Hideo Shoji

Statistical analysis on wall shear stress of turbulent boundary layer in a channel flow using micro-shear stress imager

Abstract Measurements of wall shear stress of turbulent boundary layers in the channel flow were carried out using a micro-electro-mechanical-system (MEMS)-based micro-shear stress imaging chip. The study was carried out in a turbulent channel flow facility. One array of 25 micro-shear stress sensors in the chip that covers a length of 7.5 mm is used to measure the instantaneous span-wise distribution of the surface shear stress. The characteristics of high shear stress area (streaks) were described with statistics. Based on the measurement, the physical quantities associated with the high shear stress streaks, such as their length, width with the high shear stress level, were obtained. To further explore the relationship between the shear stress slope and the peak shear stress, the probability density function (PDF) of the ratio of peak shear stress to shear stress slope at different Reynolds number Re is examined. As for the distribution of PDF, it was found that the distribution concentrated towards a certain value in each Re . This result is extremely important because it points to the possibility of predicting the peak shear stress level based on the shear stress distribution at the leading edge of the streaks.

The Application of Coconut-oil Methyl Ester for Diesel Engine

The cold start performance and ignition characteristics of coconut-oil methyl ester (CME) were investigated by using a diesel engine. Diesel fuel and CME were mixed and the blended ratio of CME was changed. The tests were conducted at full load and 3000 min. Diesel engine could be run stably with any mixing ratio of CME, however the power was slightly reduced with increasing CME mixing ratio. In cold start condition, when the mixing ratio of CME increased, the combustion chamber wall temperature rose quickly and the ignition timing was advanced. Therefore, CME had superior compression ignition characteristics in the cold start.

A Study of Knocking Using Ion Current and Light Emission

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.

A Study of HCCI Combustion Using a Two-Stroke Gasoline Engine with a High Compression Ratio

In this study, it was shown that Homogeneous Charge Compression Ignition (HCCI) combustion in a 4-stroke engine, operating under the conditions of a high compression ratio, wide open throttle (WOT) and a lean mixture, could be simulated by raising the compression ratio of a 2-stroke engine. On that basis, a comparison was then made with the characteristics of Active Thermo-Atmosphere Combustion (ATAC), the HCCI process that is usually accomplished in 2-stroke engines under the conditions of a low compression ratio, partial throttle and a large quantity of residual gas. One major difference observed between HCCI combustion and ATAC was their different degrees of susceptibility to the occurrence of cool flames, which was attributed to differences in the residual gas state. It was revealed that the ignition characteristics of these two combustion processes differed greatly in relation to the fuel octane number. Specifically, a correlation was observed between the octane number and ignition timing in HCCI combustion that took place under a low level of residual gas, but no such relationship was seen for ATAC. External exhaust gas recirculation (EGR) and internal EGR were then separately applied to HCCI combustion conditions (a high compression ratio and WOT). It was found that the correlation between the fuel octane number and ignition timing diminished as the internal EGR rate was increased, with the ignition characteristics coming to resemble those of ATAC. When external EGR was applied, the ignition characteristics of HCCI combustion were maintained and a correlation was observed between the fuel octane number and ignition timing.

A study of the influence of intermediate combustion products on knocking

Abstract This study investigated the behavior of the OH (characteristic spectrum of 306.4 nm), CH (431.5 nm) and C 2 (516.5 nm) radicals that are intermediate products of combustion and preflame reactions. Spectroscopic measurements were made of the absorption behavior of the radicals in the end and center zones of the combustion chamber of a spark-ignition engine. Two types of test fuels were used, iso-octane (100 RON) and n-heptane (0 RON). The results showed that the behavior of the OH, CH and C 2 radicals in preflame reactions differed depending on the octane number of the fuels and between normal and abnormal combustion.

Study on combustion characteristics of lean mixture ignited by diesel fuel injection

Abstract We have investigated combustion characteristics of lean gasoline–air pre-mixture ignited by diesel fuel injection using a high compression direct injection diesel engine. Gasoline was supplied as a uniform lean mixture by using carburetors, and diesel fuel was directly injected into the cylinder. It was confirmed that the lean mixture of air–fuel ratio between 150 and 35 could be ignited and burned by this ignition method. As the diesel fuel 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 diesel fuel injection.

Simultaneous measurement of light absorption and emission of end gas during the knocking operation

Abstract Simultaneous measurements were made in the same cycle of the light absorption and emission behavior of the OH (characteristic spectrum of 306.4 nm), CH (431.5 nm) and C 2 (516.5 nm) radicals in the end-gas region using spectroscopic methods. The data obtained with absorption spectroscopy in particular made the following clear. First, the changes observed in the absorbance behavior of the radicals during combustion in an actual engine corresponded to the phenomena associated with the degeneracy of a cool flame. Secondly, prior to the occurrence of auto-ignition under abnormal combustion characterized by knocking, the absorbance waveforms showed distinct evidence of a behavior corresponding to that of a blue flame.

A Comparative Study of HCCI and ATAC Combustion Characteristics Based on Experimentation and Simulations Influence of the Fuel Octane Number and Internal EGR on Combustion

Controlled Autoignition (CAI) combustion processes can be broadly divided between a CAI process that is applied to four-stroke engines and a CAI process that is applied to two-stroke engines. The former process is generally referred to as Homogeneous Charge Compression Ignition (HCCI) combustion and the later process as Active Thermo-Atmosphere Combustion (ATAC). The region of stable engine operation differs greatly between these two processes, and it is thought that the elucidation of their differences and similarities could provide useful information for expanding the operation region of HCCI combustion. In this research, the same two-stroke engine was operated under both the ATAC and HCCI combustion processes to compare their respective combustion characteristics. The results indicated that the ignition timing was less likely to change in the ATAC process in relation to changes in the fuel octane number than it was in the HCCI combustion process. It was also observed that the combustion state and the operation range of HCCI combustion approached those of ATAC under the application of internal exhaust gas recirculation.

Analysis of the Characteristics of HCCI Combustion and ATAC Combustion Using the Same Test Engine

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.

The Influence of Hot Gas Jet on Combustion Enhancement for Lean Mixture in Plasma Jet Ignition

This study clarified the influence of hot gas jet on the combustion enhancement effect in plasma jet ignition. The hot gas jet was ejected after the plasma jet finished injecting. The hot gas jet and combustion processes were visualized and combustion pressure was measured. Both initial flame kernel and hot gas jet were similarly developed in initial stage of combustion. The combustion enhancement effect and the hot gas jet velocity were increased as the characteristic length increased, although the plasma existing duration decreased. Therefore the combustion enhancement effect of plasma jet ignition was mainly caused by the hot gas jet.