Abstract
The late-injection strategy can decrease peak pressure rise rate (PRR) in reactivity controlled compression ignition (RCCI) and enables RCCI operation to high engine load. However, excessive soot emission is a limiting factor in this high engine load extension. Recently, polyoxymethylene dimethyl ethers (PODE) emerges as a promising direct-injection fuel for RCCI load extension due to the properties of high reactivity and high oxygen content. The late-injection RCCI strategy features a two-stage high-temperature heat release (HTHR) that has not been well understood. In this study, we investigated the effects of direct-injection fuel properties types and proportion on late-injection RCCI strategy on a light-duty optical engine using multiple optical diagnostic techniques. [so-octane served as the premixed fuel and the in-cylinder direct-injection timing was set at the crank angle of -10 degrees after the top dead center. Firstly, the combustion characteristics of RCCI with direct-injection fuels of n-heptane, PODE and cetane were compared. The NFL images prove that the PODE case shows less tendency of soot formation. The OH planar laser-induced fluorescence (PLIF) imaging indicates that the OH radical is widely distributed in the combustion chamber after the HTHR including the central part of the combustion chamber near the injector nozzle for all three cases. Secondly, the effects of direct-injection fuel proportion on the combustion characteristics of the late-injection RCCI strategy was evaluated using PODE as the direct-injection fuel. The NFL images show that the combustion regime in the low reactivity region of RCCI tends to change from auto-ignition to flame front propagation with the decreasing of the direct-injection fuel proportion from 30% to 6%. The introduction of the flame front propagation in the second-stage HTHR reduces the peak pressure rise rate of RCCI. We conclude that the peak PRR of RCCI at high engine load can be controlled by modulating the ratio between the auto-ignition and flame front propagation through tuning the proportion of the high-reactivity fuel in the direct injection. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.