Air-fuel mixing and combustion in a small-bore direct injection optically accessible diesel engine using a retarded single injection strategy

In this paper, the air-fuel mixing and combustion in a small-bore direct injection optical diesel engine were studied for a retarded single injection strategy. The effects of injection pressure and timing were analyzed based on in-cylinder heat release analysis, liquid fuel and vapor fuel imaging by Laser induced exciplex fluorescence technique, and combustion process visualization. NOx emissions were measured in the exhaust pipe. Results show that increasing injection pressure benefits soot reduction while increases NOx emissions. Retarding injection timing leads to simultaneous reduction of soot and NOx emissions with premixed homogeneous charge compression ignition (HCCI) like combustion modes. The vapor distribution in the cylinder is relatively homogeneous, which confirms the observation of premixed combustion in the current studies. The postulated path of these combustion modes were analyzed and discussed on the equivalence ratio-temperature map.     

Charge stratification to control HCCI: Experiments and CFD modeling with n-heptane as fuel

An optimized reduced mechanism of n-heptane including 42 species and 58 elementary reactions adapted to charge stratification combustion is developed first in this study. Some engine experiments and a fully coupled CFD and reduced chemical kinetics model with n-heptane as fuel are adopted to investigate the combustion processes of HCCI-like charge stratification combustion aimed at diesel HCCI application. For premixed/direct-injected stratification combustion, the low temperature reaction occurs in the regions with homogeneous fuel first and high temperature reaction begins from high fuel concentration regions involved in the spray process. With the increase of the injection ratio, the high temperature reaction occurs in advance, the pressure rise rate reduces, UHC emissions decrease and CO emissions increase. At larger injection ratio, the onset of the high temperature reaction advances and the maximum pressure rise rate decreases with the retarding of injection timing. UHC and CO emissions have relation to the fuel spray penetration at different injection timings. NO_x emissions increase rapidly with the increase of the stratification degree.     

Simultaneous reduction of NOx emission and smoke opacity of biodiesel-fueled engines by port injection of ethanol

In the present study, the detailed combustion characteristics and emissions of biodiesel-fueled engines with premixed ethanol by port injection were investigated. The experiments were carried out on a single-cylinder, four-stroke, natural aspirated direct injection engine at a fixed speed. The heat release analysis indicates that, with the introduction of ethanol fuel by port injection, the ignition timing of the overall combustion event delays remarkably, while the maximum heat release rate increases smoothly. At a leaner fuel/air mixture, the peak value of the heat release rate (HRR) increases slightly, the maximum in-cylinder gas pressure and temperature decrease, and the indicated thermal efficiency (ITE) deteriorates with the increase of ethanol proportion. While at a rich fuel/air mixture, with the increase of the ethanol proportion, the maximum HRR increases rapidly, the overall combustion event is completed at an earlier crank angle. Moreover, the maximum values of the HRR reach the peak point in a certain premixed ratio which ranges from 20% to 40%. Also, the ITE reaches the largest value at this operation point. Due to the introduction of the ethanol fuel by port injection, both the NO_x emission and smoke opacity decrease to a very low level under overall operation conditions. During the experimental points of this test, NO_x and smoke opacity simultaneously decrease about 35-85% compared to those of the neat biodiesel-fueled engines.     

Homogeneous charge compression ignition of LPG and gasoline using variable valve timing in an engine

The combustion characteristics and exhaust emissions in an engine were investigated under homogeneous charge compression ignition (HCCI) operation fueled with liquefied petroleum gas (LPG) and gasoline with regard to variable valve timing (VVT) and the addition of di-methyl ether (DME). LPG is a low carbon, high octane number fuel. These two features lead to lower carbon dioxide (CO₂) emission and later combustion in an LPG HCCI engine as compared to a gasoline HCCI engine. To investigate the advantages and disadvantages of the LPG HCCI engine, experimental results for the LPG HCCI engine are compared with those for the gasoline HCCI engine. LPG was injected at an intake port as the main fuel in a liquid phase using a liquefied injection system, while a small amount of DME was also injected directly into the cylinder during the intake stroke as an ignition promoter. Different intake valve timings and fuel injection amount were tested in order to identify their effects on exhaust emissions and combustion characteristics. Combustion pressure, heat release rate, and indicated mean effective pressure (IMEP) were investigated to characterize the combustion performance. The optimal intake valve open (IVO) timing for the maximum IMEP was retarded as the λ_TOTAL was decreased. The start of combustion was affected by the IVO timing and the mixture strength (λ_TOTAL) due to the volumetric efficiency and latent heat of vaporization. At rich operating conditions, the θ_90-20 of the LPG HCCI engine was longer than that of the gasoline HCCI engine. Hydrocarbon (HC) and carbon monoxide (CO) emissions were increased as the IVO timing was retarded. However, CO₂ was decreased as the IVO timing was retarded. CO₂ emission of the LPG HCCI engine was lower than that of the gasoline HCCI engine. However, CO and HC emissions of the LPG HCCI engine were higher than those of the gasoline HCCI engine.     

Effect of a narrow fuel spray angle and a dual injection configuration on the improvement of exhaust emissions in a HCCI diesel engine

The aim of this work was to investigate the effect of narrow fuel spray angle injection and dual injection strategy on the exhaust emissions of a common-rail diesel engine. To achieve successful homogeneous charge compression ignition by an early timing injection, a narrowed spray cone angle injector and a reduced compression ratio were employed. The combination of homogeneous charge compression ignition (HCCI) combustion and conventional diesel combustion was studied to examine the exhaust emission and combustion characteristics of the engine under various fuel injection parameters, such as injection timings of the first and second spray.The results showed that a dual injection strategy consisting of an early timing for the first injection for HCCI combustion and a late timing for the second injection was effective to reduce the NO_x emissions while it suppress the deterioration of the combustion efficiency caused by the HCCI combustion.     

Experimental and numerical study on the combustion characteristics of partially premixed charge compression ignition engine with dual fuel

The objective of this work is to investigate the effect of premixed fuel ratio on the combustion and emission characteristics in diesel engine by the experimental and numerical method. In order to investigate the effect of various factors such as the premixed ratio, EGR rate, and equivalence ratio on the exhaust gas from the premixed charge compression ignition diesel engine, the injection amount of premixed fuel is controlled by electronic port injection system. The range of premixed ratio between dual fuels used in this study is between 0 and 0.85, and the exhaust gas is recirclulated up to 30 percent of EGR rate.     

Experimental study on the auto-ignition and combustion characteristics in the homogeneous charge compression ignition (HCCI) combustion operation with ethanol/n-heptane blend fuels by port injection

This article investigates the auto-ignition, combustion, and emission characteristics of homogeneous charge compression ignition (HCCI) combustion engines fuelled with n-heptane and ethanol/n-heptane blend fuels. The experiments were conducted on a single-cylinder HCCI engine using neat n-heptane, and 10%, 20%, 30%, 40%, and 50% ethanol/n-heptane blend fuels (by volume) at a fixed engine speed of 1800 r/min. The results show that, with the introduction of ethanol in n-heptane, the maximum indicated mean effective pressure (IMEP) can be expanded from 3.38 bar of neat n-heptane to 5.1 bar, the indicated thermal efficiency can also be increased up to 50% at large engine loads, but the thermal efficiency deteriorated at light engine load. Due to the much higher octane number of ethanol, the cool-flame reaction delays, the initial temperature corresponding the cool-flame reaction increases, and the peak value of the low-temperature heat release decreases with the increase of ethanol addition in the blend fuels. Furthermore, the low-temperature heat release is indiscernible when the ethanol volume increases up to 50%. In the case of the neat n-heptane and 10% ethanol/n-heptane blends, the combustion duration is very short due to the early ignition timing. For 20–50% ethanol/n-heptane blend fuels, the ignition timing is gradually delayed to the top dead center (TDC) by the ethanol addition. As a result, the combustion duration prolongs obviously at the same engine load when compared to the neat n-heptane fuel. At overall stable operation ranges, the HC emissions for n-heptane and 10–30% ethanol/n-heptane blends are very low, while HC emissions increase substantially for 40% and 50% ethanol/n-heptane blends. CO emissions show another tendency compared to HC emissions. At the engine load of 1.5–2.5 bar, CO emissions are very high for all fuels. Beside this range, CO emissions decrease both for large load and light load. In terms of operation stability of HCCI combustion, for a constant energy input, n-heptane shows an excellent repeatability and light cycle-to-cycle variation, while the cycle-to-cycle variation of the maximum combustion pressure and its corresponding crank angle, and ignition timing deteriorated with the increase of ethanol addition.     

A fundamental study on the control of the HCCI combustion and emissions by fuel design concept combined with controllable EGR. Part 1. The basic characteristics of HCCI combustion

This article investigates the basic combustion parameters including start of the ignition timing, burn duration, cycle-to-cycle variation, and carbon monoxide (CO), unburned hydrocarbon (UHC), and nitric oxide (NO_x) emissions of homogeneous charge compression ignition (HCCI) engines fueled with primary reference fuels (PRFs) and their mixtures. Two primary reference fuels, n-heptane and iso-octane, and their blends with RON25, RON50, RON75, and RON90 were evaluated. The experimental results show that, in the first-stage combustion, the start of ignition retards, the maximum heat release rate decreases, and the pressure rising and the temperature rising during the first-stage combustion decrease with the increase of the research octane number (RON). Furthermore, the cumulative heat release in the first-stage combustion is strongly dependent on the concentration of n-heptane in the mixture. The start of ignition of the second-stage combustion is linear with the start of ignition of the first-stage. The combustion duration of the second-stage combustion decreases with the increase of the equivalence ration and the decrease of the octane number. The cycle-to-cycle variation improved with the decrease of the octane number.     

Biodiesel combustion in an optical HSDI diesel engine under low load premixed combustion conditions

An optically accessible single-cylinder high-speed direct-injection (HSDI) diesel engine was used to investigate the spray and combustion processes for biodiesel blends under different injection strategies. The experimental results indicated that the heat release rate was dominated by a premixed combustion pattern and the heat release rate peak became smaller with injection timing retardation. The ignition and heat release rate peak occurred later with increasing biodiesel content. Fuel impingement on the wall was observed for all test conditions. The liquid penetration became longer and the fuel impingement was stronger with the increase of biodiesel content. Early and late injection timings result in lower flame luminosity due to improved mixing with longer ignition delay. For all the injection timings, lower soot luminosity was seen for biodiesel blends than pure diesel fuel. Furthermore, NO_x emissions were dramatically reduced for premixed combustion mode with retarded post-TDC injection strategies.     

Effect of biofuels combustion on the nanoparticle and emission characteristics of a common-rail DI diesel engine

This study was performed to investigate the effect of biogas-biodiesel fuel combustion on the emissions reduction and nanoparticle characteristics in a direct injection (DI) diesel engine. In order to apply the two biofuels, biogas was injected into a premixed chamber during the intake process by using two electronically controlled gas injectors, and biodiesel fuel was directly injected into combustion chamber by a high-pressure injection system. The in-cylinder pressure and rate of heat release (ROHR) were investigated under various fuel conditions for single-fuel (biodiesel) and dual-fuel (biogas-biodiesel) combustions. To evaluate the engine performances and exhaust emissions characteristics, the indicated mean effective pressure (IMEP) and exhaust emissions were also investigated under various test conditions. Furthermore, the particle number concentration and the size distribution of nanoparticles were analyzed by using a scanning mobility particle sizer (SMPS).In the case of dual-fuels, the peak combustion pressure and ROHR were gradually decreased with the increase of the biogas fraction in the dual-fuels. As the premixed ratios increased, ignition delay and combustion durations were prolonged compared to single-fuel mode. The dual-fuels combustion showed that the IMEP decreased slightly and maintained similar levels up to 20° BTDC due to the retarded combustion phase. The concentrations of NO_x emissions were decreased for all injection timings as the premixed ratio (r_p) increased. The soot emissions in dual-fuel operations were significantly lower than those in the single-fuel mode (r_p = 0), and decreased gradually as the premixed ratio increased, regardless of injection timing. A lower nanoparticle size distribution was observed at all premixed ratios for dual-fuel combustion compared to those of the single fuel mode. The number distribution of both nuclei and accumulation modes also decreased with an increase in the biogas fraction. A slight reduction in the total particle number and total volume for all premixed ratios was observed as the injection timing increased from TDC up to 20° BTDC.     

Study of HCCI-DI combustion and emissions in a DME engine

HCCI combustion demonstrates the capability of simultaneously reducing NO_x and PM emissions and having a high brake thermal efficiency. However, there are still many challenges such as combustion control to overcome before full HCCI operation can be used reliably over the full engine operation range. Recently, the HCCI-DI compound combustion concept is presented, which is a compromise to full HCCI in that only a portion of the fuel is premixed and a portion of combustion is still controlled by the direct injection timing. Investigations towards HCCI-DI combustion in a DME engine were carried out in this paper. HCCI engine performances were presented to make a comparison. The peak in-cylinder pressure and the maximum heat release rate for HCCI-DI were lower than those for HCCI combustion and they decreased with a decrease in port DME aspiration quantity. Moreover, combustion duration was longer for HCCI-DI combustion and it would elongate with a decrease in port DME aspiration quantity. Engine experimental results showed HCCI-DI combustion could extend the operating range with a comparatively high brake thermal efficiency in comparison to HCCI combustion. CO and HC emission for HCCI-DI were lower than those of HCCI engine. As for NO_x emissions for HCCI-DI operation, it decreased remarkably at low loads with an increase in port DME aspiration quantity, while showed an increasing trend at high loads. To control the ignition and combustion phase of HCCI, the effect of cooled EGR on HCCI-DI was evaluated. As a result, NO_x emission decreased and the engine’s operating range enlarged for HCCI-DI combustion with cooled EGR.     

Improved emission characteristics of HCCI engine by various premixed fuels and cooled EGR

This work investigates partial HCCI (homogeneous charge compression ignition) combustion as a control mechanism for HCCI combustion. The premixed fuel is supplied via a port fuel injection system located in the intake port of DI diesel engine. Cooled EGR is introduced for the suppression of advanced autoignition of the premixed fuel. The premixed fuels used in this experiment are gasoline, diesel, and n-heptane. The results show that with diesel premixed fuel, a simultaneous decrease of NO_x and soot can be obtained by increasing the premixed ratio. However, when the inlet charge is heated for the improved vaporization of diesel fuel, higher inlet temperature limits the operational range of HCCI combustion due to severe knocking and high NO_x emission at high premixed ratios. Gasoline premixing shows the most significant effects in the reductions of NO_x and soot emissions, compared to other kinds of premixed fuels.     

Effects of valve events on the engine efficiency in a homogeneous charge compression ignition engine fueled by dimethyl ether

Combustion characteristics of a homogeneous charge compression ignition (HCCI) engine were investigated with regard to the residual gas, i.e. internal exhaust gas recirculation (IEGR), by changing the intake and exhaust maximum opening points (MOP) and the exhaust cam lifts. Three different exhaust camshafts were used and had 2.5 mm, 4.0 mm and 8.4 mm exhaust valve lift. In-cylinder gas was sampled at the intake valve immediately before ignition to measure the IEGR rate. The heat release, fuel conversion efficiency and combustion efficiency were calculated using the in-cylinder pressure and composition of exhaust gases to examine the combustion features of the HCCI engine. The negative valve overlap (NVO) was increased as exhaust valve lift was reduced. Longer NVO made an increased IEGR through exhaust gas trapping. The IEGR rate was increased as the exhaust valve timing advanced while it was affected more by exhaust valve timing than by intake valve timing. Combustion phase was advanced by lower exhaust valve lift and early exhaust and intake MOP. It was because of higher amount of IEGR gas and effective compression ratio. The fuel conversion efficiency with higher exhaust valve lift was higher than that with lower exhaust valve lift. The late exhaust and intake MOP made the fuel conversion efficiency improve.     

Effect of dimethoxy-methane and exhaust gas recirculation on combustion and emission characteristics of a direct injection diesel engine

The effect of fuel constituents and exhaust gas recirculation (EGR) on combustion characteristics, fuel efficiency and emissions of a direct injection diesel engine fueled with diesel-dimethoxymethane (DMM) blends was investigated experimentally. Three diesel-DMM blended fuels containing 20%, 30% and 50% by volume fraction of DMM, corresponding to 8.5%, 12.7% and 21.1% by mass of oxygen in the blends, were used. By the use of DMM, it is observed that CO and smoke emissions as well as the total number and mass concentration of particulate reduce significantly, while HC emissions and particulate number with lower geometric mean diameters (D_i < 0.039 μm) increase slightly. For each fuel, there is an increase of ignition delay whereas a decrease of cylinder pressure and heat release rate in the premixed combustion phase when the diesel engine was operated with EGR system. The brake thermal efficiency fluctuates at small EGR ratio, while decreases with the further increase of EGR ratio. With an increase of EGR ratio, NO_x emission is reduced at the cost of increased smoke, HC and CO emissions as well as the total number and mass of particulates for each fuel.     

含氨燃料预混火焰的层流火焰速度及NO排放特性

将甲烷或氢气与氨气共燃可以克服NH₃火焰的点火能量高、燃烧速度慢的缺点。为了解NH₃作为燃料的燃烧特性,对含NH₃燃料进行一维层流预混火焰数值模拟,研究其层流火焰速度及NO排放特性。采用文献中5个简化反应机理进行数值计算,发现Okafor机理模拟NH₃/CH₄/air火焰精度更高;Xiao机理模拟NH₃/H₂/air、NH₃/air精度适中,计算时间较短。此外,开展了当量比、燃料混合物组分比例、压力等参数对含NH₃燃料燃烧时烟气中NO浓度影响的研究。研究发现：含NH₃燃料燃烧时NO主要通过OH、H、O自由基和O₂分子的消耗而生成,主要通过与NH_i（i=0, 1, 2）自由基反应消耗;含NH₃燃料在富燃状态下燃烧可有效减少NO排放,但富燃燃烧效率低,可采用富燃-贫燃分级燃烧技术来提高燃烧效率,同时保持NO的低排放;掺有较多NH₃的含NH₃燃料在中高压下燃烧时可有效减少NO排放。     

预分解系统分级燃烧技术的工业试验研究 及工程应用

分析了国外公司在新型干法水泥生产线应用分级燃烧技术降低NOx排放方面存在的问题,并针对性的开发了适合已有水泥生产线改造的分级燃烧技术,并将其应用于工业试验和工程实践中。实践表明,在满足原燃料和操作条件下,分级燃烧系统可以有效降低NOx排放,其脱氮效率为10%~15%。     

Pilot ignited premixed combustion of dimethyl ether in a turbodiesel engine

This paper describes combustion studies of dimethyl ether in a common rail turbodiesel engine wherein the dimethyl ether was fumigated into the intake air and the conventional diesel injection was used with the intention of igniting the premixed DME-air charge. This combustion process is referred to here as a “mixed mode” process and is similar in some respects to what is commonly referred to as “dual fuel” combustion. In contrast to “dual fuel” combustion, however, in which the gaseous fuel is often natural gas or biogas, in this process with DME the gaseous charge ignites largely independently of the diesel injection. The diesel injection was accomplished with a single, main injection. The engine was operated at a single speed and load. Gaseous and particulate emissions were monitored and heat release analysis was performed to examine how the fuels burn and the impact on emissions formation at various levels of substitution of diesel fuel with fumigated DME, at as high as 44% of the fuel energy from DME. Reductions in NO_x emissions and increases in particulate matter emissions are observed with DME fumigation. The increase in PM emissions is attributed to enrichment of the diesel fuel spray, due to displacement of intake oxygen by the fumigated DME, despite the widely observed soot suppressing effect of DME.     

An investigation of the effects of spray angle and injection strategy on dimethyl ether (DME) combustion and exhaust emission characteristics in a common-rail diesel engine

An experimental investigation was performed on the effects of spray angle and injection strategies (single and multiple) on the combustion characteristics, concentrations of exhaust emissions, and the particle size distribution in a direct-injection (DI) compression ignition engine fueled with dimethyl ether (DME) fuel. In this study, two types of narrow spray angle injectors (θ_spray = 70° and 60°) were examined and its results were compared with the results of conventional spray angle (θ_spray = 156°). In addition, to investigate the optimal operating conditions, early single-injection and multiple-injection strategies were employed to reduce cylinder wall-wetting of the injected fuels and to promote the ignition of premixed charge. The engine test was performed at 1400 rpm, and the injection timings were varied from TDC to BTDC 40° of the crank angle.The experimental results showed that the combustion pressure from single combustion for narrow-angle injectors (θ_spray = 70° and 60°) is increased, as compared to the results of the wide-angle injector (θ_spray = 156°) with advanced injection timing of BTDC 35°. In addition, two peaks of the rate of heat release (ROHR) are generated by the combustion of air-fuel premixed mixtures. DME combustion for all test injectors indicated low levels of soot emissions at all injection timings. The NO_x emissions for narrow-angle injectors simultaneously increased in proportion to the advance in injection timing up to BTDC 25°, whereas BTDC 20° for the wide-angle injector. For multiple injections, the combustion pressure and ROHR of the first injection with narrow-angle injectors are combusted more actively, and the ignition delay of the second injected fuel is shorter than with the wide-angle injector. However, the second combustion pressure and ROHR were lower than during the first injection, and combustion durations are prolonged, as compared to the wide-angle injector. With advanced timing of the first injection, narrow-angle injectors with multiple injections could achieve low NO_x levels and soot levels similar to single-injection cases.     

低NOx燃气燃烧技术研究进展

气体燃料具有易于点火、燃烧迅速、燃烧完全等特点,且氮、硫、灰分低,因此燃烧后产生的污染物相对较少,属于较清洁的燃料,且国家燃气补贴政策的实施,使气体燃料燃烧近年来有很好的发展前景。但随着国家对大气污染物的控制更加严格,控制气体燃料燃烧过程中NOx的生成至关重要。笔者介绍了不同种类NOx的产生机理及影响因素,并基于NOx的产生机理提出控制措施,分析目前应用较广泛的燃气燃烧技术的低氮原理及应用现状,最后提出燃气燃烧器应用的展望。燃气燃烧过程中主要以热力型NOx及快速型NOx为主,温度和过量空气系数是影响NOx生成的主要影响因素。燃烧温度高于1 500℃时,热力型NOx呈指数型增长,温度是影响NOx生成的最重要因素。根据NOx产生机理,低NOx燃烧技术的实质是降低最高燃烧温度,控制燃烧区燃料浓度以及氧浓度,缩短烟气在高温区的停留时间,破坏NOx生成的最佳条件,最终抑制NOx的生成。低NOx燃烧技术一定程度降低了NOx的生成,但又会破坏整个燃烧进程,对燃烧和放热过程造成不利影响,降低了燃烧效率和传热效率,因此如何解决这些矛盾是亟需解决的问题。在实际应用中,应根据需求选择合适的燃烧技术,同时可将不同燃烧技术相结合起到稳燃、低氮的效果。应用较广泛的燃气燃烧技术主要是阶段型燃烧技术、烟气再循环燃烧技术、无焰燃烧技术等,其中催化燃烧技术发展前景较好,目前已应用于多个领域,其催化剂的热稳定性和寿命问题是限制其工业上广泛应用的核心问题。     

Influence of ethanol blends on the combustion performance and exhaust emission characteristics of a four-cylinder diesel engine at various engine loads and injection timings

The aim of this work is to investigate the effect of ethanol blending to diesel fuel on the combustion and exhaust emission characteristics of a four-cylinder diesel engine with a common-rail injection system. The overall spray characteristics, such as the spray tip penetration and the spray cone angle, were studied with respect to the ethanol blending ratio. A spray visualization system and a four-cylinder diesel engine equipped with a combustion and emission analyzer were utilized so as to analyze the spray and exhaust emission characteristics of the ethanol blending diesel fuel. Ethanol blended diesel fuel has a shorter spray tip penetration when compared to pure diesel fuel. In addition, the spray cone angle of ethanol blended fuels is larger. It is believed that the lower fuel density of ethanol blended fuels affects the spray characteristics. When the ethanol blended fuels are injected around top dead center (TDC), they exhibit unstable ignition characteristics because the higher ethanol blending ratio causes a long ignition delay. An advance in the injection timing also induces an increase in the combustion pressure due to the sufficient premixed duration. In a four-cylinder diesel engine, an increase in the ethanol blending ratio leads to a decrease in NO_x emissions due to the high heat of evaporation of ethanol fuel, however, CO and HC emissions increase. In addition, the CO and HC emissions exhibit a decreasing trend according to an increase in the engine load and an advance in the injection timing.