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1 An all fiber-optic sensor has been developed to measure H2O mole fraction and gas temperature in an HCCI engine. This absorption-spectroscopy-based sensor utilizes a broad wavelength (1320 to 1380 nm) source (supercontinua generated by a microchip laser) and a series of fiber Bragg gratings (19 gratings centered on unique water absorption peaks) to track the formation and temperature of combustion water vapor. The spectral coverage of the system promises improved measurement accuracy over two-line diode-laser based systems. Meanwhile, the simplicity of the fiber Bragg grating chromatic dispersion approach significantly reduces the data reduction time and cost relative to previous supercontinuum-based sensors. The data provided by the system is expected to enhance studies of the chemical kinetics which govern HCCI ignition as well as HCCI modeling efforts.

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.

Internal combustion engines present a harsh environment for optical sensors, owing to challenges such as high pressures, multiple phases, and window fouling. Fortunately, a new class of “wavelength-agile” light sources that rapidly scan through a broad wavelength range facilitates accurate absorption spectroscopy in engines. Gas temperature and species concentrations can be monitored with fast time response (down to ∼ 1 μs). Here, two strategies for generating wavelength-agile light are introduced. We present in-cylinder results obtained by applying one of the strategies to monitor gas temperature and H2O concentration during compression in a single-cylinder optical engine operating in HCCI mode. Such data will be useful for improving the understanding of engine phenomena and offer the potential for active engine control.