The design of IC engine inlet ports and combustion chambers, both spark and compression ignition requires a deep understanding of the in-cylinder flow field. Generation of large-scale flow structures during the intake process clearly influences the engine combustion behaviour. Swirl and tumble have been identified as important flow structures: for the usual procedure the evaluation of these fluid motion characteristics is based on steady-flow measurements. However, these methods cannot accurately describe flows which are neither pure swirl nor pure tumble but a combination of the two; besides, the steady flow rig cannot examine the effect of valve timing on air motion and its interaction with the prison movement.The paper describes the design and development of both, a steady and a dynamic water flow analogy rig for the characterisation of the in-cylinder flow field during the intake stroke in standard production engines. These two rigs allow a qualitative and quantitative evaluation of the local flow velocity components: for this reason the digitised images are imported into dedicated software developed at C.R.F. for post-processing. The use of water instead of air under Reynolds similarity conditions ensures good visualisation results and higher temporal resolution.This technique was applied to an 8V JTD engine cylinder head with helical intake port: in both steady and dynamic flow visualisations, the intake swirl flow field was found to be different from the solid-body structure assumed in the standard steady-flow measurements. Moreover, the steady flow visualisation results showed an evident unsteadiness and a precession motion of the intake flow superimposed to the swirl rotation. The dynamic flow analysis highlighted fairly different characteristics compared to the steady flow rig results and notable cycle-to-cycle variations in the flow structures.Furthermore the level of detail provided by these experiments is much deeper than the conventional swirl and tumble ratio estimates, obtained with steady- flow test rigs.During is work, CFD simulations were carried on with the STAR-CD code in order to compare the two difference methods. Calculations have been performed both under steady and dynamic flow conditions (these last ones with moving piston and valve) following a procedure currently adopted at CRF. Experimental and analytical results are presented and discussed: the comparisons show a good agreement, not only qualitative but also quantitative for both conditions.
Analysis of intake flow fields in ic engines using steady and dynamic water flow visualisation under reynolds similarity conditions
1999
Aufsatz (Konferenz)
Englisch
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