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This paper presents a new measurement technique for in-cylinder gas temperature and residual gas concentration during the compression stroke of an internal combustion (IC) engine. This technique is based on the infrared absorption of water vapor by a wavelength modulated laser. Wavelength modulation spectroscopy with second harmonic detection (WMS-2f) was adopted to enable the short-path measurements over a wide range of temperatures and pressures corresponding to the late compression stroke in a typical automotive engine. The WMS-2f signal is detected through a bandpass filter at a width of 7.5 kHz, enabling crank angle-resolved measurements. The temperature is determined from the ratio of optical absorption for two overtone transitions of water vapor in the intake gas mixture, and the H2O concentration is determined from this inferred temperature and the absorption for one of the transitions.

Advanced environmental monitoring and control technologies are required for future human exploration of the solar system, where the spacecraft air supply must be recycled for continuous use over a period of months to years. Primary target compounds and 180-day Spacecraft Maximum Acceptable Concentration (SMAC's) limits have been identified for several classes of contaminants. For the identified major species, O2, CO, CO2 and CH4, room-temperature, near-IR tunable diode laser absorption sensors have already been demonstrated with ∼1 ppm-meter detection limits (at or below many of the current SMAC limits) and continuous response band widths on the order of 1 Hz. Additional sensitivity can be achieved using compact long pathlength multipass optical cells. These new generation diode laser sensors use communications-type diode lasers in compact, fiber-coupled packages ideal for remote and autonomous sensor applications.

Room-temperature, tunable diode lasers operating in the visible and near-IR spectral region have been shown to meet detection sensitivity and time response requirements for most major species and rapid-response pyrolysis product measurements identified for spacecraft environmental monitoring applications. Multi-laser sensors with multiplexed fiber-delivery systems appear to be feasible for simultaneous monitoring of several individual contaminants. Work is underway to extend this approach to a number of volatile organic compounds using overtone and combination band absorption in the 1.5 to 2.0 μm wavelength region. The first quantitative measurements of these absorption bands have been obtained in pure vapors using a sensitive, near-IR FTIR spectrometer. These results are being used to guide the selection of custom-fabricated diode lasers for sensitive and rapid detection of trace contaminant levels.