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.     

Experimental study of combustion and emission characteristics of ethanol fuelled port injected homogeneous charge compression ignition (HCCI) combustion engine

The homogeneous charge compression ignition (HCCI) is an alternative combustion concept for in reciprocating engines. The HCCI combustion engine offers significant benefits in terms of its high efficiency and ultra low emissions. In this investigation, port injection technique is used for preparing homogeneous charge. The combustion and emission characteristics of a HCCI engine fuelled with ethanol were investigated on a modified two-cylinder, four-stroke engine. The experiment is conducted with varying intake air temperature (120–150 °C) and at different air–fuel ratios, for which stable HCCI combustion is achieved. In-cylinder pressure, heat release analysis and exhaust emission measurements were employed for combustion diagnostics. In this study, effect of intake air temperature on combustion parameters, thermal efficiency, combustion efficiency and emissions in HCCI combustion engine is analyzed and discussed in detail. The experimental results indicate that the air–fuel ratio and intake air temperature have significant effect on the maximum in-cylinder pressure and its position, gas exchange efficiency, thermal efficiency, combustion efficiency, maximum rate of pressure rise and the heat release rate. Results show that for all stable operation points, NO_x emissions are lower than 10 ppm however HC and CO emissions are higher.     

Exploring the potentials of lean-burn hydrogen SI engine compared to methane operation

The global rush for decarbonization and the more restrictive emission regulations are pushing the research for cleaner powertrains to the transport sector. In this sense, this work contributes with an experimental investigation of the performance and emissions of a single-cylinder SI engine operating under lean-burn hydrogen combustion. Its performance, combustion parameters, exhaust emissions, and indicated efficiency for a wide range of mixture dilutions are then compared to methane under similar engine load conditions. Hydrogen achieved stable combustion up to lambda 3.4, presenting zero CO emission and very low HC emission for all tested operating conditions. Hydrogen operation also presented zero NOx emissions for conditions leaner than lambda 2.2 and 3.0 at 2000 and 3000 rpm, respectively, however, the NOx emissions increase as the mixture is enriched. The high in-cylinder pressure rise rate limited the operation at mixtures richer than lambda 1.3 at 2000 rpm. When compared to methane, the hydrogen allows de-throttle the engine to burn lean mixtures maintaining a proper flame speed, resulting in lower pumping losses, lower pollutants emissions for most of the conditions tested, and higher indicated efficiency, making hydrogen a promising fuel to replace conventional fuels on cleaner SI engines.     

Study of methane enrichment in a biogas fuelled HCCI engine

Biogas has been a promising alternative fuel for IC engines. However, its CO₂ content reduces calorific value and ignitability. The CO₂ fraction of raw biogas can be separated out by various techniques, which are collectively called methane enrichment. The present study explores the effect of methane enrichment on the output parameters of a Homogeneous Charge Compression Ignition (HCCI) engine. A single cylinder CI engine is altered for this purpose. Biogas (CH₄ + CO₂) is supplied along with air. Diethyl Ether (DEE) is used as the secondary fuel to initiate auto-ignition. The effects of injecting DEE at the inlet port and upstream in the intake manifold are also compared. Performance, emission and combustion characteristics such as brake thermal efficiency, equivalence ratio, HC, CO, CO₂, NO_x and smoke emissions, start and duration of combustion, in-cylinder pressure and maximum heat release rate are compared for operation with raw biogas (50% methane) and methane enriched biogas (100% methane) for various biogas flow rates and engine torques. Results show that methane enrichment enhances brake thermal efficiency by up to 2% compared to raw biogas. Methane enrichment advances and speeds up combustion. HC, CO and CO₂ emissions, maximum cylinder pressure and maximum heat release rate are also improved with methane enrichment. Ultra-low NO_x and smoke emissions can be obtained using raw biogas as well as methane enriched biogas. Low biogas flow rates provide better brake thermal efficiency and HC emissions. Manifold injection of DEE enhances brake thermal efficiency by up to 2% compared to port injection by virtue of greater mixture homogeneity.     

Experimental study of the performances of a modified diesel engine operating in homogeneous charge compression ignition (HCCI) combustion mode versus the original diesel combustion mode

Homogeneous charge compression ignition (HCCI) combustion mode provides very low NO_x and soot emissions; however, it has some challenges associated with hydrocarbon (HC) emissions, fuel consumption, difficult control of start of ignition and bad behaviour to high loads. Cooled exhaust gas recirculation (EGR) is a common way to control in-cylinder NO_x production in diesel and HCCI combustion mode. However EGR has different effects on combustion and emissions, which are difficult to distinguish. This work is intended to characterize an engine that has been modified from the base diesel engine (FL1 906 DEUTZ-DITER) to work in HCCI combustion mode. It shows the experimental results for the modified diesel engine in HCCI combustion mode fueled with commercial diesel fuel compared to the diesel engine mode. An experimental installation, in conjunction with systematic tests to determine the optimum crank angle of fuel injection, has been used to measure the evolution of the cylinder pressure and to get an estimate of the heat release rate from a single-zone numerical model. From these the angle of start of combustion has been obtained. The performances and emissions of HC, CO and the huge reduction of NO_x and smoke emissions of the engine are presented. These results have allowed a deeper analysis of the effects of external EGR on the HCCI operation mode, on some engine design parameters and also on NO_x emission reduction.     

HCCI甲醇发动机的燃烧与排放特性

在Ricardo Hydra单缸四冲程发动机上利用内部废气再循环策略实现了甲醇燃料的HCCI燃烧.通过调整HCCI发动机的过量空气系数和转速,研究了HCCI甲醇发动机的燃烧和排放特性.结果表明,甲醇燃料的HCCI燃烧不同于普通汽油,其着火更早、燃烧更快,但在低转速时,平均指示压力相对较低.甲醇燃料可以在更稀的混合气条件下实现HCCI燃烧.在相同的转速和过量空气系数下,甲醇燃料的NOx和HC排放低于汽油.     

Effect of hydrogen enrichment in the intake air of diesel engine fuelled with honge biodiesel blend and diesel

In the current investigation, the enrichment of hydrogen with the honge biodiesel blend and diesel is used in a compression ignition engine. The biodiesel is derived from the honge oil and mixed with diesel fuel by 20% (v/v). Thereafter, hydrogen at different volume flow rates (10 and 13 lpm) is introduced into the intake manifold. The outcomes by enrichment of hydrogen on the performance, combustion and emission characteristics are investigated by examining the brake thermal efficiency, fuel consumption, HC, CO, CO₂, NOₓ emissions, in-cylinder pressure, combustion duration, and rate of heat release. The engine fuelled with honge biodiesel blend is found to enhance the thermal efficiency, combustion characteristics. Compare to diesel, the BTE increased by 2.2% and 6% less fuel consumption for the HB20 + 13H₂ blend. Further, reduction in the emission of exhausts gases like CO and HC by 21% and 24%, respectively, are obtained. This is due to carbon-free structure in hydrogen. Moreover, due to high pressure in the cylinder, there is a slight increase in oxides of nitrogen emission compare to diesel. The combustion characteristics such as rate of heat release, combustion duration, and maximum ₂rate of pressure rise and in-cylinder pressure are high due to hydrogen.     

A computational study to analyze the effect of equivalence ratio and hydrogen volume fraction on the ultra-lean burning of the syngas-fueled HCCI engine

This computational study investigates the equivalence ratio and hydrogen volume fraction effect on the ultra-lean burning of the syngas-fueled homogeneous charge compression ignition (HCCI) engine. In this research, low calorific syngas, composed of different compositions of H2, CO, and CO2, is used as a fuel in the HCCI engine that is operated under an overly lean air-fuel mixture. ANSYS Forte CFD package with Gri-Mech 3.0 chemical kinetics was used to analyze the in-cylinder combustion phenomena, and the simulation results were validated with experimental tests in the form of in-cylinder pressure and heat release rate at different equivalence ratios.The results indicate that changing the equivalence ratio produces a negligible change in combustion phasing, while it positively impacts the combustion and thermal efficiency of this syngas-fueled HCCI engine under lean conditions due to the high burning rate in the squish region. Moreover, an increased equivalence ratio increases MPRR due to the rich mixture combustion. The results also represent that the high-volume fraction of H2 in syngas fuel causes an advanced burning phase, improves the combustion performance of the HCCI engine at all equivalence ratio conditions, and causes slightly high NOx emissions.     

Inhibition effect of doping methyl tert-butyl ether compounds to n-heptane on homogenous charge compression ignition combustion

This article reports an experimental study on the combustion characteristics and emissions of homogenous charge compression ignition (HCCI) combustion using n-heptane doped with methyl tert-butyl ether (MTBE). The experiments were conducted on a single cylinder HCCI engine using neat n-heptane and 10%, 20%, 30%, 40% and 50% (by volume) MTBE/n-heptane blends at constant engine speed. The experimental results reveal that the ignition timing of the low temperature reaction (LTR) gets retarded, the peak values of heat release during the LTR decrease and the negative temperature coefficient (NTC) duration gets prolonged with the increase of MTBE in the blends. Consequently, the ignition timing of the high temperature reaction (HTR) gets delayed and both the attainable maximum indicated mean effective pressure (IMEP) and the lowest stable IMEP increase. Parametric studies on CO and HC emissions reveal that the maximum combustion temperature, pressure rise rate, IMEP, ignition timing of the HTR, combustion duration and fuel components have important impacts on HC emission, while the main parameters that show an important influence on CO emissions are the maximum combustion temperature, pressure rise rate, IMEP and combustion duration. Moreover, in order to suppress the CO and HC emissions to a low level, the maximum combustion temperature should be higher than 1500 K, the maximum pressure rise rate larger than 0.5 MPa/°CA, the IMEP above 0.3 MPa and the combustion duration shorter than 9 °CA.     

均质压燃发动机燃烧与排放的多区模型模拟

应用一个有质量交换的6区模型模拟正庚烷在HCCI发动机中的燃烧和排放特性．通过把缸内划分为缝隙区、边界层区、外核心区和内核心区,加入Woschni传热模型计入了缸内的温度和浓度的不均匀分布．全部计算基于正庚烷燃烧的包含57种组分290个反应的详细机理,结果表明,该多区模型合理地模拟了HCCI发动机的燃烧过程,并可满意地预测出HC、CO和NO的排放．最后采用此多区模型分别讨论了缝隙容积、边界层厚度和壁面温度对HCCI发动机的燃烧和排放的影响．     

Study on combustion and emissions of a turbocharged compression ignition engine fueled with dimethyl ether and biodiesel blends

In this study, combustion and emissions characteristics of a turbocharged compression ignition engine fueled with dimethyl ether (DME) and biodiesel blends are experimentally investigated. The effects of nozzle parameter on combustion and emissions are evaluated. The result shows that with the increase of DME proportion, ignition delay, the peak in-cylinder pressure, peak heat-release rate, peak in-cylinder temperature decrease, and their phases retard. Compared to the nozzle 6 × 0.40 mm, the peak cylinder pressure and peak heat-release rate are higher with nozzle 6 × 0.35 mm, and their phases are advanced. Increased DME proportion in fuel blends causes greater differences. Compared to biodiesel, NO_x emissions of blends significantly decrease; HC emissions and CO emissions increase slightly. DME–biodiesel blends can be used as an alternative in a turbocharged CI engine. To obtain low NO_x emissions and a soft engine operation, for high DME proportion blended fuels, nozzle of 6 × 0.40 mm adopted.     

Effects of second hydrogen direct injection proportion and injection timing on combustion and emission characteristics of hydrogen/n-butanol combined injection engine

Hydrogen and n-butanol are superior alternative fuels for SI engines, which show high potential in improving the combustion and emission characteristics of internal combustion engines. However, both still have disadvantages when applied individually. N-butanol fuel has poor evaporative atomization properties and high latent heat of vaporization. Burning n-butanol fuel alone can lead to incomplete combustion and lower temperature in the cylinder. Hydrogen is not easily stored and transported, and the engine is prone to backfire or detonation only using hydrogen. Therefore, this paper investigates the effects of hydrogen direct injection strategies on the combustion and emission characteristics of n-butanol/hydrogen dual-fuel engines based on n-butanol port injection/split hydrogen direct injection mode and the synergistic optimization of their characteristics. The energy of hydrogen is 20% of the total energy of the fuel in the cylinder. The experimental results show that a balance between dynamics and emission characteristics can be found using split hydrogen direct injection. Compared with the second hydrogen injection proportion (IP2) = 0, the split hydrogen direct injection can promote the formation of a stable flame kernel, shorten the flame development period and rapid combustion period, and reduce the cyclic variation. When the IP2 is 25%, 50% and 75%, the engine torque increases by 0.14%, 1.50% and 3.00% and the maximum in-cylinder pressure increases by 1.9%, 2.3% and 0.6% respectively. Compared with IP2 = 100%, HC emissions are reduced by 7.8%, 15.4% and 24.7% and NOx emissions are reduced by 16.4%, 13.8% and 7.9% respectively, when the IP2 is 25%, 50% and 75%. As second hydrogen injection timing (IT2) is advanced, CA0-10 and CA10-90 show a decreasing and then increasing trend. The maximum in-cylinder pressure rises and falls, and the engine torque gradually decreases. The CO emissions show a trend of decreasing and remaining constant. However, the trends of HC emissions and NOx emissions with IT2 are not consistent at different IP2. Considering the engine's dynamics and emission characteristics, the first hydrogen injection proportion (IP1) = 25% plus first hydrogen injection timing (IT1) = 240°CA BTDC combined with IP2 = 75% plus IT2 = 105°CA BTDC is the superior split hydrogen direct injection strategy.     

Experimental study of fuel stratification for HCCI high load extension

Fuel stratification has the potential to extend the high load limits of homogeneous charge compression ignition (HCCI) combustion by improving the control over the combustion phase as well as reducing the maximum rate of pressure rise. In this work, experiments were carried out on a single-cylinder engine equipped with a dual-fuel-injection system – a port injector for preparing a homogeneous charge and a direct in-cylinder injector for creating the desired fuel stratification. The homogeneous charge was prepared using gasoline fuel while the fuel stratification was created with the in-cylinder injection of either gasoline or methanol during the compression stroke. The test results indicate that high load extension using gasoline for fuel stratification is limited by the trade-off between CO and NO_x emissions. Weak gasoline stratification leads to an advanced combustion phase and an increase in NO_x emission, while increasing the stratification with a higher quantity of gasoline direct injection, results in a significant deterioration in both the combustion efficiency and the CO emission. Engine tests using methanol for the stratification retarded the ignition timing and prolonged the combustion duration, resulting in a substantial reduction in the maximum rate of pressure rise and the maximum cylinder pressure – a prerequisite for HCCI high load extension. Further tests were then conducted with methanol stratification to extend the HCCI load limit and to optimize the stratified methanol-to-gasoline fuel ratio. Compared to gasoline HCCI, a 50% increase in the maximum IMEP attained was achieved with an acceptable maximum pressure rise rate of 0.5 MPa/°CA while maintaining a high thermal efficiency.     

Active fuel design—A way to manage the right fuel for HCCI engines

Homogenous charge compression ignition (HCCI) engines feature high thermal efficiency and ultralow emissions compared to gasoline engines. However, unlike SI engines, HCCI combustion does not have a direct way to trigger the in-cylinder combustion. Therefore, gasoline HCCI combustion is facing challenges in the control of ignition and, combustion, and operational range extension. In this paper, an active fuel design concept was proposed to explore a potential pathway to optimize the HCCI engine combustion and broaden its operational range. The active fuel design concept was realized by real time control of dual-fuel (gasoline and n-heptane) port injection, with exhaust gas recirculation (EGR) rate and intake temperature adjusted. It was found that the cylinderto- cylinder variation in HCCI combustion could be effectively reduced by the optimization in fuel injection proportion, and that the rapid transition process from SI to HCCI could be realized. The active fuel design technology could significantly increase the adaptability of HCCI combustion to increased EGR rate and reduced intake temperature. Active fuel design was shown to broaden the operational HCCI load to 9.3 bar indicated mean effective pressure (IMEP). HCCI operation was used by up to 70% of the SI mode load while reducing fuel consumption and nitrogen oxides emissions. Therefore, the active fuel design technology could manage the right fuel for clean engine combustion, and provide a potential pathway for engine fuel diversification and future engine concept.     

The influence of Exhaust Gas Recirculation (EGR) on combustion and emissions of n-heptane/natural gas fueled Homogeneous Charge Compression Ignition (HCCI) engines

In Homogeneous Charge Compression Ignition (HCCI) combustion, a lean premixed charge combusts simultaneously in multiple sites. Utilizing highly diluted mixtures, and lack of any significant flame propagation, in-cylinder NO_x formation is reduced. Making HCCI engine a feasible alternative to conventional engines requires several challenges to be resolved. Combustion timing control is one of the most important of these items. It should be done in order that heat is released at the most optimum phasing for efficiency and emissions. In this study, a Waukesha Cooperative Fuel Research (CFR) single cylinder research engine was used to be operated in HCCI combustion mode fueled by natural gas and n-heptane. The main goal of the experiments was to investigate the possibility of controlling combustion phasing and combustion duration using various Exhaust Gas Recirculation (EGR) fractions. For the analysis of the results, a modified apparent heat release model was developed. The influence of EGR on emissions was discussed. Results indicate that applying EGR reduces mean charge temperature and has profound effect on combustion phasing, leading to a retarded Start of Combustion (SOC) and prolonged burn duration. Heat transfer rate decreases with EGR addition. Under examined condition EGR addition improved fuel economy, reduced NO_x emissions and increased HC and CO emissions.     

Effects of premixed diethyl ether (DEE) on combustion and exhaust emissions in a HCCI-DI diesel engine

In this study, the effects of premixed ratio of diethyl ether (DEE) on the combustion and exhaust emissions of a single-cylinder, HCCI-DI engine were investigated. The experiments were performed at the engine speed of 2200 rpm and 19 N m operating conditions. The amount of the premixed DEE was controlled by a programmable electronic control unit (ECU) and the DEE injection was conducted into the intake air charge using low pressure injector. The premixed fuel ratio (PFR) of DEE was changed from 0% to 40% and results were compared to neat diesel operation. The percentages of premixed fuel were calculated from the energy ratio of premixed DEE fuel to total energy rate of the fuels. The experimental results show that single stage ignition was found with the addition of premixed DEE fuel. Increasing and phasing in-cylinder pressure and heat release were observed in the premixed stage of the combustion. Lower diffusion combustion was also occurred. Cycle-to cycle variations were very small with diesel fuel and 10% DEE premixed fuel ratio. Audible knocking occurred with 40% DEE premixed fuel ratio. NO_x-soot trade-off characteristics were changed and improvements were found simultaneously. NO_x and soot emissions decreased up to 19.4% and 76.1%, respectively, while exhaust gas temperature decreased by 23.8%. On the other hand, CO and HC emissions increased.     

An experimental study on the effect of hydrogen enrichment on diesel fueled HCCI combustion

This paper experimentally investigates the influence of hydrogen enrichment on the combustion and emission characteristics of a diesel HCCI engine using a modified Cooperative Fuel Research (CFR) engine. Three fuels, n-heptane and two middle distillates with cetane numbers of 46.6 and 36.6, are studied.The results show that hydrogen enrichment retards the combustion phasing and reduces the combustion duration of a diesel HCCI engine. Besides, hydrogen enrichment increases the power output and fuel conversion efficiency, and improves the combustion stability. However, hydrogen enrichment may narrow the operational compression ratio range and increase the knocking tendency. Both the overall indicated specific CO emissions (isCO) and CO emissions per unit burned diesel fuel mass are reduced by hydrogen enrichment. Although hydrogen enrichment decreases the overall indicated specific unburned hydrocarbon emissions (isHC), it does not significantly affect the HC emissions per unit burned diesel fuel mass.     

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.     

代理汽油在均质压燃内燃机中燃烧特性的模拟研究

在均质压燃（HCCI）内燃机中,燃烧主要由化学动力学控制。研究燃料的化学动力学反应机理对了解和控制HCCI具有重要意义。本文利用CHEMKIN多区模型,研究了由正庚烷、异辛烷和甲苯混合而成的代理汽油的燃烧特性。计算结果显示,多区模型弥补了单区模型中出现的温度压强陡升缺点,能更好地反映缸内真实燃烧过程。多区温度分布区间越广,则燃烧提前,燃烧持续期长。各区NOx排放和温度分布趋势类似。HC和CO排放主要集中在燃烧不完全的第1区。     

Characteristics and energy distribution of modulated multi-pulse injection modes based diesel HCCI combustion and their effects on engine thermal efficiency and emissions

Cycle fuel energy distribution and combustion characteristics of early in-cylinder diesel homogenous charge compression ignition (HCCI) combustion organized by modulated multi-pulse injection modes are studied by the engine test. It is found that heat loss due to unburned fuel droplets and CO emission can be decreased effectively by injection mode regulation, and thermal efficiency can be potentially increased by 4%–12%. From the analyses of combustion process, it is also found that diesel HCCI combustion is a process with a finite reaction rate and is very sensitive to injection timing and injection mode. At injection timing of −90°CA ATDC, extra low NO_x emissions can be obtained along with high thermal efficiency. __________ Translated from Transactions of CSICE, 2006, 24(6): 385–393 [译自: 内燃机学报]