The stricter worldwide emission legislation and growing demands for lower fuel consumption and anthropogenic CO2 emission require significant efforts to improve combustion efficiency while satisfying the emission quality demands. Ethanol fuel combined with gasoline provides a particularly promising and, at the same time, a challenging approach. Ethanol is widely used as an alternative fuel or an effective additive of gasoline due to the advantage of its high octane number and its self-sustaining concept, which can be supplied regardless of the fossil fuel. As a result, vast study has been carried out to study its effects on engine performance and emission.
The first part of this article discusses prospect of fuel ethanol as a gasoline substitute. Then it discusses comparative physicochemical properties of ethanol and gasoline. The slight differences in properties between ethanol and gasoline fuels are enough to create considerable change to combustion system as well as behaviors of SI engines. These effects lead to several complex and interacting mechanisms, which make it difficult to identify the fundamentals of how ethanol affects NOx emission. After that, general NOx forming mechanisms are discussed to create a fundamental basis for further discussion. Finally, the article discusses different fuel composition, engine parameter and engine modification effects on NOx formation as well as mathematical approach for NOx prediction using ethanol.
Many studies have been done on SI engines as well as flex-fuel vehicles using either pure ethanol or ethanol–gasoline blends as a fuel. Section 3 shows some dissimilarity in properties between ethanol and gasoline. These properties may affect in fuel con-sumption, combustion speed, combustion temperature, mass burn fraction etc. as well as NOx emission. Many literatures [56,82–87] have shown increase in NOx emission for ethanol. Opposite trend was also observed by many researchers [29,88–90] as well. Few literatures [91,92] also found irregularity in NOx emission with ethanol. In this section, NOx emission causes will be discussed for ethanol–gasoline blends, emphasizing on different fuel blends, different engine parameters and engine modifications as well as different vehicle conditions.
Effect of fuel composition
Effect of blend concentration
Researchers have tested ethanol–gasoline blends from 5 vol%ethanol to as high as 100 vol% i.e. pure ethanol in SI engines. The physicochemical properties of different ethanol–gasoline blends are summarized in Table 5. These results presented have been obtained by different test methods as done by the researchers. From Table 5 it can be seen that the addition of ethanol to gasoline simultaneously increases the octane number, density, and latent heat of vaporization and decreases the heating value of the ethanol–gasoline blend. Many investigations have been carried out to identify the effect of these changes on emission character-istics especially on NOx emission due to variation in ethanol contents.
