天然气发动机点火系统研发的理论与实践(上)

点火系统对发动机性能有非常重要的影响,点火系统的研发是一个跨电磁学和内燃机燃烧学科的课题。本文回顾了汽车点火系统的发展历程,对一汽锡柴的天然气发动机点火系统技术水平进行定位;研究了点火系统各变量之间的关系及其对发动机燃烧和性能的影响;研究了天然气发动机与汽油发动机点火系统所需击穿介质的物理和化学特性的差异;参考点火能量模拟计算和测试结果,确定一汽锡柴6SN1天然气发动机点火系统点火能量;综合平衡点火系统性能、寿命、成本,确定一汽锡柴6SN1天然气发动机点火系统的主要零件结构参数、性能参数;为满足更高排放要求,针对不同排放控制策略、提出未来点火系统的发展方向。     

天然气发动机点火系统研发的理论与实践(下)

点火系统对发动机性能有非常重要的影响,点火系统的研发是一个跨电磁学和内燃机燃烧学科的课题。本文回顾了汽车点火系统的发展历程,对一汽锡柴的天然气发动机点火系统技术水平进行定位;研究了点火系统各变量之间的关系及其对发动机燃烧和性能的影响;研究了天然气发动机与汽油发动机点火系统所需击穿介质的物理和化学特性的差异;参考点火能量模拟计算和测试结果,确定一汽锡柴6SN1天然气发动机点火系统点火能量;综合平衡点火系统性能、寿命、成本,确定一汽锡柴6SN1天然气发动机点火系统的主要零件结构参数、性能参数;为满足更高排放要求,针对不同排放控制策略、提出未来点火系统的发展方向。     

火花点火天然气发动机CFD变参数研究

应用AVL三维CFD软件FIRE,对一台柴油/天然气双燃料发动机改成的单一燃料天然气火花点火发动机进行了多维数值模拟计算。分别针对压缩比、过量空气系数、点火提前角进行了模拟研究,为大型天然气发动机的开发研究提供了有价值的参考数据。     

气体机点火时刻和燃料当量比对气耗影响的研究

研究了天然气(LNG)发动机点火提前角与燃料当量比对燃料消耗率的综合影响,对点火过程的各阶段进行了分析,对不同当量比的混合气燃烧过程进行了对比,通过改变点火提前角和燃料当量比,得到不同工况下发动机的点火提前特性和燃料调整特性,本次试验数据表明:在各工况下,要获得最低的燃料消耗率,点火提前角与燃料当量比均存在一个最佳值,选择较合适的点火提前角与燃料当量比,使天然气发动机可以兼顾低速动力性与高速经济性。     

天然气发动机点火正时的自适应控制策略研究

提出了一种新的天然气发动机点火正时自适应控制策略。通过对点火正时实施抖动形成寻优控制因素的变化、测量发动机转速波动差作为寻优控制响应的判据 ,从而实现以燃料经济性为寻优目标的闭环优化控制策略 ,弥补了点火正时开环控制方式的不足。通过软件和硬件控制系统的设计 ,在一台单缸天然气发动机上进行了应用 ,验证了其可行性。     

不同点火时刻下天然气掺氢缸内直喷发动机燃烧与排放特性

在缸内直喷火花点火发动机上开展了天然气掺混0％-18％氢气的混合燃料不同点火时刻下的试验研究。结果表明：对于给定的喷射时刻和喷射持续期,点火时刻对发动机性能、燃烧和排放有较大影响,喷射结束时刻与点火时刻的间隔对直喷天然气发动机极为重要,喷射结束时刻与点火时刻的间隔缩短时,混合气分层程度高,燃烧速率快,热效率高。最大放热率等燃烧特征参数随点火时刻的提前而增加。HC排放随点火时刻的提前而下降,CO2和NOx排放随点火时刻的提前而增加,NOx排放的增加在大点火提前角下更明显。掺氢可降低HC排放,对CO和CO2排放影响不大。掺氢量大于10％时可提高天然气发动机热效率。     

点火室直喷汽油机的电控点火与缸压采集系统的开发

开发了一套应用在点火室直喷汽油机上的电控点火与缸压采集系统,阐述了它的硬件组成、软件程序与实验验证.该系统具有实时修改点火提前角的电控点火功能和精确的缸压采集功能,也可以应用在天然气发动机等其他火花点火式发动机的开发试验中.     

压缩天然气发动机点火驱动的设计与研究

介绍了基于电控调压器技术的压缩天然气发动机电控系统.为了实现点火正时和点火能量的精确控制,设计了单缸高能点火驱动系统.基于电路仿真的方法,采用智能IGBT元件简化了点火驱动硬件电路设计.基于32位微控制器的TPU时间处理单元,可以精确实现点火正时的控制、点火提前角的控制及点火能量的控制,软件设计了点火正时和能量的修正方法.另外设计了用点火提前角进行调速的方法.开发的CNG发动机ECU及相应的点火驱动模块,已经成功应用于CNG发动机台架试验.试验结果表明:点火驱动硬件工作正常可靠,可以精确控制点火正时和点火能量,通过调整点火提前角可以实现怠速中调速的效果.     

柴油机改为火花点火式天然气发动机技术研究

结合天然气的特性,通过理论分析,论述了柴油机改为火花点火式天然气发动机所涉及的技术问题,如进气系统、燃烧系统、点火系统、控制策略等。     

电晕点火控制系统在天然气发动机上的应用研究

研究了电晕点火控制系统在天然气发动机上的应用,重点介绍了电晕点火的原理、电晕驱动器的控制方式、电晕控制系统信号流.通过对驱动器相关参数的标定优化,促进电晕放电的形成且最小化电弧形成的几率,得到了更高点火能量,达到更高燃烧效率.     

氮气稀释气对天然气着火特性的影响

为获得氮气稀释气对天然气着火特性的影响规律,对不同当量比、初始压力、初始温度以及氮气稀释系数下天然气的着火延迟时间进行了试验测试与数值计算,并分析了氮气稀释气对其着火特性的影响机理。结果表明,随初始温度的变化天然气着火延迟时间呈线性变化趋势;天然气的着火延迟时间随初始温度或初始压力的升高逐渐缩短,且受当量比的影响不明显;随着氮气稀释系数的增加,天然气的着火延迟时间逐渐延长,且促进天然气着火的关键反应步,■的敏感性系数将大幅度下降。     

Experimental study on ignition and combustion of coal-rice husk blends pellets in air and oxy-fuel conditions

The ignition and combustion characteristics of anthracite-rice husk (AC-RH) and bituminous coal-rice husk (BC-RH) pellets were investigated in a vertical heating tube furnace under different experimental condition, for gas temperature (873 K–1073 K) and under air and different oxygen concentration (21–70%) in CO₂/O₂ atmosphere. The investigation of the ignition and combustion characteristics focused on ignition mechanism, ignition delay, ignition temperature and combustion process. AC-RH pellets had two ignition mechanism in CO2/O2 atmosphere: homogeneous ignition of volatile and heterogeneous ignition of char. Heterogeneous ignition region decreased while homogeneous ignition increased as rice husk blending ratio increased in oxygen concentration-gas temperature plane. Only homogeneous ignition was observed when rice husk blending ratio was 30%. As for BC-RH pellets, only homogeneous ignition occurred in all experimental conditions. The effect of the rice husk blending on the anthracite was more pronounced than the bituminous coal for ignition mechanism. As oxygen concentration increased, a significant reduction in ignition delay and ignition temperature was observed at low rice husk blending ratio and low gas temperature. but at 1073 K, high oxidizer temperature weakened the effect of biomass blending and oxygen concentration on ignition delay and ignition temperature. Meanwhile, at 20% and 30% rice husk blending ratio, it also weakened the effect of oxygen concentration and oxidizer temperature on ignition delay and ignition temperature. In contrast, blending ratio had a more significant effect on ignition behavior. The replacement of N₂ by CO₂ at the same oxygen concentration contributed to an increase in ignition delay time and internal ignition temperature, which suppressed the ignition behavior. Different ignition mechanisms corresponded to different combustion processes.     

船用天然气发动机关键技术发展及应用

论述了船用天然气发动机技术的发展情况及关键技术,包括空燃比的控制、点火技术和安全设计等。并就天然发动机的应用中问题进行了探讨,包括燃料的标定和机油的选择等。     

基于转速反馈的CNG发动机点火提前角标定

点火正时对发动机运行时的动力性、经济性和排放具有很大影响,因此最佳点火提前角的选取十分重要。以发动机转速为反馈信号,利用MATLAB/Simulink软件建立CNG发动机点火提前角标定控制模型,算法实施平台为dSPACE公司的MicroAutoBox,在实验室NQ150N型天然气发动机上进行了算法测试。实验结果表明,以转速为反馈信号的点火提前角标定算法简单可行,并且能较快地找到对应工况下的最佳点火提前角。     

Criteria for piloted ignition of combustible solids

The onset of piloted ignition (flash point) of combustible polymers is predicted by a gas phase combustion energy density of 1.9 MJ/m³ that describes the lower flammability limit of fuel vapor-air mixtures. The mass flux (1 g/m² s) and (virtual) heat release rate (24 kW/m²) of the solid at incipient ignition derived from this observation are in general agreement with experimental data for piloted ignition of plastics. Sustained ignition marks the transition from lower-limit to stoichiometric burning and the critical heat release rate increases proportionally to 66 kW/m². A critical heat release rate, unique among ignition criteria, is independent of material composition because of the implicit coupling between the gas and condensed phase processes.     

An experimental investigation of iso-octane ignition phenomena

High-speed digital imaging has been used in rapid compression facility (RCF) studies to investigate ignition phenomena of iso-octane/air mixtures. Sequential images were captured for each experiment. The results indicate the existence of two ignition regimes. In one domain, ignition is rapid, typically less than 76 μs, and ignition occurs simultaneously throughout the test volume. In the other domain, reaction fronts form and propagate within the test volume prior to volumetric ignition. The data span equivalence ratios from ?=0.20 to 1.98, with inert/O₂ gas ratios from 1.38 to 5.89, pressures from 8.7 to 16.6 atm, and temperatures from 903 to 1020 K. The transition between the two regimes is discussed in the context of the mixture composition and experimental conditions. The analysis shows that the fuel mole fraction is a key parameter dictating the boundary between the modes of ignition. Below a critical mole fraction limit, volumetric ignition is observed; above the critical limit, reaction fronts are consistently present prior to volumetric ignition. The ignition delay times for both ignition regimes are well reproduced using a homogeneous simulation with detailed reaction chemistry, when the state conditions are modified to account for the presence of the reaction fronts. The results are discussed in terms of proposed reaction chemistry, ignition theory, and previous studies of iso-octane ignition.     

Ignitability limits for combustion of unintended hydrogen releases: Experimental and theoretical results

The ignition limits of hydrogen/air mixtures in turbulent jets are necessary to establish safety distances based on ignitable hydrogen location for safety codes and standards development. Studies in turbulent natural gas jets have shown that the mean fuel concentration is insufficient to determine the flammable boundaries of the jet. Instead, integration of probability density functions of local fuel concentration within the quiescent flammability limits, termed the flammability factor, was shown to provide a better representation of ignition probability. Recent studies in turbulent hydrogen jets showed that the envelope of ignitable gas composition (based on the mean hydrogen concentration), did not correspond to the known flammability limits for quiescent hydrogen/air mixtures. The objective of this investigation is to validate the flammability factor approach to the prediction of ignition in hydrogen leak scenarios. The ignition probability within a turbulent hydrogen jet was determined using a pulsed Nd:YAG laser as the ignition source. Laser Rayleigh scattering was used to characterize the fuel concentration throughout the jet. Measurements in methane and hydrogen jets exhibit similar trends in the ignition contour, which broadens radially until an axial location is reached after which the contour moves inward to the centerline. Measurements of the mean and fluctuating hydrogen concentration are used to characterize the local composition statistics conditional on whether the laser spark results in a local ignition event or complete light-up of a stable jet flame. The flammability factor is obtained through direct integration of local probability density functions. A model was developed to predict the flammability factor using a presumed probability density function with parameters obtained from experimental data and computer simulations. Intermittency effects that are important in the shear layer are incorporated in a composite probability density function. By comparing the computed flammability factor with the measured ignition probability we have validated the flammability factor approach for application to ignition of hydrogen jets.     

A comparison of various models in predicting ignition delay in single-particle coal combustion

In this paper, individual coal particle combustion under laminar conditions is simulated using models with various levels of complexity for the particle and gas phase chemical kinetics. The mass, momentum and energy governing equations are fully coupled between the particle and the gas phase. In the gas phase, detailed chemical kinetics based on GRI3.0 and infinitely-fast chemistry are considered and compared. For the particle phase, models for vaporization, devolatilization and char oxidation/gasification are considered, and the Kobayashi–Sarofim devolatilization model is compared to the Chemical Percolation Devolatilization (CPD) model. Ignition delay is used as a quantitative metric to compare the simulation prediction with experimental data, with careful attention given to the definition of ignition delay in the simulations. The effects of particle size, coal type and gas-phase temperature on the ignition delay are studied and compared with experimental data.     

Experimental investigation on unconfined hydrogen explosion with different ignition height

Experimental research is performed to investigate the effects of ignition height on explosion characteristics in a 27 m³ hydrogen/air cloud. With the ignition height decreasing, the flame propagation velocity increases gradually. The flame travels in oscillating mode and the average oscillating frequency lies between 145Hz and 155Hz. An original parameter τ, which involves flame scale and flame propagation velocity, is proposed to measure the effect of buoyancy. The higher the value of τ, the more obvious the buoyancy effect. As the ignition height increases, the critical flame scale for flame deceleration increases. The middle ignition height in the gas cloud causes the highest overpressure peak, overpressure impulse, overpressure rising and decreasing rate. As the ignition point approaches the initial gas boundary, the explosion intensity would decrease gradually. For the open space outside the flame, overpressure peak for the lower space is higher, while, the middle space experiences higher overpressure impulse.