Legislative restrictions on the currently limited exhaust gas components and the future CO2 emissions limits have led to intensive research in the field of alternative fuels and innovative combustion approaches. Increased homogeneity of air-fuel mixture through advanced injection is one combustion approach, which potentially reduces engine-out nitrogen oxide and particulate emissions, with good fuel consumption in certain load ranges. Ignition characteristics under homogenous combustion conditions differ from those under heterogeneous conditions. Among other reasons, this is due to the increased role of low temperature chemistry with increasing homogeneity.
The ignition behaviour of diesel fuels is characterised by the Cetane number (CN), which is, however, determined at significant higher temperatures than those prevalent during ignition under homogenous combustion. As a result, its relevance as a fuel characteristic number requires evaluation. In this work, the relevance of CN and other candidates as a characteristic number has been analysed under partly homogeneous combustion conditions.
The investigations have been performed on a single cylinder heavy duty diesel engine. Increased mixture homogeneity was achieved through multiple pilot injections in addition to a main injection in the vicinity of ignition top dead centre (TDC). In order to identify a global characteristic number, fuels with different properties, including types of diesel, kerosene and naphtha-based fuels, were selected for the study. The ignition delay measured at the engine were validated with shock tube measurements, The correlation between the ignition delay and the candidate characteristic numbers like initial boiling point, molecular weight and activation energy have been analytically evaluated.