The Simulation Study On The Working Process Of Air-assisted Direct Injection Gasoline Engine

Compared with conventional port fuel injection gasoline engine, GDI engine hasadvantages in improving fuel economy, power output and cold-start emissions, thusbecomes the main development direction of today’s automotive gasoline engine. Thispaper aims to study the working process of a low-pressure air-assisted direct injectiongasoline engine based on3d simulation software AVL-FIRE, and then provides thebasis for GDI engine combustion system design optimization.Firstly, the geometry and computational grid of the intake port and combustionchamber were established for air-assisted direct injection gasoline engine. The flow ofworking fluid in intake port and cylinder were analyzed, the mixture formationprocess between the original engine and the new combustion chamber scheme underdifferent injection timings were compared based on the validation of spray model, andthe difference of combustion characteristic between the original engine and the newscheme was discussed. Results show that:(1)The in-cylinder air motion of the original engine was analyzed: Two rollingvortexes with opposite directions are respectively formed below both ends of theintake valve with the increasing of valve opening magnitude, then they meet andsqueeze each other, only one direction of cylinder tumble motion is existed in thecompression stroke. The cylinder tumble motion becomes insignificant when pistonapproaches the top dead center(700°CA), the squish strength is high at the time,which helps speed up the flame propagation.(2)Compared with the original engine, the cylinder average turbulent kineticenergy of the new scheme increases dramatically at the later stage of compressionstroke, and the maximum increase achieves20.3%, which enhances the overalluniformity of cylinder mixture concentration distribution at20°BTDC. the mixtureconcentration for stable ignition could be formed in the two spark plugs gap of thenew scheme with the injection timings of420°CA and440°CA.(3)The cylinder flame propagation of the original engine is uneven and obviouslylagged behind the new scheme, the original engine has a serious phenomenon ofafter-burning. And the cylinder flame propagation has been greatly improved in thenew scheme, the combustion phase advances, the maximum cylinder pressure and the maximum pressure rise rate increase drastically, as a result, the thermal efficiency ofthe engine has been improved.(4)The overall uniformity of the temperature distribution in the cylinder issignificantly higher than the original engine in the combustion process for theimprovement of cylinder flame propagation.