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Investigation of Fuel Effects on Combustion Characteristics of Partially Premixed Compression Ignition (PPCI) Combustion Mode at Part-Load Operations
Abstract:
Fuel effects on combustion characteristics, including combustion robustness/stability, for partially premixed compression ignition (PPCI) combustion was investigated using Delphi’s second-generation gasoline direct-injection compression ignition (Gen2 GDCI) multi-cylinder engine. Three high-reactivity RON 80 gasoline fuels were evaluated in this study. First, the effect of octane sensitivity (RON-MON) was investigated by comparing two non-oxygenated gasolines with octane sensitivities of 2.4 and 5.1. The octane sensitivity difference of the two fuels arose from different hydrocarbon compositions. Second, the effect of octane sensitivity origin was evaluated with two fuels having the same octane sensitivity of 2.4-one fuel was non-oxygenated, while the other one contains ethanol. The engine performance and emissions comparison was focused on part-load operations (1500 rpm, 6 bar IMEP and 800 rpm, 2 bar IMEP) that implemented PPCI low temperature combustion.
Better combustion robustness when using higher octane sensitivity fuel was observed, independent of the hydrocarbon compositions or oxygenate concentrations. Combustion robustness was enhanced through a faster main combustion process, along with lower engine-out nitrogen oxides (NOx) and carbon monoxide (CO) emissions for the higher octane sensitivity fuel. The ignition delay time (IDT) calculated by using toluene reference fuel (TRF) surrogate fuels with different octane sensitivities supported the findings from the experimental results. When ignition occurs within the intermediate temperature regime, in-cylinder temperatures and pressures were found to be more representative of the MON test conditions. As a result, higher octane sensitivity and lower MON fuels showed less pronounced negative temperature coefficient (NTC) behavior and higher reactivity, thereby, contributing toward the more robust combustion process observed experimentally. These results demonstrated the benefits of optimizing fuel composition for the PPCI combustion process to achieve faster, more robust combustion along with lower NOx and CO engine-out emission levels.