Search Results

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.

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.

Improving the thermal efficiency of internal combustion engines requires operation under a lean combustion regime and a higher compression ratio, which means that the causes of autoignition and pressure oscillations in this operating region must be made clear. However, there is limited knowledge of autoignition behavior under lean combustion conditions. Therefore, in this study, experiments were conducted in which the ignition timing and intake air temperature (scavenging temperature) of a 2-stroke optically accessible test engine were varied to induce autoignition under a variety of conditions. The test fuel used was a primary reference fuel with an octane rating of 90. The results revealed that advancing the ignition timing under lean combustion conditions also advanced the autoignition timing, though strong pressure oscillations on the other hand tended not to occur.

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.

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 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.

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.

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.

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.

There are strong demands today to further improve the thermal efficiency of internal combustion engines against a backdrop of various environmental issues, including rising carbon dioxide (CO2) emissions and global warming. One factor that impedes efforts to improve the thermal efficiency of spark ignition engines is the occurrence of knocking. The aim of this study was to elucidate the details of knocking based on spectroscopic measurements and visualization of phenomena in the combustion chamber of a test engine that was operated on three primary reference fuels with different octane ratings (0 RON, 30 RON, and 50 RON). The ignition timing was retarded in the experiments to delay the progress of flame propagation, making it easier to capture the behavior of low-temperature oxidation reactions at the time knocking occurred.

This study examines the effects of ethanol content on engine performances and the knock characteristics in spark ignition gasoline engine under various compression ratio conditions by cylinder pressure analysis, visualization and numerical simulation. The results confirm that increasing the ethanol content provides for greater engine torque and thermal efficiency as a result of the improvement of knock tolerance. It was also confirmed that increasing the compression ratio together with increasing ethanol content is effective to overcome the shortcomings of poor fuel economy caused by the low calorific value of ethanol. Further, the results of one dimensional flame propagation simulation show that ethanol content increase laminar burning velocity. Moreover, the results of visualization by using a bore scope demonstrate that ethanol affects the increase of initial flame propagation speed and thus helps suppress knock.

To suppress knock in small gasoline engines, the coolant flow of a single-cylinder engine was improved by using two methods: a multi-dimensional knock prediction method combining a Flamelet model with a simple chemical kinetics model, and a method for predicting combustion chamber wall temperature based on a thermal fluid calculation that coupled the engine coolant and the engine structure (engine head, cylinder block, and head gasket). Through these calculations as well as the measurement of wall temperatures and the analysis of combustion by experiments, the effects of wall temperature distribution and consequent unburnt gas temperature distribution on knock onset timing and location were examined. Furthermore, a study was made to develop a method for cooling the head side, which was more effective to suppress knock: the head gasket shape was modified to change the coolant flow and thereby improve the distribution of wall temperatures on the head side.

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 in 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.

Fuel temperature in the injector of small direct injection gasoline engine is high. On some conditions it is higher than saturated temperature. Over saturated temperature spray characteristics greatly change. In order to predict in-cylinder phenomena accurately, it is important to understand spray behavior and mixture process above saturated temperature. Therefore spray shape, mixture formation process and Sauter mean radius were (SMR) measured in a constant volume chamber. And based on the measurement result initial spray boundary conditions were arranged so that spray characteristics over saturated temperature could be represented by using CFD code KIVA-3[1]. Moreover KIVA-3 code was combined with detailed chemical kinetics code Chemkin II to predict combustion products. [2] Calculated combustion process was validated with visualization of chemiluminescence. As a result, spray shape and penetration length have good agreement with measured ones for each fuel temperature.

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). A four-cycle, air-cooled, single-cylinder gasoline engine with a side valve arrangement was used as the test engine. Light emission behavior was simultaneously observed at two positions (the end zone and the center zone) in the combustion chamber. The test fuel used was n-heptane (0 RON). The test engine was operated at three speed levels (1400, 1800 and 2200 rpm). As a result, preflame reactions were observed. It was also observed that the tendencies seen for the preflame reaction interval varied depending on the engine speed.

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).

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.