Principle of determining the optimal operating parameters based on fuel properties and initial conditions for RCCI engines

Both the gasoline/diesel (G/D) and methanol/diesel (M/D) reactivity controlled compression ignition (RCCI) engines are optimized by coupling the fast non-dominated sorting genetic algorithm (NSGA) with the three-dimensional computational fluid dynamics (CFD) model at a low-to-mid load. The results indicate that G/D RCCI demonstrates better overall engine performance than M/D RCCI when the in-cylinder temperature at intake valve closing (TIVC) is below 400 K. By comparing the optimized parameters of G/D and M/D RCCI, the impacts of fuel properties on the determination of the optimal operating parameters are understood. It is found that homogeneous fuel/air distribution with high premixed ratio and advanced start of injection (SOI) of diesel is beneficial to improve the engine performance of both G/D and M/D RCCI. Meanwhile, high TIVC is indispensable to avoid the excessively late combustion phasing and the deteriorated combustion efficiency. Fuel reactivity and combustion rate are two important properties of the fuel in RCCI engines. Due to the lower reactivity and the faster combustion rate of methanol than gasoline, lower premixed ratio and more retarded SOI of diesel are adopted in M/D RCCI than G/D RCCI to avoid the excessively late CA50 and alleviate the fast combustion rate. Moreover, higher exhaust gas recirculation (EGR) rate is required for M/D RCCI to reduce its fast combustion rate. Boosting the in-cylinder pressure at IVC (PIVC) has two contradictory effects on combustion temperature and fuel efficiency. Owing to the later CA50 in M/D RCCI, the higher PIVC is favorable for M/D RCCI, while lower PIVC is preferable for G/D RCCI. When the TIVC is elevated above 400 K, the engine performance of both G/D and M/D RCCI is improved, especially for M/D RCCI.