Effects of Low Temperature Reforming (LTR) Products of Low Octane Number Fuels on HCCI Combustion

摘要

In order to achieve high-efficiency and clean combustion in HCCI engines, combustion must be controlled reasonably. A great variety of species with various reactivities can be produced through low temperature oxidation of fuels, which offers possible solutions to the problem of controlling in-cylinder mixture reactivity to accommodate changes in the operating conditions. In this work, in-cylinder combustion characteristics with low temperature reforming (LTR) were investigated in an optical engine fueled with low octane number fuel. LTR was achieved through low temperature oxidation of fuels in a reformer (flow reactor), and then LTR products (oxidation products) were fed into the engine to alter the charge reactivity. Primary Reference Fuels (blended fuel of n-heptane and iso-octane, PRFs) are often used to investigate the effects of octane number on combustion characteristics in engines. Then PRF0 (n-heptane) and PRF50 (mixture of 50% n-heptane and 50% iso-octane by volume) were chosen as representative low octane number fuels. LTR products were quantitatively detected using online gas chromatograph (GC). High-speed imaging was conducted to illustrate the flame development. A single-zone model was used to evaluate the reactivity of LTR products. The GC measurements indicate that PRF0 and PRF50 cannot chemically react at low reformer temperature of 423 K. When the reformer temperature rises up to 523 K, LTR products mainly include hydrogen, carbon monoxides, aldehydes, alcohols, ketones, alkanes, olefins and alkynes. Due to the higher fuel reactivity, PRF0 produces more reformates than PRF50. According to the experimental engine analysis, the ignition timing is retarded significantly via LTR for both PRFs. The ignition timing difference of PRF0 due to LTR is larger than PRF50. The high-speed images reveal that LTR can lead to a slower flame development. Soot formation persists because of in-cylinder inhomogeneities, and can be lowered by LTR. The reactivity evaluation using the chemical modeling approach manifests that for PRF0 most of the LTR products inhibit mixture reactivity, while there is a large increase in the species enhancing reactivity for PRF50. The impacts of LTR products on ignition depend on both the chemical structure and the concentration in the mixture. The concentration of individual LTR product usually changes along with the reforming conditions. Thus LTR has the potential to control autoignition flexibly in HCCI engines.