Mixture Preparation and Combustion via LIEF and LIF of Combustion Radicals in a Direct-Injection, HCCI Diesel Engine

The influence of piston geometry on the in-cylinder mixture distribution and combustion process in an optically-accessible, direct injection HCCI Diesel engine has been investigated. A new, purpose-designed piston which allows optical access directly into the combustion chamber bowl permitted the application of a number of optical diagnostic techniques. Firstly, laser-induced exciplex fluorescence (LIEF) has been applied in order to characterize the fuel spray and vapor development within the piston bowl. Subsequently a detailed study of the auto-ignition and two-stage Diesel HCCI combustion process has been conducted by a combination of direct chemiluminescence imaging, laser-induced fluorescence (LIF) of the intermediate species formaldehyde (CH₂O) which is present during the cool flame and LIF of the OH radical later present in the reaction and burned gas zones at higher temperature. Finally, spectrometry measurements were conducted with the objective of determining the emitting species of the chemiluminescence signal. The experiments were performed on a single cylinder optical engine equipped with a conventional common rail injection system and narrow angle injector.

Results obtained for two re-entrant piston bowl designs (flat piston and bowl-dome piston) show that the geometry plays a significant role on mixture preparation and subsequently the combustion and emissions characteristics for late injection in HCCI mode particularly in such cases where liquid impingement is unavoidable. Planar LIF 355 imaging revealed the presence of the intermediate species formaldehyde allowing the temporal and spatial detection of auto-ignition precursors prior to the signal observed by chemiluminescence in the early stages of the cool flame. Formaldehyde was then rapidly consumed at the start of the ‘hot’ combustion which was marked not only by the increase in the main heat release but also by the first detection of OH, present within the reaction and burned gas zones. In the case of the flat piston geometry, soot precursors were also detected, as indicated by the strong PAH fluorescence signal observed later in the cycle (from 375 CAD).