Spectral IR images of direct-injection diesel combustion by high-pressure fuel injection

摘要

Instantaneous successive spectral infrared (IR) images were obtained from a spray plume in a direct injection (DI) type compression-ignition (CI) engine during the compression and combustion periods. The engine equipped with a high-pressure electronic-controlled fuel injector system was operated by using D-2 diesel fuel. In the new imaging system used for the present study, four high-speed IR cameras (with a respective band filters in front) were lined up to a single optical arrangement containing three spectral beam splitters to obtain four spectral images at once. Two band filters were used for imaging the water vapor distribution and another two band filters were placed for capturing images of combustion chamber wall or soot formation. The simultaneous imaging was successively triggered by signals from an encoder connected to the engine. The fuel injection parameters were precisely controlled and the pressure-time (p-t) history was obtained for individual sets of images. The start of fuel injection was varied through four different crank angle positions. Mentioning some results from the study, the spectral IR images had no resemblance with the ones obtained using a visible-range camera from a comparable engine system as reported by others. In general, the present spectral images taken at the same crank angle were not mutually comparable. A highly unexpected phenomenon was observed in the study, i.e., the images recorded during the ignition delay period exhibit some strong preflame reactions over the fuel spray plume. The preflame reactions appear to have started immediately after the fuel injection. This newly observed phenomenon is interpreted to represent strong chemiluminescence seemingly followed by exothermic reactions. The heat release during the ignition delay period is expected to be offset by the latent heat of evaporation to cause no measurable change in p-t history compared with the same measured from a hot-motoring condition.