The HCCI Fuel Number - Measuring and Describing Auto-ignition for HCCI Combustion Engines

Sammanfattning: HCCI is an advanced combustion concept, using premixed fuel and air with a diluted charge. This is thermodynamically favorable, leading to high efficiency and therefore lower CO2 emissions. The well-premixed and diluted fuel charge gives lower cylinder temperatures than conventional diesel compression ignition and spark ignited gasoline combustion, resulting in low engine out emissions of both nitrogen oxides and soot.
To be able to optimize and use the advanced combustion concepts in commercial engines, knowledge of fuel behavior is needed, and a way to describe it. This thesis work provides detailed information about HCCI auto-ignition by studying parameters such as low temperature heat release and auto-ignition temperatures.
An HCCI Fuel Number is presented, developed with the purpose to describe fuel performance. By comparing fuels such as full distillate gasolines or biofuels to the required compression ratio for auto-ignition for reference fuels, a measure on HCCI fuel performance is gained. This fuel number was shown to correlate well with pre-reactions in the fuels.
The thesis work is based on CFR engine experiments, studying over 40 different reference fuels consisting of blends of n-heptane, iso-octane, toluene, and ethanol, which are model surrogates for gasoline. In addition, 21 different full distillate gasoline fuels prepared from refinery feedstocks, some with addition of single components, were tested as well. Five different inlet air temperatures ranging from 50°C to 150°C were used to achieve different temperature-pressure histories, and the compression ratio was changed accordingly to keep a constant combustion phasing, CA50, of 3±1° after TDC. The main parts of the experiments were carried out in lean operation with a constant equivalence ratio of 0.33 and with an engine speed of 600 rpm. Additional experiments were performed at higher engine speeds.
Studied fuel effects include low temperature heat release quenching effects, were ethanol was found to quench low temperature heat release at all conditions, and toluene had an in comparison very weak effect on these pre-reactions.
All conditions and fuels with extensive low temperature heat release showed similar auto-ignition temperatures. When LTHR diminished, either due to fuel quenching from ethanol or toluene, or when the inlet air temperature was increased, the auto-ignition temperature was increased.