Soloiu, V., Muinos, M., and Harp, S., "Investigation of Dual Fuel PCCI (PFI of n-Butanol and DI-ULSD) Compared with DI of Binary Mixtures of the Same Fuels in an Omnivorous Diesel Engine," SAE Technical Paper 2015-01-0857, 2015, doi:10.4271/2015-01-0857.
In this study, a Premixed Charge Compression Ignition (PCCI) obtained by sequential dual fueling strategy of n-butanol port fuel injection (PFI) and direct injection of ULSD#2 was investigated against binary mixtures combustion (defined as premixed in the tank) of n-butanol and ultra-low sulfur diesel (ULSD#2) with the same n-butanol to diesel ratios (35%, 50%, 65% by mass) in an omnivorous compression ignition engine. The hypothesis of the study is that combustion phasing (respectively CA50) can be successfully controlled by the above named strategies. Both fueling strategies controlled the high reactivity of the ULSD#2 and slowed down the chemical reactions with the low cetane number fuel, n-butanol. These processes led to fuel reactivity stratification and an increase in the ignition delay observed as the amount of n-butanol increased. In the experiments the dual fuels strategies resulted in CN stratification, the combustion started with the auto-ignition of ULSD#2, followed by flame propagation throughout the homogenous n-butanol-air mixtures for the PCCI case, or entrainment of air and n-butanol for the binary mixtures. The n-butanol fueling strategies resulted in a staggered and stratified combustion process controlled by the mixture reactivity stratification. As the n-butanol mass content increased the ignition delay increased because of the greater distribution of the low reactivity fuel in the combustion chamber. For the binary mixtures with 65% n-butanol by mass, the lowest local & global reactivity mixtures were obtained, resulting in a longer ignition delay and allowing an over-mixing of the high reactivity fuel resulting in a late high temperature heat release. ULSD#2 exhibited the lowest ringing intensity, with a value of 1.3 MW/m2. When butanol was added into combustion the ringing intensity increased. Bu65-DI exhibited the largest ringing intensity with a value of 16.6 MW/m2 and very heavy knock. For the binary mixtures, the ringing intensity increased as the percentage of butanol increased. Bu35-PFI exhibited the largest ringing intensity for the PFI strategy at 14.9MW/m2 for the PFI strategy. The ringing intensities for both fueling strategies exhibited opposite trends and they correlate well with AHRR. As the mass of n-butanol increased, the soot emissions drastically reduced for all tested blends and fueling strategies due to longer ignition delay, better mixing, and an increase of the lower reactivity fuel amount with lighter molecules of alcohol that produce less soot. Incomplete oxidation of over-lean and cold mixture zones near the wall and crevice phenomena were factors that may have caused elevated unburned hydrocarbons (UHC) emissions. The CA10 results indicate that the PCCI strategy starts combustion faster than the binary mixtures by 10CAD, with a 5CAD lead at CA50 and a combination of these strategies can give higher combustion flexibility and control.