Experimental And Numerical Investigation On Mixture Formation In A HDDI Diesel Engine With Different Combustion Chamber Geometries

One of the most important phases in thedevelopment of direct-injected Diesel engines is theoptimisation of the fuel spray evolution within thecombustion chamber, since it strongly influences boththe engine performance and the pollutant emissions.Aim of the present paper is to provide information aboutmixture formation within the combustion chamber of aheavy duty direct injection (HDDI) diesel engine formarine applications. Spray evolution, in terms of tippenetration, is at first investigated under quiescentconditions, both experimentally and numerically,injecting the fuel in a vessel under ambient temperatureand controlled gas back-pressure. Results of penetrationand images of the spray from the optically accessiblehigh pressure vessel are used to investigate thecapabilities of some state of the art spray models withinthe STAR-CD software in correctly capturing sprayshape and propagation.The experimental investigation is carried outusing a mechanical injection pump equipping a heavyduty eight cylinder engine. Only one of its plungers isactivated, and the fuel is discharged through a sevenhole nozzle, 0.40 mm in diameter, mounted on amechanical injector. Tests are carried out at two differentload fuel amounts, representing 50%, and 100%respectively, and results are used as data base for theCFD setup. CFD analyses of the intake andcompression strokes are at first performed in order tocompare two different combustion chambers anddifferent jet orientations with respect to the combustionchamber cavities, running the engine under motoredconditions and injecting 50% load fuel amount. Both thetwo tested pistons show two-stage deep valve pocketshollowed under the valve seats projections, but somerelevant differences exist in the piston outer region andin the squish area.Subsequently, full CFD analyses of the intake,compression and combustion processes are performedfor the two different combustion chambers and thepreviously optimised jet orientation, operating the engineat full load, maximum revving speed. Numericalpredictions are used to assess the influence of bothcombustion chamber shapes on the mixture formationeffectiveness and the engine-out emissions.