Fuel Energy Density and Vehicle Mileage

Energy density is the usable chemical energy contained in a unit mass or volume of fuel. Because vehicle fuel economy is commonly measured in kilometres per litre, ethanol's lower volumetric energy density means that each litre can provide less energy for propulsion than a litre of petrol.

How lower energy density affects mileage

A vehicle must obtain a certain amount of energy to overcome rolling resistance, aerodynamic drag, acceleration and drivetrain losses. Ethanol contains only about two-thirds of the energy of petrol for a comparable quantity, so an engine must burn a larger volume of ethanol-containing fuel to produce similar useful work.

• When less energy enters the engine per litre, the distance travelled per litre generally falls, reducing fuel economy measured in km/litre.
• The engine's fuel-control system supplies more fuel to maintain proper combustion because ethanol is an oxygenated fuel and requires a different air-fuel ratio from petrol.

Effect of blending with petrol

In an ethanol-petrol blend, the energy per litre declines as the ethanol share rises. E20, which contains 20 per cent ethanol and 80 per cent petrol by volume, therefore normally gives lower km/litre than unblended petrol in an otherwise comparable vehicle.

• The reduction in mileage is not necessarily proportional to the blending percentage because engine efficiency, calibration, driving conditions and fuel properties also influence consumption.
• Comparisons should use the same vehicle, route and operating conditions because congestion, speed, load and driving behaviour can obscure the energy-density effect.

Why engine design matters

Ethanol has a high octane rating, which permits greater resistance to engine knocking. Engines specifically designed with suitable compression, combustion control and fuel systems can exploit this property to improve thermal efficiency and partly offset the mileage loss.

• Higher octane does not itself add energy to each litre; it enables engine designs or operating strategies that may use the fuel more efficiently.
• For consumers, the relevant economic measure is often cost per kilometre, which depends on both mileage and the retail price of the blend.