It’s commonly quoted that because gasoline has a much higher energy density (111,500 vs 81,000 BTUs) than E-85 (a mixture of 85% ethanol and 15% gasoline), that to replace the 140 billion gallons of gas Americans burn annually would require 192 billion gallons of E-85 based on a linear decrease in fuel economy proportional to the decrease in energy density.
On the other hand, there seems to be an argument that E-85 has a significantly higher octane rating (between 100 and 105) which, if I understand the reasoning correctly, would allow more efficient combustion so that, in the end, you can get more motion out of fewer BTUs.
I’m obviously no expert on internal combustion engines - what’s the straight dope here? If we all woke up tomorrow and E-85 were ubiquitous and we started designing cars to take advantage of that, how much of a hit would those cars take, mileage-wise, using E-85 instead of gasoline?
As an ethanol defender, I’ve been scouring every type of resource from industry to gearhead to find an answer. I can’t locate anything definitive.
Best guess – a standard E-85 FFV engine (able to burn regular unleaded straight gasoline) can expect anywhere from 12%-25% lower gas mileage with E-85. A high performance engine, tuned for 100+ octane fuel might experience a loss of something like 5% if using E-85 instead of racing-formula gasoline.
If E-85 is 25% cheaper than regular unleaded, and you have a flexible fuel engine, you’ll either break even or come out slightly ahead. If it’s less than 25% cheaper, you may will lose money in the cost per mile area.
Saab’s been experimenting with a high-octane, turbocharged engine. They can get better performance with E-85, but so far they haven’t been able to improve mileage.
To exploit the higher octane rating the compression ratio needs to be increased, or as with the
Saab example, a forced induction system can increase the boost. Increasing boost yields more power, increasing compression ratio gives a bit more power, but significant increases in fuel economy.
Increasing the compression ratio requires substitution or modification of internal engine components *…pretty involved, and just as difficult to reverse.