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Comment by fetus8 | Hacker News Reader

And it generally gets much worse MPG compared to regular gasoline!

I've been tracking my flex-fuel usage - mostly E85 fillips - for a while now, tracking price and MPG. At least with the prices I've been getting, I typically get around 15% less MPG, but the price is between 10-15% cheaper. My 'cents per mile' price has generally remained the same over the last year or so I've been tracking (been tracking longer than a year, but have been using E85 more regularly in the last year).

Now... I've got a small car/engine. The MPG might be markedly worse on a bigger car. Don't know. But at least in my case, cents/mile is about the same.

So then do you fill up with E0 or E10 so you can fill up less frequently?

Last... year or so I've been getting E85 when possible/convenient, and defaulting to regular 87 octane when E85 isn't available. Those tend to be the only reliably available options - haven't seen E10 around here. Last 6 months have been about 80% E85 and 20% 87 regular.

In the US anyways, E10 is the regular gas. E0 (ethanol-free) is usually marked as being ethanol-free.

Can't edit now, so I'll add.

With my typical city/town driving, I get 32-35 mpg with 87 octane, and typically 26-30 with E85.

Regular 87 octane is $3.12 around me here, and E85 is $2.71. E85 works out around 12% cheaper.

This is probably a fudged statistic, because you have to add oxygenates to what's called 'pure gasoline' to get it to burn cleanly and efficiently, which bulks out the volume considerably. (These kinds of arguments were also trotted out in favor of tetra-ethyl-lead to provide the 'oxygenate' which only needs to be added at 1% or IIRC, compared to 10% ethanol blend or similar usually).

Fuel blending for optimal combustion characteristics is a complicated business, most of what people claim about this issue is not very reliable.

I wonder if this doesn't need to be the case. With the higher octane rating couldn't you in theory run a much more efficient and higher compression engine?

It's a simple case of it's energy density. The energy content of ethanol is about 33% less than pure gasoline. Higher octane doesn't even necessary equate to higher gas mileage. As soon as octane is high enough for your engine tuning, there's no mileage benefit.

> As soon as octane is high enough for your engine tuning, there's no mileage benefit.

That's exactly my point. Would it be possible to design an engine from the ground up for very high octane fuels?

Yeah, this is how most E85 engines generally work. They have a E15 engine map, and an E85 and map. And use sensors to figure out which fuel is in the tank and which map to use.

Stock cars generally don't take full advantage of the octane benefits of E85 because they will blow up if someone fills the tank with E15. But in the aftermarket world, "corn powered" cars make substantially more power, especially turbocharged ones. They have the benefit of owners that understand to manually switch maps when changing fuels.

They don’t have to manually switch maps, modern ECUs can figure out the blend and compensate.

Agreed, there are several aftermarket ethanol content analyzer sensors available, easily installable onto the fuel line of any car. These supply an ethanol % reading to the ECU and allow for aftermarket tunes to adjust timings based on the current fuel blend. No manual swapping necessary.

That is the theory behind "flex fuel" vehicles. In practice, the optimizations don't materialize in real world usage. This is likely because of safety factors built into the ECM preventing the engine from running enough timing advance to take advantage of the octane of a blended fuel.

The total amount you can get from changing the timing isn't very much. Even if you put in a whole new camshaft. Not to mention timing effects emissions as well.

The more typical way to do this is have a turbocharger (supercharger... I can never remember the difference and which you want when). You and run more boost which increases the compression ratio. However this requires an expensive turbo and so isn't a popular option.

I'm not sure why turbo cars have not made these modifications though. There are probably other trade offs I'm not aware of.

Exactly. More ICE cars are coming standard with turbochargers which would enable more to take advantage of the extra octane, but it's an optimization problem with constraints: fuel efficiency, reliability, drive-ability, emissions to name a few.

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