Calorific Value of Ethanol

Calorific value is the heat released when a unit mass or volume of fuel is completely burned. Ethanol contains less chemical energy per litre than petrol, so an ethanol-petrol blend generally supplies less energy from each litre. Consequently, the vehicle may travel fewer kilometres per litre even when combustion remains efficient.

From lower energy content to lower mileage

A vehicle needs a certain amount of useful mechanical energy to overcome rolling resistance, air resistance and other loads. Because ethanol has a lower volumetric calorific value, more litres of an ethanol-containing fuel must be burned to deliver broadly the same energy to the engine.

• As ethanol's share rises, the blend's energy per litre generally falls relative to petrol.
• The fuel-management system may increase the quantity injected to maintain the required power and combustion conditions.
• Higher volumetric fuel consumption translates into lower kilometres per litre, other conditions remaining similar.

Why the mileage loss is not mechanically fixed

The reduction in fuel economy need not equal the reduction in calorific value because mileage also depends on engine efficiency, calibration and operating conditions. Ethanol's high octane quality can support efficiency-enhancing engine designs, which may partly offset its lower energy content.

• An engine specifically designed and calibrated for ethanol blends can use the fuel more effectively than an unmodified petrol engine.
• Driving cycle, vehicle load, blend level and the composition of the base petrol influence the observed mileage.
• Lower fuel economy per litre does not necessarily imply lower thermal efficiency, since thermal efficiency measures useful work obtained from the fuel's energy.

Implication for blended fuels

In a blend such as E20, ethanol constitutes 20 percent of the fuel by volume. Its lower energy content can therefore increase litres consumed for a given journey, while the exact effect depends on vehicle technology and calibration.