Computational study on recompression reaction of the pilot-injected fuel during negative valve overlap by using a cycle-coupled,isooctane-fueled homogeneous charge compression ignition engine model

Abstract

In residual-effected homogeneous charge compression ignition, substantial exhaust-gas-recirculation is applied to limit the high rate of pressure rise from rapid combustion process. However, significant dilution, especially at low load, may lead to incomplete combustion and unstable operation. To improve the low-load-limit behavior, direct-fuel-injection during negative valve overlap was suggested by many researchers. Due to the high temperature and pressure conditions during negative valve overlap, the injected fuel experiences chemical reactions with trapped exhaust, referred to as recompression reaction. In this study, the possibility of the low-load-limit extension using this injection strategy during negative valve overlap is demonstrated from our previous experiments using isooctane fuel, and simulation with cycle-coupled, zero-dimensional homogeneous charge compression ignition engine model is conducted to understand the effects of recompression reaction. There are three major conclusions from our simulation. First, thermal consequence of the recompression reaction depends on the leftover oxygen in the trapped exhaust gas. Two equivalence ratios, 0.85 and 0.95, are tested in our simulation, and the overall recompression reaction is net-exothermic at 0.85, while net-endothermic at 0.95. Both conditions show significant fuel pyrolysis and reforming reactions during negative valve overlap, which are mostly endothermic, but sufficient oxidation reaction at 0.85 overcompensates for the endothermicity, leading to net-exothermicity in the end. Second, during the main compression stroke, the resultant fuel mixture from recompression reaction has overall higher specific heat ratio than the fresh fuel mixture in comparable operating conditions, which achieves higher mixture temperature rise at a given compression ratio and reaches ignition temperature earlier. Finally, in order to understand the chemical effect of the recompression reaction, ignition delays of the fuel mixture with or without recompression reaction are compared. The results show that the former has overall shorter ignition delay than the latter, which demonstrates overall improved ignitability of the fuel mixture by recompression reaction.