The government has amended the Central Motor Vehicles Rules, 1989, to clear the way for flex-fuel vehicles that can run on up to E100, which is almost purely ethanol.The rule change does not mean that every petrol car on Indian roads can now use E100. The government has basically expanded the range of fuels that specially designed vehicles can be certified to use. A conventional petrol car approved for E20 — petrol containing 20% ethanol — will not be filled with E85 or E100.The amendment also changes certain technical and vehicle-classification provisions, including raising the weight threshold for light petrol and diesel vehicles from 3,000 kg to 3,500 kg.What exactly are flex fuels?“Flex fuel” is short for flexible fuel. It refers to vehicles that can run on petrol, ethanol, or different mixtures of the two, depending on what the vehicle has been designed and certified to use.Petrol is refined from crude oil extracted from the earth. Ethanol is a type of alcohol that can be produced by fermenting sugars or starches from plants. In India, common sources include sugarcane and grains. Ethanol can also be made from agricultural residues and other plant material. The two fuels can be mixed.Ethanol behaves differently from petrol, and a vehicle’s engine, fuel-delivery system and other components must be designed to handle the fuel safely.A flex-fuel vehicle can adjust how its engine operates to accommodate different ethanol-petrol mixtures. As the US Department of Energy explains, flexible fuel vehicles “have one fuel system, and most components are the same as those found in a conventional gasoline-only car. Some special ethanol-compatible components are required to compensate for the different chemical properties and energy content in ethanol, such as modifications to the fuel pump and fuel injection system. The engine control module (ECM) is also calibrated to accommodate the higher oxygen content of ethanol.”Story continues below this adCan cars run safely on E100? What needs to be different in their engines for it?Yes. An engine can run on almost pure ethanol, provided the vehicle is designed for it. Ethanol is a combustible fuel, just as petrol is. The challenge is that the two fuels have different properties.Three things matter particularly:Fuel delivery: Ethanol contains less energy per litre than petrol. The engine must generally inject more fuel to produce comparable power. The US Department of Energy estimates that E100 contains about 30% less energy per gallon than petrol. This means a car may travel a shorter distance on the same volume of high-ethanol fuel, although engine design and fuel prices affect the overall cost of driving.Fuel-system materials: Ethanol can affect certain metals, plastics, rubber seals and other components. Fuel pumps, pipes, seals and injectors must be compatible with high-ethanol concentrations.Also in Explained | Brazil’s masterclass in biofuel adoption offers takeawaysElectronic controls: Sensors and the engine control unit must adjust the air-fuel mixture and ignition timing to suit the fuel. Starting and running the engine reliably under different temperatures also require appropriate calibration.Story continues below this adEthanol has an advantage: its high-octane rating helps resist engine knocking, the uncontrolled combustion that can damage an engine. An engine specifically optimised for ethanol can make use of this property.Why blend petrol at all?If ethanol can power a car on its own, why mix it with petrol?First, petrol makes the transition easier. Most cars already run on petrol, and much of the existing fuel infrastructure is built around it. Mixing ethanol into petrol allows countries to replace some fossil fuel without immediately replacing their entire vehicle fleet.Second, ethanol can reduce dependence on imported oil. India imports much of the crude oil it consumes. Replacing a portion of petrol with domestically produced ethanol can reduce the amount of crude oil the country needs to buy from abroad. It can also create a market for farmers and ethanol producers. This is particularly relevant when international conflicts threaten energy supplies. But ethanol is not without problems.Story continues below this adWhat are some concerns about blended fuel?There are four broad concerns.Mileage and running costs: Because ethanol contains less energy per litre, a vehicle may deliver fewer kilometres per litre as the ethanol proportion increases. Whether this makes driving more expensive depends on the price of the fuel, the vehicle’s efficiency and how much mileage falls. Cheaper fuel per litre does not automatically mean cheaper travel per kilometre.Compatibility with vehicles: Ethanol can affect some materials used in older or conventional fuel systems, while high-ethanol blends require appropriate engine calibration.Water and agricultural resources: Ethanol is made from crops or other plant material. Producing it at a large scale therefore raises questions about how much land and water are needed, which crops are used, and whether fuel production competes with food production.Story continues below this adSugarcane, for instance, requires substantial water in its cultivation. The environmental benefits of ethanol also depend on how it is produced, including the energy used in manufacturing it and emissions generated across its supply chain. The International Energy Agency cautions that crop-based biofuels can have consequences for land use, biodiversity and food prices.Cleaner does not mean pollution-free: Ethanol can reduce dependence on fossil fuels. The use of ethanol blending also lowers harmful pollutants such as carbon monoxide, sulphur, and carbon and nitrogen oxides. But it does not eliminate emissions from a vehicle. Nor does replacing petrol with ethanol automatically guarantee lower greenhouse gas emissions overall: the result depends on how the ethanol is grown, produced and transported.