Characterization of Knock Tendency and Onset in a GDI Engine by Means of Conventional Measurements and a Non-Conventional Flame Dynamics Optical Analysis 2017-24-0099

Gasoline direct injection (GDI) allows knock tendency reduction in spark-ignition engines mainly due to the cooling effect of the in-cylinder fuel evaporation. However, the charge formation and thus the injection timing and strategies deeply affect the flame propagation and consequently the knock occurrence probability and intensity. In particular, split injection allows a reduction of knock intensity by inducing different AFR gradient and turbulent energy distribution.

Present work investigates the tendency to knock of a GDI engine at 1500 rpm full load under different injection strategies, single and double injections, obtained delivering the same amount of gasoline in two equal parts, the first during intake, the second during compression stroke. In these conditions, conventional and non-conventional measurements are performed on a 4-stroke, 4-cylinder, turbocharged GDI engine endowed of optical accesses to the combustion chamber. Imaging in the UV-visible range is carried out by means of a high spatial and temporal resolution camera through a wide transparent window in the piston head allowing the view of the whole combustion chamber almost until the cylinder walls, to include the end-gas zones. Optical data are correlated to in-cylinder pressure-based indicated analyses and ion current data, on a cycle resolved basis. Normal flame front propagation before knock onset, end-gas auto-ignition and the effect of in-cylinder pressure waves of knock on the residual flame are deeply investigated. This synergic analysis is used to explore the effect of modulating injection on the charge formation optimization and the deriving improvement of combustion and reduction of knock tendency. Split injection reduces engine cycle-by-cycle variability with respect to the single injection case, all the others relevant parameters remaining unchanged. It is also able to increase the resistance to knock also changing relevantly the location of the knock onset.

A new methodology based on high temporal resolution optical diagnostics for the determination of the knock intensity and onset has been proposed. This procedure is focused on the cycle resolved analysis of the centroid of luminosity dynamics under the knock pressure waves. It allows the precise determination of knock onset timing and spatial location within the combustion chamber. Furthermore, the amplitude of the luminous centroid oscillation has been found in good agreement with the knock index obtained by in-cylinder pressure and the ion current based analyses.

This work and the proposed methodology can contribute to give a further insight to knock mechanism under real engine conditions.