A Computational Study of the Effect of Fuel Reforming, EGR and Initial Temperature on Lean Ethanol HCCI Combustion

Homogeneous charge compression ignition (HCCI) engines have great potential in ultra-low NO_x emissions, high efficiency and low particulates. The major disadvantage of HCCI lies in a narrow operating range with low power output. We investigated the expansion of the acceptable operating range (AOR) using fuel reforming complemented by exhaust gas recirculation (EGR), to control the chemical kinetics which dominates HCCI combustion. The study is carried out using a single-zone well-stirred reactor model and established reaction mechanisms. The HCCI engine is fueled with ethanol of equivalence ratio (Ф) of 0.2, 0.4 and 0.5. The (AOR) must meet both the complete combustion and the maximum NO_x limit. It is found that reforming enhances combustion and extends the complete combustion limit to lower initial temperatures, but also increases NO_x emissions. For Ф's of 0.5 and 0.4, the NO_x limit cannot be met without the complementary use of EGR to lower the NO_x emission. It is found that reforming is not as effective as EGR in widening the operating range at the Ф's studied. However, reforming may still be useful in HCCI combustion, since hydrogen is reported by others to lower cycle-to-cycle variation [1].