Mixture Stratification for CA50 Control of LTGC Engines with Reactivity-Enhanced and Non-Additized Gasoline 2021-01-0513

Low-temperature gasoline combustion engines can provide high efficiencies with very low NO_x and particulate emissions, but rapid control of the combustion timing (50% burn point, CA50) remains a challenge. Partial Fuel Stratification (PFS) was recently demonstrated [2019-01-1156] to control CA50 over a wide range at some selected operating conditions using a regular-grade E10 gasoline. PFS was produced by a double direct injection (D-DI) strategy using a gasoline-type direct injector. For this D-DI-PFS strategy, the majority of the fuel is injected early in the intake stroke, establishing the minimum equivalence ratio in the charge, while the remainder of the fuel is supplied by a second injection at a variable time (SOI2) during the compression stroke to vary the amount of stratification. Adjusting the stratification changes the combustion timing, and this can be done on a cycle-to-cycle basis by adjusting SOI2. The current work expands the understanding of D-DI-PFS by investigating the effects of global equivalence ratio, variations in the fuel-fraction split between the two injections, and intake pressure on the ability of D-DI-PFS to control CA50, for both a regular-grade gasoline and this same gasoline additized with 2-ethylhexyl nitrate (EHN) to enhance its autoignition reactivity. Moreover, the understanding gained from these fixed-load experiments was applied to determine the ability of D-DI-PFS to control CA50 through a load sweep from 0.42 ≥ φ ≥ 0.30 at intake pressures equal to 1.0 bar and 1.3 bar for both the non-additized and EHN-additized gasolines. For each load, SOI2 was adjusted to keep CA50 between the knock and misfire limits, while also considering emissions constraints. For both fuels, D-DI-PFS was shown to be effective for maintaining appropriate CA50 phasing over the sweep. Furthermore, NO_x emissions were kept close to the US-2010 HD limit and combustion stability (COV-IMEP_g) was kept to a reasonable value.