Effects of a Low Octane Gasoline Blended Fuel on Negative Valve Overlap Enabled HCCI Load Limit, Combustion Phasing and Burn Duration

Abstract

Homogeneous charge compression iginition (HCCI) combustion allows for the use of fuels with octane requirements below that of spark-ignited engines. A reference gasoline was compared with iso-octane and a low octane blend of gasoline and 40% n-heptane, NH40. Experiments were conducted on a single cylinder engine operating with negative valve overlap (NVO). The fuel flow rate per cycle was compensated based on the lower heating value to maintain a constant energy addition across fuels. Iso-octane and gasoline demonstrated similar maximum load, achieving a gross IMEPg of ~430 kPa, whereas the NH40 demonstrated an increased IMEPg of ^460 kPa. The NH40 could be operated at a later phasing compared with the higher octane fuels, and exhibited a shorter burn duration at a given fueling rate and phasing. These results could be due to compositional differences, as NH40 required less NVO compared to iso-octane and gasoline, leading to less thermal and compositional stratification, as well as a higher O2 concentration and less residual gas. Additionally, the NH40 fuel demonstrated a higher intermediate temperature heat release than the higher octane fuels, potentially contributing to the shorter burn duration. Overall, these results demonstrate clear benefits to NVO enabled HCCI combustion with low octane fuels.

Thermally Stratified Compression Ignition: A new advanced low temperature combustion mode with load flexibility

“…a distribution of in-cylinder gas temperatures prior to ignition) that stagger the ignition timing of various regions based on their local temperature [7][8][9][10][11][12][13][14][15][16][17]. Depending on the valve strategy and the fuel injection strategy, there can also be compositional gradients of either residual gases or fuel fraction, respectively, that can also stagger the autoignition timing of various regions [18][19][20][21][22][23]. With this new information, an improved understanding of LTC was achieved: 1) regions autoignite sequentially based on their local temperature and reactivity, and 2) the early-igniting regions release their energy first, compressing the remaining unburned regions and causing their subsequent autoignition.…”