Study On Combustion And Emission Of DME HCCI Engine

As the first decade of the 21st century passing away the two problems ,crisis of resource and environment pollution , are approaching gradually .In order to keep pace with the society development the internal combustion engine which is a important factor of resource consuming and pollutant releasing will have to improve the fuel situation and combustion mode. Dimethyl ether(DME) is regarded as the ideal substitute of the diesel engine because of its high cetane number,low boiling point and wide source .Homogeneous charge compression ignition(HCCI), which combines the characteristics of original spark ignition engine and compression ignition engine, can conduct lean burn and has the advantages of low pollution of HC and NOx.In this article the main reaction progresses of DME combustion are analyzed and the temperature sensitivity analysises of every elementary reaction are implemented by using the chemical kinetic software CHEMKI N. Based on the above work a reduced chemical mechanism is figured out with its accuracy being verified by certain method. This reduced mechanism which containing 31 species and 49 elementary reactions consists of three sub- mechanism: low temperature and NTC region,formic acid formation and consumption,h igh temperature region .Additionally the 2D chamber CFD model is built according to ZS1100 diesel engine by using the computation fluid dynamic software FLUENT. The DME HCCI combustion is concurrently computed with selecting suitable sub- model and coupling the reduced mechanism.By using numerical simulation method the combustion characteristics of DME HCCI are studied. The studying contents and conclusions are exhibited as follows:1. The varying process of cyl inder temperature,pressure and radical species are analyzed. The study presents that the low temperature combustion begins in the clearance volume and chamber pit where the temperature reaches to 750K to 800K, in these areas the two oxygen addition reactions behaviors fiercely; in the NTC region the radical CH2O is generated numerously, the heat releasing rate decreases and the temperature grows slowly; when the temperature reaches above 1000K the high temperature combustion which including blue flame and hot flame starts and releases great deal of heat.2. The influences of the intake charge tem perature,fuel air equivalence ratio,compression ratio,CO2 to combustion and pollution are studied. The study presents that raising the intake charge temperature leads to the advance of the starting point of low and high temperature combustion, increase of top temperature, first up and then down of the top pressure, decrease of overall HC and CO emission and the increase of NO emission; raising the fuel air equivalence ratio leads to the growth of pressure and temperature, advance of high temperature combustion starting point, decrease of HC and NO emission and the growth of NO emission; Under the condition of normal combustion, raising the compression ratio leads to the advance of combustion starting point,top pressure and temperature point; when CO2 is added to intake charge the combustion is delayed and the pressure and temperature in cylinder grows down.3. The important intermediate species and pollutant are analyzed. The study presents that CH2 O,CO and OH are important radical elements of the combustion; the main combustion pollutants are formic acid,DME,formaldehyde and carbon monoxide; the NO emission of DME HCCI is very low.