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Lean combustion strategies are commonly employed to reduce NOx in stationary large-bore
natural gas engines and gas turbines; however, overly lean conditions can result in degraded
engine performance and higher misfire rates. A feedback scheme based on fast measurements of
individual cylinder equivalence ratio would allow the users to operate each cylinder (and
each cycle) at its optimum equivalence ratio. Currently, equivalence ratio measurements are
made either by an oxygen sensor placed in the common exhaust or by spark plug ion sensing.
The former method is not cylinder resolved and can suffer from slow warm-up and response times.
Ion sensing can provide cycle-by-cycle and cylinder resolved equivalence ratio measurements;
however, the method inherently requires conventional electric spark plugs that may not be
suitable for operating future high BMEP engines. |
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Since a practical laser ignition system requires a clean optical window to the combustion
chamber (engine cylinder in our case), we can use this optical access to perform LIBS to
monitor the light emitted from a laser spark. A natural concern is whether soot or
particulates will foul the window, but a fortuitous effect of the passage of the pulsed
beam through the window is to keep the window clean. Air-fuel ratios can be determined from
LIBS by taking appropriate ratios of spectral line intensities, for example, lines of
atomic nitrogen (originating from the air) to lines of atomic hydrogen (originating from the
fuel). With calibration, these line ratios can be used to infer air-fuel ratios of
combustion mixtures ignited with laser sparks. |
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We used laser sparks formed from the 1064 nm beam of a Nd:YAG laser to both ignite the charge
mixture in a single cylinder variable compression ratio Cooperative Fuel Research (CFR)
engine and simultaneously provide emission spectra for LIBS equivalence ratio measurements of
methane-air mixtures. We quantified Hα to Ntot and Hα
to O777 atomic emission line ratios and examined their dependence on equivalence ratio
(measured by an oxygen sensor placed at the exhaust). Ntot is the sum of emission line
areas of N atomic lines at 742, 744 and 746 nm. The tests were performed at two laser pulse energy
levels and two compression ratios. |
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A linear dependence and non-zero slope was observed between LIBS signal (Hα/O777 or Hα/Ntot)
and equivalence ratio at all compression ratios, thereby providing proof of principle for
simultaneous laser ignition and LIBS monitoring of in-cylinder equivalence ratios. |
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| References: |
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- S. Joshi, D.B. Olsen, C. Dumitrescu, P.V. Puzinauskas, A.P. Yalin, "Laser-
Induced Breakdown Spectroscopy for In-Cylinder Equivalence Ratio Measurements in
Laser-Ignited Natural Gas Engines", Applied Spectroscopy, 63
5, pp. 549 (2009)
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