High-Speed Photography of Stratified Combustion in an Optical GDI Engine for Different Triple Injection Strategies

To contribute to knowledge required to meet new emission requirements, relationships between multiple injection parameters, degrees of fuel stratification, combustion events, work output and flame luminosity (indicative of particulate abundance) were experimentally investigated using a single-cylinder optical GDI engine. A tested hypothesis was that advancing portions of the mass injected would enhance the fuel-air mixing and thus reduce flame luminescence. An outward-opening piezo actuated fuel injector capable of multiple injections was used to inject the fuel using different triple injection strategies, with various combinations of late and earlier injections leading to various degrees of fuel stratification. Sprays and combustion events were captured using two high-speed cameras and cylinder pressure measurements. The data were analyzed to assess effects of fuel stratification on yellow flame luminescence(assumed to be dominated by soot luminescence), flame propagation, jet flames, pool fires and heat release. The combustion phasing, amount of fuel injected and engine speed were kept constant and the engine was unthrottled for all tested cases. Image sectorization was used to analyze events captured in different parts of the cylinder. The results show that the injection strategy influences fuel spray behavior and the combustion in terms of both flame luminescence patterns and work output. Injecting some of the fuel earlier results in increased spray liquid penetration, streakier sprays (due to the lower backpressure), and less intense yellow flame luminescence, but also reductions in work output. The greater portions of fuel injected close to the ignition results in increased soot luminescence