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What Is Flex Fuel?

Posted by Auto.com Staff | Jan 2, 2019

Flex fuel is the ability for a vehicle to run on either gasoline or E85, or any combination thereof. Different fuels require different air/fuel ratios to burn efficiently. Flex fuel vehicles (FFVs) are not new – the original Ford Model T was designed to run on ethanol, gasoline or kerosene. But the technology used on a modern flex fuel engine ensures reliability and efficiency that was unheard of in the Model T’s heyday. Flex-fuel vehicles are designed with ethanol-friendly alternative fuel systems and an engine management system that can adapt to the varying fuel types and run accordingly.

To understand a bit more about vehicles with flex fuel systems, we should first talk about the fuels that they’re designed for: gasoline and ethanol. You may have seen E85 at the pump, but what is it? E85 is a gasoline-ethanol flexible-fuel blend that consists of approximately 85% Ethanol and 15% conventional unleaded gasoline. Ethanol is a grain alcohol commonly derived from corn or sugar cane. Because of its source, ethanol can be considered a renewable resource.

The resulting fuel blend is popular for its high octane, which is 113 for 100% ethanol – E85 is often cited as 105-108 octane, but blends vary. On the other hand, it is derided by some for its low energy output, with a gallon of E85 producing roughly 73-83% versus a gallon of conventional gasoline. This means FFVs often require a richer air/fuel ratio, thus reducing fuel economy by about 10% to 25%. With its higher octane, it is widely used as a more accessible racing fuel with the aftermarket having stepped up to support flex fuel technology in older vehicles by using additional and upgraded components to retrofit the older regular gas fuel systems to work with ethanol.

A flex fuel vehicle uses a special sensor to read the ethanol content in the fuel and allows the engine management to adjust control strategies to suit. Performance may improve with E85 due to the ability of the engine management to be more aggressive with ignition advance or boost pressure on turbocharged engines.  E85 fuel is not as widely available as regular gasoline, but a flex fuel vehicle will allow you to fill up with whichever fuel is convenient and most applicable for you.