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Visualization and Spectroscopic Measurement of Knocking Combustion Accompanied by Cylinder Pressure Oscillations in an HCCI Engine
Abstract
Combustion experiments were conducted with an optically accessible engine that allowed the entire bore area to be visualized for the purpose of making clear the characteristics that induce extremely rapid HCCI combustion and knocking accompanied by cylinder pressure oscillations. The HCCI combustion regime was investigated in detail by high-speed in-cylinder visualization of autoignition and combustion and emission spectroscopic measurements. The results revealed that increasing the equivalence ratio and advancing the ignition timing caused the maximum pressure rise rate and knocking intensity to increase. In moderate HCCI combustion, the autoignited flame was initially dispersed temporally and spatially in the cylinder and then gradually spread throughout the entire cylinder. In contrast to that behavior, in extremely rapid HCCI combustion, the autoignited flame was dispersed in the cylinder in its initial stage, but the remaining unburned end gas rapidly autoignited at a certain point. That gave rise to knocking combustion accompanied by cylinder pressure oscillations.
... Knocking combustion is in result of rapid burning of mixture that originates from multiple locations inside the combustion chamber simultaneously and results in higher heat releases rates (Blumreiter and Edwards 2014;Iijima et al. 2013Iijima et al. , 2016Wang et al. 2014;Bhaduri et al. 2017). In presence of knocking combustion, in-cylinder pressure oscillates during combustion and results in vibration and high level of noise and leads to excessive load on piston surface and other components and finally causes to structural damages (Sheppard et al. 2002). ...
... Hence, combustion duration is long enough and burning process is smooth. But knocking combustion in these engines is due to rapid combustion that occurs in the different regions of the combustion chamber, simultaneously (Iijima et al. 2013). The rapid combustion cause local pressure growth in addition to the general pressure generated from combustion process. ...
The aim of current study is mathematical modeling of the knocking combustion in HCCI engines. A thermodynamic multi zone model (MZM) is used to simulate HCCI engine combustion. Ringing intensity parameter is used to distinguish the start of knock phenomenon and then acoustic wave equation is applied to the MZM for computation of pressure waves that are the main cause of rough sound production and engine vibration during knock occurrence. In order to compute the in-cylinder total pressure, pressure waves are calculated for each zone separately, which are added to the general in-cylinder pressure calculated using MZM. Intensity of pressure amplitudes depends on the general in-cylinder pressure and pressure rise rate. Pressure differences produced in the cylinder during the knocking combustion are predictable so that the maximum pressure amplitude for each zone is individual and the central zones have the highest amplitudes. The results show that the MZM can predict knocking combustion in HCCI engines respectively. Knocking combustion results in higher reaction rates and higher concentration of in-cylinder radicals.
... Optical studies have demonstrated that if in-cylinder thermal states are maintained at relatively mild levels, by e.g. managing in-cylinder flows, end gas AI without high frequency pressure fluctuations typical of knocking can take place (Iijima et al. 2013), which is the basis of spark-assisted CAI engines. In fact, LTHR can also have a knock inhibiting effect if it is staged appropriately to exploit the temperature drop induced by NTC reactions (Splitter et al. 2019). ...
... At present, there is still a big emission problem in the combustion process of the traditional diesel engine. During the combustion process, the emission of NOX and PM can not be effectively suppressed at the same time, because NOX is mostly generated under high temperature and oxygen enrichment conditions, but the generation of NOX is suppressed by reducing the combustion and emission temperature, which is not good to the oxidation reaction of PM [3]. Therefore, although this method can suppress NOX production, it makes carbon smoke emissions increase instead [4]. ...
Under the pressure of energy and environmental problems, peoples demand for high-efficiency and low-pollution power sources is becoming more and more urgent. In this case, the HCCI combustion engine was developed. A new combustion method combining premixed combustion gas and low-temperature combustion was developed: it relies on a uniform formed mixture by premixed combustion gas and the lower cylinder temperature when combustion happened to reduce PM and NOX emissions simultaneously. The HCCI combustion adopts high compression ratio ignition and multi-point combustion in the cylinder, which makes its lean mixture have high thermal efficiency, and the engine performance can reach a better condition.The research and development goal of HCCI technology is to surpass compression ignition and spark ignition engines in performance and emissions. Because HCCI engine has the advantages of the first two, and its emission control system only needs to rely on its own operating characteristics and emission characteristics to reduce pollutant emissions.So if HCCI combustion can be truly mature and commercialized on a large scale, it will be a major innovation in the development history of internal combustion engines.
div class="section abstract"> Homogeneous Charge Compression Ignition (HCCI) engines have attracted much attention and are being widely researched as engines characterized by low emissions and high efficiency. However, one issue of HCCI engines is their limited operating range because of the occurrence of rapid combustion at high loads and misfiring at low loads. It is known that knocking accompanied by in-cylinder pressure oscillations also occurs in HCCI engines at high loads, similar to knocking seen in spark-ignition engines. In this study, HCCI combustion accompanied by in-cylinder pressure oscillations was visualized by taking high-speed photographs of the entire bore area. In addition, the influence of internal exhaust gas circulation (EGR) on HCCI knocking was also investigated. The visualized combustion images revealed that rapid autoignition occurred in the end-gas region during the latter half of the HCCI combustion process when accompanied by in-cylinder pressure oscillations. It was also found that applying internal EGR had the effect of moderating combustion by delaying the ignition timing, thereby suppressing in-cylinder pressure oscillations.
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The knock combustion and pollutant emission of heavy-duty diesel engines at low temperatures are still unclear, especially under different injection timings. Therefore, this study illustrates the above issues through CONVERGE simulation. The results show that with the start of injection (SOI) sweeps from −7°CA to −32°CA, a large amount of liquid-phase fuel adheres to the wall, and the wet-wall ratio of fuel at SOI = −32°CA is as high as nearly 30%. The fuel film evaporates slowly, coupled with the effect of low temperature on chemical reactions, the high-temperature ignition (HTI) is delayed seriously until the end of injection. The amount of premixed mixture formed during long ignition delay is significantly increased, but its uniformity is better and the concentration is more suitable for ignition. Once HTI is triggered, high-frequency and strong pressure oscillation occurs in the cylinder, and the maximum oscillation amplitude is as high as nearly 10 MPa, far exceeding the threshold of destructive knock combustion. Delayed fuel injection can effectively alleviate the above problems, such as the best when the SOI in this study is −17°CA. In addition, HC emissions are positively correlated with the amount of fuel film, but the trend of CO quantity with injection timing shows the opposite result. NOx emission increases as the injection timing advances, while soot is the opposite, because the mixture concentration is leaner at the earlier SOI and the expanded high-temperature region leads to an accelerated oxidation rate of soot.
Pressure oscillation often occurs in high-load homogeneous charge compression ignition (HCCI) combustion, which is a challenge in the development of HCCI engines for automobiles. This work proposes a novel method of reducing the pressure oscillation in HCCI combustion at high loads. The proposed technique injects air into homogeneous mixtures before compression, thereby giving local fuel concentration gradient. The fuel concentration gradient is expected to suppress a rapid pressure rise, resulting in reduced pressure oscillation. High-load HCCI combustion was simulated via a rapid compression machine with a high compression ratio. Varying the period from air injection to compression, that is, the waiting time, controlled the magnitude of fuel concentration gradient. The pressure oscillation was quantified and evaluated via the knock intensity (KI) and the averaged pressure rise rate. For the short waiting time; in other words, when the local fuel concentration gradient was large, the KI was very lower than that for no air injection. The KI, however, increased with the waiting time to approach that for no air injection. The oscillation modes were also different with and without air injection according to a modal analysis. The in-cylinder temperature distribution was visualized via the infrared radiometry to better understand the effect of air injection. For no air injection, the temperature in the cylinder uniformly increased, and the whole mixtures were ignited instantaneously. With air injection and for the short waiting time, on the other hand, hot spots developed on the rim of the injected air where the specific heat ratio was higher and then gradually spread throughout the chamber. Therefore, retarded auto-ignition and subsequently slow spread would limit a rapid pressure rise, resulting in reduced pressure oscillation in HCCI combustion. In conclusion, the proposed technique is effective for reducing the pressure oscillation in high-load HCCI combustion only for the short waiting time.
Preliminary studies have been done in a framework of the “Symbolic Project of College of Science and Technology, Nihon University” on the new technologies potentially capable of improving energy conversion efficiency. New concepts for thermal engine, solar and nuclear energy utilization, and for their associated numerical simulation were proposed. Through the basic studies, their feasibility and impact on the future practice have been examined. The low-temperature combustion engine, HCCI (Homogeneous Charge Compression Ignition) engine, Gasoline Sabathe-cycle engine, SOFC jet-engine are evaluated as the new engine concept. For the utilization of solar heat, chemical pumping and Rankine cycle based system are examined. A catalytic nuclear fusion concept are also evaluated. The highlights of these individual studies are presented. In addition to the diversity of the studies, synergy between combustion technology and plasma technology have been promoted. This brought a novel plasma-assisted HCCI engine concept and its potential are also introduced.
This study investigated the effect of streamer discharge on autoignition and combustion in a Homogeneous Charge Compression Ignition (HCCI) engine. A continuous streamer discharge was generated in the center of the combustion chamber of a 2-stroke optically accessible engine that allowed visualization of the entire bore area. The experimental results showed that the flame was initiated and grew from the vicinity of the electrode under the application of a streamer discharge. Subsequently, rapid autoignition (HCCI combustion) occurred in the unburned mixture in the end zone, thus indicating that HCCI combustion was accomplished assisted by the streamer discharge. In other word, ignition timing of HCCI combustion was advanced after the streamer discharging process, and the initiation behavior of the combustion flame was made clear under that condition.
This study investigated the origin of knocking combustion accompanied by pressure wave and strong pressure oscillations in a Homogeneous Charge Compression Ignition (HCCI) engine. Experiments were conducted with a two-stroke single cylinder optically accessible engine that allowed the entire bore area to be visualized. The test fuel used was n-heptane. The equivalence ratio and intake temperature were varied to induce a transition from moderate HCCI combustion to extremely rapid HCCI combustion accompanied by in-cylinder pressure oscillations. Local autoignition and pressure wave behavior under each set of operating conditions were investigated in detail on the basis of high-speed in-cylinder visualization and in-cylinder pressure analysis. As a result, under conditions where strong knocking occurs, a brilliant flame originates from the burned gas side in the process where the locally occurring autoignition gradually spreads to multiple locations. The flame proceeds at high speed toward the unburned end gas, which burns in a short interval of less than 1 crank angle degree. The autoignited flame at that time appears to propagate at high speed from the burned gas side to the unburned end gas side.
Crank angle resolved imaging in the UV-visible spectral range was used to investigate flame front characteristics during normal combustion, surface ignition and light knock conditions. 'Line of sight' measurements provided information on local wrinkling: the evaluation was based on a statistical approach, with multiple frames taken at the same crank angle during consecutive cycles. This allowed the results during normal combustion to be representative for the specific operational conditions and to a good degree independent from the effects of cyclic variation. Abnormal combustion on the other hand, was investigated on a cycle-to-cycle basis, given the stochastic nature of such phenomena. The experimental trials were performed at fixed engine speed on an optically accessible direct injection spark ignition (DISI) engine equipped with the cylinder head of a four cylinder 16-valves commercial power unit. The absolute intake pressure was fixed at 1.5 bar and the air-fuel mixture was set close to stoichiometric. Commercial gasoline with an octane rating of 95 RON was used for all investigated conditions. Different flame contour characteristics were identified in different combustion phases, suggesting that the influence of local fluid dynamics is exerted differently as the fuel oxidation progresses from the central kernel initiated by the spark towards the cylinder walls. Surface ignition and auto-ignition were found to occur near the outer walls of the combustion chamber and the main flame front was found to be influenced to a relatively reduced degree by these secondary ignition sites.
Knocking combustion experiments were conducted in this study using a test engine that allowed the entire bore area to be visualized. The purpose was to make clear the detailed characteristics of knocking combustion that occurs accompanied by cylinder pressure oscillations when a Homogeneous Charge Compression Ignition (HCCI) engine is operated at high loads. Knocking combustion was intentionally induced by varying the main combustion period and engine speed. Under such conditions, knocking in HCCI combustion was investigated in detail on the basis of cylinder pressure analysis, high-speed photography of the combustion flame and spectroscopic measurement of flame light emissions. The results revealed that locally occurring autoignition took place rapidly at multiple locations in the cylinder when knocking combustion occurred. In that process, the unburned end gas subsequently underwent even more rapid autoignition, giving rise to cylinder pressure oscillations. In addition, when the engine speed and main combustion period were varied, it was found that the intensity of the cylinder pressure oscillations, i.e., knocking intensity PKI, correlated strongly with the maximum pressure rise rate per unit time dP/dtmax [MPa/msec]. The frequency characteristics of the knocking were the same even when the engine speed was varied. The primary vibration mode was the (1,0) mode in which pressure waves traverse the cylinder in the bore direction. Under operating conditions causing strong pressure oscillations, pressure oscillations at higher frequencies were observed.
In this study, optical measurements were made of the combustion chamber gas during operation of a Homogeneous Charge Compression Ignition (HCCI) engine in order to obtain a better understanding of the ignition and combustion characteristics. The principal issues of HCCI engines are to control the ignition timing and to optimize the combustion state following ignition. Autoignition in HCCI engines is strongly influenced by the complex low-temperature oxidation reaction process, alternatively referred to as the cool flame reaction or negative temperature coefficient (NTC) region. Accordingly, a good understanding of this low-temperature oxidation reaction process is indispensable to ignition timing control. In the experiments, spectroscopic measurement methods were applied to investigate the reaction behavior in the process leading to autoignition. Two types of spectroscopic measurements were obtained: measurement of spontaneous light emission from the flame in the combustion chamber and measurement of light absorption. The former measurement facilitated detection of light emission from chemical species produced by the reactions; the latter method facilitated measurement of the production and consumption behavior of the chemical species even when no light emission occurred. Following ignition, combustion (hot flame) proceeds rapidly under a high load, giving rise to knocking and the production of nitrogen oxides (NOx) by the high-temperature gas. Engine operation under a low load is restricted by incomplete combustion and misfiring due to the lower temperature. Therefore, the characteristics of the hot flame following ignition were investigated by measuring the light emission spectrum of the flame and by visualizing the conditions in the cylinder. Using these methods, the light emission spectrum was measured during operation of the experimental HCCI engine. The results made clear the light emission spectrum of the hot flame following ignition was made clear. The main emission was found to be a continuous spectrum from 300 nm to 500 nm, presumably attributable to the recombination reaction of carbon monoxide and oxygen atoms and known as CO-O glow. The results also revealed that the time of light emission in a wavelength region of approximately 300-500 nm, corresponding to the CO-O glow, coincided well with the time of heat release. This implies that spectroscopic measurement of light emission in the wavelength region corresponding to the CO-O glow can facilitate detection of local heat release in the cylinder. In-cylinder visualization was used to obtain two-dimensional flame images across the entire cylinder bore area in order to investigate the relationship between the local state of hot flame development and the rapidity of combustion. The results revealed that the time for the occurrence of autoignition showed little local difference under conditions conducive to rapid combustion; autoignition occurred almost simultaneously throughout the unburned region and the mixture burned all at once.
In order to extend the HCCI high load operational limit, the effects of the distributions of temperature and fuel concentration on pressure rise rate (dP/dθ) were investigated through theoretical and experimental methods. The Blow- Down Super Charge (BDSC) and the EGR guide parts are employed simultaneously to enhance thermal stratification inside the cylinder. And also, to control the distribution of fuel concentration, direct fuel injection system was used. As a first step, the effect of spatial temperature distribution on maximum pressure rise rate(dP/dθmax) was investigated. The influence of the EGR guide parts on the temperature distribution was investigated using 3-D numerical simulation. Simulation results showed that the temperature difference between high temperature zone and low temperature zone increased by using EGR guide parts together with the BDSC system. Experiments were conducted by using a four-cylinder gasoline engine equipped with the BDSC with EGR guide system to investigate the effect of the EGR guide on the heat release rate and the in-cylinder pressure rise rate. Experimental results showed that 50% and 90% mass fraction burned timing (CA50 and CA90) were delayed and combustion duration became longer when the EGR guide was used to enhance thermal stratification. As a result, the maximum pressure rise rate could be decreased and the HCCI high load limit successfully extended. Meanwhile10% mass fraction burned timing (CA10) was not affected by the thermal stratification generated by the EGR guide. This is probably because the fuel is also spatially stratified such that the fuel concentration becomes lean in the high temperature zone. Next, the effect of the fuel distribution on high load HCCI operation was investigated. Numerical analysis using a multi-zone combustion model considering detailed chemical reactions was carried out. The simulation results showed that the maximum pressure rise rate was decreased by 27% when the fuel distribution was uniform with temperature distribution generated by the BDSC with EGR guide system. Then, to obtain a uniform fuel distribution while keeping the temperature distribution generated by the BDSC with EGR guide system, a direct injection system was employed and the effect of fuel direct injection on maximum pressure rise rate was experimentally investigated. As a result, if 20% of the total fuel was injected directly into the cylinder during the exhaust stroke, the spatial distribution of the fuel concentration (G/F; fuel mass ratio to the total mass of in-cylinder mixture) became more homogeneous and maximum pressure rise rate was decreased by 10%. And also, 20% of the total fuel directly injected during the compression stroke, maximum pressure rise rate was decreased by 20%. Finally, a simple method to predict the ignition timing using Livengood-Wu integral and 1-D numerical simulation code was examined. It was found that the proposed method can predict the ignition timing with small deviation around ±2 degrees.