Spectroscopic techniques for the evaluation of the cylinder air fuel ratio in a small optical SI engine fueled with methane and hydrogen/methane blends

The local air fuel ratio (AFR) is a key factor in the internal combustion engines for monitoring the pollutant emissions as well as the engine operation. An experimental activity was carried out to evaluate the in-cylinder AFR from the flame chemiluminescence measurements. The study was performed on a small optical spark ignition (SI) engine. The engine was fueled with methane and blends of 20%v/v and 40%v/v of hydrogen in methane. The engine was operated at 2000 rpm wide open throttle (WOT), and at stoichiometric condition. UV-visible spectroscopic measurements were performed to analyze the combustion process with high spatial and temporal resolution. The optical system was composed by two intensified CCD cameras, band pass filters centered on 309 nm and 431 nm to detect the OH* and CH* luminosity, respectively, and a dichroic filter. The local AFR was evaluated from OH* and CH* chemiluminescence emissions using empirical equations. The measurement of OH* and CH* was performed both simultaneously, using the dichroic filter and not. The analysis of the local AFR provided interesting information also on the flame propagation speed and the cycle to cycle variability. The results point out that the simultaneous measurements allow a better evaluation of the local AFR.