Phase Doppler Particle Analyzer Tests Fuel-Injector Systems

Ruth A. Mendonsa

Now that those long waits at the gas station of the 1970s are just a memory, and drivers can relax and enjoy their bigger, but still more-fuel-efficient cars, auto manufacturers are facing new emission standards scheduled to go into effect in 1998. Researchers struggling with the perpetual battle to improve fuel efficiency are focusing on fuel-injection systems for spark injection engines, and photonic methods are being studied to perform the spray diagnostics involved.

Fuel-injection systems place special requirements on measurement systems because of the transient nature of the particles. Though drop size and number density can vary greatly for each injection cycle, exact measurement of volumetric values is important for increasing the efficiency of spark injection engines.

To aid in its research, a major German automobile company recently purchased a Phase Doppler Particle Analyzer (PDPA) from Aerometrics Inc. of Sunnyvale, Calif. This system, capable of precise measurement of fuel droplets, comprises a laser-based optical transmitter, an optical receiver, an electronic signal processor and a PC with data-acquisition and -analysis software.

With the phase Doppler method, which is based on the principles of light-scattering interferometry, measurements are made at the intersecting point of two laser beams. As a particle passes through, it scatters light from the beams and creates an interference fringe pattern. A receiving lens projects a portion of this fringe pattern onto several detectors, and each detector produces a Doppler burst signal with a frequency proportional to the speed of the particle. The phase shift between the Doppler burst signals from the different detectors is proportional to the size of the spherical particles.

Aerometrics has developed a method to directly measure the sample volume simultaneously with the particle size and speed. This enables accurate determination of how many fuel particles are in the area and how the fuel is moving. The phase Doppler method requires no calibration because the particle size and velocity are dependent only on the laser wavelength and optical configuration. The PDPA measurements are not based on the scattered light intensity and therefore are not subject to errors in high fuel flows and can be used in working engines. The optical transmitter and receiver can be moved to relocate the optical probe for spatial mapping of the flow field and the particle size distributions.

With the automobile industry promoting research of this caliber in preparation for the new emission standards, it's probably a pretty safe bet that we can count on the cars of the future consuming less and polluting less.