Fuel reactivity effects on the efficiency and operational window of dual-fuel compression ignition engines

An experimental engine efficiency study was conducted that explores the effects of direct injected fuel properties on gross thermal efficiency and operational authority as functions of intake pressure and temperature, and equivalence ratios (premixed and global) using the Reactivity Controlled Compression Ignition (RCCI) combustion strategy. The experiments were conducted in a heavy-duty single-cylinder engine at constant net IMEP of 8.45 bar, 1300 rev/min engine speed, with 0% EGR, and a CA50 combustion phasing of 0.5 CA ATDC. The engine was port fueled with E85 for the low reactivity fuel and direct injected with either #2 ULSD or 3% 2-ethylhexyl nitrate doped into 91 Anti-Knock Index (AKI) gasoline for the high reactivity fuel. The reactivity of the EHN enhanced fuel has been correlated to an AKI of approximately 56 and a cetane number of approximately 28. Intake pressure and temperature were swept independently of each other while combustion phasing and load were maintained by adjusting the global fuel reactivity and DI fuel timing as needed. The results demonstrate that for fixed cycle thermodynamics, sources of engine inefficiency are functions of the premixed and global equivalence ratios. At extremes in either, excessive losses occur, decreasing the gross thermal efficiency. The study's findings demonstrate that losses can be minimized through proper balancing of the intake pressure and temperature, which are affected by fuel reactivity differences. Specifically, when the reactivity difference between the high and low reactivity fuel streams is reduced, with the EHN + Gas/E85 strategy, the control and effectiveness of the DI fuel is reduced, making combustion more abrupt. To reduce the peak pressure rise rate, leaner conditions were required, and the peak gross thermal efficiency increased. The results demonstrate that through proper optimization of both engine conditions, and fuels, increases in engine efficiency are possible with RCCI.