Chemical Kinetic Model Research Articles

On multi-stage autoignition of n-propylcyclohexane at low temperatures

n-Propylcyclohexane (nPCH) has been widely used as a surrogate component representing alkylated cycloalkanes in conventional transportation fuels. Since the autoignition characteristics of nPCH at low temperatures have not been extensively studied in the literature, a series of rapid compression machine (RCM) experiments were conducted using nPCH/air mixtures over a wide range of engine-relevant conditions. Specifically, ignition delay times were measured over the compressed temperature range of TC = 615‒750 K, compressed pressures of pc = 10‒30 bar, and equivalence ratios of φ = 0.25‒2.0. For the first time, a three-stage ignition phenomenon of nPCH was observed at ultra-fuel-lean conditions (φ = 0.25). Using the chemical kinetic model of nPCH developed in our recent work (Combustion and Flame 238 (2022) 111944), experimental results were simulated to better understand the changes in temperature and heat release rate at each stage of autoignition. In addition, the simulated concentrations of key species (ȮH, HȮ2, CO, and CO2) and values of the heat production rate per reaction were analyzed to identify the controlling chemistry at each stage. The current model can well capture the three-stage, two-stage, and one-stage ignition for different mixtures.Based on experimental pressure traces, heat release rate (HRR) profiles of representative one-, two-, and three-stage ignition cases were derived, showing that the trends of the deduced HRR characteristics agree well with those predicted by RCM simulations using the current model. Moreover, the main heat production chemistry of different stages in the multi-stage ignition is summarized in this work. For the three-stage ignition, the low-temperature chain-branching reactions of nPCH oxidation dominate at the first stage, while the intermediate-to-high temperature chain termination reaction of Ḣ + O2 (+M) 〈=〉 HȮ2 (+M), the chain branching reaction of H2O2 (+M) 〈=〉 ȮH + ȮH (+M), and the reaction sequence of ĊH2CHO → CH2O → HĊO → CO play the major role at the second and third stages. When increasing the equivalence ratio from 0.25 to 0.5, a two-stage ignition is observed and its heat production chemistry is different from those of the three-stage ignition. The reactions related to the Ḣ-atom abstraction reaction from small species by O2 and ȮH radical and the formation reaction of C2 species (CHX1*O2J =〉 Ċ2H3 + CH2CO + C2H4) are dominant at the first stage, while the high-temperature reactions involving the competitive reactions of Ḣ + O2 (+M) 〈=〉 HȮ2 (+M) and Ḣ + O2 〈=〉 Ö + ȮH and the exothermic reaction of CO + ȮH 〈=〉 CO2 + Ḣ play an essential role at the second stage. When pressure increases from 10 bar to 20 bar, only one-stage ignition behavior is observed. The heat production analysis also shows that the high-temperature chemistry is dominated by the Ḣ-atom associated reactions, namely Ḣ + O2 (+M) 〈=〉 HȮ2 (+M) and Ḣ + O2 〈=〉 Ö + ȮH, as well as the CO conversion reactions such as CO + ȮH 〈=〉 CO2 + Ḣ.

Large-scale thermochemistry calculations for combustion models

Accurate thermochemical properties for chemical species are of vital importance in combustion chemistry research. Group additivity approaches are widely used to generate thermochemistry data used in chemical kinetic models, but this approach has limited accuracy. Following on previous studies by the combustion community, we performed electronic structure calculations to obtain reliable thermochemistry data for a large set of molecules taken from a well-established chemical kinetic model. The developed database consists of 1340 species that contain up to 18 and 5 carbon and oxygen atoms, respectively. The M06-2X/aug-cc-pVTZ level of theory was used for the geometry optimizations, vibrational frequency calculations, and dihedral angle scans. The potential energy of the different species was further refined with different composite methods, and the G3 method with the atomization reaction approach was selected to calculate the enthalpy of formation at 0 K. This information was then used in statistical thermodynamics calculations to obtain standard enthalpies of formation and entropy, as well as heat capacities at different temperatures. Our thermochemistry data exhibits good agreement with existing values in the literature, verifying the accuracy of our approach. Comparisons against group additivity (GA) method are also presented. The database of thermochemistry quantities developed in this study can be used to improve the training GA or machine learning models. The impact of the developed dataset is illustrated by examining the variation in ignition delay times using the updated thermochemistry values.

Reaction kinetics of OH radicals with 1,3,5-trimethyl benzene: An experimental and theoretical study

Alkylated aromatics are ubiquitous in transportation fuels. 1,3,5-Trimethyl benzene (135TMB) is a popular surrogate for the aromatic content of distillate fuels due to its symmetry (point group, C3h), which facilitates the construction of an accurate chemical kinetic model. The reaction of OH radicals with 135TMB plays a crucial role in the oxidation kinetics of 135TMB. In this work, the reaction kinetics of OH-initiated oxidation of 135TMB were investigated behind reflected shock waves over 975–1318 K and atmospheric pressure. The reaction was followed by monitoring OH radicals near 307 nm. The rate coefficients were extracted from detailed chemical kinetic modeling of OH concentration-time profiles. Our measured data clearly showed a positive temperature dependence, in contrast to the negative temperature dependence of the literature low-temperature data. At 1000 K, our measured rate coefficient is 1.3 × 10−11 cm3 molecule−1 s−1, which is roughly a factor of 5 lower than the room-temperature data reported in the literature. This observation reflects the complex nature of the OH + 135TMB reaction, similar to that observed for various aromatics + OH chemical systems. Our measurements did not show any discernible pressure dependence over the narrow pressure range of 870 – 1148 Torr. The title reaction has several possible channels in the reactive potential energy surface. The importance of each channel was characterized using ab initio/RRKM-ME calculations over T = 200–2000 K and P = 0.76 -7600 Torr. Our analyses revealed that addition channels and hydrogen abstraction from the methyl site have negative energy barriers. The reaction was found to undergo almost exclusively (∼93%) via the addition channel under ambient conditions. However, beyond 600 K, the abstraction channels take the lead, yielding the positive T-dependence of the overall rate coefficient. Although addition channels display a sharp fall-off behavior beyond 500 K, the general rate coefficients are pressure-independent. The title reaction shows a complex kinetic behavior due to competing channels whose contribution changes significantly with temperature. Our theoretical calculations nicely reproduced the complex T-dependence of the reaction. After adjusting the barrier height, our theory remarkably captured the positive T-dependence of our high-T kinetic data and the negative T-dependence of the low-T literature data. To our knowledge, this is the first detailed study on the reaction kinetics of 135TMB with OH radicals. The reported rate data will be helpful for the combustion modeling of alkylated aromatic species.

Effect of heat transfer enhancement on pyrolysis coking process for n-decane at supercritical pressure in corrugated cooling channels

The safety of engine cooling systems is compromised by hydrocarbon fuel pyrolytic coking. In this study, a numerical model based on a dynamic grid technique was developed at a supercritical pressure of 4 MPa, the model comprised a detailed chemical kinetic model of n-decane and an empirical pyrolytic coking kinetic model. The chemical kinetic model included 16 species and 26 elementary reactions. The flow and heat transfer in smooth and corrugated channels of length 500 mm, mass flow rate 1.5 g/s, inlet temperature 300 K, and wall heat flux 1.3 MW/m2 were studied after 20 min of transient coking. Compared with that in the smooth channel, the temperature of the corrugated external wall decreased by a maximum of 160K, and the amount of coke decreased by 51.89%. The convective heat transfer and Nusselt coefficients for the corrugated channel increased by a maximum of 36.3% and 42.4%, respectively. The pressure drop in the cooling channel changed after pyrolytic coking. The change in the pressure drop after 20 min of pyrolytic coking in the smooth channel was 9.89%, whereas it was only 4.39% in the corrugated channel. This indicates that the coking in the corrugated channel had a smaller effect on the flow distribution.

Atmospheric oxidation mechanism and kinetics of indole initiated by ●OH and ●Cl: a computational study

Abstract. The atmospheric chemistry of organic nitrogen compounds (ONCs) is of great importance for understanding the formation of carcinogenic nitrosamines, and ONC oxidation products might influence atmospheric aerosol particle formation and growth. Indole is a polyfunctional heterocyclic secondary amine with a global emission quantity almost equivalent to that of trimethylamine, the amine with the highest atmospheric emission. However, the atmospheric chemistry of indole remains unclear. Herein, the reactions of indole with ⚫OH and ⚫Cl, and subsequent reactions of resulting indole radicals with O2 under 200 ppt NO and 50 ppt HO2⚫ conditions, were investigated by a combination of quantum chemical calculations and kinetics modeling. The results indicate that ⚫OH addition is the dominant pathway for the reaction of ⚫OH with indole. However, both ⚫Cl addition and H abstraction are feasible for the corresponding reaction with ⚫Cl. All favorably formed indole radicals further react with O2 to produce peroxy radicals, which mainly react with NO and HO2⚫ to form organonitrates, alkoxy radicals and hydroperoxide products. Therefore, the oxidation mechanism of indole is distinct from that of previously reported amines, which primarily form highly oxidized multifunctional compounds, imines or carcinogenic nitrosamines. In addition, the peroxy radicals from the ⚫OH reaction can form N-(2-formylphenyl)formamide (C8H7NO2), for the first time providing evidence for the chemical identity of the C8H7NO2 mass peak observed in the ⚫OH + indole experiments. More importantly, this study is the first to demonstrate that despite forming radicals by abstracting an H atom at the N site, carcinogenic nitrosamines were not produced in the indole oxidation reaction.

Unraveling the role of EGR olefins at advanced combustion conditions in the presence of nitric oxide: Ethylene, propene and isobutene

The role of EGR (exhaust gas recirculation) olefinic constituents at advanced combustion conditions in the presence of nitric oxide is unraveled in this study through experimental and modeling efforts using a twin-piston rapid compression machine operating at a stochiometric fuel loading with 20% EGR by mass, pressures of 20 and 40 bar, and temperatures from 680 to 950 K. Five different levels of olefin addition, focusing on ethylene, propene and isobutene, with a fixed amount of NO at 70 ppm are doped into test mixtures of PACE-20, a multi-component gasoline surrogate, where olefin addition effects are characterized through changes in ignition times and heat release rates. Experiments indicate that all three EGR olefins inhibit autoignition reactivity and low-temperature heat release at Tc < 850 K, with isobutene exhibiting the greatest impact, while at Tc > 850 K, low ethylene and propene additions promote reactivity. A recently updated chemical kinetic model, with detailed gasoline/NOx interacting and olefin/NOx interacting chemistry incorporated, is adopted to simulate the experiments. Simulation results are somewhat inconsistent with the experiments, where the model captures the inhibiting effects of all olefins on first-stage ignition reactivity, while consistently predicting a promoting effect on main ignition reactivity. Sensitivity and rate of production analyses reveal that adding olefins greatly alters the role of the consuming pathways for the olefins and their primary derivatives at the initial stage of the oxidation process, particularly with the presence of NO, where the olefins and their derivatives interact with both NOx species such as NO2 and other species such as OH and HO2. The olefin/NOx interactions are particularly pronounced with propene and isobutene addition, where these lead to increased ignition reactivity by facilitating NO production and are mostly responsible for the disagreement between the model and experiments. Further investigations of olefin interacting chemistry, particularly those with NOx species, are needed for chemistry models to accurately predict the complicated effects of EGR.

Experimental Study and Kinetic Modeling for the Laminar Flame Speed of Methyl Octanoate and n-Nonane

ABSTRACT Biodiesel is one of the most important alternative fuels due to limited conventional resources. Methyl octanoate with long alkyl chain and mono-alkyl ester is one of the surrogate fuels of actual biodiesel. In order to clarify distinctive combustion characteristics of methyl octanoate, comparison with alkane in traditional diesel is necessary. Therefore, this work chose n-nonane with same carbon number as methyl octanoate. The laminar flame speeds of two fuels were measured at different conditions (T = 423 K, P = 1 and 3 atm, and φ = 0.7–1.5) in a constant volume combustion bomb. Results show that laminar flame speed of n-nonane is higher than that of methyl octanoate in most conditions, but lower in fuel-rich conditions. The combined chemical kinetic model of methyl octanoate and n-nonane was developed through eliminating unnecessary reactions, which can well predict the experimental results of laminar flame speed and ignition delay time. The adiabatic flame temperature results from the updated kinetic model show that the temperatures of n-nonane are higher than methyl octanoate in whole conditions, which is unexpectedly opposite to their laminar flame speeds order in fuel-rich conditions. In overall conditions, the intermediates of methyl octanoate tend to produce CH3, which consumes H radicals, thereby reducing the laminar flame speed, which can explain the higher laminar flame speed of n-nonane in most conditions. Meanwhile, the oxidation reactions with oxygen molecules of intermediates of n-nonane are suppressed due to low combustion temperature at φ = 1.5 unlike at φ = 0.7 and 1.0. On the contrary, those of methyl octanoate have similar reactions between φ = 0.7, 1.0, and φ = 1.5. It means that the laminar flame speed of n-nonane decreases more significantly than methyl octanoate as the equivalence ratio increases from φ = 1.0 to φ = 1.5.

The effect of ozone addition on the combustion characteristics of hydrogen-jet fuel

Ozone (O3) addition combustion is a trending technique to enhance the ignition characteristics of hydrocarbon fuel. In this study, the spray ignition characteristics of jet fuel along with hydrogen (H2) and O3 seeding were experimentally investigated and numerically simulated with an updated chemical kinetic model. The ignition delay (ID) period of jet propellant 5 (JP-5) with H2/O3 addition was measured using a constant volume combustion chamber (CVCC) at 600 K to 818 k, the pressure of 15 bar, equivalence ratio of 0.5. The intake manifold was added with air, H2 (10%, and 20% volume fraction), and 2000 ppm ozone (O3). In general, pure JP-5 has a shorter ID than those obtained with H2 addition. At 600 K, with 10% and 20% H2 to JP-5, the ID was increased by 18.49%, and 35.12% respectively. When O3 was added, the ID of JP-5 was shortened by 10%, while for 10% and 20% H2 addition, the ID was shortened by 19.25% and 22.5%, respectively. An existing jet fuel mechanism was chosen and validated with the previous and the present study experimental ID. The model was in agreement with the practical ID. The H2-mechanism rate constants were updated with the recently optimized values available in the literature to reflect the newly obtained experimental data at low temperatures. An existing sub-mechanism of O3 was adopted and combined with the JP-5 model. The updated model was validated with the present study ID data. Good predictions were noticed between the model and JP-5 experimental ID. However, a small deviation of 5.01% and 0.71% was found for 10%, and 20% H2 ID at 600 K. The reaction H2 + OH = H2O + H and HO2 + HO2 = H2O2 + O2 were found to be the critical reactions responsible to reduce the systems reactivity and makes the fuels ID to become longer. The O-atom from O3 was decomposed through the reaction O3 + N2 = O2 + O + N2, and O3 + O2 = O2 + O + O2 at an early stage of combustion. Due to H2 addition to O3 and air, the early formation of OH radicals is due to O3 + H = O2 + OH, which further accelerates the oxidation of the fuel, resulting in a shorter ID.

폐수방류수 오존처리시 오존 및 OH 라디칼 노출량 기반 미량유기오염물질 제거 예측

Municipal wastewater is a major source of contaminants of emerging concern in aquatic environments. Many studies have proven the effectiveness of ozonation for the removal of micropollutants, though the composition of the wastewater matrix influences the oxidation potential during this process. The ozone decomposition rate consists of two stages: instantaneous ozone consumption and slower ozone decay. In particular, with instantaneous ozone consumption, 62.8-79.7% of the initial ozone injection concentration is consumed. Determining the appropriate ozone dosage for wastewater ozonation is complicated by the complexity of the wastewater matrix composition. The purpose of this study was to propose a chemical kinetic model for the prediction of micropollutant removal during the ozonation of wastewater effluent. A kinetics approach based on the measurement of ozone and hydroxyl radical(·OH) exposure was proposed to predict the micropollutant removal efficiency. In this study, a batch-type ozone reactor was set up to measure the ozone and OH radical exposure during wastewater ozonation. Ozone and OH radical exposure was proportional to the initial dose of ozone, while OH radical exposure was found to be proportional to ozone exposure, though the deviation was relatively high at 1.0 to 1.5 gO³/gDOC. The calculated OH radical exposure was 3.0×10^-10 to 5.3×10^-10M·s. Of the target micropollutants, acetaminophen, sulphamethoxazole, andnaproxen, which are highly reactive with ozone and OH radicals, exhibited a removal rate of more than 80% at an ozone injection rate of 0.75 gO³/gDOC.

Ozone-assisted oxidation of ethylene in a jet-stirred reactor: An experimental and modeling study

Ozone addition is a promising method to achieve improvement and control of combustion/ignition processes due to ozone-derived chemistry. However, the complexity induced by ozonolysis reactions make investigations on O3-assisted oxidation with alkenes still scarce. In this work, the ozone-assisted oxidation of ethylene was studied in a jet-stirred reactor from 300 to 1000 K. The main products were identified and quantified by mass spectrometry/gas chromatography analysis. The mole fractions were then compared to those computed with a kinetic model from the literature. This work provides data for the development of detailed chemical kinetic models relevant to atmospheric and combustion chemistry studies.

The role of source emissions in sulfate formation pathways based on chemical thermodynamics and kinetics model

Sulfate is a major PM2.5 constituent and poses a significant threat to ecosystems and human health, which has attracted lots of attention to the sulfate formation mechanism. In recent years, there has been great scientific interest in the multiphase oxidation of SO2 in aqueous aerosol particles. Many factors are involved in the reaction process, including precursor SO2, oxidants/catalysts, and aerosol acidity, which are three channels closely related to the source emission. The conjoint analysis of source emissions and sulfate aqueous formation can provide a scientific basis for designing effective strategies, though the related research is extremely limited. Here, we applied an improved solute strength-dependent chemical Thermodynamics & Kinetics model (for aqueous pathway contribution) and the Partial Target Transformation-Positive matrix factor model (for source apportionment) to explore the role of source emission in sulfate aqueous formation. The results indicated H2O2 aqueous oxidation was the dominant pathway (65.9 %), and secondary nitrate source may grow together with sulfate formation from H2O2 pathway. H2O2 and TMI pathways were related to higher SOR (sulfur oxidation rate). TMI pathway was significant in summer (54.6 %) and increased with secondary sources and vehicle exhaust. NO2 pathway was more significant at low secondary source and high coal combustion (higher contribution of NO2 pathway appeared in winter, 24.7 %). While high formation rate of the O3 pathway always occurred at low source levels. Coal combustion and vehicle exhaust showed obvious effects on sulfate aqueous formation. Notably, aerosol acidity is a significant factor related to sources and plays a key role in sulfate formation. The result also suggested aerosol pH may be more important than the amounts of substances involved in the oxidation reaction. The findings in this work can provide useful information for better understanding sulfate aqueous formation and offer a scientific basis for designing strategies for air pollution control and sulfate mitigation.

Octane Requirements of Lean Mixed-Mode Combustion in a Direct-Injection Spark-Ignition Engine

This study investigates the octane requirements of a hybrid flame propagation and controlled autoignition mode referred to as mixed-mode combustion (MMC), which allows for strong control over combustion parameters via a spark-initiated deflagration phase. Due to the throughput limitations associated with both experiments and 3-D computational fluid dynamics calculations, a hybrid 0-D and 1-D modeling methodology was developed, supported by experimental validation data. This modeling approach relied on 1-D, two-zone engine simulations to predict bulk in-cylinder thermodynamic conditions over a range of engine speeds, compression ratios, intake pressures, trapped residual levels, fueling rates, and spark timings. Those predictions were then transferred to a 0-D chemical kinetic model, which was used to evaluate the autoignition behavior of fuels when subjected to temperature–pressure trajectories of interest. Finally, the predicted autoignition phasings were screened relative to the progress of the modeled deflagration-based combustion in order to determine if an operating condition was feasible or infeasible due to knock or stability limits. The combined modeling and experimental results reveal that MMC has an octane requirement similar to modern stoichiometric spark-ignition engines in that fuels with high research octane number (RON) and high octane sensitivity (S) enable higher loads. Experimental trends with varying RON and S were well predicted by the model for 1000 and 1400 rpm, confirming its utility in identifying the compatibility of a fuel’s autoignition behavior with an engine configuration and operating strategy. However, the model was not effective in predicting (nor designed to predict) operability limits due to cycle-to-cycle variations, which experimentally inhibited operation of some fuels at 2000 rpm. Putting the operable limits and efficiency from MMC in the context of a state-of-the-art engine, the MMC showed superior efficiencies over the range investigated, demonstrating the potential to further improve fuel economy.

Numerical prediction of research octane numbers via a quasi-dimensional two-zone cylinder model

Sustainably produced synthetic fuels offer great potential for a fast reduction of the greenhouse gas emissions of the transport sector. For an immediate application within the existing infrastructure and vehicle fleet, synthetic fuels need to comply with existing standards such as the EN 228 for gasoline. Beyond these standards and with optimized fuel design, certain properties can be improved compared to conventional fuels. For this purpose, methods for evaluating the properties are needed.This work discusses the development of a simplified numerical quasi-dimensional two-zone cylinder model, combined with chemical kinetic models, for the estimation of octane numbers. The model emulates fuel specific operating conditions of the standardized procedure for the determination of octane numbers in the cooperative fuel research engine with variable compression ratios. The two zones represent the burned and unburned in-cylinder volume and are modeled with homogeneous reactors. The octane numbers are determined by the identification of the critical compression ratio, for which premature ignition occurs in the unburned reactor.A two-zone cylinder model is validated against experimental data for various primary reference fuels, blended with toluene, ethanol, isobutanol and ethyl tert-butyl ether. The successful application of different kinetic models is demonstrated and enables the application on a wide range of fuels. It is shown that the simulated research octane numbers are in good agreement with the experimental data.

Research on the Effect of Water Injection Pressure on Knocking Combustion of a Direct Injection Gasoline Engine

In this study, the research method of numerical simulation is used to explore the inhibition of different water injection pressures on knock combustion of turbocharged direct injection gasoline (GDI) engines by coupling computational fluid dynamics with a chemical-kinetics model. First, the ignition advance angle and compression ratio are increased to induce the GDI engine to knock, and then the influence of the water injection pressure on the in-cylinder, evaporation of water, and the knock of the gasoline engine are analyzed. The simulation results show that, compared with no water injection, the direct injection of water in the cylinder can significantly reduce the knock intensity. When the water injection pressure is greater than 40 bar, the knock intensity is less than 2 and the knocking is completely suppressed. In this work, the effects of different water injection pressures on knocking are explored by analyzing the effects of water injection pressure on water atomization, in-cylinder combustion, and the knocking mechanism. On the one hand, the evaporation rate of water increases with increasing water injection pressure and the quality of the liquid film generally improves. On the other hand, direct water injection can significantly reduce the distribution of CH2O in the end mixture, thereby reducing the generation of H2O2 and further suppressing the spontaneous combustion of the end mixture. At the moment of knock, when the water injection pressure is greater than 40 bar, the detonation mechanism of the no. 7 monitoring point does not produce a sudden change in HCO radicals. The water spray can effectively reduce the NOx emission, and the NOx emission under the water spray pressure of 120 bar is the lowest. However, after spraying water, it will increase CO emissions.

Experimental and modeling study of water time histories during H2S-N2O combustion in a shock tube

With the interest in directly burning sour gas in gas turbines, and the fact that even small amounts of H2S or its combustion products can alter combustion characteristics, many research studies have been performed to better understand the combustion chemistry of H2S. In the present study, the water formation was followed by laser absorption with N2O as an oxidant, instead of O2. Nitrous Oxide being essentially consumed via N2O (+M) ⇌ N2 + O (+M), the water formation via the H2S + O route can then be probed to further validate the models. Three H2S/N2O mixtures diluted in 98% Ar were studied to cover the following range of equivalence ratios: 0.5, 1.0, and 2.0, over a wide range of temperatures (1580–1940 K) around atmospheric pressure. A chemical kinetic model was then developed first by validating the base N2O kinetics mechanism, then by investigating the experimental results presented herein. The N2O kinetics mechanism was updated with recent work on the low- and high-pressure limits for N2O decomposition (N2O (+M) ⇌ N2 + O (+M)) as well as a review of rate coefficients for N2O + H ⇌ N2 + OH from the literature. Good agreement is shown for H2/N2O mixtures. Updates were then made to the H2S kinetics mechanism, specifically, an update from the literature on SO2 + H (+M) ⇌ SO + OH (+M) and an adjustment to SO + SH ⇌ S2 + OH. Additionally, reactions between SH and N2O were determined using W1BD data and transition state theory which required the addition of an NNS sub-mechanism. With these updates, the mechanism provides good agreement with the H2S/N2O experiments.

Exhaust emissions from gasoline vehicles after parking events evaluated by chassis dynamometer experiment and chemical kinetic model of three-way catalytic converter

Despite the worldwide trend of introducing of zero-fuel-based vehicles to the market, the emissions of air pollutants and greenhouse gases from passenger vehicles are likely to remain a concern for the coming 20 to 30 years. In this study, exhaust emissions of gasoline engines running after varying parking durations were measured using a chassis dynamometer. The experimental results showed that exhaust emissions of hydrocarbons, nitrogen oxides, and carbon monoxide from most vehicles increased dramatically following 60 to 120 min of parking, and were higher than cold-start (1040 + min parking) emissions, indicating the impact of parking duration on atmospheric pollutant emissions. The after-treatment capacity of the three-way catalytic converter was evaluated by chemical kinetic modeling of the chemical reactions on the catalyst coupled with a time-dependent energy conservation equation. The results of the model calculation indicated that both the initial temperature of the three-way catalytic converter and the inlet engine gas temperature are critical factors impacting exhaust pollutants after parking; therefore, proper management to reduce the emissions after middle-term parking durations should be developed to mitigate air pollution.

The numerical simulation of nanosecond-pulsed discharge-assisted ignition in lean-burn natural gas HCCI engines

A plasma-assisted internal combustion engine model is established based on detailed plasma kinetics, combustion kinetics, and physical compression/expansion processes. The effects of nanosecond repetitively pulsed discharge (NRPD) on plasma-assisted ignition characteristics of mixtures under different fuel concentrations are studied under HCCI engine-relevant conditions. The coupled plasma and chemical kinetic model are validated with experiments. The comparison between NRPD and thermal ignition with a certain amount of input energy is carried out, and the results show that the former can ignite a mixture owing to the kinetic effect of nonequilibrium plasma, but the latter cannot ensure ignition. Path flux analysis shows that excited states and electrons react with fuel, providing O and H directly, increasing the possibility of ignition at a low temperature. The effect of NRPD on combustion performance under various equivalence ratios (φ) is investigated. It was found that in ICEs with NRPD, the ignition delay time under the lean-burn condition (φ = 0.5) is the shortest among three demonstrative cases. Even though the leaner mixture case with φ = 0.2 is more favorable for the production of O and OH during the discharge, after discharge, the heat release in case 2 with φ = 0.5 dramatically increases, resulting in the temperature exceeding that in the ultra-lean case. As the piston moves up, the higher amounts of CH3, HO2, and H2O2 as well as higher temperature for the lean-burn (φ = 0.5) case lead to the rapid increase of OH and O, which accelerates the consumption of methane and finally the earliest hot ignition near TDC. Finally, a series of parameter studies are performed to show the effects of E/N, current density, φ, and discharge timing on the ignition process. The results suggest that discharge parameters E/N and current density together with discharge timings and equivalence ratios can improve ignitability in internal combustion engines.

The secondary chemistry of synthetic fuel oxymethylene ethers unraveled: Theoretical and kinetic modeling of methoxymethyl formate and formic anhydride decomposition

Replacing fossil fuels by oxymethylene ethers (OMEs) produced from renewable resources could help to reduce the rising CO2 levels. In this work, the thermal decomposition chemistry of methoxymethyl formate (CH3OCH2OCHO) and formic anhydride (OCHOCHO) is investigated by means of a combination of quantum chemical calculations and kinetic modeling. The latter compounds are two important intermediates formed during the thermal decomposition chemistry of synthetic fuel OMEs. Two detailed kinetic models are developed based on first principles to describe the radical decomposition chemistry of methoxymethyl formate and formic anhydride, which are ultimately incorporated into the OME-2 model from De Ras et al. (Combustion and Flame, 2022). This newly obtained kinetic model describes experimental measurements for pyrolysis from literature significantly better than the model from the original study, without any fitting of thermodynamic or kinetic parameters. More particularly, some minor compounds are now satisfactorily reproduced within the experimental uncertainty margin of 10 mol% relative. Methoxymethyl formate and formic anhydride are found to be more reactive compared to OMEs. Both a reaction pathway analysis and sensitivity analysis reveal the important decomposition pathways under pyrolysis conditions.

Numerical analysis of performance and soot emissions of a natural gas engine operating in HPDI and SPC combustion modes

High-pressure direct injection (HPDI) natural gas (NG) engines can help significantly improve thermal efficiency and reduce harmful emissions. In this study, we established a three-dimensional (3D) model combined with a multicomponent reduced chemical kinetic model and a phenomenological soot model. Using the 3D simulation model, we investigated the influence of pilot diesel mass on the combustion and emission characteristics of an HPDI NG engine at different start of injection of NG (NSOI). Subsequently, we evaluated the influence of diesel and NG injection interval (DNT) on the combustion characteristics and soot emissions in the HPDI and slightly premixed combustion (SPC) combustion modes at different NSOI. The simulation results indicated that with an increase in the pilot diesel mass at different NSOI, the peak values of cylinder pressure (Pmax) and nitrogen oxides (NOx) emissions increase, but carbon monoxide (CO) and soot emissions decrease simultaneously. Advancing NSOI leads to higher NOx emissions and increased Pmax and maximum pressure rise rate (MPRR) at different pilot diesel masses; by contrast, delaying NSOI leads to decreased indicated thermal efficiency (ITE). In the HPDI mode, combustion parameters such as Pmax and CA50% are not sensitive to variations in the DNT at different NSOI. Furthermore, the variations in the DNT do not notably influence the reduction of soot emissions. In the SPC mode, the ignition delay significantly increases with the decrease in the DNT, whereas CA50% moves away from the top dead center (TDC) and Pmax decreases. Meanwhile, the peak values of pyrene (A4) and acetylene (C2H2) simultaneously decrease, and the OH radicals are more extensively distributed in the cylinder, thereby resulting in a considerable reduction in final soot emissions. Considering the combined influence of Pmax, MPRR, ITE, and soot emissions, the operating point with the NSOI of −12°CA ATDC and DNT of −4°CA is considered the optimized point.

Experimental study of the formation of CO during ethanol pyrolysis and dry reforming with CO2

To use piston engines and their pressure and temperature histories as a chemical rector to produce valuable chemicals, accurate chemical kinetics models are necessary so the engine parameters and mixtures can be designed. Using piston engines in such a way could lead to the production of target chemicals with net-negative CO2 emissions. This benefit is especially true for ethanol, a very common biofuel. To validate models, the dry reforming of ethanol and CO2 was investigated experimentally in a shock tube (50/50 ethanol/CO2 in 99.75% dilution) by following the formation of CO with a laser absorption setup. The effect of the presence of CO2 in the mixture was also investigated by comparing the results of the 50/50 ethanol/CO2 mixture with those from the pyrolysis of ethanol (without CO2) at various concentrations (99.75 and 98% dilution). The 98% dilution results compare extremely well with results from the literature, whereas the other conditions had never been investigated prior. The experimental data were compared to modern detailed kinetics mechanisms, and the experimental CO profiles were poorly predicted overall. Recent literature developments on ethanol pyrolysis were incorporated into the models, and the predictions were improved in some cases and conditions, although further improvements are still necessary. A numerical analysis (rate of production and sensitivity) was performed for the most accurate modified model to highlight the most important reactions involved in the formation of CO under our conditions.