Ethanol-blended petrol is now the standard fuel across the UK, with E10 petrol containing up to 10% ethanol replacing the previous E5 standard. Understanding how ethanol affects your car helps you make informed fuel choices and recognise potential issues with ethanol-compatible and non-compatible vehicles. ## E10 Petrol in the UK E10 petrol, containing up to 10% ethanol by volume, became the standard grade at UK filling stations from September 2021. This change was part of the government's commitment to reducing transport carbon emissions, as ethanol from renewable sources cuts lifecycle CO2 emissions by around 2-3% compared to pure petrol. E5 petrol, with a maximum of 5% ethanol, is still available as the super-unleaded premium grade at most stations. Drivers of older vehicles that are not compatible with E10 should use E5, which remains widely available. The Society of Motor Manufacturers and Traders estimates that around 85% of petrol-powered cars on UK roads are approved to run on E10. The government provides an online checker to verify whether your vehicle is compatible. ## How Ethanol Affects Engines Ethanol has different chemical properties than pure petrol that affect engine operation. Ethanol has a higher octane rating than petrol, typically around 113 RON compared to 95 for standard unleaded. This higher octane rating actually benefits high-compression engines, allowing more advanced ignition timing and slightly better performance. However, ethanol has lower energy content per litre than petrol. E10 contains approximately 3% less energy than E5, meaning fuel consumption increases by roughly 3% when using E10 compared to pure petrol. Drivers may notice slightly worse miles-per-gallon figures on E10. Ethanol is hygroscopic, meaning it absorbs water from the air. This can cause fuel system corrosion and phase separation, where water and ethanol separate from petrol and sink to the bottom of the tank. Phase separation is rare in sealed fuel systems but can occur in vehicles stored for extended periods with partially filled tanks. ## Vehicles Not Compatible With E10 Classic cars and older vehicles manufactured before around 2003 often have fuel system components not designed for ethanol. Pure rubber fuel hoses, cork gaskets, aluminium fuel tanks, and fibreglass fuel tank linings can all be damaged by ethanol. Symptoms of ethanol incompatibility include fuel leaks from deteriorated rubber components, fuel pump failures from corrosion, blocked fuel filters from debris released by tank lining degradation, and engine running problems from varnish and deposits on fuel system components. Motorcycle fuel systems are particularly sensitive to ethanol, with many manufacturers recommending E5 fuel only. Check your motorcycle handbook for specific fuel requirements before using E10. If your vehicle is not compatible with E10, use E5 premium unleaded. While E5 costs more per litre, it is compatible with all petrol vehicles and remains available at all UK filling stations. ## Storing Ethanol-Blended Fuel Ethanol-blended fuel has a shorter shelf life than pure petrol because ethanol absorbs moisture. Fuel stored in jerry cans or tanks for more than three months may develop water contamination and phase separation problems. For vehicles stored over winter, adding a fuel stabiliser helps prevent ethanol-related degradation. Run the engine after adding stabiliser to ensure the treated fuel circulates through the fuel system. Alternatively, fill the tank completely full to minimise air space and moisture absorption. Boat fuel systems are particularly vulnerable to ethanol problems because of exposure to water. Many boat owners use E10 at their peril, discovering fuel system damage when launching in spring after winter storage. ## Frequently Asked Questions **Does E10 damage car engines?** For vehicles approved for E10 use, the fuel causes no damage. The standard ethanol content of 10% is well within the tolerance of modern fuel systems. Older vehicles may experience problems, which is why compatibility checking is essential. **Is E10 fuel cheaper than E5?** E10 is typically 3-5p per litre cheaper than E5 at most filling stations because of reduced fuel duty on ethanol. However, the 3% lower energy content offsets some of this saving in increased fuel consumption. **Can I mix E10 and E5 fuel?** Yes. Mixing the two fuels simply results in an ethanol content between 5% and 10%. This is perfectly safe and has no detrimental effects beyond the normal properties of ethanol-blended fuel.