Study of Second Generation Biofuels in Internal Combustion Engines
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The search for alternative fuels to replace the existing petroleum-based products has intensified. Fossil fuels are non-renewable and the uncertainty in oil prices puts a strain on several national economies. In addition, the negative impacts of the emissions resulting from the combustion of fossil-hydrocarbons on environment and human health are causes for concern. These noxious emissions may be curtailed by adopting different approaches of fuel modification, modification of combustion chamber design and by after-treatment of the exhaust gas. This piece of research focused on the fuel modification approach to emissions reduction. In order to address the food-fuel-fiber conflicts, the alternative fuel has to have its source preferably from non-food crops; a second generation bio-fuel in other words. Therefore, non-food-crop-based blends of biomass-derived biodiesel (BD), derived from Jatropha curcus, is considered a good choice. Trade-off is inevitable, however this bio-diesel has a high viscosity and thereby problems associated with its spray characteristics. In order to tide over these challenges, it is blended with low-viscosity fuels such as ethanol or Fischer-Tropsch (FT) synthetic diesel. This research studied the feasibility and usage of bio-fuels in IC engines to address the aforesaid concerns, understand what the fundamentally, acceptable emission levels are and also determine the optimal blending limits of bio-fuels. In this context, the experimental works are carried out as two cases. First, the experimental investigations on blends of Fischer-Tropsch (FT) synthetic diesel and Jatropha methyl ester (JME), as fuel in a diesel engine, are carried out. For this purpose, the JME was produced by the trans-esterification process and the blends are tested against EN14214, or in some cases, ASTM standards. The volumetric blending percentages of BD to FT fuel are 0:100, 25:75, 50:50, 75:25 and 100:0 respectively. Fourier Transform Infra-Red (FTIR) spectroscopy results indicate that the BD and FT fuels are mainly aliphatic and ester compounds. The gas chromatography (GC) result shows that the neat BD (B100) contains more than 97% ester. The experiment was conducted with a four-stroke, six-cylinder, turbo-charged, direct injection (DI) diesel engine with an optimum engine speed of 1450 rpm and a dynamic fuel injection timing of 20 crank angle degrees (CAD) before the top dead centre (TDC). The experimental results showed that the exhaust emissions including carbon monoxide (CO), total unburnt hydrocarbon (THC), smoke, total particulate matter (TPM) and oxides of nitrogen (NOx) were reduced with FT fuel, vis-à-vis neat diesel fuel (DF). The CO, THC, smoke and TPM emissions were reduced significantly, while NOx emissions were somewhat higher with BD blended fuels compared to neat FT fuel. From this study, the reductions in CO, THC, smoke and TPM emissions with BD blends were mainly due to the oxygen content in the BD blended fuel, while the increases in NOx emissions with BD fuels were due to advances in injection timing, higher percentages of fatty acids with double bonds in the carbon chain and higher heat release in the pre-mixed combustion. As regards engine performance and emissions, non-edible renewable BD blends can be excellent competitors as alternate fuels for diesel engines. Secondly, a four-stroke, single-cylinder, naturally-aspirated (NA), direct-injection (DI) diesel engine with 8 BHP at 1500 rpm coupled with water-cooled, eddy current dynamometer was used for the experiments. Ethanol (5% by volume) was injected into the intake manifold by the port injection method with the assistance of a mechanical fuel injection pump. Therefore, the volumetric blending percentages of ethanol, BD and diesel fuels (E:D:JME) are (0:100:0), (5:95:0), (5:75:20), (5:55:40), (5:35:60), (5:15:80) (5:0:95) and (0:0:100) respectively. Ethanol pre-mixed with intake air, assisted in improving combustion in both diesel and the JME blends. The addition of ethanol to high-viscosity Jatropha methyl ester (JME) through port injection is investigated in order to determine its effect on the fuel’s viscosity and thereby on the diesel engine performance. In addition to viscosity alteration, the impact of ethanol addition on combustion characteristics such as combustion duration, ignition delay and emissions levels from diesel engines fuelled with blends of ethanol, diesel and JME was studied in particular. It was found that blending of oxygenated fuels with diesel modifies the chemical structure and physical properties which in turn, alter the engine’s operating conditions, combustion parameters and emissions levels. However, the injection of only 5% ethanol through port injection allows for up to 25% blending of diesel with biofuels, while retaining the fuel characteristics. Furthermore, a two-zone combustion model was used to perform numerical analysis. Both the experimental and numerical results showed that 5% ethanol addition in JME-blended diesel results in a slight increase in fuel consumption and improved the engine performance about 3-5%. Also, the combustion characteristics were improved which includes the maximum in-cylinder pressure, cumulative heat release (CHR) rate of heat release (ROHR), in-cylinder peak temperature and combustion duration. Regarding emission characteristics, the experimental results showed a significant reduction in emissions like carbon monoxide (CO), total hydrocarbon (THC), by 51%, 40% and 43% respectively. However, oxides of nitrogen (NOx) emissions were found to increase at high loads although the common trade-off between smoke and NOx is found to be more prominent for the oxygenated fuels; presumably owing to the presence of oxygen atoms in the fuel’s molecular structure. Finally, the results showed that the optimum blending limits are FT:BD (25:75) and E:D:JME (5:75:20). The thesis presents the conclusions along with recommendations for further research.