Combined Effects of Fuel and Dilution Type on Efficiency Gains of Lean Well-Mixed DISI Engine Operation with Enhanced Ignition and Intake Heating for Enabling Mixed-Mode Combustion

Well-mixed lean or dilute SI engine operation can provide efficiency improvements relative to that of traditional well-mixed stoichiometric SI operation. However, the realized gains depend on the ability to ensure stable, complete and fast combustion. In this work, the influence of fuel type is examined for gasoline, E30 and E85. Several enabling techniques are compared. For enhanced ignition stability, a multi-pulse (MP) transient plasma ignition system is compared to a conventional high-energy inductive spark ignition system. Combined effects of fuel type and intake-gas preheating are examined. Also, the effects of dilution type (air or N-2-simulated EGR) on lean efficiency gains and stability limits are clarified. The largest efficiency improvement is found for lean gasoline operation using intake preheating, showing the equivalent of a 20% fuel-economy gain relative to traditional non-dilute stoichiometric operation. The reason for gasoline's larger efficiency improvement is its lower octane number which facilitates the use of end-gas autoignition to produce mixed-mode combustion. For these conditions, such mixed-mode combustion is required for rapid completion of the inherently slow lean combustion event prior to piston expansion. The fuel-economy gains are somewhat smaller for both E30 and E85 because of higher resistance to end-gas autoignition under lean conditions. To avoid knocking cycles when mixed-mode combustion is used, the deflagration-based combustion must be very repeatable to ensure consistent compression of the end-gas reactants. Multi-pulse transient plasma ignition is used beneficially to stabilize the combustion, especially for dilute operation which suffers from low flame speeds. However, even with an enhanced ignition system, the best fuel-economy gains of dilute stoichiometric operation with mixed-mode combustion are on the order of 11-12%, which is substantially less than for lean operation.