The aim of the present study was to produce a theoretical method for estimating air swirl speeds in 4-stroke diesel engines fitted with poppet valves and purely directional inlet ports. This was achieved by building a steady flow test rig and observing the effects of such parameters as inlet port geometry, valve lift, and pressure ratio on the polar velocity outlet profile around the valve head and the swirl speed in the cylinder. From these observations, two theories were developed which enabled the swirl speed in the steady flow rig to be estimated with some degree of accuracy; particularly in inlet ports with inclination angle between 45° and 60°. The two theories were then adapted for inclusion into an existing step-by-step engine synthesis program to find out how swirl speeds vary throughout the engine cycle, and what effect engine speed and boost ratio have on the swirl speeds attained. The synthesised results are not backed up by any experimental data, but the curve shapes obtained were similar to those observed by other workers. Engine speed and boost ratio did not have a great effect on swirl ratio (swirl speed/engine speed).
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