Characterization of low load PPC operation using RON70 fuels

The concept of Partially Premixed Combustion is known for reduced hazardous emissions and improved efficiency. Since a low-reactive fuel is required to extend the ignition delay at elevated loads, controllability and stability issues occur at the low-load end. In this investigation seven fuel blends are used, all having a Research Octane Number of around 70 and a distinct composition or boiling range. Four of them could be regarded as 'viable refinery fuels' since they are based on current refinery feedstocks. The latter three are based on primary reference fuels, being PRF70 and blends with ethanol and toluene respectively. Previous experiments revealed significant ignition differences, which asked for further understanding with an extended set of measurements. Experiments are conducted on a heavy duty diesel engine modified for single cylinder operation. In this investigation, emphasis is put on idling (600 rpm) and low load conditions. In particular, the so-called low-temperature heat release (LTHR) is studied. The LTHR is known to be an indicator for ignition behavior in cold conditions as it is the precursor of the main combustion event due to the chain-branching reactions occurring during this phase. LTHR is found to decrease with elevated intake temperatures. This effect is strongly fuel specific and is closely linked to the n-paraffins present in the fuel. It should be noted that the LTHR effect diminishes if ignition delays shorten. Both ethanol and toluene enhance the LTHR phase. This observation is opposite to effects observed in homogeneous combustion, which implies a significant effect of mixture strength.