An Optical Study of Spark Ignition and Flame Kernel Development Near the Lean Limit at Elevated Pressure

Abstract

Spark ignition of lean and dilute fuel-air mixtures provides emission reductions of NOx. Furthermore, operation at the lean-dilute limit increases engine efficiency through reduced pumping loses and reduced heat transfer. However, ignition near the lean flammability limit becomes more stochastic and exhibits substantially decreased flame propagation rates. In this work, spark ignition and the subsequent flame kernel development and propagation are studied in a constant volume optical combustion vessel. The vessel provides full field orthogonal and line-of-site optical access via sapphire windows. Additionally, an automated process controller with a versatile gas system enables the creation of a wide range of fuel-air mixtures, including lean and dilute mixtures of hydrocarbons, oxygen, nitrogen, carbon dioxide, and other gases. Ambient conditions including in-chamber temperature and pressure levels, along with dilution conditions, can be set independently. Ignition is provided by an automotive spark plug in the chamber. Optical diagnostics including simultaneous CH* chemiluminescence and shadowgraph imaging are utilized to characterize initial kernel growth and flame development under elevated pressure conditions, from atmospheric to 17.3 bar. Chemiluminescence images are quantified to determine flame intensity and kernel radius to understand the success of initial flame kernel development and propagation. Increasing the pressure yields a slower rate of flame kernel development and propagation, with a thickening flame front, which in turn increases the effects of buoyancy and heat loss. Leaning the mixture can yield unsuccessful kernel development due to heat loss to the large electrode which may cause a failed sustaining of combustion. This knowledge on kernel development near the lean limit benefits the engine community by characterizing the importance of ambient conditions including pressure and mixture properties in sustaining flame growth and propagation.