Abstract
Research and/or Engineering Questions/Objective
The autoignition characteristics of fuel affect the combustion in the different combustion regimes, such as spark ignition, compression ignition and partial premixed compression ignition. The autoignition property depends on the physical and chemical properties of fuel (e.g. hydrocarbon composition , distillation range and viscosity). Experiments using primary reference fuels conducted in a heated constant volume vessel have shown that ignition delay and burn time are related to premixed versus non-premixed combustion (Rabl, Davies et al. 2015). The engineering question to be answered in this work is whether this useful relationship holds for refinery based fuels.
Methodology
A range of refinery based fuels were tested in a constant volume vessel to find the relationship between physical and chemical properties against combustion characteristics, such as ignition delay, burn period, together with the pressure history and the associated heat release analysis to provide information on the combustion modes. Visualisation using chemiluminescence and high speed imaging provided further information on mixing, ignition and combustion (as in Founti, Hardalupas et al. 2015). The fuels tested were diesel like fuels with cetane numbers between 43 and 72, gasoline with 10% Ethanol spanning different combinations of RON (92/95) and MON (77-87) to vary the octane sensitivity, along with two naphtha type fuels. These were injected in a constant volume vessel with ambient conditions varied between temperatures of 500-590°C, pressures between 28-75 bar, injection pressures between 400-1600 bar and oxygen content between 15%-21%.
Results
The results show the relationship between ignition delay and burn duration for these refinery based fuels, giving an indication of whether the mode is premixed or non-premixed combustion, with the properties of the fuels as parameter. The effect of different operating conditions helps to isolate the fuel property which has the biggest impact on the ignition delay. Also, this provides further information towards better understanding of the outcome of the subsequent single engine testing using the same set of fuels.
Limitations of this study
The limitations of the study are related to
1. the use of a constant volume vessel, rather than an engine, the primary limitation relating to the difference in the volume of the two configurations
2. there is no turbulence in this vessel.
In defence of limitation 1, such vessels are widely used in industry not only for their convenience but also because the conditions can be well controlled as opposed to the differences that could arise which are related to particular engine designs.
In defence of limitation 2, a substantial amount of the turbulence which relates to conditions at autoignition is generated through the shear associated with the high momentum of the liquid spray rather than the turbulence which is generated through the gas motion near top dead centre.
What does the paper offer that is new in the field in comparison to other works of the author
This work extends the work of the authors to refinery based fuels. References Founti, M., et al. (2015). An Experimental Investigation on the Effect of Diluent Addition on Flame Characteristics in a Single Cylinder Optical Diesel Engine, SAE Technical Paper. Rabl, S., et al. (2015). "Understanding the relationship between ignition delay and burn duration in a constant volume vessel at diesel engine conditions." Proceedings of the combustion institute 35(3): 2967-2974.