Investigation Of Combustion Cyclic Variation From A Diesel/CNG Dual Fuel Engine

The world faces energy shortage and environmental pollution problems since the21stcentury, looking for alternative sources of energy and alleviating the pressure of the oilshortage become the urgent matter. In the meantime, haze pollution has become the prominentproblems affecting residents’ health, and will restrict the economic and social development.Natural gas as a substitute fuel of petroleum energy, become one of the most promisingalternative energy sources because of its rich resources, high energy density, the advantages ofefficient clean.Combustion cyclic variation is spark ignition engine at low speed and low load acommon irregular combustion phenomenon. In this paper the research of diesel/natural gasdual fuel engine especially has its particularity. When the engine is in the dual fuel mode, asmall amount of diesel burns in advance then produces the many fires to ignite around theCNG-air mixture. During the combustion process contains diesel premixed combustion phaseand CNG premixed combustion by the fire ignited of pilot diesel fuel. Flame propagation path,initial velocity and to the entire development process of the combustion chamber is decided bysteady fire of previous diesel, the number and distribution of ignition source energy in thecylinder, which kind of combustion processes will appear obvious cyclic variation. Theresearch object is a diesel/CNG dual fuel engine and bench test is established forexperimenting, controlling engine and collecting data. The concept of deviation degree isintroduced to determine the reasonable economy of sampling sample size in this study. Benchtest is experimented at1334r/min,25%load condition, the natural gas energy substitutionrate(SR) and ignition diesel injection timing on the influence rule of engine combustion cyclicvariations are studied respectively, the different parameters of combustion cyclic variations iscompared, the applicability of different evaluation index of dual fuel engine combustion cyclicvariations is evaluated, and combustion in cylinder ignition process under the influence of thetwo boundary conditions are described.When the study is on natural gas energy substitution rate, main conclusions are asfollowing. At1334r/min,25%load condition, with the increase of natural gas energysubstitution rate, the peak combustion pressure in cylinder is gradually reduced, and theposition of the crankshaft angle of the peak combustion pressure appears in advance. In cylinder pressure curve appears obvious bimodal phenomenon, the first peak appears in thecompression on the check points. The second peak is significantly higher than the first peak,the peak pressure is produced by the mutual coupling. Coefficient of variation (COV) of thepeak combustion pressure offers upgrade firstly than descending latter tendency with theincreasing of SR. The position of the crankshaft angle of the peak combustion pressure movesgradually close to TDC, and the sample standard deviation also gradually increased.With medium or small SR, IMEP basically remains unchanged; COVPmiis between2%~4%. When the SR increases, IMEP slightly reduces and COVPmiincreases obviously.When the ultimate SR arrives96.1%, IMEP is6.0bar and COVPmiis10.96%. Compare withthe dates under normal single diesel operation, which pmiis6.1bar and COVPmiis1.3%, IMEPis almost the same, but COVPmiis increased by about seven times.As the SR increased, ignition delay period increased gradually. In single diesel mode,ignition delay timing is the shortest, and COV of ignition delay period increases, the ignitiondelay period fluctuation is more and more intense. When SR is close to the limit, due to theoccurrence of large area misfire phenomenon, COV of ignition delay period has some reduce.With the increase of SR, CA50moves gradually away from TDC, and its standard deviation isbigger and bigger.When the study is on ignition diesel injection timing, main conclusions are as following:When the SR is90%, along with the delay of ignition of diesel fuel injection timing, IMEPdeclines gradually and COVPmidecreases a little first before it increases again. When ignitiondiesel injection timing is before top dead center, COVPmiis almost the same. When the SR is80%and70%, the IMEP and COVPmiis equal to the data when the SR is90%. On theseconditions the optimal values of the ignition diesel injection timing well can be obtained,which will give rise that the COVPmiare the lowest and IMEP are relatively large. When SR is90%,80%and70%, with the delay of ignition of diesel fuel injection timing, the peakpressure gradually decreases and COVPmaxfirst increases and then decreases.When SR is90%, Along with the delay of ignition diesel injection timing, ignition delayperiod decreases a little first before it increases again. When SR is80%and70%, the changetrend of ignition delay period are similar. The optimal values of the ignition diesel injectiontiming well can be obtained, which will give rise that the ignition delay period are the shortestand COV are relatively lower.Along with the delay of ignition diesel injection timing, CA50moves gradually awayfrom TDC, and its standard deviation increased. When SR is90%,80%and70%, with the delay of ignition of diesel fuel injection timing, the duration of the fuel combustion in cylinderbecomes longer; the COV of the duration of combustion gradually decreases.