Study On The Mixture Formation And Combustion Characteristics On Common Rail Diesel Engine

With the number of vehicles people owned，it has cause great damage to theenvironment. Nowadays Vehicle cars has become a public nuisance. The high-pressurecommon-rail system had greatly enhanced performance of the diesel engine, on thepower、economy and emissions. The diesel engine enter the era of efficient and clean.But the trade-off relationship between NOx and PM is still a difficult problem to solve，although many emission control technologies have been applied to meet the currentregulations，with the increasingly demanding regulatory requirements, forcing peopleto adopt a more advanced technology to control the combustion process andemissions generated.The mixture formation have an important influence on the combustion process，further affect the performance of the diesel engine. As the high-speed operation ofdiesel engine, plus the entire mixture formation and combustion process occurs in thesmall space near the top dead point.After the fuel was sprayed into the chamber，it wasthe relative motion between air and fuel，which made the fuel crushing, fog,evaporation, diffusion and thus to form the mixture. So the airflow in the narrow spacewas important to the mixture formation and combustion process. The combined actionof the air intake system, high pressure common rail fuel injection system andcombustion chamber shape decided cylinder airflow conditions. Intake swirl、thesqueeze flow produced byωchamber, high-speed jet of high-pressure injection madethe entire cylinder air movement disorder irregular. Under certain intake port shape，the spray parameters especially inject pressure and combustion chamber shape hadgreat impact on the inner flow. The combustion process of the diesel engine includepremixed combustion and diffusion combustion. premixed combustion was affected byinject pressure, inject timing and cylinder airflow movement, and diffusion combustionwas affected by airflow strength and its retention. In this paper, study on how combustion chamber shape and injection pressureaffect mixture formation and combustion process by FIRE. By establishing thesampling points around the spray in the cylinder, extraction and analysis of thesespatial positions speed, equivalence ratio and the reaction rate, two dimensionlessparameters as evaluation criteria, Da number (Damkohler number) and Jet number,from different the angle to evaluate the mixture formation and combustion process. DAnumber reflects a characteristic time of the chemical reaction of the combustionprocess and the time needed to form mixture. thereby to study the pre-mixedcombustion and diffusion combustion. Jet number reflect the crushing effect of thefriction of the convective motion of the gas-liquid, Researching from the perspective ofatomization.The ratio of diameter to depth had an impact on the airflow strength and itsdistribution, It was lager, the air flow would be weak in the middle of the cylinder.Otherwise, the airflow would be weak at the bottom of chamber, which was notconducive to the diffusion of the fuel. The lager one would promote fuel diffusion, theexothermic became ahead and peak higher, shorten exothermic duration, socombustion efficiency was improved. High-speed diesel engine appropriatelyincreased diameter depth ratio is conducive to the formation of the mixture andoptimize the combustion process.Boss slope shape would guide the direction of airflow and have an impact on theshape of the spray. Convex slopes upward movement of the airflow along the slope,hinder the development of the spray. The concave slope formed eddy under the sprayto promote the spread of the spray to the pits. By analysis of the Da number，convexslopes promote fuel diffusion, shorten the ignition delay period, suppressed thepremixed combustion facilitated diffusion combustion process. Convex slopes due tothe formation of a large number of mixed gas during the ignition delay, premixedcombustion process was intense.Under different condition, engine performance was different with the change of compression ratio. when small load, increasing the compression ratio appropriately,could reduced the fuel consumption rate. With the speed increased, fuel consumptionrate of low compression ratio would increased and fuel consumption rate had a peakwhen compression ratio is large.NO formation decreased with the speed increased.The injection pressure affect fuel atomization and spatial distribution. The higher theinjection pressure, the faster the diffusion rate and the wider spatial distribution, whichis conducive to the formation of the mixture. By analysis of Jet number, the frictionbetween air and fuel would be intense under high pressure spray. And also, thecrushing effect on the droplet was strong,this could do good to fuel atomization.Different injection pressure had an impact of the on mixture formation, thus affectingNO production.