Flexible-Fuel Vehicles

A flex-fuel vehicle (FFV) is designed to run on petrol, ethanol, or their mixtures within a specified range of proportions. It uses a compatible fuel system and electronically controlled engine to adjust combustion as the petrol-ethanol blend changes, rather than requiring a separate engine for each blend.

How the engine adapts to different blends

Petrol and ethanol are mixed in the same fuel tank. The driver normally does not need to select a different operating mode when refuelling with another blend within the vehicle's approved range.

• The engine control unit (ECU) determines the fuel mixture's characteristics using fuel-composition sensing or information derived from engine sensors.
• The ECU changes fuel injection because ethanol requires more fuel than petrol to burn with the same quantity of air.
• The engine-management system also adjusts ignition timing and other operating settings according to the fuel blend and driving conditions.
• An exhaust oxygen sensor provides feedback so that fuel delivery can be corrected and combustion maintained near the required air-fuel ratio.

Why a purpose-built fuel system is needed

Flex-fuel capability involves both compatible materials and suitable engine calibration; it is not simply permission to pour more ethanol into an ordinary petrol vehicle.

• Fuel tanks, pipes, seals and other components must tolerate the ethanol concentrations for which the vehicle is designed.
• Injectors and fuel-delivery components must accommodate the higher fuel flow needed for ethanol-rich operation.
• Ethanol-rich fuels can make cold starting more difficult, so suitable starting strategies are needed.
• The designation E100 denotes ethanol fuel, but an FFV is not automatically capable of using it; the manufacturer's approved fuel range determines compatibility.

What changes for efficiency and energy use

Ethanol's higher octane rating improves resistance to engine knocking, but its lower energy content per litre means that mileage generally falls as the ethanol share rises.

• Higher octane can support efficiency improvements when the engine is designed and calibrated to exploit it; it does not automatically mean better kilometres per litre.
• Using ethanol can substitute for part of petrol consumption and diversify transport-fuel supply.
• The climate benefit depends on the ethanol feedstock, production energy and land-use effects, so it cannot be inferred from the blending percentage alone.