Toulson, E., Huisjen, A., Chen, X., Squibb, C. et al., "Visualization of Propane and Natural Gas Spark Ignition and Turbulent Jet Ignition Combustion," SAE Int. J. Engines 5(4):1821-1835, 2012, doi:10.4271/2012-32-0002.
This study focuses on the combustion visualization of spark ignition combustion in an optical single cylinder engine using natural gas and propane at several air to fuel ratios and speed-load operating points. Propane and natural gas fuels were compared as they are the most promising gaseous alternative fuels for reciprocating powertrains, with both fuels beginning to find wide market penetration on the fleet level across many regions of the world. Additionally, when compared to gasoline, these gaseous fuels are affordable, have high knock resistance and relatively low carbon content and they do not suffer from the complex re-fueling and storage problems associated with hydrogen. Although both propane and natural gas offer unique lean burn benefits for spark ignition combustion, a novel Turbulent Jet Ignition pre-chamber system was also evaluated, with several Turbulent Jet Ignition optical images compared to the spark ignition images in order to provide insight into the lean limit extension provided by the pre-chamber combustion system. Turbulent Jet Ignition enables very fast burn rates due to the ignition system producing multiple, widely distributed ignition sites, which consume the main charge rapidly. This high energy ignition results from the partially combusted (reacting) pre-chamber products initiating combustion in the main chamber. The distributed ignition sites enable relatively small flame travel distances enabling short combustion durations and ultra lean engine operation exceeding lambda 2. Experimental findings from the spark ignition study highlight faster flame propagation with propane when compared to natural gas across all speed load points tested. However, across all fuels, the optical images revealed rapid spark ignition combustion deterioration with lean engine operation exceeding lambda 1.4. When comparing ignition systems, the Turbulent Jet Ignition pre-chamber system demonstrated significant benefits relative to the spark ignition system, with the optical images revealing stable combustion past lambda 1.8 resulting in the near elimination of NOx emissions in-cylinder and significant improvements in efficiency and fuel economy.