On applied CFD and model development in combustion systems development for DI diesel engines

Detta är en avhandling från Stockholm : Maskinkonstruktion

Författare: Lars Dahlén; Kth.; [2002]

Nyckelord: ;

Sammanfattning: Soot mediated oil thickening, normally referred to asoilsoot, is a well known problem that can cause increased wearon lubricated moving or rubbing components in diesel engines.The ambition of this thesis is to understand the mechanisms ofcombustion and soot formation that pave the way for thisproblem and use this knowledge in the development of apredictive model for soot contamination of the oil. Anadditional task, however, is the use of Computational FluidDynamics (CFD) as the platform for development of the oilsootmodel.A preliminary hypothesis for soot contamination of the oilis formulated from a combination of theoretical reasoning andprevious experimental evaluations. This states that the oilsootgrowth rate is the result of particle deposition in the oilfilm on the cylinder liner, followed by scraping ofcontaminated oil to the crank case by the piston rings. Acontrolling factor here is assumed to be thermophoresis, whichhas been identified in the past as the dominating particletransport mechanism within the viscous sublayer at combustionchamber surfaces and hence governs the rate of particledeposition in the oil film on the liner.A thermophoretic particle deposition model is suggested forthe calculation of soot deposition on the liner from local sootdistribution as predicted by the standard CFD code. The modelis evaluated through parameter studies comprising the influenceof injection timing, liner temperature, topland height,injected fuel quantity, equivalence ratio, boost pressure andspeed, and the agreement between simulations and measurementsis generally good. It is concluded that the peak in-cylindersoot concentration is more important to oilsoot growth than theexhaust soot level. Moreover, changes in in-cylinder sootdistribution has strong effect on soot mediated oil thickening.Consequently, an important practical guideline to reduce sootmediated oil thickening is to end injection?ontime?, i.e. before the spray guided diffusion flame isdirected into the squish region rather than into the pistonbowl.