Abstract
In spark ignition engines, knock can be defined as self-ignition of a certain portion of the unbumed gas beyond the flame front. This abnormal combustion releases a chemical energy that excites within the cylinder volume pressure oscillations that are responsible of engine noise and sometimes damage. Therefore, knock is undesirable process that has to be avoided. The present paper aims to evaluate the knocking combustion by an ion current system and optical diagnostics of radical species. The ion signal was also compared with the in-cylinder combustion pressure and the engine vibration. The optical measurements were based on 2D UV-Visible digital imaging of combustion process, UV chemiluminescence and spectrally resolved emissions from radical species. The optical techniques were applied for their potential to provide accurate measurements over a wide range. In particular, in this paper the spatial and temporal evolution of flame front and radical species such as OH and HCO were correlated to pressure and ion-current sensing during knocking combustion. The engine used during experiments was an optically accessible single cylinder, ported fuel injection, four-stroke spark-ignition engine with a four-valve production head. All the measurements were realized in the transparent combustion chamber equipped with a wide quartz window (diameter=57 mm) in the bottom of the chamber. The engine operated at 1000 rpm with stoichiometric mixture and wide-open throttle.
