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How-Tos of Flex-Fuel Engines

We have all seen the little badge on many vehicles announcing flex-fuel capabilities. You may even own one yourself. To anyone involved with agriculture, those two words should bring a smile to your face; flex fuel means agri-fuel. 

This begs the question, what is involved in making an engine flex-fuel compatible? To be good ambassadors of renewable agriculture-based fuels, we need to understand the function of the flex-fuel system. 

The definition of a flex-fuel vehicle is one that can use gasoline with an ethanol content of zero (E0) to 85% ethanol (E85) or anything in between. A modern nonflex-fuel labeled application is currently certified to use E0 to E10. Yet, in reality, E15 or E20 would be fine.

ethanol is not corrosive

Since auto firms need to engineer for reliability, they take into consideration ethanol’s ability to wick in moisture to a higher extent than gas. Though many believe ethanol to be corrosive, it is actually no more so than pure petroleum-based gasoline. An issue occurs with ethanol’s propensity to wick in moisture at a higher rate, thus, it is the water that attacks the fuel system and causes corrosion, not the ethanol.

This concern is addressed simply with fuel system components that are corrosion-proof. It is my opinion that even nonflex-fuel vehicles have the same corrosion-resistant parts as those labeled for up to E85. The economies of scale would not allow for one specific engine in a vehicle family to have dedicated fuel lines and gas tank. 

The complex part of the system is the calibration of the engine’s ignition and fuel delivery since ethanol in any amount in the fuel has different combustion and energy characteristics. Whereas a gallon of pure gasoline has around 114,000 Btu’s of energy, the same amount of E85 contains only 81,800 units. 

When the engine is running on E85, the injector time has to be longer to supply more fuel due to the lower energy content and stoichiometric value (this describes the air:fuel ratio for the greatest chemical energy release). The stoichiometric rating of E0 is 14.67:1 (nearly 15 parts of air to one part of fuel). E85 has a stoichiometric rating of 9.765:1. To compensate for that, the fuel injector needs to increase the opening time to create that mixture strength.

This would be simple if the engine would only be run on E0 or E85, but that is not the case. A modern engine controller is math-based. It uses an algorithm for the desired air:fuel ratio to be created by calculating the mass of the fuel, the mass of the air, and the cylinder fill. This is identified as a volumetric efficiency (VE) table. The amount of air ingested by the engine will not change with the amount of ethanol in the fuel. The desired air:fuel ratio will. Since a flex-fuel vehicle can have any amount of ethanol in the fuel (up to 85%), it needs to determine the ethanol content and then set a mathematical correction factor based on the engine’s requirements with no ethanol. This is identified as a trim table in the software.

For example, you have a flex-fuel pickup truck. The tank is nearly empty, and you fill it with E85. Since the last fill was with E85, the ethanol content of the fuel is 85%. A sensor mounted in the fuel line monitors this. The engine controller (via the trim table) delivers the proper injector opening and spark ignition timing (ethanol burns faster than gas) for the engine to start and run fine. Due to ethanol’s chemical composition, it doesn’t start an engine in extremely cold weather as easily.

Now let’s say the truck has a little less than a half tank and you fill it up with E10. The E85 is mixed with E10, and the ethanol content is somewhere in between. The flex-fuel sensor reads this, and the trim table adjusts the engine calibration accordingly. 

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