Ignition Diagnostics in EGR- and Air-diluted Methane/Air Mixtures Using Spark Induced Breakdown Spectroscopy

Abstract

A diagnostic tool based on Spark Induced Breakdown Spectroscopy (INSIDE -INduced Spectroscopy for Ignition Diagnostics in Engines), designed and developed for providing information on the mixture composition at the spark plug location during spark timing, was successfully used for diagnostics in diluted methane/air mixtures at engine relevant conditions. The INSIDE technique allows deriving information on local fuel to air equivalence ratio in the vicinity of the spark plug and on operating pressure based on the features of the spectral emissions (peaks intensity ratios and peaks width). Aiming to future application of the INSIDE technique in running engines, the possibility to diagnose also the amount of exhaust gas recirculation (EGR) in the spark plug location is needed. In the present work, a capacitive discharge ignition (CDI) system is used in Spark Induced Breakdown Spectroscopy (SIBS) in a constant volume ignition cell with the aim to exploit the faster discharge at higher power when compared to an inductive discharge ignition system. To the authors' knowledge, this is the first time a CDI system is used for SIBS-based ignition diagnostics in spark-ignition engines' relevant conditions. The methane/air mixture dilution is performed following four different approaches: dilution with synthetic EGR mixture consisting of 20 vol.% CO2 and 80 vol.% nitrogen, dilution with pure nitrogen, dilution through fuel to air equivalence ratio variation without EGR addition, and dilution through equivalence ratio variation with constant EGR addition. Spectral signatures of the different mixtures are measured in the spark plug location, and the peaks intensities of four atomic lines (hydrogen Balmer-α at 656 nm, nitrogen at 744-746 nm, carbon at 765-769 nm and oxygen at 777 nm) are measured and compared to the baseline (stoichiometric methane/air mixture with no dilution). The atomic peak intensity ratios versus nitrogen percentage addition show distinct trends depending on the dilution strategy selected. Therefore, the atomic line ratios can be used to extract information on the local mixture composition also in the case of EGR dilution.