SPECTROMETERS: In situ monitoring chases automotive emissions

Researchers at the University of Manchester (England) have developed a multispecies near-infrared (NIR) diode-laser spectrometer for in situ measurements of carbon monoxide, carbon dioxide, and methane in the exhaust of internal-combustion engines. Unlike exhaust measurements taken with traditional extractive techniques, the in situ method enabled the researchers to monitor variations in gas concentration across changes in engine operating conditions such as engine start-up, increasing and decreasing the throttle, and adjusting the air-fuel ratio. The concept offers promise toward the development of both remote and onboard exhaust-monitoring systems for automotive emissions.

Commonly used extractive sensors (such as the nondispersive IR spectroscopy systems used for carbon monoxide and carbon dioxide measurements) have several drawbacks that include a relatively slow response time, a tendency to adsorb or condense exhaust gases in the extraction tubing, and susceptibility to spectral interference caused by optical-bandpass filters.

As a potential alternative, the researchers applied multispecies NIR tunable diode-laser absorption spectroscopy directly to the exhaust of a gasoline engine (a four-cylinder Rover K series 1.4-liter, 16-valve version with a double overhead cam and single-point fuel-injection system) mounted on a test bed. Two NIR distributed-feedback (DFB) diode lasers were used to measure carbon monoxide, carbon dioxide, and methane simultaneously at 1579.737 and 1579.574 nm, respectively. Methane was monitored using a second DFB diode laser at 1650.95 nm (see Fig. 1).

The laser beams were combined using a 50:50 beamsplitter and directed through the exhaust to a 3 in. retroreflector via three plane mirrors. The returning beam was focused by a gold-coated off-axis parabola onto a 1 mm indium gallium arsenide photodiode. The actual monitoring was performed using wavelength-modulation-division multiplexing, in which both lasers were on continuously, but wavelength modulated and detected at different frequencies. A background-subtraction technique was used to eliminate the signals from ambient carbon monoxide, carbon dioxide, and methane in the atmosphere.

The effect of different engine operating conditions on the concentrations of exhaust gases could be clearly observed, according to Philip Martin, coauthor of a recently published paper on the work. “Such advances make it feasible to have a flexible onboard vehicle engine-diagnostics system for control purposes,” said Martin. “The dependence of the emissions on the vehicle operating conditions is also useful for the analysis of remote vehicle-emission measurements.”¹

Moving forward, the researchers also plan to include nitrogen oxides among the monitored exhaust gases (nitrogen oxides are currently measured using chemiluminescence, which cannot be applied in situ). The researchers are looking into various diode-laser approaches that include measuring nitric oxide at 1800 nm and nitrogen dioxide in either the visible or mid-IR range. They have also constructed an all-fiber spectrometer that allows up to four lasers to be coupled together into a single-mode optical fiber, providing a more rugged and portable instrument than the experimental setup.

REFERENCE

1. Y. Gérard et al., Applied Optics46(19), 3937 (July 1, 2007).