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。     

不同喷射时刻下缸内直喷天然气发动机的燃烧特性

开展了天然气高压缸内直喷发动机不同喷射时刻时的燃烧特性研究。研究结果表明：燃料喷射时刻对发动机性能及排放有较大影响,喷射太迟会导致天然气和空气混合时间短,混合效果差,燃烧持续期长,放热速率慢。喷射过早会导致充量系数下降,燃料容易进入燃烧室狭缝间隙处,造成较高的HC排放。对于给定转速,发动机存在一个最佳燃料喷射提前角,此时缸内最高压力值最大,最大压力升高率和最大放热率最大,放热速率快,燃烧过程等容度好,火焰发展期、快速燃烧期和燃烧持续期短,发动机热效率高,HC、CO排放也维持较低水平。     

喷油时刻对汽油/乙醇发动机性能的影响

通过一台Yamaha YBR250发动机原机,进行乙醇直喷(EDI)与汽油进气道喷射(GPI)的改装,研究了EDI喷油时刻对发动机缸内混合气形成、燃烧和排放的影响,同时建立了EDI结合GPI发动机的三维计算模型,对试验工况进行了数值模拟.分别对喷雾模型和燃烧模型进行了试验标定,结果表明:通过改变EDI喷油时刻,进气涡流与喷雾带动气流的运动共同作用于缸内燃料的蒸发雾化,可以在火花塞附近形成不同程度的燃料浓区;延迟EDI喷油时刻至100°CA BTDC,能够有效地协同壁面传热以及乙醇蒸发作用,降低点火时刻缸内温度,从而降低最大爆发压力和缸内燃烧温度;相对于早喷工况,EDI喷油时刻为100°CA BTDC的工况能够有效降低缸内HC及NO排放.     

喷射参数对二甲醚PPCCI发动机燃烧与排放特性的影响

在一台6缸增压电控共轨二甲醚发动机上进行试验,研究了预喷时刻、预喷燃料量、喷射压力、主喷时刻等喷射参数对二甲醚部分预混合充量压缩燃烧(PPCCI)发动机燃烧与排放特性的影响。试验结果表明:随预喷时刻提前,缸内压力峰值降低,二甲醚发动机缸内燃烧由两阶段放热转变为PPCCI三阶段放热,氮氧化物(NOx)排放显著降低,HC和CO排放升高;随预喷射燃料量增加,缸内压力峰值及预混合燃烧的冷焰反应和热焰反应速率明显增大,NOx排放逐渐降低,HC和CO排放显著升高;随喷射压力降低,预混合燃烧热焰反应速率增加,主喷扩散燃烧始点推迟,扩散燃烧放热率峰值和NOx排放明显降低,HC和CO排放升高;随主喷时刻推迟,预喷预混合燃烧几乎没有变化,主喷扩散燃烧延后,缸内压力峰值和放热率峰值降低,NOx排放显著降低,HC和CO排放升高。     

船用直喷式天然气发动机喷射策略的优化

利用CONVERGE软件基于L23/30DF型船用天然气发动机建立了双天然气喷嘴、双引燃柴油喷嘴的直喷天然气发动机的缸内燃烧过程的CFD计算模型,计算了不同的柴油和天然气喷射时刻和间隔下发动机缸内燃烧和排放过程。结果表明:引燃柴油的喷射时刻及其与天然气喷射时刻的间隔,对直喷式天然气发动机燃烧和排放性能有重要影响。当喷射时刻为-25℃A时,发动机具有较高的缸内爆发压力和良好的排放性能。在引燃柴油和天然气喷射间隔为16℃A时,可获得最高的缸内爆发压力,此时soot排放降低了22%。     

不同进气温度条件下燃料特性对均质压燃燃烧过程影响的试验研究

为了研究不同进气温度(Tin)条件下燃料特性对均质压燃(HCCI)燃烧特性、工况范围和排放特性的影响,在一台改装的4缸直喷柴油机上进行了不同辛烷值(RON)基础燃料(PRF)、汽油和含氧燃料的进气加热发动机试验.结果表明,Tin较低时,PRF着火时刻最早,缸内最大爆发压力和峰值放热率最高,其次是汽油和含氧燃料,但Tin较高时,则是汽油燃料着火时刻最早,燃烧效率和能够实现的最高指示效率最高;Tin升高,所有燃料主燃烧着火时刻提前,缸内最大爆发压力和燃烧效率升高,HCCI正常运转工况范围向小负荷区域拓展,但Tin对指示效率的影响与燃料的RON有关.排放测试表明,Tin升高,所有燃料的HC和CO排放均降低,Tin较高时,燃料特性对两者的影响减小;Tin高于363 K时,汽油的HC和CO排放均最低;Tin和RON都相同,汽油燃料的Nox排放值相对较高,Tin对所有燃料的Nox排放的影响均不明显.     

乙醇DI/汽油PFI发动机性能与排放特性

基于一台点燃式发动机,对缸内直喷(DI)乙醇和进气道喷射(PFI)汽油的复合喷射方式进行了研究.与传统喷油模式相比,采用乙醇-汽油复合喷射能够提升发动机动力性.随直喷乙醇比例增加,缸内爆发压力升高;受乙醇燃烧速率和缸内冷却效果的综合影响,着火滞燃期和燃烧持续期先缩短后延长.最佳点火时刻下,单一汽油喷射(PFI和GDI)爆震频次超过10%,,发动机发生轻微爆震,而复合喷射乙醇比例超过20%,可消除爆震;随直喷乙醇比例增加,循环波动系数降低,当量燃油消耗率降低,指示热效率提高,复合喷射相对PFI可提高发动机热效率3.8%,;同时,能够有效降低NOx和HC常规气体排放物.通过采用相对较高的缸内直喷乙醇比例,复合喷射能够提高发动机热效率及抑制爆震并降低常规气体排放物.     

过渡闪沸喷雾对缸内直喷汽油机性能影响研究

利用CONVERGE软件仿真分析了缸内直喷汽油机中速大负荷工况下过渡闪沸喷雾对整机性能的影响规律,对比分析了过渡闪沸喷雾状态和冷态喷雾状态下汽油机的缸内混合气形成、燃烧及排放特性。结果表明：随着燃料温度升高,燃油喷雾液滴粒径减小,喷雾破碎和雾化速度加快,当燃料温度达到380 K时,喷雾在速燃期之前索特平均粒径已降低至0 mm附近。过渡闪沸状态下,发动机缸内平均温度、缸压峰值和放热率峰值均高于冷态喷雾状态。燃料温度为380 K时与360 K时缸压峰值相差不大。过渡闪沸状态下发动机的soot排放低于冷态喷雾,且NO_x排放较高,燃料温度为380 K时相对于360 K时soot排放下降了80%以上,而NO_x排放仅上升8%。在低温冷态喷雾下,燃料温度的提高对发动机缸内混合气形成、缸内燃烧状态以及soot排放生成均有显著影响。过渡闪沸喷雾状态下,缸内气流运动对于缸内混合气形成的影响效果相比于喷雾破碎程度的影响效果更为显著。过渡闪沸喷雾状态下继续提高燃料温度对于发动机混合气形成、燃烧状态和发动机排放改善效果逐渐减弱。     

高辛烷值燃料对HCCI增压发动机燃烧和排放影响的试验研究

在一台经改装的4缸直喷式柴油机上进行了不同辛烷值(RON)基础燃料(PRF)和93号汽油的进气增压(pin)对均质压燃(HCCI)燃烧特性、性能和排放影响的试验研究.结果表明,进气压力增加,发动机缸内最大爆发压力提高,着火时刻提前.增压后,RON对PRF着火时刻的影响减小,汽油的着火时刻滞后于PRF.进气压力增加,HCCI正常运转工况范围向大负荷和小负荷区域都得到拓展.增压后汽油燃料所能达到的最大负荷比PRF高.相同供油量下,进气压力提高,燃烧效率和净指示热效率先增大后减小;最高燃烧效率和净指示热效率均增大.进气压力增加,HCCI发动机的HC和Nox排放降低,CO排放升高.增压后,RON对PRF的HC、CO和Nox排放影响变小,汽油的HC、CO和Nox排放较PRF高.     

喷油正时和压力对混合燃料燃烧影响的试验

在一台4缸柴油机上对比研究喷油策略对柴油/汽油/正丁醇混合燃料燃烧和排放特性的影响.试验中发动机转速固定为1,600,r/min,使用的4种燃料为纯柴油(D100)、柴油/汽油混合燃料(D70G,30)、柴油/正丁醇混合燃料(D70B30)和柴油/汽油/正丁醇混合燃料(D70B15G15).结果显示:与D100相比,3种混合燃料的soot排放大幅降低,其中D70B30最低.汽油或正丁醇的混入导致缸内压力峰值、放热率峰值和最大压力升高率(MPRR)增大,滞燃期延长,主燃烧放热时刻(CA,50)推迟,燃油经济性恶化.然而,CO排放升高,喷油时刻提前,可以明显削弱这一现象.D100与混合燃料的NOx排放之间基本没有差异.并且,混合燃料的soot排放对喷油策略的敏感程度远低于D100.但推迟喷油能够大幅度抵消掺混汽油或正丁醇所引起的MPRR升高趋势.此外,喷油压力对soot排放的影响大于喷油正时.     

柴油/汽油反应活性控制压燃发动机喷油协调控制与排放特性研究

对某4缸高压共轨柴油机进气道进行改造,搭建了柴油/汽油双燃料反应活性控制压燃(reactivity controlled compression ignition,RCCI)发动机专用试验台架,设计了柴油/汽油双燃料RCCI燃烧汽油喷射控制策略,实现了全工况下汽油与柴油的协调喷射控制,系统地研究了不同运行工况下,不同汽油替代率对柴油机燃烧与排放性能的影响规律。结果表明:采用柴油/汽油双燃料RCCI燃烧控制策略,发动机可在其运行工况范围内实现高效清洁燃烧,随着汽油替代率的增加,发动机缸内最高压力逐渐增大,缸压峰值出现时刻推迟,放热率峰值降低,燃烧持续期延长,燃油消耗率降低,有效热效率升高,全碳氢、CO排放增加,NOx和碳烟排放降低。     

喷油定时对柴油机超多喷孔预混合燃烧影响研究

柴油机低温预混合燃烧能够同时大幅度降低NOx和碳烟(soot)排放,本研究采用大量废气再循环(EGR)实现低温燃烧来降低NOx排放,采用超多喷孔喷油嘴并结合高压喷射来缩短喷油持续期,实现预混合燃烧从而降低soot排放,主要对喷油定时如何影响柴油机超多喷孔预混合燃烧性能进行了试验研究,选定4个试验工况,通过改变喷油定时来测试柴油机性能,结果显示随着喷油始点从上止点前向后推迟,各工况的NOx和soot排放都有不同程度的同时下降,有别于传统燃烧方式,但HC,CO,比油耗(BSFC)有所升高。     

Effects of fuel combination and IVO timing on combustion and emissions of a dual-fuel HCCI combustion engine

This paper experimentally and numerically studied the effects of fuel combination and intake valve opening (IVO) timing on combustion and emissions of an n-heptane and gasoline dual-fuel homogeneous charge compression ignition (HCCI) engine. By changing the gasoline fraction (GF) from 0.1 to 0.5 and the IVO timing from –15°CA ATDC to 35°CA ATDC, the in-cylinder pressure traces, heat release behaviors, and HC and CO emissions were investigated. The results showed that both the increased GF and the retarded IVO timing delay the combustion phasing, lengthen the combustion duration, and decrease the peak heat release rate and the maximum average combustion temperature, whereas the IVO timing has a more obvious influence on combustion than GF. HC and CO emissions are decreased with reduced GF, advanced IVO timing and increased operational load.     

火花点火天然气发动机起动阶段HC排放特性研究

为降低天然气发动机起动阶段的HC排放,在一台6缸火花点火天然气发动机上进行了起动试验研究。控制起动阶段的喷射脉宽和点火提前角,采集起动后1min内的转速和排放数据。试验结果表明:在一定范围内,HC排放随喷射脉宽和点火提前角的增加而增加。为保证顺利起动并有较低的HC排放,起动初始阶段应采用较大的喷射脉宽和较大的点火提前角,而转速稳定后采用逐渐减小的喷射脉宽和点火提前角。     

掺混比例和喷射定时对二甲醚-乙醇发动机燃烧和排放的影响

在一台电控共轨发动机上,试验研究了乙醇掺混比例和喷射定时对二甲醚-乙醇混合燃料燃烧及排放的影响。结果表明:随乙醇比例的增加,滞燃期延长,燃烧持续期缩短,最大压力升高率上升。随喷射推迟,滞燃期延长,燃烧相位延后,燃烧持续期在纯二甲醚时延长,而在掺混乙醇时则先延长后缩短,最大压力升高率先下降后上升。掺混乙醇和推迟喷射使预混燃烧比例增加。随喷射推迟,混合燃料的排气温度升高,喷射推迟到上止点后,排气温度随乙醇比例的增加而升高,排气温度高,则废气能量高,增压器增压比大,进气流量大,导致缸内压缩压力升高。在上止点前喷射时,掺混乙醇能使HC和CO排放保持在较低范围的同时,一定程度降低NO_x排放,掺混15%的乙醇较纯二甲醚最大降低约11%NO_x排放。随推迟喷射,NO_x排放降低,最大降幅达52%,在过分推迟燃料喷射时,因热效率低,循环喷射量增加,含15%乙醇混合燃料的NO_x排放会高于纯二甲醚。HC和CO排放随喷射推迟而升高,且升高幅度增大。     

增压直喷汽油机燃油喷射特性对排放影响的试验研究

基于某1.5L涡轮增压直喷汽油机,搭建试验测试系统,采用试验匹配测试方法研究了喷油模式、喷油时刻、喷油比例、喷油压力等决定燃油喷射特性的关键参数对碳烟排放的影响。试验结果表明:单次喷油模式下在部分负荷时,喷油越提前,碳烟排放越多;在全负荷时,喷油越推迟,碳烟排放越多。在多次喷油模式下,随第一次喷油的推迟碳烟排放降低,随第二、三次喷油的推迟碳烟排放增加。提高喷油压力对部分负荷工况燃烧及排放改善不明显,但外特性工况碳烟排放显著下降,碳氢化合物排放总量也大幅度降低,缸内燃烧速度加快,燃烧稳定性提高,有效燃油消耗率降低约2%。     

Study on combustion and emission characteristics of a n-butanol engine with hydrogen direct injection under lean burn conditions

The n-butanol fuel, as a renewable and clean biofuel, could ease the energy crisis and decrease the harmful emissions. As another clean and renewable energy, hydrogen properly offset the high HC emissions and the insufficient of dynamic property of pure n-butanol fuel in SI engines, because of the high diffusion coefficient, high adiabatic flame velocity and low heat value. Hydrogen direct injection not only avoids backfire and lower intake efficiency but also promotes to form in-cylinder stratified mixture, which is helpful to enhance combustion and reduce emissions. This experimental study focused on the combustion and emissions characteristics of a hydrogen direct injection stratified n-butanol engine. Three different hydrogen addition fractions (0%, 2.5%, 5%) were used under five different spark timing (10° ,15° ,20° ,25° ,30° CA BTDC). Engine speed and excess air ratio stabled at 1500 rpm and 1.2 respectively. The direct injection timing of the hydrogen was optimized to form a beter stratified mixture. The obtained results demonstrated that brake power and brake thermal efficiency are increased by addition hydrogen directly injected. The BSFC is decreased with the addition of hydrogen. The peak cylinder pressure and the instantaneous heat release rate raises with the increase of the hydrogen addition fraction. In addition, the HC and CO emissions drop while the NOx emissions sharply rise with the addition of hydrogen. As a whole, with hydrogen direct injection, the power and fuel economy performance of n-butanol engine are markedly improved, harmful emissions are partly decreased.     

Numerical study on effects of hydrogen direct injection on hydrogen mixture distribution,combustion and emissions of a gasoline/hydrogen SI engine under lean burn condition

A numerical study on effects of hydrogen direct injection on hydrogen mixture distribution, combustion and emissions was presented for a gasoline/hydrogen SI engine. Under lean burn conditions, five different direct hydrogen injection timings were applied at low speeds and low loads on SI engines with direct hydrogen injection (HDI) and gasoline port injection. The results were showed as following: firstly, with the increase of hydrogen direct injection timing, the hydrogen concentration near the sparking plug first increases and then decreases, reaching the highest when hydrogen direct injection timing is 120°CA BTDC: Secondly, hydrogen can speed up the combustion rate. The main factor affecting the combustion rate and efficiency is the hydrogen concentration near the sparking plug: Thirdly, in comparing with gasoline, the NO_X emissions with hydrogen addition increase by an average of 115%. For different hydrogen direct injection timings, the NO_X emissions of 120°CA BTDC is the highest, which is 29.9% higher than the 75°CA BTDC. The hydrogen addition make the NO_X emissions increase in two ways. On the one hand, the average temperature with hydrogen addition is higher. On the other hand, the temperature with hydrogen addition is not homogeneous, which makes the peak of temperature much higher. In a word, the main factor of NO_X emissions is the size of high temperature zone in the cylinder: Finally, because the combustion is more complete, in comparing with gasoline, hydrogen addition can reduce the CO and HC emissions by 32.2% and 80.4% respectively. Since a more homogeneous hydrogen mixture distribution can influence a lager zone in the cylinder and reduce the wall quenching distance, these emissions decrease with the increase of hydrogen direct injection timing. The CO and HC emissions of 135°CA BTDC decrease by 41.5% and 71.4%, respectively, compared to 75°CA BTDC.     

Effects of Fischer-Tropsch diesel fuel on combustion and emissions of direct injection diesel engine

Effects of Fischer-Tropsch (F-T) diesel fuel on the combustion and emission characteristics of a single-cylinder direct injection diesel engine under different fuel delivery advance angles were investigated. The experimental results show that F-T diesel fuel exhibits shorter ignition delay, lower peak values of premixed burning rate, lower combustion pressure and pressure rise rate, and higher peak value of diffusion burning rate than conventional diesel fuel when the engine remains unmodified. In addition, the unmodified engine with F-T diesel fuel has lower brake specific fuel consumption and higher effective thermal efficiency, and presents lower HC, CO, NO_x and smoke emissions than conventional diesel fuel. When fuel delivery advance angle is retarded by 3 crank angle degrees, the combustion duration is obviously shortened; the peak values of premixed burning rate, the combustion pressure and pressure rise rate are further reduced; and the peak value of diffusion burning rate is further increased for F-T diesel fuel operation. Moreover, the retardation of fuel delivery advance angle results in a further significant reduction in NO_x emissions with no penalty on specific fuel consumption and with much less penalty on HC, CO and smoke emissions.