Last modified: 2017-10-13
Abstract
The use of biofuels, especially in transportation and industrial processes, is seen as one of the most effective solutions to promote the reduction of the greenhouse gases and pollutant emissions, as well as to lighten the dependence from petro-fuel producers.
Biofuels are defined as a wide range of fuels in some way derived from biomass. In this category, alcohols produced in fermentation processes, such as bioethanol and biobutanol, are considered some of the most suitable for transportation purposes.
The benefits of ethanol addition to gasoline have always been recognized for practical reasons. Apart from the variety of sources which it can be produced from, ethanol can raise the octane rating of gasoline due to its better anti-knock characteristics, allowing the use of higher compression ratios and higher thermal efficiencies. However, ethanol’s high latent heat of vaporization can cause problems for cold engine starting due to excessive charge cooling and poor evaporation. On the other hand in hot climates ethanol fuelling can result in adverse effects such as vapor lock. Normal butanol has several well-known advantages when compared to ethanol, including that n-butanol has a higher energy content, greater miscibility with transportation fuels, and lower propensity for water absorption. Despite of these pros, the costs for the production of n-butanol are higher due to a lower yield than ethanol. Moreover, vaporization remains a critical aspect of this biofuel.
Some countries already consume bioethanol as a fuel in large scale, being Brazil and the United States the most important examples, using it pure or blended with gasoline in different proportions. Butanol can be considered as an emergent alternative fuel.
Understanding the effect of biofuels on in-cylinder combustion processes is a key-point for the optimization of fuel flexibility and achieving lower CO2 emissions.. To this aim, a combined thermodynamic and optical investigation was performed on a direct injection (DI) SI engine fuelled with ethanol, butanol and gasoline. All tests were performed at 2000 RPM with 1 bar intake plenum pressure and stoichiometric (ϕ = 1.0) air-fuel ratio. Fuels were compared fixing the fuel injection and spark ignition strategy.
Thermodynamic measurements were coupled with optical investigations based on the cycle resolved visualization. Optimized procedures of image processing were applied to follow the evolution of the flame front in terms of morphological parameters and to evaluate the local distribution of diffusive flames induced by fuel deposits burning in the late combustion stages. These data were correlated with exhaust gas measurements as well as opacity. The experiments confirmed that the chemical-physical specifications of the tested fuels strongly influenced the temporal and spatial evolution of the flame front. Moreover different distributions and intensities of diffusive flames were observed. These results demonstrated the effect of the fuel on the deposits amount and distribution in the combustion chamber, at fixed operative conditions.