Combustion and emission characteristics of a lean burn natural gas engine

Lean burn is an effective way to improve spark ignition engine fuel economy. In this paper, the combustion and emission characteristics of a lean burn natural gas fuelled spark ignition engine were investigated at various throttle positions, fuel injection timings, spark timings and air fuel ratios. The results show that ignition timings, the combustion duration, the coefficient of variation (COV) of the indicated mean effective pressure (IMEP) and engine-out emissions are dependent on the overall air fuel ratio, spark timings, throttle positions and fuel injection timings. With the increase of the air fuel ratio, the ignition delays and combustion duration increases. Fuel injection timings affect ignition timings, combustion duration, IMEP, and the COV of the IMEP. Late fuel injection timings can decrease the COV of the IMEP. Moreover, the change in the fuel injection timings reduces the engine-out CO, total hydrocarbon (THC) emissions. Lean burn can significantly reduce NOx emissions, but it results in high cyclic variations.     

Comparison of HC species from diesel combustion modes and characterization of a heat-up doc formulation

This study summarizes engine speed and load effects on HC species emissions from premixed charge compression ignition (PCI) and conventional diesel combustion, and it evaluates diesel oxidation catalyst (DOC) formulations on a gas flow reactor for the purpose of diesel particulate filter regeneration or lean NO_x trap desulfation. HC emissions are sampled simultaneously by a Tedlar bag for light HC species and by a Tenax TA™ adsorption trap for semi-volatile HC species, and they are analyzed by gas chromatography with a flame ionization detector. The bulk temperature and residence time during combustion are key parameters that are important for understanding the effects of speed and load on engine-out HC emissions. The degree of post-flame oxidation is higher in PCI than in conventional combustion, and it is increased for PCI with a higher speed and load, as indicated by a lower fuel alkanes/THC ratio, a higher alkenes/fuel alkanes ratio, and a higher methane/THC ratio. Ethene and n-undecane are two representative HC species, and they are used as a surrogate mixture in the gas flow reactor to simulate PCI and conventional combustion with in-cylinder post fuel injection. Among the three DOC formulations tested, the catalyst with constituent precious metals of platinum and palladium (PtPd) showed the best light-off performance, followed by PtPd with an addition of cerium dioxide (PtPd+CeO₂), and platinum (Pt), regardless of exhaust compositions. Conventional combustion exhaust composition shows a lower light-off temperature than that of PCI, regardless of catalyst formulation.     

Investigation of engine restart stability after idle stop for a mild type HEV powertrain

Stringent regulations on exhaust emissions and fuel economy for vehicles have become major issues in the automotive industry. Hybrid electric vehicles (HEV) are one of the crucial alternative plans to current conventional vehicles, but they have drawbacks, which include increases in total hydrocarbon (THC) emission from the engine and deterioration of the combustion stability with frequent stopping and restarting of the engine. Intake port fuel film is evaporated up during the deceleration state due to the fuel-cut. The λ (relative A/F ratio) at engine restart is lean because part of injected fuel is used to form the fuel film in the intake port. This study revealed the behavior of a fuel film in engine stop with fuel-cut and in engine restart with a simulation model. To investigate the fuel film characteristics, a simulation model was applied and validated with a single-cylinder engine. The simulation result shows that λ of at least 1.2 is required for a stable engine restart. The minimum injection quantity of the first cycle for stable combustion is suggested to be at least 240% of the steady-state idle condition.     

预燃室催化燃烧对HCCI发动机燃烧特性的影响

对预燃室壁涂有催化剂的均质压燃(HCCI)发动机的燃烧过程进行了数值计算,分析了催化燃烧对HCCI发动机燃烧特性的影响;同时分析了预燃室内催化剂种类、过量空气系数、进气温度、进气速度、缸径以及预燃室壁温对HCCI发动机燃烧特性的影响。结果表明,预燃室存在催化燃烧时对HCCI发动机的着火时刻有很大的影响:随着过量空气系数及预燃室进气速度的增加,HCCI发动机的着火时刻提前;催化剂种类、预燃室缸径以及预燃室壁温对HCCI发动机着火时刻影响不显著,但对缸内燃烧温度影响显著。     

Combustion instabilities and nanoparticles emission fluctuations in GDI spark ignition engine

The main challenge facing the concept of gasoline direct injection is the unfavourable physical conditions at which the premixed charge is prepared and burned. These conditions include the short time available for gasoline to be sprayed, evaporated, and homogeneously mixed with air. These conditions most probably affect the combustion process and the cycle-by-cycle variation and may be reflected in overall engine operation. The aim of this research is to analyze the combustion characteristics and cycle-by-cycle variation including engine-out nanoparticulates of a turbocharged, gasoline direct injected spark ignition (DISI) engine at a wide range of operating conditions. Gasoline DISI, turbo-intercooled, 1.6L, 4 cylinder engine has been used in the study. In-cylinder pressure has been measured using spark plug mounted piezoelectric transducer along with a PC based data acquisition. A single zone heat release model has been used to analyze the in-cylinder pressure data. The analysis of the combustion characteristics includes the flame development (0–10% burned mass fraction) and rapid burn (10–90% burned mass fraction) durations at different engine conditions. The cycle-by-cycle variations have been characterized by the coefficient of variations (COV) in the peak cylinder pressure, the indicated mean effective pressure (IMEP), burn durations, and particle number density. The combustion characteristics and cyclic variability of the DISI engine are compared with data from throttle body injected (TBI) engine and conclusions are developed.     

气缸壁涂加催化剂对均质压燃发动机燃烧特性的影响

通过修改CHEMKIN软件包中的SENKIN模块,对气缸壁涂有催化剂的HCCI燃烧过程进行了数值计算。对比分析了缸内催化燃烧对HCCI发动机的着火时刻及NOx排放的影响;讨论了当气缸壁涂有催化剂时进气温度、进气压力、压缩比、过量空气系数及催化剂种类等多种工况参数对HCCI燃烧特性的影响。     

柴油微引燃乙醇发动机燃烧、性能及排放特性研究

柴油微引燃乙醇发动机采用进气道喷射乙醇、缸内直喷微量柴油引燃的方式进行燃料供给。基于单缸四冲程柴油机,对其燃烧、性能及排放特性进行研究,固定引燃柴油喷射量为发动机能实现压燃着火的最小值,在进气压力为0.15 MPa时比较不同乙醇喷射量的工况组,通过改变柴油喷射时刻进行工况扫描。结果表明,引燃柴油的喷射时刻对发动机的燃烧、性能和排放影响显著。柴油微引燃乙醇发动机在中高负荷能够稳定运行,指示热效率可达34%以上,通过适当调节柴油喷射时刻,可以有效控制未燃碳氢(UHC)、NO_x与CO排放,同时可以实现极低的炭烟排放。柴油微引燃乙醇发动机燃烧模式为预混合或部分预混合燃烧,燃烧有两阶段放热特征,改变引燃柴油喷射时刻,可以有效控制燃烧相位。     

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。     

Effects of the throttle opening ratio and the injection timing of CNG on the combustion characteristics of a DI engine

We investigated the effects of the fuel injection timing — both for early and late injection — in conjunction with the throttle opening ratio on the fuel-air mixing characteristics, engine power, combustion stability and emission characteristics of a DI CNG spark engine and control system that had been modified and designed according to the author’s original idea. We verified that the combustion characteristics were affected by the fuel injection timing and that the engine conditions were affected by the throttle opening ratios and the rpm. The combustion characteristics were greatly improved for a complete open throttle ratio with an early injection timing and for a partial throttle ratio with a late injection timing. The combustion duration was governed by the duration of flame propagation in late injection timing scenarios and by the duration of early flame development in cases of early injection timing. As the result, the combustion duration is shortened, the lean limit is improved, the air-fuel mixing conditions are controlled, and the emissions are reduced through control of the fuel injection timing and vary according to ratio of the throttle opening.     

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.     

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。     

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.     

Effects of the fuel injection ratio on the emission and combustion performances of the partially premixed charge compression ignition combustion engine applied with the split injection method

Currently, due to the severity of world-wide air pollution by substances emitted from vehicles, emission control is being enforced more strictly, and it is expected that the regulation requirements for emission will become even more severe. A new concept combustion technology that can reduce the Nitrogen oxides (NOx) and PM in relation to combustion is urgently required. As a core combustion technology among new combustion technologies for the next generation engine, the homogenous charge compression ignition (HCCI) is expanding its application range by adopting a multiple combustion mode, a catalyst, direct fuel injection and partially premixed charge compression ignition combustion using the split injection method. This paper used a split injection method in order to apply the partially premixed charge compression ignition combustion method without significantly altering engine specifications of the multiple combustion mode and practicality by referring to the results of studies on the HCCI engine. Furthermore, the effects of the ratio of the fuel injection amount on split injection are investigated. From the test results, the adequate combination of the ratio of the fuel injection amount for the split injection method has some benefit on exhaust and fuel economy performance in a naturally aspirated single cylinder diesel engine.     

稀薄燃烧发动机改型设计及正时策略优化

基于某高速汽油机,对燃烧室结构、燃油喷射特性、凸轮型线改型设计为稀薄燃烧发动机。提出利用响应面模型对正时策略进行分析和优化的研究方法,并建立利用响应面进行多目标优化计算的流程。以提高有效功率和降低有效燃油消耗率为优化目标,以点火正时、空燃比和进排气正时为设计变量,建立了发动机性能与响应面耦合优化模型。分析与试验结果表明:较标准混合比燃烧时,稀薄燃烧发动机的进排气提前角减小,点火正时提前,最低燃油消耗率下降3.9%,最大功率提升9.7%;同时利用响应面优化方法提高了优化效率。     

Effect of the fuel injection strategy on first-cycle firing and combustion characteristics during cold start in a TSDI gasoline engine

The first firing cycle is very important during cold-start for all types of spark ignition engines. In addition, the combustion characteristics of the first firing cycle affect combustion and emissions in the following cycles. However, the first-cycle fuel-air mixing, combustion and emissions generation within the cylinder of a two-stage direct-injection (TSDI) engine during cold start is not completely understood. Based on the total stoichiometric air-fuel ratio and local richer mixture startup strategy, the first-cycle firing and combustion characteristic at cold start were investigated in a two-stage direct injection (TSDI) gasoline engine. In addition, the effects of the first injection timing, second injection timing, 1st and 2nd fuel injection proportion and total excess air ratio on the in-cylinder pressure, heat release rate and accumulated heat release were analyzed on the basis of a cycle-by-cycle analysis. It is shown that a larger 2nd fuel injection amount and later 2nd injection timing are more beneficial to the firing of the first cycle in the case of a total excess air ratio of 1.0. The optimum 1st and 2nd injection timing fuel injection proportions are 120°CA ATDC during the intake stroke, 60°CA BTDC during the compression stroke and 1:1. In addition, the firing boundary is a 2nd injection timing later than 90°CA BTDC during the compression stroke in the case of the 1st injection timing from 60°CA to 180°CA ATDC during an intake stroke and involves a 1st and 2nd fuel injection proportion of 1:1 and an excess air ratio of 1.0. The study provides a detailed understanding of cold-start combustion characteristics and a guide for optimizing the reliable first-cycle firing at cold start.     

基于喷射正时的柴油机燃烧和排放研究

基于集成电控单体泵柴油机,讨论了喷射正时对柴油机燃烧及排放特性的影响规律。喷射正时对滞燃期和速燃期有显著的影响,从而影响放热规律、缸内压力和燃烧温度,进而对NOx和PM排放特性有显著的影响。在中大负荷下,随着喷射正时的滞后,放热率曲线相对于上止点的相位滞后,缸内最高燃烧温度下降,NOx排放下降,PM排放升高。在中小负荷下,随着喷射正时的进一步滞后,NOx和PM的排放同时下降。     

气相射流点火天然气发动机的燃烧及排放特性

射流点火是实现稳定的稀薄燃烧,大幅度提升发动机热效率的有效技术途径。该文利用设计的一种射流点火器,对气相射流点火(GJI)的燃烧开展研究,揭示了主动式射流点火(射流室内有补充燃料)和被动式射流点火(射流室内无补充燃料)的燃烧和排放特性。结果表明:相比于被动式射流点火,主动式射流点火将过量空气系数拓展至2.0,热效率提升1.5%;进一步引入废气再循环(EGR)后,热效率提升至44.5%。主动式射流点火时,最高热效率点NOx排放较被动式射流点火下降低66%,THC及CO排放的增加使燃烧效率降低3%;引入EGR后,NOx进一步降低79%,燃烧效率保持稳定在96%。     

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.     

HCCI combustion characteristics during operation on DME and methane fuels

The Homogeneous Charge Compression Ignition (HCCI) engine has attracted much interest because it can simultaneously achieve high efficiency and low emissions. However, the ignition timing is difficult to control because this engine has no physical ignition mechanism. In addition, combustion proceeds very rapidly because the premixed mixture ignites simultaneously at multiple locations in the cylinder, making it difficult to increase the operating load. In this study, an HCCI engine was operated using blended test fuels comprised of dimethyl ether (DME) and methane, each of which have different ignition characteristics. The effects of mixing ratios and absolute quantities of the two types of fuel on the ignition timing and rapidity of combustion were investigated. Cool flame reaction behavior, which significantly influences the ignition, was also analyzed in detail on the basis of in-cylinder spectroscopic measurements. The experimental results revealed that within the range of the experimental conditions used in this study, the quantity of DME supplied substantially influenced the ignition timing, whereas there was little observed effect from the quantity of methane supplied. Spectroscopic measurements of the behavior of a substance corresponding to HCHO also indicated that the quantity of DME supplied significantly influenced the cool flame behavior. However, the rapidity of combustion could not be controlled even by varying the mixing ratios of DME and methane. It was made clear that changes in the ignition timing substantially influence the rapidity of combustion.     

Transient characteristics of emissions during engine start/stop operation employing a conventional gasoline engine for HEV application

In this paper, we investigate the transient characteristics of combustion and emissions during engine start/stop operations in hybrid electric vehicle (HEV) applications. Hydrocarbon (HC) emissions during the initial 2^nd∼9^th cycles are found to be significantly greater when the engine is quickly started under the original engine calibration mode. Lower intake manifold absolute pressure (MAP) was also found to cause larger residual gas dilution and poor combustion, resulting in a higher HC concentration when the cranking speed was increased. The post-catalyst HC concentration was found in the way of initially decrease and then to increase again as the cranking speed was increased. A lowest concentration value was achieved at a cranking speed of 1000 r/min. Engine shut-down by fuel cut-off was shown to produce lower emissions than shut-down by ignition cut-off as one can avoid misfire of the last fuel injection cycle. The fuel deposited during the stop process seems to impact engine restart enrichment mostly during the initial 0.7 s for this engine, whose performance is dominated by the MAP transition characteristic and the time coefficient for fuel vaporization in this time period