点火时刻对甲醇发动机燃烧及非法规排放的影响

针对甲醇发动机低温冷起动困难,在1台由1130单缸柴油机改造而成的直喷火花点火甲醇发动机上,利用CFD模拟软件AVL-Fire耦合甲醇氧化反应机理,通过电热塞将进气温度加热到283K,研究了点火时刻对甲醇发动机低温(266K)冷起动燃烧及非法规排放的影响。结果表明:提前点火时刻能够使缸内混合气得到较充分燃烧,减少未燃甲醇排放,当点火时刻由8°BTDC提前到11°BTDC时未燃甲醇排放显著减少;提前点火时刻能够降低甲醛排放,当点火时刻提前到17°BTDC、缸内最高燃烧温度超过1 300K时,甲醛快速氧化,甲醛排放显著减少。     

Effect of a 2-stage injection strategy on the combustion and flame characteristics in a PCCI engine

Recently, to reduce environmental pollution and the waste of limited energy resources, there is an increasing requirement for higher engine efficiency and lower levels of harmful emissions. A premixed charge compression ignition (PCCI) engine, which uses a 2-stage type injection, has drawn attention because this combustion system can simultaneously reduce the amount of NOx and PM exhausted from diesel engines. It is well known that the fuel injection timing and the spray angle in a PCCI engine affect the mixture formation and the combustion. To acquire two optimal injection timings, the combustion and emission characteristics of the PCCI engine were analyzed with various injection conditions. The flame visualization was performed to validate the result obtained from the engine test. This study reveals that the optimum injection timings are BTDC 60° for the first injection and ATDC 5° for the second injection. In addition, the injection ratio of 3 to 7 showed the best NOx and PM emission results.     

Simulation of mixed gas formation for a spray-wall complex guided LPG direct injection engine

To obtain an ultralean air-fuel ratio and to reduce engine-out NOX and HC emissions induced by the richer mixture near the spark plug, a spray and wall complex guided combustion system has been developed by utilizing the fuel characteristics of LPG. The new combustion system configuration is optimized by using a commercial CFD code, FIRE V2013, and the reliability of the system has been experimentally demonstrated by Plane Laser-Induced Fluorescence (PLIF). The mixture formation in the new combustion system under part load (2,000 rpm) is numerically simulated. With an injection timing of 40°CA BTDC, the LPG spray which is injected from two upper holes, reaches the ignition point, and the other part of the LPG spray which is injected from the bottom hole, is directed to the ignition point through the vertical vortices at the same time. At the ignition timing of about 20°CA BTDC, the two-part mixtures have been shown to form a stable and richer stratified mixture around the ignition point, and the maximum global air-fuel ratio reaches to 60: 1.     

Numerical Analysis of the Mixture Formation in a Two-Stroke Wall-Guided LPG DI Engine for Extended-Range Electric Vehicle

The use of automotive LPG characteristics which are easy to evaporate vaporization and carry. The paper presents a design of extended-range electric vehicle for wall-guided two stroke LPG engine with direct injection combustion system. Based on the modified vehicle LPG spray model, a database describing the characteristics of vehicle LPG fuel was built and imported into the CFD software. And the accuracy of the model is verified by the Schlieren experimental results. The concentration and velocity field of the mixture in the cylinder under different load conditions are numerically analyzed. The analyzed result indicated that the start injection time θ = 60°–70°CA BTDC under part load condition, the plug electrode near the gathering of a richer mixture is easy to be fired at spark ignition time, the obvious formation of mixture in cylinder is formed and the overall air-fuel ratio is above 40: 1. The start-transition working condition and large load conditions in the piston moves upward before closing the exhaust port to start injection LPG. The optimized LPG injection start time θ ensures that the fresh gas is locked in the cylinder when the exhaust port is closed (63°CA ABDC). In the ignition time of the spark plug, an ideal homogeneous mixture in the cylinder is realized.     

对置活塞二冲程缸内直喷汽油机混合气形成的数值研究

针对对置活塞二冲程汽油机缸径小、冲程长的特点,利用三维CFD软件AVL-Fire对缸内喷雾方向进行优化,实现全负荷工况下(6 000r/min)的缸内混合气均匀混合;并且基于优选的喷雾方向,研究部分负荷工况下(2 000r/min)二次喷射策略(不同喷油时刻和喷油比例)对缸内混合气分层分布的影响。结果显示,增大排气侧3束喷雾的中心线与气缸中心面夹角β会导致燃油蒸发率降低,而增大进气侧3束喷雾的中心线与气缸中心面夹角α有利于提高缸内混合气的均匀度;在部分负荷时,当第一次喷油时刻为内止点前140°曲轴转角,第二次喷射时刻为内止点前60°曲轴转角,第二次喷油量为总喷油量的33%时,缸内形成理想的混合气分层分布。     

Numerical analysis of homogeneous mixture formation for a direct injection liquid LPG engine

A method to form a homogeneous mixture using low pressure direct injection of liquid phase LPG, pent-roof combustion chamber, flat-top piston and center-located injector layout is presented. To validate the method, the mixture formation processes in the cylinder were investigated using the CFD code. The effect of different injection timing and engine loads on the mixture formation processes were researched. The simulated results showed that, the intake tumble for high load conditions or the inclined intake swirl for part load conditions would break into small-scale vortex (turbulence) near the end of compression stroke, which enhanced the homogeneous mixture formation. The results also showed that if the liquid phase LPG was injected at 60–80°CA ATDC in intake stroke even at different loads, the homogeneous mixture would be formed under any engine load conditions.     

Combined effect of boost pressure and injection timing on the performance and combustion of CNG in a DI spark ignition engine

There is an increasing interest in supercharging spark ignition engines operating on CNG (compressed natural gas) mainly due to its superior knock resisting properties. However, there is a penalty in volumetric efficiency when directly injecting the gaseous fuel at early and partial injection timings. The present work reports the combined effects of a small boost pressure and injection timing on performance and combustion of CNG fueled DI (direct injection) engine. The experimental tests were carried out on a 4-stroke DI spark ignition engine with a compression ratio of 14. Early injection timing, when inlet valves are still open (at 300°BTDC), and partial injection timing, in which part of the injection occurs after the inlet valves are closed (at 180°BTDC), were varied at each operating speed with variation of the boost pressure from 2.5 to 10 kPa. A narrow angle injector (NAI) was used to increase the mixing rate at engine speeds between 2000 and 5000 rpm. Similar experiments were conducted on a naturally aspirated engine and the results were then compared with that of the boosting system to examine the combined effects of boost pressure and injection timing. It was observed that boost pressure above 7.5 kPa resulted in an improvement of performance and combustion of CNG DI engine at all operating speeds. This was manifested in the faster heat release rates and mass fraction burned that in turn improved combustion efficiency of the boosting system. An increased in cylinder pressure and temperature was also observed with boost pressure compared to naturally aspirated engine. Moreover, the combustion duration was reduced due to concentration of the heat release near to the top dead center as the result of the boost pressure. Supercharging was also found to reduce the penalty of volumetric efficiency at both the simulated port and partial injection timings.     

LPG电控喷射冷起动循环的着火及HC排放影响因素分析

分析了电喷LPG发动机冷起动过程中影响着火及HC排放的主要因素。试验在一台四冲程、水冷125mL单缸电喷发动机上进行。试验结果表明：LPG发动机冷起动混合气的浓度相当于稳定燃烧混合气浓度的1．5倍左右，比汽油机稀，HC排放也低；随着混合气变稀，首次着火循环逐渐推迟；高起动转速是发动机冷起动可靠的一个主要保障因素；适当提前点火和增大火花塞间隙有利于降低冷起动循环的首次着火循环数；环境温度是影响冷起动过程的一个主要参数。     

Predicting auto-ignition characteristics of RCCI combustion using a multi-zone model

The objective of new combustion concepts is to meet emission standards by improving fuel air mixing prior to ignition. Since there is no overlap between injection and ignition, combustion is governed mainly by chemical kinetics and it is challenging to control the phasing of ignition. Reactivity Controlled Compression Ignition (RCCI) combustion aims to control combustion phasing by altering the fuel ratios of the high- and low octane fuel and injection timings. In this study the dual fuel blend is prepared with gasoline and diesel fuels. The applied injection timings of the diesel are very early (90 to 60° CA bTDC). In the detailed reaction mechanism, n-heptane and iso-octane represent diesel and gasoline fuel, respectively. A multi-zone model approach is implemented to perform RCCI combustion simulation. Ignition characteristics are analyzed by using CA50 as the main parameter. In the experiments for the early direct injection (DI) timing advancing the injection time results in a later ignition. Qualitatively, the trend effect of the diesel injection timing and the effect of the ratio gasoline/diesel are captured accurately by the multi-zone model.     

Influence of pilot injection on combustion characteristics and emissions in a DI diesel engine fueled with diesel and DME

This work experimentally investigates how the dwell time between pilot injection and main injection influences combustion and emissions characteristics (NOx, CO, THC and smoke) in a single-cylinder DI diesel engine. The experiments were conducted using two fuel injection systems according to the fuel type, diesel or dimethyl ether (DME), due to the different fuel characteristics. The injection strategy is accomplished by varying the dwell time (10°CA, 16°CA and 22°CA) between injections at five main injection timings (?4°CA aTDC, ?2°CA aTDC, 0°CA aTDC, 2°CA aTDC and 4°CA aTDC). Results from pilot-main injection conditions are compared with those shown in single injection conditions to better demonstrate the potential of pilot injection. It was found that pilot injection is highly effective for lowering heat-release rates with smooth pressure traces regardless of the fuel type. Pilot injection also offers high potential to maintain or increase the BMEP; even the combustion-timing is retarded to suppress the NOx emission formation. Overall, NOx emission formation was suppressed more by the combustion phasing retard effect, and not the pilot injection effect considered in this study. Comparison of the emissions for different fuel types shows that CO and HC emissions have low values below 100 ppm for DME operation in both single injection and pilot-main injection. However, NOx emission is slightly higher in the earlier main injection timings (?4°CA aTDC, ?2°CA aTDC) than diesel injections. Pilot injection was found to be more effective with DME for reducing the amount of NOx emission with combustion retardation, which indicates a level of NOx emission similar to that of diesel. Although the diesel pilot-main injection conditions show higher smoke emission than single-injection condition, DME has little smoke emission regardless of injection strategy.     

喷油控制参数对中置喷油器GDI发动机喷雾及燃烧的影响

针对匹配中置高压喷油器的直喷汽油光学发动机,试验研究了不同喷油时刻及喷油压力下的缸内燃烧及喷雾发展特性,分析了燃油喷射控制参数对直喷汽油机缸内喷雾及燃烧的影响规律。研究结果表明:随第三段喷油时刻(θ_(SOI3))提前,燃烧持续期与滞燃期均先减小后增大,燃烧特征参数均在θ_(SOI3)=120°BTDC时存在明显拐点,此时平均指示压力(p_(mi))的循环变动系数C_(OVpmi)相对较小;第三段喷油时刻过晚,活塞上行距上止点较近,易导致油束冲击活塞表面;提高喷油压力可缩短燃烧持续期,有助于改善燃烧定容度,但喷油压力过大,油束贯穿距进一步延长,油束冲击缸壁的倾向增加,滞燃期及燃烧持续期反而延长。     

基于2阶段喷射的缸内直喷汽油机HCCI燃烧的研究

在缸内直喷汽油机（GDI）上采用2阶段燃油喷射技术来控制缸内混合气形成和燃烧，在GDI发动机上实现了均质混合气压燃（HCCI）燃烧方式，研究了缸内2阶段汽油喷射对HCCI燃烧特性的影响。结果表明，压缩行程中的第2次喷油时间可以有效地控制燃烧始点，二次喷油持续期可以控制燃烧速率、燃烧相位和拓宽发动机负荷。     

Combustion and emission characteristics of BD20 reformed by ultrasonic energy for different injection delay and EGR rate in a diesel engine

The purpose of this study is to understand the operational characteristics of a diesel engine that uses BD20 reformed by ultrasonic energy irradiation. In particular we study the effects of tuning injection delay and EGR rate. BD containing about 10% oxygen has attracted attention due to soaring crude oil prices and environmental pollution. This oxygen decreases soot by promoting combustion, but it also increases NOx. To solve this problem, injection timing may be delayed or an EGR system may be applied. These adjustments normally lower engine power and increase exhaust emission but, in using fuel reformed by ultrasonic energy irradiation (which is changed physically and chemically to promote combustion), we may hope to circumvent this problem. To control the duration of the ultrasonic energy irradiation, the capacity of the chamber in an ultrasonic energy fuel supply system was tested at 550cc and 1100cc capacities. As for the results of the experiment, we could identify the optimum EGR rate by investigating the engine performance and the characteristics of exhaust emissions according to the injection timing and the EGR rate while ultrasonically irradiated BD20 was fed to a commercial diesel engine. With UBD20 (at an injection timing of BTDC 16°), the optimum EGR rate, giving satisfactory engine performance and exhaust emissions characteristics, was in the range of 15∼20%.     

内部EGR及增压拓展柴油HCCI燃烧负荷范围试验研究

在135单缸柴油机上对比了传统燃烧模式和HCCI燃烧模式的负荷特性,优化了HCCI燃烧模式的喷油始点,分析了内部EGR率及增压压力对HCCI燃烧负荷范围及排放的影响。试验结果表明:对于负气门重叠期喷油的HCCI燃烧模式,1 500r/min下,最佳喷油始点为370°BTDC,气门重叠期为-30°时既保证了较低的NOx排放,又可以获得较佳的负荷范围;提高增压压力不仅可以拓展HCCI燃烧的负荷上限,对负荷下限的燃烧稳定性也有利;将增压压力提高到0.18MPa时,负荷上限从传统燃烧的0.594MPa上升到0.723MPa,但负荷下限较传统燃烧模式要高,CO排放、烟度和燃油经济性都较差。     

高压共轨柴油机冷起动关键控制参数优化的试验研究

以某直列6缸柴油机高压共轨系统为研究对象,研究了柴油机冷起动控制参数和可控变量,并建立了共轨系统冷起动优化控制方案,通过改变共轨系统燃油喷射压力、格栅预热时间、喷射时刻等相关参数,改善并优化冷起动燃烧过程,采用试验标定验证了优化方案的可行性。研究结果表明,关键参数优化可以明显改善柴油机的冷起动性能,保证柴油机快速起动,满足车用柴油机的使用要求,为柴油机的低温起动标定提供依据。     

电喷汽油机起动及暖机过程HC排放的测试分析

根据实测的催化器入口、出口温度及HC排放浓度，结合示功图对电喷汽油机冷起动时HC排放量在台架上进行了模拟分析，将起动过程以节气门突开为界，划分为3个阶段，其中HC主要排放量发生在开始超导 劝到节气门开这一段时间内。适当提高空燃比及匹配合适的点火提前角。促使缸内发生不完全燃烧，则未燃HC在排气管内可继续燃烧，使得最终排出的HC量降低。在节气门开后，也可通过控制点火提前角，使缸内发生不完全燃烧，将燃烧延续到排气管内，即可降低HC排放量，也有助于加速催化器起燃。     

Influence of injection parameters on the transition from PCCI combustion to diffusion combustion in a small-bore HSDI diesel engine

In this paper, the influence of injection parameters on the transition from Premixed Charge Combustion Ignition (PCCI) combustion to conventional diesel combustion was investigated in an optically accessible High-Speed Direct-Injection (HSDI) diesel engine using multiple injection strategies. The heat release characteristics were analyzed using incylinder pressure for different operating conditions. The whole cycle combustion process was visualized with a high-speed video camera by simultaneously capturing the natural flame luminosity from both the bottom of the optical piston and the side window, showing the three dimensional combustion structure within the combustion chamber. Eight operating conditions were selected to address the influences of injection pressure, injection timing, and fuel quantity of the first injection on the development of second injection combustion. For some cases with early first injection timing and a small fuel quantity, no liquid fuel is found when luminous flame points appear, which shows that premixed combustion occurs for these cases. However, with the increase of first injection fuel quantity and retardation of the first injection timing, the combustion mode transitions from PCCI combustion to diffusion flame combustion, with liquid fuel being injected into the hot flame. The observed combustion phenomena are mainly determined by the ambient temperature and pressure at the start of the second injection event. The start-of-injection ambient conditions are greatly influenced by the first injection timing, fuel quantity, and injection pressure. Small fuel quantity and early injection timing of the first injection event and high injection pressure are preferable for low sooting combustion.     

发动机停机后曲轴停止相位的研究

针对发动机在停机过程中由于缸内气体压力的作用出现的曲轴反转现象,研究发动机停机后曲轴停止的位置对于缸内直喷汽油机实施起动—停止控制策略的重要意义。在Jetta1.6 L发动机上的研究结果表明:发动机停机后处于压缩行程的活塞停在80°CA BTDC的概率较大,约占50%;发动机是否发生反转主要取决于转动动能衰减到0时缸内气体负力矩的大小;采用两个具有一定相位差的光电传感器可以判断曲轴是否反转并检测停机后曲轴的位置。     

进气门晚关机构对高增压柴油机排放与热效率的影响

以某重型柴油机为研究对象,在低速中等负荷工况,通过对比进气门晚关机构开闭两种状态对两级增压系统匹配关系的影响,分析其对柴油机排放和热效率的影响。研究表明,喷油定时(上止点后-2.5°~8.5°)和EGR率(0%~21%)恒定时,使用进气门晚关(IVCA)机构后HC排放降低,有效热效率下降。当喷油定时不变,BNOxdif与BCOdif随着EGR率的增加而减少;当EGR率保持恒定时,随着喷油定时增加,BCOdif呈现先增加后减小的趋势,拐点在4.5°~6.5°(ATDC)之间;同时,开启IVCA机构后,EGR率在15.29%~21.16%,喷油定时在-2.5°~8.5°(ATDC)范围内,Bsootdif均小于0。在保证喷油定时恒定时,EGR率越大,Bsootdif越小。当量比在0.42~0.52范围内,保持喷油策略不变,使用IVCA机构致使进气流量减少,若当量比在原状态的基础上增加超过0.07,即可克服由于流量减少导致柴油机缸内平均温度和燃烧持续期增加对NOx生成的负面影响。     

Effect of injection timing,injector opening pressure,injector nozzle geometry,and swirl on the performance of a direct injection,compression-ignition engine fuelled with honge oil methyl ester (HOME)

Increasing petroleum prices, increasing threat to the environment from exhaust emissions and global warming have generated intense international interest in developing renewable and alternative non-petroleum fuels for engines. Evolving feasible technology and recurring energy crisis necessitated a continued investigation into the search for sustainable and clean-burning renewable fuels. In this investigation, Honge oil methyl ester (HOME) was used in a four stroke, single cylinder diesel engine. Tests were carried out to study the effect of fuel injection timing, fuel injector opening pressure (IOP) and injector nozzle geometry on the performance and combustion of CI engine fuelled with HOME. Injection timing was varied from 19°bTDC (before top dead centre) to 27°bTDC in incremental steps of 4°bTDC; injector opening pressure was varied from 210 bar to 240 bar in steps of 10 bar. Nozzle injectors of 3, 4 and 5 holes, each of 0.2, 0.25 and 0.3 mm size were selected for the study. It was concluded that retarded injection timing of 19°bTDC, increased injector opening pressure of 230 bar and 4 hole nozzle injector of 0.2 mm size resulted in overall better engine performance with increased brake thermal efficiency (BTE) and reduced HC, CO, smoke emissions. Further air-fuel mixing was improved using swirl induced techniques which enhanced the engine performance as well.