  In order to enable investigations of the fuel flow inside unmodified
injectors, we have developed a new experimental approach to measure
time-resolved vibration spectra of diesel nozzles using a three dimensional
laser vibrometer. The technique we propose is based on the triangulation of the
vibrometer and fuel pressure transducer signals, and enables the quantitative
characterisation of quasi-cyclic internal flows without requiring modifications
to the injector, the working fluid, or limiting the fuel injection pressure.
The vibrometer, which uses the Doppler effect to measure the velocity of a
vibrating object, was used to scan injector nozzle tips during the injection
event. The data were processed using a discrete Fourier transform to provide
time-resolved spectra for valve-closed-orifice, minisac and microsac nozzle
geometries, and injection pressures ranging from 60 to 160MPa, hence offering
unprecedented insight into cyclic cavitation and internal mechanical dynamic
processes. A peak was consistently found in the spectrograms between 6 and
7.5kHz for all nozzles and injection pressures. Further evidence of a similar
spectral peak was obtained from the fuel pressure transducer and a needle lift
sensor mounted into the injector body. Evidence of propagation of the nozzle
oscillations to the liquid sprays was obtained by recording high-speed videos
of the near-nozzle diesel jet, and computing the fast Fourier transform for a
number of pixel locations at the interface of the jets. This 6-7.5kHz frequency
peak is proposed to be the natural frequency for the injector's main internal
fuel line. Other spectral peaks were found between 35 and 45kHz for certain
nozzle geometries, suggesting that these particular frequencies may be linked
to nozzle dependent cavitation phenomena.
