Experimental Study On Lean Dilution Combustion For Improving Fuel Economy Of Gasoline Direct Injection Engine At Part-load

High Boost and direct injection have been becoming the future trend of gasolineengine technology due to the advantages of high power density and low fuelconsumption. As the mainstream products of the current market, the2nd generation GDIengines employ homogeneous charge and stoichiometric mixture combustion modewith smaller specific heat ratio and higher pumping loss compared to lean burn mode,which constrains the improvement of thermal efficiency and fuel economy.Exhaust gas dilution can reduce the pumping loss and improve the break thermalefficiency of gasoline engine at part-load. The lean burn mode of the1st generation GDIengines is also a kind of dilution and can be called ’air dilution’, which leads to theincrease of the intake charge volume. In this paper, lean dilution combustion (LDC)concept, namely the combination of air dilution and exhaust gas dilution, was proposedto improve the thermal efficiency of gasoline engine. LDC can combine and make useof lean burn and EGR’s three effects of ’volume dilution’,’density dilution’ and’reactivity dilution’, can thus improve engine thermal efficiency to the largest extentwith NOXemission constrained.The influences of dilution combustion on the part-load fuel efficiency, combustioncharacteristics and engine emissions are studied. Lean combustion can reduce fuelconsumption by maximum7~8%. And LDC can improve fuel economy by4~6%atpart-load. The engine can have a significant reduction of fuel consumption with NOXemissions not deteriorated too much by employing LDC, therefore LDC mode proves tobe feasible. The thermal efficiency improvement for dilution combustion is due to thereduction of the heat transfer energy loss and the pumping loss. The increase of intaketemperature results in the reduction of pumping loss, accelerating of combustion, anddecrease of cyclic variation. As a result, the fuel consumption of dilution combustion isreduced further and engine’s dilution limit is extended.The stratified mixture of two-zone homogeneity can improve the combustionstability of dilution combustion. Split injection, accompanied by the combustionchamber geometry of volcano-ring pit piston, can help to form an advantageousstratified mixture in cylinder. For the dilution conditions with unstable combustionunder single injection mode, dual injection can reduce the cyclic variation effectively. Fuel economy of dilution combustion can be improved by7~8%under the mostfavorable split injection strategy.