Negative Temperature Coefficient Behavior Research Articles

Evaluating the Potential of a Direct-Injection Constant-Volume Combustion Chamber as a Tool to Validate Chemical-Kinetic Models for Liquid Fuels

ABSTRACTChemical-kinetic mechanisms for low-volatility hydrocarbons and alternative fuels are mostly scarce or not validated, primarily due to the lack of a convenient experimental platform to investigate fundamental ignition and combustion behavior for such fuels. This work reports experimental measurements of ignition delays and related combustion behavior of n-heptane in a direct-injection constant-volume combustion chamber, for pressures ranging from 0.2 MPa to 1.0 MPa and temperatures ranging from approximately 390°C to 500°C while holding the global equivalence ratio constant at 0.62. Results showed clear two-stage ignition and negative temperature coefficient behavior, and data at higher pressures agreed closely with predictions from a perfectly-stirred-reactor model. At lower pressures, impingement of liquid fuel on the chamber wall likely caused experimental data to deviate significantly from model predictions. Overall, this work strongly suggests that this experimental platform can be used to validate chemical kinetics of liquid fuels.

The role of low temperature chemistry in combustion mode development under elevated pressures

Negative Temperature Coefficient (NTC) behavior is an essential feature of low-temperature oxidation for large hydrocarbon fuels, which is of particular relevance to cool flame and auto-ignition. In this study, using n-heptane as a typical fuel exhibiting NTC, combustion phenomena involving both auto-ignition and flame propagation are computationally studied at initial temperatures within and above NTC regime under elevated pressures in a one-dimensional planar constant-volume configuration, with detailed kinetics and transport. Multi-staged flame structures representing cool flame and hot flame are observed, and consequently, different types of auto-ignition are identified during two-staged and single-staged flame propagation scenarios by varying initial temperature. Specially, as the initial temperature increases, the behavior of cool flame is gradually suppressed and auto-ignition position is transferred from the location ahead of flame front to end-wall region, leading to different combustion modes and peak pressure magnitudes. Moreover, attributed to the chemical reactivity processed by cool flame, the flame propagation of the cases within NTC regime is even faster than those beyond NTC regime. A recently developed two-staged Livengood–Wu integral is further utilized to predict these auto-ignition scenarios, yielding good agreement and further demonstrating the significant role of NTC chemistry in modifying the thermodynamic state and chemical reactivity at upstream of a reaction front. Finally, different combustion modes and knocking intensity for these detailed calculations are summarized in non-dimensional diagrams, which suggest that a higher initial temperature does not guarantee a higher knocking intensity, instead, the developing and developed detonation wave initiated by an auto-ignition occurring within NTC regime could even induce higher knocking intensity in comparison to the thermal explosion under the temperatures beyond NTC regime.

Comparative investigation of homogeneous autoignition of DME/air and EtOH/air mixtures at low initial temperatures

The dynamics of homogeneous isochoric explosions of dimethylether (DME)/air and ethanol (EtOH)/air mixtures were studied and compared at relatively low initial temperatures (≤ 700 K) using algorithmic tools derived from the methodology of computational singular perturbation. In the DME case, it is shown that the low-temperature oxidation is dominated by reactions involving heavy carbonaceous species, in contrast to the high-temperature case where oxidation is dominated by reactions involving light carbonaceous species. Moreover, it is demonstrated that the outcome of the competition between two specific reactions is the cause of the exhibited negative temperature coefficient (NTC). In the EtOH case, the analysis points to the importance of carbonaceous species and in particular acetaldehyde, which is different from what happens at elevated initial temperatures (above 1000 K), where hydrogen chemistry dominates the entirety of the oxidation process. The autoignition dynamics of both mixtures are shown to be pretty much independent of initial pressure, with the exception of the NTC behaviour of DME.

Investigation of Iso-octane Ignition and Validation of a Multizone Modeling Method in an Ignition Quality Tester

An ignition quality tester was used to characterize the autoignition delay times of iso-octane. The experimental data were characterized between temperatures of 653 and 996 K, pressures of 1.0 and 1.5 MPa, and global equivalence ratios of 0.7 and 1.05. A clear negative temperature coefficient behavior was seen at both pressures in the experimental data. These data were used to characterize the effectiveness of three modeling methods: a single-zone homogeneous batch reactor, a multizone engine model, and a three-dimensional computational fluid dynamics (CFD) model. A detailed 874 species iso-octane ignition mechanism (Mehl, M.; Curran, H. J.; Pitz, W. J.; Westbrook, C. K. Chemical kinetic modeling of component mixtures relevant to gasoline. Proceedings of the European Combustion Meeting; Vienna, Austria, April 14–17, 2009) was reduced to 89 species for use in these models, and the predictions of the reduced mechanism were consistent with ignition delay times predicted by the detailed chemical mechanism acr...

Interpretation of auto-ignition delays from RCM in the presence of temperature heterogeneities: Impact on combustion regimes and negative temperature coefficient behavior

Rapid compression machines (RCMs) have often been used to study auto-ignition phenomena and to validate chemical kinetic models at conditions relevant to engines. Many RCM designs were historically used for the latter purpose. First, the flat piston configuration was extensively used and contributed to valuable ignition delay data. Temperature heterogeneity in these devices was later addressed, and new designs were proposed. In particular, creviced piston configurations have been demonstrated to enhance the temperature homogeneity in the bulk volume of RCM chambers. Nevertheless, data from flat piston RCMs are still used in the community. Thus, additional understanding of experimental results from such devices is still useful, and it also helps to enhance the understanding of temperature stratification effects on auto-ignition phenomena.This paper addresses the interpretation of measured ignition delays from a flat piston RCM (without a circumferential crevice in the piston). First, we characterize local and temporal temperature evolution with a toluene planar laser induced fluorescence (PLIF) technique applied in oxygen-free gas under different test conditions. At a compression pressure (Pc) of 11bar and an adiabatic core temperature (Tc) of 750K, we find that the maximum measured temperature is very close to Tc up to 30ms after the compression stroke. In the second step, we qualitatively compare temperature fields with chemiluminescence images during the auto-ignition of n-hexane and an n-heptane/methyl-cyclohexane (MCH) blend for a fuel-air equivalence ratio of 0.4, Pc values of 11bar and 16bar and Tc values of 700–900K. The chemiluminescence results of n-hexane show a fast sequential auto-ignition which strongly suggest that the combustion propagation regime is mainly a reaction fronts regime. In the n-heptane/MCH case, the tracking of ignition kernels demonstrates that the auto-ignition initiates in the cold vortex at Tc values of 787K and 834K and in the adiabatically compressed region at Tc values of 742K and 900K.In this study, flat piston RCM experiments demonstrate temperature heterogeneities with ranges up to 120K and gradients of approximately 160K/mm. This significantly influences the ignition delay measurement in the negative temperature coefficient (NTC) range. Through the example of n-heptane/MCH auto-ignition, it seems that despite temperature heterogeneities, the lower limit of the NTC range is well detected, whereas the higher limit is over-estimated. The determination of temperature thresholds in which ignition delay data may be confounded highly depends on the stratification level and on complex thermo-kinetic interactions. Outside these thresholds, when combustion is dominated by auto-ignition in regions at the adiabatic core temperature, the experimental data are easier to compare with simulation frameworks assuming a homogeneous hypothesis. In chemical kinetics-oriented studies, temperature heterogeneity should be considered when flat piston RCMs are used and when the piston crevice is not adequate.

Effects of iso-octane/ethanol blend ratios on the observance of negative temperature coefficient behavior within the Ignition Quality Tester

An ignition delay study investigating the reduction in low temperature heat release (LTHR) and negative temperature coefficient (NTC) region with increasing ethanol concentration in binary blends of ethanol/isooctane was conducted in the Ignition Quality Tester (IQT). The IQT is advantageous for studying multi-component fuels such as iso-octane/ethanol which are difficult to study at lower temperatures covering the NTC region in traditional systems (e.g., shock tubes, rapid compression machines, etc.). The high octane numbers and concomitant long ignition delay times of ethanol and iso-octane are ideal for study in the IQT allowing the system to reach a quasi-homogeneous mixture; allowing the effect of fuel chemistry on ignition delay to be investigated with minimal impact from the fuel spray due to the relatively long ignition times. NTC behavior from iso-octane/ethanol blends was observed for the first time using an IQT. Temperature sweeps of iso-octane/ethanol volumetric blends (100/0, 90/10, 80/20, 50/50, and 0/100) were conducted from 623 to 993K at 0.5, 1.0 and 1.5MPa and global equivalence ratios ranging from 0.7 to 1.0. Ignition of the iso-octane/ethanol blends in the IQT was also modeled using a 0-D homogeneous batch reactor model. Significant observations include: (1) NTC behavior was observed for ethanol/iso-octane fuel blends up to 20% ethanol. (2) Ethanol produced shorter ignition delay times than iso-octane in the high temperature region. (3) The initial increase in ethanol from 0% to 10% had a lesser impact on ignition delay than increasing ethanol from 10% to 20%. (4) The 0-D model predicts that at 0.5 and 1.0MPa ethanol produces the shortest ignition time in the high-temperature regime, as seen experimentally.

Thermal and Ludwig–Soret diffusion effects on near-boundary ignition behavior of reacting mixtures

The autoignition behavior is key towards providing insight into a fuel's behavior and performance and equally important towards developing kinetic models. However, there has been evidence in past literature that under certain conditions legacy reactor experiments may exhibit inhomogeneities that could potentially reduce the scientific value of the reported data. Additionally, the initiation of knock in spark ignition engines has been observed to occur close to the cylinder wall. In this study, detailed one-dimensional simulations were carried out in order to provide additional insight into the aforementioned observations. The main focus of the present investigation is on the effects of a colder wall relatively to the core of the reacting mixture, and the attendant development of thermal boundary layers. It is determined that the thermal stratification could alter the ignition behavior for fuels that exhibit distinct negative temperature coefficient behavior and could cause also light and heavy species concentration stratification due to Ludwig–Soret diffusion. More specifically, simulations performed for n-heptane/air mixtures revealed that localized exothermic centers could develop for a range of initial mixture temperatures. Furthermore, simulations for hydrogen/oxygen/argon mixtures showed that species stratification caused by Ludwig–Soret diffusion could lead to increased local heat release rates.

A theoretical study of cyclic ether formation reactions

Cyclisation reactions of hydroperoxyl–alkyl radicals forming cyclic ethers and hydroxyl radicals play an important role in low temperature oxidation chemistry. These reactions contribute to the competition between radical chain propagation and chain branching reaction pathways which dominate the reactivity of alkanes at temperatures where negative temperature coefficient (NTC) behaviour is often observed. This work is motivated by previous experimental and modelling evidences that current literature rate coefficients for these reactions are in need of refinement and/or re-determination. In light of this, the current study presents quantum-chemically-derived high-pressure limit rate coefficients for all cyclisation reactions leading to cyclic ether formation in alkanes ranging in size from C2 to C5. Ro-vibrational properties of each stationary point were determined at the M06-2X/6-311++G(d,p) level of theory. Coupled cluster (CCSD(T)) and Møller–Plesset perturbation theory (MP2) methods were employed with various basis sets and complete basis set extrapolation techniques to compute the energies of the resulting geometries. These methods, combined with canonical transition state theory, have been used to determine 43 rate coefficients, with enough structural diversity within the reactions to allow for their application to larger species for which the use of the levels of theory employed herein would be computationally prohibitive. The validity of an alternative, and computationally less expensive, technique to approximate the complete basis set limit energies is also discussed, together with implications of this work for combustion modelling.

Compositional effects on the ignition of FACE gasolines

As regulatory measures for improved fuel economy and decreased emissions are pushing gasoline engine combustion technologies towards extreme conditions (i.e., boosted and intercooled intake with exhaust gas recirculation), fuel ignition characteristics become increasingly important for enabling stable operation. This study explores the effects of chemical composition on the fundamental ignition behavior of gasoline fuels. Two well-characterized, high-octane, non-oxygenated FACE (Fuels for Advanced Combustion Engines) gasolines, FACE F and FACE G, having similar antiknock indices but different octane sensitivities and chemical compositions are studied. Ignition experiments were conducted in shock tubes and a rapid compression machine (RCM) at nominal pressures of 20 and 40atm, equivalence ratios of 0.5 and 1.0, and temperatures ranging from 650 to 1270K. Results at temperatures above 900K indicate that ignition delay time is similar for these fuels. However, RCM measurements below 900K demonstrate a stronger negative temperature coefficient behavior for FACE F gasoline having lower octane sensitivity. In addition, RCM pressure profiles under two-stage ignition conditions illustrate that the magnitude of low-temperature heat release (LTHR) increases with decreasing fuel octane sensitivity. However, intermediate-temperature heat release is shown to increase as fuel octane sensitivity increases. Various surrogate fuel mixtures were formulated to conduct chemical kinetic modeling, and complex multicomponent surrogate mixtures were shown to reproduce experimentally observed trends better than simpler two- and three-component mixtures composed of n-heptane, iso-octane, and toluene. Measurements in a Cooperative Fuels Research (CFR) engine demonstrated that the multicomponent surrogates accurately captured the antiknock quality of the FACE gasolines. Simulations were performed using multicomponent surrogates for FACE F and G to reveal the underlying chemical kinetics linking fuel composition with ignition characteristics. A key discovery of this work is the kinetic coupling between aromatics and naphthenes, which affects the radical pool population and thereby controls ignition.

Comparative study on monitoring structural damage in fiber-reinforced polymers using glass fibers with carbon nanotubes and graphene coating

This paper reported the preparation of glass fibers (GFs) with carbon nanotubes (CNTs) and graphene coating by using an electrophoretic deposition (EPD) process. The developed GFs were embedded into epoxy to illustrate the role of different coatings on monitoring the damage in fiber reinforced polymers (FRPs). Piezoresistive effects of GFs with different coatings and corresponding composites were studied and compared. The results showed that GFs with both coatings exhibited a negative temperature coefficient behavior, and graphene coated sample was more sensitive to temperature and strain than that of CNTs. Huge dissimilarities on piezoresistivity of FRPs were also observed when the samples were subjected to the mechanical loading, which originated from the different interphases generated between the fibers and polymer matrix. GFs with graphene coating provided more details on the damage accumulation in FRPs and could be more practical for the early warning of structural damage in FRPs.

A comprehensive experimental and kinetic modeling study of butanone

Through a large interdisciplinary approach the “Tailor Made Fuels from Biomass” (TMFB) cluster of excellence has been pursuing the identification of next generation biofuels. By first using chemical synthesis to procure suitable chemical components from biomass followed by initial screening experiments a large information database is compiled and can be used to guide fuel design. As a result of this method butanone has been identified as a particularly interesting target owing to its potential usage as a spark-ignition fuel, thanks to its impressive knock resistant properties. This has motivated this study to consider the fundamental combustion chemistry controlling its reactivity under engine relevant conditions. A detailed chemical kinetic model which includes both high- and low-temperature oxidation reaction pathways for butanone is developed. This model (PCFCbutanone_v1) is validated using the available experimental data from the literature (included as supplemental material) and newly measured data collected during the course of this study. Ignition delay times are measured using both a shock tube and a rapid compression machine. They are measured for φ=1.0 butanone/air mixtures covering a range of temperatures (850–1280K) and pressures (20 and 40bar) that incorporates engine relevant conditions. In addition, laminar burning velocities for butanone are measured for a range of equivalence ratios (0.7–1.3) and pressures (1 and 5bar), an important parameter considering the potential use of butanone within spark ignition engines.The new model also incorporates new quantum chemical calculations of the thermodynamic parameters for butanone and the low-temperature species associated with its oxidation. The thermodynamic parameters used in the model are necessary to calculate the reverse rate constants in the model making them important parameters, in particular in predicting the low-temperature ignition delay times presented here.Butanone shows no evidence of negative temperature coefficient behavior. However, inclusion of the low-temperature oxidation pathways is shown to be important to accurately predict the low-temperature ignition delay times of butanone. Of particular importance are the radical β-scission and HȮ2 elimination reactions which are essential in accurately predicting the ignition delay times.

First-stage ignition delay in the negative temperature coefficient behavior: Experiment and simulation

The existence of the first-stage ignition delay (FID) negative temperature coefficient (NTC) behavior was confirmed by rapid compression machine experiments using iso-octane and methyl-cyclohexane. The first-stage NTC behavior of iso-octane is observed in the temperature range 757–782K under 20bar and φ=1. For methyl-cyclohexane, the observed first-stage NTC temperature range is 750–785K under 15bar and φ=0.5. In further iso-octane experiments, the FID is found to be sensitive to the O2 concentration and insensitive to the dilution gas and fuel concentrations. The effects of the FID and its NTC behavior on the total ignition delay NTC were analyzed using a detailed n-heptane mechanism. The contributions to the total ignition delay NTC from the reduced second-stage initial temperature, pressure, and less reactive species pool, together with the NTC of FID were discussed quantitatively. For the first-stage NTC behavior, five competing reactions were identified as being important based on sensitivity analysis, reaction pathway analysis, and simplified mechanism method. They are the backward reaction of second O2 addition, RO2 ⇔ alkene+HO2, QOOH ⇔ cyclic-ether+OH, QOOH ⇔ alkene+HO2, and the beta scission reaction of the alkyl radical. Their competition with the low-temperature branching channel finally leads to the first-stage NTC behavior.

Synthesis, characterization, and transport properties of single-layer pure and molybdenum-doped vanadium oxide thin films on metallic conductive substrates

Single-layer undoped and 10mol% molybdenum (Mo)-doped vanadium oxide (V2O3) thin films with thicknesses of approximately 342nm are fabricated by an aqueous sol–gel method and then deposited onto 316L stainless steel conductive substrates. The influence of various annealing temperatures (in a nitrogen atmosphere) on the structural and electrical properties of undoped and Mo-doped vanadium oxide thin films is investigated. Through a controlled annealing process, the electrical resistances of the single-layer thin films are optimized to attain the required amount of Joule heating for cold-start fuel cell applications within an ambient temperature range (273.15 to 253.15K). The films show a negative temperature coefficient (NTC) behavior and a transition from a metal to an insulator at sub-zero temperatures. The highest electrical resistivities are measured to be 0.032Ω·cm and 0.071Ω·cm for undoped and Mo-doped vanadium oxide films, respectively, after annealing under 20sccm N2 at 673.15K. Consequently, the equilibrium surface temperature of the single-layer Mo-doped vanadium oxide thin film increases from 253.15K to 299.46K upon induced Joule heating at a current density of 0.1A·cm−2. Thus, it is concluded that single-layer NTC Mo-doped vanadium oxides can be effectively used for cold-start fuel cell applications.

Resistivity and PTC behaviour of electro-conductive PVOH/MWNT and EVOH/MWNT nanocomposites

Electro-conductive poly(vinyl alcohol)/multi-walled carbon nanotube (PVOH/MWNT) and poly(ethylene-co-vinyl alcohol) (EVOH)/MWNT nanocomposites were prepared by precipitation saponification method. The MWNT was functionalised by electron beam irradiation in air at 1200 kGy doses. The electrical resistivity, thermal and mechanical properties, and positive temperature coefficient (PTC) behaviour of these nanocomposites were investigated. The melting and crystallisation peak temperatures of both nanocomposite systems were shifted at a higher temperature with the increase in saponification time. Their crystallinity and mechanical strength also increased with saponification time, indicating an increase in intermolecular hydrogen bond between vinyl alcohol groups. With the saponification time, PTC peak temperature of EVA28/MWNT and EVA40/MWNT nanocomposites was shifted at a higher temperature and followed by a negative temperature coefficient (NTC) of resistivity. However, the saponified PVAc/MWNT nanocomposites showed only NTC behaviour over a temperature range of 30–140°C.

Study of the Ignition Characteristics of Light Fractions of Crude Oil and Their Surrogate Fuels under a Range of Low-to-Medium Temperatures

To improve the self-ignition capability of oil reservoirs and build a detailed reaction kinetics mechanism of in situ combustion (ISC), ignition and reaction characteristics of light fractions of crude oil (boiling point of <100 °C, 100–125 °C, 125–150 °C, 150–175 °C, 175–200 °C, and 200–225 °C) were investigated on a rapid compression machine at compressed pressures of 20 bar and a compressed temperature range of 660–900 K, with a stoichiometric ratio of 1.0. According to our analysis with a gas chromatography–mass spectrometry instrument (GC-MS), cyclohexane, methyl-cyclohexane, p-xylene, and mesitylene were selected as surrogate fuels and were tested under the same conditions on a rapid compression machine (RCM). The experimental results show that three groups of light fractions, <100 °C, 100–125 °C, and 175–200 °C, display strong reactions at low and high temperatures and negative temperature coefficient (NTC) behavior. These phenomena imply that these reactive components can be used as possible promoters to accelerate the development of an ISC project and to improve the reaction speed of the entire process. Two groups of 125–150 °C and 200–225 °C fractions are difficult to ignite in the low-to-medium temperature range under the compressed pressure of 20 bar, but they can be ignited at relatively higher top dead center (TDC) temperature ranges under the compressed pressure of 30 bar. Moreover, the logarithm of ignition delays and the reciprocal temperature clearly show a linear relationship, meaning that the reactivity of these two groups is poor. The results obtained with these surrogate fuels match well with the experimental results of each light fraction.

Inversion of temperature–resistance relationship by enlarged structural defects in graphene oxide

Here we report a novel means to transform a positive temperature coefficient behavior (i.e., electrical resistance increase with temperature) of water-soluble, reduced edge-oxidized graphene oxide (rEOGO) into a negative temperature coefficient (NTC) material (i.e., resistance decrease with temperature). The temperature-versus-resistance relationship of this material could be reversed by introducing enlarged structural defects in the basal plane of rEOGO using nitric acid as an etchant. We found that the degree of the NTC behavior of rEOGO could be correlated to the number of structural defects. Similar effects of structural defects were also observed in influencing the NTC behavior of reduced graphene oxide (rGO) produced by the Hummer's method and thermal treatment. These results suggest that structural defects function as thermally activated electron traps, and can be used to precisely tune the temperature-versus-resistance relationship of GO materials desired for a variety of applications.

Reduction and validation of a chemical kinetic mechanism including necessity analysis and investigation of CH4/C3H8 oxidation at pressures up to 120 bar using a rapid compression machine

Cost efficiency and the ecological footprint are becoming more important in the locomotive and maritime business. To overcome these needs, manufacturers are forced to develop highly efficient internal combustion engines suitable for alternative fuel, such as biogas. In the development process of a new engine, simulation tools offer economic benefits compared to engine bench tests. To manage the trade-off between enhanced efficiency by increasing the compression ratio and knocking combustion, a reaction mechanism is necessary to capture the ignition behavior of various fuel blends and conditions during the simulation. Since previous studies only investigated methane/propane oxidation at pressures up to 50bar, experimental investigations have been conducted to include pressure regimes, which are similar to those of a knocking combustion. Therefore, rapid compression machine (RCM) experiments were performed using methane/propane mixtures at lean conditions with air-to-fuel ratios of 1.5, 1.7 and 1.9. Furthermore a range of compression temperatures from 800 to 1000K at pressures of 80, 100 and 120bar were conducted. The experiments for mixtures containing 30mol% propane showed a negative temperature coefficient (NTC) behavior, which is consistent with experiments for other alkane fuels presented in the literature. In addition, the experimental results were compared with the recently published detailed chemical mechanism Aramco Mech 1.3. This mechanism was used to simulate the ignition delay times with consideration of the facility effects of the rapid compression machine. The simulation results, based on a zero-dimensional homogeneous batch reactor showed good agreement with the measurements over a wide range of the investigated conditions. In a further step the detailed mechanism has been reduced by necessity analysis in order to minimize computational efforts for future combined CFD (Computational Fluid Dynamics) and kinetic simulations. The results of this novel work show the first set of high pressure screening experiments in a RCM for CH4/C3H8 mixtures combined with a systematic method for mechanism reduction.

On the controlling mechanism of the upper turnover states in the NTC regime

Using n-butane, n-heptane and iso-octane as representative fuels exhibiting NTC (negative temperature coefficient) behavior, comprehensive computational studies with detailed mechanisms and theoretical analysis were performed to investigate the upper stationary point, denoted as turnover states, on the NTC curve near the higher temperature regime, where the ignition delay τ exhibits a local maximum. It is found that the global behavior of the turnover states exhibits distinctive thermodynamic and kinetic characteristics under different pressures, in that the ignition delay at the turnover states shows an Arrhenius dependence on the temperature T and an approximate inverse quadratic power law dependence on the pressure P. These global behaviors imply that the temperature and pressure of the turnover states are not independent and can be correlated by Arrhenius dependence, as ln P ∝ 1/T. Further theoretical analyses demonstrate that such turnover states result from the competition between the low-temperature chain branching reactions and the decomposition of the intermediate species, and therefore correspond to a critical value, α, of the ratio of OH production from low-temperature chemistry. In addition, the ignition delay at the turnover state can be well correlated by the analytical expression derived by Peters et al., with the further demonstration that the pressure dependence of the turnover ignition delay mainly result from the H2O2 decomposition reaction. Comparison of the present results with the literature experimental data of n-heptane ignition delay time shows very good agreement.

An NTC Zone Compliant Knock Onset Prediction Model for Spark Ignition Engines

Pollutant emissions reduction and energy saving policies increased the production of Spark Ignition (SI) engines operated with gaseous fuels. Natural Gas (NG) and Liquefied Petroleum Gas (LPG), thanks to their low cost and low environmental impact represent the best alternative. Bi-fuel engines, which may run either with gasoline or with gas (NG or LPG), widely spread in many countries thanks to their versatility, high efficiency and low pollutant emissions: gas fueled vehicles, as example, are allowed to run in many limited traffic zones. In the last years, supercharged SI engines fueled with either gasoline or gaseous fuel, spread in the market. Thermodynamic simulations, widely used to reduce costs during engine development and optimization process, require proper combustion and knock onset prediction models. In particular the fuel knocking resistance is a crucial issue in supercharged engines development. Starting from these considerations the authors developed and calibrated an original knock onset prediction model for knock-safe performances optimization of engines fueled by gasoline and gaseous fuels. The proposed model, despite its very simple formulation, takes into account the Negative Temperature Coefficient (NTC) behavior exhibited by many hydrocarbons fuels such as gasoline, propane and methane. The knock prediction model has been calibrated by a great number of light-knocking pressure cycles sampled using a Cooperative Fuel Research (CFR) engine. The engine Compression Ratio (CR), inlet mixture temperature and spark advance have been varied to obtain very different operative conditions for model calibration; as a result the model can be used in the development of different kind of engines, i.e. naturally aspirated or supercharged. Five fuels have been tested: gasoline, LPG, NG, propane and methane. The calibrated model showed a very high reliability with a maximum knock onset prediction error of only 4 crank angle degrees (CAD) and an overall mean absolute error lower than 1 CAD, that are negligible quantities from an engine control point of view.

A Refined Model for Knock Onset Prediction in Spark Ignition Engines Fueled With Mixtures of Gasoline and Propane

In the last decade, gaseous fuels, such as liquefied petroleum gas (LPG) and natural gas (NG), widely spread in many countries, thanks to their prerogative of low cost and reduced environmental impact. Hence, bi-fuel engines, which allow to run either with gasoline or with gas (LPG or NG), became very popular. Moreover, as experimentally demonstrated by the authors in the previous works, these engines may also be fueled by a mixture of gasoline and gas, which, due to the high knock resistance of gas, allow to use stoichiometric mixtures also at full load, thus drastically improving engine efficiency and pollutant emissions with respect to pure gasoline operation without noticeable power loss. This third operation mode, called double fuel combustion, can be easily introduced in series production engine, since a simple electronic control unit (ECU) programing is required. The introduction into series production would require the availability of proper models for thermodynamic simulations, nowadays widely adopted to reduce research and development efforts and costs. To this purpose, the authors developed a quite original knock onset prediction model for knock-safe performances optimization of engines fueled by propane, gasoline, and their mixtures. The ignition delay model has been properly modified to account for the negative temperature coefficient (NTC) behavior exhibited by many hydrocarbon fuels such as gasoline and propane. The model parameters have been tuned by means of a considerable amount of light knocking in-cylinder pressure cycles acquired on a modified cooperative fuel research (CFR) engine, fueled by gasoline–propane mixtures. The adoption of many different compression ratios (CRs), inlet mixture temperatures, spark advances (SAs), and fuel mixture compositions allowed to use a very differentiated set of pressure and temperature curve, which gives the calibrated model a general validity for using different kinds of engines, i.e., naturally aspirated or supercharged. As a result, the model features a maximum knock onset prediction error around four crank angle degrees (CAD) and a mean absolute error always lower than 1 CAD, which is a negligible quantity from an engine control standpoint.