High-speed diode laser measurements of temperature and water vapor concentration in the intake manifold of a diesel engine

A diode laser–based sensor system, utilizing absorption spectroscopy, has been developed to provide high-speed (5 kHz) simultaneous measurements of temperature and water vapor concentration in the intake manifold of a diesel engine. A fiber-coupled 1.38 µm diode laser was used to probe absorption transitions of water vapor for the high-speed gas temperature and water vapor concentration measurements. Water vapor readily absorbs in the near-infrared region, and distributed feedback diode lasers as well as optics for near-infrared region are readily available because of their use in telecommunications. Fresh charge and combustion products are the only sources of water vapor in an engine’s gas exchange path; therefore, water vapor concentration at various locations in the intake manifold is a useful measure of the recirculated exhaust gas distribution in the intake manifold. Measurements were performed on a six-cylinder Cummins diesel engine using compact fiber optic–coupled connectors. The chosen water vapor absorption transitions provided good absorption strength even without exhaust gas recirculation and water vapor concentration of as low as 0.7 vol.% could be measured with a signal-to-noise ratio of ∼35 leading to very good spectral fits. The sensor output was within 2% of the thermocouple readings and within 10% of the water vapor concentration derived from mean CO₂ analyzer measurements for steady-state engine operation. For transient engine operation, the time response of the diode laser sensor was shown to be vastly superior to that of the installed thermocouple and the gas analyzer system.