Homogeneous Charge Compression Ignition Conditions Research Articles

Effect of nitric oxide on the performance and emissions of a hydrogen-fueled HCCI engine

Homogeneous charge compression ignition (HCCI) engines have historically generated interest due to its potential to achieve high efficiency and low emissions. This paper presents an experimental investigation of hydrogen port fuel injection in a modified Cooperative Fuel Research (CFR) engine operated under HCCI conditions. The effect of adding nitric oxide (NO) to the intake air on engine performance, emissions, and combustion characteristics was investigated at various inlet temperatures. The experiments considered the operating regime and limitations for hydrogen HCCI engines with and without NO addition. The engine can operate over a wide range of air–fuel ratios i.e., λ = 2.7 to 4.0 at 150 OC inlet air temperature without NO addition.NO also acts as a knock promoter in reciprocating engines, limiting the engine's output. The results indicate that adding NO significantly promotes the auto-ignition of the hydrogen-air mixture, thereby extending the engine's operable range, and an ultra-lean λ of 5.0 can be reached. Our findings indicate that NO has the most notable effect on engine performance at an inlet air temperature of 90 OC. Furthermore, the results show that the addition of NO increases the amount of NOx emissions in the exhaust gas; about 50 ppm of NO was converted into NO2 during the combustion process. Adding NO to the hydrogen-air mixture has the potential to extend the engine's range of operation in HCCI providing a promising avenue for further research into HCCI combustion engines and the potential to improve efficiency.

Understanding the effects of doping a regular E10 gasoline with EHN in an HCCI engine: Experimental and numerical study

In this study, the effects of doping a regular E10 gasoline with 2-ethlyhexyl nitrate (EHN) are investigated under homogeneous charge compression ignition conditions. Experiments are performed in a 1-liter single-cylinder engine fueled with both straight and EHN-doped E10 gasoline. Numerical studies are performed with an internal combustion engine single-zone reactor utilizing detailed chemical kinetic mechanism with EHN and NOx chemistry and a surrogate fuel. The kinetic model reproduces the experimental data well for the straight fuel at high initial temperature (TBDC) conditions, whereas the low temperature heat release (LTHR) is under-predicted. Adding EHN reduces the required TBDC, while EHN thermal decomposition rate has to be significantly reduced to accurately reproduce the experimental result, by preventing over-estimation of EHN effect and LTHR. EHN decomposition generates NO2 and 3-heptyl radicals. Using the well-matched mechanism, the numerical results indicate that among the product of EHN decomposition, NO2 decreases the autoignition reactivity whereas the production of 3-heptyl radical is the main source for enhancing the low-to-intermediate chemistry by which OH production is accelerated. The production of 3-heptyl radical is highly sensitive to the EHN decomposition reactions. Despite the reactivity enhancement, increase in NOx emission is observed when the fuel-doping increases.

Validation of Kinetic Mechanisms against Various Ignition Delay Data and the Development of Ignition Delay Correlations for Ethanol, Natural Gas, and Primary Reference Fuel Blends under Homogeneous Charge Compression Ignition Conditions

Validation of Kinetic Mechanisms against Various Ignition Delay Data and the Development of Ignition Delay Correlations for Ethanol, Natural Gas, and Primary Reference Fuel Blends under Homogeneous Charge Compression Ignition Conditions

Exergy loss characteristics of DME/air and ethanol/air mixtures with temperature and concentration fluctuations under HCCI/SCCI conditions: A DNS study

The exergy loss characteristics of combustion processes under homogeneous-charge compression ignition (HCCI) and stratified-charge compression ignition (SCCI) conditions are numerically investigated by analyzing two-dimensional (2-D) direct numerical simulation (DNS) data. Two fuels, dimethyl ether and ethanol, together with the initial conditions of different mean temperatures, and levels of temperature and concentration fluctuations relevant to HCCI/SCCI conditions were investigated. It is found that the prevalent deflagration mode significantly decreases the maximum exergy loss rates and spreads out the exergy loss rate for all the cases regardless of fuel types, temperature regimes, and temperature and/or concentration fluctuations. The primary irreversible sources of exergy loss are also identified. The chemical reaction is found to be the primary contributor to the total exergy loss, followed by heat conduction and mass diffusion, regardless of the fluctuation levels. It is also found that the relative change of exergy loss due to chemical reactions, ELchemrel, correlates strongly with the heat release fraction by deflagration. The maximum ELchemrel is found to be less than 10%. Chemical pathway analysis reveals that the exergy loss induced by low-temperature reactions, represented by the decomposition of hydroperoxy–alkylperoxy and the H-abstraction reactions of the fuel molecule, is much lower under the SCCI conditions than that under the HCCI conditions. Generally, the dominant reactions contributing to the exergy loss in the high-temperature regime are nearly identical for the HCCI and SCCI combustion. Key reactions, including the H2O2 loop reactions, the reactions of the H2–O2 mechanism, and the conversion reaction of CO to CO2, CO+OH=CO2+H, are found to contribute more than 50% of the total exergy loss. Due to locally higher reactivities by temperature and concentration fluctuations inducing deflagration dominance, these reactions occur at a relatively higher temperature (1600 K–1900 K) compared with the homogeneous zero-dimensional cases (∼1400 K), resulting in a net reduction in exergy loss.

Detection of low Temperature heat release (LTHR) in the standard Cooperative Fuel Research (CFR) engine in both SI and HCCI combustion modes

To this date, extensive research has been conducted to understand the low-temperature auto-ignition chemistry of gasoline. The detection of low-temperature chemical reactions under Spark Ignition (SI) combustion cannot be detected, as they are hidden by the flame propagation. Alternatively, Homogeneous Charge Compression Ignition (HCCI) combustion has a two-stage combustion involving low and high-temperature heat release (LTHR and HTHR respectively). Both Knocking SI and HCCI combustion involve auto-ignition and are governed by fuel characteristics and the pressure-temperature (P-T) history. Therefore, HCCI combustion might provide an alternative to understand the knocking behavior and LTHR in modern SI engines. A standard Cooperative Fuel Research (CFR) engine was operated at lean HCCI conditions (lambda 3), as well as SI conditions at stoichiometry. For SI combustion, the CFR engine was operated with RON-like conditions, but at late spark timing to induce LTHR prior to flame propagation. Three RON 90 binary fuel blends were investigated, being composed of n-heptane with isooctane, toluene, or ethanol. This work demonstrated that the CFR engine under stoichiometric SI with late spark timing and HCCI combustion mode can help to detect LTHR which is not possible in the standard RON test. The intake pressure and temperature sweeps showed similar effects on LTHR for both combustion modes. The linking of auto-ignition behavior of SI and HCCI was dependent primarily on intake valve closing (IVC) conditions. The high exhaust temperature in SI lead to high IVC temperatures. In order to match the IVC temperatures and to overlap the P-T trajectories, the intake temperature for HCCI was increased.

An automated target species selection method for dynamic adaptive chemistry simulations

The relative importance index (RII) method for determining appropriate target species for dynamic adaptive chemistry (DAC) simulations using the directed relation graph with error propagation (DRGEP) method is developed. The adequacy and effectiveness of this RII method is validated for two fuels: n-heptane and isopentanol, representatives of a ground transportation fuel component and bio-alcohol, respectively.The conventional method of DRGEP target species selection involves picking an unchanging (static) set of target species based on the combustion processes of interest; however, these static target species may not remain important throughout the entire combustion simulation, adversely affecting the accuracy of the method. In particular, this behavior may significantly reduce the accuracy of the DRGEP-based DAC approach in complex multidimensional simulations where the encountered combustion conditions cannot be known a priori with high certainty. Moreover, testing multiple sets of static target species to ensure the accuracy of the method is generally computationally prohibitive. Instead, the RII method determines appropriate DRGEP target species solely from the local thermo-chemical state of the simulation, ensuring that accuracy will be maintained. Further, the RII method reduces the expertise required of users to select DRGEP target species sets appropriate to the combustion phenomena under consideration.Constant volume autoignition simulations run over a wide range of initial conditions using detailed reaction mechanisms for n-heptane and isopentanol show that the RII method is able to maintain accuracy even when traditional static target species sets fail, and are even more accurate than expert-selected target species sets. Additionally, the accuracy and efficiency of the RII method are compared to those of static target species sets in single-cell engine simulations under homogeneous charge compression ignition conditions. For simulations using more stringent DRGEP thresholds, the RII method performs similarly to that of the static target species sets. With a larger DRGEP threshold, the RII method is significantly more accurate than the static target species sets without imposing significant computational overhead.Furthermore, the applicability of the RII method to a DRG-based DAC scheme is discussed.

Validation of a Newly Developed n-Heptane Reduced Chemistry and Its Application to Simulations of Ignition Quality Tester, Diesel, and HCCI Combustion

A skeletal mechanism (144 species) and a corresponding reduced mechanism (62 species) were developed on the basis of the most recent detailed n-heptane mechanism by Lawrence Livermore National Laboratories (LLNL, version 3.1, 2012) (Mehl et al., 2011, “Kinetic Modeling of Gasoline Surrogate Components and Mixtures Under Engine Conditions,” Proc. Combust. Inst., 33, pp. 193–200), in order to assess the mechanism's performance under various practical combustion conditions. These simplified mechanisms were constructed and validated under shock tube conditions. Three-dimensional computational fluid dynamics (3D CFD) simulations with both simplified mechanisms were conducted for the following modeling applications: ignition quality tester (IQT), diesel engine, and homogeneous charge compression ignition (HCCI) engine. In comparison with experimental data, the simulation results were found satisfactory under the diesel condition but inaccurate for both the IQT and HCCI conditions. For HCCI, the intake temperature used in the simulation had to be increased 30 K in order to be consistent with the engine data provided by Guo et al. (2010, “An Experimental and Modeling Study of HCCI Combustion Using n-Heptane,” ASME J. Eng. Gas Turbines Power, 132(2), 022801). Exploration of possible causes is conducted leading to the conclusion that refinement in the mechanism is needed for accurate prediction of combustion under IQT and HCCI conditions.

Gray-Box Modeling for Performance Control of an HCCI Engine With Blended Fuels

High fidelity models that balance accuracy and computation load are essential for real-time model-based control of homogeneous charge compression ignition (HCCI) engines. Gray-box modeling offers an effective technique to obtain desirable HCCI control models. In this paper, a physical HCCI engine model is combined with two feed-forward artificial neural network models to form a serial architecture gray-box model. The resulting model can predict three major HCCI engine control outputs, including combustion phasing, indicated mean effective pressure (IMEP), and exhaust gas temperature (Texh). The gray-box model is trained and validated with the steady-state and transient experimental data for a large range of HCCI operating conditions. The results indicate that the gray-box model significantly improves the predictions from the physical model. For 234 HCCI conditions tested, the gray-box model predicts combustion phasing, IMEP, and Texh with an average error of less than 1 crank angle degree, 0.2 bar, and 6 °C, respectively. The gray-box model is computationally efficient and it can be used for real-time control application of HCCI engines.

Investigation on effect of equivalence ratio and engine speed on homogeneous charge compression ignition combustion using chemistry based CFD code

Combustion in a large-bore natural gas fuelled diesel engine operating under Homogeneous Charge Compression Ignition mode at various operating conditions is investigated in the present paper. Computational Fluid Dynamics model with integrated chemistry solver is utilized and methane is used as surrogate of natural gas fuel. Detailed chemical kinetics mechanism is used for simulation of methane combustion. The model results are validated using experimental data by Aceves, et al. (2000), conducted on the single cylinder Volvo TD100 engine operating at Homogeneous Charge Compression Ignition conditions. After verification of model predictions using in-cylinder pressure histories, the effect of varying equivalence ratio and engine speed on combustion parameters of the engine is studied. Results indicate that increasing engine speed provides shorter time for combustion at the same equivalence ratio such that at higher engine speeds, with constant equivalence ratio, combustion misfires. At lower engine speed, ignition delay is shortened and combustion advances. It was observed that increasing the equivalence ratio retards the combustion due to compressive heating effect in one of the test cases at lower initial pressure. Peak pressure magnitude is increased at higher equivalence ratios due to higher energy input.

Global reaction mechanism for the auto-ignition of full boiling range gasoline and kerosene fuels

Compact reaction schemes capable of predicting auto-ignition are a prerequisite for the development of strategies to control and optimise homogeneous charge compression ignition (HCCI) engines. In particular for full boiling range fuels exhibiting two stage ignition a tremendous demand exists in the engine development community. The present paper therefore meticulously assesses a previous 7-step reaction scheme developed to predict auto-ignition for four hydrocarbon blends and proposes an important extension of the model constant optimisation procedure, allowing for the model to capture not only ignition delays, but also the evolutions of representative intermediates and heat release rates for a variety of full boiling range fuels. Additionally, an extensive validation of the later evolutions by means of various detailed n-heptane reaction mechanisms from literature has been presented; both for perfectly homogeneous, as well as non-premixed/stratified HCCI conditions. Finally, the models potential to simulate the auto-ignition of various full boiling range fuels is demonstrated by means of experimental shock tube data for six strongly differing fuels, containing e.g. up to 46.7% cyclo-alkanes, 20% napthalenes or complex branched aromatics such as methyl- or ethyl-napthalene. The good predictive capability observed for each of the validation cases as well as the successful parameterisation for each of the six fuels, indicate that the model could, in principle, be applied to any hydrocarbon fuel, providing suitable adjustments to the model parameters are carried out. Combined with the optimisation strategy presented, the model therefore constitutes a major step towards the inclusion of real fuel kinetics into full scale HCCI engine simulations.

Analysis and modeling of exhaust gas temperature in an ethanol fuelled HCCI engine

Low exhaust temperature in homogeneous charge compression ignition (HCCI) significantly limits efficiency of an exhaust aftertreatment system to mitigate high HC and CO emissions in HCCI engines. This article aims to understand the effect of varying input parameters on HCCI exhaust gas temperature (Texh) for an ethanol fuelled engine. A single cylinder engine is used to collect experimental data at 100 different HCCI conditions. The results indicate that variation in combustion parameters such as start of combustion (SOC), burn duration (BD) and maximum in-cylinder pressure (Pmax) are not effectively correlated with variations of Texh, but the indicated mean effective pressure (IMEP) and constant-volume adiabatic flame temperature (Tad) are strongly related to Texh. These experimental findings were then used to design an artificial neural network (ANN) model to predict Texh. The model was validated with the experimental data, indicating an average error less than 4.5°C between predicted and measured Texh.

Characterization of exhaust particulates from diesel fueled homogenous charge compression ignition combustion engine

Engine exhaust particulates undergo different processes in the ambient environment such as agglomeration, coagulation, surface condensation, adsorption, and oxidation before evolving as mature particles. Conventional CI engines emit a significant mass/number of particulates due to heterogeneous combustion. However this problem can be resolved by using an advanced combustion technology named as Homogeneous Charge Compression Ignition (HCCI), which has potential to substantially reduce particulates and NOx simultaneously and deliver efficiencies comparable to conventional CI combustion. In the present study involving homogeneous mixture of diesel and air, an electrically heated diesel vaporizer was developed. Experiments were performed at different relative air-fuel ratios and EGR levels. Enrichment of the mixture increases the peak in-cylinder temperature, which was effectively controlled by EGR under leaner HCCI conditions. A partial flow dilution tunnel was used to collect particulate samples for trace metal content and Benzene Soluble Organic Fraction (BSOF), which is considered to be a marker of toxicity. This analysis showed that the trace metals detected were comparatively lower in HCCI particles. Trace metal concentration increased with application of EGR in the HCCI engine. BSOF content of the HCCI particulates increased when the mixture becomes leaner as well as with increasing EGR. Physical characterization of particulates was also carried out using engine exhaust particle sizer (EEPS), which measures the particle size-number distribution for the nano-particles in the exhaust. The particles collected on the filter paper were also analyzed for morphology using scanning electron microscopy (SEM).

A thermodynamics-based homogeneous charge compression ignition engine model for adaptive nonlinear controller development

Low-temperature combustion modes, such as homogeneous charge compression ignition, represent a promising means to increase the efficiency and to reduce significantly the emissions of internal combustion engines. Implementation and control are difficult, however, owing to the dependence of the combustion event on the chemical kinetics rather than an external trigger. This work describes a nonlinear control-oriented model developed for a single-cylinder homogeneous charge compression ignition engine, which is physically based on a five-state thermodynamic cycle. This model is aimed at capturing the behavior of an engine which utilizes fully vaporized gasoline-type fuels, exhaust gas recirculation, and intake air heating in order to achieve homogeneous charge compression ignition operation. The onset of combustion, which is vital for control, is modeled using an Arrhenius reaction rate expression which relates the combustion timing to both the charge dilution and the temperature. Despite the fact that homogeneous charge compression ignition combustion is indeed fast, it is not perfectly instantaneous and therefore requires some finite amount of time to occur. To account for this phenomenon within the model, a Δθ term is added which shifts the point of instantaneous combustion from the start of combustion to a point of very-high-energy release based on experimental heat release data. The model is validated against experimental data from a single-cylinder compression ignition engine operating under homogeneous charge compression ignition conditions at two different fueling rates. Parameters relevant to control such as the combustion timing, peak in-cylinder pressure, and pressure rise rates from the simulation agree very well with the experiment in both operating conditions. The extension of the model to other fuels is also investigated via the octane index of several different gasoline-type fuels. Since this nonlinear model is developed from a controls perspective, both the output and the state update equations are formulated such that they are functions of only the control inputs and the state variables, therefore making them directly applicable to state-space methods for control. The result is a discrete-time nonlinear control model which provides a platform for developing and validating various nonlinear control strategies.

A DNS study of ignition characteristics of a lean iso-octane/air mixture under HCCI and SACI conditions

The effect of thermal stratification, spark-ignition, and turbulence on the ignition of a lean homogeneous iso-octane/air mixture at constant volume and high pressure is investigated by direct numerical simulations (DNS) with a new 99-species reduced kinetic mechanism developed for very lean mixtures from a detailed mechanism. Two-dimensional DNS are performed in a fixed volume with two-dimensional isotropic velocity spectrums, temperature fluctuations, and an ignition source superimposed on the initial scalar fields. The influence of variations in the initial temperature field imposed by changing the variance of temperature, the ignition-timing by changing the time at which the ignition source is superimposed, and the turbulence intensity and length scale on ignition of a lean iso-octane/air mixture is elucidated. The mean heat release rate increases more slowly and ignition delay decreases with increasing thermal stratification under homogeneous charge compression-ignition (HCCI) conditions since the present mean temperature lies far outside of the negative temperature coefficient (NTC) regime. The spark-ignition induces relatively short ignition delay under spark-assisted compression ignition (SACI) conditions while mildly spreading out the mean heat release rate. For SACI combustion, high turbulence intensity decreases the ignition delay more by increasing the turbulent flame area. Displacement speed and Damköhler number analyses reveal that the high degree of thermal stratification induces deflagration at the reaction fronts, and hence, the mean heat release rate is smoother subsequent to thermal runaway occurring at the highest temperature regions in the domain. For SACI combustion, the heat release rate occurs solely by deflagration prior to the occurrence of the maximum heat release rate and subsequently by mixed modes of deflagration and spontaneous ignition. These results suggest that thermal stratification provides smooth operation of HCCI engines and moreover, spark-ignition can precisely control the ignition timing for SACI combustion.

The autoignition of practical fuels at HCCI conditions: High-pressure shock tube experiments and phenomenological modeling

Delay times for first- and second-stage ignition of n-heptane and two practical kerosene-like fuels have been measured in a heated high-pressure shock tube at conditions similar to those found in homogeneous charge compression ignition (HCCI) engines. Initial reflected shock conditions covered temperatures from 700 to 1100K, pressures from 20 to 65bar, equivalence ratios of 0.5, 0.67, and 1, and exhaust gas recirculation rates (EGR) of 0%, 30%, and 50%. EGR is simulated by introducing additional N2 in the test gas mixture. Because detailed chemical kinetics models are not available for practical fuels so far, we propose a global ignition time correlation that is fitted to the measured data. The empirical model describes both first- and second-stage ignition delay as a function of temperature, pressure, equivalence ratio and EGR. It is based on a three-stage Arrhenius approach, which has been extended to capture the first-stage ignition times and the influence of EGR. For each of the fuels, even for the fuels that contain cyclo-alkanes and aromatics, good agreement between model and experiment is observed, especially at lean and high-EGR conditions relevant for HCCI. Based on this extensive set of ignition data, interesting correlations between fuel composition and ignition behavior could be identified, which may prove useful in matching the fuel to the engine application.

A Skeletal Chemical Kinetic Model of PRF Oxidation for HCCI Engines

A skeletal chemical kinetic mechanism, including 42 species and 71 reactions for the oxidation of primary reference fuel (PRF), was developed and applied to model homogeneous charge compression ignition (HCCI) combustion after various experiments and available mechanisms for PRF oxidation being reviewed and the performance of mechanisms applied to experiments relevant to HCCI engines being analyzed. The ignition delay predicted by the skeletal mechanism showed good agreement with experiments for different fuels over the temperature range 667-1350K. Validation was also performed with experiments from HCCI engines, and good agreement was obtained for the primary reference fuels. The results show that the present PRF mechanism gives reliable performance for HCCI combustion predictions. Sensitivity analysis indicates that for PRF70 and PRF91.8 under HCCI conditions, H-atom abstraction from iso-octane molecule reaction and OH addition to n-heptane molecule have large influence, and CH2O, CH3 are very important intermediate species.

Experimental Characterization of Ethyl Acetate, Ethyl Propionate, and Ethyl Butanoate in a Homogeneous Charge Compression Ignition Engine

The homogeneous charge compression ignition (HCCI) engine can be run on a large range of fuels if the appropriate operating conditions are chosen. This can improve the efficiency of biofuel production from low-value biomass by suppressing the need for the transformation process to obtain products that are compatible with spark ignition or compression ignition engines. A simple biochemical process that includes acidogenic fermentation and produces a mixture of various esters can take advantage of this flexibility. However, the behavior of this mixture under HCCI conditions needs to be characterized. It can also have a great impact on the HCCI operating limits and its successful implementation. Using an HCCI engine, we investigated how the operating limits are modified by the combustion characteristics of three of these esters: ethyl acetate, ethyl propionate, and ethyl butanoate. This paper reports the experimental results for each of these products and for ethanol taken as the reference fuel. It also analyzes their effects on the ignition timing and the combustion rate. For the selected operating conditions, stable HCCI operations on a large range of equivalence ratios were obtained for every fuel The difference in specific heats of the air/fuel mixtures and in the ignition kinetics both contributed to the ignition characteristics. Ethanol ignites earlier, which leads to a low upper limit, whereas the late ignition of ethyl acetate shifts the operating zone upward due to smoothed high loads but unstable low loads. As a consequence, these low-grade products can be used in an HCCI engine. Fuel blends of these products may take advantage of the different combustion characteristics to extend the HCCI zone. Still, the range of this extension is difficult to estimate and the research of the optimal fuel blend composition will, therefore, remain the focus of future work.

Multi-timescale modeling of ignition and flame regimes of n-heptane-air mixtures near spark assisted homogeneous charge compression ignition conditions

The flame regimes of ignition and flame propagation as well as transitions between different flame regimes of n-heptane-air mixtures in a one-dimensional, cylindrical, spark assisted homogeneously charged compression ignition (HCCI) reactor are numerically modeled using a multi-timescale method with reduced kinetic mechanism. It is found that the initial mixture temperature and pressure have a dramatic impact on flame dynamics. Depending on the initial temperature gradient, there exist at least six different combustion regimes, an initial single flame front propagation regime, a coupled low temperature and high temperature double-flame regime, a decoupled low temperature and high temperature double-flame regime, a low temperature ignition regime, a single high temperature flame regime, and a hot ignition regime. The results show that the low temperature and high temperature flames have distinct kinetic and transport properties as well as flame speeds, and are strongly influenced by the low temperature chemistry. The pressure and heat release rates are affected by the appearance of different flame regimes and the transitions between them. Furthermore, it is found that the critical temperature gradient for ignition and acoustic wave coupling becomes singular at the negative temperature coefficient (NTC) region. The results show that both the NTC effect and the acoustic wave propagation in a closed reactor have a dramatic impact on the ignition front and acoustic interaction.

Impact of Ethylene and NO Addition on Fuel Oxidation Under Simulated HCCI Conditions

We present an investigation on the chemical influence of some exhaust gas recirculation (EGR) compounds on fuel oxidation, especially the effects of NO and ethylene. Two experimental setups were used: a homogeneous charge compression ignition (HCCI) engine and a jet-stirred reactor (JSR). The first one was chosen for its representativeness and the second one for its well-controlled operating conditions. NO and ethylene were selected because NO is known for its high reactivity and ethylene has a high concentration in the EGR gases. A complex impact of NO and little influence of C2H4 on the fuel oxidation kinetics were observed, in accordance with the literature. However, a synergistic effect on fuel oxidation was observed by coupling between NO and ethylene chemistry. For the fuel oxidation, a considerable inhibition at low temperatures and a significant promotion at higher temperatures were observed. These results tend to demonstrate the importance of taking into account the complexity of EGR composition.

Ignition delay time correlations for a diesel fuel with application to engine combustion modelling

New diesel engine combustion concepts, such as the homogeneous charge compression ignition (HCCI), have encouraged the development of ignition delay correlations allowing the reliable prediction of the chemical auto-ignition time of the fuel, which depends on its oxidation kinetics. These correlations permit one to design and optimize the most adequate control strategies leading to both low emission levels of pollutants and proper engine performance. Although some diesel ignition correlations can be found in the literature, most of them consider the physical delay time and have been obtained only for high temperature values, which are usual at the end of the compression stroke in traditional diesel engines. However, there is still a significant lack of information regarding the complex three-stage oxidation (low-, intermediate-, and high-temperature range) of a diesel fuel under HCCI conditions. Thus the proposal of delay time correlations for the three temperature ranges is the objective of the present work. The correlations have been assumed as Arrhenius-type equations and consider the effect of the main parameters affecting the auto-ignition time, i.e. pressure, temperature, and equivalence fuel/air ratio. The adjustment coefficients have been calculated by using multiple linear regression and least-squares techniques, and a very good fit between modelling-predicted and correlation-predicted delay values has been obtained ( R2 higher than 0.95 in all cases).