What is the energy efficiency of ethanol as a fuel?

The energy efficiency of ethanol as a fuel is relatively low compared to fossil fuels like petrol and diesel.

Ethanol is a type of biofuel, which is derived from plant materials, primarily corn and sugarcane. It is considered a renewable energy source as it is produced from crops that can be grown annually. However, when it comes to energy efficiency, ethanol doesn't perform as well as fossil fuels.

Energy efficiency is measured by the amount of usable energy that can be extracted from a certain amount of fuel. For ethanol, this is around 34-35%, which means that only about a third of the potential energy in the fuel is actually converted into useful work. This is significantly lower than the efficiency of petrol and diesel, which is around 45% and 40% respectively.

The lower energy efficiency of ethanol is due to its lower energy density. Energy density is the amount of energy stored in a given system or region of space per unit volume or mass. Ethanol has an energy density of about 24 megajoules per litre, while petrol has an energy density of about 35 megajoules per litre. This means that you need more ethanol to get the same amount of energy as you would from petrol.

Furthermore, the production of ethanol also requires a significant amount of energy. This includes the energy used in growing the crops, harvesting them, transporting them to the processing plant, and then converting them into ethanol. When you factor in this energy input, the net energy gain from ethanol is even lower.

In conclusion, while ethanol is a renewable and cleaner-burning fuel, its energy efficiency is relatively low compared to fossil fuels. This is one of the reasons why it is often blended with petrol or diesel, rather than being used on its own.