Investigation of Mixture Formation Process in a HDDI Diesel Engine by CFD and Imaging Technique

The paper aims at providing information about the spray structure and its evolution within the combustion chamber of a heavy duty direct injection (HDDI) diesel engine. The spray penetration is investigated, firstly under quiescent conditions, injecting the fuel in a vessel under ambient temperature and controlled back pressure by both numerical and experimental analyses using the STAR-CD code and the imaging technique, respectively. Experimental results of fuel injection rate, fuel penetration, and spray cone angle are used as initial conditions to the code and for the comparison of predictions.

The experimental investigation is carried out using a mechanical injection pump equipped by the heavy duty eight cylinder engine. Only one of its plungers has been activated and the fuel is discharged through a seven holes mechanical injector, 0.40 mm in diameter. Measurements of fuel injection rate have been performed at 900 rpm pump speed by the AVL Bosch tube at engine loads ranging between 10 to 100% that correspond to the injected fuel from 85 to 600 mg/str. Spray tip penetrations have been measured by an imaging technique in an optically accessible high pressure vessel at different instant from the start of injection and different load conditions.

CFD analysis is first focused on in-cylinder flow structure during the intake and compressions strokes to evaluate the swirl and turbulence intensity, as well the tangential profile of the air velocity within two combustion chambers having a different geometry. The prediction of liquid fuel and vapour mass fraction is carried out at 50 and 100% spray load rates considering different jet orientation with respect to the combustion chamber cavities. The predictions are carried to estimate the influence of both shape and jet orientation on the spray behaviour.