Analysis of surrogate fuels effect on ignition delay and low temperature reaction during partially premixed combustion

Fuel effects on ignition delay and low temperature reactions (LTR) during partially premixed combustion (PPC) were analyzed using Design of Experiments (DoE). The test matrix included seventeen mixtures of n-heptane, isooctane, toluene and ethanol covering a broad range of ignition quality and fuel chemistry. Experiments were performed on a light duty diesel engine at 8 bar IMEPg, 1500 rpm with a variation in combustion phasing, inlet oxygen concentration and injection pressure. A single injection strategy was used and the start of injection and injection duration were adjusted to achieve the desired load and combustion phasing. The experimental data show that fuels with higher Research Octane Number (RON) values generally produced longer ignition delays. In addition, the alcohol content had significantly stronger effect on ignition delay than the aromatic content. Fuels with more ethanol gave longer ignition delays and a combination of high level of ethanol and toluene produced the longest ignition delay. An increased inlet oxygen concentration shortened the ignition delay. Surprisingly and in contradiction to what has been reported for HCCI combustion, ethanol amplified the LTR phase, while n-heptane suppressed it. Finally, the LTR phase was proportional to the ignition delay. Longer ignition delay resulted in higher fraction of LTR.