Progress is reported in a project concerned with simple modeling and laboratory experiments to elucidate the mechanisms whereby trace amounts of unburned hydrocarbons may persist after the combustion event in Otto-cycle-type internal-combustion-engine cylinders, and the fate of these residual hydrocarbons during the power-stroke and exhaust-event portions of the cycle. The motivation for the research is that a highly fuel-lean fast-burn design for the spark-ignition homogeneous-charge, four-stroke engine may permit exceptionally fuel-efficient operation of this highly driveable, relatively well-understood automotive engine. Work during this period concentrated on the mathematical modelling of wall quenching and turbulent flame propagation. (LCL).