Experimental Analysis Of The Effects Of Atkinson Cycle Combined With EGR On Gasoline Engine Performance

Through using a new intake camshaft and VVT (variable valve timing)mechanism on an Otto cycle engine, we realize Atkinson cycle which adopts LateIntake Valve Closing strategy. We also change the Otto cycle engine’s pistons to newpistons which have a higher compression ratio. Comparative analysis of externalcharacteristic and universal performance between Otto cycle and Atkinson cycle iscarried out, and this two kinds working cycles’ combustion characteristic is alsoinvestigated. Then we add an external EGR (Exhaust Gas Recirculation) system to theAtkinson engine, and we attempt to study the EGR’s influence on fuel economy,combustion characteristic and emission characteristics.At full loads points, closing Atkinson engine’s intake valve in advance can makeit have an excellent output performance which is nearly as the same as the originalOtto engine. Atkinson engine’s full loads output only have a slight decline when therotate speed is under3000rpm, but the decline is smaller than5%. At all operatingpoints, Atkinson engine’s BSFC (Brake Specific Fuel Consumption) is lower than theoriginal Otto engine. The area enclosed by Atkinson engine’s250g/(kW h) contourline is nearly twice than the original Otto engine’s. The effective compression ratio ofAtkinson engine at high-speed&high-load points is higher than the original Ottoengine, thus Atkinson engine has a higher thermal efficiency.At the operating point of2000rpm/0.2MPa BMEP, Atkinson engine’s BSFCdeclines10.2%than the original Otto engine. Because Atkinson engine’s pump lossreduced28.5%, and it’s expansion ratio is higher than the original Otto engine. Butcompared to the original Otto engine, Atkinson cycle’s maximum combustiontemperature in cylinder declines, the maximum cylinder pressure declines3.92%,andthe peak of heat release ratio declines17.4%.The total combustion period has beenextended16%.But the decline of combustion phase results in more heat releasefocusing on the working stroke and improves the thermal efficiency.Guiding right amount of EGR to the Atkinson cycle can make fuel economybetter. At the speed of2000rpm, three load points (0.2MPa,0.4MPa,0.6MPa;BMEP)have been selected. Every one of these three points has an optimal EGR ratio, whichcan results the BSFC of the point to a biggest drop. The optimal EGR ratios of thethree operating points are5%,10%,15%respectively. On the circumstance ofadopting MBT(Minimum spark advance for Best Torque),when the load increases, a higher EGR ratio leads to a higher maximum cylinder pressure, but the peak of heatrelease ratio remains decline. A higher EGR ratio can extend the flame developingperiod, but have little influence in rapid burning period as the load increased, andpositions of CA50corresponding to different EGR ratios are hardly changed. With theintroduction of EGR, NOx emissions drop significantly, HC emissions increaseslightly, but CO emissions show some volatility.