Study liquid length penetration results obtained with a direct acting piezo electric injector

Abstract

A state of the art prototype common rail injector featuring direct control of the needle by means of a piezo stack (direct acting) has been tested. Liquid phase penetration of the sprays in diesel engine-like conditions has been studied via imaging technique in a novel continuous flow test chamber that allows an accurate control on a wide range of thermodynamic conditions (up to 1000 K and 15 MPa). This state of the art injector fitted with a 7-hole nozzle, allows a fully flexible control on the nozzle needle movement, enabling various fuel injection rates typology. The temporal evolution of the seven sprays has been studied recording movies of the injection event in evaporative conditions via Mie scattering imaging technique and using a high speed camera. The results showed a strong influence of needle position on the stabilized liquid length while the effect of the injection pressure is negligible: the decrease of the needle lift causes a pressure drop in the needle seat and thus a reduction in the effective pressure upstream of the orifices (in the nozzle sac). According to known literature the stabilized liquid-length depends mainly on effective diameter, spray cone-angle and fuel/air properties and does not depend on fuel velocity at the orifice outlet. Therefore, due to small change in the spray cone-angle, higher injection pressures give slightly lower liquid length. However, partial needle lifts has an opposite effect: when needle is partially lifted a dramatic increase of the spray cone-angle and a consequent reduction of the liquid length are observed. A deeper analysis revealed that low charges are linked also to higher hole to hole dispersion and flow instabilities. Needle vibrations caused by the fuel-needle interactions with fuel flow at partial needle lift and the onset of cavitation in the needle seat are likely the causes of this unexpected behavior. Finally, the effect of injection rate shaping on the transient liquid penetration is presented, showing the capability of the injector to control the liquid length along the injection event. This feature, when applied in a real engine, yields to develop new injection strategies to avoid fuel wall impingement.