Optical Characterization of Biodiesel and Diesel Fuel Sprays From a Cr Injection Apparatus

The fuels from renewable resources have obtained an increasing interest for transport application in the last decade because of their biodegradability, potential improvements on exhaust emissions and benefits on the virtuous CO₂ cycle of the earth.

The different physic-chemical characteristics of the biofuel, respect to diesel fuel, affect the combustion phenomenon in diesel engines being different the droplets distribution in the combustion chamber and, consequently, the air-fuel mixture preparation in the ignition delay ready to be burned. The recent trend to enhance the spray atomization, increasing the injection pressure and the hole number in the nozzle to better distribute the fuel, imposes a deep understatement of the spray characteristics in terms of tip penetration, cone-angle, droplet velocity, fragmentation and vaporization.

The modern Common-Rail (CR) injection apparatus enables a management of injection strategy both in terms of injection pressure and injection number and timing per cycle. They allow to exploit all the potentiality of modulated combustion in engine for NO_x and noise reduction, acting on pilot and pre-injections, and matter particulate using post and late injections.

In this paper a study of overall behavior of spray from rapeseed methylester (RME) biofuel and diesel fuel has been carried out in an optically accessible vessel filled with inert gas (N₂) with pressure ranging between 0.1 to 5.0 MPa. The injections were obtained by a CR apparatus driven by a Programmable Electronic Control Unit (PECU) enabling different strategies performances. The injector mounted an axially disposed cylindrical single hole nozzle (0.18 mm in diameter and 1.0 mm in thickness) and the investigated injection pressures have been 60, 90 and 120 MPa. The sprays have been lightened by a pulsed sheet (100 μm in thickness and 10 ns in duration) generated by the second harmonic of an Nd-YAG laser and matched at different instant from the start of injection. The jet images have been captured by a CCD camera collecting the light scattered at right angle and synchronized with the light sheet. A digital pulser/delayer has allowed finely investigating the entire spray duration (1.0 ms). The spray characteristics have been extracted by a digital image processing software. Tip penetration and spray cone angle have been strictly measured while droplet velocities and nozzle discharge coefficient have been derived from the data.

The diverse characteristics of the investigated fuels have produced quite differences in sprays global performances in terms of penetrations and cone angles. These differences have shown a no-monotonic behavior during the spray duration and their relationships with the injection pressure have been observed.