Effects of Direct Injection DI on Knocking Combustion in Spark Ignition SI Engine Operated on 75-RON and Ethanol Fuels

摘要

The study compares the effect of simultaneous Port Fuel Injection(PFI)and Direct Injection(DI)on heavy knock in Optical Spark Ignition(SI)engine operated on gasoline(75-RON)and ethanol.It involved evaluation of the Knocking Intensities(KI),In-cylinder Pressure,and Mass Fraction Burned(MFB)of the fuels.The engine has full bore and operated at typical stoichiometric part-load condition with fixed spark timing.We used high-speed natural light(or chemiluminescence)imaging and the engine operating thermodynamics properties to analyze the influence of a particular combustion cycle on the preceding cycle.We observed the combustion process until multiple centred auto-ignition occurred with varied knocking intensities of up to ~50bar.During heavy knock,we adopted fast over-fueling approach and injected varied amounts of fuel into the end-gas region.We found the fast over-fueling to consistently suppressed autoignition and eliminated knock for both fuels,especially ethanol.We attributed the effect to reduced autoignition delay time and charge cooling effect associated with DI.We compared the cyclic variations of both fuels using the in-cylinder pressures.The result showed that ethanol had higher pressure than RON-75 at both PFI and DI approaches.We attributed the higher pressure of ethanol to its higher affinity to preignition.In terms of MFB,75-RON demonstrated fastest burning rate than the ethanol at both PFI and DI.We attributed the lower burning rate of ethanol to the increased air mixture dilution effect associated with alcohol fuels.We also attributed it to the higher latent heat of vaporization of the ethanol.Between 50 to 90% mass of the mixture burned,Ethanol proved a higher MFB than RON-75.We likened the improved combustion of ethanol to the presence of oxygen or hydroxyl ion(OH-)in its molecular structure.