Low-load extension of residual-effected homogeneous charge compression ignition using recompression reaction.

摘要

Homogeneous charge compression ignition (HCCI) is a promising combustion strategy for internal combustion engines, with its high efficiency and low nitric oxide (NO) emissions compared to conventional spark ignition. The combustion process in an HCCI engine is spontaneous and occurs throughout the cylinder volume, and its rate is limited mainly by chemical kinetics. In residual-effected HCCI, exhaust gas from the previous cycle is used to mitigate combustion rate as well as to provide the sensible energy required for autoignition. This intrinsic need for dilution results in a limited load range for HCCI operation compared to its conventional counterparts.;In this dissertation, in-cylinder pre-processing (or recompression reaction) of direct-injected fuel during negative valve overlap (NVO) is investigated for low-load-limit extension of retention-mode HCCI, both experimentally and computationally. Isooctane and n-heptane are chosen as model fuels both because of the extensive work that has been undertaken to develop their chemical kinetic mechanisms (thus enabling modeling study) and because these fuels span the range of ignitability that is likely to be of interest for HCCI engines.;In the experiments, the possibility of low-load-limit extension (as low as ∼1 bar NMEP) using recompression reaction is demonstrated at high residual fraction and low equivalence ratio conditions, although overall indicated efficiency is reduced due to increased heat loss and decreased combustion efficiency. Near the low-load-limit, combustion is stable, with slightly advanced timing, relatively low UHC, very low NO (5 ppm), and slightly increased CO emissions. Optimization of recompression reaction is performed by varying geometric compression ratio, pilot injection timing, and (pilot/main) split ratio. The compression ratio study shows that there are optimum values of equivalence ratio and extent of recompression reaction that achieve the widest stable low-load operation. Pilot injection timing and split ratio variations demonstrate good controllability of the extent of recompression reaction and the subsequent main combustion phasing by affecting fuel residence time and concentration available during NVO, respectively.;Model calculations of recompression reaction and ignition delay of the recompression products using detailed mechanism are performed to elucidate the chemical and thermal effects involved in recompression reaction. The results demonstrate that recompression reaction is limited by kinetics, not thermodynamics, and that residual oxygen during NVO is a determining species for the extent and speciation of recompression reaction. The recompression product mixture exhibits improved ignitability except under lean conditions when significant oxidation during NVO leaves very low reactant concentration available for main ignition. Competing effects between thermal and chemical consequences of recompression reaction are observed on mixture ignitability, which leads to an optimum oxygen concentration (equivalence ratio) for reducing ignition delay and extending HCCI operability.