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Spark plugs instrumented with a ring of optical fibers in the threaded-body region have seen considerable use in the past ten years, and it is expected that their application to unmodified production engines will increase in the years to come. Interpretation of the optical signals obtained with the probe is often difficult, particularly under lean operating conditions where the low luminosity of the flame leads to imprecise flame arrival detection. A systematic look at the optical signals, along with direct imaging of the flame, has been undertaken to calibrate and optimize the determination of flame arrival times. In addition, an evaluation of the different models available for the analysis of the flame arrival data is made. Data fits are compared with real flame images, to determine which model best estimates the convective velocity of the flow and the expansion speed of the flame kernel.

Ionization probes installed in the head gasket of an engine have been used to infer the shape of the burned volume from measurements of when the flame contacts the gasket. It is demonstrated that the technique can be extended to infer fluid motion by using one of the ionization probes as the ignition site, with the ensuing flame serving as a flow marker. It is shown that swirl motion, and its direction, can be detected, and that flame propagation velocities can be measured. A comparison of estimated swirl velocities with laser Doppler velocimeter measurements show remarkably good agreement. The most valuable feature of the technique is that it can be applied to any production engine without modification.