**1. Introduction**

Around the world, energy is one of the major sources for the improvement and development of human beings life standards and its sustainable development. With rapid growth in the world population during the past few decades, the energy requirements that have also increased at an even large rate in industrialization and transportation sectors lead to an increase in crude oil prices, which is directly affected by global economic activity [1]. Now a days, worldwide 80% of fossil fuels consumed as primary energy, of which 58% of fossil fuels are consumed by the transportation sector only [2]. Since 1973, worldwide the primary energy demand has been increased at the rate of 2.0% on average per every year. As well as, still around one third of world's population are dependent on non-commercial fuels (United Nations 2007, 1). One of the significant environmental related issues is the widespread usage or burning of fossil fuels in many industries and transportation which are major contributors to air pollution, ozone depletion, global warming, climatic changes and human health-related problems. However, CO2 is an important pollutant, which is produced by improper combustion of fuel and other major pollutant is NO*x*, which is produced from both natural and man-made processes.

Moreover, SO2 is one of the major air pollutants; it is released by the burning of fossil fuels like coal, petroleum and other factory combustibles.

In the 1960s, environmental awareness was brought to public attention, when smog became a major issue in developed cities such as Mexico, Los Angeles and Tokyo City. According to National Energy Strategy in February, 1992, The United States of America used more than 185 million busses, cars and trucks for transportation, which consume two-thirds of the oil used by the United States. Likewise, in India 50% of oil was consumed for transportation in 1991; it has been increased to up to 61% by the year 2010. Day to day the oil usage was rapidly increased, it causes the severe damage of environment. In the year of 1992 June, Earth Summit conducted by the United Nations on Environment and Development (UNCED). In this summit mainly the delegates all over the world aimed to decrease the global warming.

Thus, the search for possible alternatives to fossil fuels becomes essential. In this regard, an ideal replacement would be characterized by renewable, sustainable, efficient, and cost effective energy sources with fewer emissions [3, 4]. Among many energy alternatives, alternative fuels are the most environment friendly energy source. A worrying statistical analysis is that, the global oil and gas production is approaching its maximum production level and the world is now finding one new barrel of oil for every four it consumes. Therefore, alcohol fuels are the best alternative to fossil fuels; alcohol fuels have been represented as a future leading supplier of energy sources that have the ability to increase the security of supply, reduce the amount of vehicle emissions, and offered a stable income for farmers. Right now, alcohol fuel used as alternative fuel instead of fossil fuels in different motor vehicles (busses, cars, trucks, etc.) in most of the countries and **Figure 1** shows the bus running with alcohol fuel [5].
