An experimental and numerical analysis of the HCCI auto-ignition process of primary reference fuels,toluene reference fuels and diesel fuel in an engine,varying the engine parameters

For a future HCCI engine to operate under conditions that adhere to environmental restrictions, reducing fuel consumption and maintaining or increasing at the same time the engine efficiency, the choice of the fuel is crucial. For this purpose, this paper presents an auto-ignition investigation concerning the primary reference fuels, toluene reference fuels and diesel fuel, in order to study the effect of linear alkanes, branched alkanes and aromatics on the auto-ignition. The auto-ignition of these fuels has been studied at inlet temperatures from 25 to 120 °C, at equivalence ratios from 0.18 to 0.53 and at compression ratios from 6 to 13.5, in order to extend the range of investigation and to assess the usability of these parameters to control the auto-ignition. It appeared that both iso-octane and toluene delayed the ignition with respect to n-heptane, while toluene has the strongest effect. This means that aromatics have higher inhibiting effects than branched alkanes. In an increasing order, the inlet temperature, equivalence ratio and compression ratio had a promoting effect on the ignition delays. A previously experimentally validated reduced surrogate mechanism, for mixtures of n-heptane, iso-octane and toluene, has been used to explain observations of the auto-ignition process.     

An experimental and numerical analysis of the influence of the inlet temperature,equivalence ratio and compression ratio on the HCCI auto-ignition process of Primary Reference Fuels in an engine

In order to understand better the auto-ignition process in an HCCI engine, the influence of some important parameters on the auto-ignition is investigated. The inlet temperature, the equivalence ratio and the compression ratio were varied and their influence on the pressure, the heat release and the ignition delays were measured. The inlet temperature was changed from 25 to 70 °C and the equivalence ratio from 0.18 to 0.41, while the compression ratio varied from 6 to 13.5. The fuels that were investigated were PRF40 and n-heptane. These three parameters appeared to decrease the ignition delays, with the inlet temperature having the least influence and the compression ratio the most. A previously experimentally validated reduced surrogate mechanism, for mixtures of n-heptane, iso-octane and toluene, has been used to explain observations of the auto-ignition process. The same kinetic mechanism is used to better understand the underlying chemical and physical phenomena that make the influence of a certain parameter change according to the operating conditions. This can be useful for the control of the auto-ignition process in an HCCI engine.     

Selection of a diesel fuel surrogate for the prediction of auto-ignition under HCCI engine conditions

Homogeneous charged compression ignition (HCCI) is a promising combustion concept able to provide very low NO_x and PM diesel engine emissions while keeping good fuel economy. Since HCCI combustion is a kinetically controlled process, the availability of a kinetic reaction mechanism to simulate the oxidation (low and high temperature regimes) of a diesel fuel is necessary for the optimisation, control and design of HCCI engines. Motivated by the lack of information regarding reliable diesel fuel ignition values under real HCCI diesel engine conditions, a diesel fuel surrogate has been proposed in this work by merging n-heptane and toluene kinetic mechanisms. The surrogate composition has been selected by comparing modelled ignition delay angles with experimental ones obtained from a single cylinder DI diesel engine tests. Modelled ignition angle results are in agreement with the experimental ones, both results following the same trends when changing the engine operating conditions (engine load and speed, start of injection and EGR rate). The effect of EGR, which is one of the most promising techniques to control HCCI combustion, depends on the engine load. High EGR rates decrease the n-heptane/toluene mixture reactivity when increasing the engine load but the opposite effect has been observed for lower EGR rates. A chemical kinetic analysis has shown that the influence of toluene on the ignition time is significant only at low initial temperature. More delayed combustion processes have been found when toluene is added, the dehydrogenation of toluene by OH (termination reaction) being the main kinetic path involved during toluene oxidation.     

A kinetic modeling study of self-ignition of low alkylbenzenes at engine-relevant conditions

The self-ignition of low alkylbenzenes at engine-relevant conditions has been studied with kinetic modeling. A previously developed chemical kinetic model for gasoline surrogate fuels [J.C.G. Andrae, R.A. Head, Combust Flame 156 (2009) 842-51] was extended with chemistry for ethylbenzene and m-xylene resulting in an overall model consisting of 150 species and 759 reactions. In model validation, comparisons were made between model predictions and experimental data of ignition delay times measured behind reflected shock waves, laminar burning velocities collected at elevated temperature and pressure and species profiles in a high-pressure single pulse shock tube. Generally good agreement was found and the model is sensitive to changes in mixture strength, pressure and temperature. Shock tube ignition delay modeling results for ethylbenzene and m-xylene also compare well to the ones for toluene. The rate controlling step for the ignition of ethylbenzene in the current mechanism is the reaction with ethylphenyl radical and oxygen. Ignition delay time for m-xylene was found to be very sensitive to reactions involving hydrogen atom abstraction from fuel by hydroxyl and oxygen and to branching reactions where methylbenzyl reacts with oxygen and hydroperoxide. The validated mechanism was used to study fuel chemistry effects when blending ethylbenzene with the paraffinic fuels iso-octane and n-heptane. A sensitivity- and flow path analysis showed that a higher consumption of hydroperoxide by ethylphenyl than expected from the contribution of neat ethylbenzene in a fuel mixture with iso-octane inhibits both iso-octane and ethylbenzene ignition. This can explain the observed increase in ignition delay time and octane number for fuel mixtures compared to neat fuels.     

Detailed approach for apparent heat release analysis in HCCI engines

Homogeneous charge compression ignition (HCCI) combustion is a spontaneous multi-site auto-ignition of a lean premixed fuel-air mixture, which has high heat release rate, short combustion duration and no evidence of flame propagation. In HCCI engines, there is no direct control method for the time of auto-ignition. Auto-ignition timing should be controlled in order to make heat release process take place at the appropriate time in the engine cycle. Heat release analysis is a diagnostic tool which aids engine experimenters. It facilitates the endeavors being conducted in obtaining a control method by investigating heat release rate and also cumulative heat release. This study can be divided into two parts. First, traditional first law heat release model which is widely used in engine combustion analysis was presented and the applicability of this model in HCCI engines was investigated. Second, a new heat release model based on the first law of thermodynamics accompanying with a temperature solver was developed and assessed. The model was applied in four test conditions with different operating conditions and a variety of fuel compositions, including i-octane, n-heptane, pure NG, and at last, a dual fueled case of NG and n-heptane. Results of this work indicate that utilizing the modified first law heat release model together with a solver for temperature correction will guarantee obtaining a well-behaved and accurate apparent heat release trend and magnitude in HCCI combustion engines.     

Investigations on surrogate fuels for high-octane oxygenated gasolines

Gasoline is a complex mixture that possesses a quasi-continuous spectrum of hydrocarbon constituents. Surrogate fuels that decrease the chemical and/or physical complexity of gasoline are used to enhance the understanding of fundamental processes involved in internal combustion engines (ICEs). Computational tools are largely used in ICE development and in performance optimization; however, it is not possible to model full gasoline in kinetic studies because the interactions among the chemical constituents are not fully understood and the kinetics of all gasoline components are not known. Modeling full gasoline with computer simulations is also cost prohibitive. Thus, surrogate mixtures are studied to produce improved models that represent fuel combustion in practical devices such as homogeneous charge compression ignition (HCCI) and spark ignition (SI) engines. Simplified mixtures that represent gasoline performance in commercial engines can be used in investigations on the behavior of fuel components, as well as in fuel development studies. In this study, experimental design was used to investigate surrogate fuels. To this end, SI engine dynamometer tests were conducted, and the performance of a high-octane, oxygenated gasoline was reproduced. This study revealed that mixtures of iso-octane, toluene, n-heptane and ethanol could be used as surrogate fuels for oxygenated gasolines. These mixtures can be used to investigate the effect of individual components on fuel properties and commercial engines performance.     

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.     

Development of a detailed kinetic model for gasoline surrogate fuels

A detailed chemical kinetic model to describe the autoignition of gasoline surrogate fuels is presented consisting of the fuels iso-octane, n-heptane, toluene, diisobutylene and ethanol. Model predictions have been compared with shock tube ignition delay time data for surrogates of gasoline over practical ranges of temperature and pressure, and the model has been found to be sensitive to both changes in temperature and pressure. Moreover, the model can qualitatively predict the observed synergistic and antagonistic non-linear blending behaviour in motor octane number (MON) for different combinations of primary reference fuels (PRFs) and non-PRFs by correlating calculated autoignition delay times from peak pressures and temperatures in the MON test to experimental MON values. The reasons for the blending behaviour are interpreted in terms autoignition chemistry.     

Structure of an <Emphasis Type="Italic">n</Emphasis>-heptane/toluene flame: Molecular beam mass spectrometry and computer simulation investigations

Molecular beam mass spectrometry was used to measure mole fraction profiles of the reactants, major reaction products and intermediates, including precursors of polycyclic aromatic hydrocarbons, in a premixed fuel-rich (equivalence ratio of 1.75) n-heptane/toluene/O₂/Ar flame stabilized on a flat burner at atmospheric pressure. The ratio of the liquid volumes in the n-heptane/toluene mixture was 7: 3. The chemical structure of the flame was modeled using a detailed mechanism of chemical reactions tested against experimental data of other authors on n-heptane/toluene flames and comprising the reactions of formation of polycyclic aromatic hydrocarbons. The mechanism was extended with cross-reactions involving derivatives of n-heptane and toluene. Overall, the new experimental data are in satisfactory agreement with the numerical simulation results; however, there are differences between the measured and calculated mole fraction profiles of some species. Analysis shows that in the n-heptane/toluene flame, the main reactions leading to the formation of low-aromatic compounds (benzene and phenyl) are reactions typical of the pure toluene flame.     

A soot formation embedded reduced reaction mechanism for diesel surrogate fuel

A reduced diesel surrogate fuel chemical reaction mechanism of n-heptane/toluene was developed, the reduced mechanism (referred as the “THU mechanism”) includes 60 species and 145 reactions, and it contains soot formation reactions. The THU mechanism was developed from the existing n-heptane/toluene mechanism (70 species and 313 reactions) of Chalmers University of Technology (referred as the “CTH mechanism”). SENKIN and XSENKPLOT were used to analyze the important reactions and species during n-heptane, toluene oxidation and soot formation processes to formulate the reduced mechanism. Ignition delays of n-heptane and toluene predicted by the THU mechanism match well with the CTH mechanism and shock-tube test data under different conditions. The THU and CTH mechanisms also show similar soot concentration prediction. The global reaction of diesel fuel decomposed into n-heptane and toluene with mole fraction 7:3 was built to accelerate the decomposition and advance ignition timing. Kinetic constants of soot oxidation reactions were adjusted to reduce the soot oxidation rate. The THU mechanism was coupled with the KIVA-3V Release 2 code to model diesel combustion processes in the constant-volume combustion vessel and optical diesel engine of Sandia. The predicted ignition delay, in-cylinder pressure and heat release rate match the experimental results well. The predicted spatial and temporal soot concentration distributions have similar trend with the experiments.     

Development and calibration of a reduced chemical kinetic model of n-heptane for HCCI engine combustion

A new reduced chemical kinetic model for the Homogeneous Charge Compression Ignition (HCCI) combustion of n-heptane in an engine has been developed. The new model is based on two previous reduced kinetic models for alkane oxidation, from which some reactions have been eliminated and with enhanced treatment of the oxidization of CO and CH₃O. The kinetic parameters of the key reactions in the new model were adjusted by using a genetic algorithm optimization methodology to improve ignition timings predictions over the range of equivalence ratios from 0.2 to 1.2, temperature from 300 to 3000 K. The final model contains 40 species and 62 reactions and was validated under HCCI engine conditions. The results showed the well-known two-stage ignition characteristics of n-heptane, which involve low and high temperature regimes followed by a branched chain explosion. The optimized reduced model generally agrees well with those of the detailed chemical kinetic model (544 species and 2446 reactions); the computational time of using the former is less 1/1000 that of the latter.     

Modelling of the auto-ignition angle in diesel HCCI engines through D-optimal design

The HCCI (Homogeneous Charge Compression Ignition) process is one of the most promising combustion processes developed to reduce pollutant emissions from automotive vehicles. However, there are practical difficulties concerning the control of the onset of ignition, and thus the availability of simple models which allows to simulate the auto-ignition phenomena may be very interesting for the development of new HCCI engines. In this work, the onset of ignition in a HCCI engine and the auto-ignition angle were modelled (OAM and AAM respectively) through experimental plans based on the D-optimal criterion. The experimental values were obtained by using the chemical kinetic code CHEMKIN together with an appropriate diesel fuel surrogate. The models developed have an acceptable goodness-of-fit and predictive capability (differences lower than 3 CAD were obtained between modelled and real auto-ignition angles for all the cases). The relative fuel/oxidant ratio and the intake temperature were the most significant engine parameters affecting the onset of auto-ignition, while the intake temperature and pressure appear as the most important parameters determining the auto-ignition angle. These models could be used by the Engine Control Unit (ECU) as an on-board diagnostic technique to control the HCCI combustion in real time. The optimal engine parameters for five specific operating conditions (chosen to cover the most common light duty diesel vehicles operating modes) were also calculated by using the above mentioned models (OAM and AAM) and by solving two non-linear optimization problems. To achieve optimization, a desirability function was defined. The optimization methodology proposed can be used to obtain the optimum engine parameters, which are used by the ECU, matching different vehicle requirements.     

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.     

Reduced chemical reaction mechanisms: experimental and HCCI modelling investigations of autoignition processes of iso-octane in internal combustion engines

A semi-reduced (70 species, 210 reactions) and a skeletal (27 species, 29 reactions) chemical reaction mechanism for iso-octane are constructed from a semi-detailed iso-octane mechanism (84 species, 412 reactions) of the Chalmers University of Technology in Sweden. The construction of the reduced mechanisms is performed by using reduction methods such as the quasi-steady-state assumption and the partial equilibrium assumption. The obtained reduced iso-octane mechanisms show, at the mentioned conditions, a perfect coherence with another more detailed iso-octane mechanism of ENSIC-CNRS (2411 reactions and 473 species) and the semi-detailed iso-octane mechanism of Chalmers. The validity of this mechanism with regard to the ignition delay is determined for several engine parameters adhering to HCCI conditions: inlet temperature (303–363 K), equivalence ratio (0.2–0.7) and compression ratio (10–16). The ignition delay is found to be decreased by an increase in the inlet temperature, a decrease in the equivalence ratio and an increase in the compression ratio. In order to validate the effects of the inlet temperature, compression ratio on the auto-ignition delay, experiments are performed on a CFR engine. A good agreement is obtained between experimental results and calculations.     

A chemical kinetics model of iso-octane oxidation for HCCI engines

The necessity of developing a practical iso-octane mechanism for homogeneous charge compression ignition (HCCI) engines is presented after various different experiments and currently available mechanisms for iso-octane oxidation being reviewed and the performance of these mechanisms applied to experiments relevant to HCCI engines being analyzed. A skeletal mechanism including 38 species and 69 reactions is developed, which could predict satisfactorily ignition timing, burn rate and the emissions of HC, CO and NO_x for HCCI multi-dimensional modeling. Comparisons with various experiment data including shock tube, rapid compression machine, jet stirred reactor and HCCI engine indicate good performance of this mechanism over wide ranges of temperature, pressure and equivalence ratio, especially at high pressure and lean equivalence ratio conditions. By applying the skeletal mechanism to a single-zone model of HCCI engine, we found out that the results were substantially identical with those from the detailed mechanism developed by Curran et al. but the computing time was reduced greatly.     

Liquid–liquid equilibrium for the systems hydrocarbon–thiophene or pyridine–1-hexyl-3,5-dimethylpyridinium bis(trifluoromethylsulfonyl)imide

Liquid–liquid equilibrium for eight ternary systems involving one hydrocarbon (n-hexane, n-heptane, i-octane or toluene), thiophene or pyridine and an ionic liquid (1-hexyl-3,5-dimethylpyridinium bis(trifluoromethylsulfonyl)imide) was experimentally determined at atmospheric pressure and 25°C. Equilibrium data are presented with binodal curves as well as with tie lines. The suitability of ionic liquid (IL) for extractive desulfurization and denitrification was evaluated in terms of solute distribution ratio and selectivity. Extraction experiments with three-component and seven-component (n-hexane, n-heptane, i-octane, toluene, thiophene, pyridine and IL) systems have been performed. The equilibrium data in three-component systems were well described with Non-Random Two-Liquid (NRTL) and Universal Quasi-Chemical (UNIQUAC) models.     

A fundamental study on the control of the HCCI combustion and emissions by fuel design concept combined with controllable EGR. Part 2. Effect of operating conditions and EGR on HCCI combustion

In Part 1, the effects of octane number of primary reference fuels and equivalence ration on combustion characteristics of a single-cylinder HCCI engine were studied. In this part, the influence of exhaust gas recirculation (EGR) rate, intake charge temperature, coolant temperature, and engine speed on the HCCI combustion characteristics and its emissions were evaluated. The experimental results indicate that the ignition timing of the first-stage combustion and second-stage combustion retard, and the combustion duration prolongs with the introduction of cooled EGR. At the same time, the HCCI combustion using high cetane number fuels can tolerate with a higher EGR rate, but only 45% EGR rate for RON75 at 1800 rpm. Furthermore, there is a moderate effect of EGR rate on CO and UHC emissions for HCCI combustion engines fueled with n-heptane and RON25, but a distinct effect on emissions for higher octane number fuels. Moreover, the combustion phase advances, and the combustion duration shorten with the increase of intake charge temperature and the coolant out temperature, and the decrease of the engine speed. At last, it can be found that the intake charge temperature gives the most sensitive influence on the HCCI combustion characteristics.     

Production of synthesis gas by autothermal reforming of iso-octane and toluene over metal modified Ni-based catalyst

The reforming process of gasoline is an attractive technique for fuel processor or hydrogen station applications. We investigated catalytic autothermal reforming (ATR) of iso-octane and toluene over transition metal supported catalysts. The catalysts were prepared by an incipient wetness impregnation method and characterized by N₂ physisorption, XRD, and TEM techniques before and after the reaction. Many of the tested catalysts displayed reasonably good activity towards the reforming reactions of iso-octane. Especially, Ni/Fe/MgO/Al₂O₃ catalyst showed more activity than the other catalysts tested in this study including commercial HT catalyst. Ni/Fe/MgO/Al₂O₃ catalyst showed good stability for 700 h in the ATR of iso-octane. No major change was observed in catalytic activity in ATR of iso-octane or in the structure of catalyst. Since iso-octane, toluene are surrogates of gasoline, Ni/Fe/MgO/Al₂O₃ catalyst can be considered as ATR catalyst for gasoline fuel processor and hydrogen station systems.     

A parametric study on natural gas fueled HCCI combustion engine using a multi-zone combustion model

Homogenous Charge Combustion Ignition (HCCI) is a good method for higher efficiency and to reduce NOx and particulate matter simultaneously in comparison to conventional internal combustion engines. In HCCI engines, there is no direct control method for auto ignition time. A common way to indirectly control the ignition timing in HCCI combustion engines is varying engine’s parameters which can affect the combustion. In this work, a parametric study on natural gas HCCI combustion is conducted in order to identify the effect of inlet temperature and pressure, compression ratio, equivalence ratio and engine speed on combustion and engine performance parameters. In this paper, two kinds of parameters will be discussed. First, in-cylinder pressure diagrams and variation of start of combustion which are combustion parameters will be presented and then the second category, indicated mean effective pressure and thermal efficiency which are performance parameters will be studied. A six zone model coupled with detailed chemical kinetics code is used to simulate HCCI combustion. Both heat and mass transfer was considered in the modeling procedure. Results revealed that among the considered parameters, the equivalence ratio and inlet pressure are the most valuable parameters which can improve the combustion and performance characteristics of the HCCI engine.     

An experimental study for the effects of boost pressure on the performance and exhaust emissions of a DI-HCCI gasoline engine

As an alternative combustion mode, the HCCI combustion has some benefits compared to conventional SI and CI engines, such as low NO_x emission and high thermal efficiency. However, this combustion mode can produce higher UHC and CO emissions than those of conventional engines. In the naturally aspirated HCCI engines, the low engine output power limits its use in the current engine technologies. Intake air pressure boosting is a common way to improve the engine output power which is widely used in high performance SI and CI engine applications. Therefore, in this study, the effect of inlet air pressure on the performance and exhaust emissions of a DI-HCCI gasoline engine has been investigated after converting a heavy-duty diesel engine to a HCCI direct-injection gasoline engine. The experiments were performed at three different inlet air pressures while operating the engine at the same equivalence ratio and intake air temperature as in normally aspirated HCCI engine condition at different engine speeds. The SOI timing was set dependently to achieve the maximum engine torque at each test condition. The effects of inlet air pressure both on the emissions such as CO, UHC and NO_x and on the performance parameters such as BSFC, torque, thermal and combustion efficiencies have been discussed. The relationships between the emissions are also provided.