Homogeneous Charge Compression Ignition Engine Research Articles

Combustion characteristics, performance and emissions of an acetone/n-heptane fuelled homogenous charge compression ignition (HCCI) engine

Recently, there has been renewed interest in homogenous charge compression ignition (HCCI) engines due to their high thermal efficiency and low NOx and particulate matter. However, knocking and misfiring are the classic problems in HCCI engines. The present study aimed to explore the effects of acetone addition into the n-heptane on the combustion, performance and emission characteristics of HCCI engine. Four different fuels were used in this research: N100 (pure n-heptane), A20N80 (acetone 20% – n-heptane 80%), A40N60 (acetone 40% – n-heptane 60%), A60N40 (acetone 60% – n-heptane 40%). The experiments were carried out at 1000 rpm, 60 °C inlet air temperature and various lambda values (between 1.61 and 2.82). The research presented here confirms that in-cylinder pressure and heat release rate significantly decreased with the usage of A60N40 fuel blend. Besides, knocking disappeared with increasing amount of acetone into the mixture owing to the high octane number of acetone. The brake torque increased by 53.5% and specific fuel consumption (SFC) was lower by 44.8% for A60N40 compared to n-heptane at λ = 2.1.

High-efficiency conversion of natural gas fuel to power by an integrated system of SOFC, HCCI engine, and waste heat recovery: Thermodynamic and thermo-economic analyses

A novel natural gas utilization system for power generation including solid oxide fuel cell (SOFC) and homogeneous charge compression ignition (HCCI) engine is proposed and modeled to evaluate the thermodynamic and thermo-economic performance in this paper. The results show that the SOFC-HCCI engine hybrid system has a net electrical efficiency of approximately 59% and the exergy efficiency of 53.5%, both of which are more than single fuel cell system and comparable to other hybrid fuel cell systems. It is also found that the SOFC contributes to the total power of 80% and has a relatively low exergy destruction. Through the parametric analysis, the power distribution between SOFC and engine can be adjusted by controlling SOFC fuel utilization to optimize the system overall performance. Besides, the specific electric energy cost of the proposed hybrid system is calculated to be 6.91 ¢/kW h, which is comparable to the specific cost of electric power (6.92 ¢/kW h) generating from a biogas-fueled fuel cell hybrid system. The payback period and annual return on investment can reach 1.53 year and about 6.41%. These results reveal that the proposed conversion technology of natural gas fuel to power is efficient and economical, which could be a promising natural gas fuel utilization.

An Experimental Research on the Effects of Negative Valve Overlap on Performance and Operating Range in a Homogeneous Charge Compression Ignition Engine With RON40 and RON60 Fuels

Abstract In this study, the effects of negative valve overlap (NVO) on homogenous charge compression ignition (HCCI) combustion and engine performance were experimentally investigated. A four stroke, single cylinder, port injection HCCI engine was operated at −16 deg crank angle (CA), −8 deg CA, and +8 deg CA valve overlap values and different lambda values and engine speeds at wide open throttle. RON40 and RON60 were used as test fuels in view of combustion and performance characteristics in HCCI mode. The variations of indicated mean effective pressure (IMEP), residual gas, CA50, indicated thermal efficiency (ITE), indicated specific fuel consumption (ISFC), maximum pressure rise rate (MPRR) and ringing intensity (RI) were observed on HCCI combustion. The results showed that NVO caused to trap residual gases in the combustion chamber. Hot residual gases showed heating and dilution effect on HCCI combustion. Combustion was retarded with the presence of residual gas at −16 deg CA NVO. Test results showed that higher imep and maximum in-cylinder pressure were obtained with RON60 according to RON40. As expected, CA50 was obtained later with RON60 compared to RON40 due to more resistance of auto-ignition. RON60 residual gas prevented the rapid and sudden combustion due to higher heat capacity of charge mixture. RI decreased with the usage of RON60 compared to RON40. Significant decrease was seen on RI with RON60 especially at lower lambda values. It was seen that HCCI combustion can be controlled with NVO and operating range of HCCI engines can be extended.

Autoignition behavior of gasoline/ethanol blends at engine-relevant conditions

Ethanol is an attractive oxygenate increasingly used for blending with petroleum-derived gasoline yielding beneficial combustion and emissions behavior for a range of internal combustion engine schemes, including stoichiometric spark-ignition and low temperature combustion (LTC). As such, it is important to fundamentally understand the autoignition behavior of gasoline/ethanol blends. This work utilizes a rapid compression machine (RCM) and a homogeneous charge compression ignition (HCCI) engine to experimentally quantify changes in fuel reactivity, through ignition delay times and preliminary heat release, for blends of 0 to 30% vol./vol. into a full boiling range research gasoline (FACE-F). Diluted/stoichiometric and undiluted/fuel-lean conditions are explored covering a wide range of compressed temperatures and pressures relevant to conventional and advanced, gasoline combustion engines. Detailed chemical kinetic modeling is undertaken using a recently updated gasoline surrogate model in conjunction with a five-component surrogate to model the RCM experiments and provide chemical insight into the perturbative effects of ethanol on the autoignition process.The diluted/stoichiometric RCM measurements reveal that within the low-temperature regime ethanol retards first-stage and main ignition delay times, and suppresses both the rates and extents of low-temperature heat release (LTHR), while within the intermediate-temperature regime ethanol only causes slight changes. Good agreement of ignition delay time and preliminary heat release prediction is found between model and experimental results. Sensitivity and flux analyses further show that ethanol blending effects are dominated by the competition between the H-atom abstraction from ethanol and other fuel components by OH radical at low temperatures, and by HO2 radical at intermediate temperatures. These findings are consistent across both fuel loading conditions explored in this study. In addition, when HCCI engine experiments are mapped onto undiluted/lean RCM measurements under a constant combustion phasing scenario, good correspondence between the two apparatuses is observed for LTHR and start of high-temperature heat release. The current study highlights the importance of characterizing LTHR in predicting fuel behaviors in high-boost/low-temperature engines, and demonstrates that RCM experiments can provide an alternative, and more-efficient avenue for such characterization.

A Numerical Study to Investigate the Effect of Syngas Composition and Compression Ratio on the Combustion and Emission Characteristics of a Syngas-Fueled HCCI Engine

Abstract The purpose of this work is to investigate syngas composition (of constituents H2, CO, and CO2) and compression ratio (CR) effects on the combustion and emissions characteristics of a syngas-fueled homogenous charge compression ignition (HCCI) engine, which operates in very lean air–fuel mixture conditions for power plant usage. Investigations were conducted using ansys forte cfd package at low (3 bar indicated mean effective pressure (IMEP)) and medium (5 bar IMEP) loads, and the calculated results were compared with the Aceves et al.’s multi-zone HCCI model, using the same chemical kinetics set (Gas Research Institute (GRI)-Mech3.0). All calculations were carried out at maximum brake torque (MBT) conditions by sweeping the air–fuel mixture temperature at intake valve closing (IVC) (TIVC).This study found out that the H2 consumption rate is slightly high in a low-temperature range in the early period of combustion while the CO consumption rate is high in a high-temperature range in the later period of combustion. The results reveal that the change of H2 /CO ratio and inert gas volume fraction according to fuel composition affects combustion, but the TIVC is the dominant factor affecting combustion phasing at MBT conditions. For each fuel and load condition, the TIVC was significantly reduced with the increase of CR (17.1–22.3) to get MBT conditions, which causes to retard combustion phasing and lowers in-cylinder peak temperature. The oxides of nitrogen (NOx) emissions reduced with increasing the CR due to the lowering of the in-cylinder peak temperature.

Simulation of the ignition mechanisms of low and high octane number blended fuels in HCCI engine

Homogenous Charge Compression Ignition (HCCI) is an alternative combustion concept for in reciprocating engines which offers significant benefits in terms of its high efficiency and low emissions. HCCI is the most commonly used name for the auto-ignition of various fuels and one of the most promising alternatives to SI and CI combustion. This study focus on the ignition reactions of low and high octane number of fuel blends through comprehensive simulation. This study was carried out by using n-heptane as a base fuel and toluene as a sub fuel use as a fuel mixture in this simulation. Furthermore, for numerical analysis, MATLAB Software has been used to design simplified model of reaction mechanism for n-heptane. The simplified model has been discussed in this study. The highest value of hydroxyl radicals OH was achieved at approximately 0.23 at NTF 10 (Toluene mixture 10%) and the line decreased until 0 This value is gradually decreased when the mixture of toluene (NTF) as sub fuel is elevated until NTF60 Due to the content percentage of toluene added 10% consecutively, HCHO production increased as well. It is because HCHO consumes OH and at the same time affects the amount of OH. By doing this method (mixing n-heptane with toluene), the ignition delay of the fuel becomes longer is described. It is also shows that the simplified model constructed with a consideration of the property of reaction happen in nheptane (base fuel) added with toluene (sub fuel) in which OH reproduction and fuel + OH reaction plays important role. The purpose of this study is to figure out the reaction mechanism of compression ignition at Low Temperature Oxidation (LTO) and design the simplified model of reaction mechanism for n-heptane + toluene (NTF).

Numerical study on the performance of a homogeneous charge compression ignition engine fueled with different blends of biodiesel

The increasing need for fossil fuels along with their limited resources and also environmental concerns caused by pollution from internal combustion engines have always led researchers to improve the technology. The use of homogeneous compression ignition combustion technology as well as alternative fuel sources such as biodiesel is proposed as a novel and promising solution to achieve goals including improving efficiency and reducing environmental pollutants specifically NOx and CO2 that are emerging in the industry. In this study, the effect of using different blends of diesel/biodiesel fuel on thermal performance along with output pollution of a homogeneous fuel compression ignition engine is investigated. The fuels range from B00 to B40, and parameters such as engine thermal efficiency, maximum cylinder pressure and exhaust emissions have been evaluated. The results show retarded and reduced maximum pressure with a decrease in engine thermal efficiency, a reduction in NOx and CO2 emissions and an increase in CO emissions, while the mass fraction of biodiesel in the fuel composition is increased. Moreover, in the investigated engine, the B20 is considered as the optimum composition with a reduction of 4.2% in maximum cylinder pressure, 5% in thermal efficiency and 26% in NOx emissions compared to B00.

Study on the effect of water addition on combustion characteristics of a HCCI engine fueled with natural gas

The coordinated control of combustion boundary conditions and fuel chemistry is the core academic idea of homogenous charge compression ignition (HCCI) combustion. Realizing controlled combustion is the current development goal of HCCI engine and adding additives to the cylinder cooperate with the change of boundary conditions is an effective way to achieve it. In the paper, using a multi-zone thermodynamic model coupled with a detailed chemical reaction mechanism (including 53 species and 325 reactions) to study. First, the rationality of the multi-zone model was verified for six different operating conditions. Then seven different mass fraction of water were added to the cylinder and examined its effect on natural gas (NG) HCCI combustion. The thermal, dilution and chemical effects of water on start of combustion (SOC) were carried out using artificial inert species method. It was determined that in-cylinder pressure and heat release rate (HRR) decreases, SOC retards with the increase of water addition at different loads and boundary conditions. Under high load which is knock and abnormal combustion, water addition controls SOC and reduces ringing intensity (RI) to normalize the combustion process. The results show that the sensitivity of combustion to water addition increases and the tolerance of engine to water decreases as the intake temperature (Tin) reduces and excess air ratio (λ) rises. The thermal effect of water on SOC is greater than that its dilution and thermal effect. It is easier to achieve controlled combustion by changing Tin than λ in NG HCCI engine with water to mitigate knock. In addition, the effect of water on important species in chamber was proposed to prove the above conclusion from internal mechanism.

A Review on New Technology in Internal Combustion Engine- HCCI engine

In present study a concise literature review on Homogeneous Charge Compression Ignition (HCCI) engine have been performed by taking the many practical problem which are hurdle for implementation of this concept. HCCI combustion is also termed as Low Temperature Combustion (LTC) technology because of its prospective to attain high efficiency and low Nox and particulate matter emission. The typical quality of HCCI technique is the need of preparation of homogeneous mixture early at combustion. Combustion processes in HCCI engine were controlled by chemical kinetics and proper mixing of fuel. Numerous researchers are working towards the challenges and solution for commercialization of this engine. Present review covers the elementary feature for the evaluation of HCCI engine, its working principle and challenges for practical implementation. This study elaborates the prospective solution and future growth in HCCI engine. The most critical challenges for HCCI engine is controlling method for direct ignition of combustion. Therefore in HCCI combustion engine implementation of this problem is way for practical availability of this concept.

Misfire detection of homogeneous charge compression ignition engines using matter‐element extension theory and thermodynamic multi zone model

AbstractNowadays, researchers tend to simulate and model the studied phenomena to reduce the calculation time and cost. Matter‐element extension model is a useful method that can be used to evaluate a wide range of data that belong to different subjects. In this study, this model is used to diagnose different engine's performance modes including misfire, normal, knock, and middle modes in which both of the mentioned modes are occurred simultaneously. To achieve this purpose, a set of performance and emission parameters acquired for a multi zonal model are chosen as input data to the model. Statistical analysis is done on a set of experimental data. First, ANOVA (analysis of variance) is used to determine parameters significant level. Then the proportional weights are given for the chosen parameters with respect to their importance. To do this, linear regression analysis is used for determination of weights. Two kinds of single objective and two objectives matter‐element extension models are employed to diagnose the engine's operation mode. The results show that both the single objective and two objectives models show a good conformity with the corresponding experimental results without acquiring empirical data. Single objective model can predict misfire, normal combustion, and knocking combustion. Meanwhile the two objectives model can predict the middle modes containing normal‐misfire, misfire‐knock, and knock‐normal accurately.

The comparison of combustion, engine performance and emission characteristics of ethanol, methanol, fusel oil, butanol, isopropanol and naphtha with n-heptane blends on HCCI engine

Recently, there has been increased emphasis on the homogenous charge compression ignition (HCCI) engine which offers higher thermal efficiency and ultra-low NOx and soot emissions. The central thesis of this paper is that the effects of different alternative fuels with different physical and chemical properties on combustion, performance and emissions in a HCCI engine. Ethanol (E25), methanol (M25), fusel oil (F25), butanol (B25), isopropanol (IP25) and naphtha (N25) were used alternative fuels blended with n-heptane 25% by volume. The experiments were performed at 333 K intake air temperature (IAT) and with various excess air coefficient in a single cylinder SI-HCCI test engine. The parameters such as in-cylinder pressure, heat release rate (HRR), start of combustion (SOC), combustion duration, CA50, indicated thermal efficiency (ITE), COVIMEP, maximum pressure rise rate (MPRR), HC and CO emissions were investigated. In addition, operating range of the fuels were also defined. The results showed that N25 has the largest operating range among all the test fuels. The maximum indicated mean effective pressure (IMEP) value was obtained for E25 as 5.71 bar at 800 rpm engine speed and λ = 1.3. Knocking tendency were determined for all test fuels but it decreased with increasing lambda value. The highest HC and CO emissions were obtained for F25 due to the water content of the fuel. On the contrary, the lowest HC and CO emissions were obtained for N25.

The effects of diisopropyl ether on combustion, performance, emissions and operating range in a HCCI engine

In the current study, the effects of diisopropyl ether were experimentally investigated on combustion, performance, emissions and operating range in a homogeneous charged compression ignition (HCCI) engine. For this purpose, a single cylinder, four stroke, port injection test engine was run with different lambda values between 1.69 and 3.08 on HCCI mode with pure n-heptane, 20% diisopropyl ether 80% n-heptane (D20N80), and 40% diisopropyl ether 60% n-heptane (D40N60) fuel blends at full load. HCCI engine was operated between 800 and 1600 rpm engine speed at constant inlet air temperature of 60 °C and wide open throttle (WOT). The effects of diisopropyl ether addition were observed on cylinder pressure, heat release rate (HRR), combustion duration (CD), indicated mean effective pressure (imep), brake torque, power output, specific fuel consumption (SFC) and exhaust emissions. Test results showed that the increase of lambda leads to lower in-cylinder pressure and HRR. The addition of diisopropyl ether caused to retard combustion. Indicated thermal efficiency (ITE) increased 4.92% with D40N60 compared that n-heptane at 1200 rpm and λ = 2. Brake torque and power output increased by about 1.03% and 1.18% with D20N80 according to pure n-heptane at 1200 rpm respectively. On the contrary, SFC decreased 24.08% with D40N60 compared to n-heptane at 1200 rpm and λ = 2. HC and CO increased with the addition of diisopropyl ether. The test results also showed that the addition of diisopropyl ether expanded the HCCI combustion towards to knocking and partial combustion zone.

Experimental investigation on pressure and heat release HCCI engine operated with chicken fat oil/diesel-gasoline blends

Low-temperature combustion is the individuality of Homogenous charge compression ignition (HCCI) engines where the combustion is achieved at the temperature lower than the temperature at which NOx is formed. By achieving this type of combustion in the engine it is possible to reduce the formation of Thermal NOx and PM. But on the other hand, the performance. In this current study, the combustion parameters of the engine are studied by varying the input parameters of the engine that has been modified to run in HCCI mode. The engine is fuelled with the blends of chicken fat oil biodiesel (CFOB) and gasoline. In the present work, the engine is tested for combustion properties such as in-cylinder pressure, heat release rate and the rate of pressure rise for different blend ratios. The ignition delay has persistently elevated for an increase in the gasoline concentration in blends. Also, the knock is consistently seen at higher engine loads with the blends of gasoline and diesel as well as gasoline with biodiesel. Diesel fuel exhibited lower peak pressure whereas, pure biodiesel exhibits higher peak pressure and the peak cylinder pressure within the engine at higher load varies from 40 to 75 bar. On comparing the heat release rate, the diesel fuel exhibit lower heat release as compared to fuel blended with chicken fat oil. Due to the delay in the start of low-temperature reactions the peaks on the rate of heat release is reduced. During higher engine load conditions, the engine seizes to operate in HCCI mode instead it operates in a conventional pattern which is clearly proved by the heat release curves obtained for the indicated pressure and crank angle data of the particular operating mode of the engine.

Experimental and kinetic study of diesel/gasoline surrogate blends over wide temperature and pressure

Blending diesel and gasoline is a promising way to stabilize combustion process and reduce emissions of NOx and soot in the Homogeneous Charge Compression Ignition (HCCI) engine. In order to computationally study diesel/gasoline blends for their autoignition characteristics, a five-component diesel surrogate (21.6% n-hexadecane, 15.5% n-octadecane, 26.0% isocetane, 20.7% 1-methylnaphthalene, 16.2% decalin, by mol.) and a five-component gasoline surrogate (11.0% n-heptane, 30.8% isooctane, 38.2% toluene, 10.3% diisobutylene, 9.7% cyclohexane, by mol.) are blended in three volume ratios of 30%/70%, 50%/50%, and 70%/30% (denoted as SD30, SD50, and SD70) to compare measured ignition delay with published data of diesel/gasoline blends. A heated shock tube and a heated rapid compression machine (RCM) are used to measure the ignition delay of three surrogate blends under compressed pressures of 6, 10, and 20 bar for fuel-lean (ϕ = 0.5), fuel-rich (ϕ = 1.5), and stoichiometric mixtures. Experimental results indicate that surrogate blends match well with fuel-lean and stoichiometric diesel/gasoline/air mixtures while get longer ignition delays for the fuel-rich case, which is thought of as the consequence of fitting the surrogate to fuel-lean conditions in the engine. The kinetic simulation for ignition delays is performed with a comprehensive mechanism. Due to the mechanism overpredicting ignition delays in the intermediate-to-high temperature region, proper refinement is done to the original mechanism based on sensitivity analyses at 1000 and 660 K. A further comparison of ignition delay predictive ability for two mechanisms is conducted subsequently. The modified mechanism not only simulates ignition delays in the intermediate-to-high temperature region successfully but also maintains the good performance of the original mechanism in the negative-temperature-coefficient (NTC) and low-temperature region. Besides, species and temperature profiles after the compression stroke in RCM are simulated using the modified mechanism for three surrogate blends at equal equivalence ratio, compressed pressure, and compressed temperature. Sensitivity analysis is also conducted to identify reactions dominating the system reactivity.

Investigation on the effect of reformer gas on availability terms and waste heat recovery from exhaust gases of an HCCI engine considering radiation heat transfer

The main purpose of the current study is to investigate the different effects of reformer gas on different availability terms and the capacity of waste heat recovery from a homogeneous charge compression ignition engine. A validated multizone model is utilized for HCCI engine simulation. Mass transfer and conductive heat transfer are considered between zones, and convection and radiation are considered between near-wall zone and combustion chamber walls. Four different percentages of reformer gas (0–30%) are added to main fuel, and its effects on the different availability terms are discussed. Thermomechanical availability, chemical availability, availability of work, availability loss due to convection heat transfer, availability loss due to radiation heat transfer and irreversibility are calculated. Thermal, dilution and chemical effects of reformer gas are computed separately. The results indicated that by reformer gas addition to main fuel, inlet chemical availability and availability of work reduce. Heat loss availability reduces by reformer gas addition. Irreversibility decreases by reformer gas addition and second law efficiency increases slightly. The total utilization efficiency of HCCI engines increases by reformer gas addition. The results showed that the dilution effect of RG on availability terms is more significant than its chemical and thermal effects. The dilution effect reduces both engine-produced work and second law efficiency.

Effects of partitioned fuel distribution on auto-ignition and knocking under spark assisted compression ignition conditions

Spark-assisted compression ignition (SACI) is a potential way to enhance ignition-control robustness and extend the load range of homogeneous charge compression ignition (HCCI) engines. However, the mechanism underlying combustion mode transition without knocking is not completely elucidated. Using direct high-speed photography and simultaneous pressure acquisition, the proposed study examines different auto-ignition and engine knock scenarios under SACI conditions. A small amount of heptane was directly injected into premixed methane-air mixture to modify local mixture reactivity. Additionally, late side injection was adopted to achieve stable SACI combustion whilst suppressing knocking combustion. Results obtained demonstrate that SACI combustion is essentially determined by the stochastic auto-ignition of the unburned end-gas mixture. End-gas mixture auto-ignition becomes stable thus normal SACI under conditions of high mixture reactivity, but a further rise in mixture reactivity causes engine knock prevail. To suppress SACI knocking, partitioned distribution of heptane via side injection has been found to be significantly effective. Results demonstrate that partitioned fuel distribution via late injection can greatly reduce the knock intensity, thereby implying a transition from SACI knocking to normal SACI combustion. Further analysis demonstrates that knock intensity is determined by the peak heat release rate of auto-ignition and there exists distinct boundaries between different combustion modes. The corresponding threshold value characterizing knocking combustion is approximately 202.30 J/CAD under current conditions. Besides, the peak heat release rate is related to the auto-ignition propagation speed and can be adjusted by the partitioned fuel distribution. The proposed study provides great insight into realizing and controlling SACI combustion to improve engine efficiency.

Experimental investigation of ethanol/diesel and ethanol/biodiesel on dual fuel mode HCCI engine for different engine load conditions

Homogeneous charge compression ignition (HCCI) engine has fuel flexibility and potential to reduce NOX and soot emissions. The aim of this work is to investigate the effect of different mass flow rates of ethanol on dual fuel mode HCCI engine working under different load conditions. Ethanol (primary fuel) was supplied through the carburetor at the time of suction and diesel /biodiesel blend (secondary fuel) was injected to start the combustion at the end of the compression. Mass flow rate of ethanol varied by the use of different fuel jets (Jet 40, Jet 60, Jet 70, Jet 80, and Jet 90) in the carburetor. The results showed that with an increase in the mass flow rate of ethanol, ignition delay (ID) increases, combustion duration (CD), in-cylinder pressure and temperature are reduced; which leads to reduce NOX emission and smoke opacity of ethanol/diesel (E + D) and ethanol/biodiesel (E + B20) dual-fuel mode HCCI engine compared to neat diesel engine. However, HC and CO emissions are highly increased for dual fuel mode HCCI engines compared to neat diesel engine. Instead of diesel, biodiesel (B20) triggers the combustion and reduces ID (by 4 deg. crank angle (CA)) for 100% load with the use of jet 90 in the carburetor. Finally, it is concluded that Jet 40 and Jet 60 are suitable for 20% load and 40% load of engine respectively and also Jet 70 is suitable for 60%, 80% and 100% load conditions in order to optimize higher thermal efficiency, low NOX emissions and low smoke opacity.

Degree centrality of combustion reaction networks for analysing and modelling combustion processes

Combustion research still needs more advanced fundamental understanding of combustion chemistry and dynamics from molecule scale to particle. The latter is also needed for soot and nanoparticles formation and combustion system control such as homogeneous charge compression ignition engine, and flame regimes and instability. The complex interactions between hundreds of species linked within thousands of reactions continue to be a challenge to analyse and model. The focus on this paper is to develop a method to facilitate the modelling and analysing of the detailed kinetics chemistry of fuels combustion. Through the use of combustion reaction networks (CRNs) analysis of degree centrality, principal species are identified during a combustion process by exploiting the introduced definition of principal species. A principal, central or the more active species of a combustion process, at a specific time step or cell of area/volume mesh, is the more tied up to other species in the CRN and so have the largest value of degree centrality. The accuracy of the dynamic identification of principal species, locally adapted to the thermochemical conditions at each time step/cell of the simulated combustion process, used by the employed directed relation graphs method of mechanisms reduction, is proved. The simulations were carried out using an adjusted dynamic adaptive chemistry approach of detailed chemistry implementing. It is demonstrated that an ‘active’ species in a combustion system would not necessary be considered as a part of important species set needed for its predictive simulations.

Effects of water addition on the combustion of iso-octane investigated in laminar flames, low-temperature reactors, and an HCCI engine

The effect of H2O injection on the combustion process of iso-octane was investigated with the aim to better understand the suitability of water addition as a potential engine control parameter for homogeneous-charge compression ignition (HCCI) combustion. Several experiments were combined including premixed low-pressure flames, a jet-stirred reactor (JSR) and a plug-flow reactor (PFR), both at atmospheric pressure, and a single-cylinder research engine (SCRE) operated with either iso-octane or RON 98 gasoline. The thermal effect of H2O addition was determined in laminar premixed iso-octane/O2/Ar flames (equivalence ratio Φ=1.4, 40 mbar) with H2O mole fractions of 0 to 0.22, where water addition reduced the temperature measured by laser-induced fluorescence (LIF) by up to 110 K. Speciation data were obtained from these flames as well as in the JSR (Φ=0.65, 933 mbar) and PFR experiments (Φ=0.65, 970 mbar) with and without H2O addition in the low- to intermediate temperature regime from 700–1100 K. The chemical analysis in these flame and reactor experiments was performed using molecular-beam mass spectrometry (MBMS) employing either electron ionization (EI) in the PFR and premixed flame or single-photon ionization (PI) by tunable vacuum-ultraviolet radiation in the JSR. The effects on species mole fractions were small which is supported by predictions from chemical-kinetic simulations. Quantitative speciation data of the exhaust gas of the SCRE were obtained by using Fourier-transform infrared (FTIR) spectroscopy. A very similar species pool was detected in the laboratory-scale experiments and for the engine operation. It is thus assumed that these results could assist in guiding both the improvement of fundamental chemical-kinetic as well as HCCI engine control models.

An experimental examination of the effects of n-hexane and n-heptane fuel blends on combustion, performance and emissions characteristics in a HCCI engine

Homogenous charge compression ignition (HCCI) engines are low-temperature combustion engines with high thermal efficiency. The operation range of HCCI engines, which are not directly controlled on combustion, is limited by the problems of knocking and misfiring. At this point, it is aimed to eliminate the problem of knocking at the high engine loads by the charge mixture composition and various operating parameters. In this study, the experiments were performed in a single cylinder, four stroke, port injection HCCI gasoline engine to investigate the effects of n-hexane, n-heptane and n-hexane/n-heptane blends. Combustion characteristics, engine performance and exhaust emissions were determined at different engine speeds (800–1800 rpm), lambda (λ = 1.5–3.0) and inlet air temperatures (60 and 80 °C). Operating range of the HCCI engine was determined by using both pure n-hexane and pure n-heptane fuels. The experiments showed that n-hexane, which has higher octane number than that of n-heptane, has more resistance to knocking. In-cylinder pressure decreased with the increase of lambda for all test fuels. Thermal efficiency increased about 28% by using n-hexane compared to n-heptane at constant lambda (λ = 2.35). The results showed that H75N25 provided larger operating range compared with other test fuels.