Effects of CO_2 Addition in Intake Air on Combustion and Unregulated Exhaust Emissions of CI Engine Operated with Dimethyl Ether

An experimental study of the effects of CO2 addition in intake air on combustion and unregulated emissions was carried out with a direct inject compression ignition dimethyl ether fuelled engine.The results indicate that increasing CO2 addition rate will result in a delayed ignition timing and a prolonged main combustion duration.When CO2 addition rate reaches 25%,the peak cylinder pressure decreases by nearly 1.0 MPa and the timing of peak pressure occurrence is postponed by 5° CA.The pressure rise rate at this CO2 addition rate is then 0.15 MPa/(° CA) lower than that when there is no CO2 addition,and the timing of the peak rate of pressure rise is postponed as CO2 addition rate increases.For given speed and load,the emission of formaldehyde increases linearly as CO2 addition gets higher.The emission of formic acid decreases with the increase of CO2 addition.The emission of methyl formate is higher at the CO2 addition rates of 0 and 25% and lower at the rates of 10% and 15%.In conclusion,the trade-off of the emissions of three unregulated pollutants can be reached at the best CO2 addition rate of 15%.The effects of engine speed and load on unregulated emissions were also investigated.