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Comment by 2OEH8eoCRo0 | Hacker News Reader

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?

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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.

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.

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