On the combustion of premixed gasoline - natural gas dual fuel blends in an optical SI engine

Natural Gas (NG) is a promising alternative fuel. Historically, the slow burning velocity of NG poses significant challenges for its utilisation in energy efficient Spark Ignited (SI) engines. It has been experimentally observed that a binary blend of NG and gasoline has the potential to accelerate the combustion process in an SI engine, resulting in a faster combustion even in comparison to that of the base fuels. The mechanism of such effects remains unclear. In this work, an optical diagnosis has been integrated with in-cylinder pressure analysis to investigate the mechanism of flame velocity and stability with the addition of NG to gasoline in a binary Dual Fuel (DF) blend. Experiments are performed under a sweep of engine load, quantified by the engine intake Manifold Air Pressure (MAP) (0.44, 0.51. 0.61 Bar), and equivalence air to fuel ratio (Φ = 0.8, 0.83, 1, 1.25). NG was added to a gasoline fuelled engine in three different energy ratios 25%, 50% and 75%. The results showed that within the flamelet combustion regime, the effect of Markstein length is dominating the lean burn combustion process both from a stability and velocity prospective. The effect of the laminar burning velocity on the combustion process gradually increases as the air fuel ratio shifts from stoichiometric to fuel rich values.