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Laser monitors exhaust emissions

03 Jul 2007

A portable near-IR spectrometer could directly measure emissions from a moving vehicle.

A team from the University of Manchester, UK, has constructed a near-IR diode laser spectrometer that directly measures carbon dioxide, carbon monoxide and methane in the exhaust emissions of a four-cylinder gasoline engine.

This approach is claimed to be faster and more sensitive than the extractive techniques normally used to monitor emissions while cars are stationary, and could in future allow measurements to be made unobtrusively as a vehicle drives past (Applied Optics 46 19 3937).

"This is the first application of near-IR diode-laser sensors to in-situ exhaust measurements, and they offer several advantages," Philip Martin of the University's School of Chemical Engineering and Analytical Science told optics.org. "The components are readily available, the narrow bandwidth enables high specificity and reduces interference, and the collimated beam can be readily directed."

The biggest hurdle is that the narrow wavelength tuning range generally only allows one species to be measured with one laser. For multi-species measurements, the lasers must be multiplexed.

The device therefore used two distributed feedback diode lasers, one operating at 1580 nm and the other at 1650 nm. Wavelength modulation-division multiplexing, where both lasers were on constantly but were wavelength modulated and detected at different frequencies, enabled one source to detect CO and CO2, and the second to detect CH4. Background measurements taken before and after the engine was run allowed atmospheric concentrations of the gases to be eliminated.

The team measured the emissions of a Rover K-series motor mounted on a test bed, but they have also taken their process outside the laboratory. "Components combining the high sensitivity and robustness required to apply this approach in the real world are only now becoming available," Martin said. "We've already constructed a battery-powered roadside unit using the same technology, employing ruggedized telecommunications components."

The next steps will be to fully quantify the technique, and to add additional lasers for other key emissions such as NOx and specific hydrocarbons. Once optimized the process could be incorporated into onboard diagnostic systems that would monitor the emissions as a car drives along.

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