Control of Early Combustion by Means of Suitable Ignition

An innovative concept for improving the control of the early combustion in s.i. engines has been investigated. The excitation of small cavities (few mm3) by means of a very fast energy release (~100 mJ) is proposed as an effective solution, that provides enhancement of the flame development, resulting from the displacement of the ignition kernel far from the electrodes, and from the intense local mixing between the hot gas pocket and the fresh mixture. Imaged laser shadowgraphy has been applied to describe the ignition of propane/air mixtures in a constant volume reactor. A pulsed laser beam has been used as the ignition source, since it allows to deliver a well-known amount of energy to the ignition kernel in the desired position within the combustion chamber, while retaining all the properties of electrical breakdown sparks. Experimental results are presented showing the superior performance of the proposed solution over wall-located ignition, both in quiescent and turbulent atmospheres. A set of representative behaviors is illustrated, as emerged from parametric investigations carried out on these ignition devices; experimental data are supported by a theoretical model of the cavity depletion, that allows to define the thermo- and fluid-dynamic properties of the outflowing excited gas.