Diesel Spray Behaviour and Air Entrainment

Formulation of diesel spray tip penetration is one of the basic research works of diesel spray. Since physical relationships among various injection parameters of diesel spray are included in the equation of spray tip penetration, it becomes a fundamental “tool-equation” for diesel spray analysis and improvement of diesel spray combustion. There have been many proposals of spray tip equation but most of them were derived from experimental data obtained by classical jerk injection system. They have received some critical comments of mismatch with experimental results of up-to-date injection system. Since diesel spray injection system has been changed from jerk pump system to high pressure common rail system, empirical equation of spray tip penetration have to be modified or optimized for new injection system. In this report, spray tip penetration equation obtained by Hiroyasu and Arai was modified to match for common rail injection system with high pressure rise. It shows that initial injection rate has strong influence on spray tip penetration. Similarity law of tip penetration was derived by determinative model of breakup time and breakup length. Combustion of diesel spray is considered to be controlled by spray angle and air entrainment, and both of spray angle and air entrainment have direct relationship with spray tip penetration. Then the additional derivation of spray angle and air entrainment was carried out based on the new equation of spray tip penetration and momentum conservation. Formulated analysis shows that spray angle is controlled by contraction ratio of fuel jet at nozzle exit and air entrainment has strong relationship with tip penetration.