays

2009-04-20
2009-01-0839
Diesel low-temperature combustion strategies often rely on early injection timing to allow sufficient fuel-ambient mixing to avoid NOx and soot-forming combustion. However, these early injection timings permit the spray to penetrate into a low ambient temperature and density environment where vaporization is poor and liquid impingement upon the cylinder liner and piston bowl are more likely to occur. The objective of this study is to measure the transient liquid and vapor penetration at early-injection conditions. High-speed Mie-scatter and shadowgraph imaging are employed in an optically accessible chamber with a free path of 100 mm prior to wall impingement and using a single-spray injector. The ambient temperature and density within the chamber are well-controlled (uniform) and selected to simulate in-cylinder conditions when injection occurs at -40 crank-angle degrees (CAD) or fewer before top-dead center (TDC).
Technical Paper

Influence of Injection Timing for Split-Injection Strategies on Well-Mixed High-Load Combustion Performance in an Optically Accessible Spark-Ignition Direct-Injection (SIDI) Engine

2017-03-28
2017-01-0657
One major drawback of spark-ignition direct-injection (SIDI) engines is increased particulate matter (PM) emissions at high load, due to increased wall wetting and a reduction in available mixture preparation time when compared to port-fuel injection (PFI). It is therefore necessary to understand the mechanics behind injection strategies which are capable of reducing these emissions while also maintaining the performance and efficiency of the engine. Splitting the fuel delivery into two or more injections is a proven way of working towards this goal, however, many different injection permutations are possible and as such there is no clear consensus on what constitutes an ideal strategy for any given objective. In this study, the effect of the timing of the first and second injections for an evenly split dual injection strategy are investigated in an optical SIDI engine running at 1200 RPM with an unthrottled intake.
Technical Paper

In-Cylinder Soot Reduction Using Microwave Generated Plasma in an Optically Accessible Small-Bore Diesel Engine

2018-04-03
2018-01-0246
The present study explores the effect of in-cylinder generated non-thermal plasma on hydroxyl and soot development. Plasma was generated using a newly developed Microwave Discharge Igniter (MDI), a device which operates based on the principle of microwave resonation and has the potential to accentuate the formation of active radical pools as well as suppress soot formation while stimulating soot oxidation. Three diagnostic techniques were employed in a single-cylinder small-bore optical diesel engine, including chemiluminescence imaging of electronically excited hydroxyl (OH*), planar laser induced fluorescence imaging of OH (OH-PLIF) and planar laser induced incandescence (PLII) imaging of soot. While investigating the behaviour of MDI discharge under engine motoring conditions, it was found that plasma-induced OH* signal size and intensity increased with higher in-cylinder pressures albeit with shorter lifetime and lower breakdown consistency.
Technical Paper

Optimisation of Image Processing Parameters for Flame Image Velocimetry (FIV) Measurement in a Single-Cylinder, Small-Bore Optical Diesel Engine

2019-04-02
2019-01-0719
High-speed soot luminosity movies are widely used to visualise flame development in optical diesel engines thanks to its simple setup and relatively low cost. Recent studies demonstrated the high-speed soot luminosity movies are not only effective in showing the overall distribution and temporal evolution of sooting flames but also flow fields within the flame through the application of combustion (or flame) image velocimetry. The present study aims to improve this imaging technique by systematically evaluating key image processing parameters based on high-speed soot luminosity movies obtained from a single-cylinder, small-bore optical diesel engine. The raw soot luminosity movies are processed using PIVlab - a Matlab-based open-source code widely used for particle image velocimetry (PIV) applications.
Technical Paper

Triple Injection Strategies for Gasoline Compression Ignition (GCI) Combustion in a Single-Cylinder Small-Bore Common-Rail Diesel Engine

2019-04-02
2019-01-1148
Implementing triple injection strategies in partially premixed charge-based gasoline compression ignition (GCI) engines has shown to achieve improved engine efficiency and reduced NOx and smoke emissions in many previous studies. While the impact of the triple injections on engine performance and engine-out emissions are well known, their role in controlling the mixture homogeneity and charge premixedness is currently poorly understood. The present study shows correspondence between the triple injection strategies and mixture homogeneity/premixedness through the experimental tests of second/third injection proportion and their timing variations with an aim to explain the observed GCI engine performance and emission trends. The experiments were conducted in a single cylinder, small-bore common-rail diesel engine fuelled with a commercial gasoline fuel of 95 research octane number (RON) and running at 2000 rpm and 830 kPa indicated mean effective pressure conditions.
Journal Article

In-Flame Soot Sampling and Morphology Analysis in an Optical Spark-Ignition Direct-Injection (SIDI) Engine

2018-04-03
2018-01-1418
Stringent particulate emission regulations are applied to spark-ignition direct-injection (SIDI) engines, calling for a significant in-cylinder reduction of soot particles. To enhance fundamental knowledge of the soot formation and oxidation process inside the cylinder of the engine, a new in-flame particle sampling system has been developed and implemented in a working optical SIDI engine with a side-mounted, wall-guided injection system. Using the sampling probes installed on the piston top, the soot particles are directly sampled from the petrol flame for detailed analysis of particle size distribution, structure, and shape. At the probe tip, a transmission electron microscope (TEM) grid is stored for the soot collection via thermophoresis, which is imaged and post-processed for statistical analysis. Simultaneously, the flame development was recorded using two high-speed cameras to evidence the direct exposure of the sampling grids to the soot-laden diffusion flames and pool fires.
Technical Paper

Combustion Control Using Two-Stage Diesel Fuel Injection in a Single-Cylinder PCCI Engine

2004-03-08
2004-01-0938
A diesel-fueled premixed charged compression ignition (PCCI) combustion technique using a two-stage injection strategy has been investigated in a single cylinder optical engine equipped with a common-rail fuel system. Although PCCI combustion has the advantages of reducing NOx and PM emissions, difficulties in vaporization of a diesel fuel and control of the combustion phase hinder the development of the PCCI engine. A two-stage injection strategy was applied to relieve these problems. The first injection, named as main injection, was an early direct injection of diesel fuel into the cylinder to achieve premixing with air. The second injection was a diesel injection of a small quantity (1.5 mm3) as an ignition promoter and combustion phase controller near TDC. Effects of injection pressure, injected fuel quantity and compression ratio were studied with variation of an intake air temperature.
Technical Paper

The Effect of Swirl Ratio and Fuel Injection Parameters on CO Emission and Fuel Conversion Efficiency for High-Dilution, Low-Temperature Combustion in an Automotive Diesel Engine

2006-04-03
2006-01-0197
Engine-out CO emission and fuel conversion efficiency were measured in a highly-dilute, low-temperature diesel combustion regime over a swirl ratio range of 1.44-7.12 and a wide range of injection timing. At fixed injection timing, an optimal swirl ratio for minimum CO emission and fuel consumption was found. At fixed swirl ratio, CO emission and fuel consumption generally decreased as injection timing was advanced. Moreover, a sudden decrease in CO emission was observed at early injection timings. Multi-dimensional numerical simulations, pressure-based measurements of ignition delay and apparent heat release, estimates of peak flame temperature, imaging of natural combustion luminosity and spray/wall interactions, and Laser Doppler Velocimeter (LDV) measurements of in-cylinder turbulence levels are employed to clarify the sources of the observed behavior.
Technical Paper

High-Speed Imaging of Soot Luminosity and Spectral Analysis of In-Cylinder Pressure Trace during Diesel Knock

2014-04-01
2014-01-1259
The present study focuses on the observation of knock phenomena in a small-bore optical diesel engine. Current understanding is that a drastic increase of pressure during the premixed burn phase of the diesel combustion causes gas cavity resonances, which in turn induce a high frequency pressure ringing. The frequency and severity of this ringing can be easily measured by using a pressure transducer. However, visual information of flames under knocking conditions is limited especially for a small-bore diesel engine. To fill this gap, high-speed imaging of soot luminosity is performed in conjunction with in-cylinder pressure measurement during knocking cycles in an automotive-size optical diesel engine. From the experiments, flames were observed to oscillate against the direction of the swirl flow when the pressure ringing occurred.
Technical Paper

Uncertainty in Sampling and TEM Analysis of Soot Particles in Diesel Spray Flame

2013-04-08
2013-01-0908
For better understanding of soot formation and oxidation processes applicable to diesel engines, the size, morphology, and nanostructure of soot particles directly sampled in a diesel spray flame generated in a constant-volume combustion chamber have been investigated using Transmission Electron Microscopy (TEM). For this soot diagnostics, the effects of the sampling processes, TEM observation methodology and image processing methods on the uncertainty in the results have not been extensively discussed, mainly due to the complexity of the analysis.
Journal Article

Size Distribution and Structure of Wall-Deposited Soot Particles in an Automotive-Size Diesel Engine

2013-10-14
2013-01-2534
Wall-deposition of soot particles occurs during the cylinder liner wall/flame interaction, which can potentially deteriorate engine oil quality and alter the heat loss rate in a diesel engine. These issues motivate a detailed study on structure and size of the wall-deposited soot particles. A morphological difference between the wall-deposited soot and in-flame soot particles is another focus of this study. We performed thermophoretic soot sampling in the cylinder liner wall using an in-liner-type sampler. Obtained soot particles were imaged by a transmission electron microscope and post-processed to acquire the number of particles, projection area on the sampling grid, and size distribution. The same set of data was also obtained for soot particles within the diesel flame using a probe-type sampler.
Technical Paper

Multiple Injection Strategy Investigation for Well-Mixed Operation in an Optical Wall-Guided Spark-Ignition Direct-Injection (WG-SIDI) Engine through Flame Shape Analysis

2016-10-17
2016-01-2162
One major drawback of spark-ignition direct-injection (SIDI) engines is increased particulate matter (PM) and unburned hydrocarbon emissions at high load, due to wall wetting and a reduction in available air/fuel mixing time when compared to port-fuel injection (PFI). It is therefore necessary to understand the mechanics behind injection strategies which are capable of reducing these emissions while also maintaining the performance and efficiency of the engine. This study investigates the effect of varying the number fuel injection events and equivalence ratio on the operation of a wall-guided SIDI (WG-SIDI) engine. Of particular interest is how increased mixture homogeneity achieved by the double injection events impacts in-cylinder conditions and flame development.
Technical Paper

Effects of Multiple Injections in a HSDI Diesel Engine Equipped with Common Rail Injection System

2004-03-08
2004-01-0127
Diesel fuel injection system is the most important part of the direct-injection diesel engine and, in recent years, it has become one of the critical technologies for emission control with the help of electronically controlled fuel injection. Common rail injection system has great flexibility in injection timing, pressure and multi-injections. Many studies and applications have reported the advantages of using common rail system to meet the strict emission regulation and to improve engine performance for diesel engines. The main objective of this study is to investigate the effect of pilot-, post- and multiple-fuel injection strategies on engine performance and emissions. The study was carried out on a single cylinder optical direct injection diesel engine equipped with a high pressure common rail fuel injection system. Spray and combustion evolutions were visualized through a high speed charge-coupled device (CCD) camera.
Technical Paper

Soot Formation Modelling of Spray-A Using a Transported PDF Approach

2015-09-01
2015-01-1849
Numerical simulations of soot formation were performed for n-dodecane spray using the transported probability density function (TPDF) method. Liquid n-dodecane was injected with 1500 bar fuel pressure into a constant-volume vessel with an ambient temperature, oxygen volume fraction and density of 900 K, 15% and 22.8 kg/m3, respectively. The interaction by exchange with the mean (IEM) model was employed to close the micro-mixing term. The unsteady Reynolds-averaged Navier-Stokes (RANS) equations coupled with the realizable k-ε turbulence model were used to provide turbulence information to the TPDF solver. A 53-species reduced n-dodecane chemical mechanism was employed to evaluate the reaction rates. Soot formation was modelled with an acetylene-based two-equation model which accounts for simultaneous soot particle inception, surface growth, coagulation and oxidation by O2 and OH.
Technical Paper

A Numerical Study of the Influence of Different Operating Conditions on the Combustion Development in an Automotive-Size Diesel Engine

2015-09-01
2015-01-1852
In this paper, numerical simulations of an automotive-size optical diesel engine have been conducted employing the Reynolds-Averaged Navier-Stokes (RANS) equations with the standard k-ε turbulence model and a reduced n-heptane chemical mechanism implemented in OpenFOAM. The current paper builds on a previous work where the model has been validated for the same engine using optical diagnostic data. The present study investigates numerically the influence of different operating conditions - relevant for modern diesel engines - on the mixture formation development under non-reactive conditions as well as low- and high-temperature ignition behaviour and flame evolution in the presence of strong jet-wall interactions typically encountered in automotive-size diesel engines. Also, emissions of CO and unburned hydrocarbons (UHC) are considered.
Technical Paper

Studying the Influence of Direct Injection on PCCI Combustion and Emissions at Engine Idle Condition Using Two Dimensional CFD and Stochastic Reactor Model

2008-04-14
2008-01-0021
A detailed chemical model was implemented in the KIVA-3V two dimensional CFD code to investigate the effects of the spray cone angle and injection timing on the PCCI combustion process and emissions in an optical research diesel engine. A detailed chemical model for Primary Reference Fuel (PRF) consisting of 157 species and 1552 reactions was used to simulate diesel fuel chemistry. The model validation shows good agreement between the predicted and measured pressure and emissions data in the selected cases with various spray angles and injection timings. If the injection is retarded to -50° ATDC, the spray impingement at the edge of the piston corner with 100° injection angle was shown to enhance the mixing of air and fuel. The minimum fuel loss and more widely distributed fuel vapor contribute to improving combustion efficiency and lowering uHC and CO emissions in the engine idle condition.
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