End-gas Autoignition Research Articles

The Effect of EGR Dilution on the Heat Release Rates in Boosted Spark-Assisted Compression Ignition (SACI) Engines

<div class="section abstract"><div class="htmlview paragraph">This paper presents an experimental investigation of the impact of EGR dilution on the tradeoff between flame and end-gas autoignition heat release in a Spark-Assisted Compression Ignition (SACI) combustion engine. The mixture was maintained stoichiometric and fuel-to-charge equivalence ratio (<i>ϕ</i><sup>′</sup>) was controlled by varying the EGR dilution level at constant engine speed. Under all conditions investigated, end-gas autoignition timing was maintained constant by modulating the mixture temperature and spark timing. Experiments at constant intake pressure and constant spark timing showed that as <i>ϕ</i><sup>′</sup> is increased, lower mixture temperatures are required to match end-gas autoignition timing. Higher <i>ϕ</i><sup>′</sup> mixtures exhibited faster initial flame burn rates, which were attributed to the higher laminar flame speeds immediately after spark timing and their effect on the overall turbulent burning velocity. The increasing trends in peak heat release rate and peak autoignition rate as a function of <i>ϕ</i><sup>′</sup> were consistent for all intake pressures ranging from 80 kPa to 150 kPa. For a constant spark timing, the mass fraction burned at the onset of autoignition correlated well with <i>ϕ</i><sup>′</sup>, regardless of intake pressure. Reducing the unburned mass fraction at the onset of autoignition was effective at limiting the peak heat release rates. This behavior was attributed to the reduced energy content of the end-gas and also to the reduced end-gas burn rates. Possible explanations for the latter observation were the greater buffer gas effect due to a larger burned gas mass, and the higher reactivity stratification of the end-gas being closer to the walls.</div></div>

Investigation of Homogeneous Lean SI Combustion in High Load Operating Conditions

<div class="section abstract"><div class="htmlview paragraph">Homogeneous lean combustion (HLC) can be utilized to substantially improve spark ignited (SI) internal combustion engine efficiency. Higher efficiency is vital to enable clean, efficient and affordable propulsion for the next generation light duty vehicles. More research is needed to ensure robustness, fuel efficiency/NO<sub>x</sub> trade-off and utilization of HLC. Utilization can be improved by expanding the HLC operating window to higher engine torque domains which increases impact on real driving. The authors have earlier assessed boosted HLC operation in a downsized two-litre engine, but it was found that HLC operation could not be achieved above 15 bar NMEP due to instability and knocking combustion. The observation led to the conclusion that there exists a lean load limit. Therefore, further experiments have been conducted in a single cylinder research DISI engine to increase understanding of high load lean operation. HLC is known to suppress end-gas autoignition (knock) by decreasing reactivity and temperatures, but during the experiments knock was observed to be prominent and increasing in severity when engine load was increased despite operating ultra-lean close to lambda 2. Knock is normally mitigated by spark retardation which decreases peak cylinder pressure. However, to maintain stable combustion at lambda = 2 the combustion phasing had to be kept close to TC which resulted in high peak cylinder pressures. Therefore, the combustion event had to be balanced in a window where early combustion promoted knock and late resulted in instability and partial burns. A tumble flap was introduced to increase in-cylinder tumble which reduced knock and improved combustion stability. It could be observed that for most load-points assessed; increased tumble could suppress knock and increase the air-dilution limit which proved beneficial in decreasing the NO<sub>x</sub> emissions. The highest engine load that could be achieved with highly diluted combustion was 16 bar NMEP.</div></div>

Demonstration of knock intensity mitigation through dielectric barrier discharge reformation in an RCEM

A knock intensity mitigation effect resulting from the application of dielectric barrier discharge (DBD) was experimentally demonstrated. The DBD was utilized to reform fuel–air premixtures. A rapid compression and expansion machine (RCEM) was used for the demonstration experiment. A rectangular combustion channel was installed in the RCEM's cylinder to observe flame propagation and end-gas auto-ignition behavior. The effect of the DBD was investigated by installing a plug-shaped DBD reactor in the combustion chamber. Part of the fuel-air mixture was reformed by the DBD and diffused in the chamber, and the combustion behavior was observed by a color and a monochrome high-speed camera with several different interference filters. In ordinary end-gas auto-ignition, a hot flame rapidly appears throughout the end-gas region, and generates strong pressure oscillation; whereas, in the present study, when the DBD was applied, the magnitude of the pressure oscillation decreased and a blue flame was generated in the end gas before full end-gas auto-ignition. The onset time of the blue flame, and the interval between the onset and the hot flame's appearance, depended on the fuel and initial temperature. The effect was investigated in the case of a primary reference fuel, surrogate gasoline, and n-butane lean mixture; however, though the magnitude of the effect varied, the mitigation effect was demonstrated for every fuel-air mixture. The proposed method is therefore expected to mitigate knocking in internal combustion engines and contribute to greater thermal efficiency.

Quantifying uncertainty in kinetic simulation of engine autoignition

Combustion chemistry models have been developed with inherent uncertainties in them. Whether a model that is developed using fundamental combustion experiments is capable to reproduce practical combustion processes within typical levels of measurement uncertainty is an open question. This paper quantifies the uncertainty of engine autoignition simulation using the uncertainties in the selected chemical kinetic model and then minimizes the model prediction uncertainty using various experiments. The method adopts a deterministic framework of uncertainty quantification, termed bound-to-bound data collaboration, and applies it to simulate the autoignition of n-pentane in a standard octane rating experiment. The results show that simulation of the end-gas autoignition using a comprehensively tested n-pentane model coupled with a two-zone engine combustion model yields an uncertainty substantially higher than that of engine experiment (as indicated by the cycle-to-cycle variation of the autoignition timing measurement). In-cylinder thermochemical conditions are found to be less important than the kinetic parameters in determining the model uncertainty. The large model uncertainty can be reduced by constraining the simulation with consistent experimental data and their measurement uncertainties, including those from fundamental experiments that measure ignition delays, species concentrations, flame speeds, and more significantly from autoignition experiments in well-calibrated engines.

Relationship of flame propagation and combustion mode transition of end-gas based on pressure wave in confined space

The main objective of this work is to identify the end-gas combustion mode transition under different initial thermodynamic conditions and to focus on the role of pressure waves in autoignition formation and detonation development in the confined space by a group of two-dimensional (2D) numerical simulations with detailed chemistry of H2/air mixture. Pressure waves with different strengths are obtained by the flame acceleration in closed chambers with and without obstacles under three different initial temperatures. The results indicate that with the increase of initial temperature, there exist three different end-gas autoignition transition modes: autoignition promoted by strong flame acceleration, autoignition suppressed by weak flame acceleration and autoignition independent of flame acceleration. It is also shown that there are three types of end-gas autoignition-induced detonation initiation: (1) detonation initiated directly by the pressure wave generated from the flame propagation; (2) detonation initiated directly by the pressure wave generated from other hot-spot autoignition; (3) autoignition to detonation transition based on the reactivity gradient theory. Meanwhile, to further identify the autoignition transition mode under continuous variation of pressure wave intensity and initial temperature, an idealized physical model, with the Mach number of the pressure wave as the link between the flame propagation duration and autoignition time of the end-gas, is proposed. It is shown that the autoignition can be suppressed by the elevated flame speed when the pressure wave is weak, while it cannot be prevented intrinsically when the temperature of the end-gas is high or the pressure wave is strong enough. Moreover, different autoignition propagation modes are identified from Bradley's diagram, including deflagration, developing detonation and thermal explosion, and the combined effects of reactivity and pressure wave strength are discussed as well.

Optical study on autoignition and knocking characteristics of dual-fuel engine under CI vs SI combustion modes

Uncontrollable autoignition with engine knock has been the main obstacle for high thermal efficiency and low CO2 emissions in downsized engines. Therefore, it is necessary to study the autoignition and knocking characteristics and reveal the similarities/differences between different combustion modes. In this study, synchronization measurement was performed through simultaneous pressure acquisition and high-speed natural flame acquisition, and knocking experiments were comparatively conducted under spark-ignition (SI) and compression-ignition (CI) conditions in a high-strength optical engine. The CI experimental results show that early ignited fuel injection leads to advanced combustion phase thus concentrated heat release. The advanced combustion phase can mitigate cyclic variation at low energy density while induce knocking combustion at high energy density. The stochastic analysis shows that CI knocking intensity mainly depends on the combustion rate whereas SI knock intensity is more random because of the stochastic end-gas autoignition. Visualized combustion images show that compared to SI combustion, the burning rate of CI combustion is much higher due to the multipoint autoignition. At low energy density, there is no pressure oscillation of CI and SI and the main reason is the low value of peak heat release rate (HRR) regardless of the autoignition. Whereas at high energy density, there are obvious two-stage HRRs for the CI knocking combustion and the high second peak HRR results in the engine knock. Further flame comparison shows that the AI flame speeds in CI and SI modes are similar but much higher than traditional SI flame speed.

Analytical studies on pressure wave characteristics in end-gas auto-ignition

Analytical studies on pressure wave characteristics in end-gas auto-ignition

Development of High-Accuracy End-Gas Autoignition Prediction Model (Third Report)

Development of High-Accuracy End-Gas Autoignition Prediction Model (Third Report)

The use of partial fuel stratification to enable stable ultra-lean deflagration-based Spark-Ignition engine operation with controlled end-gas autoignition of gasoline and E85

Lean operation of Spark-Ignition engines can provide higher thermal efficiency compared to standard stoichiometric operation. However, for a homogeneous lean mixture, the associated reduction of flame speeds becomes an important issue from the perspective of robust ignition and fast flame spread throughout the charge. This study is focused on the use of a lean partial fuel stratification strategy that can stabilize the deflagration, while sufficiently fast combustion is ensured via the use of end-gas autoignition. The engine has a spray-guided Direct-Injection Spark-Ignition combustion system and was fueled with either a high-octane certification gasoline or E85. Partial fuel stratification was achieved using several fuel injections during the intake stroke in combination with a small pilot-injection concurrent with the Spark-Ignition. The results reveal that partial fuel stratification enables very stable combustion, offering higher thermal efficiency for parts of the load range in comparison to well-mixed lean and stoichiometric combustion. The heat release and flame imaging demonstrate that the combustion often has three distinct stages. The combustion of the pilot-injected fuel, ignited by the normal spark, acts as a “super igniter,” ensuring a very repeatable initiation of combustion, and flame incandescence reveals locally rich conditions. The second stage is mainly composed of blue flame propagation in a well-mixed lean mixture. The third stage is the compression autoignition of a well-mixed and typically very lean end-gas. The end-gas autoignition is critical for achieving high combustion efficiency, high thermal efficiency, and stable combustion. Partial fuel stratification enables very effective combustion-phasing control, which is critical for controlling the occurrence and intensity of end-gas autoignition. Comparing the gasoline and E85 fuels, it is noted that achieving end-gas autoignition for the higher octane E85 requires a more aggressive compression of the end-gas via the use of a more advanced combustion phasing or higher intake-air temperature.

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>

On the Role of Nitric Oxide for the Knock-Mitigation Effectiveness of EGR in a DISI Engine Operated with Various Gasoline Fuels

<div class="section abstract"><div class="htmlview paragraph">The knock-suppression effectiveness of exhaust-gas recirculation (EGR) can vary between implementations that take EGR gases after the three-way catalyst and those that use pre-catalyst EGR gases. A main difference between pre-and post-catalyst EGR gases is the level of trace species like NO, UHC, CO and H<sub>2</sub>. To quantify the role of NO, this experiment-based study employs NO-seeding in the intake tract for select combinations of fuel types and compression ratios, using simulated post-catalyst EGR gases as the diluent. The four investigated gasoline fuels share a common RON of 98, but vary in octane sensitivity and composition. To enable probing effects of near-zero NO levels, a skip-firing operating strategy is developed whereby the residual gases, which contain trace species like NO, are purged from the combustion chamber.</div><div class="htmlview paragraph">Overall, the effects of NO-seeding on knock are consistent with the differences in knock limits for preand post-catalyst EGR gases. This suggests that for a majority of the conditions studied here, variations in NO concentration dominate the autoignition-influencing role of trace species that are present in pre-and post-catalyst EGR gases. The specific response of the knock limits to NO vary greatly between fuels, and the response also varies with compression ratio for a given fuel. Generally, the greatest sensitivity to NO exists in the 0 - 50 ppm range for all fuels, with increasing [NO] strongly promoting end-gas autoignition. For higher levels of NO, the autoignition-enhancing effect of NO saturates. This is particularly true for the Alkylate fuel with low octane sensitivity, for which addition of NO beyond 100 ppm reverses the trends, with further NO addition actually suppressing knock, although not beyond the level achieved without NO present.</div></div>

Effect of rapid combustion on engine performance and knocking characteristics under different spark strategy conditions

Rapid combustion is considered as an effective way to improve engine performance but there is lack of investigations about the effect of rapid combustion on engine knock. In this study, a highly-strengthened engine, equipped with double spark system, was chosen for investigating the influence of rapid combustion on engine performance and knocking characteristics. The results show that under synchronous double spark ignition condition, output power and effective thermal efficiency are improved because of shortened combustion duration. However, engine knock is more prone to occur and knock intensity is more sensitive to spark timing. Then, considering the ability of solving detailed chemical kinetics, an engine simulation software was chosen to simulate auto-ignition and knocking characteristics at different spark intervals and locations. The results show that the compressing and heating effects of two ignited flames are the main reasons for end-gas auto-ignition. Further investigation on asynchronous ignition shows that early heat release of end-gas mixture before auto-ignition can reduce maximum explosive pressure and knock intensity can be decreased by more than 50%. Besides, the results of different spark location show that additional spark is more effective in reducing knock intensity if the location is closed to the position where auto-ignition occurs.

Understanding the correlation between auto-ignition, heat release and knocking characteristics through optical engines with high compression ratio

Improving compression ratio is an effective way to meet higher thermal efficiency and lower CO2 emissions for internal combustion engines. However, the severe thermodynamic condition under low speed and high road may promote the possibility of auto-ignition and damage engine body if it is uncontrollable. Therefore, further study on the correlation between auto-ignition and knocking characteristics are necessary for modern engines with high compression ratio. In this study, combined with high-speed direct photography and simultaneous pressure acquisition, flame images and in-cylinder pressures characterizing auto-ignition, heat release rate and knocking combustion were studied based on optical engine experiments. The results show that auto-ignition occurs in all engine cycles under current critical knocking conditions, and fast SI flame speed cycles is more likely to facilitate end-gas auto-ignition. However, the auto-ignition flame speed is comparable to the SI flame and auto-ignition is not a sufficient condition for knock: for some cycles multipoint auto-ignitions occur near the SI flame front and result in two peak heat release rate, which is beneficial to engine thermal efficiency. Further analysis shows that there is no obvious pertinence between knock intensity and auto-ignition onset while knock intensity strongly depends on the peak HRR per unit volume. High peak HRR per unit volume always results in knocking combustion. The current study shall give insights into utilizing auto-ignition in high compression ratio SI engines.

Numerical Analysis of End-Gas Autoignition and Pressure Oscillation in a Downsized SI Engine Using Large Eddy Simulation

Knock and super-knock are abnormal combustion phenomena in engines, however, they are hard to study comprehensively through optical experimental methods due to their inherent destructive nature. In present work, the methodology of large eddy simulation (LES) coupled with G equations and a detailed mechanism of primary reference fuel (PRF) combustion is utilized to address the mechanisms of knock and super-knock phenomena in a downsized spark ignition gasoline engine. The knock and super-knock with pressure oscillation are qualitatively duplicated through present numerical models. As a result, the combustion and onset of autoignition is more likely to occur at top dead center (TDC), which causes end gas at a higher temperature and pressure. It is reasonable to conclude that the intensity of knock is not only proportional to the mass fraction of mixtures burned by the autoignition flame but the thermodynamics of the unburned end-gas mixture, and the effect of thermodynamics is more important. It also turns out that two auto-ignitions occur in conventional knock conditions, while only one auto-ignition takes place in super-knock conditions. However, the single autoignition couples with the pressure wave and they reinforce each other, which eventually evolves into detonation combustion. This work gives the valuable insights into knock phenomena in spark ignition gasoline engines.

Experimental investigation of the stochastic nature of end-gas autoignition with detonation development in confined combustion chamber

In the present work, end-gas autoignition formation, and the effects of oxygen concentration on the flame/shock waves propagation and pressure oscillation, are investigated in a self-designed constant-volume chamber equipped with a perforated plate. A hydrogen–oxygen–nitrogen mixture with adjustable oxygen to nitrogen ratio is chosen as the test fuel. In an oxygen-enriched condition, the probability of an end-gas autoignition occurrence increases significantly. End-gas autoignition with detonation development is further investigated, with a special emphasis on the stochasticity of the detonation development. In a low-oxygen condition, detonation occurs randomly owing to its stochastic physical behavior. However, when the oxygen concentration increases to 28%, the stochastic factors have a lower impact, and the detonation occurrence is certain. Nevertheless, the pressure and pressure oscillation in the autoignition exhibit random behaviors and are unrelated to the oxygen concentration. The variation tendency of the flame tip velocity remains constant under different oxygen concentrations. However, an increase in the oxygen concentration improves the flame tip velocity, thereby inducing stronger shock waves and promoting autoignition. Based on the start time of the autoignition, two types of autoignition modes were identified: Mode 1 and Mode 2. In Mode 2, the unburnt mixture experiences merely one compression by the shock wave before autoignition and only occurs under high oxygen concentrations of 27–28%. Under equal oxygen concentrations, the pressure and pressure oscillation of Mode 2 are higher than those of Mode 1 owing to the larger amount of unburnt mixture. Finally, the exhaust gas was introduced into the initial unburnt mixture to investigate the effect of an inert gas on the combustion.

CFD Analysis of Port Water Injection in a GDI Engine under Incipient Knock Conditions

This paper investigates, through computational fluid dynamics (CFD) simulations, the knock resistance improvements that can be obtained in a turbo-charged GDI engine with water injection. In a first step, water and gasoline injector models are validated comparing the results with experimental data from constant volume chamber tests. Then, multi-cycle simulations are performed using the G-equation turbulent combustion model focusing on spray evolution and wall film dynamics. The main intent is analyzing the effectiveness of different water injection timings and injection pressures in a port water injection (PWI) installation. Combustion rates are validated against experimental engine data, with and without water injection. Afterwards, in order to predict autoignition behavior with different spark advance (SA) timings, the extended coherent flamelet model (ECFM) combined with a tabulated kinetic ignition (TKI) dataset is used. End-gas autoignition delays are calculated using a reduced mechanism for toluene primary reference fuel (TPRF), which revealed essential for capturing actual gasoline ignition characteristics. Results indicate that the water atomization quality, i.e., injection pressure, is significant in a PWI installation allowing a reduction of the water wall film formation in the ports. Water injection timing needs also to be carefully chosen for optimized performance. As the injected water allows the SA to be increased, the overall benefits on indicated mean effective pressure and fuel consumption are quantified under the same knock safety margin, matching adequately well the available measurements.

Ignition characteristics of a temporally evolving n-heptane jet in an iso-octane/air stream under RCCI combustion-relevant conditions

The ignition characteristics of a temporally-evolving n-heptane jet in an iso-octane/air stream under reactivity controlled compression ignition (RCCI) combustion-relevant conditions are investigated using 2-D direct numerical simulations (DNSs) with a 116-species primary reference fuel (PRF)/air reduced mechanism. For the DNSs of RCCI combustion, iso-octane and n-heptane are chosen as two different fuels delivered by the port-fuel and direct-fuel injections, respectively. Therefore, the ignition characteristics of both fuels can be investigated by simulating the ignition of a temporally-evolving n-heptane jet with relative jet velocity, U0, within iso-octane/air charge. It is found that the first-stage ignition kernels governed by the low-temperature chemistry first develop primarily within the n-heptane jet near the mixing layer regardless of U0, and evolve into low-temperature flames, propagating into relatively fuel-rich mixtures in the n-heptane jet. High-temperature flames also develop first in the n-heptane jet, following the trajectories of low-temperature flames, and then, propagate towards both relatively fuel-lean mixtures of the iso-octane/air charge and fuel-rich mixtures of the n-heptane jet. They keep propagating into fuel-lean mixtures and finally end-gas auto-ignition occurs. It is also found that the first-stage ignition occurs more quickly with increasing U0 due to enhanced mixing between relatively cold n-heptane jet and hot iso-octane/air charge, and consequently, the second-stage ignition also advances in time with increasing U0, which are opposite to the results found in previous DNSs of RCCI combustion. Such ignition characteristics are more likely to prolong the overall combustion duration and reduce the peak of heat release rate with increasing U0. In addition, chemical explosive mode analysis (CEMA) identifies important variables and reactions for the low-, intermediate-, and high-temperature chemistries under such RCCI conditions.

End-gas autoignition characteristics of PREMIER combustion in a pilot fuel-ignited dual-fuel biogas engine

To improve the thermal efficiency of internal combustion engines at high loads, premixed mixture ignition in the end-gas region (PREMIER) combustion is proposed as a precursor to knocking. In the current work, a pilot fuel-ignited dual-fuel gas engine was operated at constant speed under different intake pressures (101, 150, and 200 kPa). A simulated biogas was served as the primary fuel and diesel as the pilot fuel. The maximum mean effective pressure and thermal efficiency were evident during PREMIER operation because of autoignition in the end-gas region. Similar to knocking combustion, PREMIER combustion features two stages but neither pressure oscillation nor a rapid pressure rise is observed. We here define a new parameter, the PREMIER intensity (PI), which reflects the strength of PREMIER combustion. As the injection timing was advanced and the pressure boosted, more cycles underwent end-gas autoignition; the associated heat release increased and, consequently, the PI value rose. When the pressure and temperature of a premixed fuel mixture rose as injection timing was advanced, end-gas autoignition commenced earlier. The end-gas autoignition delay became shorter as intake pressure was increased and injection timing advanced.

Mechanism of end-gas autoignition induced by flame-pressure interactions in confined space

The main objective of this work is to comprehensively provide a fundamental understanding of the entire process of the flame-pressure wave interactions with end-gas autoignition and detonation development in a confined chamber by two-dimensional numerical simulations with a stoichiometric hydrogen/air mixture. The flame dynamics, pressure wave propagation, and its structure evolution, together with the mechanism of autoignition and detonation development in the end gas, are analyzed in detail. Six stages, including spherical flame, finger flame, flame with its skirt touching the sidewalls, flame-pressure wave interactions, end-gas autoignition induced by the flame-pressure wave interactions, and detonation development, are observed for the flame development in the confined space. The results demonstrate that the flame-pressure wave multi-interactions result in violent oscillations of the flame shape and speed. Three stages of flame shape evolution during each interaction, backward propagation of the flame front, stretch of the flame front at the boundary layer, and formation of the tulip flame, are captured. A new mechanism in terms of combined effects of the viscous boundary layer and pressure waves is provided for the formation of the tulip flame. It is also found that the velocity distributions in the boundary layer show the trend of increase first and then decrease after the pressure waves pass the fields twice in the opposite directions. The autoignition occurrence and detonation initiation at different positions and different moments in the end-gas region are analyzed. It is indicated that the nonuniform temperature distribution induced by the reflections of pressure waves and the specific pressure wave structures can be responsible for this phenomenon.

Auto-ignition characteristics of end-gas in a rapid compression machine under super-knock conditions

Spark ignition induced super-knock was generated in a rapid compression machine using stoichiometric iso-octane/O2/N2 mixture. In addition to the pressure traces, the combustion processes were also recorded using high-speed photography, from which the characteristics of the end-gas auto-ignition were analyzed. The effect of negative temperature coefficient (NTC) on the end-gas auto-ignition was investigated using detailed and reduced reaction mechanisms. The end-gas auto-ignition of diluted methanol/O2/Ar mixture without NTC behavior was also tested for comparison. The results showed that the end-gas auto-ignition of iso-octane/O2/N2 mixture exhibited a two-stage ignition process. During the second ignition stage of the end-gas, two auto-ignition events with very short time interval were sequentially observed in the end-gas region. The first auto-ignition event generated a weak shock wave, and the second one initiated detonation. Both the two auto-ignition events occurred near the wall but at different sites. Due to the heat loss to the wall, the near-wall region is generally considered to be colder than the adiabatic core region, and thus the near-wall auto-ignition of end-gas was usually considered as a result of fuel's NTC behavior. However, in this study the chemical kinetic calculation showed that the evolution of the end-gas almost bypassed the NTC region in the ignition delay-temperature diagram. Furthermore, for the methanol/O2/Ar mixture the end-gas auto-ignition also started at the very near-wall region. Considering that methanol fuel does not has an NTC behavior, the near-wall auto-ignition of methanol/O2/Ar mixture should be a result of other factors than NTC. Therefore, it was concluded that NTC might not play a dominant role in the near-wall auto-ignition of the end-gas.