A study of mixture formation in a lean burn research engine using laser fluorescence imaging

摘要

Lean burn in spark-ignition engines offers a significant efficiency advantage comparedwith stoichiometric operation. The lean operation is restricted by increasing cyclicfluctuation in torque. In order to make use of the efficiency advantage and meet themandatory emission standards the lean operation limit has to be further extended. Thisrequires particular control of the mixing of fuel and air.To study the effect of mixture formation on cyclic variability and to providequantitative information on the mixing of air and fuel planar laser-induced fluorescence(PLIF) was developed and applied to an operating SI engine. The method is based onimaging the fluorescence of a fluorescent marker (3-pentanone) mixed with the fuel(iso-octane). 3-pentanone was found to have similar vaporisation characteristics tothose of iso-octane as well as low absorption and suitable spectral properties.The technique was applied to an one-cylinder SI engine with a cylinder headconfiguration based on the Honda VTEC-E lean burn system. The mixture formationprocess during the inlet and compression stroke could be described by measuring theaverage fuel concentration in four planes, between 0.7 and 15.2 mm below the sparkplug, in a section of the cylinder orthogonal to the cylinder axis. The results showedthat for 4-valve pent-roof cylinder head systems with swirl inlet flows, fuel impingingon the cylinder wall opposite to the inlet valves has a major influence on the mixtureformation process.In order to quantify the cyclic variability in the mixture formation process and itscontribution to cyclic variability in combustion the fuel concentration in a plane nearthe spark plug was measured on a large number of cycles. It could be shown, that thefuel concentration in a small region close to the spark plug has a dominating effect onthe subsequent pressure development for lean mixtures. Variations in the mixtureconcentration in the vicinity of the spark plug contribute significantly to cyclicvariations in combustion.In order to address the issue of nonuniformity in residual gas concentration prior toignition a laser induced fluorescence method was developed to measure nitric oxide(NO) concentrations in the unburned charge in the same one-cylinder research engine.Measurements of average and instantaneous NO concentrations revealed, that theresidual gas is not homogeneously mixed with the air and that significant cyclicvariations in the local residual gas concentration exist.