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Engine knock is the one of the main issues to be addressed in developing high-efficiency spark-ignition (SI) engines. In order to improve the thermal efficiency of SI engines, it is necessary to develop effective means of suppressing knock. For that purpose, it is necessary to clarify the mechanism generating pressure waves in the end-gas region. This study examined the mechanism producing pressure waves in the end-gas autoignition process during SI engine knock by using an optically accessible engine. Occurrence of local autoignition and its development process to the generation of pressures waves were analyzed under several levels of knock intensity. The results made the following points clear. It was observed that end-gas autoignition seemingly progressed in a manner resembling propagation due to the temperature distribution that naturally formed in the combustion chamber. Stronger knock tended to occur as the apparent propagation speed of autoignition increased.

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.

Technologies for further improving vehicle fuel economy have attracted widespread attention in recent years. However, one problem with some approaches is the occurrence of abnormal combustion such as low-speed pre-ignition (LSPI) that occurs under low-speed, high-load operating conditions. One proposed cause of LSPI is that oil droplets diluted by the fuel enter the combustion chamber and become a source of ignition. Another proposed cause is that deposits peel off and become a source of ignition. A four-stroke air-cooled single-cylinder engine was used in this study to investigate the influence of Ca-based additives having different properties on abnormal combustion by means of in-cylinder visualization and absorption spectroscopic measurements. The results obtained for neutral and basic Ca-based additives revealed that the former had an effect on advancing the time of autoignition.

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.

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.

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.

Although metallic compounds are widely known to affect combustion in internal combustion engines, the potential of metallic additives in engine oils to initiate abnormal combustion has been unclear. In this study, we investigated the influence of combustion chamber deposits derived from engine oil additives on combustion in a spark-ignited engine. We used a single-cylinder four-stroke engine, and measured several combustion characteristics (e.g., cylinder pressure, in-cylinder ultraviolet absorbance in the end-gas region, and visualized flame propagation) to evaluate combustion anomalies. To clarify the effects of individual additive components, we formed combustion products of individual additives in a combustion chamber prior to measuring combustion characteristics. We tested three types of metallic additives: a calcium-based detergent, a zinc-based antiwear agent, and a molybdenum-based friction modifier.

Supercharged direct-injection engines are known to have a tendency toward abnormal combustion such as spontaneous low-speed pre-ignition and strong knock because they operate under low-speed, high-load conditions conducive to the occurrence of irregular combustion. It has been hypothesized that one cause of such abnormal combustion is the intrusion of engine oil droplets into the combustion chamber where they become a source of ignition. It has also been reported that varying the composition of engine oil additives can change susceptibility to abnormal combustion. However, the mechanisms involved are not well understood, and it is not clear how the individual components of engine oil additives affect autoignition. In this study, abnormal combustion experiments were conducted to investigate the effect on autoignition of a calcium-based additive that is typically mixed into engine oil to act as a detergent.

Spontaneous low-speed pre-ignition, strong knock and other abnormal combustion events that occur in supercharged direct-injection engines are viewed as serious issues. The effects of the engine oil and the components of engine oil additives have been pointed out as one cause of such abnormal combustion. However, the mechanisms involved have yet to be elucidated, and it is unclear how the individual components of engine oil additives influence autoignition. This study investigated the effect on autoignition of boundary lubricant additives that are mixed into the engine oil for the purpose of forming a lubricant film on metal surfaces. A high-speed camera was used to photograph and visualize combustion through an optical access window provided in the combustion chamber of the four-stroke naturally aspirated side-valve test engine. Spectroscopic measurements were also made simultaneously to investigate the characteristics of abnormal combustion in detail.

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.

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.

Engine downsizing with a turbocharger / supercharger has attracted attention as a way of improving the fuel economy of automotive gasoline engines, but this approach can be frustrated by the occurrence of abnormal combustion. In this study, the factors causing abnormal combustion were investigated using a supercharged, downsized engine that was built by adding a mechanical supercharger. Combustion experiments were conducted in which the fuel octane number and supercharging pressure were varied while keeping the engine speed, equivalence ratio and intake air temperature constant. In the experiments, a visualization technique was applied to photograph combustion in the combustion chamber, absorption spectroscopy was used to investigate the intermediate products of combustion, and the cylinder pressure was measured. The experimental data obtained simultaneously were then analyzed to examine the effects on combustion.

A Homogeneous Charge Compression Ignition (HCCI) engine was operated under a continuous firing condition in this study to visualize combustion in order to obtain fundamental knowledge for suppressing the rapidity of combustion in HCCI engines. Experiments were conducted with a two-stroke engine fitted with a quartz observation window that allowed the entire bore area to be visualized. The effect of varying the compression ratio and fuel octane number on HCCI combustion was investigated. In-cylinder spectroscopic measurements were made at compression ratios of 11:1 and 15:1 using primary reference fuel blends having different octane numbers of 0 RON and 50 RON. The results showed that varying the compression ratio and fuel octane number presumably has little effect on the rapidity of HCCI combustion at the same ignition timing when the quantity of heat produced per cycle by the injected fuel is kept constant.

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 in the combustion reaction process of hydrocarbon fuels. Attention was focused on light emission behavior at wavelengths corresponding to those of formaldehyde (HCHO), Vaidya's hydrocarbon flame band (HCO) and the OH radical in a forced progression from normal combustion to a knocking state. Light emission behavior was measured simultaneously in the center and in the end zone of the combustion chamber when the engine was operated on two different test fuels. The test fuels used were n-heptane (0 RON) and a blended fuel (70 RON) consisting of n-heptane (0 RON) and iso-octane (100 RON).

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. The test engine used was a 2-stroke, air-cooled engine fitted with an exhaust pressure control valve in the exhaust manifold. 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.

We have investigated combustion characteristics of lean gasoline-air pre-mixture ignited by gas-oil injection using a high compression D.I. diesel engine. Gasoline was supplied as an uniform lean mixture by using carburetors, and gas-oil was directly injected into the cylinder. Two different types of combustion chamber were attempted. It was confirmed that the lean mixture of air-fuel ratio between 150 and 35 could be ignited and burned by this ignition method. An engine with the re-entrant type combustion chamber had an advantage for combustion and ignition. The brake mean effective pressure increased when relatively rich mixture was provided with a small amount of the gas-oil injection. As the gas-oil injection increased, HC concentration decreased, and NO and CO concentration increased. The exhaust gas emission of pollutants could be reduced when lean mixture was ignited by an optimum gas-oil injection.

The aim of this research was to investigate the mechanism causing autoignition and the effect of exhaust gas recirculation (EGR) on combustion by detecting the behavior of the OH radical and other excited molecules present in the flame in a spark ignition engine. The test equipment used was a 2-cycle engine equipped with a Schnürle scavenging system. Using emission spectroscopy, the behavior of the OH radical was measured at four locations in the end zone of the combustion chamber. The OH radical plays an important role in the elemental reactions of hydrocarbon fuels. When a certain level of EGR was applied according to the engine operating conditions, the unburned gas became active owing to heat transfer from residual gas near the measurement positions on the exhaust port side and the influence of excited species in the residual gas, and autoignition tended to occur.

The use of a higher compression ratio is desirable for improving the thermal efficiency and specific power of spark-ignition engines, but it gives rise to a problem of engine knock. In the present research, an investigation was made of the role of the preflame reaction region of a spark-ignition engine in the occurrence of autoignition. Emission spectroscopy was used to measure the behavior of formaldehyde (HCHO) in a cool flame. In addition, measure the behavior of the faint light attributed to the HCO radical in a blue flame with the concurrent measurement of the OH radical. The emission waveforms measurements obtained for HCHO when n-heptane (ORON) was used as the fuel, It is thought that these tendencies correspond to the passage and degeneracy of a cool flame. Further, the emission waveforms measured for the HCO radical when blended fuels (6ORON, 8ORON) were correspond to that of a blue flame.

Emission intensity was measured at a wavelength of 395 2 nm (corresponding to the characteristic spectrum of the HCHO radical) and absorbance was measured at 306 4 nm (corresponding to that of the OH radical). The emission intensity and absorbance waveforms recorded during engine operation on n-heptane show behavior indicative of the passage and degeneracy of cool flame in the preflame reaction interval. As the combustion chamber wall temperature approached an overheated state in the transition from normal combustion to knocking operation different preflame reaction behavior was observed which is thought to correspond to the presence of a negative temperature coefficient region related to the ignition delay time.

The investigation regarding performance of plasma jet igniters was explored by using a constant volume vessel. This study focused on investigating the relationship between the jet effect, the hot gas jet issued from the igniter, and combustion enhancement. The hot gas penetration was visualized by the schlieren system with CCD camera and image intensifier. In the cases of small energies, 0.63 and 0.90 J, the combustion enhancement effect is similar to that of combustion jet igniter. In cases of supplied energies, 2.45 and 5.00 J, the jet effect influences on the combustion enhancement effect for small characteristic length of the igniter.