Two diagnostics were developed that are particularly suitable
for use with natural gas-fuelled reciprocating engines that are
used for power generation applications.
The first diagnostic relates flame chemiluminescence to
thermodynamic metrics relevant to engine combustion - Heat Release
Rate (HRR) and in-cylinder bulk gas temperature. Studies were
conducted in a single-cylinder natural gas-fired reciprocating
engine that could simulate turbocharged conditions with Exhaust Gas
Recirculation. Crank-angle-resolved spectra (266 to 795 nm) of
flame luminosity were measured for various operational conditions
by varying the ignition timing for MBT conditions and by holding
the speed at 1800 rpm and Brake Mean Effective Pressure (BMEP) at
12 bar. The effect of dilution on CO₂* chemiluminescence
intensities was studied, by varying the global equivalence ratio
(0.6 - 1.0) and by varying the Exhaust Gas Recirculation rate.
Though the measured CO₂* chemiluminescence intensities did not
correlate with in-cylinder bulk gas temperature and Heat Release
Rate (HRR) on a crank-angle-resolved basis, the peak values
correlated extremely well with the peak cycle heat release rates
and to a lesser extent with the peak cycle temperatures. Such
observations point towards the potential use of flame
chemiluminescence to monitor peak bulk gas temperatures, as well
as, peak heat release rates in natural gas-fired reciprocating
engines.
A second diagnostic based on Laser-Induced Breakdown
Spectroscopy (LIBS) was evaluated to measure in-cylinder
equivalence ratio. Following the laser-induced spark ignition event
in a single-cylinder natural gas-fuelled engine that was equipped
with an Exhaust Gas Recirculation (EGR) system, the broadband
emission from the spark kernel was collected and spectrally
resolved. The peaks corresponding to Hα, N and O atoms were
measured for a range of conditions with global equivalence ratios
ranging between 0.6 and 1.0, and for Exhaust Gas Recirculation
fractions up to 29%. The (Hα/O) and (Hα/N) peak intensity ratios
from the spectral scans correlated extremely well (R₂ ≻ 0.97) with
local oxygen-based equivalence ratios. From the relations that were
developed, it appears that for homogenous intake charge, such a
LIBS diagnostic enables estimation of EGR rate with the knowledge
of the global equivalence ratio.