Absorption spectra of H₂O (ν₂ + ν₃ band, R branch) and iC₈H₁₈ (C–H stretch overtone, entire band) were measured in the harsh and highly transient environment of a combusting piston engine using a lamp and spectrometer. Spectra were taken at a rate of 900 spectra second^–1 over the 1600 nm–1850 nm range with a resolution of 0.75 nm (3.0 cm^–1). A grating spectrometer based on an extended indium gallium arsenide (x-InGaAs) linear array camera was used. The engine is an isooctane(2,2,4-trimethylpentane)-fueled homogeneous charge compression ignition (HCCI) engine operating at 1000 RPM. Spectra were post processed for in-cylinder temperature, H₂O density, and fuel density. Fuel spectra measured near autoignition conditions differ slightly from room-temperature spectra as expected. Averaging was employed (1000 engine cycles) to mitigate the challenges introduced by measuring spectra in an engine (for example, beamsteering). With this averaging we were able to achieve a broadband minimum detectable absorbance of less than 1%.

Keywords: near infrared, HCCI isooctane, 2,2,4-trimethylpentane, multispecies