Co-Evaporative Tracer-PRF Mixtures For LIF Measurements in Optical HCCI Engines

Frequent use is made of planar laser-induced fluorescence (LIF) to quantify in-cylinder fuel-air mixing in optical engines. This diagnostic typically relies on one or more fluorescent tracers mixed with a non-fluorescing fuel. An important consideration is the evaporation behavior of the fuel-tracer mixture: the liquid fuel and tracer must co-evaporate so that the tracer can properly track fuel molecules in the vapor phase. Previous work matched tracers to fuels frequently used in direct-injection, spark-ignition research engines. The goal of the current research is to identify appropriate LIF tracers for the primary reference fuels (PRF) commonly used in homogeneous-charge, compression-ignition (HCCI) research engines. A bench-top evaporation experiment characterizes the evaporation of four selected tracers blended with a range of PRF fuels with octane numbers from 0 to 100. Fuel-tracer mixtures are then tested in an optical HCCI engine to assess their performance during LIF measurements of in-cylinder equivalence ratio. Recommendations on the suitability of the four candidate tracers for HCCI engine research are presented.