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Internal combustion engines have been required to achieve even higher efficiency in recent years in order to address environmental concerns. However, knock induced by abnormal combustion in spark-ignition engines has impeded efforts to attain higher efficiency. Knock characteristics during abnormal combustion were investigated in this study by in-cylinder visualization and spectroscopic measurements using a four-stroke air-cooled single-cylinder engine. The results revealed that knock intensity and the manner in which the autoignited flame propagated in the end gas differed depending on the engine speed.

In-cylinder visualization of the entire bore area at an identical frame rate was used to investigate knocking conditions under spark ignition (SI) combustion and under Homogeneous Charge Compression Ignition (HCCI) combustion in the same test engine. A frequency analysis was also conducted on the measured pressure signals. The results revealed that a combustion regime accompanied by strong pressure oscillations occurred in both the SI and HCCI modes, which was presumably caused by rapid autoignition with attendant brilliant light emission that took place near the cylinder wall. It was found that the knocking timing was the dominant factor of this combustion regime accompanied by cylinder pressure oscillations in both the SI and HCCI combustion modes.

One issue of downsized and supercharged engines is low-speed pre-ignition (LSPI) that occurs in the low-speed and high-load operating region. One proposed cause of LSPI is the influence of the engine oil and its additives. However, the effect of engine oil additives on pre-ignition and the mechanism involved are still not fully understood. This study investigated the influence of engine oil additives on abnormal combustion in a spark-ignition engine. A four-stroke air-cooled single-cylinder engine with a side valve arrangement was used in conducting combustion experiments. The research methods used were in-cylinder pressure analysis, in-cylinder visualization and absorption spectroscopic analysis. Engine oil additives were mixed individually at a fixed concentration into a primary reference fuel with an octane number of 50 and their effect on knocking was investigated.

The purpose of this study is to elucidate flame propagation behavior of homogeneous propane-air mixture under application of non-uniform electric field. A needle-shaped electrode was attached to the ceiling and a plate electrode was set at bottom of combustion chamber, so that the electric field was applied in the direction of the chamber's vertical axis. A homogeneous propane-air mixture was supplied at equivalence ratio of 1.0 and was ignited by leaser induced breakdown under atmospheric pressure and room temperature. It was found that the flame front and plate electrode were repelled each other and a thin air layer was formed between the flame and plate electrode when a relatively low positive DC non-uniform electric field was applied to the needle-shaped electrode. It might be thought that the induced current was generated in the flame front, so that the flame front and plate electrode repelled each other.

Unresolved issues of Homogeneous Charge Compression Ignition (HCCI) combustion include an extremely rapid pressure rise on the high load side and resultant knocking. Studies conducted to date have examined ways of expanding the region of stable HCCI combustion on the high load side such as by applying supercharging or recirculating exhaust gas (EGR). However, the effect of applying EGR gas to supercharged HCCI combustion and the mechanisms involved are not fully understood. In this study, the effect of EGR gas components on HCCI combustion was investigated by conducting experiments in which external EGR gas was applied to supercharged HCCI combustion and also experiments in which nitrogen (N2) and carbon dioxide (CO2) were individually injected into the intake air pipe to simulate EGR gas components. In addition, HCCI combustion reactions were analyzed by conducting chemical kinetic simulations under the same conditions as those of the experiments.

This study clarified the influence of hot gas jet on combustion enhancement effect for lean mixture in the plasma jet ignition. The hot gas jet was generated by the high temperature plasma and was ejected from igniter after plasma jet finished issuing. In combustion tests, propane-air mixture at equivalence ratio of 0.6 was used and the mixture was filled in the combustion chamber at atmosphere pressure and room temperature. For generation of the hot gas jet, the standard air was filled in chamber at same conditions and the hot gas jet was visualized by schlieren method in the absence of combustion. The combustion development processes were also visualized and the combustion pressure was measured. The discharge voltage, discharge current and the plasma luminescence were also measured. The plasma luminescence disappeared within 0.05 ms for any experimental conditions. When cavity depth was deep and orifice diameter was small, the maximum plasma luminescence height was short.

Homogenous Charge Compression Ignition (HCCI) combustion experiments were conducted in this study using a single-cylinder test engine fitted with a sapphire observation window to facilitate visualization of the entire cylinder bore area. In addition to in-cylinder visualization of combustion, spectroscopic measurements were made of light emission and absorption in the combustion chamber to investigate autoignition behavior in detail. Engine firing experiments were conducted to visualize HCCI combustion over a wide range of compression ratios from 12:1 to 22:1. The results showed that increasing the compression ratio advanced the ignition timing and increased the maximum pressure rise rate, making it necessary to moderate combustion. It was also found that autoignition can be induced even in a mixture lean enough to cause misfiring by raising the intake air temperature so as to advance the overall combustion process.

This study examined Homogeneous Charge Compression Ignition (HCCI) combustion characteristics in detail on the basis of in-cylinder combustion visualization, spectroscopic measurements of light emission and absorption and chemical kinetic simulations. Special attention was focused on investigating and comparing the effects of the fuel octane number and residual gas on combustion characteristics. The results made clear the relationship between the production/consumption of formaldehyde (HCHO) in the HCCI autoignition process and flame development behavior in the cylinder. Additionally, it was found that both the fuel octane number and residual gas have the effect of moderating low-temperature oxidation reactions. Furthermore, it was observed that residual gas has the effect of shifting the temperature for the occurrence of the hot flame to a higher temperature range.

The flame propagation behavior of homogeneous hydrogen-air mixture under application of high-voltage uniform or non-uniform electric field was explored by using combustion vessel. When a uniform electric field was applied, two plate electrodes were attached to ceiling and bottom of combustion chamber and, to apply a non-uniform electric field, an electrode in ceiling was needle-shaped and an electrode in bottom was plate-shaped. The positive or negative polarity DC high voltage was applied for an electrode in ceiling. When a positive polarity non-uniform electric field was applied to the mixture at any equivalence ratios and the input voltage was higher than 12 kV, the flame propagation was enhanced in the downward direction. This is because the corona wind was generated from the tip of needle-shaped electrode to grounded electrode by the brush corona.

This study examined the effects of fuel composition and intake pressure on two-stage high temperature heat release characteristics of a Homogeneous Charge Compression Ignition (HCCI) engine. Light emission and absorption spectroscopic measurement techniques were used to investigate the combustion behavior in detail. Chemical kinetic simulations were also conducted to analyze the reaction mechanisms in detail. Blended fuels of dimethyl ether (DME) and methane were used in the experiments. It was found that the use of such fuel blends together with a suitable intake air flow rate corresponding to the total injected heat value gave rise to two-stage heat release behavior of the hot flame, which had the effect of moderating combustion. The results of the spectroscopic measurements and the chemical kinetic simulations revealed that the main reaction of the first stage of the hot flame heat release was one that produced CO from HCHO.

This study was conducted to investigate the influence of low-temperature reactions on the Homogeneous Charge Compression Ignition (HCCI) combustion process. Specifically, an investigation was made of the effect of the residual gas condition on low-temperature reactions, autoignition and the subsequent state of combustion following ignition. Light emission and absorption spectroscopic measurements were made in the combustion chamber in order to investigate low-temperature reactions in detail. In addition, chemical kinetic simulations were performed to validate the experimental results and to analyze the elemental reaction process. The results made clear the formation behavior of the chemical species produced during low-temperature HCCI reactions.

The composition ratio of saturated and unsaturated fatty acid methyl esters (FAME) is depended on feedstock. Three FAMEs: soybean (SME), palm (PME) and coconut oil (CME) methyl esters were mixed to make fuels which have different composition ratio. The ignitability of fuel which mainly consisted of unsaturated FAME was inferior. Power was slightly reduced with increasing of mixing ratio of CME; however exhaust gas emissions were improved because CME contained a lot of oxygen atoms. Fuel which was equal mixture SME and CME indicated almost the same ignition characteristic as that of PME because they have same composition ratio.

In this study, light emission and absorption spectroscopic measurement techniques were used to investigate the Homogeneous Charge Compression Ignition (HCCI) combustion process in detail, about which there have been many unclear points heretofore. The results made clear the formation behavior and wavelength bands of the chemical species produced during low-temperature reactions. Specifically, with a low level of residual gas, a light emission band was observed from a cool flame in a wavelength range of 370–470 nm. That is attributed to the light emission of formaldehyde (HCHO) produced in the cool-flame reactions. Additionally, it was found that these light emission spectra were no longer observable when residual gas was applied. The light emission spectra of the combustion flame thus indicated that residual gas has the effect of moderating cool-flame reactions.

The purpose of this study is to elucidate the flame propagation behavior under the electric field application by using the constant volume vessel. The laser induced breakdown applies the ignition and Nd:YAG laser is used. A homogeneous propane-air mixture is used and three equivalence ratios, 0.7, 1.0 and 1.5 are tested. In the uniform electric field, the premixed flame rapidly propagates toward both upward and downward directions and the flame front becomes a cylindrical shape. The maximum combustion pressure decreases with an increase of input voltage because of an increase of heat loss to the electrode, however the combustion duration is hardly affected by the input voltage. In the non-uniform electric field, the flame propagation velocity of downward direction increases. The combustion enhancement effect is remarkably when the input voltage is larger than 12 kV because the brush corona occurs and intense turbulence is generated on the flame front.

The purpose of this study is to elucidate the development process of hot kernel generated by the laser induced breakdown and to clarify the relationship between corona discharge application and flame propagation. The mixture can be ignited by the laser induced breakdown. Nd:YAG laser is used for the ignition and laser light is optically focused on the central part of combustion chamber by a plano convex lens. The hot kernel is observed in the absence of combustion and is rapidly developed into the laser incidence side. The homogeneous propane-air mixture is used and six equivalence ratios between 0.7 and 1.5 are tested. For generating the positive corona discharge in the combustion chamber, a non-uniform electric field is applied by the needle to plane gap. In a lean mixture, the whole flame front shifts to downward from the breakdown point and, in the rich mixture region, the combustion is strongly enhanced.

The new fuel injection method which is using the high-voltage electrical discharge has been proposed. The plasma jet ignition technology is applied to this injection system, and the component parts of fuel injector are similar to the plasma jet igniter. The purpose of this study is to elucidate the spray characteristics and the fuel injection development processes of this injection method. To obtain the influence of injector configuration and supplied electrical discharge energy on the fuel spray, the fuel is ejected into the open atmosphere and fuel injection development process is visualized by the schlieren method. The penetration depth, maximum width and projected area of fuel spay increase with increasing in the electrical discharge energy and an orifice diameter. In the case at which the large electrical discharge energy is provided, the fuel injection is finished within a short duration and the mean fuel spray velocity becomes fast.

Homogeneous Charge Compression Ignition (HCCI) combustion has attracted considerable interest in recent years as a new combustion concept for internal combustion engines. On the other hand, two combustion concepts proposed for two-cycle spark-ignition (SI) engines are Active Thermo-Atmosphere Combustion (ATAC) and Activated Radical (AR) combustion. The authors undertook this study to examine the similarities and differences between HCCI combustion and ATAC (AR) combustion. Differences in the low-temperature oxidation reaction behavior between these two combustion processes were made clear using one test engine.

In this study, the effect of corona discharge on the combustion phenomenon has been made clear. A homogeneous propane-air mixture was used and six equivalence ratios were tested. For generating the positive and negative corona discharge, a non-uniform electric field was applied to the combustion chamber by the needle to plane gap. One or five needle-shaped electrodes were used to change the corona discharge state. When the positive corona discharge was applied, the luminescence from corona with five electrodes was weak as compared with that of one needle-shaped electrode. When the negative corona discharge was applied, the luminescence from corona and combustion were not affected by the number of electrode. When the positive corona discharge was applied by low voltage, the combustion was improved in the case of one needle-shaped electrode, but the index of combustion with one needle-shaped electrode was almost equal to that of five electrodes when the high voltage was applied.

In this research, the effect of high-temperature residual gas, resulting from the application of a certain level of EGR, on combustion was investigated using a two-stroke engine and alcohol fuels (ethanol and methanol) and gasoline as the test fuels. Measurements were made of the light emission intensity of the OH radical on the intake and exhaust port sides of the combustion chamber and of the combustion chamber wall temperature (spark plug washer temperature) and the exhaust gas temperature. Data were measured and analyzed in a progression from normal combustion to autoignited combustion to preignition and to knocking operation.

This research focused on the light emission behavior of the OH radical (characteristic spectrum of 306.4 [nm]) that plays a key role in combustion reactions, in order to investigate the influence of the residual gas on autoignition. Authors also analyzed on the heat release and thermodynamic mean temperature due to research activity state of unburned gas. The test engine used was a 2-stroke, air-cooled engine fitted with an exhaust pressure control valve in the exhaust manifold. Raising the exhaust pressure forcibly recirculated more exhaust gas internally. When a certain level of internal EGR is forcibly applied, the temperature of the unburned end gas is raised on account of heat transfer from the hot residual gas and also due to compression by piston motion. As a result, the unburned end gas becomes active and autoignition tends to occur.