OMAHA, Neb. (AP) — The Environmental Protection Agency cleared the way Friday for a higher blend of ethanol to be sold nationwide for the third summer in a row, citing global conflicts that it says are putting pressure on the world’s fuel supply.
The agency announced an emergency waiver that will exempt gasoline blended with 15% ethanol from an existing summertime ban. Gasoline with 10% ethanol is already sold nationwide, but the higher blend has been prohibited in the summer because of concerns it could worsen smog during warm weather.
The tricky thing about ethanol blends is that you end up getting into similar issues to literal rocket science. In modern (and modernish) engines, up to E15 is generally well-handled, though does potentially increase wear. The tricky part is “why stop at E15?”. The reason for that is energy density.
You know that mention of rocket science? Well, one of the important things to consider in chemical rockets is the mass and energy density of the fuel. It needs to move both the rocket and itself. When it comes to automotive fuel, energy density is also vital and ethanol has about half of the energy density of petrol. The tipping point where the reduction in energy density overcomes the excess energy stored in the petrol is about 15%.
After the tipping point, the ethanol blend tends to create more pollution per mile/km because if its impact on the fuel’s energy density - more fuel is required to travel a particular distance, causing more pollutants from the petrol to be emitted. At that point, it’s probably less polluting to run a 100% ethanol “blend” in a motor designed for it.