Effect of injection strategies on the turbulence intensity and combusiton characteristics in an optical GDI engine

Abstract

In the present study, effect of injection strategies on the intensity of the turbulence in the combustion chamber and the combustion characteristics have been investigated. The injection strategies were divided into two parts: single injection with various injection timing during the intake stroke and double injection with a short split injection during the compression stroke. The combustion speed was analyzed using in-cylinder pressure and flame radius which was measured from the flame images. The flame propagation procedure in an optical GDI engine was captured by a high-speed camera, and the intensity of the turbulence in the combustion chamber was calculated by the KIVA simulation. Experimental results showed that retarding the injection timing caused increase of the in-cylinder pressure and combustion speed because the later injection made the stronger turbulence at the ignition timing. Turbulence occurred by the intake flow was weakened by friction with the cylinder wall and piston head, but the additional turbulence supplied by the late injection was kept strong because it lost less energy. The higher turbulence intensity made the flame speed faster, so the in-cylinder pressure increased. The short split injection right before the ignition timing also supplied the additional turbulence. The in-cylinder pressure increased as the portion of the split injection increased because the longer injection produced the higher intensity of turbulence.