High-speed diode-laser-absorption measurements of gas dynamics for diesel engines
Abstract
A diode laser based sensor system, utilizing absorption spectroscopy, has been developed and applied to perform simultaneous measurements of gas temperature, gas pressure, and H2O concentration with 5 kHz measurement bandwidth. A 1387.2 nm tunable distributed feedback diode laser was used to scan across multiple H2O absorption transitions and the direct absorption signal was recorded with the help of a high-speed data acquisition system. H2O was chosen because the target molecule as it is a major product of combustion and absorbs strongly in the 1 – 1.5 &mgr;m region, enabling the use of low-cost telecommunications band diode lasers and optics. Compact optical connectors were designed to conduct measurements in the intake manifold, the EGR cooler exit, and the turbocharger inlet of a 6-cylinder Cummins 6.7L diesel engine featuring a unique cam-less variable valve actuation (VVA) system. These custom optical connectors survived gas temperatures as high as 800 K without the need for any active water or oil cooling. A spectral fitting code employing a Levenberg-Marquardt based least square fitting routine was developed to calculate the gas properties from the recorded absorption data. Diode laser measurements performed during steady state engine operation were within 5% of the thermocouple and pressure sensor measurements, and within 10% of the H2O concentration values derived from the CO2 gas analyzer measurements. Measurements were also performed in the engine during transient events such as step changes in the EGR and exhaust valve openings. In one such transient event, where the EGR valve was rapidly closed to decrease the EGR fraction from 40% to 0%, the diode laser sensor measurements in the intake manifold were able to accurately capture the 300 millisecond change; the thermocouple, on the other hand, required 60 seconds to accurately reflect the change in gas conditions, while the gas analyzer required nearly 2 seconds. This highlights the superior transient response of the diode laser sensor over thermocouples and the gas analyzer.
Degree
Ph.D.
Advisors
Lucht, Purdue University.
Subject Area
Mechanical engineering
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