Optical characterization of combustion processes in a DISI engine equipped with plasma-assisted ignition system

Currently, direct injection (DI) for gasoline engines is a primary trend for improving efficiency allowing reduced fuel consumption and the possibility to meet stringent emission standards. A large temporal and spatial variation of mixture composition near the spark location during low speed and high load operation are driving the demands for significantly improved ignition systems. Recently, plasma-assisted ignition systems received increasing interest, as they are used for combustion initiation and control and show the potential for flame stabilization. The aim of this paper is to improve the understanding of the processes related to the use of a plasma-assisted ignition system and its influence on combustion development. Experiments were performed in an optical spark ignition (SI) engine equipped with the cylinder head and injection system of a commercial gasoline turbocharged engine. The effects of spark plug geometry and different plasma configurations were investigated. UV–visible digital imaging was applied; the optical data were correlated to in-cylinder pressure traces and exhaust emission measurements. Significantly higher stability, quantified through the coefficients of variation for the indicated mean effective pressure and flame radius, was obtained using the alternative ignition system in stoichiometric and lean burn conditions. An improvement in engine performance and pollutant emissions was also achieved. •Plasma-assisted ignition (PAI) was studied in an optical direct injection SI engine.•The effect of the spark plug geometry and plasma duration were investigated.•In-cylinder pressure measurements and combustion chamber optical diagnostics were applied.•PAI increased engine stability, in particular using superficial charge plug.•PAI allowed an extension of lean burn limits and reduced partial burned.