Low Temperature Heat Release Research Articles

The effect of 2-ethyl-hexyl nitrate on HCCI combustion properties to compensate ethanol addition to gasoline

Stable HCCI combustion requires a proper level of fuel reactivity. This study shows that adding ethanol as a renewable fraction to low octane gasoline decreases the reactivity of the gasoline, while adding 2-ethyl hexyl nitrate (2-EHN) can enhance the reactivity of the blend and counter the effect of ethanol.The experimental apparatus consisted of a modified CFR engine for HCCI combustion equipped with two port fuel injectors and an intake air heater. Gasoline blended with ethanol (10% v/v) was used as the base fuel. Different percentages of 2-EHN (0.25%, 0.50%, 1%, and 2.5%) were added to the base fuel as an ignition improver. The blends were tested at operating points defined for HCCI number at two different engine speeds (600 and 900 rpm) and three different intake temperatures (50, 100, and 150 °C) to investigate the effect of 2-EHN on the auto-ignition behavior of the fuel.Combustion, emissions, and performance parameters of HCCI combustion of the blends were measured. The presence of 2-EHN in the blends improved the auto-ignitability of the blends in a nonlinear manner. It was also found that 0.25% of 2-EHN can compensate for the effect of ethanol on the required compression ratio and remove the quenching effect of ethanol on low temperature heat release. The results show that for the same fuel, a higher compression ratio is needed to maintain the combustion phasing constant at a higher engine speed.

Fischer-Tropsch coal-to-liquid fuel negative temperature coefficient region (NTC) and low-temperature heat release (LTHR) in a constant volume combustion chamber (CVCC)

The purpose of the study was to investigate the correlation between thermal characteristics of neat Iso-Paraffinic Kerosene (IPK) in relation to Low Temperature Heat Release (LTHR), ignition delay, and combustion delay within the Negative Temperature Coefficient region (NTC), utilizing a Box-Behnken design matrix. There were 15 trials of 3 control input parameters which were strategic manipulations of ASTM standard D7668-14a. The standard parameters are: 1000 bar for injection pressure, a combustion chamber temperature of 595.5 °C, and an injection pulse width of 2.5 ms. It was observed that, at ASTM standard, the Derived Cetane Number of neat Ultra-Low Sulfur Diesel (ULSD) and IPK was 47.38 and 25.9, respectively. Results show that the LTHR duration and energy release were reduced when two parameters were simultaneously increased in ULSD operation. Meanwhile, for both fuels, the duration and energy release in LTHR has increased when two parameters were simultaneously reduced. It was found that the lower set wall temperature resulted in increased NTC region and LTHR increased to over 10% of the total energy release. The maximum combustion pressure for both ULSD and IPK where increased by 17.2% and 16.1% respectively from the standard due to the increase in injection pressure and pulse width.

Formation of combustion wave in lean propane-air mixture with a non-uniform chemical reactivity initiated by nanosecond streamer discharges in the HCCI engine

The propagation of combustion wave, which is formed as a result of impact of a filamentary pulsed periodic non-equilibrium discharge on a mixture that is not ignited by only compression in the HCCI engine, has been simulated. The discharge for a short time locally activated the mixture in the engine cylinder at a certain crankshaft rotation angle at the compression stage. The approach to evaluation of the temperature and composition in the region activated by high-frequency corona discharge has been proposed. Multi-pulse and multi-channel nature of discharge was accounted for in our model. The densities of chemically active species produced during streamer propagation were found using two-dimensional axially symmetric fluid model. The obtained composition and temperature in the activated zone were the initial conditions for modeling the propagation of combustion wave along the cylinder radius in 1D approximation. Special terms were included in equations responsible for the compression along the axis of cylinder to mimic the real change in pressure in cylinder. The calculations were performed for a lean C3H8-air mixture, which is characterized by the presence of low-temperature heat release (LTHR) stage, and intermediate and high temperatures heat release (ITHR and HTHR) stages. It was shown that the ignition timing in activated zone depends on the specific energy input into the streamer channel, the radius of activated region coupled with the volume fraction of activated region treated by discharge, and the moment of discharge initiation relative to the top dead center (TDC). The speed of combustion wave, the occurrence of auto-ignition in the end gas and transition to combustion, accompanied by “knocking” or strong pressure fluctuations are largely determined by the moment of discharge initiation and the specific energy input. The significant effect of discharge is explained by the stimulation of oxidation kinetic mechanisms at LTHR and ITHR stages.

Spray combustion simulation study of waste cooking oil biodiesel and diesel under direct injection diesel engine conditions

Spray combustion characteristics of waste cooking oil biodiesel (WCO) and conventional diesel fuels were simulated using a RANS (Reynolds Averaged Navier Stokes) based model. Surrogates were used to represent WCO and diesel fuels in simulations. N-tetradecane (C14H30) and n-heptane (C7H16) were used as surrogates for diesel. Furthermore for WCO, surrogate mixtures of methyl decanoate, methyl-9-decenoate and n-heptane were used. Thermochemical and reaction kinetic data (115 species and 460 reactions) were implemented in the CFD code to simulate the spray and combustion processes of the two fuels. Validation of the spray liquid length, ignition delay, flame lift-off length and soot formation data were performed against previous published experimental results. The modeled data agreed with the trends obtained in the experimental data at all injection pressures. Further investigations, which were not achieved in previous experiments, showed that prior to main ignition, a first stage ignition (cool flame) characterized by the formation formaldehyde (CH2O) species at low temperature heat release occurred. The main ignition process occurred at high temperature with the formation of OH radicals. Furthermore, it was observed that the cool flame played a greater role in stabilizing the downstream lifted flame of both fuels. Increase in injection pressure led to the cool flame location to be pushed further downstream. This led to flame stabilization further away from the injector nozzle. WCO had shorter lift-off length compared to diesel as a result of its cool flame which being closer to the injector. Soot formation followed similar trends obtained in the experiments.

Uncertainty quantification from raw measurements to post-processed data: A general methodology and its application to a homogeneous-charge compression–ignition engine

Internal combustion engines have been improved for many decades. Yet, complex phenomena are now resorted to, for which any optimum might be unstable: noise, low-temperature heat release timing, stratification, pollutant sweet spots, and so on. In order to make reliable statements on an improvement, one must specify the uncertainty related to it. Still, uncertainty quantification is generally missing in the piston engine experimental literature. Therefore, we detailed a mathematical methodology to obtain any engine parameter uncertainty and then used it to derive the uncertainty expressions of the physical quantities of the most generic homogeneous-charge compression–ignition research engine (mass-flow-induced mixture with [Formula: see text] fuel). We then applied those expressions on an existing hydrogen homogeneous-charge compression–ignition test bench. This includes the uncertainty propagation chain from sensor specifications, user calibrations, intake control, in-cylinder processes, and post-processing techniques. Directly measured physical quantities have uncertainties of around 1%, depending on the sensor quality (e.g. pressure, volume), but indirectly measured quantities relying on modelled parameters have uncertainties higher than 5% (e.g. wall heat losses, in-cylinder temperature, gross heat release, pressure rise rate). Other findings that such an analysis can bring relate, for example, to the physical quantities driving the uncertainty and to the ones that can be neglected. In the case of the homogeneous-charge compression–ignition engine considered, the effects of blow-by, bottle purity and air moisture content were found negligible; the post-processing for effective compression ratio, effective in-cylinder temperature, and top dead centre offset were found essential; and the pressure and volume uncertainties were found to be the main drivers to a large extent. The obtained numeric values serve the general purpose of alerting the experimenter on uncertainty order of magnitudes. The developed methodology shall be used and adapted by the experimenter willing to study the uncertainty propagation in their setup or willing to assess the adequacy of a sensor performance.

Spark Assisted Compression Ignition Engine with Stratified Charge Combustion and Ozone Addition

<div class="section abstract"><div class="htmlview paragraph">Performance and emissions characteristics for stratified charge spark assisted compression ignition (SACI) with 30 ppm of added ozone (O<sub>3</sub>) were explored in a single-cylinder, optically accessible, spray-guided, research engine. For the present study, intake pressure and temperature were fixed at 1.0 bar and 42°C respectively, with a range of engine loads (1.5 – 5.5 bar indicated mean effective pressure) and speeds (800 – 1600 revolutions per minute) explored. Fuel stratification achieved by a late-cycle injection of ~ 10–25% of the total fuel was used to maintain stable operation at lower engine loads. For each condition spark timing, second injection SOI, and fuel split ratio between the main and second injection were optimized to maximize engine performance while maintaining nitrogen oxide emissions (NOx) below 5 g/kg-fuel.</div><div class="htmlview paragraph">Ozone addition was found to decrease specific fuel consumption by up to 9%, with across the board improvement in combustion stability relative to similar conditions without O<sub>3</sub>. The effect of O<sub>3</sub> addition was most substantial for the lowest loads. Moreover, because a higher fraction of the fuel burned was due to end-gas auto-ignition, specific NOx emissions likewise decreased by up to 30%. From complementary measurements of in-cylinder O<sub>3</sub> decomposition acquired via an ultraviolet light absorption diagnostic, it was observed that rapid decomposition of O<sub>3</sub> into molecular and atomic oxygen coincided with the onset of end-gas auto-ignition. The burst of resultant atomic oxygen was thought to accelerate low-temperature heat release (LTHR) reactions in the end gas. Optimal end-gas auto-ignition started between 20 and 30 crank angles before top dead center with temperatures at LTHR onset estimated to be between 575 and 700 K. An included analysis indicates that the spark deflagration was needed to add between 10 and 40 J of additional thermal energy to the end gas to achieve optimal auto-ignition.</div></div>

Chemical Kinetics Study on Combustion of Ethanol/biodiesel/n-heptane

Methyl decanoate (MD), methyl-9-decanoate (MD9D), and n-heptane (H) as alternative blends of biodiesel (B) were used to build a detailed chemical kinetic mechanism containing 3,324 components and 11,053 elementary reactions. This condition verifies that the ignition delay time of the detailed mechanism in the experiment conditions is reasonable. MD and MD9D will produce methyl-2-palmitate (MP2D) and finally be dehydrogenated as CH2O. Furthermore, R4 (O + H2O→OH + OH) and R228 (CH2CHO + O2→CH2O + CO + OH) are the key reactions, which will influence the ignition delay and heat release. According to the simulation result, the rate of BH (the volume ratio of the biodiesel/n-heptane mixture is fixed at 20%/80%) in constant volume bomb (Cetane Ignition Delay 510, CID 510) is the highest. However, with the development of ethanol, the rates decreased. The reactors of BHE5 (BHE5 refers to a blend of 5% ethanol and 95% biodiesel/n-heptane) have the highest rate among the ethanol blends. In addition, the reaction rate of the intermediate substance of ketohydroperoxide (KHP) in a modified cooperative fuel research engine (CFR) during combustion decreased with ethanol addition. However, the KHP rate of BHE15 (BHE15 refers to a blend of 15% ethanol and 85% biodiesel/n-heptane) and BHE20 (BHE20 refers to a blend of 20% ethanol and 80% biodiesel/n-heptane) is similar, causing the closed onset of low-temperature heat release. The rate of CH2O and MP2D of BH is the highest over the others in CID 510. The rate of CH2O and MP2D of BHE5 is lower than that of BHE10 (BHE10 refers to a blend of 10% ethanol and 90% biodiesel/n-heptane), BHE15, and BHE20.

Experimental study on the effect of pre-ignition heat release on GCI engine combustion

Interactions between fuel and engine are basic issues for gasoline compression ignition (GCI) engine, and recent engine experiment results show that the gasoline fuel with low-octane number RON = 60–80 is optimal for GCI engines. However, gasoline fuel with low-octane ratings exhibits significant low-temperature combustion behavior, which will significantly affect the entire combustion process. In this study, different toluene primary reference fuels (TPRF) with identical research octane number (RON) but various octane sensitivities were employed. Optimized start of injection (SOI) was obtained based on indicated mean effective pressure (IMEP), combustion stability limits and engine emissions. Under the optimized SOI, the role of fuel sensitivity in combustion processes was investigated. With addressing low-temperature heat release (LTHR) before main ignition, the effect of intake conditions and fuel sensitivity on combustion phasing was clarified. The results indicate that the LTHR only occurs when the SOI is sufficiently advanced, and an earlier injection timing can achieve stable combustion, higher IMEP and lower NOX emission. More obvious LTHR promotes the advancement of combustion phasing, and the magnitude of LTHR is negatively correlated with intake temperature but positively correlated with intake pressure. The effect of increasing LTHR magnitude on combustion phasing is similar to that of increasing intake temperature. Finally, the effect of octane sensitivity on LTHR was also studied, which shows that the impact of LTHR between different sensitivity fuels is mainly reflected in combustion phasing, and the onset of LTHR is more advanced for low sensitivity fuels.

Modeling pre-spark heat release and low temperature chemistry of iso-octane in a boosted spark-ignition engine

Recent trends among automotive manufacturers towards downsized, boosted engines make it imperative to understand specific fuel chemistry interactions encountered in this new operating regime. At these elevated pressure conditions a phenomenon called pre-spark heat release has recently been discovered, and is characterized by kinetically controlled heat release before spark, with resultant changes in end-gas thermodynamic state and composition. These reactions typically occur in the end-gas during normal operation, but are obscured by the deflagration heat release and therefore cannot be easily studied. A 2-zone spark-ignition engine model was utilized to determine whether chemical kinetic mechanisms are able to predict this phenomenon, and whether they accurately capture end-gas thermodynamic history. Experimental engine data at a range of boosted operating conditions demonstrating pre-spark heat release were compared with simulations using mechanisms representing the latest developments in gasoline kinetic modeling. The results demonstrated significant discrepancies between mechanisms, and between experimental and simulated results in terms of low-temperature heat release magnitude, end-gas thermodynamic state, and autoignition propensity. The results highlight shortcomings in low-temperature reaction pathways, and indicate the necessity of simultaneously matching first-stage ignition delay and heat release magnitude, in addition to second-stage ignition delay, in order to accurately predict end-gas thermodynamics and autoignition.

Experimental study on combustion and emission characteristics of iso-butanol/diesel and gasoline/diesel RCCI in a heavy-duty engine under low loads

This work experimentally studies the combustion and emission of iso-butanol/diesel RCCI (Reactivity Controlled Compression Ignition) under low loads and compares its results with gasoline/diesel RCCI. The results show that iso-butanol/diesel RCCI has better power recovery ability than gasoline/diesel RCCI. When Rp (Premixed Ratio) is 50% and 60%, IMEP (Indicated Mean Effective Pressure) of iso-butanol/diesel RCCI is 1.4% and 3.9% higher than maximum IMEP of CCM (Conventional Combustion Mode) respectively. Both iso-butanol/diesel and gasoline/diesel RCCI contain LTHR (Low Temperature Heat Release) and HTHR (High Temperature Heat Release). With increase of Rp in both RCCI combustion modes, combustion phase retard, peak value of heat release and maximum PRR (Pressure Rising Rate) decrease, ID (ignition delay), CD (combustion duration), CA50-CA10, maximum mean in-cylinder temperature and ITE (Indicated Thermal Efficiency) increase. Compared with the gasoline/diesel RCCI, iso-butanol/diesel RCCI has later combustion phase, longer ID, CD and CA50-CA10, higher maximum mean in-cylinder temperature and ITE, as well as lower maximum PRR. Furthermore, with increase of Rp, CO (Carbon Monoxide), NOx (Nitrogen Oxides) and PM (Particulate Matter) emissions decrease, HC (Hydrocarbon) increases for both gasoline/diesel and iso-butanol/diesel RCCI. Compared with the gasoline/diesel RCCI, iso-butanol/diesel RCCI achieves lower CO, HC, NOx and PM emissions.

The oxidation of C2-C4 diols and diol/TPGME blends in a motored engine

Inspired by the announcement that liquid biofuel will be used nationally by 2020 in China, the autoignition of surrogates from hydrogenated biomass pyrolysis oil, namely, the C2-C4 diols, including ethanediol, 1,2-propanediol, and 1,2-butanediol has been studied using a modified CFR engine. From the oxidation process before critical compression ratio (CCR), it is observed that C2-C4 diols had similar oxidation reactivity as ethanol, indicating that C2-C4 diols had the potential for use as a supplement for ethanol. In addition, both of ethanol and C2-C4 diols did not display low temperature heat release. The ignition characteristics of C2-C4 diols suggest that diol/TPGME blends could be used, as TPGME has high ignition quality and could be generated from diols with a carbon selectivity as high as 75%. Based on this idea, diol/TPGME blends with volume ratio 20:80 were prepared and tested using the modified CFR engine. The blends showed strong low temperature oxidation behavior, even under a low intake temperature at 120 °C and a low compression ratio at 4, the low temperature heat release peak could still be found for the blends. Finally, the physical delay and chemical delay times were tested by a modified CID instrument between 540 and 640 °C, indicating that chemical process played a more important role on the combustion process than physical process, and the influence of temperature was more important for physical processes than chemical processes.

Two-stage ignition behavior and octane sensitivity of toluene reference fuels as gasoline surrogate

Current approaches to improve the efficiency of Spark-Ignition (SI) gasoline engines have been focusing on turbocharging, increasing the compression ratio, and pursuing advanced low-temperature combustion concepts. In order to maximize these strategies, it is important to optimize the knock resistance of the fuel, and therefore knowledge of the sensitivity of the ignition process under a wide range of engine operating conditions is required. Octane sensitivity (OS), which is defined as the difference between Research Octane Number (RON) and Motored Octane Number (MON), has been introduced to represent how fuel's ignition reactivity changes relative to the primary reference fuels (n-heptane/iso-octane) within RON/MON conditions. Previous works have indicated that OS is intimately related to low temperature reactivity of the fuel, which can be revealed as two-stage heat release characteristics during an ignition event. Prompted by these findings, in this paper, we investigate the relationship between two-stage ignition behavior and OS, using chemical kinetic simulations of 24 Toluene Reference Fuels (TRFs)/ethanol blends. TRFs are ternary mixtures of n-heptane/iso-octane/toluene, which is capable of capturing aromatic content and positive values of OS of real gasoline fuels. Simulation results show that fuels with weak or no two-stage ignition behavior tend to have high OS, due to their lack of Negative Temperature Coefficient (NTC) effect and high sensitivity in ignition delay time. Leveraging such observations, we develop a correlation between two-stage behavior and OS as an OS prediction method. Two metrics that represent the strength of the two-stage ignition behavior are proposed and used as OS predictors, which are Low Temperature Heat Release percentage (LTHR%) and Heat Release Rate at the end of first stage (HRRinf) calculated from a simple kinetic simulation. Regression analysis shows a clear trend between decreases in the proposed two-stage behavior metrics and increases in the value of OS of the fuel. We also test the new metric (LTHR%) using simulation results of 0-D reactors with imposed pressure time histories obtained from engine experiments, as well as using different TRF kinetic mechanisms. The results demonstrate the effectiveness of the metric as a representation of the two-stage ignition behavior in practical combustion systems, highlighting the importance of the proposed relationship, and its potential as a simple and effective OS predictor.

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.

A comprehensive study of fuel reactivity on reactivity controlled compression ignition engine: Based on gasoline and diesel surrogates

Based on three-component gasoline surrogates and five-component diesel surrogates, the effects of fuel reactivity of port injection fuel and direct injection fuel on combustion and emission characteristics of reactivity controlled compression ignition (RCCI) mode were systematically studied. The research octane number of port injection fuels and cetane number of direct injection fuels were modulated by changing the proportions of n-alkanes and iso-alkanes in surrogates. The results show that both the cetane number of direct injection fuel and research octane number of port injection fuel played the key role in the combustion phase of RCCI mode. When port injection fuel with research octane number ≤ 90, the low temperature heat release would occur. For premixed ratio of 0.8 and with RON70/CN55 as fuel, the CO emissions were the lowest (as low as 1585 ppm). Increasing the premixed ratio was an effective solution for suppressing NOx and particulate matter emissions. For premixed ratio increased from 0.4 to 0.8, the peak value of nuclear mode particulates decreased from 109 to 107. Increasing the cetane number of direct injection fuels or lowering the research octane number of port injection fuels inhibited the emissions of formaldehyde, acetaldehyde, ethylene, propylene, acetylene and 1,3-butadiene effectively. Port injection of low research octane number was beneficial to reducing indicated specific fuel consumption. Fuel combination with port injection of RON70, direct injection of CN55 and premixed ratio of 0.8 was recommended taking into account of fuel consumption, and regulated/unregulated gas emissions.

On pre-ignition heat release of fuels with various octane sensitivities under compression ignition conditions

Fuel and engine interactions are the essential issues for advanced compression ignition engines, and low-octane gasoline is considered as prospective fuel under compression ignition conditions. However, with enhanced low-temperature chemistry, low-octane gasoline may exhibit distinctive characteristics affecting ignition phenomena. In this study, the ignition phenomena of low-octane gasoline was investigated under gasoline compression ignition conditions using validated chemical kinetic mechanism. The toluene primary reference fuel blends were employed to obtain the same RON = 75 with various octane sensitivity. With addressing ignition characteristics and pre-ignition heat release, the role of fuel sensitivity, inlet conditions, and exhaust gas recirculation on fuel reactivity was clearly demonstrated. The results show that the fuel sensitivity is positively correlated with fuel reactivity in high-temperature low-pressure regime, whereas there is a negative correlation in low-temperature high-pressure regime. The opposite correlation between fuel sensitivity and fuel reactivity is closely related to the pre-ignition heat release, with decreasing low-temperature heat release at high fuel sensitivity. Finally, the effects of exhaust gas recirculation on low-temperature heat release were investigated. It shows that affected by low-temperature heat release, distinct characteristics in combustion phasing are observed with the addition of the exhaust gas recirculation.

Solid-state high-power photo heat output of 4-((3,5-dimethoxyaniline)-diazenyl)-2- imidazole/graphene film for thermally controllable dual data encoding/reading

Solid-state high-power heat output at a relatively low temperature is a core problem for photo thermal batteries (PTB) films because of the fundamental contradiction between fast heat release and low-rate isomerization. To overcome this problem, imidazole-containing azoheteroarene (i-Azo-h) is introduced into a graphene-templated assembly for low-temperature time-resolved heat release. Intramolecular pull-push electronic interaction of i-Azo-h in the assembly film results in high-rate and high-degree trans-cis isomerization compared with azobenzene. The uniform i-Azo-h/graphene PTB film reaches very high power density (2380 W kg−1) and high energy density (105 Wh kg−1). A “tortoise”-like integrated system is designed as a high-power photo-heat source to realize a reversible and controllable temperature rise up to 3.3–4.1 °C on the “back” with a temperature gradient of the “leg.” Results show that heat release rate at 60 °C and under room-temperature green-light irradiation at 535 nm are higher than those at 45 °C, indicating controllability in time-resolved temperature rise. Based on these results, a thermally controllable dual encoding/reading strategy based on reversible time-resolved thermochromic-patterned display is developed for the first time. Dual encoding/reading information (response time or a specific pattern) can be selectively changed by optimizing isomerization degree or stimulus.

Low-temperature exothermic oxidation characteristics and spontaneous combustion risk of pulverised coal

To investigate the spontaneous combustion risk of pulverised coal, the low-temperature oxidation heat flow of three metamorphic pulverised coals was measured using a C80 microcalorimeter system at various oxygen concentrations to investigate the spontaneous combustion risk of pulverised coal. The results indicated that low-temperature exothermic oxidation of pulverised coal occurred in stages that had distinct characteristics. When the temperature increased, the heat flow curve of RNM and QM pulverised coals first decreased, then increased, and finally decreased, but the heat flow curve of the PSM pulverised coal first decreased and then increased. A considerable lag was observed in the heat flow curves with the decrease in the oxygen concentration, and the characteristic temperature increased. Stage 1 released the least heat, whereas stage 3 released the most heat. A decrease in the oxygen concentration considerably reduced the heat release of pulverised coal. A spontaneous combustion risk index was proposed on the basis of low-temperature oxidation heat release of pulverised coal. Decreased oxygen concentration and high metamorphism of pulverised coal considerably reduced the spontaneous combustion risk index.

Low temperature auto-ignition characteristics of methylcyclohexane/ethanol blend fuels: Ignition delay time measurement and kinetic analysis

New ignition delay times (IDTs) of binary methylcyclohexane (MCH)/ethanol fuels were provided by using a rapid compression machine. Results show that MCH exhibits typical negative temperature coefficient (NTC) regime bounded by two turnover temperatures (Tupper and Tlower). As the ethanol blending ratio increases, IDTs increase, NTC regime shrinks and shifts to the lower temperature. Measured data are then used to validate the most updated kinetic model (Bissoonauth et al. Proc Combust Inst 2018), which well captures the IDT and NTC behavior dependence on the ethanol blending ratio. The kinetic reason for the experimentally observed ignition behavior is that ethanol addition reduces the low temperature heat release and slows down the temperature rise and H2O2 decomposition pathway activation. The mole fraction of ROO and QOOH decrease rapidly due to the reduction of MCH initial concentration, and the NTC behavior becomes less apparent consequently. Further comparison among the ignition behavior of the different binary fuels have been conducted to access the effect of ethanol addition on different structure fuels. Results indicate a generalized effect of ethanol addition and the kinetic reason is that the presence of ethanol just simply reduces the hydrocarbon concentration without fuel-to-fuel interactions.

Numerical investigation of reactivity controlled compression ignition (RCCI) using different multi-component surrogate combinations of diesel and gasoline

The role of chemical mechanisms in studying the combustion phenomenon is very important. These mechanisms, if formulated properly, can make a numerical investigation more accurate and time-saving. In this study, a multi-component surrogate has been presented after integrating two previous mechanisms to study the combustion chemical kinetics. The mechanism contains a total of six components, 168 species and 680 reactions and has been validated extensively. The validations have been carried out for individual and mixture of components over a wide range of temperatures and pressures. The mechanism showed a good overall agreement with the experimental data. In order to do an application-based kinetic study, this mechanism was later used to study the combustion characteristics of Reactivity Controlled Compression Ignition (RCCI), which is a low-temperature and dual fuel combustion concept. The combustion characteristics have then been investigated considering the impact of single and multi-component surrogates on chemical and physical properties. First, only chemical properties and then physical properties have been changed followed by a change in both properties by a multi-component mechanism. The in-cylinder pressure and heat release rates have been captured well by multi-component surrogate fuel representing both physical and chemical properties of multi-component surrogate. The NOx and soot emissions have been predicted well by using a combination of physical and chemical properties of multi-component and single component surrogates for diesel and gasoline, respectively. The chemical kinetics analysis was performed at points of five percent (CA5) and fifty percent (CA50) of total heat release using sensitivity and consumption reaction pathways analyses. These analyses showed an overall more production of OH, CH2O and CO2 in multi-component surrogate fuel which enhanced the overall heat release rate. Toluene and cyclohexane were observed to inhibit the reactivity of multi-component surrogate fuel. They, on the other hand, also control the low-temperature heat release rates and enhance the high-temperature heat release rates.

Insights into the Reactions of Hydroxyl Radical with Diolefins from Atmospheric to Combustion Environments.

Hydroxyl radicals and olefins are quite important from a combustion and an atmospheric chemistry standpoint. Large amounts of olefinic compounds are emitted into the earth's atmosphere from both biogenic and anthropogenic sources. Olefins make a significant share in transportation fuels (e.g., up to 20% by volume in gasoline), and they appear as important intermediates during hydrocarbon oxidation. As olefins inhibit low-temperature heat release, they have caught some attention for their applicability in future advanced combustion engine technology. Despite their importance, the literature data for the reactions of olefins are quite scarce. In this work, we have measured the rate coefficients for the reaction of hydroxyl radicals (OH) with several diolefins, namely 1,3-butadiene, cis-1,3-pentadiene, trans-1,3-pentadiene, and 1,4-pentadiene, over a wide range of experimental conditions ( T = 294-468 K and p ∼ 53 mbar; T = 881-1348 K and p ∼ 1-2.5 bar). We obtained the low- T data in a flow reactor using laser flash photolysis and laser-induced fluorescence (LPFR/LIF), and the high- T data were obtained with a shock tube and UV laser-absorption (ST/LA). At low temperatures, we observed differences in the reactivity of cis- and trans-1,3-pentadiene, but these molecules exhibited similar reactivity at high temperatures. Similar to monoolefins + OH reactions, we observed negative temperature dependence for dienes + OH reactions at low temperatures-revealing that OH-addition channels prevail at low temperatures. Except for the 1,4-pentadiene + OH reaction, which shows evidence of significant H-abstraction reactions even at low-temperatures, other diolefins studied here almost exclusively undergo addition reaction with OH radicals at the low-temperature end of our experiments; whereas the reactions mainly switch to hydrogen abstraction at high temperatures. These reactions show complex Arrhenius behavior as a result of many possible chemical pathways in such a convoluted system.