Detailed examination of the combustion of diesel and glycerol emulsions in a compression ignition engine

David Robert Emberson,Jan Wyndorps,Ahfaz Ahmed,Karl Oskar Pires Bjørgen,Terese Løvås

doi:10.1016/j.fuel.2021.120147

David Robert Emberson, Jan Wyndorps + Show 3 more

Open Access

https://doi.org/10.1016/j.fuel.2021.120147

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Abstract

This study examines glycerol as an additive to diesel fuel to demonstrate it has the potential to suppress the formation of soot/PM. The investigation of a diesel/glycerol emulsion included an engine trial, high-speed imaging in an optical combustion chamber and a fundamental chemical kinetic study examining soot precursor formation. The emulsion had a longer ignition delay but higher AHRR with increasing load. There was no impact on the brake thermal efficiency. CO and THC were higher with the emulsion at the lower engine loads. The emulsion emitted a smaller number of particles with diameters greater than 25 nm, with a significant drop in the number of particles at 60 nm. The number of particles with diameters greater than 25 nm is reduced by 61% at 20 Nm, by 56% at 80 Nm, and by 11% at 140 Nm. A large peak of sub 10 nm particles, 2 orders of magnitude greater than with diesel alone, was observed, hypothesised to be semi-volatile organic compounds that have started to condense. A thermogravimetric analysis supported a larger semi-volatile content. Ignition delay time, determined from the OH* flame emission, was always longer for the emulsion at all conditions. In-flame soot was always lower with the emulsion at all conditions. Flame lift-off length decreased with increasing temperature and pressure of the ambient gas whilst soot increased. The concentration of known soot precursors, C2H2 and C2H4 was reduced but the concentrations of C3H6 and PC3H4 were not significantly affected.

Highlights

Particulate matter (PM) emissions from compression ignition (CI) engines pose a considerable environmental and public health problem
Glycerol is immiscible with diesel, diesel/glycerol emulsions were prepared using a mixture of the surfactants Span 80 and Tween 80 with a lipophilic–hydrophilic balance of hydrophiliclipophilic balance (HLB) = 6.4, as suggested by Sidhu [23]
Homogeneous constant pressure reactor cases were run with both the fuels to observe the evolution of soot precursors such as ethene (C2H4), ethyne (C2H2), propene (C3H6) and propyne (PC3H4) during the combustion process and the concentrations of these species long after the ignition event

Summary

Introduction

Particulate matter (PM) emissions from compression ignition (CI) engines pose a considerable environmental and public health problem. Soot emissions represent the main portion of PM and can be reduced using oxygenated fuels; the presence of oxygen in the fuel aids the soot oxidation process and changes the structure of the soot in a way that facilitates oxidation [2,3] This means it is worth while to investigate potential oxygenatedfuel-additives for use in large CI engines with the goal to reduce soot emissions. The use of oxygenated fuels in CI engines has been studied extensively and shows a general trend of reducing PM emissions, usually attributed to the oxygen content in the fuel [4] This effect has been observed in studies examining in-flame soot production [5,6,7] and engine out emissions [4,8,9,10], though size distribution of the PM in the exhaust is not commonly reported [11]. If there is an increase in the smaller sizes of PM, which are more harmful to human health, the perceived benefit of PM mass reduction may be negated

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