Experimental Studies on Methanol Sprays for IC Engine Applications
In this work, studies have been conducted on sprays of methanol and its emulsions with diesel towards assessing the potential of methanol as an alternative fuel for IC engines. Methanol can be used in compression ignition engines as a methanol-in-diesel emulsion, and in spark ignition engines as a direct gasoline-substitute. Evaporating spray characteristics of methanol-in-diesel emulsions and pure methanol are studied in a high-pressure chamber with optical access. In the first part of the study, a comparison between two measurement methods (DBI and scattering) along with two post-processing methods (line-fit and threshold) to measure liquid length of a diesel spray is presented. The second part of the study involved assessing the stability of methanol-in-diesel emulsions with conventional surfactants such as sorbitan monooleate (Span-80®) and polyoxyethylene sorbitan monooleate (Tween-80®). The hydrophilic-lipophilic balance (HLB) values of the surfactant were varied from 7 to 15 to investigate the role of the surfactant on stability of the macroemulsion. It was observed that macroemulsions with up to 10 wt.% of methanol were stable. The macroemulsion with an HLB value of 10 gave the best stability results. In the third part of the study, the emulsion sprays were characterized in a constant volume chamber at injection pressures of 500 bar, 1000 bar, and 1500 bar in an inert atmosphere of nitrogen at 50-bar and 900-K. Interestingly, the liquid-length of the methanol-in-diesel emulsion spray with 10 wt.% of methanol using a mixture of Span-80® and Tween-80® as a surfactant is higher by around 25% as compared to that of the diesel spray, even though methanol is far more volatile than the components of diesel. This is attributed to the higher boiling point of the surfactant used, as confirmed by experiments with a low boiling point surfactant, i.e., 1-dodecanol, and recent observations from the literature. Next, microemulsions of diesel-methanol were produced by using various surfactants such as 1-dodecanol, pentanol, and butanol. Among these, 1-dodecanol was chosen as the most suitable surfactant for the microemulsion owing to its ability to create microemulsions with up to 25 wt.% of methanol, and its high cetane number. The evaporating spray liquid-length is observed to be insensitive to the methanol percentage in the emulsion when 1-dodecanol is used as the surfactant. The fourth part of the study focused on evaluating whether the phenomenon of micro-explosion in emulsion fuel droplets occurs at engine-relevant conditions of pressure and temperature. In a first-of-its-kind observation, emulsion droplets in the size range of 50 to 150-µm in a high-pressure, high temperature (50 bar, 900 K) environment did not exhibit the phenomenon of micro-explosion even up to 11 milliseconds, which is close to the timescales of relevance in practical engines. This is possibly because these timescales are not sufficient for phenomena such as coalescence of the dispersed phase and internal bubble formation to occur, which are normally precursors to micro-explosions. In the fifth part of the study, pure methanol sprays were characterized at injection pressures of 200 bar, 300 bar, 400 bar, and 480 bar in an inert atmosphere of nitrogen at three engine-relevant conditions. Data generated on liquid and vapour penetration at these conditions highlight the effect of the surrounding gas temperature and density on the spray physics. In the last part, a validated computational spray model of methanol sprays is developed using a commercial CFD solver, namely, CONVERGE-CFD. Overall, the data generated in the present work is expected to aid engine designers in adapting both compression ignition and spark-ignition engines for methanol fuel.