Two-Dimensional Visualization of Premixed-Charge Flame Structure in an IC Engine - SP-715 870454
Flame fronts were examined in a premixed-charge, spark-ignition, ported engine using a two-dimensional visualization technique with 10 nanoseconds time resolution and 200 microns best spatial resolution. The engine had a pancake chamber, a compression ratio of 8, a TDC swirl number of 4 and was operated at 300 to 3000 rpm with stoichiometric and lean propane/air mixtures. The measurements were made far from, and near to, the cylinder wall. A pulsed laser sheet was passed through the engine and the light scattered by sub-micron TiO2 or ZrO2 seeding particles was collected by a 100 x 100 diode array with fields of view of 1 cm x 1 cm, 2 cm x 2 cm, and 9 cm x 9 cm. The thickness of the flame front is as small as, or smaller than, the 200 micron best resolution of the measurements thus confirming that premixed-charge engine turbulent flames generally appear to be wrinkled laminar flames. Ignition kernels as small as 2 mm were distorted by turbulence as early as 100 micro-seconds after ignition. Except for very small flames, the degree of wrinkling is fairly independent of flame size and wall proximity but increases with engine speed and for lean flames. The structure of lean flames varies with equivalence ratio and spark timing and from being similar to that of stoichiometric flames to suggesting local extinction. The area of the wrinkled flame increases rapidly for small flames and slowly for larger ones. It also increases less than proportionally to engine speed between 300 and 1200 rpm and proportionally to it between 1200 and 2400 rpm. The ratio of the turbulent velocity to the laminar burning velocity measured by this novel, direct technique in a ported engine is close to the higher values deduced in valved engines by other, indirect techniques. The distributions of the radius of curvature of the flame front are symmetric and similar at 1200 and 2400 rpm (and have half-widths of about 1.6 mm) and are somewhat broader and asymmetric at 300 rpm.