Abstract: The present invention provides a method of controlling engine performance that includes obtaining at least one optical wavelength-dependent measurement from at least one combustion event in at least one combustion chamber. The method further includes analyzing the optical wavelength-dependent measurement for determining adjustments to the at least one combustion event. Additionally, the method includes adjusting the at least one combustion event or at least a next combustion event by changing at least one physical parameter, at least one constituent parameter, or at least one physical parameter and at least one constituent parameter to control the engine performance. The engine can include steady-flow engines or periodic flow engines, and the engine performance can be selected by an engine user.