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The purpose of this study is to elucidate autoignition characteristics of HCCI of natural gas mixtures during the compression process in the engine. A rapid compression and expansion machine was used to reproduce and visualize autoiginition combustion. The test results show that initial temperature and density, and hydrocarbon contents other than methane, are important parameters influencing autoignition timing and the successive combustion. A detailed chemical kinetics model combined with a multi-dimensional CFD code supports these autoignition characteristics.

Fuel properties play a dominant role in the spray, mixture formation and combustion process, and are a key to emission control and efficiency optimization. This paper deals with the influence of the fuel properties on the spray and combustion characteristics in a high-pressure and temperature chamber. Light diesel fuel spray and combustion images were taken by using a high-speed video camera and analyzed by their penetration and evaporation characteristics in comparison with current diesel fuel. Then, a single-cylinder DI engine was used to investigate combustion and exhaust characteristics. The mixture formation of the light diesel fuel is faster than that of the current fuel depending on physical properties like boiling point, density, viscosity and surface tension. Engine test results show that smoke is reduced without an increase in other emissions.

It is recognized that alternative fuels such as liquid petroleum gas (LPG) have less polluting combustion characteristics than diesel fuel. In this study, LPG was injected into a high pressure and temperature chamber to reproduce the stratification processes in an engine. The spray images were taken by the use of a PLIF method with Nd:YAG laser to analyze their penetration and evaporation characteristics. Also the characteristics of combustion were investigated by simultaneous visualization of OH radical and flames. The results show that the mixture moves along the impingement wall that reproduced the piston bowl and reaches the ignition spark plug. Also, OH fluorescence rises sharply and then decreases gradually when the combustion is carried out actively.

It has been recognized that alternative fuels such as dimethyl ether (DME) and liquefied petroleum gas (LPG) have less polluting combustion characteristics than diesel fuel. In this study, such alternative fuels were injected into a high pressure and temperature chamber by means of an electronically controlled common rail injection system to reproduce their charge stratification processes in the engine. Their spray images were taken by using a high-speed video camera and copper-vapor laser to analyze their penetration and evaporation characteristics in comparison with normal-dodecane (n-dodecane) which represent the physical and chemical characteristics as a replacement for diesel fuel. The mixture formation of DME and LPG sprays is faster than that of n-dodecane due to their flash boiling characteristics at high pressures and temperatures.

This paper deals with a diesel engine dual-fueled with natural gas. This system can achieve a high thermal efficiency at higher loads by utilizing the premixed lean natural gas mixture ignited by pilot-injected diesel fuel. At low loads, however, high THC emission and low thermal efficiency were observed. To resolve these problems, effects of EGR and intake heating with an exhaust to intake heat exchanger on engine performance and exhaust gas emissions were investigated, especially at low loads. In addition, 3-CFD simulations were conducted to analyze the combustion process in the combustion chamber, using the KIVA-3 code. The result indicates that thermal efficiency and THC emission at lower loads can be improved by intake air heating combined with EGR.

An experimental study was carried out to measure the evaporating processes of fuel sprays having different boiling points. n-dodecane was used as the higher boiling point fuel, and iso-octane as the lower boiling point fuel. A laser induced fluorescence method was used to separate vapor and liquid-phases of the spray. The measurement of fluorescence wavelength indicates that dimethyl-anilene (5 wt%)+naphthalene (5 wt%) should be added to each fuel. As a result, it was confirmed that n-dodecane in a spray consisting of n-dodecane and iso-octane evaporates more rapidly than an n-dodecane spray.

1. (1) Slit nozzles which concentrate methanol spray around the glow-plug improve combustion at low load although ignition delay was slightly long, which can favorably reduce unburned methanol emissions. At high load, there is room for improvement in utilizing air after ignition. 2. (2) At low load, the glow-plug temperature must be kept at more than 900°C to avoid misfire.