火花点火天然气掺氢发动机结合EGR时的循环变动规律

在火花点火式天然气掺氢发动机上,开展天然气掺氢结合EGR时发动机循环变动的试验研究,分析了不同EGR率和掺氢比时发动机燃烧循环变动规律.结果表明,对于给定的燃料,随EGR率的增加,缸内最高压力和最大压力升高率下降,循环变动增加,缸内最高压力和最大压力升高率与其对应的曲轴转角之间的相关性减弱.平均指示压力下降且分布趋于分...     

发动机燃用天然气掺二氧化碳混合燃料的循环变动试验研究

在一台单缸火花点火发动机上开展了燃用不同组分配比的天然气掺二氧化碳混合气体燃烧循环变动的试验研究.研究结果表明:随着混合气中二氧化碳体积比的增加,燃烧稳定性下降,发动机循环中出现部分燃烧和失火等不正常燃烧现象.通过分析最高缸压和其对应曲轴转角的关系、平均指示压力与最高缸压对应曲轴转角的关系以及平均指示压力和最高缸压的关系等,考察了发动机快速燃烧循环与慢速燃烧循环在特征参数之间关系中的发展规律,混合气中二氧化碳体积比的增加,燃烧放热变慢,导致平均指示压力的循环变动系数增大.     

利用定容燃烧弹研究天然气掺氢混合燃料直喷燃烧循环变动

利用定容燃烧弹开展了天然气掺混0%～40%氢气混合燃料直喷燃烧循环变动研究,高压气体燃料(8.0 MPa)喷入定容燃烧弹模拟直喷发动机燃烧条件.在整体当量比为0.6和0.8下,试验采集了火焰发展图片和燃烧过程容弹内压力,从火焰发展图片和燃烧特征参数两个方面分析了掺氢和混合气分层分布对天然气直喷燃烧循环变动的影响.结果表明:燃烧循环变动起始于火焰发展初期阶段.随着掺氢比增加,火焰形态更规则且更集中于点火电极.同时,由于直喷燃烧方式混合气分层分布,能够实现低循环变动的稳定稀燃.循环变动随着掺氢比的增加而减小,这种趋势在稀燃工况((b=0.6)下更加明显.在直喷燃烧方式下,由于混合气分层分布减弱了火焰发展初期阶段对后续燃烧过程的影响,因此燃烧特征参数间呈现相互独立的关系.     

电喷汽油机燃用丁醇-汽油混合燃料的循环变动特性

对某电控进气道多点喷射汽油机分别燃用国Ⅳ汽油、纯丁醇,以及丁醇体积比分别为10%、15%、20%、50%、85%的汽油-丁醇混合燃料的循环变动特性进行了试验研究,试验中未对发动机进行任何改动。研究结果表明:与汽油的燃料特性相比,随着丁醇-汽油混合燃料中丁醇含量的升高,汽油机燃用丁醇-汽油混合燃料的缸内压力峰值及其对应曲轴转角的循环变动率、压力升高率峰值循环变动率及放热率峰值及其对应曲轴转角的循环变动率增大。     

沼气掺氢发动机燃烧稳定性试验

在一台单缸火花点火发动机上开展了燃用不同组分配比的沼气模拟气体的掺氢混合气的燃烧稳定性试验研究.研究结果表明:在15%～35%的掺氢比范嗣内,随着混合气中掺氢比的增加,发动机循环变动变小,燃烧稳定性提高.掺氢导致平均指示压力的循环变动系数减小,燃烧放热率加快,火焰发展期缩短.其中35%掺氢比的混合气比15%掺氢比的混合...     

典型工况下的稀燃天然气发动机燃烧循环变动研究

通过分别记录稀燃天然气发动机在怠速、低速大负荷及高速大负荷工况下的缸内压力数据,提取并计算各工况下缸内燃烧特征参数的循环变动率,进而研究各工况下稀燃天然气发动机燃烧循环变动的特性。结果表明:在怠速工况下,天然气发动机燃用稀混和气时,燃烧循环变动十分明显;与低速大负荷工况相比,稀燃天然气发动机在高速大负荷工况下的平均指示压力燃烧循环变动系数上升了一倍。     

预燃室式天然气掺氢发动机燃烧及排放模拟

为探索掺氢对预燃室式大功率中速天然气发动机燃烧和排放的影响,采用计算流体动力学耦合化学动力学方法,在一台6ACD320型天然气发动机上,对氢气体积分数为0～ 30％的天然气-氢气混合燃料的燃烧过程进行了数值模拟.结果表明:在天然气中掺氢促使缸内产生了更多的0、OH等活性自由基,从而加速了缸内火焰传播,发动机的指示燃气消...     

稀燃天然气发动机燃烧循环变动影响因素研究

通过对一台点燃式多点电喷稀燃天然气发动机进行试验,获得了不同工况下的平均指示压力循环变动系数,以此为基础研究了燃空当量比、节气门开度、转速及点火时刻对稀燃天然气发动机燃烧循环变动的影响趋势。结果表明:混合气燃空当量比越小,燃烧循环变动越明显,当燃空当量比降低到一定值时,平均指示压力循环变动系数的增长会突然加大;节气门开度越小燃烧循环变动越明显,节气门开度小于30%后,其对燃烧循环变动影响更加明显;燃烧循环变动量随转速上升有增加的趋势,在高转速工况下燃烧循环变动的加强尤其明显;在工况一定的条件下存在一个最优的点火时刻可使稀燃天然气发动机的燃烧循环变动最小。     

氢气缸内直喷发动机燃烧优化及热效率提升研究北大核心CSCD

基于一台1.5 L氢气缸内直喷发动机,通过试验研究了氢气燃料的燃烧特性,分析了深度增压和喷氢时刻对燃烧及热效率的影响。研究表明:稀燃模式下,随着过量空气系数增加,缸内最高燃烧压力越来越高且对应曲轴转角提前,放热率峰值逐渐下降同时放热始点提前,放热时长变长,缸内燃烧温度和压升率下降。深度增压后,有效热效率可提升3%。靠近上止点喷射时,缸内喷射背压增加,喷射流量减小,提前喷射容易出现早燃,优化喷射策略后的有效热效率达43.5%。     

不同点火提前角时HCNG发动机的燃烧与排放特性

在一台火花点火天然气发动机上开展了在不同点火提前角下燃用不同体积掺氢比(O%～50%)的天然气掺氢燃料(HCNG)的试验研究,进行热效率、燃烧放热率、循环变动及排放特性的分析.结果表明:与原天然气发动机相比,HCNG发动机的最大扭矩点火提前角(MB了)减小,MBT时指示热效率变化不大;点火提前角增大时,火焰发展期增长,最大压力变动率减小,快速燃烧期和平均指示压力变动率先减小后增大;在相同点火提前角时,以上4个参数均随掺氢比的增加而减小.N0x、CO排放浓度随掺氢比增加而增大,CH4排放則相反.     

Cycle-by-cycle variations in a spark ignition engine fueled with natural gas–hydrogen blends combined with EGR

Study of cycle-by-cycle variations in a spark ignition engine fueled with natural gas–hydrogen blends combined with exhaust gas recirculation (EGR) was conducted. The effects of EGR ratio and hydrogen fraction on engine cycle-by-cycle variations are analyzed. The results show that the cylinder peak pressure, the maximum rate of pressure rise and the indicated mean effective pressure decrease and cycle-by-cycle variations increase with the increase of EGR ratio. Interdependency between the above parameters and their corresponding crank angles of cylinder peak pressure is decreased with the increase of EGR ratio. For a given EGR ratio, combustion stability is promoted and cycle-by-cycle variations are decreased with the increase of hydrogen fraction in the fuel blends. Non-linear relationship is presented between the indicated mean effective pressure and EGR ratio. Slight influence of EGR ratio on indicated mean effective pressure is observed at low EGR ratios while large influence of EGR ratio on indicated mean effective pressure is demonstrated at high EGR ratios. The high test engine speed has lower cycle-by-cycle variations due to the enhancement of air flow turbulence and swirls in the cylinder. Increasing hydrogen fraction can maintain low cycle-by-cycle variations at high EGR ratios.     

柴油机预混合燃烧循环变动特性研究

在发动机台架进行了直喷式柴油机预混合燃烧中低负荷循环变动的试验研究。分析了EGR、喷油始点和喷油压力对预混合燃烧循环变动的影响。研究结果表明:预混合燃烧的燃烧持续期短,放热迅速,最大压力升高率较大,增大EGR和推迟喷油降低了最高燃烧压力和最大压力升高率,最大压力升高率的循环变动系数随EGR增大而增大,随喷油推迟而减小。燃烧发展期与最高燃烧压力、最大压力升高率、最大放热率、燃烧持续期和燃烧重心等参数具有很强的相关性。燃烧循环变动小的点与排放较好的点参数相吻合。利用空燃比很好地反映了EGR、喷油率始点等参数对燃烧循环变动系数的影响。平均指示压力的循环变动系数在正常情况均小于10%。并提出了减小循环变动进一步降低排放可能采取的措施。     

Strategies to improve the performance of a spark ignition engine using fuel blends of biogas with natural gas,propane and hydrogen

This work presents the strategies applied to improve the performance of a spark ignition (SI) biogas engine. A diesel engine with a high compression ratio (CR) was converted to SI to be fueled with gaseous fuels. Biogas was used as the main fuel to increase knocking resistance of the blends. Biogas was blended with natural gas, propane, and hydrogen to improve fuel combustion properties. The spark timing (ST) was adjusted for optimum generating efficiencies close to the knocking threshold. The engine was operated on each blend at the maximum output power under stable combustion conditions. The maximum output power was measured at partial throttle limited by engine knocking threshold. The use of biogas in the engine resulted in a power derating of 6.25% compared with the original diesel engine (8 kW @ 1800 rpm). 50% biogas + 50% natural gas was the blend with the highest output power (8.66 kW @1800 rpm) and the highest generating efficiency (29.8%); this blend indeed got better results than the blends enriched with propane and hydrogen. Tests conditions were selected to achieve an average knocking peak pressure between 0.3 and 0.5 bar and COV of IMEP lower than 4% using 200 consecutive cycles as reference. With the blends of biogas, propane, and hydrogen, the output power obtained was just over 8 kW whereas the blends of biogas, natural gas, and hydrogen the output power were close to 8.6 kW. Moreover, a new approach to evaluate the maximum output power in gas engines is proposed, which does not depend on the engine % throttle but on the limit defined by the knocking threshold and cyclic variations.     

Effect of ignition timing and hydrogen fraction on combustion and emission characteristics of natural gas direct-injection engine

An experimental study on the combustion and emission characteristics of a direct-injection spark-ignited engine fueled with natural gas/hydrogen blends under various ignition timings was conducted. The results show that ignition timing has a significant influence on engine performance, combustion and emissions. The interval between the end of fuel injection and ignition timing is a very important parameter for direct-injection natural gas engines. The turbulent flow in the combustion chamber generated by the fuel jet remains high and relative strong mixture stratification is introduced when decreasing the angle interval between the end of fuel injection and ignition timing giving fast burning rates and high thermal efficiencies. The maximum cylinder gas pressure, maximum mean gas temperature, maximum rate of pressure rise and maximum heat release rate increase with the advancing of ignition timing. However, these parameters do not vary much with hydrogen addition under specific ignition timing indicating that a small hydrogen fraction addition of less than 20% in the present experiment has little influence on combustion parameters under specific ignition timing. The exhaust HC emission decreases while the exhaust CO₂ concentration increases with the advancing of ignition timing. In the lean combustion condition, the exhaust CO does not vary much with ignition timing. At the same ignition timing, the exhaust HC decreases with hydrogen addition while the exhaust CO and CO₂ do not vary much with hydrogen addition. The exhaust NO_x increases with the advancing of ignition timing and the behavior tends to be more obvious at large ignition advance angle. The brake mean effective pressure and the effective thermal efficiency of natural gas/hydrogen mixture combustion increase compared with those of natural gas combustion when the hydrogen fraction is over 10%.     

Experimental study of a single-cylinder engine fueled with natural gas–hydrogen mixtures

The objective of this study is to evaluate the power, efficiency and emissions of an electronic-controlled single-cylinder engine fueled with pure natural gas and natural gas–hydrogen blends, respectively. Replacing the nature gas with hydrogen/methane blend fuels was found to have a significant influence on engine performance. The effects of excess air ratio and spark timing were discussed. The results show that under certain engine conditions the maximum cylinder gas pressure, maximum heat release rate increased with the increase of hydrogen fraction. The increase of hydrogen fraction in the blends contributed to the increase of NO_x and the decrease of HC and CO. The brake specific fuel consumption decreased with the increase of hydrogen fraction. Using HCNG at relatively leaner fuel–air mixtures and retarded spark timing totally improved the engine emissions without incurring the performance penalty.     

Effect of natural gas enrichment with hydrogen on combustion characteristics of a dual fuel diesel engine

Environmental benefits are one of the main motivations encouraging the use of natural gas as fuel for internal combustion engines. In addition to the better impact on pollution, natural gas is available in many areas. In this context, the present work investigates the effect of hydrogen addition to natural gas in dual fuel mode, on combustion characteristics improvement, in relation with engine performance. Various hydrogen fractions (10, 20 and 30 by v%) are examined. Results showed that natural gas enrichment with hydrogen leads in general to an improved gaseous fuel combustion, which corresponds to an enhanced heat release rate during gaseous fuel premixed phase, resulting in an increase in the in-cylinder peak pressure, especially at high engine load (4.1 bar at 70% load). The highest cumulative and rate of heat release correspond to 10% Hydrogen addition. The combustion duration of gaseous fuel combustion phase is reduced for all hydrogen blends. Moreover, this technique resulted in better combustion stability. For all hydrogen test blends, COV_IMEP does not exceed 10%. However, no major effect on combustion noise was noticed and the ignition delay was not affected significantly. Regarding performance, an important improvement in energy conversion was obtained with almost all hydrogen blends as a result of improved gaseous fuel combustion. A maximum thermal efficiency of 32.5%, almost similar to the one under diesel operation, and a minimum fuel consumption of 236 g/kWh, are achieved with 10% hydrogen enrichment at 70% engine load.     

利用快速压缩装置研究天然气直喷燃烧循环变动

利用快速压缩装置研究了天然气直喷燃烧循环变动，研究结果表明：借助于分层燃烧和由燃料喷射的湍流引发的快速火焰传播，天然气直喷燃烧在小当量比条件下能实现良好的燃烧稳定性，低的压力峰值循环变动，低的压力升高率峰值循环变动和低的燃烧放热率峰值循环变动，研究发现燃烧期和燃烧产物的循环变动。CO和未燃碳氢的循环变动依赖于后续燃烧期的循环变动，NOx的循环变动依赖于快速燃烧期的循环变动。在燃烧最佳喷射条件下，天然气直喷燃烧的循环变动随当量比的变化并不敏感。     

Combustion behaviors of a direct-injection engine operating on various fractions of natural gas–hydrogen blends

Combustion behaviors of a direct injection engine operating on various fractions of natural gas–hydrogen blends were investigated. The results showed that the brake effective thermal efficiency increased with the increase of hydrogen fraction at low and medium engine loads and high thermal efficiency was maintained at the high engine load. The phase of the heat release curve advanced with the increase of hydrogen fraction in the blends. The rapid combustion duration decreased and the heat release rate increased with the increase of hydrogen fraction in the blends. This phenomenon was more obviously at the low engine speed, suggesting that the effect of hydrogen addition on the enhancement of burning velocity plays more important role at relatively low cylinder air motion. The maximum mean gas temperature and the maximum rate of pressure rise increased remarkably when the hydrogen volumetric fraction exceeds 20% as the burning velocity increases exponentially with the increase of hydrogen fraction in fuel blends. Exhaust HC and _CO2${\mathrm{CO}}_2$ concentrations decreased with the increase of the hydrogen fraction in fuel blends. Exhaust _NOx${\mathrm{NO}}_x$ concentration increased with the increase of hydrogen fraction at high engine load. The study suggested that the optimum hydrogen volumetric fraction in natural gas–hydrogen blends is around 20% to get the compromise in both engine performance and emissions.     

Effect of ignition timing and hydrogen fraction on combustion and emission characteristics of natural gas direct-injection engine

An experimental study on the combustion and emission characteristics of a direct-injection spark-ignited engine fueled with natural gas/hydrogen blends under various ignition timings was conducted. The results show that ignition timing has a significant influence on engine performance, combustion and emissions. The interval between the end of fuel injection and ignition timing is a very important parameter for direct-injection natural gas engines. The turbulent flow in the combustion chamber generated by the fuel jet remains high and relative strong mixture stratification is introduced when decreasing the angle interval between the end of fuel injection and ignition timing giving fast burning rates and high thermal efficiencies. The maximum cylinder gas pressure, maximum mean gas temperature, maximum rate of pressure rise and maximum heat release rate increase with the advancing of ignition timing. However, these parameters do not vary much with hydrogen addition under specific ignition timing indicating that a small hydrogen fraction addition of less than 20% in the present experiment has little influence on combustion parameters under specific ignition timing. The exhaust HC emission decreases while the exhaust CO₂ concentration increases with the advancing of ignition timing. In the lean combustion condition, the exhaust CO does not vary much with ignition timing. At the same ignition timing, the exhaust HC decreases with hydrogen addition while the exhaust CO and CO₂ do not vary much with hydrogen addition. The exhaust NOx increases with the advancing of ignition timing and the behavior tends to be more obvious at large ignition advance angle. The brake mean effective pressure and the effective thermal efficiency of natural gas/hydrogen mixture combustion increase compared with those of natural gas combustion when the hydrogen fraction is over 10%. __________ Translated from Transactions of CSICE, 2006, 24(5): 394–401 [译自:内燃机学报]     

Characteristics of direct injection combustion fuelled by natural gas–hydrogen mixtures using a constant volume vessel

The effects of hydrogen addition and turbulence intensity on the natural gas–air turbulent combustion were studied experimentally using a constant volume vessel. Turbulence was generated by injecting the high-pressure fuel into the vessel. Flame propagation images and combustion characteristics via pressure-derived parameters were analyzed at various hydrogen volumetric fractions (from 0% to 40%) and the overall equivalence ratios of 0.6, 0.8 and 1.0. The results showed that the turbulent combustion rate increased remarkably with the increase of hydrogen fraction in fuel blends when hydrogen fraction is over 11%. Combustion rate was increased remarkably with the introduction of turbulence in the bomb and decreased with the decrease of turbulence intensity. The lean flammability limit of natural gas–air turbulent combustion can be extended with increasing hydrogen fraction addition. Maximum pressure and maximum rate of pressure rise increased while combustion duration decreased monotonically with the increase of hydrogen fraction in fuel blends. The sensitivity of natural gas/hydrogen hybrid fuel to the variation of turbulence intensity was decreased while increasing the hydrogen addition. Maximum pressure and maximum rate of pressure rise increased while combustion duration decreased with the increase of turbulent intensity at stoichiometric and lean-burn conditions. However, slight influence on combustion characteristics was presented with variation of hydrogen fraction at the stoichiometric equivalence ratio with and without the turbulence in the bomb.