Crank angle resolved imaging in the UV-visible spectral range was used to investigate flame front characteristics during normal combustion, surface ignition and light knock conditions. ‘Line of sight’ measurements provided information on local wrinkling: the evaluation was based on a statistical approach, with multiple frames taken at the same crank angle during consecutive cycles. This allowed the results during normal combustion to be representative for the specific operational conditions and to a good degree independent from the effects of cyclic variation. Abnormal combustion on the other hand, was investigated on a cycle-to-cycle basis, given the stochastic nature of such phenomena.
The experimental trials were performed at fixed engine speed on an optically accessible direct injection spark ignition (DISI) engine equipped with the cylinder head of a four cylinder 16-valves commercial power unit. The absolute intake pressure was fixed at 1.5 bar and the air-fuel mixture was set close to stoichiometric. Commercial gasoline with an octane rating of 95 RON was used for all investigated conditions.
Different flame contour characteristics were identified in different combustion phases, suggesting that the influence of local fluid dynamics is exerted differently as the fuel oxidation progresses from the central kernel initiated by the spark towards the cylinder walls. Surface ignition and auto-ignition were found to occur near the outer walls of the combustion chamber and the main flame front was found to be influenced to a relatively reduced degree by these secondary ignition sites.