Abstract 

    Thermal engines have become a monopoly and uncompromising regarding 
transportation systems.  Since their introduction, the internal combustion en- 
gines have been developed with the first objective to reduce consumption and 
improve performance and drivability.         Over the time,  pollution has become a 
critical  factor  for  governments  resulting  in  the  introduction  of  regulations  to 
reduce pollutant emissions of combustion engines. 

    With constant technological progresses required by the anti-pollution stan- 
dards, direct injection has become indispensable as regards Diesel engines.  The 
introduction of fuel directly into the combustion chamber allows a high level of 
control on the energy released by the exothermic process that is combustion. 
With the emergence of new combustion strategies, the process of injection took 
the lead and the spray has become the main factor. 

    The work done throughout this study to analyze the process of fuel-air mix- 
ing was based upon the development of powerful laser diagnostic techniques. 
As a first step, the Diesel spray has been studied in an isothermal atmosphere 
in order to avoid the evaporation of the fuel (n-dodecane) through the use of 
structured  illumination  to  get  the  mass  distribution  of  mixing  by  controlling 
the multiple Mie scattering.  The application of Rayleigh scattering in the va- 
porized region of a jet injected into a high temperature chamber has allowed 
the extraction of the mixing field of the jet under evaporative conditions. 

    The analysis and comparison of the jet introduced in vaporizing conditions 
or  not  permits  to  understand  better  the  process  of  mixing  during  a  Diesel 
injection  event.   In  addition,  depending  on  the  state  of  the  fluid,  liquid  or 
gaseous,  the  experimental  conditions  have  different  effects  and  therefore  the 
air-fuel mixing behaves in distinct ways.