Stratified Mixture Formation Research Articles

Computational study of nozzle spray-line distribution effects on stratified mixture formation, combustion and emissions of a high compression ratio DISI methanol engine under lean-burn condition

In this paper, five different spray-line distribution nozzles were designed to evaluate the effect of the nozzle spray-line distribution on stratified-charge combustion of methanol engine. Nozzle spray-line distribution characteristics, the stratified mixture formation, combustion and emissions of a high compression ratio direct-injection spark-ignition methanol engine for different spray-line distribution nozzles under lean-burn condition were numerically simulated. Simulation results showed that a non-uniform spray-line distribution nozzle of a 10-hole × 0.30 mm was conducive to form an ideal stratified mixture and obtain better combustion and engine performance. The maximum in-cylinder pressure of nozzle A was approximately 50.5% higher than that of nozzle E. The ignition delay and combustion duration of nozzle A were approximately both 20% lower than nozzle E. Nozzle A had lower unburned methanol, CO and soot emissions. The nozzle order of the highest NO emission was: A ≫ B > C > D > E. NO emission of nozzle A was approximately 13.6 times higher than that of nozzle E. According to the combustion and emission performance tradeoff, the nozzle order was: A > B > D > C > E. Nozzle A was preferred for actual spark ignition methanol engine applications.

Flow characteristics of natural-gas from an outward-opening nozzle for direct injection engines

In this research article, we present for the first time an experimental investigative study on the flow characteristics of compressed natural-gas (CNG) issued from a direct injector with an outward-opening nozzle operating at a wide range of pressures. High speed schlieren imaging is used to capture the growth of the highly turbulent transient gaseous jet with high spatial and temporal resolution in a quiescent chamber at atmospheric conditions. The existing penetration scaling correlations for gas jets issued from single round nozzles were found to apply to outward-opening nozzle as well. The penetration constant is found to be 1.15 ± 0.05 for fuel pressure ranging from 20 bar to 160 bar. A new method to calculate combined penetration for radially spreading jets such as the one used in this study is presented which can also be used for jets of irregular shape. The data from these tests show that the axial penetration from a 90° conical annular nozzle is roughly about 3 times lower than round nozzles for similar injection and ambient conditions. The lower penetration rate of such annular nozzles can be beneficial for stratified mixture formation in spray-guided direct-injection spark-ignition engines.

3D Numerical Simulation of Stratified Mixture Formation of DI LPG Engine

Low-carbon economy is a necessary requirement for sustainable development. LPG Direct Injection (DI) method can greatly improve the engines power performance, fuel economy and emission characteristics, hence a new wall-guided combustion system with a specially designed piston cavity used to form lean stratified mixture for DI LPG engine was proposed. In this paper, the stratified mixture formation process of a DI LPG engine was simulated. Before this, the model and method used in the simulation was firstly validated by simulating mixture formation process in an optical engine with conventional piston. The simulation results of the stratified mixture formation showed that, the mixture could gradually move upward along the combustion chamber wall under the guide of the spray induced entrainment vortex and the wall. At the same time, the rich mixture would diffuse to its surroundings. Finally, by the time to ignition, the ignitable stratified lean mixture could be formed.

Effect of a split-injection strategy on the performance of stratified lean combustion for a gasoline direct-injection engine

Fuel economy can be significantly improved in a gasoline direct-injection (GDI) engine owing to the stratified mixture formation strategy which guarantees stable combustion under ultra-lean air–fuel mixture conditions. In the GDI engine, it is widely accepted that the spray-guided direct-injection system, which is characterized by a close configuration between the centrally mounted injector and the spark plug, is regarded as a promising technology among the efforts to realize stratified lean combustion. With this configuration, the split injection can be an effective fuel injection scheme because of its potential to modulate the spray characteristics as well as to create a combustible mixture in the proximity of the spark plug. In this study, the split-injection strategy was assessed as a way to achieve ultra-lean operation in a GDI engine and the resulting lean-combustion characteristics were investigated in terms of the engine performance and emissions. The engine was tested at constant operating conditions (an engine speed of 3000 r/min and an indicated mean effective pressure of 0.4 MPa) with a change in the fuel injection pressure ranging from 10 MPa to 20 MPa. The excess air ratio λ varied from stoichiometry to the lean flammability limit, which turned out to be extended in the GDI approach. The engine test results show that the split injection of fuel allowed the formation of an adequately stratified mixture in lean-combustion conditions, and thus stable combustion was guaranteed. These results indicate that an excess air ratio was one of the important factors affecting the thermal efficiency and nitrogen oxide (NO x) emissions and was significantly extended to λ = 2.0 by the split-injection scheme. In addition, several fuel injection parameters such as the injection quantity, the split ratio, and the number of splits were examined with an emphasis on their impact on the fuel economy and emissions. The results demonstrate that there exists a trade-off between the NO x reduction and the efficiency, and therefore it is required to choose an appropriate injection strategy depending on the engine operating conditions in order to satisfy emission regulations.

ファンスプレー直接噴射ガソリン機関における混合気形成のガス流動および燃焼室形状による制御―３次元数値シミュレーションによる検討

A physical model for a fan spray from a slit injector is proposed. The behavior of a fan spray in a constant volume vessel, and in a direct-injection gasoline engine, has been numerically analyzed by means of the authors' GTT code. By comparing these calculated results with the experimental results, the spray model's validity has been confirmed. Three-dimensional numerical simulation using the GTT code has shown that mixing of air and fuel which has been injected early during the compression stroke progresses better in the case of the inclined swirl flow than in the case of the tumble flow. Furthermore, a modification to the piston head shape can improve the stratified mixture formation.

An Experimental Study on the Combustion and Emission Characteristics of a Stratified Charge Compression Ignition (SCCI) Engine

Stratified charge compression ignition (SCCI) combustion, which is developed to extend the homogeneous charge compression ignition operating range, has a good potential to improve fuel economy and reduce emissions. In this study, SCCI combustion and emissions characteristics were investigated in a single-cylinder gasoline engine with a direct-injection system. From the experimental results, we elucidated the effects of air -fuel ratio, intake temperature, and injection timing including early and late injection on the attainable SCCI combustion region. Injection timing during the intake process was found to be an important parameter that affects the extension of the SCCI operating region. The effects of stratified mixture formation can be utilized to extend the operating range for suitable SCCI combustion under different engine speeds and compression ratio conditions. In this study, we found that the higher intake temperature is very effective to evaporate the injected fuel, and this effect helps to enhance SCCI combustion. The intake temperature can be controlled by the compression ratio. Finally, the injection timing and stratified mixture formation played an important role in achieving stable combustion and extending the operating range.

Characterization of the flow field and stratification effects of fuel spray in a visualization engine using DPIV and entropy analysis

The objective of this study was to analyze the spray characteristics according to the injection duration under ambient pressure conditions and the injection timing in the visualization engine. In order to investigate spray behavior, the spray velocity is obtained through the PIV method a useful optical diagnostics technology and the vorticity calculated from the spray velocity component. These results elucidated the relationship between vorticity and entropy which plays an important role in the diffusion process for early injection and the stratification process for late injection cases. In addition, the homogeneous diffusion rate of spray was quantified using the entropy analysis based on Boltzmann’s statistical thermodynamics. Using these methods, it was found that the homogeneous mixture distribution is more effective as a momentum dissipation of surrounding air than that of the spray concentration to increase the injection timing. It is found that the homogenous diffusion rate increased as the injection timing moved to the early intake stroke process, and BTDC 60° was the most efficient injection timing for the stratified mixture formation during the compression stroke.

정적연소기내에서의 분위기 온도 및 압력에 따른 혼합기 분포에 관한 성층화 정도 특성

It is well known that a lean bum engine caused by stratified mixture formation has many kinds of advantages to combustion characteristics, such as higher thermal efficiency and lower CO, NOx levels than conventional homogeneous mixture combustion. Although this combustion can achieve low fuel consumption technology, It produces much unburned hydrocarbon and soot because of heterogeneous equivalence ratio in the combustion chamber Therefore, the stratified mixture formation technology is very important to obtain the stable lean combustion In this paper, fundamental studies for stratified combustion were earned out using a constant volume combustion chamber The local effect of mixture formation according to control air-fuel distribution in the chamber was examined experimentally In addition, the effect of turbulence on stratified charge combustion process was observed by schlieren photography. From this study, we found that the flame propagation speed Increase With swirl flow and the swirl promotes the formation of fuel and au mixture<br/>

A Study on the Effect of Stratified Mixture Formation on Combustion Characteristics in a Constant Volume Combustion Chamber

It is well known that a lean burn engine caused by stratified mixture formation has many kinds of advantages to combustion characteristics, such as higher thermal efficiency and lower CO, NOx levels than conventional homogeneous mixture combustion. Although this combustion can achieve low fuel consumption, it produces much unburned hydrocarbon and soot because of inhomogeneity of the charge mixture in the combustion chamber. Therefore, it is necessary to investigate the effect of mixture formation on combustion characteristics in order to obtain the stable lean combustion. In this paper, fundamental studies for stratified combustion were carried out using a constant volume combustion chamber. The effect of mixture formation on the combustion characteristics in the chamber was examined experimentally. In addition, the effect of turbulence on stratified charge combustion process was observed by schlieren photography. From this study, as the swirl intensity increases, (Sv)max is rapidly enhanced and the period of combustion is shortened. We also find that the stratification degree can be quantified by using burning velocity and it was controlled by induced air pressure and turbulent intensity.

Numerical Analysis of Mixture Formation of a Direct Injection Gasoline Engine.

A direct injection gasoline engine employing a new stratified combustion system has been developed. A fan-shaped fuel spray and a shell-shaped piston cavity achieved the combustion strategy. The process of stratified mixture formation and consequent stratified combustion is affected by the fuel spray characteristics, therefore numerical analysis (CFD) was applied to understand the phenomena of mixture formation process. From the results of CFD, it was clarified that the vaporization characteristic is important in realizing a suitable stratified mixture formation, in addition to the spray liquid characteristics, and CFD enables prediction and analysis of actual phenomena.

New concept of a direct injection SI gasoline engine: a study of stratified charge combustion characteristics by radical luminescence measurement

A new stratified charge system, which employs a thin fan–shaped fuel spray and a shell–shaped position cavity, has been developed for direct injection gasoline engines. In order to clarify the characteristics of stratified mixture formation and combustion, spray motion visualization, local fuel concentration in piston cavity, and frame behavior observation were performed. The newly developed system achieves stable stratified combustion in the high-speed region due to optimized piston cavity shape and fuel spray characteristics, especially suitable spray penetration with high injection pressure which ensures the mixture formation is “robust” against air flow and accelerating mixture diffusion in the cavity.

A numerical study of the effects of piston head configurations on stratified mixture formation in gasoline direct-injection engines

In this paper, the characteristics of flow and spray motions affected by from piston head configurations were investigated numerically. Calculations were carried out from intake process to the end of compression. GTT (Generalized Tank and Tube method) code, which includes a third order upwind Chakravarthy-Osher TVD scheme andk-e turbulence model with fuel spray analysis was used for the calculations. As a results, piston heads with smaller radii of curvature were found to give stronger reverse tumble than those with larger radii of curvature. Similar results are shown in the convection and diffusion of fuel sprays.