Wavelength-agile sensor applied for HCCI engine measurements

Wavelength-agile laser sources enable continuous monitoring of multiple spectral features, and are of high interest for spectroscopic sensing of combustion processes. Here, we present a tunable external-cavity diode laser (ECDL) based on a rapidly vibrating diffraction grating. The laser scanned from 1374 to 1472 nm every 85 mu s; the wavelength range was chosen to encompass the P-branch of the nu(1) + nu(3) band of H2O, and the scan rate was fast enough to monitor transient processes as in internal combustion engines. During combustion, gas temperature and H2O mole fraction can be inferred from each scan of the laser by use of absorption spectroscopy techniques. In this case, we directed the laser through an engine operating on heptane/air at an equivalence ratio of 0.16 in homogeneous-charge compression ignition (HCCI) mode. During the final 35 crank-angle degrees of a single compression stroke, the measured temperature and H2O mole fraction rose from 800 K and 0.3% to 1350 K and 2.7%, respectively. The details of such records show promise for contributing to the fundamental understanding of HCCI combustion processes. In addition, the sensing strategy shows promise in its own right: although the ECDL we built exhibits high intensity noise, and multi-spectral and -spatial-mode output, the wavelength-agile approach and robust data reduction techniques form a reliable sensor appropriate to a wide range of combustion applications. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.