Total Ignition Delay Times Research Articles

An experimental study of the spray ignition of alkanes

Spray ignition experiments were carried out in a constant volume spray combustion chamber (CVSCC) for determination of first-stage and total ignition delay times for pure alkane fuel components in air at 650–825K and 1–4MPa, providing insight into their oxidative reactivity under spray combustion conditions. Experiments for n-alkanes (heptane, decane, dodecane, and hexadecane) illustrate strong single-stage ignition at pressures of 2.14 and 4.0MPa and two-stage ignition at pressures of 1.0MPa. The total ignition delay for n-alkanes was found to decrease with increasing chain length according to τ∝carbonnumber-0.7. Highly branched alkanes, iso-octane and iso-cetane, were found to have very distinct two stage ignition where the ratio of the time scales of total ignition to first-stage ignition delay ranges from approximately two to three. The lightly branched 2,6,10-trimethyldodecane was found to behave similarly to n-alkanes with reactivity between n-heptane and n-decane, indicating that n-alkanes suitably emulate the spray ignition behavior of lightly branched alkanes for engineering experiments or predictions. Cycloalkanes, cyclohexane and methylcyclohexane, were found to have long total ignition delay times, of the order of the highly branched alkanes, but to ignite in a single stage. All ignition delay times were found to decrease with increasing temperature and pressure, with reduction in overall activation energy at temperatures around 750–825K indicative of the influence of competition between chain branching and propagation that occurs at the low-temperature onset to the negative-temperature-coefficient regime. Comparisons between the results from the CVSCC and those from the Ignition Quality Tester (IQT) show good correlation for fuels with nearly single-stage ignition. However, this is not the case for fuels with strong two-stage ignition behavior, where ignition delay determinations are shown to be sensitive to the definition of ignition delay and experimental parameters.

Effect of gas temperature and oxygen concentration on single particle ignition behavior of biomass fuels

This work focuses on single particle ignition behavior of biomass fuels. Five biomass fuels were studied, namely, wheat straw, kiwi branches, vine branches, sycamore branches and pine bark. Coal was also tested for comparison purposes. Tests were carried out in an optical flat flame McKenna burner able to produce a confined laminar flow of combustion products, in which single particles were injected upward through a central hole. The equivalence ratio and the thermal input of the burner were adjusted to yield the following operating conditions in the ignition zone: five mean temperatures (1500, 1575, 1650, 1700 and 1800K) and three mean dry oxygen concentrations (3.5, 5.1 and 6.5vol. %). The ignition mode and the ignition delay time were evaluated through the analysis of images obtained by means of a CMOS high-speed camera. The effects of gas temperature and oxygen concentration on the ignition mode and ignition delay time were assessed. For the studied conditions, the ignition of the biomass particles generally occurred in the gas-phase, although surface ignition was also observed in some cases. The results suggest that the critical diameter for the ignition mode transition varies with the fuel type. All particles show a consistent trend of decreasing ignition delay time with increasing gas temperature, with the biomass particles presenting slightly higher values than the coal tested. The estimated drying times were subtracted from the total ignition delay time and the trends shifted, with the pine bark and wheat straw presenting a lower remaining time than the coal. Finally, for the studied oxygen concentration range, its variation did not have a significant impact on the ignition mode and delay time at high temperatures.

A fundamental investigation into the relationship between lubricant composition and fuel ignition quality

A fundamental experiment involving the use of an ignition quality tester (IQT) was carried out to elucidate the effects of lubricant oil composition which could lead to low speed pre-ignition (LSPI) processes in direct injection spark ignition (DISI) engines. Prior to the IQT tests, lubricant base oils were analyzed using ultra-high resolution mass spectrometry to reveal their molecular composition. High molecular-weight hydrocarbons such as nC16H34, nC17H36, and nC18H38 were selected as surrogates of lubricant base oil constituents, and then mixed with iso-octane (iC8H18-gasoline surrogate) in proportions of 1vol.% (iC8H18=99vol.%) and 10vol.% (iC8H18=90vol.%) for the IQT experiments. In addition, lubricant base oils such as SN100 (Group I) and HC4 and HC6 (Group III) and a fully formulated lubricant (SAE 20W50) were mixed with iso-octane in the same proportions. The IQT results were conducted at an ambient pressure of 15bar and a temperature range of 680–873K. In the temperature range of 710–850K, the addition of 10vol.% base oils surrogates, base oils, and lubricating oil to the 90vol.% iC8H18 reduces the average total ignition delay time by up to 54% for all mixtures, while the addition of 1vol.% to 99vol.% iC8H18 yielded a 7% reduction within the same temperature range. The shorter total ignition delay was attributed to the higher reactivity of the lubricant base oil constituents in the fuel mixtures. A correlation between reactivity of base oils and their molecular composition was tentatively established. These results suggest that the lubricants have the propensity of initiating LSPI in DISI engines. Furthermore, similar results for n-alkanes, lubricant base oils, and fully formulated commercial lubricants suggest that it is the hydrocarbon fraction that contributes primarily to enhanced reactivity, and not the inorganic or organometallic additives.

An on-line monitoring system for wire electrical discharge turning process

This article presents a new pulse discriminating system for process monitoring of wire electrical discharge turning. The pulse discriminating device, including a spark gap monitor and a digital signal processor card, has been developed to identify four major gap states classified as open circuit, normal spark, arc discharge and short circuit based on the characteristics of gap voltage waveform as well as compute ignition delay time for each open circuit and normal spark. By means of this system, the proportion of open circuit, normal spark, arc discharge and short circuit to total sparks and average ignition delay time within a time interval of 200 ms can be analyzed quantitatively during the wire electrical discharge turning process. It has been observed that thread cutting using wire electrical discharge turning exhibited more stable machining than cylindrical wire electrical discharge turning under a servo feed machining condition because servo feed control of the cylindrical wire electrical discharge turning process only focused on keeping forward sparking gap at a proper width, but it failed to respond to the variation of lateral gap width due to an independent motion of the rotary axis with respect to the translational axis. Experimental results also demonstrate that these pulse train data and average ignition delay time can substitute for average gap voltage as sensing parameters for on-line monitoring and control of the wire electrical discharge turning process.

Effects of buffer gas composition on low temperature ignition of iso-octane and n-heptane

Experimental and numerical studies have been performed on the thermal and chemical effects of buffer gas composition on low temperature ignition of iso-octane and n-heptane. Experiments were conducted using a recently developed rapid compression machine in the temperature range of 600–850K. Three buffer gases were studied including nitrogen (N2), argon (Ar), and a mixture of Ar and carbon dioxide (CO2) at a mole ratio of 65.1%/34.9%. Iso-octane was studied at 20bar, ϕ=1, and a dilution level of buffer gas to O2 of 3.76:1 (mole ratio). n-Heptane was studied at 9bar, ϕ=1, and a dilution level of buffer gas to O2 of 5.63:1 (mole ratio). For experiments where two-stage ignition was observed, the buffer gas composition had no impact on the first-stage ignition time but, as expected, it caused differences in the total heat release, pressure and temperature rise after the first-stage ignition. As a consequence, significant differences were observed for the total ignition delay time as a function of the buffer gas composition, with up to 40% and 42.5% faster total ignition time for iso-octane and n-heptane, respectively, by using Ar instead of N2. The chemical effects of the buffer gas composition were studied experimentally by comparing the results of the N2 and Ar/CO2 (65.1%/34.9%) mixtures, recognizing that while the Ar/CO2 mixture has the same heat capacity as N2, its predicted combined third-body collision efficiency is about 76% higher than N2. The experimental results showed negligible chemical effects on the first-stage and total ignition delay times. Numerical simulations were carried out over a wider range of temperatures for pure N2, Ar, and CO2 as buffer gases. Results showed that thermal effects are very pronounced and dominated at the negative temperature coefficient and two-stage ignition conditions, which is consistent with the experimental results and previous studies in the literature. However, the simulation results also showed at temperatures higher than 850K, the chemical effects of CO2 became more important than the thermal effects.

Autoignition and combustion of hydrocarbon-hydrogen-air homogeneous and heterogeneous ternary mixtures

A numerical simulation of the ignition and combustion of hydrocarbon-hydrogen-air homogeneous and heterogeneous (gas-drop) ternary mixtures for three hydrocarbon fuels (n-heptane, n-decane, and n-dodecane) is for the first time performed. The simulation is carried out based on a fully validated detailed kinetic mechanism of the oxidation of n-dodecane, which includes the mechanisms of the oxidation of n-decane, n-heptane, and hydrogen as constituent parts. It is demonstrated that the addition of hydrogen to a homogeneous or heterogeneous hydrocarbon-air mixture increases the total ignition delay time at temperatures below 1050 K, i.e., hydrogen acts as an ignition inhibitor. At low temperatures, even ternary mixtures with a very high hydrogen concentration show multistage ignition, with the temperature dependence of the ignition delay time exhibiting a negative temperature coefficient region. Conversely, the addition of hydrogen to homogeneous and heterogeneous hydrocarbon-air mixtures at temperatures above 1050 K reduces the total ignition delay time, i.e., hydrogen acts as an autoignition promoter. These effects should be kept in mind when discussing the prospects for the practical use of hydrogen-containing fuel mixtures, as well as in solving the problems of fire and explosion safety.

Experimental and surrogate modeling study of gasoline ignition in a rapid compression machine

The use of gasoline in Homogeneous Charge Compression Ignition engines has propelled the need to better understand compression ignition processes for gasoline under engine-like conditions. In order to quantify low-temperature heat release and to provide fundamental validation data for chemical kinetic models, it is imperative to study autoignition phenomena under well-controlled conditions. However, there is a significant lack of autoignition delay data in the low temperature regime. Recognizing the need for kinetic information at high pressures and low-to-intermediate temperatures, this work aims to fill this void by conducting an experimental study of gasoline autoignition in a Rapid Compression Machine (RCM) to characterize the ignition response of gasoline+air mixtures over a wide range of compression temperatures at compression pressures of 20 and 40bar with equivalence ratios ranging from 0.3 to 1.0. Results from the RCM experiments are also simulated using a four-component gasoline surrogate model which includes n-heptane, iso-octane, toluene, and 2-pentene. For the conditions investigated, good agreement between the experiments and the four-component surrogate model, in terms of first-stage and total ignition delay times as well as the comparison of measured and simulated pressure traces, is demonstrated. Kinetic analysis is further conducted to understand the role of the different hydrocarbon classes present in gasoline in controlling autoignition.

Radiation effect on thermal explosion in a gas containing evaporating fuel droplets

The dynamics of thermal explosion in a fuel droplets/hot air mixture is investigated using the geometrical version of the method of integral manifolds. The results are applied to the modelling of the ignition process in diesel engines. Effects of the thermal radiation, semi-transparency of droplets and oxidizer are taken into account. In contrast to the previous studies, the difference between gas temperature (responsible for convective heating of droplets) and external temperature (responsible for radiative heating of droplets) is taken into account. The dynamics of the explosion is presented in terms of the dynamics of a multi-scale, singularly perturbed system. The relevant parametric regions of this system are analyzed. Explicit analytical formulae for the ignition delay in the presence of thermal radiation are derived. It is shown that the effect of thermal radiation can lead to considerable reduction (up to about 30%) of the total ignition delay time.

Highly accurate ignition delay apparatus for hypergolic fuel research

The efficient development of hypergolic fuels requires an interdisciplinary approach involving ab initio modeling, synthesis, and experimental physical chemistry. Candidate molecules must exhibit hypergolic ignition delay times that are fast enough to warrant further testing for safety and performance criteria. Hypergolic ignition delay apparatus has been mentioned in the open literature for six decades, but accurate, detailed, modern ignition delay hardware that uses inexpensive laboratory building blocks and a minimum of custom circuitry is still needed. This article details line-of-sight electro-optical circuitry with direct digital readout and additional oscilloscope recording that can be used to measure total ignition and chemical delay times for screening candidate fuels. We also illustrate the value of high speed video and quantum chemical calculations to supplement the ignition delay measurements for a comprehensive approach to hypergolic fuel research.

Autoignition of n-pentane and 1-pentene: Experimental data and kinetic modeling

Autoignitions of n-pentane and 1-pentene are studied by rapid compression between 600 and 900 K at high pressure. Both hydrocarbons show a two-stage ignition and a negative temperature coefficient region (NTC). However, 1-pentene is less reactive. Ignition temperature limit is 50 K higher; cool flames and NTC are weaker and confined to a narrower temperature range. Chemical analyses are performed on the reacting mixture for fuel consumption and cyclic ethers. n-Pentane and 1-pentene give very different distribution patterns for cyclic ethers. 2-Methyltetrahydrofuran dominates the n-pentane pattern, whereas propyloxirane is by far the major cyclic ether formed by 1-pentene. Detailed mechanisms based on a common skeleton scheme are developed and used to simulate the experiments. They are validated for ignition delay times, cool flame intensities, and cyclic ether distributions. Good results are obtained for 1-pentene only if (1) direct addition channels of OH and HO2 to the double bond are included and (2) if a higher rate constant for the decomposition of the hydroperoxyalkyl radicals into cyclic ethers is used when this radical is formed by direct HO2 addition instead of isomerization of alkylperoxy radicals. The sensitivity analysis of the low-temperature scheme for 1-pentene points out that the total ignition delay time is dependent upon the competition between the decomposition channels of hydroperoxyalkyl radical into the branching sequence and into alkenes. The cool flame delay time is less sensitive but depends mainly upon the decomposition rate of unsaturated ketohydroperoxides.

Pulse combustion: The quantification of characteristic times

Measurements of the total ignition delay time in a pulse combustor have been made for several chemical kinetic ignition delay times and several fluid dynamic mixing times. These measured total ignition delay times are compared with calculated values of the characteristic time for mixing and with calculated values for the homogeneous ignition delay time. A chemical kinetic model was used to calculate the homogeneous chemical kinetic ignition delay time for conditions typical of an operating pulse combustor. Similarly, a fluid dynamic mixing model was used to estimate characteristic times for a transient jet of cold reactants to mix with an ambient environment of hot products to an ignition temperature. These calculated time scales compared well with measured values in both trend and magnitude. It has also been shown that a simple sum of the characteristic mixing times and chemical kinetics times provides a good first-order approximation to the total ignition delay time.