Flex fuel vehicles are cars and trucks designed to run on gasoline, ethanol fuel, or many mixtures of the two. E85 is a common ethanol blend that contains mostly ethanol and some gasoline, although the exact ethanol percentage can vary. This matters because ethanol can be made from plant materials such as corn or sugarcane, giving drivers another fuel option.
Understanding flex fuel helps students connect chemistry, engine control, and transportation technology.
Key Facts
- E85 usually contains about 51% to 83% ethanol, depending on season and location.
- Ethanol formula: C2H5OH.
- Air fuel ratio for gasoline is about 14.7:1 by mass, while pure ethanol is about 9.0:1.
- Flex fuel vehicles use an engine computer to adjust fuel injection based on the detected ethanol blend.
- Ethanol has less energy per gallon than gasoline, so E85 often gives lower miles per gallon.
- Power = torque x angular speed, so proper fuel and ignition control helps maintain engine performance.
Vocabulary
- Flex fuel vehicle
- A vehicle designed to operate on gasoline, ethanol blends, or a range of mixtures between them.
- E85
- A fuel blend that contains mostly ethanol and a smaller amount of gasoline.
- Ethanol
- An alcohol fuel with the chemical formula C2H5OH that can burn in an engine.
- Fuel injector
- A device that sprays a controlled amount of fuel into the engine for combustion.
- Engine control unit
- The vehicle computer that adjusts fuel injection, ignition timing, and other engine settings.
Common Mistakes to Avoid
- Assuming E85 always means exactly 85% ethanol is wrong because the ethanol percentage can change with temperature, season, and fuel standards.
- Putting E85 in a non flex fuel vehicle is wrong because ordinary fuel systems and engine controls may not be designed for high ethanol content.
- Expecting the same miles per gallon on E85 as gasoline is wrong because ethanol contains less chemical energy per gallon than gasoline.
- Thinking the engine physically changes parts when switching fuels is wrong because most adjustments are electronic, mainly changes to fuel injection and ignition control.
Practice Questions
- 1 A flex fuel car gets 30 miles per gallon on gasoline. If it gets 24 miles per gallon on E85, how many gallons of E85 are needed to drive 240 miles?
- 2 A fuel blend contains 12 gallons total and 9 gallons are ethanol. What is the ethanol percentage of the blend?
- 3 Explain why a flex fuel vehicle must adjust fuel injection when switching from gasoline to E85, using the idea of air fuel ratio.