Numerical and Experimental Analysis of Diesel Air Fuel Mixing 931948

The air fuel mixing process of a small direct injection (d.i.) diesel engine, equipped with two different re-entrant combustion chambers and two nozzles having unlike spray angles, has been studied by integrated use of in-cylinder laser Doppler velocimetry (LDV) measurements, engine tests, and KIVA simulations. The LDV measurements have been carried out in an engine with optical access motored at 2200 rpm. The engine tests have been performed on a similar engine at the same speed, at fixed start of combustion, and different air-fuel ratio. The KIVA-II simulations have been made using as initial conditions the parameters determined by LDV and engine tests.

The re-entrant bowl with higher levels of air velocity and turbulent kinetic energy at the time of injection gives the best performance. The nozzle having a spray angle of 150° which injects the fuel into the regions at higher turbulent kinetic energy lowers the smoke emission levels.

Low fuel consumption levels of modern direct injection diesel engines make their use promising to meet stringent regulations of CO₂ emissions even though there are inconveniences about NOx and soot emissions [1, 2, 3]. Therefore, a deeper knowledge of the emission formation process is needed. To obtain this insight, researchers and designers are now seeking to understand what occurs inside the cylinder during the combustion, rather than examine the emissions at the engine exhaust [4, 5, 6, 7]. In particular, the understanding of the mechanisms of fuel spray evolution, dispersion (turbulence, atomization, vaporization, etc.), and its interaction both with the fluidynamic field and the chamber wall is also a critical point [8, 9]. Moreover, the chemical and physical properties of liquid fuel droplets within the fuel spray directly affect the efficiency of the combustion process. Turbulence and air-fuel mixing also influence how emissions are made and how completely the fuel is burnt [10]. However, it is not easy to observe and measure what occurs within the hostile environment of an operating diesel engine.

Today, many efforts are being devoted following two ways: experimental and numerical. The experimental way aims at developing non intrusive in-cylinder diagnostics based principally on laser techniques. The numerical way, started over the last decade, aims at developing experimentally validated multidimensional computer models that simulate the complex processes occurring in engines. These models are used to analyze in more details the early stage of pre-combustion mechanisms in terms of fluid motion and mixture preparation. Recently, the KIVA-II code, released in 1989 [11], allows to evaluate the effects of the combustion system geometry before actually manufacturing the engine. In this context, the objective of the present work is to examine the fuel spray-air flow interactions in a small d.i. diesel engine by integrated use of engine tests, LDV measurements, and KIVA simulations.

Engine tests have been performed at fixed start of combustion and different load. The swirl ratio, used as input for KIVA simulations, has been determined by LDV in-cylinder measurements of tangential velocity and turbulence made during the compression stroke in an engine optically accessed from the head.