Investigation of Compression Ignition Combustion of Glycerol-Diesel Fuel Emulsions

With Biodiesel production having established in the past century, there is an ongoing interest to add value to its abundant by-product glycerol. With regard to a shortly closed product cycle, its utilisation in the fuel sector is favourable. Beside the usage as a stand-alone fuel and chemical conversions to tailor-made oxygenated fuel blends, purified glycerol can be added to diesel fuel in form of an emulsion. The increased oxygen content of the fuel is promising to reduce soot formation when used in compression ignition engines. The objective of this work is the profound investigation of the combustion behaviour of glycerol-diesel emulsions when used as a diesel fuel. To begin with, previous studies on glycerol utilisation in internal combustion engines are summarised. Based on this knowledge the emulsion preparation method is determined and potential issues due to the properties of glycerol are identified. To obtain a broad view, the experimental investigation is conducted on two independent setups. Global parameters like performance and engine-out emissions are assessed with an instrumented six-cylinder engine which is equipped with a particulate matter and an exhaust gas analyser. Additionally, an optical accessible one-cylinder engine is used for profound investigation of flame properties and in-flame soot using OH*-chemiluminescence and light extinction high-speed imaging. The data acquired on both test rigs are analysed in detail to gain a greater understanding of the combustion properties. In the end, the results are discussed with regard to potential benefits and issues when using glycerol-diesel emulsions

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

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

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

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