Experiments Study On The Effects Of Fuel Physical And Chemical Properties On Homogeneous Charge Compression Ignition Combustion

Combined the advantages of Gasoline engine and Diesel engine, with ultra-low NOx and PM emissions and high thermal efficiency, homogeneous charge compression ignition (HCCI) was considered as a new combustion process with the strongest potential in internal combustion engine and a focus in the field of combustion study in the world. Its ignition and combustion are mainly controlled by combustion chemistry reaction kinetics of mixture; therefore, the fuel properties and combustion boundary condition have dominant effect on HCCI combustion process.To learn the combustion reaction mechanism of different characteristic fuel and the effects of fuel physical and chemical properties on HCCI combustion, find effective controlling strategy of HCCI combustion, the effects of fuel properties and combustion boundary condition on the HCCI combustion characteristics, operation region, engine performance and emission characteristics were systematically investigate by experiments. The purpose of this study is to realize the exact control to HCCI ignition timing and combustion reaction rate and the extended of operation region.Firstly, Primary Reference Fuels (PRFs) with different octane number (ON), obtained by mixing with n-heptane and isooctane, were applied to learn the effects of ON and combustion boundary condition on HCCI combustion process, operation region and emission characteristics. The HCCI combustion process of PRFs was characteristic by two-stage heat release. The lower the ON is, the more obvious the heat release of low temperature reaction. The influence of ON of fuel on HCCI combustion process has relations with combustion boundary condition, and change with the variation of combustion boundary condition. With the load lower, the engine speed and EGR rate higher, the influence becomes more obvious. The increase of RON of fuel and ratio of EGR would expend the HCCI operation region to high load range. The demanded ON of fuel for HCCI is between that of gasoline fuel and diesel fuel. In the experiment condition of this paper, both the HCCI operation load region and the high efficiency operation region get the broadest area when the fuel ON is 60, the highest indicated efficiency is the highest, too. The HC emissions of HCCI are related to fuel ON and operation condition. At high load condition HC emissions mainly come from crevice in the cylinder while at the low load condition mainly from quenching. Different to HC, CO emissions are related to the in-cylinder temperature, and there is the good consistent relation between CO and in-cylinder temperature.Fuels, with the almost same Research Octane Number (RON) but different physical and chemical properties, were used to learn the effects of fuel property and combustion boundary condition on HCCI combustion process, operation region and emission characteristics. With the same combustion boundary condition, the fuels with same RON but different physical and chemical characteristics have large differences in combustion characteristic parameters. If combustion boundary condition changed, the fuel has different effects on HCCI combustion characteristic parameters. It means that the effects of fuel properties of HCCI combustion process are relate with combustion boundary condition. Octane Index (OI) is relatively measurement of antiknock performance, more comprehensive than RON and Motor Octane Number (MON). No matter what operation condition, it could relatively indicate the antiknock performance of fuel in the same operation condition. In the same condition, the higher OI is, the stronger the capability is, the later the ignition timing is.Finally, simulated boost was applied to learn the effects of intake pressure and fuel properties on HCCI combustion process, operation region and emission characteristics. The intake pressure increased, the main combustion ignition timing advanced, the maximum cylinder pressure rise. But with different intake pressures, the fuel properties have different effects on HCCI combustion process. The increase of intake pressure would expand the normal operation region of HCCI to both high load condition and low load condition. The effects of intake pressure on combustion efficiency are related to the ON of fuel and operation condition. For the combustion efficiency and indicated efficiency, exits an optimum intake pressure. When operation condition changed, the optimum intake pressure changed. At different intake pressure conditions, the influences of fuel properties on both combustion efficiency and indicated efficiency are related to OI. In different intake pressure conditions, its influences on HC and CO emissions are also related to OI, while the influences of intake pressure and fuel properties are not obvious on NOx emissions.