End-gas Region Research Articles

A numerical study on knock combustion suppression using targeted fuel injection in an SI engine

A novel concept to suppress knock combustion using the targeted fuel injection in a spark-ignition (SI) engine is proposed, which utilizes a customized injector to inject a small portion of the total fuel toward the end-gas region in the late compression stroke so that charge cooling effect by fuel vaporization is imposed on the local mixtures. CFD studies were carried out to evaluate the effects of various targeted fuel injection strategies on the processes of charge cooling, air-fuel mixing and combustion in a 1.2-L gasoline SI engine with a high compression ratio. A knock prediction model adopting a level set G-equation model coupled with a transported Livengood-Wu integral correlation was applied to predict the knock event by capturing the occurrence of end-gas auto-ignition. It was found that the local gas temperatures in the sprayed region decreased and knock combustion tendency was suppressed indeed, and the degree of the reduction in local gas temperature and knock tendency was affected by the selected targeted injection strategy. However, the results also indicated that excessive fuels injected could result in worsen air-fuel mixing that led to incomplete combustion. Hence, the injection strategy needs to be optimized by balancing the knock tendency and combustion efficiency. The study further shown that the spark timing could be advanced by 3.3° crank angle in a particular injection strategy case by comparing with the non-targeted-injection case, which demonstrated the effectiveness of the proposed concept.

Experimental and numerical study of split injection strategy on balancing the ultra-lean burn and high compression ratio engine design

In this study, the aim of split injection is to strike a balance between the ultra-lean burn mode and high compression ratio method by experimented and numericized. Under ultra-lean condition, no matter single injection mode or split injection mode, both knock percentage and combustion stability does not monotonically decrease with the retarded injection timing. Considering the combustion stability and knocking, first injection timing of 300 oCA BTDC and second injection timing of 190 oCA BTDC is choose as optimized strategy, which achieves knock frequency of 16 % and COVIMEP of 3.69 %. Based on optimized injection strategy, the indicated specific fuel consumption achieves 184.53 g/KWh. With the split injection, a deeper ultra-lean burn can be achieved without the severe knock and combustion fluctuation and obtain ultra-low NOx emissions of 2.8 ppm. The numericized result also demonstrates that because of the ultra-lean conditions, the flame grows more slowly, and then the temperature of the un-burn zone rises significantly as a result of the primary flame stream. Meanwhile, the propagation speed to the exhaust valve side appears to be substantially slower than in other directions. As a result, auto-ignition can be seen near the exhaust valve. When using split injection, the flame kernels do not propagate as fast and deep as in the case of single injection because of the cooling effect. This drops temperature in the unburned end-gas region and thus reduces knocking tendency.

Effects of non-thermal termolecular reactions on detonation development in hydrogen (H[formula omitted])/methane (CH[formula omitted]) - air mixtures

The binary fuel blend of H2/CH4 is one of the most promising hydrogen-enriched hydrocarbon fuels in spark-ignition (SI) engines. Yet, the undesirable phenomenon of super-knock, which can severely and instantaneously damage an SI engine, limits its widespread adoption. Moreover, there is still a lack of consensus on the precise mechanism by which this phenomenon occurs i.e. via flame acceleration or spontaneous ignition, despite numerous previous investigations. At the same time, recent studies [M. P. Burke, S. J. Klippenstein, Nat. Chem. 9 (2017) 1078 - 1082, Y. Tao, A. W. Jasper, Y. Georgievskii, S. J. Klippenstein, R. Sivaramakrishnan, Proc. Combust. Inst. 38 (2021) 515–522] have demonstrated a high probability of occurrence of non-thermal reactions in premixed flames of such H2/CH4 fuel blends with air due to the presence of non-trivial amounts of highly reactive radicals including H, O and OH apart from O2. The present study focuses on the evolution of an initial deflagration front to a detonation wave in H2/CH4-air mixtures under SI engine relevant conditions through fully resolved, constant volume 1D simulations with and without non-thermal reactivity. Non-thermal reactions were included in the macroscopic kinetics model as chemically termolecular reactions facilitated by the H + CH3 and H + OH radical-radical recombination and the H + O2 radical-molecule association reactions. The nonthermal reactions result in a corresponding decrease in the reaction fluxes of the incipient recombination/association reactions. Therefore, an additional set of simulations were performed by applying corrections to the respective incipient recombination/association rate constants using the methodology demonstrated by Tao et al. [Y. Tao, A. W. Jasper, Y. Georgievskii, S. J. Klippenstein, R. Sivaramakrishnan, Proc. Combust. Inst. 38 (2021) 515–522]. Compared to the baseline case, the onset of spontaneous ignition in the end-gas region was observed to be delayed in the presence of non-thermal termolecular reactions. Concurrently, the developing detonation was observed to be significantly stronger. In contrast, applying corrections to the recombination/association rate constants resulted in a completely different behavior. Specifically, detonation was observed to occur due to self acceleration of the primary flame in the absence of spontaneous ignition in the end-gas region. Sensitivity analysis was performed to quantify the effects of non-thermal reactions on the duration of heat release rate and thereby the mechanism of detonation formation. In addition, chemical explosive mode analysis (CEMA) was performed to identify the dominant species/reactions responsible for the observed results.

Effects of charge motion on knocking combustion under boosted high load condition of a medium-duty gasoline engine

Due to higher power density and strong swirl flow field, knock tendency is more severe in medium duty (MD) gasoline engines derived from diesel engine platforms. How to design the charge motion reasonably, so as to increase the thermal efficiency and suppress knock simultaneously needs to be further discussed in combination with the actual combustion boundary conditions. The investigations in this study focus on the charge motion roles on knock and thermal efficiency under a high load/low speed operating condition using numerical simulations. The results show that the mechanisms of inducing knocking combustion and the effects of tumble ratio (TR) on knock intensity (KI) are different under swirl, inclined swirl and tumble motion conditions. With strong swirl configurations, weaker turbulence intensity at the bottom of the combustion chamber reduces the convective heat transfer between the fresh charge and hot exhaust gas, resulting in an obvious high temperature region, and inducing the first hot spot. Increasing TR can effectively suppress knock by shortening the residence time of end gas. With inclined swirl and strong tumble configurations, the position of hot spots is determined by the asymmetrical shape of the flame front, which is induced by large-scale tumble motion around the spark plug close to TDC. The end gas reactivity is higher and the temperature distribution in the end gas region is more uniform with the increase of TR, resulting in a sharp increase in auto-ignition velocity and thus KI. Due to the suppression of knock, reduction of heat losses as well as promotion of flame speed, an optimized inclined swirl motion based on the flat cylinder head is more appropriate for MD gasoline engines.

Effect of active ignition on the end-gas autoignition with detonation combustion in confined space

In spark-ignition engines, the presence of knock significantly prevents the engines from further improving thermal efficiency. In this work, the effect of active ignition in the end-gas region on the end-gas autoignition with detonation combustion was optically observed in the self-designed constant volume combustion bomb (CVCB-TJU). The different combustion modes together with pressure oscillation were presented. The results indicate that advancing the ignition timing of the end-gas spark can reduce the pressure oscillation intensity and even suppress the occurrence of autoignition. In addition, the present work observed a statistically positive relationship between unburned mass fraction (UMF) and detonation intensity for the knock induced by developing detonation. The underlying influencing mechanism of end-gas ignition on auto-ignition was presented. The end-gas ignition influences the occurrence and intensity of detonation and pressure oscillation by two means: 1) reducing the UMF burned by autoignition, 2) influencing the flame acceleration, thereby reducing the flame tip velocity, and the subsequent shock intensity. Furthermore, the burned mass fraction (BMF) consumed by end-gas flame when jet flames emerge is applied to substitute the ignition timing when measuring its influence at different oxygen concentrations of 29% and 33%. It is found there exists a critical BMF above which the autoignition would be suppressed. And this critical value increases with increased oxygen concentration.

Effects of intake air conditions and micro-pilot (MP) injection timing on micro-pilot dual fuel (MPDF) combustion characteristics in a single cylinder optical engine

The effects of intake air conditions and micro-pilot injection timing on the characteristics of micro-pilot dual fuel combustion were examined to enhance the combustion stability in marine engines. A minimum amount of diesel fuel was injected as the ignition source, and most of the fuel energy was obtained from methane gas. First, the intake air flow rate was varied. In addition, the methane gas flow rate and micro-pilot injection timing were changed depending on the intake air flow rate. Second, the micro-pilot injection timing was varied under intake air temperatures of 35 °C and 55 °C. The intake air and methane gas flow rates were maintained. Results indicated that increasing the intake air and methane gas flow rates promoted autoignition in the end-gas region. Nevertheless, the standard deviation of the peak cylinder pressure was within 3.1 bar. Under the intake air temperature of 35 °C, advancing and retarding the micro-pilot injection led to misfiring, which increased the standard deviation of the peak cylinder pressure to 10.2 bar and 9.6 bar, respectively. Knocking combustion was occurred when the micro-pilot injection was retarded under the intake air temperature of 55 °C. This phenomenon increased the standard deviation of the peak cylinder pressure to 6.9 bar. Premixed ignition in the end-gas region combustion corresponded to the optimal engine operating conditions for marine engines because of the low combustion variation even under high cylinder pressures of approximately 170 bar to 220 bar. However, high NOx emissions occurred under these conditions.

PREMIER combustion characteristics of a pilot fuel-ignited dual-fuel biogas engine with consideration of cycle-to-cycle variations

Biogas is a promising alternative to fossil fuels for internal combustion (IC) engines. The purpose of this study is to investigate the effect of PREmixed Mixture Ignition in the End-gas Region (PREMIER) combustion on biogas engine combustion. As the main component of biogas is methane (CH4), methane slip is of concern; improvement of combustion efficiency is important and PREMIER combustion may be useful to this end. Experiments were carried out using a dual-fuel gas engine under supercharged conditions, with an intake pressure of 200 kPa. Simulated biogas served as the primary fuel and diesel as the pilot fuel. The pilot fuel injection timing was varied. Cycle-to-cycle variations were considered to improve the accuracy of temperature estimations at the time of end-gas autoignition. As the addition ratio of carbon dioxide (CO2) to CH4 in the intake increased, the temperature at end-gas autoignition increased under the micro-pilot injection timing before knocking. The total energy input was reduced as the CO2 proportion increased. The fuel mass in the end-gas region decreased and the intensity of end-gas autoignition was reduced. The results indicated that knocking is unlikely even when micro-pilot injection timing is advanced when the CO2 proportion is high. As the injection timing advanced, combustion efficiency was improved. As the CO2 proportion increased, the combustion efficiency under the micro-pilot injection timing conditions before knocking was approximately 81 % for all conditions. This resulted in a high indicated thermal efficiency of approximately 42 % even when the CO2 proportion increased.

Visualization of end-gas auto-ignition during PREMIER combustion in a dual-fuel gas engine

The purpose of this study is to visualize PREMIER combustion phenomenon in a micro-pilot diesel fuel ignited dual-fuel engine with differentiating the causes of knocking and PREMIER combustion. The auto-ignition characteristics in the end-gas region were investigated with combustion visualization and in-cylinder pressure analysis using a compression-expansion machine (CEM) fueled with a natural gas. The micro-pilot diesel fuel injection timing used to generate PREMIER combustion occasionally induced knocking and pressure oscillations, while normal and PREMIER combustion did not cause the pressure to oscillate. The auto-ignition in the end-gas region of both knocking and PREMIER combustion was found to cause a second rise in pressure before the propagation premixed flame reached the cylinder wall. As the auto-ignition in PREMIER combustion was later than in knocking, the area and mass of the end-gas region were smaller. The KI factors, which represented the intensity of the pressure oscillations, were higher in the case of knocking than in the case of PREMIER combustion. As the knock intensity increases in CEM, the dominant frequencies of 10 kHz and 14.5 kHz are generated in addition to the dominant frequency of 6.4 kHz. The auto-ignited flame area spread faster during knocking than during PREMIER combustion. Knocking caused a sudden pressure difference and imbalance between the flame propagation and end-gas regions, followed by pressure oscillations.

Performance, emissions and end-gas autoignition characteristics of PREMIER combustion in a pilot fuel-ignited dual-fuel biogas engine with various CO2 ratios

Biogas, a renewable and alternative energy, has recently gained attention as an environmentally friendly fuel for power generation in internal combustion (IC) engines. This study aimed to investigate the effect of the carbon dioxide (CO2) to methane (CH4) ratio in a dual-fuel gas engine. It is known that gaseous fuel combustion at high loads is accompanied by knocking if an appropriate combustion controlling strategy is not applied. Therefore, premixed mixture ignition in the end-gas region (PREMIER) combustion was proposed under high load conditions. Experiments were carried out using a dual-fuel gas engine under supercharged conditions, with an intake pressure of 200 kPa. Simulated biogas consisting of CH4 and CO2 was used as a primary fuel and diesel was used as a pilot fuel. The pilot fuel injection timing was varied during the experiments. The indicated mean effective pressure (IMEP) and thermal efficiency increased as injection timing was advanced. With the addition of CO2, the thermal efficiency of PREMIER combustion was slightly increased, although the IMEP was slightly decreased. The unburned gas temperature for PREMIER combustion increases as a function of the CO2 content of the mixture. Due to compression by flame propagation, high unburned gas temperatures in dual-fuel gas engines are important for achieving autoignition inside the end-gas region under PREMIER combustion conditions.

Role of low-temperature oxidation in non-uniform end-gas autoignition and strong pressure wave generation

This study highlights the importance of heat release rate in low-temperature oxidation (LTO) on non-uniform end-gas autoignition and strong pressure wave generation, which are substantially relevant to knocking combustion. The simulations are conducted using the compressible Navier–Stokes equations with detailed transport and chemical kinetics models in a one-dimensional constant-volume reactor. Four fuel/air stoichiometric mixtures, n-butane, i-octane, n-heptane, and dimethyl ether (DME)/air mixtures, are simulated. The results show that larger knocking intensities are produced with n-heptane and DME in their negative temperature coefficient (NTC) regimes because of the stronger non-uniformity of end-gas autoignition. The non-uniformity of end-gas autoignition is enhanced by a pressure wave disturbance that is caused by the rapid temperature rise of the end-gas region in LTO. In particular, the high heat release rate with the DME/air mixture generates a distinct pressure wave disturbance in the reactor, which considerably enhances the non-uniformity of end-gas autoignition through the reflection of the wave at the wall. In contrast, the heat release rate in the n-heptane case is milder than that in the DME case, and therefore, the knocking intensity in the n-heptane case is smaller compared to that of DME due to less enhancement of the non-uniform end-gas autoignition. No large knocking intensities are produced with n-butane and i-octane, which have weak NTC, because of the absence of a temperature rise in LTO. Thus, this study concludes that the high heat release rate in LTO and the generated pressure wave disturbance play a significant role in the generation of large knocking intensities through the enhancement of non-uniform end-gas autoignition.

Split diesel injection effect on knocking of natural gas/diesel dual-fuel engine at high load conditions

Advancing the start of diesel injection timing is an effective way to enhance thermal efficiency and reduce greenhouse gas (GHG) emissions of natural gas/diesel dual-fuel (NDDF) engine. However, severe thermodynamic conditions under high engine load conditions may increase the propensity for engine knocking when advancing the start of diesel injection (SODI). In this study, the strategy of split diesel injection (two-pulse injection) is used and its feasibility as a method to reduce knocking intensity and improve thermal efficiency of NDDF engine is investigated. The results reveal that advancing single diesel injection timing significantly increases knocking intensity, whereas split diesel injection strategy decreases knocking intensity. The results also show that, when using split diesel injection, the flame kernels do not propagate as fast and deep as in the case of single diesel injection. This slows down the pressure and temperature rise rate in the unburned end-gas region and thus reduces knocking tendency. Moreover, the early partially burning of the premixed natural gas – air mixture in the squish region dilutes the unburned end-gas and consequently makes it resistant to auto-ignition. NDDF engine with split diesel injection can reach a maximum thermal efficiency that is comparable to that observed under knocking conditions of single diesel injection. Using either, single or split, diesel injection strategy reduces GHG emissions of NDDF engine (up to 12%) compared to its counterpart diesel engine. However, the lowest GHG emissions of NDDF engine with single diesel injection strategy is recorded under knocking conditions.

Direct Numerical Simulation of Flame Propagation and Deflagration to Detonation Transition in Confined Space with Different Perforated Plate Positions

ABSTRACT Flame acceleration and the deflagration to detonation transition (DDT) are important physical phenomena in combustion theories. The main objective of this work is to provide a comprehensive understanding of the effect of the positions of obstacles when a flame propagates in obstructed confined channels, using two-dimensional direct numerical simulations (DNSs) with the detailed chemical kinetics mechanism of H2. The underlying mechanism for the flame propagation and detonation, affected by the obstacles with the flame-shockwave and flame–vortex interactions, is investigated. In 1000 K cases, two different detonation modes associated with flame propagation are observed, where for the case with obstacles near the ignition kernel, autoignition takes place in the end-gas region as predicted, while for the case with obstacles far away from the ignition kernel, autoignition occurs, instead, in front of the flame and triggers a DDT immediately. Both of two modes consist three processes: flame propagation stages, flame-pressure wave/shockwave interaction, and detonation development. The evolutions of centerline temperature and pressure are employed to explicitly demonstrate the detonation process. The differences in the shockwave formation, peak temperature, and pressure values can be clearly observed. The mechanisms of the different detonation modes induced by different obstacle positions are analyzed in detail for the first time. The results indicate that the compressed and preheated temperature of the unburned mixture determines the detonation combustion mode. The conclusion is further proved in the flame-shockwave number variation and detonation modes transition when initial temperature is changed. This study will provide new insight into the DDT and detonation phenomena in engines.

Effects of the injection timing on knock and combustion characteristics in dual-fuel dual-injection engines

To utilize ethanol fuel in spark ignition engines more efficiently and flexibly, a new ethanol/gasoline dual-direct injection concept in gasoline engine is proposed. Therefore, based on the dual-fuel dual-direct injection system, the effects of different injection timings and two injector positions on the characteristics of combustion were studied comprehensively, and the effects of different octane numbers and temperature stratifications on knock and combustion were explored. The results show that as for Position A (ethanol injecting toward spark plug), with the delay of injection timing, knock tendency and its intensity increase initially and then decrease due to the comprehensive effect of ethanol evaporation and fuel stratification; on the contrary, for Position B (ethanol injecting toward end-gas region), retarding the injection timing of ethanol can effectively reduce the knock propensity. As for the engine performance, a dual-direct injection performs best, especially the retarded injection timing of ethanol for Position A. It can be found that with the delay of the fuel injection timing, the torque first increases and then decreases. The brake-specific fuel consumption decreases initially and then increases at maximum brake torque spark timing.

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.

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.

End-gas autoignition behaviors under pressure wave disturbance

The present study addresses effects of pressure wave disturbance on end-gas autoignition behavior and subsequent pressure wave development, which are of particular interest for understanding a fundamental process of knocking combustion in spark-assisted ignition engines. Such pressure wave disturbance is initiated by a compression wave generated from spark-like ignition and continuously exists in a combustion chamber. The investigation is conducted through a direct numerical simulation with a 1-D constant volume reactor, where the compressible Navier–Stokes equations are solved with detailed reaction mechanisms of n−C7H16 and n−C4H10/air mixtures. The width of an ignition kernel parametrically changes in order to control the strength of an initial compression wave and thus pressure wave disturbance. The result showed that stronger compression waves with larger kernel widths enhance the progress of the autoignition process at the wall through the transient increase in pressure and temperature due to the wave reflection. Consequently, stronger pressure waves such as a developing detonation are generated with the enhanced inhomogeneous end-gas autoignition starting from the wall. In contrast, weak pressure wave disturbance produces no strong pressure waves due to the lower inhomogeneity of autoignition timing in an end-gas region. An analysis with temperature variation in an end-gas region indicates that cool flame generation helps maintain the superiority of the wall for earlier autoignition through the heat release at an earlier stage. Therefore, a stronger pressure wave associated with inhomogeneous end-gas autoignition is likely generated around 650 K in the case of an n−C7H16/air mixture, which corresponds to a distinct peak generation of knocking intensity in the negative temperature coefficient regime. For lower temperature conditions such as 500 K, the end-gas autoignition first takes place in a region away from the wall. This is because the initially induced temperature inhomogeneity in the end-gas region is considerably weakened due to wave interactions and dissipation effects during longer ignition delay times. Thus, multiple end-gas points including the wall can become a preferred point for earlier end-gas autoignition, resulting in a variation of end-gas autoignition locations. The result for an n−C4H10/air mixture is finally shown and the difference from that for the n−C7H16/air mixture is highlighted.

Differences between PREMIER combustion in a natural gas spark-ignition engine and knocking with pressure oscillations

PREMIER (PREmixed Mixture Ignition in the End-gas Region) combustion occurs with auto-ignition in the end-gas region when the main combustion flame propagation is nearly finished. Auto-ignition is triggered by the increases in pressure and temperature induced by the main combustion flame. Similarly to engine knocking, heat is released in two stages when engines undergo this type of combustion. This pattern of heat release does not occur during normal combustion. However, engine knocking induces pressure oscillations that cause fatal damage to engines, whereas PREMIER combustion does not. The purpose of this study was to elucidate PREMIER combustion in natural gas spark-ignition engines, and differentiate the causes of knocking and PREMIER combustion. We applied combustion visualization and in-cylinder pressure analysis using a compression–expansion machine (CEM) to investigate the auto-ignition characteristics in the end-gas region of a natural gas spark-ignition engine. We occasionally observed knocking accompanied by pressure oscillations under the spark timings and initial gas conditions used to generate PREMIER combustion. No pressure oscillations were observed during normal and PREMIER combustion. Auto-ignition in the end-gas region was found to induce a secondary increase in pressure before the combustion flame reached the cylinder wall, during both knocking and PREMIER combustion. The auto-ignited flame area spread faster during knocking than during PREMIER combustion. This caused a sudden pressure difference and imbalance between the flame propagation region and the end-gas region, followed by a pressure oscillation.

The Effects of Equivalence Ratio on Pressure Wave Development during Knocking Combustion

A knocking combustion in a one-dimensional constant volume reactor has been simulated with a detailed chemical kinetic mechanism of n-heptane premixed gases by using the compressible Navier-Stokes equations. This study focuses on the impact of the various equivalence ratios (0.6–1.4) in the pressure wave development during knocking combustion in an account of different initial temperature (600–900K). The result demonstrates the autoignition process in the end-gas region and explains the knocking phenomenon on different equivalence ratio. In an adiabatic wall condition, the largest knocking intensity occurs in equivalence ratio 1.4 whereas the lowest knocking intensity found in equivalence ratio 0.6. Regarding to initial temperatures, a strong peak of knocking is generated around 650K in all the equivalence ratios (0.8–1.4). However, yet a small perceptivity of knocking is found in around 750K for each equivalence ratio. In case of smaller equivalence ratios, a weak knocking occurs, which can be identified by the behavior of the pressure wave generation in the end-gas region.