OH Absorption Spectroscopy to Investigate Light-Load HCCI Combustion

Abstract

Absorption spectroscopy of the OH molecule was used to examine the light-load limit of the HCCI combustion process. Optical results were compared to cylinder pressure and emissions data, with a focus on the transition from low to high engine-out CO emissions. The goal of the work was to experimentally verify a low-load emissions limit to practical HCCI combustion that has been predicted by detailed kinetic simulations. Two diluent mixtures were used along with 100% air to create intake charges with varying specific heats in order to highlight the temperature dependence of OH formation. Peak OH concentration data show a correlation with temperature, which agrees with theory. In general, OH absorption decreased monotonically with the mass of fuel injected per cycle for all diluent cases. The absorption spectra, which were taken with 400 ?s time resolution (~1.8 CAD at 600 RPM) show that OH forms during the secondstage heat release and remains in the cylinder well into the expansion stroke. Spectral resolution did not allow a temperature measurement from absorption, so temperature was measured separately. Emissions data showed low exhaust CO concentrations at high fuel rates, and higher CO concentrations at low fueling rates. Qualitatively, the detection limit for OH coincided with the onset of increased engine-out CO emissions and the transition from strong to weak second stage heat release. These data suggest that the three phenomena are linked. As increased CO emissions are the practical limitation to the light-load operation of the HCCI engine, this transition is the light-load limit. Due to noise issues in the temperature data, the temperatures at which these transitions happened could not be determined exactly.