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Raman Spectroscopy Helps to Decrease Engine Emissions

Photonics Spectra
May 1999
Michael D. Wheeler

Engineers at Vanderbilt University are using Raman spectroscopy to analyze combustion in direct injection gasoline engines -- research that could lead to decreased pollution.

Direct injection engines differ from conventional spark ignition engines in that they draw fuel into the cylinder late in the compression stroke, making them run more efficiently. But they do not burn the gasoline as completely as conventional engines, so they produce more pollution.

Determining the precise amount of gases such as carbon monoxide, oxygen and methane that are present during combustion may go a long way toward helping engineers develop a cleaner-burning direct injection engine. With the help of a three-year grant from the US Department of Energy, a research team led by Robert W. Pitz investigated methods to make these measurements. Nonphotonic techniques such as gas chromatography/mass spectroscopy are accurate in diagnosing emissions from exhaust but cannot withstand the high temperatures at the point of combustion.

Previously, Pitz's team had used Raman scattering to perform combustion diagnostics using a krypton fluorine laser emitting at 248 nm as the excitation source. Exciting molecules in the UV led to one major problem: Polycyclic aromatic hydrocarbons tended to skew results.

For this application, the group made some changes. It replaced the excimer laser with a Continuum frequency-doubled Nd:YAG laser emitting at 532 nm and a custom-built burner that simulated the fuel-air stratification that occurs inside a direct injection engine.

Laser light travels through the burner via a quartz window on the burner wall. When the laser strikes a gas molecule, the molecule momentarily absorbs photons, and vibrates. This vibration produces light scattering, or a "red-shifted" wavelength. Cassegrain optics collect the light, sending it to a spectrometer that separates the wavelengths and focuses the diffracted images to a Princeton Instruments Inc. back-illuminated charge-coupled device camera. A computer processes the results, showing the wavelengths as a series of lines that are proportional to the species present in the flames.

The team hopes its research will lead to computer models demonstrating how direct injection engines can burn fuel cleanly.

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