Hello,
When used in “regular” cars, ethanol (E85) tends to decrease the mileage compared to gazoline. This increased consumption suggests a lower energy density.
If that’s the case, why is ethanol used on tuned cars to increase their performance?
Thanks !
In: 2
Ethanol contains less energy per unit of volume than gasoline. This reduced energy density means that replacing a volume of gasoline with ethanol reduces the overall amount of energy which can be extracted, lowering mileage.
On the other hand ethanol has a higher octane rating than gasoline. Octane is a measure of how much heat and pressure the fuel can take before igniting itself. If you are trying to get more power out of an engine you want to push as much fuel and air into the cylinder before igniting it in order to get a bigger bang. There is a limited amount of volume within the cylinder though and if your fuel explodes before it is compressed to the right point your engine tries to spin backwards and generally destroys itself.
So higher octane means more compression is possible and a bigger bang, meaning more power output. Ethanol has a higher octane and so using more ethanol in the mix allows those greater levels of compression. You do end up using more liquid volume so your mpg goes down, but performance goes up if you tune the engine to use it.
Not suggests. It’s well known that ethanol has a lower energy density than gasoline. Like 75%.
Small amounts of ethanol help gas combustion because it burns so clean and provides some additional oxygen.
To increase performance an ethanol based engine can run a very high compression ratio whereas gasoline cannot run at similar ratios without a high risk of preignition or knocking (which would destroy the engine)
It isn’t as simple as changing the fuel – the engine itself has to be purpose built for alcohol in order to maximize performance.
Ethanol has about 75% (give or take) of the energy density of gasoline but it requires much less oxygen to burn efficiently since it brings some of it’s own oxygen to the combustion chamber. That means you can put more E85 in the combustion chamber, have it burn efficiently, and end up with a net increase in power the engine produces but with reduced mileage.
I assume you’re referring to ‘fuel’ race cars burning alcohol. In this case, they’re actually burning methanol rather than ethanol. Methanol has a lower energy density than ethanol but you can put even more of it in the combustion chamber so the engines produce considerably more power than gasoline or ethanol/gasoline mix. For methanol, you need a ratio of 7 parts air to 1 part fuel. For gasoline, you need 14 parts air for 1 part fuel in the combustion chamber for efficient combustion. That’s what creates the large difference in power between alcohol and gasoline.
Thank you all for the clear explanation !
Latest Answers