Effects of Fuel Properties on Combustion and Emission Characteristics of a Direct-Injection Diesel Engine

This study investigates the effects of fuel properties on combustion characteristics and emissions such as NOx, smoke, THC and particulates in a direct-injection diesel engine. Fuel properties, such as cetane number and aromatic content, are varied independently in the experiments to separate their effects. The engine tests are carried out at steady operation with changed load, injection timing and injection pressure.

The results show that reducing cetane number results in the increase of NOx and decrease of particulate emission at high load. This is because the low cetane number fuel has the long ignition delay and causes the high maximum heat release rate and the short combustion duration. However, high THC emission is produced at low load for the low cetane number fuel. The aromatic content has little effect on combustion characteristics such as the history of heat release rate and the cylinder pressure, while increasing aromatic content results in high NOx and particulate emissions. In particular, high aromatic content fuel with low cetane number produces high soluble organic fraction as well as THC emission at low load of retarded injection timing condition. These results suggest that local nonhomogeneity of temperature and fuel distribution affects the exhaust emissions. When aromatic content is high in the fuel, pyrolysis of fuel will not progress well and locally rich and high temperature region is formed. Soot is produced in this region. In the case of high injection pressure, fuel properties show little effects on particulate emissions compared with the low injection pressure condition because high pressure injection promotes turbulence mixing and makes uniform distribution of fuel and temperature. The short combustion duration and low particulate emission can be obtained in the high injection pressure condition and, furthermore, low particulate emission can be achieved.