An experimental and mathematical study on the effects of ignition energy and system on the flame kernel development

A constant volume combustion chamber is used to investigate the flame kernel development of gasoline air mixtures under various ignition systems, ignition energies and spark plugs. Three kinds of ignition systems are designed and assembled, and the ignition energy is controlled by the variation of the dwell time. Several kinds of spark plugs are also tested. The velocity of flame propagation is measured by a laser deflection method, and the combustion pressure is analyzed by the heat release rate and the mass fraction burnt. The results represent that as the ignition energy is increased by enlarging either dwell time or spark plug gap, the heat release rate and the mass fraction burnt are increased. The electrodes materials and shapes influence the flame kernel development by changing he transfer efficiency of electrical energy to chemical energy. The diameter of electrodes also influences the heat release rate and the burnt mass fraction.     

Numerical Calculation of Minimum Ignition Energy for Hydrogen and Methane Fuels

Minimum ignition energies of hydrogen/air and methane/air mixtures have been investigated numerically by solving unsteady one-dimensional conservation equations with detailed chemical kinetic mechanisms. Initial kernel size needed for numerical calculation is a sensitive function of initial pressure of a mixture and should be estimated properly to obtain quantitative agreement with experimental results. A simple macroscopic model to determine minimum ignition energy has been proposed, where the initial kernel size is correlated with the quenching distance of a mixture and evaluated from the quenching distance determined from experiment. The simulation predicts minimum ignition energies of two sample mixtures successfully which are in a good agreement with the experimental data for the ranges of pressure and equivalence ratio.     

Effects of turbulent mixing on spray ignition in spray system

When a column of droplets freely falling from an ultrasonic atomizer was ignited behind a reflected shock, no ignition occurred at a temperature below 1100 K, even if the pressure was as high as IMPa. Although, a higher temperature condition ensured ignition, no luminous flame was observable by high-speed photography, and even if a luminous flame lump appeared at an extremely high temperature, it disappeared without spreading over the entire column of droplets in this case. It is known however that, if a fuel is injected into a diesel cylinder or an electric furnace, ignition occurs even at a temperature as low as 650 K with a luminous flame spreading over the entire spray. These differences could be caused by the effects of turbulent mixing between fuel droplets and hot air, in fact, turbulence-generating rods were placed on the upstream side of the spray column. Experimental results indicates that the ignition limit was lowered to 840 K, and the ignition delay period was decreased by increasing the intensity of turbulence. Furthermore, the light emission of the flame was intensified, and normal spray combustion was maintained in the low-temperature atmosphere after the shock tube ceased its operation.     

Flow direction characteristics in the vicinity of the spark plug in an S. I. Engine

The flame speed may be decomposed into the burning speed and the flame transport speed. The flame transport speed is affected considerably by the flow direction, variation rate of flow direction, and flow speed in the combustion chamber. Especially, the flow direction and the variation rate of flow direction at the spark plug location during the ignition period have an important effect on the ignition process and the early flame propagation process. We measured the flow direction component and the variation rate of flow direction with a hot wire probe at the spark plug location. It was shown that the representative flow direction of ignition period is the right-vertical direction of crank shaft and it was used to investigate the variation rate of flow direction.     

Improvement and experimental validation of a multi-zone model for combustion and NO emissions in CNG fueled spark ignition engine

This article reports the experimental and theoretical results for a spark ignition engine working with compressed natural gas as a fuel. The theoretical part of this work uses a zero-dimensional, multi-zone combustion model in order to predict nitric oxide (NO) emission in a spark ignition (SI) engine. The basic concept of the model is the division of the burned gas into several distinct zones for taking into account the temperature stratification of the burned mixture during combustion. This is especially important for accurate NO emissions predictions, since NO formation is strongly temperature dependent. During combustion, 12 products are obtained by chemical equilibrium via Gibbs energy minimization method and nitric oxide formation is calculated from chemical kinetic by the extended Zeldovich mechanism. The burning rate required as input to the model is expressed as a Wiebe function, fitted to experimentally derived burn rates. The model is validated against experimental data from a four-cylinder, four-stroke, SI gas engine (EF7) running with CNG fuel. The calculated values for pressure and nitric oxide emissions show good agreement with the experimental data. The superiority of the multizone model over its two-zone counterpart is demonstrated in view of its more realistic in-cylinder NO emissions predictions when compared to the available experimental data.     

Cycle resolved no emissions and its relation with combustion chamber pressure in an s.i. engine with fast response no analyzer

A fast response NO analyzer was applied to investigate the relation between cycle-by-cycle NO emissions and combustion chamber pressure. NO emissions were sampled at an isolated exhaust manifold of 4-stroke spark ignition engine to avoid the interference of exhaust gas from other cylinders. The linear correlation analysis was performed with collected data of NO emissions and combustion chamber pressure with respect to the various air-fuel mixture ratios and engine loads. The sampled data sets were obtained during 200 cycles at each operating condition. The results showed that there was a typical pattern in NO emissions from an exhaust port through a cycle. It was possible to set a block of crank angle in which the linear correlation coefficient between NO emissions and combustion chamber pressure was high. As the engine load increased, NO emissions were more dependent on combustion chamber pressure after TDC. It was also analyzed that the correlation between two parameters with respect to air-fuel mixture ratio tended to increase as mixture went leaner. Furthermore, this correlation coefficient for the mixture near the lean limit seemed to be kept high even though combustion was unstable.     

Large-scale Experimental System for Multiphase Fuel/Air Explosions

A large-scale experimental system for multiphase combustion and explosion study was developed and manufactured.The explosion tank consists of a 2 m diameter,3.5 m long tube and ellipsoidal domes on both ends.The volume of the experimental tank is 10 m3.Pressure histories of the explosion pressure can be measured at different locations in the tank.High pressure glass windows of 200～300 mm were used to have access to the visualization of the explosion process.The explosion process of methane/air mixture and methane/coal dust/air mixture initiated by a 40 J electric spark at the center of the tank was studied in the large-scale experimental system.Five pressure sensors were arranged in the tank with different distances from the ignition point.Ten dust dispersion units were equipped to eject dust into the tank.A high-speed camera system was used to visualize the flame propagation during the explosion process.The characteristics of the pressure wave and flame propagated in methane/air mixtures and methane/coal dust/air mixtures have been studied and analyzed.     

A probe to estimate the local fuel concentration in spark ignition engines: design and validation study of catalytic hot wire probe

A catalytic hot wire probe (CHWP) technique has been developed to estimate the local fuel concentration near the spark plug of a spark ignition engine. Knowledge of this local concentration is highly useful in studying the combustion process. The small fuel concentration variation is measured by superimposing a catalysis effect in the thermal balance of the hot wire. Various parameters such as pressure, temperature and sampling velocity have been tested for their effect on the hot wire catalytic response.To validate this CHWP technique, local fuel concentration was also measured by two optical diagnostic techniques: planar laser-induced fluorescence (PLIF) and fuel/air ratio laser-induced exciplex fluorescence (FARLIEF). Comparison with PLIF measurements shows good agreement, and the capabilities and limitations of both techniques. With the FARLIEF/CHWP study, we can characterise the time and spatial variation of the fuel/air ratio in the vicinity of the spark plug in the gasoline direct injection engine.     

Effect of spark plug protrusion on the performance and emission characteristics of an engine fueled by hydrogen-natural gas blends

Mixtures of hydrogen and natural gas are promising for improving efficiency and reducing harmful emissions in spark ignition engines, since limits of flammability can be extended while stable combustion is secured. In this research, the combustion characteristics of long electrode spark plugs were evaluated in a hydrogen blended with natural gas (HCNG) engine. Decreases in the flame propagation distance through the use of spark plugs can lead to increased burning rates and further improvement of fuel economy in HCNG engines. An 11-liter heavy duty lean burn engine was employed and performance characteristics including emissions were assessed according to the spark timing of the minimum advance for best torque (MBT) for each operating condition. Retarded MBT spark advance timing with long electrode spark plugs due to increased burning speed supported increases in engine efficiency and reductions of nitrogen oxide (NO_x) emissions. The lower positions of initial flame kernels due to the use of long electrode spark plugs were preferable to improvements of cyclic variability due to reduced flame front quenching, and carbon monoxide (CO) emissions at the flammability limit were also improved.     

直接起动阶段直喷汽油机运动特性的模拟

建立了直喷汽油机的三维数值模型和运动学模型,并进行了试验验证。模拟了直喷汽油机在直接起动过程中不同喷油策略和点火时刻下的燃烧特性、反转和正转过程的运动特性。结果表明：与单次喷油相比,采用两次喷油策略时,首个着火气缸内混合气燃烧后的最大气缸压力较大,而且其大小受到点火时刻的影响;首个着火气缸内混合气燃烧后的最大气缸压力较大,则直喷汽油机反转过程中转过的最大角度较大;在各种喷油条件下,第2个着火气缸在反转到其最大转角前2°左右点火,正转过程转速较高,有利于直喷汽油机的直接起动。     

An experimental study on the effects of impingement-walls on the spray and combustion characteristics of SIDI CNG

Compressed natural gas (CNG) is regarded as one of the most promising alternative fuels, and maybe the cleanest fuel for the sparkignition (SI) engine. In the SI engine, direct injection (DI) technology can significantly increase the engine volumetric efficiency and decrease the need of throttle valve. During low load and speed conditions, DI allows engine operation with the stratified charge, and the use of extremely lean fuel-air mixture enables relatively higher combustion efficiency. In this study, a combustion chamber with a visualization system is designed. The spray development and combustion propagation processes SIDI CNG were digital recorded. It was found that high injection pressure reduced the ignition probability significantly because of quenching of flame kernel. To improve the ignition probability, three kinds of impingement-walls were designed to help the mixture preparation. It was found that the CNG-air mixture can be easily formed after spray-wall impingement and the ignition probability was also improved. The results of this study can contribute important data for the design and optimization of spark-ignition direct injection (SIDI) CNG engine.     

Analysis of combustion and flame propagation characteristics of LPG and Gasoline fuels by laser deflection method

This work is to investigate the combustion characteristics and flame propagation of the LPG (liquified petroleum gas) and gasoline fuel. In order to characterize the combustion processes of the fuels, the flame propagation and combustion characteristics were investigated by using a constant volume combustion chamber. The flame propagation of both LPG and gasoline fuels was investigated by the laser deflection method and the high-speed Schlieren photography. The result of laser deflection method show that the error of measured flame propagation speed by laser method is less than5% compared with the result of high-speed camera. The flame propagation speed of the fuel is increased with the decrease of initial pressure and the increase of initial temperature in the constant volume chamber. The results also show that the equivalence ratio has a great effect on the flame speed, combustion pressure and the combustion duration of the fuel-air mixture.     

A study on the characteristics of soot formation and oxidation in free fuel droplet array

In this study, it was attempted to obtain the fundamental data for the formation and oxidation of soot from a diesel engine. Combustion of spray injected into a cylinder is complex phenomenon having physical and chemical processes, and these processes affect each other. There are many factors in the mechanism of the formation and oxidization of soot and it is necessary to observe spray combustion microscopically. In order to observe with that view, free fuel droplet array was used as an experimental object and the droplet array was injected into an atmospheric combustion chamber with high temperature. Ambient temperature of the combustion chamber, interdroplet spacing, and droplet diameter were selected as parameters, which affect the formation and oxidation of soot. In this study, it was found that the parameters also affect ignition delay of droplet. The ambient temperature especially affected the ignition delay of droplet as well as the flame temperature after self-ignition. As the interdroplet spacing that means the local equivalence ratio in a combustion chamber was narrow, formation of soot was increased. As diameter of droplet was large, surface area of the droplet was also broad, and hence evaporation of the droplet was more active than that of a droplet with relative small diameter.     

An experimental investigation of the engine operating limit and combustion characteristics of the RI-CNG engine

In this paper, the radical induced (RI) ignition method was applied into a compressed natural gas (CNG) engine to achieve rapid bulk combustion. The experimental RI-CNG engine was modified from a diesel engine. The combustion chamber of the modified diesel engine was divided into a sub-chamber and a main-chamber. The sub-chamber is physically separated from the main-chamber above the piston and is connected to the main-chamber via several passage holes. CNG is injected into the sub-chamber during the intake stroke and then ignited before the top dead center (TDC) by a spark plug. As the ignition occurs in the sub-chamber, the pressure rises, forcing the gases which contain a number of active radicals out into the main-chamber to ignite the unburned mixture. The purpose of this paper is to study the engine operating limit and the combustion characteristics of the RI-CNG engine. The engine operating limit was accessed with different engine speeds and injection timings. The obtained data including the coefficient of variation (COV), brake specific fuel consumption (BSFC), mass fraction burned and emissions were analyzed.     

预燃室射流点火对汽油机性能影响研究

通过试验手段对比分析了预燃室射流点火模式及火花塞点火模式 （SI）对燃烧性能的影响,结果表明:SI点火模式的发动机受高负荷爆震的限制,仅在中等负荷达到最佳的油耗率和热效率。压缩比（CR）的增加仅在中小负荷对油耗率和热效率有改善效果;相比于SI点火模式,预燃室射流点火模式可实现更快的燃烧速度和火焰传播速度,对SI发动机的爆震有较好的抑制效果,在中等负荷具有更低的油耗率和更高的热效率,但在低负荷及高负荷阶段,油耗率和热效率恶化;采用预燃室射流点火模式,能有效增加缸内燃烧速率,减轻CA50推迟对油耗率恶化的效果,通过提高压缩比实现降低油耗率的潜力和效果更好。     

Numerical modeling of combustion processes and pollutant formations in direct-injection diesel engines

The Representative Interactive Flamelet (RIF) concept has been applied to numerically simulate the combustion processes and pollutant formation in the direct injection diesel engine. Due to the ability for interactively describing the transient behaviors of local flame structures with CFD solver, the RIF concept has the capabilities to predict the auto-ignition and subsequent flame propagation in the diesel engine combustion chamber as well as to effectively account for the detailed mechanisms of soot formation, NO_X formation including thermal NO path, prompt and nitrous NO_X formation, and reburning process. Special emphasis is given to the turbulent combustion model which properly accounts for vaporization effects on the mixture fraction fluctuations and the pdf model. The results of numerical modeling using the RIF concept are compared with experimental data and with numerical results of the commonly applied procedure which the low-temperature and high-temperature oxidation processes are represented by the Shell ignition model and the eddy dissipation model, respectively. Numerical results indicate that the RIF approach including the vaporization effect on turbulent spray combustion process successfully predicts the ignition delay time and location as well as the pollutant formation.     

一种研究柴油甲醇双燃料的定容燃烧弹试验装置

设计并研制了一种定容燃烧弹试验装置,用于对柴油在甲醇/空气预混均质混合气中燃烧特性的基础研究。介绍了该试验装置的各子系统的原理、结构及特点。定容燃烧弹试验结果表明:甲醇抑制了柴油的着火燃烧,随着甲醇/空气混合气浓度的增大,燃烧火焰变暗,碳烟生成受到抑制。与空气热氛围相比,甲醇/空气混合气氛围延长了柴油的滞燃期,加长了火焰的浮起长度。火焰稳定后,甲醇氛围中火焰的浮起长度随时间的变化比在纯空气氛围中大。     

Numerical analysis of the combustion process in a four-stroke compressed natural gas engine with direct injection system

In this paper, a numerical study to simulate and analyze the combustion process occurred in a compressed natural gas direct injection (CNG-DI) engine by using a multi-dimensional computational fluid dynamics (CFD) code was presented. The investigation was performed on a single cylinder of the 1.6-liter engine running at wide open throttle at a fixed speed of 2000 rpm. The mesh generation was established via an embedded algorithm for moving meshes and boundaries for providing a more accurate transient condition of the operating engine. The combustion process was characterized with the eddy-break-up model of Magnussen for unpremixed or diffusion reaction. The modeling of gaseous fuel injection was described to define the start and end of injection timing. The utilized ignition strategy into the computational mesh was also explained to obtain the real spark ignition timing. The natural gas employed is considered to be 100% methane (CH₄) with three global step reaction scheme. The CFD simulation was started from the intake valves opening until the time before exhaust valves opening. The results of CFD simulation were then compared with the data obtained from the single-cylinder engine experiment and showed a close agreement. For verification purpose, comparison between numerical and experimental work are in the form of average in-cylinder pressure, engine power as well as emission level of CO and NO. This paper was presented at the 9^th Asian International Conference on Fluid Machinery (AICFM9), Jeju, Korea, October 16–19, 2007.     

Simplified Modeling of Deflagration in Vessels

A simplified method that models the deflagration process occurring in closed or vented vessels is described. When combustion occurs within the spherical or cylindrical vessels, the flame moves spherically or segmentally to the vessel periphery. The volume and area of each element along the propagating flame front are calculated by using simple geometrical rules. For instabilities and turbulence resulting in enhanced burning rates, a simple analysis results in reasonable agreement with the experimental pressure transients when two burning rates (a laminar burning rate prior to the onset of instability and an enhanced burning rate) were used. Pressure reduction caused by a vent opening at predetermined pressure was modeled. Parameters examined in the modeling include ignition location, mixture concentration, vented area, and vent opening pressure. It was found that venting was effective in reducing the peak pressure experienced in vessels. The model can be expected to estimate reasonable peak pressures and flame front distances by modeling the enhanced burning rates, that is, turbulent enhancement factor.     

双火花塞发动机性能的仿真分析

应用发动机三维性能模拟分析软件,计算出单火花塞点火发动机缸内循环工作压力,将其与实验值进行对比并修正,确定了合理的仿真参数。在此基础上,分别对单火花塞和双火花塞点火发动机的缸内燃烧状态和缸内循环工作压力变化趋势进行瞬态仿真对比,分析了双火花塞点火对发动机性能的影响。研究结果表明,双火花塞的火焰传播距离短、缸内燃烧迅速、最高爆发压力大,其最高爆发压力可提高4%以上。