Toyota Supra mkiv

E85 vs Gasoline fuel usage by Dan Bailey

A little while ago I had a huge speel on working out how much your fuelling requirements change when you get ethanol involved in a setup previously tuned for petrol but I don't think I have actually shared it as a post specifically, but as usual I've been chatting away with people about ethanol and all things wonderful about it so figured I should actually give it it's own post in here in case it's of interest. Enjoy - or move on if you don't want to deal with a long winded LithRant! ☺

When people decide to move from using petrol to using E85 the topic of using more fuel or needing a bigger fuel system comes up - but there seems to be a fair bit of confusion or hearsay on how much more or why so while this is no authoritative definition of the whole thing... this is ultimately an ultra verbose description of the kind of thought process I go through when confronted with situations where I need to work this kind of thing out. I'm not going to be ultra specific about units of measurement and mathematical precision so much as just explain the basic concepts.

So - first things first, you need to know about the stoichiometric ratio of the fuels in question are. That word is just an epic sounding word which really just means "How much air is needed to burn ALL of the fuel and ALL of the air I am using to make a fire.". Normal petrol has a stoichiometric ratio of 14.7 to 1 which means you need 14.7 times as much air as you need fuel to make sure everything gets burnt, whereas ethanol you only need 9 times as much air. By the way, that "14.7 times" means how much more mass - or effectively how much heavier, not how much space it takes up. The reason for this is realistically where chemical reactions are concerned what matters is how much mass you actually have to react with another mass.

Firstly I'll go through a comparatively basic process of pretending we're going to burn some petrol - ☺ So for my test case, lets assume I have a mass of 100 "airs" and I want to burn them all. We know that petrol has a stoichiometric ratio of 14.7:1, or 14.7 airs for every petrol. The first step of this is really easy to work out, 100 / 14.7 = 6.8 - so I need to put 6.8 petrols in amongst the air before I start a fire to ensure that everything gets burnt. Mint! Or is it?

The trick is that we have no scales so we can't measure the mass, but we DO have a way of controlling the volume of fuel we are moving so what we need to do is work out how much space 6.8 petrols take up. There is a term called "specific gravity" which basically defines how heavy a given volume of mass is relative to water - so the specific gravity of water is 1.0, meaning 1 cup of water weighs 1.0x as much as a cup of water. Wicked - except ethanol has a specific gravity of .79, so a cup of ethanol weighs 79% as much as a cup of water... and petrol has a specific gravity of .75, see where I'm going with this? ☺

Now we know we need 6.8 petrols and we know it's specific gravity, all we have to do is work out how much to pour into cups to move make up that mass using it's specific gravity - so, 6.8 / .75 = 9.1! Sorted, we need to pour 9.1 cups of petrol into our pile of air before we start the fire and we can be fairly sure that it's all going to burn as we have a roughly 14.7:1 ratio of air mass versus petrol mass to work with. ☺

Now for the slightly trickier part, we want to work with E85 and we can only find the specific gravity (.79) or stoichiometric ratio (9.0) of ethanol - so how does one deal with this? Easy, maths!

Seeing as this fuel we are using is 85% ethanol, I multiply it's stoichiometric ratio of 9.0 with .85 to get 7.65. What this means is that 85% of our fuel needs 7.65 airs to completely burn. I can do the same thing with petrol, so 15% of our fuel needs 14.7 airs to completely burn - so 14.7 x .15 = 2.21. I can now add those parts together to get the stoichiometric ratio of my new fuel, or 7.65 + 2.21 = 9.86. Now I know that I need 9.86 airs for every E85 I have to make sure everything gets burnt, mint. ☺

The same maths can be used to work out the specific gravity for E85, so:

85% of E85 has a specific gravity of .79 (ethanol), or .79 x .85 = .6715 15% of E85 has a specific gravity of .75 (petrol), or .75 x .15 = .1125

If I take the ethanol contribution and the petrol contribution to and add them together I get a number to use as the specific gravity for E85 - or .6715 + .1125 = .784 ☺

We now have the same data on E85 as we had for petrol, it's stoichiometric ratio is 9.86:1 and the specific gravity is .784. We have 100 airs and we need 9.86 of them for each E85, so 100 / 9.86 = 10.1 - so 10.1 E85s needed to ensure a complete burn. To find out how many cups to fill I multiply that by it's specific gravity, so 10.1 / .784 = 12.9 cups of E85 required for a complete burn! ☺ Winning.

And there we have it! There are other variables which need to be considered when calculating this in the real world, but this is pretty much enough to show generally what happens in this whole tuning for different fuels process - and now we know we need 9.1 cups of petrol to burn all things things, or 12.9 cups of E85 to achieve the same thing... which means you use 12.9 / 9.1 = 1.42% - or 42% more fuel (by volume) needed when running E85 with all other things being equal.

Hope that was useful in some way! Feel free to comment or question.

Just a quick comment by Kevin on injector sizing. You need 42% more fuel by volume for the same air usage with E85, but that will make you anything from 7% to 15% more power for the same air usage depending on many different factors. As a result, you would generally need injectors around 28% to 33% bigger for the same hp with E85 as with gasoline.

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Last modified on Wednesday 4th November, 2015