Combined effects of intake flow and spark-plug location on flame development, combustion stability and end-gas autoignition for lean spark-ignition engine operation using E30 fuel

Lean or dilute spark-ignition engine operation can provide efficiency improvements relative to that of traditional well-mixed stoichiometric spark-ignition operation. However, to maintain a sufficiently short burn duration with the direct-injection spark-ignition engine hardware of the current study, mixed-mode combustion is required for operation with ϕ < 0.6. Such mixed-mode combustion uses a combination of deflagration and end-gas autoignition whereby the pressure rise of the deflagration-based combustion compresses the end-gas reactants to the point of autoignition. For better understanding of the transition from deflagration to autoignition, it is desirable to apply optical diagnostics. However, with the use of a single centrally located spark plug, the end-gas is found at the periphery of the combustion chamber, where it is difficult to examine optically. To overcome this, two additional spark plugs were mounted in the pent-roof gables (called East and West). Performance testing was performed for five different spark strategies: Central Only, East-West, ALL Three, East Only, and West Only. The five spark strategies are combined with swirl or no-swirl operation for a total of 10 ϕ-sweeps. A high-octane E30 fuel is used here, and intake heating is used to promote both lean combustion stability and end-gas autoignition. The best lean combustion stability is found for the ALL Three spark strategy, followed by the East-West and Central spark strategies, enabling stable mixed-mode spark-ignition combustion for ϕ down to 0.50 and 0.55, respectively. Here, operation without swirl provides the most stable combustion. High-speed imaging of ultra-lean operation without swirl at ϕ = 0.55 using the East-West spark strategy reveals that the transition from deflagration to end-gas autoignition frequently occurs within the view offered by the small piston-bowl window. These results encourage future optical investigations of fuel effects on this transition process, but a larger piston-bowl window is recommended.