In-cylinder combustion gas temperature and rate of heat release are important combustion metrics in internal combustion engines. Such metrics play an important role in the development of strategies for reduced emissions and improved engine performance. The traditional practice has been to measure in-cylinder pressures and deduce the metrics using simple thermodynamic treatments. However, with the availability of improved optics, especially in the field of fiber-optics, optical access to in-cylinder combustion becomes possible. Two diagnostics that measure spectral emission from CO2* and OH* species within the combustion environment of a natural gas fueled IC engine were evaluated. The combustion temperatures were varied by operating the engine over an EQR of 0.6–1.0 and different levels of exhaust gas recirculation. CO2* emission does not lend itself to prediction of crank-angle resolved combustion metrics. However its peak values in a combustion cycle correlated well with both rate of heat release as well as peak combustion temperature during the cycle. In contrast temperatures derived from curve fitting to OH* emission spectra enabled measurement of crank angle resolved in-cylinder temperatures.

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