森吉 泰生

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.

Predictions of the mixture formation process in gasoline DI engines are strongly required. Numerical simulations of free sprays are carried out on the basis of DDM. In this report, know problems such as settings of the initial conditions and droplet breakup model are discussed. Solutions to these problems are introduced in a condition with high ambient pressure. To improve the spray modeling quantitatively, both experimental and theoretical studies are still required.

In order to improve the drawbacks of current schnürle-type two-stroke gasoline engine, a reverse uniflow-type two-stroke direct injection engine was designed and developed. The scavenging port and piston geometry combined with a direct injection system were designed to realize both stratified charge combustion in light to medium load and higher power output in high load range. In-cylinder gas motion and spray behavior were analyzed and optimized by numerical simulation. The experimental result shows high BMEP in WOT operation achieved by a high delivery ratio, high trapping efficiency and good scavenging. In light to medium load a stable combustion was achieved by good stratified combustion utilizing a spray and wall guided mixture preparation. Copyright © 2001 ATA, SAE International and SAE of Japan.

A quantitative measurement method of two-dimensional fuel in both the liquid and gas phases concentration was tested with the use of scanning of a two-color laser beam and a combination of three measurement techniques of absorption, fluorescence, and scatter. It was found that the levels of the fluorescence intensity were proportional to the levels of NO concentration up to 8000 ppm under the ordinary temperature and pressure field. 2

In a stratified charge engine, a glow plug pilot flame ignition system has been compared with a spark-ignition system for a model stratified charge Wankel combustion chamber. A motored two-stroke diesel engine was operated as a rapid compression and expansion machine with the cylinder head replaced by a model Wankel combustion chamber designed to simulate the temporal changes of air flow and pressure fields inside the chamber of an actual engine. It was found that the pilot flame ignition system had better ignitability and improved combustion characteristics, especially in the lean mixture range, relative to the spark-ignition system.

A direct injection gasoline engine system which employs a unique combustion system with enhanced gas motion is evaluated. Enhanced gas motion is produced by employing both a moderately strong swirl flow and a cavity in the piston. Advantages of this system are that the injection timing or spark timing need not be controlled severely and that since the injection timing can be set at near the intake BDC, time for evaporation can be gained to reduce soot emissions. Problems to be improved are that the Nox emissions level is worse than other lean burn systems and full load operation is not evaluated. According to the numerical calculations, the problems may be solved by enhancing the in-cylinder gas motion with axial stratification of swirl intensity at intake BDC; strong swirl near the cylinder head and weak swirl near the piston surface. Copyright © 1998 Society of Automotive Engineers, Inc.

A new idea for controlling the in-cylinder mixture formation in SI engines is proposed. This concept was developed by applying the results of numerical calculations. Fuel that is directly injected into the cylinder is transferred toward the cylinder head to form a mixture stratification by using the in-cylinder gas motion that is generated by the interaction between the swirl and squish flows inside a combustion chamber. At first, the flow characteristics were measured in the whole in-cylinder space using an LDV system. Also, numerical calculations of the in-cylinder flow were made using measured data as the initial conditions. Secondly, the local equivalence ratio at several points inside the combustion chamber was measured by using a fast gas sampling device. The results showed that this idea generates the desired stratified charge when the fuel is injected with a higher injection pressure (about 1.3 MPa) at near the intake-BDC timing than with a usual pressure of 0.2 MPa in the port-injection case. Further numerical calculations were performed to examine other factors that might enhance mixture stratification during the compression stroke. © Copyright 1995 Society of Automotive Engineers, Inc.

A new DISC combustion system with a pilot flame for ignition was analyzed by using a model combustion chamber of a Wankel type rotary engine. A two-stroke diesel engine's cylinder head was replaced with this combustion chamber to simulate temporal changes of air flow and pressure fields inside the chamber as in actual engines. Two types of fuel injection systems were tested to obtain combustion characteristics such as the heat release rate. Direct photographs of spray and combustion were analyzed to understand the mixture-formation process of the main spray and to see the flame temperature distribution and flame moving velocity vectors. In order to understand the mixture-formation process, numerical calculations were made using a gaseous fuel. Finally, the effect of the fuel characteristics on combustion was examined using diesel fuel and n-hexane. © Copyright 1994 Society of Automotive Engineers, Inc.

In our previous study, it was found experimentally by LDV measurements that a large-scale annular vortex is formed during the compression stroke at a swirl case, and this vortex generates and transports turbulence. This report elucidates the above phenomena by means of a mathematical calculation: when an axial stratification of swirl intensity exists, a large-scale annular vortex is formed during the compression stroke by an interaction between the piston motion and the axial pressure gradient. This trend is enhanced when the swirl intensity is the highest at the piston top surface and the lowest at the bottom surface of the cylinder head. Based on this result, a new concept is proposed to control the turbulence generation during the compression stroke.