Ignition Delay and Combustion Duration in H₂-Enriched Gasoline SI Engines

The effect of hydrogen addition on ignition delay and combustion duration in spark ignition engines has been experimentally investigated for a wide range of operation conditions. The ignition delay and combustion duration were derived from the cylinder pressure traces with the aid of a two-zone thermodynamic analysis. Two different types of engines were employed: A 4-cylinder, 4-stroke passenger car engine and a single-cylinder, 2-stroke Shnürle-type engine. Both the ignition delay and the combustion duration were correlated with the engine speed and the laminar burning velocity of the mixture. The laminar burning velocity of the hydrogen enriched mixture at any specified operation conditions, was estimated by using a detailed computer program. The results are presented together with processed results of Rauckis and McLean. For combustion duration, a very good correlation with the “eddy burning” model of Tabaczynski et at has been obtained. However, for ignition delay the theory underestimates the pronounced effect of hydrogen addition. In the authors’ opinion, this is attributed to the rapid dissociation of the hydrogen molecules into active radicals (in particular in the spark neighborhood), which dominates the very early stages of the ignition process. In its present form, the “eddy burning” theory does not consider the role of the radicals in the ignition process.