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Fluorescence

Simultaneous SLIF


Flame-front equivalence ratio measurement during stratified combustion

The major benefit of direct-injection spark-ignition engines is their ability to operate at overall lean equivalence ratios, which reduces throttling losses. In order to achieve this a stratified mixture is required to keep the local mixture within the flammability limits. However, then it is not possible to know the true combustion equivalence ratio which is what controls the combustion rate and pollutant emissions. In this project PLIF was performed to simultaneously measure the fuel concentration (visualized with 3-pentanone added to the fuel and shown in blue-green) and combustion products as measured by the OH radical (shown in red). A single laser at 282.92 nm (frequency doubled output of a Nd:YAG-pumped dye laser) and two intensified cameras were used to acquire the data. Two sets of data are shown – one for a homogeneous mixture and one with direct injection and an overall equivalence ratio of 0.52 – both were acquired at 1200 rpm. In the homogeneous case the unburned region is uniformly filled with fuel, whereas in the direct-injection case the fuel is inhomogeneous, and the assumption that the burned region is the part devoid of fuel is incorrect. These data have been used to determine the probability density function of the flame-front equivalence ratio from the calibration of the fuel field.

Acknowledgements: This work was performed by David Rothamer, and was supported by the National Science Foundation

Publications: SAE Paper 2003-01-0069.

For more information contact Prof. Ghandhi

Homogeneous
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Stratified
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Spray mixing in DISI engines

Mixing plays a critical role in all engine processes, and it is especially important for fuel mixture preparation in direct-injection spark-ignition engines. In this study, PLIF was performed to study the critical parameters that control fuel-air mixing in DISI engines. The images shown were acquired from 3-pentanone doped into the base fuel (isooctane) and excited with the 4th harmonic of an Nd:YAG laser (266 nm). Each image in the sequence corresponds to the fuel distribution in a different cycle with all of the other parameters held constant. In this study very high spatial resolution and very low noise levels were achieved by using a cooled, slow-scan camera. This allowed the entire range of turbulence length scales to be resolved.

A dual-metric approach to categorize the state of mixedness was developed and a wide range of experiments were performed to isolate the effects of the fuel injector and the in-cylinder flow field on mixing. Two injectors were investigated a pressure swirl injector operated at two different pressures, and an air-assisted injector to evaluate the role of the fuel injector on the mixing rate. The engine flowfield was varied by using different shrouded valves to promote different in-cylinder flows, and by using valve deactivation, wherein both valves were deactivated following the intake stroke, and injection was inhibited for several cycles to allow the in-cylinder flowfield to dissipate. Injection was performed in a later cycle. The deactivation case retains the time-varying geometry and thermodynamic conditions of the standard case but the flowfield is nearly quiescent. The results show that the cylinder flowfield is the dominant factor in the mixing of the fuel spray.

Acknowledgements: This work was performed by Matt Wiles, and was supported by the National Science Foundation

Publications: SAE Paper 2005-01-0096

For more information contact Prof. Ghandhi

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Fuel film thickness measurements on the piston crown of a DISI engine

In this study a fiber-optic-based fluorescence technique was applied to measure the fuel film thickness and temperature on the piston crown of a firing DISI engine. A single fiber, polished flush with the piston surface is used to transmit the laser and to collect the fluorescence signal. The fiber was introduced to the optical engine through the extended piston as shown in the figure. An involved calibration procedure was undertaken to separately measure the fuel film thickness and temperature. The film temperature was measured using a two wavelength fluorescence approach with BTBP as the dopant. Because BTBP does not co-evaporate with isooctane, 2,3 hexanedione was used to measure the film thickness, with a correction provided by the measured temperature. A calibrated fuel film thickness measurement is shown for a specific fiber location for both motored and fired operation. The increase in the film thickness in the fired case after TDC is the result of fluorescent interference from the combustion products and corresponds to the time of flame passage.

Acknowledgements: This work was performed by Soochan Park, and was supported by the Dept. of Energy

Publications: SAE Paper 2005-01-0649

For more information contact Prof. Ghandhi

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