PDA Characterisation of Dense Diesel Sprays Using a Common-Rail Injection System

To meet the future low emission targets for Diesel engines, engineers are optimising both the fuel injection and after treatment systems fitted to Diesel engines. In order to optimise the fuel injection system there is a need to characterize the fuel spray for a given injection nozzle geometry and injection pressure/duration. Modern Diesel common rail systems produce very dense sprays, making in-cylinder investigation particularly difficult. In this study the measurement of droplet sizes and velocities in dense Diesel sprays has been investigated using Phase Doppler Anemometry (PDA). PDA has been proven to be a valuable technique in providing an understanding of the structure and characteristics of liquid sprays in many studies. It is often applied to finely atomised and dispersed particle flows. However, the application of PDA to dense sprays is complex and therefore the measurements reported in the literature are performed under conditions that are not representative of modern Diesel engines.

This paper reports both on the processes undertaken to optimise a classic PDA system so that it may be used to gather data in such difficult conditions and on the interpretation of the results obtained. The PDA technique was applied to the instantaneous measurement of Diesel droplet sizes and velocities in a rapid compression machine operated at realistic engine conditions. Results are presented for in-cylinder pressures ranging from 1.6 MPa to 6 MPa and injection pressures from 60 to 160 MPa.