Gas Temperature Measurements During Ignition in an HCCI Engine

Bulk gas temperature in an HCCI engine was measured using a novel optical sensing technique. A wavelength-agile absorption sensor using a fiber-coupled LED was used to measure the in-cylinder gas temperature. H₂O absorption spectra spanning 1380-1420nm were recorded once every 63 μs using this sensor. The gas temperature was inferred from a least-squares fit of the integrated absorbance areas of H₂O absorption features in this spectral region to those from simulated spectra. The primary source of the H₂O was the humidity in the intake air. Measurements were made during the compression and early portion of the combustion phase of an n-heptane fueled HCCI engine. The measured pressure-temperature history was compared to kinetic calculations of the ignition delay, and showed the traversal of the negative temperature coefficient regime.