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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.

Conditional sampling of cylinder-pressure data is used to investigate cyclic variability in a premixed-charge spark-ignited engine operating under fuel-lean conditions. Unlike straight ensemble averaging of pressure data, conditional sampling applies a set of constraints to the pressure data such that like combustion events can be identified and grouped together. Ensemble averaging of pressure data from an engine that exhibits significant cycle-to-cycle variation is shown to produce a mean pressure history that is not representative of the combustion process. Conditional sampling provides a means of identifying and analyzing the different groups of pressure histories and therefore the different types of combustion processes that occur in an engine that exhibits cyclic variability.