Preflame Reactions Research Articles

Optical study on the auto-ignition and knocking behaviors of ammonia/PODE RCCI mode

Employing high-reactivity fuels such as polyoxymethylene dimethyl ethers (PODE) are effective ways to improve ammonia-based engine performance. However, abnormal combustion (i.e., knocking combustion) may be encountered at reactivity-controlled compression ignition (RCCI) mode. In this study, the auto-ignition and knocking behaviors of ammonia/PODE RCCI mode were studied in a highly-strengthened optical engine at different injection timing conditions, with synchronization measurement of in-cylinder pressure and high-speed photography. The port-injection of ammonia and direct injection of PODE were employed at an energy ratio of 80/20. Results shows that knock intensity is significantly increased with the advancement of injection timings under elevated thermodynamic conditions, accompanied by steep pressure peaks and pressure oscillations. Combustion visualization elucidates the relationship between auto-ignition (AI) progress and knock intensity. Pressure oscillations under knocking combustion are closely correlated to the interplay between different AI flame fronts. Further analysis identifies two distinct combustion characteristics, manifested in the most AI locations and the initial AI flame speed. Numerical results indicate that different AI behaviors are primarily determined by the degree of inhibition in PODE pre-flame reactions by ammonia dehydrogenation, which affects AI kernel development. The slow initial auto-ignition flame results in the formation of a sufficiently active premixed mixture in the unburned region before the initial AI flame front arrives. Once the second AI flame occurs, the active premixed mixture undergoes rapid combustion, leading to strong pressure waves.

Investigation of the combustion process of activated fuel in an automotive diesel engine

BACKGROUND: Intensive development of the transport sphere and the extension of the fleet of tractors and cars, as well as the tightening of standards and requirements in the field of exhaust gas emissions have resulted in increased requirements for the combustion process. This problem can be solved by improving the working process in the engine by preheating the fuel to 300 C in the high pressure fuel supply system. Under external thermal influence, the conditions of mixture formation are improved, an increase in the rate of preflame reactions and a positive change in the dynamics of the combustion process are observed. When potential energy of hydrocarbon molecules increases in the activated complex system, energy is redistributed among active molecules. Along with this, the rate coefficient of a chemical reaction increases due to the concentration of foci of hydrocarbon molecules that have reached the energy barrier.
 The object of research is the process of diesel fuel combustion at preliminary external influence on it.
 AIMS: The aim is to study the combustion process of a motor-tractor diesel engine upon fuel activation. Development of a scheme of individual phases of the activated diesel fuel combustion of depending on the temperature at heating.
 METHODS: Theoretical study and analysis of the combustion process of thermally preprepared fuel for an automotive diesel engine. Theoretical determination of the dependence of the rate of formation and concentration of toxic components and dispersed particles on the rate constant of direct and reverse chemical reactions, as well as on the activation temperature of fuel at emission in exhaust gases.
 RESULTS: The actual coefficient of molecular change of the working mixture increases when diesel fuel is activated. The rate of formation of total nitrogen oxides and hydrocarbons increases by 1.79% and 3.66%. The proportion of unburned carbon in the combustion process on activated fuel varies within (24) %.
 CONCLUSIONS: Theoretically, a scheme for changing individual phases of the activated fuel combustion process by time and temperature has been developed. Fuel heating accelerates the pre-flame preparation in the liquid phase, shortens the duration of the rapid gorenje phase. The concentration of toxic indicators depends on the constant of the chemical reaction rate and the activation temperature of the fuel.

Numerical investigation on combustion processes of an aircraft piston engine fueled with aviation kerosene and gasoline

Replacing gasoline with aviation kerosene for spark-ignition (SI) piston engines applied for aircrafts can get better security and easier logistics, however, this action cannot proceed smoothly by serious knocking combustion. In this study, the in-cylinder combustion for gasoline and aviation kerosene are investigated through computed-fluid-dynamics (CFD) to analyze combustion characteristics, and try to use pre-chamber technology to improve combustion issues.All CFD results demonstrate in-cylinder combustion differences effectively: All injected aviation kerosene cannot be completely evaporated, leading to non-uniformed unburned mixture and slow flame propagation. Some odd test phenomena are caused by unevaporated kerosene. The slower turbulence flame speed, worse equivalence ratio, higher local temperature and lower spontaneous combustion temperature lead to mixture auto-ignition and knocking combustion eventually. After adding a passive pre-chamber, the mixture distribution near spark plugs is more reasonable, higher turbulence kinetic energy (TKE) inside pre-chamber accelerates burning process, the ignition delay period is reduced by about 21.6%. The pre-chamber increases main-chamber's TKE, the whole combustion process is shortened by about 9%, the pre-flame reaction of unburned mixture near cylinder is shortened, contributing to suppress knocking combustion effectively. The passive pre-chamber technology is of great significance to the popularization and application for SI aircraft piston engine fueled with aviation kerosene.

Thermoacoustic instability analysis of a laminar lean premixed flame under autoignitive conditions

To address engineering challenges encountered in developing advanced gas turbine combustors, theoretical analysis and numerical simulation were performed to investigate the thermoacoustic instability of a laminar lean premixed flame under autoignitive conditions. The analysis was carried out within the scope of weak autoignition tendencies and employed a three-step mechanism, which included the chain-initiation reaction. The numerical simulation was performed as a validation using a skeletal mechanism for diesel fuel under the typical operating conditions of gas turbines. The results showed that the three-step mechanism could qualitatively describe the high-temperature chemistry involved in autoignition and flame propagation. The analysis indicated that the flame propagation speed was accelerated by the accumulation of pre-flame reactions. Under certain conditions, the flame speed showed a linear correlation with the pre-flame length. The results were consistent with the existing literature. In addition, the equilibrium position of the flame oscillation, which was excited by velocity fluctuation, could return to the initial position, and pre-flame reactions could alter the phase delay. These results suggested a new mechanism for thermoacoustic instability because the overall heat release rate fluctuated with the flame front. The flame might amplify, damp, high-pass filter, or low-pass filter the acoustic energy based on the phase difference between the velocity and pressure oscillations.

Combustion and Emission Characteristics of a Diesel Engine Operating with Varying Equivalence Ratio and Compression Ratio - A CFD Simulation

The performance of diesel engine highly depends on atomization, vaporization and mixing of fuel with air. These factors are strongly influenced by various parameters e.g. injection pressure, injection timing, compression ratio, equivalence ratio, cylinder geometry, in cylinder air motion etc. In this study, a diesel engine has been investigated by employing a commercial CFD software (ANSYS Forte, version 18.1) especially developed for internal combustion engines (ICE) modeling; focusing primarily on the effects of equivalence ratio and compression ratio on combustion and emission characteristics. RNG k-ε model was employed as the turbulence model for analyzing the physical phenomena involved in the change of kinetic energy. In order to reduce the computational cost and time, a sector mesh of 45o angle with periodic boundary conditions applied at the periodic faces of the sector, is considered instead of using the whole engine geometry. Simulations are performed for a range of equivalence ratio varying from 0.6 to 1.2 and for three compression ratios namely, 15:1, 18:1 and 21:1. Results show that, improvement in combustion characteristics with higher compression ratio could be achieved for both lean and rich mixtures. Peak in-cylinder pressure and peak heat release nearer to TDC are achieved for compression ratio of 18:1 that could results in more engine torque. For compression ratio beyond 16:1, effects of fuel concentration on ignition delay is more pronounced. At lower compression ratio, in-cylinder temperature is not sufficiently high for atomization, vaporization, mixing of fuel with air, and preflame reactions to occur immediately after the fuel injection. NOx emission in diesel engine increases due to higher pressure and temperature inside the cylinder associated with relatively higher compression ratio. Rich mixture leads to more CO and unburnt hydrocarbon emission compared to lean mixture as result of incomplete combustion. Engine operation with too high compression ratio is detrimental as emission is a major concern.

Особенности рабочего процесса поршневого двигателя при работе на керосине

For aircraft in light multi-purpose aviation, piston engines are considered more efficient than gas turbine. The main technical requirement for such engines is to ensure trouble-free operation with the best possible fuel efficiency. At the same time, there are no requirements to emission of harmful substances in exhaust fumes except for the absence of visible smoke. When developing multi-purpose aircraft piston engines, it is important to ensure their multi-fuel operation, including opera-tion on TS-1 kerosene and diesel fuel. But the issues associated with setting engine control algo-rithms for operation on TS-1 kerosene are practically unexplored. In order to refine the control algo-rithms, the flow of the working process using such fuel was studied in this work. The effect of se-quencing the working process stages on the formation of the ignition delay period was shown. Based on the analysis of the factors affecting the ignition delay period, a map of the fuel injection advance angle values was generated. According to the experimental data, the activation energy of pre-flame reactions was adopted, which for kerosene TS-1 was 23–28 kJ/mol.

A new model for predicting antiknock quality of hydrocarbon fuel blends

A procedure for predicting the Critical Compression Ratio (CCR) and the Octane Number (ON) of hydrocarbon fuel blends is presented in this work. Compositional data as well as antiknock characteristics and thermo-physical properties of the constituent hydrocarbons are taken as input parameters for this calculation. The proposed methodology was developed by considering single-step kinetics for the pre-flame reactions taking place within the combustion chamber of the CFR engine, which is the standard apparatus used in ON evaluation tests. Furthermore, a thermodynamic model was used to describe the relevant processes the in-cylinder fuel-air mixture undergoes prior to the occurrence of knock. In order to validate the proposed method, CCRs of Primary Reference Fuel (PRF) mixtures; Motor ONs (MONs) of paraffinic fuels; and Research ONs (RONs) of Liquefied Petroleum Gas (LPG) fuels were computed and compared with experimental data reported in literature. The maximal absolute error and the root mean square deviation found were, respectively, 0.084 and 0.047 for the CCR of PRF mixtures; 1.6 and 0.60 for the MONs of paraffinic fuels; and 1.45 and 0.70 for the RONs of LPG fuels.

Effect of Catalytic Coatings on the Performance, Emission and Combustion Characteristics of Spark Ignition Engines

This study discusses the effect of copper, nickel and chromium coating on the spark ignition engine performance, emission and combustion characteristics. The maximum brake thermal efficiency for copper coated engine is about 5% higher than standard engine at full load and about 4% higher than the standard engine at 2800 rpm. Nitrogen oxides emission for catalytic coated engine is 7% to 20% higher than standard engine at full load. It was observed that carbon monoxide and carbon dioxide emissions of standard engine were higher than catalytic coated engines at all loads. Copper coated engine has the lowest hydro carbon emission. Catalytic coated engines have 6% to 12% higher cyclinder pressure when compared to the standard engine. The crank angle of heat release values and combustion parameters indicate that a faster heat release occured for catalyst coated engines. Similarly combustion duration of standard engine is higher than that of catalytic coated engines. Catalytic coatings increase the pre-flame reactions which lead to better and faster combustion.

20506 ノッキング時における低温酸化反応の分光学的解析(エネルギー・環境問題に対応するエンジン・燃料技術(1),OS.11 エネルギー・環境問題に対応するエンジン・燃料技術)

An increase of the compression ratio is enumerated as one of the methods of improving thermal efficiency in the internal combustion engine. However, when the compression ratio is only increased, the knocking is generated. With the aim of understanding a knocking better, we paid attention to preflame reactions before the autoignition. And we tested it with spectroscopic measurement. We measured a wavelength of the light emission of 306.4nm (characteristic spectrum of OH) and 395.2nm (HCHO). And we measured a wavelength of the light absorption of 293.1 nm (HCHO). The results revealed that the dependence of the time of autoignition on the fuel octane number varies depending on differences in the state of flame progression under a cool flame condition.

0711 火花点火機関におけるノッキング現象の分光学的解析(S42 エンジン燃焼の基礎,S42 エンジン燃焼の基礎)

An increase of the compression ratio is enumerated as one of the methods of improving thermal efficiency in the internal combustion engine. However, when the compression ratio is only increased, the knocking is generated. So, a knocking phenomenon is a problem for the thermal efficiency improvement. With the aim of understanding a knocking better, we paid attention to preflame reactions before the autoignition. And we tested it with spectroscopic measurement. And we measured a wavelength of the light emission of 306.4nm (an unusual range of OH) and 395.2nm (HCHO). By this experiment, we report the influence of preflame reactions.

4720 分光学的手法による火花点火機関のノッキング現象の解析(S56-4 エンジン燃焼-さらなる進化を目指して-(4),S56 エンジン燃焼-さらなる進化を目指して-)

A knocking is the big factor which prevents improvement in an internal combustion engine's thermal efficiency. With the aim of understanding knocking better, light emission spectroscopy was applied in this study to examine preflame reactions that can be observed prior to autoignition. Light emission intensity was measured at wavelengths of 306.4nm (characteristic spectrum of OH), 395.2nm (HCHO). In this research, it measured about the generating phenomenon of the preflame reactions in different engine speed and octane number.

3105 ノッキング現象における前炎反応発生挙動の実験的研究(G07-1 燃焼制御(1),G07 エンジンシステム)

A knocking is the big factor which prevents improvement in an internal combustion engine's thermal efficiency. With the aim of understanding knocking better, light emission spectroscopy was applied in this study to examine preflame reactions that can be observed prior to autoignition. Light emission intensity was measured at wavelengths of 306.4nm (characteristic spectrum of OH), 395.2nm (HCHO). Phenomena corresponding to the passage of cool flames and blue flames and behavior indicative of a negative temperature coefficient region were observed in relation to variation of the combustion chamber inner wall temperature, ignition timing and engine speed.

407 火花点火機関を用いた自己着火が前炎反応発光挙動に及ぼす影響の解析(GS-4 燃焼(1))

407 火花点火機関を用いた自己着火が前炎反応発光挙動に及ぼす影響の解析(GS-4 燃焼(1))

発光法を用いた火花点火機関でのノッキング時における前炎反応区間の解析(エンジン・システムの高性能化技術)

Knocking is one phenomenon that can be cited as a factor impeding efforts to improve the efficiency of spark-ignition engines. With the aim of understanding knocking better, light emission spectroscopy was applied in this study to examine preflame reactions that can be observed prior to autoignition. Light emission intensity was measured at wavelengths of 306.4 nm (characteristic spectrum of OH), 329.8 nm (HCO), 395.2 nm (HCHO). As a result, preflame reaction was observed, and the preflame reaction interval of HCHO showed a different tendency to that of HCO.

353 4サイクルエンジンにおける点火時期変化が前炎反応発光挙動に及ぼす影響の研究(応用熱工学(燃焼・熱機関))

353 4サイクルエンジンにおける点火時期変化が前炎反応発光挙動に及ぼす影響の研究(応用熱工学(燃焼・熱機関))

Advanced/Retarded Criterion on Ignition of Fuel/Air Mixtures with Formaldehyde Doping(HCCI, Effect of Fuel and Additives)

Formaldehyde artificially added into the mixtures is efficacious as an ignition control medium for hydrocarbon fuels in engine cylinders. The vapor added into the mixtures has given the premixed compression-ignition (HCCI) engine a stable ignition timing, which is controllable to get the MET by the amount of formaldehyde to be added. The effect of formaldehyde addition is either suppressing or promoting. A universal criterion of the formaldehyde effect resulting whether in the advanced or in the retarded hot-flame appearance from the original ignition events has been elucidated through experiments over a wide range of parameters on the fuel octane or butane rating, the equivalence ratio of the mixture, and the mixture temperature to be raised. The formaldehyde would be a suppressing additive under the cool-flame generating constituents, temperature and pressure conditions, and a promoting additive under the poor cool-flame generating conditions in the cylinder charge during the preflame reactions leading to the ignition. The added formaldehyde is superfluous to the ignition event originally belonging to the cool-flame dominant regime, which would give a suppressing effect. On the other hand, the artificially added formaldehyde could be a starting point of preflame reactions leading to the final hot ignition belonging to the blue-flame-dominant regime, where a few fuel transition and low intermediates appear, and where the high temperature chemistry dominates. It allows a short cut of the preflame induction period, in consequence, a promoting effect for the ignition. The low-temperature-ignition classification; three typical regime, "cool-flame dominant regime", "negative-temperature-coefficient regime" and "blue-flame dominant regime" would be a universal advanced/retarded criterion on ignition of fuel/air mixtures with formaldehyde doping for the ignition timing control in the piston compression engines.

分光学的手法を用いた点火時期変化による自己着火への影響の解析(機関特性と後処理,内燃機関の燃焼化学と排出物制御)

This study attempted to elucidate combustion conditions in a progression from normal combustion to knocking by analyzing the light emission intensity that occurred during this transition. With the aim of understanding the combustion states involved, the light emission intensity was measured at two positions-the center zone and the end zone of the combustion chamber. Light emission spectroscopy was applied to examine preflame reactions that are observed prior to autoignition in the combustion process of hydrocarbon fuels.This experiment reports on the result of the light emission behavior at various ignition timings.

Low NOx Combustion for Liquid Fuels: Atmospheric Pressure Experiments Using a Staged Prevaporizer-Premixer

Low emissions of NOx are obtained for a wide range of liquid fuels by using a staged prevaporizing-premixing injector. The injector relies on two stages of air temperature and fires into a laboratory jet-stirred reactor operated at atmospheric pressure and nominal ϕ of 0.6. The liquid fuels burned are methanol, normal alkanes from pentane to hexadecane, benzene, toluene, two grades of light naphtha, and four grades of No. 2 diesel fuel. Additionally, natural gas, ethane, and industrial propane are burned. For experiments conducted for 1790 K combustion temperature and 2.3±0.1 ms combustion residence time, the NOx (adjusted to 15% O2 dry) varies from a low of 3.5 ppmv for methanol to a high of 11.5 ppmv for No. 2 diesel fuel. For the most part, the NOx and CO are positively correlated with the fuel carbon to hydrogen ratio (C/H). Chemical kinetic modeling suggests the increase in NOx with C/H ratio is caused in significant part by the increasing superequilibrium concentrations of O-atom created by the increasing levels of CO burning in the jet-stirred reactor. Fuel bound nitrogen also contributes NOx for the burning of the diesel fuel. This paper describes the staged prevaporizing-premixing injector, the examination of the injector, and the NOx and CO measurements and their interpretation. Optical measurements, using beams of He-Ne laser radiation passed across the outlet stream of the injector, indicate complete vaporization and a small variation in the cross-stream averaged fuel/air ratio. The latter is determined by measuring the standard deviation and mean of the transmission of the laser beam passed through the stream. Additional measurements and inspections indicate no pressure oscillations within the injector and no gum and carbon deposition. Thus, the NOx and CO measurements are obtained for fully vaporized, well premixed conditions devoid of preflame reactions within the injector.

1918 イオン電流と発光強度の測定によるノック発生挙動の実験的研究

This study attempted to elucidate combustion conditions in a progression from normal combustion to knocking by analyzing the ion current and light emission intensity that occurred during this transition. With the aim of understanding the combustion states involved, the ion current was measured at two positions-the center zone and the end zone of the combustion chamber. Light emission spectroscopy was applied to examine preflame reactions that are observed prior to autoignition in the combustion process of hydrocarbon fuels. The results obtained by analyzing the experimental data made clear the relationship between the ion current and light emission during the transition from normal combustion to knocking operation.

Studies of Temperature Elevation Due to the Pre-flame Reaction in a Spark-ignition Engine with CARS Temperature Measurements Using Fuels of Various Octane Numbers.

The unburned end-gas temperatures in a combustion chamber of a conventional 4-cylinder DOHC spark-ignition engine were measured using the broadband CARS temperature measurement technique. The test engine was fueled with primary reference fuel 80 and gasoline with research octane numbers of 70.9, 83.4, 91.5 and 100.4. The measured CARS temperatures were compared with the adiabatic core temperatures calculated from the measured pressures. Significant heating by pre-flame reaction in the end gas zone was observed in the late part of compression stroke under both knocking and non-knocking conditions. The measured CARS temperatures when the cylinder pressures were above 1400kPa were higher than the calculated adiabatic core temperatures. These results indicate that some exothermic reactions exist in relatively low pressure and temperature regions. The CARS temperatures began to be higher than the adiabatic core temperature when the end-gas temperatures reached 700K. The temperature elevation due to the pre-flame reaction correlated well with the unburned gas CARS temperature for different research octane number fuels tested.