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Relationships between radiant emissions, as measured by an in-cylinder optical sensor, and spark-ignition engine combustion parameters are presented for possible use in engine combustion diagnostics and future engine control strategies. A monochromatic gas radiation model, developed in a previous study, was used to derive a series of relationships between the measured radiant emission characteristics and several spark-ignition engine combustion parameters, such as the amplitude and phasing of the peak heat-release rate, combustion duration, IMEP, NOx emission, pressure, trapped mass and exhaust-gas temperature. In addition, many engine parameters of interest can be estimated indirectly from the radiation signal using empirical models. Correlations of air-fuel ratio and exhaust emissions are presented which contain a combination of radiant emission parameters and known base-engine operating parameters, such as intake manifold pressure, etc.

Cyclic variation is examined by: (1) conditional grouping and heat-release analysis to reveal different modes of combustion, (2) considering the order in which the burn modes occur to establish prior-cycle effects and (3) comparing the measured variation in IMEP with data generated by simple models. Results show that several burn modes may exist, particularly under fuel-lean conditions. Prior-cycle effects also become more obvious as the air-fuel ratio is increased. Finally, comparisons with data generated by simple models show that the nature of cyclic variation may range from completely stochastic to a superposition of a non-chaotic deterministic process on a stochastic process.

An in-cylinder optical sensor has been developed and tested for use in spark-ignition engine combustion analysis and control, This sensor measures the luminous emission in the near infrared region. Results of these tests show good correlation between the measured luminosity and traditional combustion parameters, such as location and magnitude of maximum cylinder pressure, and location and magnitude of maximum heat release. Engine performance indicators, such as the indicated mean effective pressure (IMEP), also can be determined accurately with the measured luminosity combined with other engine operating parameters, e.g. intake manifold pressure. In-cylinder air-fuel ratio can be determined with accuracy over an ensemble of 100 cycles.