Abstract
Natural gas is one of the most promising alternative fuels because of lower exhaust emissions and lower CO2 emission with high thermal efficiency. Especially, Natural gas is spotlighted as the alternative fuel of heavy-duty diesel engine because of low emissions such as PM, CO2 and NOx. Almost all the CNG vehicles are being applied to PFI type engine. However, in case of the PFI type CNG engine, the brake horse power is reduced because the volumetric efficiency becomes lower due to the gaseous charge and the time losses increase owing to lower burning speed as compared with gasoline fuel. To solve these problems, various methods such as supercharging, direct injection method, etc. are proposed. Especially, a DI CNG engine has merits like the improvement of thermal efficiency and extent of lean limit. The purpose of this study is to investigate the effect of different injection conditions (early injection mode, late injection mode) on combustion and performance in the DI CNG engine. The combustion characteristics of the DI CNG engine such as in-cylinder pressure, cycle variation, combustion duration and emissions were investigated in this study. The results showed that late injection timing during the compression stroke significantly was able to extend lean limit at low load. Consequently, it could be verified that the overall combustion duration and lean limit were improved by using the variation of injection timing and the stratification was controlled by injection timing in the DI CNG engine.
Keywords:Natural gas, Direct injection, Injection timing, Stratified mixture, Emissions