Shamit Bakshi

In this paper, detailed computational study is presented which helps to understand and improve the fuel-air mixing in a new direct-mixture-injection two-stroke engine. This new air-assisted injection system-based two-stroke engine is being developed at the Indian Institute of Science, Bangalore over the past few years. It shows the potential to meet emission norms such as EURO-II and EURO-III and also deliver satisfactory performance. This work proposes a comprehensive strategy to study the air-fuel mixing process in this engine and shows that this strategy can be potentially used to improve the engine performance. A three-dimensional compressible flow code with standard k - ε turbulence model with wall functions is developed and used for this modeling. To account for the moving boundary or piston in the engine cylinder domain, a non-stationary and deforming grid is used in this region with stationary cells in the ports and connecting ducts. A flux conservation scheme is used in the domain interface to allow the in-cylinder moving mesh to slide past the fixed port meshes. The initial conditions for flow parameters are taken from the output of a three-dimensional scavenging simulation. The state of the inlet charge is obtained from a separate modeling of the air-assisted injection system of this engine. The simulation results show that a large, near-stoichiometric region is present at most operating conditions in the cylinder head plane. The state of the in-cylinder charge at the onset of ignition is studied leading to a good understanding of the mixing process. In addition, sensitivity of two critical parameters on the mixing and stratification is investigated. The suggested parameters substantially enhance the flammable proportion at the onset of combustion. The predicted P - θ from a combustion simulation supports this recommendation. Copyright © 2005 by ASME.

A Gasoline Direct Injection (GDI) engine typically operates on multiple fuel-preparation modes. In general, at higher loads a homogeneous mixture is favoured whereas a stratified mixture is preferred at part and low load conditions. This is usually achieved by altering the injection timing with respect to load and speed. In this paper the effect of injector configuration on the mixing process has been studied systematically. Two different injector configurations are considered, one with a central-hole injection and other with a 6-hole injection. The objective is to investigate the effect of initial fuel distribution inside the engine cylinder on charge preparation at the onset of ignition. This study also aims to explore a better solution for mixing in GDI engines by optimizing the GDI injector for both stratified and homogeneous mode of operations. An engine with a pentroof combustion chamber with centrally mounted injector and upright straight intake port and flat piston is selected. The computation begins from the start of the induction process and continued till the point of ignition. The dynamics of the mixing process is studied by grouping the in-cylinder charge in different bins in terms of the equivalence ratio. The temporal variation of the fraction of the mixture in different bins is studied as a function of time to understand the dynamics of the mixing process. Results from the parametric study indicate the possibility of switching the modes of mixing with respect to the operating conditions.

Experiments are carried out on an evaporating pendant droplet and the flow induced around the droplet due to evaporation is studied for different ambient gases. A pendant drop of ethanol is formed on a steel needle in a closed chamber maintained at room temperature (301 K). The chamber is filled with a pure gas (nitrogen, oxygen, argon or carbon dioxide). The evaporating droplet causes a flow due to thermal buoyancy (due to the temperature difference caused by evaporative cooling) and solutal buoyancy (due to the difference in molecular weight between the evaporating liquid and surrounding gas). This flow around the evaporating droplet is characterized by using PIV (particle image velocimetry) technique. From these experiments, it is observed that the flow characteristics such as the velocity and penetration length of the flow changes depending upon the molecular weight of the ambient gas. It is observed that the flow penetrates significantly in case of a lower molecular weight gas (e.g. nitrogen) and the penetration decreases with increasing molecular. This, in turn, affects the evaporation rate of the suspended droplet.

The combustion characteristics and emissions in IC engines are influenced by the fuel injection rate. The study of fuel injection rate profile thus becomes essential to improve engine performance. In the present study, the injection rate of GDI injectors was measured in terms of the instantaneous mass flow rate of fuel. A setup was fabricated to obtain the injection rate based on Bosch tube method. Multi-hole GDI injectors of two different manufacturers were used in this study and their performances were measured up to 175 bar of injection pressure. The injector energizing current, wave pressure and rail pressure data were acquired at the rate of 500 kHz on a common time scale. The injected mass value per pulse, obtained by integrating the discharge rate profile was compared with the average values of mass injected measured using a mass balance for both the injectors and their performance were compared. The hydraulic delay measured during injector closing was found to be more than that during injector opening for both the injectors. Injection rate values for any given energizing time was found to be higher for one of the injectors by both the rate profile method and the mass balance averaging method.

In this paper, detailed computational study is presented which helps to understand and improve the fuel-air mixing in a new direct-mixture-injection two-stroke engine. This new air-assisted injection system-based two-stroke engine is being developed at the Indian Institute of Science, Bangalore over the past few years. It shows the potential to meet emission norms such as EURO-II and EURO-III and also deliver satisfactory performance. This work proposes a comprehensive strategy to study the air-fuel mixing process in this engine and shows that this strategy can be potentially used to improve the engine performance. A three-dimensional compressible flow code with standard k−2 turbulence model with wall functions is developed and used for this modeling. To account for the moving boundary or piston in the engine cylinder domain, a non-stationary and deforming grid is used in this region with stationary cells in the ports and connecting ducts. A flux conservation scheme is used in the domain interface to allow the in-cylinder moving mesh to slide past the fixed port meshes. The initial conditions for flow parameters are taken from the output of a three-dimensional scavenging simulation. The state of the inlet charge is obtained from a separate modeling of the air-assisted injection system of this engine. The simulation results show that a large, near-stoichiometric region is present at most operating conditions in the cylinder head plane. The state of the in-cylinder charge at the onset of ignition is studied leading to a good understanding of the mixing process. In addition, sensitivity of two critical parameters on the mixing and stratification is investigated. The suggested parameters substantially enhance the flammable proportion at the onset of combustion. The predicted P - θ from a combustion simulation supports this recommendation. EF/proceedings.

Cavitation structures inside orifice of high pressure fuel injector nozzles have a major influence on internal flow and quality of spray. The present work provides a detailed understanding of the effect of injection pressure on cavitation and its shedding in the form of cloud cavities inside a cylindrical orifice. Commercial computational fluid dynamics (CFD) software, ANSYS Fluent with Reboud’s correction on the eddy viscosity term of k-ω SST turbulence model is used for the numerical investigations. Comparison of the results with literature on experimental studies shows that the employed modification of turbulent viscosity term by a user defined function helps in prediction of reentrant jet induced cavity shedding. Compressible, multi-phase simulations revealed non-cavitating, periodic cloud shedding, super cavitating and hydraulic flip regimes. Fourier transformation is performed on the time series data of fluctuating vapor fraction to predict the frequency of shedding of cavities for various cavitation numbers. It is shown that with a reduction in cavitation number, length of cavity increases and shedding frequency decreases.

This experimental work investigates the evaporation rates of liquids (water, ethanol) and the flow induced thereby for both pendant and sessile droplets in high density ambient gas (sulfur hexafluoride). The flow induced around an evaporating droplet is studied at ambient pressure, in a sealed glass chamber. A steel needle and aluminium substrate are used for the pendant and sessile droplet study, respectively. The evaporation rates of the droplets are measured using shadow imaging technique. Particle image velocimetry (PIV) technique is used to measure the flow induced around the evaporating droplet. This flow is observed for both water and ethanol droplets. The flow moves upward for both pendant and sessile droplets of ethanol. In case of water, the pendant droplet induces a flow which is initially upward, but with time, its direction changes downward. The evaporation-induced flow for a sessile droplet of water is however always in the upward direction. This indicates the relative role of thermal and solutal buoyancy in the genesis of the flow. Droplet surface temperatures are also measured and discussed in this paper.

The effect of the nature of the suspender on heat transfer into an evaporating microliter pendant droplet is studied experimentally and numerically. Experimental studies show that there is an intrusive effect of the suspender in the heat transfer during evaporation of pendant droplets. Evaporation rates of an ethanol droplet (diameter ~ 3mm) are found to be higher when suspended from a flat-tip, hollow metallic needle (steel) than from a quartz fibre. Experiments also show that replacing the metallic needle with a non-conducting (glass) needle, of similar conductivity as the quartz fibre, results in higher evaporation rates for the same initial diameter of the drop under the same atmospheric conditions. This suggests that the heat transfer to the liquid in the hollow portion inside the needle is significant in the case of a suspending needle and has a direct effect on the measured evaporation rates. Steady state simulations of the heat transfer in the evaporating ethanol droplet when suspended from the metallic needle indicate that there is significant heat transfer to the surface of the drop from the suspending needle. The path of heat flow is from the ambient to the liquid inside the needle through the needle wall and then into the droplet. The surface temperature of the droplet for the simulation is obtained from the measured data from the experiments. Simulations are performed using ANSYS Fluent 14.0 in an axisymmetric domain consisting of the needle and the exact shape of the droplet.

The possibility of multiple-injection in Common Rail Direct Injection (CRDI) engine allows achieving improved combination of oxides of nitrogen (NOx) and smoke emissions. In CRDI engines, the turbulent kinetic energy due to high pressure fuel injection is primarily responsible for fuel air mixing and hence the in-cylinder mixture formation. The air fuel mixing characteristics in the case of multiple-injection are quite different from that of single injection schedule. In this work a zero-dimensional model is proposed for mixing rate calculations with multiple-injection scheduling. The model considers generation and dissipation of in-cylinder turbulence through processes namely fuel injection, air swirl and combustion. The model constants are fine tuned with respect to the data available in existing literature. The model predictions are validated with the available data for the cylinder pressure and heat release rate histories on known single and multiple-injection schedules. These comparisons show good agreement to establish the role of mixing rate variations with multiple-injection. A single set of constants were found to match the cylinder pressure and heat release rate histories for single and multiple-injection from different sources in the literature. Further, the mixing rate and peak temperature predictions of the model are found to relate with the possible effect of specific injection scheduling on emission reductions reported in CRDI engine investigations. Copyright © 2009 by ASME.

It is becoming evident that the fuel-air mixing process in high pressure multiple-injection CRDI engines is different from the conventional single injection such that a simultaneous reduction between NOx and soot particulate emissions is realizable. As a novelty, this paper explores the physics behind the mixing processes in multiple-injection technique using a comprehensive phenomenological model developed and validated by the authors for predicting combustion and emissions characteristics of multiple-injection CRDI engines. Towards this objective, the paper predicts and relates the variations in mixing rates for double and triple injection schedules with their observed combustion and emission characteristics. These quantitative predictions of mixing rates in multiple-injection substantiate the cause of soot reduction during later part of CRDI combustion. The predictions of fuel evaporation, fuel air mixing and emission characteristics of high pressure multiple-injection CRDI engines obtained from the model are found to reveal features useful in understanding CRDI engine performance. The trends and relationship of injection and its related processes observed in this study conform to the experimental observations of several multiple-injection CRDI engine studies. The predictions from the model suggest that there must be an optimal injection schedule in order to achieve the simultaneous reduction of nitric oxide and soot particulate emissions with the minimum sacrifice on fuel economy. Copyright © 2012 by the Japan Society of Mechanical Engineers.