森吉 泰生

The phenomenon of autoignition is an important aspect of HCCI and knock, hence reliable information on local gas temperature in a combustion chamber must be obtained. Recently, several studies have been conducted by using laser techniques such as CARS. It has a high spatial resolution, but has proven difficult to apply in the vicinity of combustion chamber wall and requires special measurement skills. Meanwhile, a thermocouple is useful to measure local gas temperature even in the vicinity of wall. However, a traditional one-wire thermocouple is not adaptable to measure the in-cylinder gas temperature due to slow response. The issue of response can be overcome by adopting a two-wire thermocouple. The two-wire thermocouple is consisted of two fine wire thermocouples with different diameter hence it is possible to determine the time constant using the raw data from each thermocouple. In this study, measurements such as local gas temperature inside a constant-volume combustion chamber, pressure and visualization were achieved with and without autoignition. The relationship between the ignition delay and the gas temperature was clarified. This is a very important result to analyze the knock phenomenon. As a result, negative temperature coefficient was found to mostly affect autoignition in this experimental condition. This technique was applied at more engine-like conditions, by developing a new RCM (Rapid Compression Machine). In preliminary tests, local gas temperature inside the combustion chamber under compression without combustion was measured to examine the accuracy of two-wire thermocouple. Copyright © 2006 SAE International.

Thermocouples are widely used to measure gas temperature due to its accuracy and convenience. However, it is difficult to employ thermocouples in a transient phenomena such as reacting fields. In this study, the unsteady gas temperature inside a combustion chamber was measured by using an improved two-wire thermocouple technique. Based on previous two-wire methods, some modifications were examined. Firstly, numerical analysis of heat transfer between transient flow and thermocouple was performed to see what kind of modification is required. Secondly, a correction term was added to the basic equation, which was validated by experiments using RCEM. Finally, an improved two-wire thermocouple technique was evaluated by measuring the transient gas temperature inside a combustion chamber comparing to the estimated temperature using measured pressure data and assumptions such as chemical equilibrium to see the adaptability of this technique.

A new combustion method of high compression ratio SI engine was studied and proposed in order to achieve higher thermal efficiency of SI engine comparable to that of CI engine. Compression ratio of SI engine is generally restricted by the knocking phenomena. A combustion chamber profile and a cranking mechanism are studied to avoid knocking with high compression ratio. Since reducing the end-gas temperature will suppress knocking, a combustion chamber was considered to have a wide surface at the end-gas region. However, wide surface will lead to high heat loss, which may cancel the gain of higher compression ratio operation. Thereby, a special cranking mechanism was adopted which allowed the piston to move rapidly near TDC. Numerical simulations were performed to optimize the cranking mechanism for achieving higher thermal efficiency. An elliptic gear system and a leaf-shape gear system were employed in the simulations. A knocking index number was calculated to verify the effect for the new concept. As a result, this concept can be operated at the compression ratio of 14 using regular gasoline. A new single cylinder engine was designed and built for proving its performance. The experimental results show that a knocking limit has apparently improved and better indicated thermal efficiency has been obtained. Finally the indicated thermal efficiency has improved approximately 8% in the limited condition in the case of compression ratio of 12 by realizing this concept. Copyright © 2005 SAE International.

In order to improve thermal efficiency of spark ignition engines, the authors have studied means to improve degree of constant volume. The ideal Otto cycle realizes the maximal degree of constant volume with an instantaneous combustion at TDC. However, it is actually impossible to achieve instantaneous combustion as the combustion speed is limited. Thereby, the authors thought of an idea to increase degree of constant volume. That is to make the piston speed slow during combustion period by active piston-movement control, allowing more time for combustion. As a result, degree of constant volume was improved, but indicated thermal efficiency, estimated by integrating P-V diagram, was deteriorated. A longer expansion stroke was found to keep a longer period of high temperature and then, heat loss increased, leading to a decrease in indicated work. In this study, the authors built another test engine that has equal strokes of compression and expansion but has a slow piston speed in the first half of expansion stroke by making the length of connecting-rod extremely long and made some tests. In the previous report, because the numerical calculation predicted that the shorter the combustion period becomes, the worse the thermal efficiency does, a direct fuel injection system was employed to take a longer combustion period and a different profile of heat release rate compared to the port injection system. As a result, thermal efficiency did not depend on the length of connecting-rod very much, and the numerical simulation predicted the same tendency as experiment, depending on the profile of heat release rate. Copyright © 2005 Society of Automotive Engineers of Japan, Inc. and Copyright © 2005 SAE International.

A new combustion method of high compression ratio SI engine was studied and proposed in order to achieve higher thermal efficiency of SI engine comparable to that of CI engine. Compression ratio of SI engine is generally restricted by the knocking phenomena. A combustion chamber profile and a cranking mechanism are studied to avoid knocking with high compression ratio. Since reducing the end-gas temperature will suppress knocking, a combustion chamber was considered to have a wide surface at the end-gas region. However, wide surface will lead to high heat loss, which may cancel the gain of higher compression ratio operation. Thereby, a special cranking mechanism was adopted which allowed the piston to move rapidly near TDC. Numerical simulations were performed to optimize the cranking mechanism for achieving higher thermal efficiency. An elliptic gear system and a leaf-shape gear system were employed in the simulations. A knocking index number was calculated to verify the effect for the new concept. As a result, this concept can be operated at the compression ratio of 14 using regular gasoline. A new single cylinder engine was designed and built for proving its performance. The experimental results show that a knocking limit has apparently improved and better indicated thermal efficiency has been obtained. Finally the indicated thermal efficiency has improved approximately 8% in the limited condition in the case of compression ratio of 12 by realizing this concept. Copyright © 2005 SAE International.

In reciprocating internal combustion engines, the piston stops in a moment at top dead center (TDC), so there exists a necessary time to proceed combustion. However more slowing piston motion around TDC, does it have a possibility to produce the following effects? The slowed piston motion may expedite combustion proceed and increase cylinder pressure. This may lead to an increase of degree of constant volume. As a result, thermal efficiency may be improved. In order to verify this idea, two types of engines were tested. The first engine attained high cylinder pressure as expected. The P-V diagram formed an almost ideal Otto cycle. However, this did not contribute to the improvement in the thermal efficiency. Then the second engine with further slower piston motion by active piston control was tested in order to examine the above reason. It was revealed that the increased heat loss cancelled out all other favorable features such as lower pumping loss and increase in degree of constant volume.

In order to improve thermal efficiency of spark ignition engines, a novel method to increase degree of constant volume was considered. Because the combustion speed is not infinity as assumed in Otto cycle but limited, it is necessary to decrease the piston-movement around TDC so as to increase degree of constant volume. At first, experimental study was made to confirm this. A test engine which has longer expansion stroke than compression stroke and enables a slow piston-movement during combustion period was built. The experimental data indicated an increase in degree of constant volume, but did not show an increase in thermal efficiency. In order to clarify this reason, numerical simulations are conducted in this paper. As a result, the gain due to the increase in degree of constant volume caused by piston-motion during combustion was found not exceeding the loss by increased heat loss. Numerical analysis deduced that increasing the piston-movement near TDC rather achieve an improvement of thermal efficiency in case that a rapid combustion was realized.

A two-stroke Schnurle-type gasoline engine was modified to enable compression-ignition in both the port fuel injection and the in-cylinder direct injection. Using the engine, examinations of compression-ignition operation and engine performance tests were carried out. The amount of the residual gas and the in-cylinder mixture conditions were controlled by varying the valve angle rate of the exhaust valve (VAR) and the injection timing for direct injection conditions. It was found that the direct injection system is superior to the port injection system in terms of exhaust gas emissions and thermal efficiency, and that almost the same operational region of compression-ignition at medium speeds and loads was attained. Some interesting combustion characteristics, such as a shorter combustion period in higher engine speed conditions, and factors for the onset of compression-ignition were also examined.

In order to measure the fuel jet concentration quantitatively, a technique combining methods of fluorescence with absorbance was developed. LIF method can estimate the spatial fuel distribution qualitatively, but quantitative measurement is difficult. Meanwhile, absorbance method can quantitatively obtain the integrated concentration on the light-path. Thereby, a combination of this technique and laser-beam-scanning technique enables us to measure the quasi 2-D fuel concentration quantitatively. In this study, this measurement method was applied to fuel jet fields in a constant volume bomb. As a result, quasi 2-D measurements of gas concentration were successfully attained by adopting some compensation techniques.

Prediction of the mixture formation processes inside a gasoline direct injection (DI) engine is strongly required in order to improve both the fuel consumption and exhaust gas emissions. Swirl-type injectors, widely used for gasoline DI engines, are characterized by the fact that the spray cone angle drastically changes with ambient pressure. The present study is intended to describe the effect of ambient pressure on the characteristics of a free hollow-cone spray formed by a swirl-type injector using a numerical simulation that is based on a discrete droplet model (DDM) method. In this model, the droplet deformation is calculated for determining droplet break-up and the drag force variation of droplet due to the droplet deformation is taken into account. The simulation result was compared with an experiment focusing on the effect of ambient pressure on the spray shape. It was shown that the characteristics of a spray formed by a swirl-type injector are predicted very well by introducing a selected break-up model and a drag force model with appropriate initial conditions. The drag force of droplets affected by the droplet deformation was found to be a crucial factor in determining the spray shape at various ambient pressures. © 2004 IMechE.

When hydrogen is used in large size diesel engines, the direct-injection combustion regime appears most desirable for its low fuel consumption properties. In applying hydrogen as an alternative to diesel fuel, the behavior of hydrogen jets in swirling air needs to be clarified to provide design procedures for engine designers. This paper is intended to investigate the development of transient hydrogen jets in high-swirl flow fields achieved in a constant volume vessel by means of precise measurement of injection amount and imaging of developing jets. The result showed that when the pressure ratio of injection pressure to the back pressure is higher than the critical pressure ratio, the injection amount remained constant due to the flow choking at the nozzle exit, while jet tip penetration varied depending on the pressure ratio. The variation of jet penetration was elucidated qualitatively by the quasi-steady jet theory. The behavior of transient hydrogen jets in swirling air fields was imaged successfully at a swirl speed of 12000 rpm by a newly developed imaging technique denoted as " Oil mist scattering technique " The result obtained showed that the motion of hydrogen jets is significantly interacted by the swirl flow depending on the swirl intensity and the ambient air density. It was also revealed that the swirl can prohibit hydrogen jets from contacting the chamber walls, which is effective in reducing the heat loss from burning hydrogen jets to the chamber walls.

The stratified charge internal combustion engine system was studied due to the significant potentials for low fuel consumption and low exhaust gas emissions. The mixture formation process for a direct-injection stratified charge engine was affected by various parameters such as the atomization, the fuel evaporation, and the in-cylinder gas motion at high temperature and high pressure conditions. An experimental apparatus, which could control the mixture distribution and the gas motion at ignition timing was developed, and the effects of turbulence intensity, mixture concentration distribution, and mixture composition on stratified charge combustion were examined. In stratified charge combustion using propane-air mixture, when the overall equivalence ratio (φo) was unity, no advantages by charge stratification was observed, however, when φo was set at 0.8, combustion was enhanced by the additive effects combining turbulence with charge stratification. Original is an abstract.