Spark Spectroscopy for Spark Ignition Engine Diagnostics

The light emissions from a spark discharge were observed by inserting a fibre optic cable through the centre electrode of a spark plug, to investigate the possibility of determining the fuel-air ratio in the spark gap at ignition with spectroscopy.

The total broadband light emission from the spark and the light emission centred at 385 nm from the cyanogen radical (chemical formula CN), were observed for varied ϕ and residual gas concentrations. Additionally, the spark breakdown voltage, V_s, was monitored for the experiments. All light emissions were observed to be dependent on V_s, which is influenced by mixture composition, temperature and pressure. With a spark gap size of 0.7 mm, the CN emission shows promise for evaluation of the cyclical variation of ϕ for 0.9 < ϕ <1.1.