This paper describes the results of an experimental programme designed to investigate some effects of intake flow-generated turbulence on rates of combustion and emissions formation in a 91mm bore direct injection diesel engine. Swirl and squish were eliminated as far as possible, in order to isolate the desired effects. This was achieved by re-modelling the inlet port and by replacing the deep bowl piston cavity with a flat-topped version, with the same compression ratio.
Tests were carried out with three inlet valves: a standard engine Valve (“A”) and two drilled shrouded valves, one with 15 x 6 mm dia. holes (“B”) and one with 40 x 3.5 mm dia. holes (“C”). The turbulence characteristics associated with each valve were first determined on a steady flow rig, using LDA based surveys of velocity distributions at different downstream distances. The objective was to measure absolute levels of turbulence intensity and to study the subsequent rate of decay.
Engine test results at 1200 rev/min showed significant differences in ISFC, heat release period and NOx emissions between valves “B” and “C”, for a given air fuel ratio, whilst smoke levels remained the same. These differences were not observed at 1600 rev/min.
The results appear to confirm that intake generated turbulence (as opposed to bulk flow) chartacteristics can have significant effects on rates of fuel-air mixing combustion and pollutant formation in high speed DI diesel engines, at least under some operating conditions.