引燃柴油量及噴射間隔對直噴天然氣發(fā)動機(jī)排放的影響

李孟涵，張 強(qiáng)，李國祥※，邵思東

（1.山東大學(xué)能源與動力工程學(xué)院，濟(jì)南 250061；2.濰柴動力內(nèi)燃機(jī)可靠性國家重點(diǎn)實(shí)驗(yàn)室，濰坊 261001）

引燃柴油量及噴射間隔對直噴天然氣發(fā)動機(jī)排放的影響

李孟涵1，張 強(qiáng)1，李國祥1※，邵思東2

（1.山東大學(xué)能源與動力工程學(xué)院，濟(jì)南 250061；2.濰柴動力內(nèi)燃機(jī)可靠性國家重點(diǎn)實(shí)驗(yàn)室，濰坊 261001）

為優(yōu)化直噴天然氣發(fā)動機(jī)的噴射策略，在一臺六缸電控直噴天然氣發(fā)動機(jī)上，用試驗(yàn)方法研究了引燃柴油量及柴油/天然氣噴射間隔對發(fā)動機(jī)HC、CO和NOx排放的影響。試驗(yàn)結(jié)果表明：噴射間隔一定時，HC排放隨引燃柴油噴射量的增加而降低；在引燃柴油噴射量為4.0 mg時，HC排放隨噴射間隔的增加而增加；引燃柴油噴射量在6.0～11.5 mg范圍內(nèi)，HC排放在噴射間隔從0.5 ms變化到1.1 ms時，變化較??；噴射間隔增加到1.4 ms時，HC排放升高趨勢明顯。CO排放隨引燃柴油噴射量的變化規(guī)律為先降低后升高；在不同的柴油噴射量下增加噴射間隔，CO排放均降低。NOx排放隨引燃柴油噴射量的增加先降低后升高；在噴射間隔為0.5 ms時，NOx排放相對較小，在噴射間隔為1.4 ms時，NOx排放最高。增加引燃柴油噴射量有利于HC的減排，對CO排放的影響較小，但會導(dǎo)致NOx排放的惡化；增加噴射間隔會促使HC和NOx排放的升高，但CO排放有所降低。

發(fā)動機(jī)；柴油機(jī)；燃油噴射；引燃柴油量；噴射間隔；天然氣發(fā)動機(jī)；排放性

李孟涵，張 強(qiáng)，李國祥，邵思東.引燃柴油量及噴射間隔對直噴天然氣發(fā)動機(jī)排放的影響[J].農(nóng)業(yè)工程學(xué)報(bào)，2016，32（6）：95-100. doi：10.11975/j.issn.1002-6819.2016.06.013 http://www.tcsae.org

Li Menghan,Zhang Qiang,Li Guoxiang,Shao Sidong.Effects of diesel injection quantity and injection interval on emission characteristics of directly injected natural gas engine[J].Transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE）,2016,32(6):95-100.(in Chinese with English abstract) doi：10.11975/j.issn.1002-6819.2016.06.013 http:// www.tcsae.org

0 引言

研發(fā)并推廣高效清潔的氣體燃料發(fā)動機(jī)燃燒技術(shù)是實(shí)現(xiàn)燃料多元化、減少溫室氣體排放、減少有害污染物排放的有效途徑[1-5]。在中國，嚴(yán)重的霧霾天氣、特殊的能源消費(fèi)結(jié)構(gòu)和高速的社會經(jīng)濟(jì)發(fā)展形勢對天然氣發(fā)動機(jī)的研究提出了更緊迫的要求。柴油引燃直噴天然氣發(fā)動機(jī)采用柴油和天然氣兩套共軌系統(tǒng)，在上止點(diǎn)附近首先噴入一定量的引燃柴油，經(jīng)過一定的噴射間隔，主燃料天然氣由同一噴射器噴入氣缸后，被引燃柴油火焰點(diǎn)燃[6-9]。由于換氣和壓縮過程與柴油機(jī)類似，且天然氣主要的燃燒模式為擴(kuò)散燃燒，該型發(fā)動機(jī)可以實(shí)現(xiàn)動力性和排放性的兼顧[10-13]。

自二十世紀(jì)八十年代以來，國內(nèi)外科學(xué)家對柴油引燃直噴天然氣發(fā)動機(jī)的排放控制方法進(jìn)行了廣泛的研究。目前，比較常用的方法有：廢氣再循環(huán)[14]，天然氣成分優(yōu)化[15-16]以及噴射策略優(yōu)化[17-18]。其中，廢氣再循環(huán)技術(shù)會增加發(fā)動機(jī)結(jié)構(gòu)的復(fù)雜性，并且，由于天然氣的氫含量高，燃燒產(chǎn)生的大量水蒸氣會通過廢氣再循環(huán)進(jìn)入冷卻器及發(fā)動機(jī)，造成冷卻器及發(fā)動機(jī)燃燒系統(tǒng)的腐蝕，降低發(fā)動機(jī)的使用壽命，導(dǎo)致發(fā)動機(jī)維護(hù)成本的提高[19-25]。另外，通過添加惰性氣體、氫氣等組分優(yōu)化天然氣成分的排放控制技術(shù)仍處于試驗(yàn)室研究階段，實(shí)際應(yīng)用中涉及的氫氣等優(yōu)化成分的生產(chǎn)、儲存、供給等問題亟待解決。因此，噴射策略優(yōu)化依然是最經(jīng)濟(jì)實(shí)用的排放控制方法。對于柴油引燃直噴天然氣發(fā)動機(jī)，引燃柴油噴射量及柴油/天然氣噴射間隔與主燃料天然氣的引燃和后續(xù)的擴(kuò)散燃燒過程密切相關(guān)，繼而影響到發(fā)動機(jī)的HC、CO及NOx排放，是該型發(fā)動機(jī)排放控制的關(guān)鍵參數(shù)。然而至今為止對于柴油引燃直噴天然氣發(fā)動機(jī)噴射策略優(yōu)化的研究多集中于噴射時刻和噴射壓力，對于引燃柴油噴射量及柴油/天然氣噴射間隔對排放的影響缺乏系統(tǒng)的研究。

本文以電控雙共軌六缸高壓直噴天然氣發(fā)動機(jī)為研究對象，通過試驗(yàn)研究了引燃柴油噴射量及柴油/天然氣噴射間隔對發(fā)動機(jī)HC、CO及NOx排放的影響，對柴油引燃直噴天然氣發(fā)動機(jī)的排放控制和噴射策略優(yōu)化提供理論指導(dǎo)。

1 試驗(yàn)裝置及試驗(yàn)方案

1.1 試驗(yàn)裝置

試驗(yàn)發(fā)動機(jī)在原柴油機(jī)基礎(chǔ)上改裝而成，其缸徑為126 mm，行程為155 mm，壓縮比為17。電控系統(tǒng)為自主開發(fā)，在原柴油機(jī)電子控制單元（electric control unit，ECU）基礎(chǔ)上集成了天然噴射控制模塊，使電控系統(tǒng)可以靈活控制柴油及天然氣的噴射。增加了天然氣供給系統(tǒng)、天然氣共軌噴射系統(tǒng)和集成調(diào)壓模塊，為適應(yīng)同心雙軸針噴射器的安裝要求，原柴油機(jī)的缸蓋設(shè)計(jì)也相應(yīng)更改。所采用的排放測試設(shè)備及精度如表1所示。測試用排放分析儀的采樣方法為全流稀釋定容采樣法，HC排放的測試方法為氫離子火焰法，CO排放的測量方法為不分光紅外法，NOx排放的測量方法為化學(xué)發(fā)光法。試驗(yàn)工況穩(wěn)定5 min后開始各種排放數(shù)值的測量，并對每個工況點(diǎn)進(jìn)行3次取樣，取3次的平均值作為最終的排放結(jié)果。

表1 排放分析儀主要技術(shù)參數(shù)Table 1 Exhaust gas analyzer and main specifications

圖1 試驗(yàn)臺架布置圖Fig.1 Layout of test bed

1.2 試驗(yàn)方案

試驗(yàn)中保持進(jìn)氣溫度為22℃，環(huán)境壓力為101 kPa，柴油軌壓設(shè)定為18 MPa，為保證噴射器的密封性，天然氣的軌壓比柴油軌壓略低，為17.5 MPa。測試工況點(diǎn)發(fā)動機(jī)轉(zhuǎn)速設(shè)定為1 275 r/min,平均有效壓力為0.54 MPa（25%負(fù)荷），為十三點(diǎn)工況中較為典型的低轉(zhuǎn)速低負(fù)荷工況。為測試柴油噴射量（diesel injection quantity，DIQ）和柴油/天然氣噴射間隔（SDN,separation between diesel and natural gas injection）的影響，在整個試驗(yàn)過程中，天然氣噴射提前角固定在8°，試驗(yàn)在4.0、6.0、8.5、11.5 mg 4種引燃柴油噴射量和0.5、0.8、1.1、1.4 ms 4種噴射間隔下進(jìn)行，4種引燃柴油噴射量對應(yīng)的噴射脈寬（diesel pulse width，DPW）分別為0.25、0.3、0.5和0.7 ms，柴油噴射提前角根據(jù)引燃柴油噴射脈寬和噴射間隔的值進(jìn)行調(diào)整，天然氣噴射脈寬（natural gas pulse width，GPW）根據(jù)柴油噴射脈寬和固定的平均有效壓力進(jìn)行調(diào)整。

2 結(jié)果與分析

2.1 HC排放

圖2為引燃柴油噴射量及噴射間隔對總碳?xì)洌╰otal hydrocarbons，THC）及CH4排放的影響。如圖所示，隨著引燃柴油量的增加THC排放及CH4呈減小的趨勢。因?yàn)殡S著柴油噴射量的增加，柴油引燃火焰的強(qiáng)度增加，如圖3c所示，隨著引燃柴油噴射量的增加，柴油燃燒階段瞬時放熱率的峰值增加，增加了對主燃料天然氣的引燃能力，抑制了天然氣向遠(yuǎn)端的擴(kuò)散，且由于天然氣所需噴射量降低，天然氣噴射脈寬相應(yīng)縮短，同樣降低了天然氣的擴(kuò)散距離，減小了壁面淬熄發(fā)生的可能性，THC排放降低；另外如圖中THC和CH4的排放數(shù)據(jù)所示，在試驗(yàn)工況下，CH4在THC中占的比例為60%左右，同樣工況下柴油噴射量增加后，天然氣的比例減少，THC排放相應(yīng)降低。柴油噴射量為4.0 mg時，THC排放明顯高于其他3種柴油噴射量時的THC排放。這是因?yàn)榇藭r柴油噴射量最小，柴油引燃火焰的強(qiáng)度較弱，引燃能量不足，引燃穩(wěn)定性較差，導(dǎo)致整個燃燒過程的穩(wěn)定性降低，由于天然氣燃燒不完善生成的HC顯著增加。

圖2 引燃柴油噴射量及噴射間隔對THC及CH4排放的影響Fig.2 Effects of diesel injection quantity and injection separation on THC and CH4emissions

由圖2還可以看出，隨著柴油和天然氣噴射間隔的增加在柴油噴射量較少時THC及CH4呈升高趨勢，柴油噴射量較多時排放量差別小，但仍為噴射間隔大時排放高。因?yàn)殡S著柴油/天然氣噴射間隔角的增加，柴油火焰發(fā)展時間較充分，如圖3所示，噴射間隔較大時，天然氣噴射時刻柴油放熱量較多，柴油火焰周圍的氧氣被消耗的比較多，此刻天然氣噴入柴油燃燒后的廢氣區(qū)域，形成燃料的過濃區(qū)，噴入柴油已燃區(qū)的部分天然氣因缺氧而未參與燃燒，主要以CH4的形式排出，造成HC排放升高。柴油噴射量為4.0和6.0 mg時，柴油噴射量較小，柴油燃燒持續(xù)期短，此時由噴射間隔過大導(dǎo)致的HC排放的增加較為明顯。噴射量為8.5和11.5 mg時，柴油噴射量增加，柴油燃燒持續(xù)期相應(yīng)延長，噴射間隔對HC排放的影響減小。

圖3 引燃柴油量及噴射間隔對放熱率的影響Fig.3 Effects of diesel injection quantity and injection separation on heat release rate

2.2 CO排放

圖4為引燃柴油量及噴射間隔對CO排放的影響。如圖所示，CO排放隨柴油噴射量的變化規(guī)律為先降低后升高。分析原因?yàn)椋褐眹娞烊粴獍l(fā)動機(jī)CO來源包括不完全燃燒的引燃柴油和不完全燃燒的主燃料天然氣，引燃柴油量過小時（4.0 mg），引燃柴油噴射脈寬過短（0.25 ms），引燃能力弱，同時噴射脈寬過短容易因柴油軌壓波動等原因?qū)е碌囊疾裼蛧娚淞垦h(huán)差異性較大，直接影響整個燃燒過程的穩(wěn)定性，CO排放相對較高；引燃柴油量過大時（11.5 mg），一方面天然氣噴入柴油已燃區(qū)導(dǎo)致燃燒不充分，另外，柴油火焰在引燃時刻燃燒過度，其引燃能力下降，導(dǎo)致CO排放值的增加。在噴射間隔為0.5和0.8 ms時，柴油和天然氣之間的時間間隔相對較短，在柴油噴射量為6.0 mg時出現(xiàn)CO排放的最低值；在噴射間隔為1.1和1.4 ms時，2種燃料噴射之間的時間間隔增加，柴油噴射量為8.5 mg時可以實(shí)現(xiàn)引燃能量和引燃時刻的兼顧。

由圖4可見，增加噴射柴油/天然氣噴射間隔可以降低CO的排放值。當(dāng)引燃柴油噴射量為6.0、8.5和11.5 mg時，柴油燃燒持續(xù)期較長，噴射間隔從1.1 ms增加到1.4 ms會大幅降低CO的生成，然而，引燃柴油量為4.0 mg時，引燃柴油噴射量較小，燃燒持續(xù)期較短，增加噴射間隔對CO排放減排的效果下降。這是因?yàn)椋?種燃料噴射間隔較近時，在混合過程中較冷的天然氣噴束會影響引燃柴油的霧化，產(chǎn)生局部燃料過濃的區(qū)域，并且在引燃柴油燃燒過程中，天然氣的引燃會加劇2種燃料氧氣的爭奪，進(jìn)而造成局部缺氧，促進(jìn)了CO的生成，噴射間隔較高時在柴油的焰后區(qū)形成天然氣的過濃區(qū)，但這部分燃料不燃燒主要以CH4形式排出，對CO排放的貢獻(xiàn)較小，因此適當(dāng)增加噴射間隔有利于減小兩種燃料燃燒之間的干涉，降低CO的排放，實(shí)際標(biāo)定中根據(jù)HC和CO排放綜合考慮確定最優(yōu)的噴射間隔角。

圖4 引燃柴油量及噴射間隔對CO排放影響Fig.4 Effects of diesel injection quantity and injection separation on CO emissions

2.3 NOx排放

圖5a為引燃柴油量及噴射間隔對NOx排放的影響。NOx排放主要受缸內(nèi)工質(zhì)溫度和局部氧氣含量的影響。從4種引燃柴油在不同噴射間隔下的NOx排放數(shù)值看，NOx排放隨柴油噴射量的增加呈現(xiàn)先降低后升高的變化規(guī)律，柴油噴射量為11.5 mg時NOx排放最高，柴油噴射量為8.5 mg時次之，柴油噴射量為6.0 mg時NOx排放最低。因?yàn)橐后w燃料蒸發(fā)過程中更易出現(xiàn)混合不均勻的區(qū)域，使富氧區(qū)增加，且絕熱火焰溫度相對較高，因此柴油噴射量為11.5 mg時NOx排放最高。如圖5b所示，當(dāng)柴油量為4.0 mg時，缸內(nèi)最高平均燃燒溫度最高，促進(jìn)了NOx的生成，使其排放高于柴油噴射量為6.0 mg的情況。

噴射間隔對NOx排放的影響規(guī)律為：在同一柴油噴射量下，NOx排放在噴射間隔為0.5 ms時較低，在噴射間隔為1.4 ms時達(dá)到峰值；當(dāng)噴射間隔從0.8 ms增加到1.1 ms時，不同的引燃柴油量下NOx排放的變化規(guī)律不一致。這是因?yàn)楫?dāng)噴射間隔為0.5 ms時，從圖6中2種燃料的噴射信號（DPW和GPW）和放熱率的對比圖可以得出，柴油的燃燒階段和天然氣的噴射階段有一定的重合期，柴油蒸汽和天然氣共同和空氣形成混合氣，燃燒過程中火焰區(qū)及火焰前鋒面的高溫區(qū)氧含量相對較低，從而抑制了NOx的生成；噴射間隔為1.4 ms時，天然氣被引燃時柴油放熱已基本結(jié)束，2種燃料之間爭奪氧氣的干涉作用降低，在引燃柴油燃燒期間及主噴天然氣燃燒期間，高溫區(qū)的氧含量相對充足，使NOx生成量較大；噴射間隔為0.8和1.1 ms時，由于燃燒溫度和高溫區(qū)氧化量的影響程度不同，在不同引燃柴油噴射量呈現(xiàn)不同的規(guī)律。

圖5 引燃柴油量及噴射間隔對NOx排放及缸內(nèi)最高平均溫度的影響Fig.5 Effects of diesel injection quantity and injection separation on NOx emissions and maximum in-cylinder mean temperature

圖6 噴射過程及燃燒過程對比Fig.6 Comparison between injection process and combustion process

3 結(jié)論

以一臺六缸直噴天然氣發(fā)動機(jī)為研究對象，研究了不同引燃柴油量及柴油/天然氣噴射間隔下發(fā)動機(jī)的各項(xiàng)排放，結(jié)論如下：

1）在噴射間隔一定時，THC及CH4排放隨引燃柴油噴射量的增加而降低；在引燃柴油噴射量為4.0 mg時，THC及CH4排放隨噴射間隔的增加而增加；在引燃柴油噴射量在6.0～11.5 mg范圍內(nèi)時，THC及CH4排放在噴射間隔從0.5 ms變化到1.1 ms時，變化較小，噴射間隔增加到1.40 ms時，THC及CH4排放升高較明顯。

2）噴射間隔在0.5～1.4 ms范圍內(nèi)時，CO排放隨引燃柴油噴射量的變化規(guī)律為先降低后升高，排放的最低值出現(xiàn)在引燃柴油噴射量為6.0或8.5 mg時；在不同的柴油噴射量下增加噴射間隔均可以實(shí)現(xiàn)CO的減排。

3）在噴射間隔一定時，NOx排放隨引燃柴油噴射量的增加呈現(xiàn)先降低后升高的變化規(guī)律；不同引燃柴油噴射量下，NOx排放隨噴射間隔無一致的變化規(guī)律，在噴射間隔為0.50 ms時，NOx排放相對較小，在噴射間隔為1.40 ms時，NOx排放最高。

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Effects of diesel injection quantity and injection interval on emission characteristics of directly injected natural gas engine

Li Menghan1,Zhang Qiang1,Li Guoxiang1※,Shao Sidong2
(1.School of Energy and Power Engineering,Shandong University,Jinan 250061,China;2.State Key Laboratory for Reliability of Internal Combustion Engines at Weichai Power,Weifang 261001,China)

Adjustment of injection strategy has been considered as an effective and reliable way for the emission control of directly injected natural gas engines.However,the system investigation on the effects of pilot diesel quantity and injection separation is scarce.In order to achieve better emission characteristics by optimizing the injection strategy of directly injected natural gas engine,experimental investigation was conducted on a 6-cylinder pilot-ignited natural gas engine to study the effects of diesel injection quantity(DIQ)and separation between diesel and natural gas injection(SDN).The test engine was modified from a diesel engine with bore diameter of 126 mm,stroke of 155 mm and compression ratio of 17.The design of cylinder head was redesigned to adapt to the installation of the dual fuel injector,which had two concentric needles and two electronically controlled solenoid valves.An integrated pressure regulating module was added to controlthe injection pressure of diesel and natural gas.The supply system of natural gas,including compressor and buffer tank,was also added to provide compressed pipeline natural gas.Besides,the control module of natural gas injection was integrated into the original electronic controlled unit to realize the accurate control of both diesel and natural gas injection.In the experiment process,the intake temperature was fixed at 22°CA and the intake air pressure was maintained at 101 kPa while the diesel rail pressure was adjusted to 18MPa.To prevent natural gas leaking into diesel,the rail pressure of natural gas was slightly lower than that of diesel(17.5 MPa).The tested operating condition was at engine speed of 1 275 r/min with brake specific effective pressure of 0.54Mpa,which was a typical operating point of European steady state cycle.To evaluate the effects of diesel injection quantity(DIQ)and separation between diesel and natural gas injection(SDN),the injection timing of natural gas was kept constant at 8°BTDC while diesel injection quantity was varied from 4.0 to 11.5 mg under four different injection separations(0.5 ms,0.8 ms,1.1 ms and 1.4 ms).The natural gas pulse width was adjusted in accordance with the diesel injection quantity to maintain the fixed engine brake specific power.The emissions were measured by a Horiba MEXA 7 200 exhaust gas analyzer.CO emissions were tested by nondispersive infrared technology. HC emissions were tested by flame ionization detector.NOx emissions were tested by chemiluminescent detector(CLD). The emissions of each operating point were collected after 5 minutes of steady operation and all the emissions were recorded three times to obtain the averaged values for further analysis.The experimental results showed that HC emissions decreased with the increase of diesel injection quantity at the same injection separation and increased with increasing injection separation at the diesel injection quantity of 4.0 mg;in the diesel injection quantity range of 6.0～11.5 mg.HC emissions changed slightly when the injection separation varied from 0.5 ms to 1.1ms,however,when injection separation extended to 1.4 ms,the rising trend of HC emissions became more obvious.CO emissions exhibited a first decrease then increase trend with the increase of diesel injection quantity,and the minimum value occurred at the diesel injection quantity of 6.0 mg or 8.5 mg;a decrease with the increase of injection separation at all diesel injection quantities can also be observed.NOx emissions firstly declined and then rise with the increasing diesel injection quantity;additionally,at the injection separation of 0.5 ms,NOx emissions were relatively lower while reached the peak value at the injection separation of 1.4 ms.It can be concluded that the increase of diesel injection quantity has beneficial effects on HC emissions and negative effects on NOx emissions while exerts little influence on CO emissions;the extension of injection separation results in higher HC and NOx emissions as well as reduced CO emissions.

engines;diesel engines;fuel injection;diesel injection quantity;injection separation;natural gas engine; emission characteristics

10.11975/j.issn.1002-6819.2016.06.013

TK421.5

A

1002-6819（2016）-06-0095-06

2015-10-20

2016-01-18

國家高技術(shù)船舶科研項(xiàng)目（2060303）

李孟涵（1990-），女，山東淄博人，博士研究生，主要從事天然氣發(fā)動機(jī)燃燒及排放控制研究。濟(jì)南 山東大學(xué)能源與動力工程學(xué)院內(nèi)燃機(jī)研究所，250061。Email：sdulmh@163.com；

※通信作者：李國祥（1965-），男，山東蓬萊人，博士，教授，博士生導(dǎo)師，主要從事發(fā)動機(jī)燃燒及排放控制研究。濟(jì)南 山東大學(xué)能源與動力工程學(xué)院內(nèi)燃機(jī)研究所，250061。Email：liguox@sdu.edu.cn