zlacker

Fundamentally, ethanol will always be less efficient per gallon than gasoline. It has significantly less specific energy compared to gasoline. This is a fact known by anyone that has tried e85 either for racing or for flex fuel compatible normal vehicles.

>>Kirby6+(OP)
But look at figure 8. At 40%, volumetric efficiency is at its peak. It's very important to note that these results are neither monotonic nor linear, so while the public only has access to E85 and E10/E15 (at least where I've been), those are almost ends of extremes and a middle ground can synergize. Yes some metrics always get worse the more ethanol you use, but hopefully you looked at the paper and saw that isn't nearly the full story.

>>Neywin+Bm
I'm not sure what that proves? Volumetric efficiency is just about air efficiency. Higher VE is better, yes, but you need to compensate for higher VE by adding more fuel to utilize the air, otherwise you run lean, which is bad. Turbocharging a car, for instance, directly increases VE.

None of this directly equates to fuel efficiency although it can be related, which is what the GP was asking about presumably. You still need to content with the fact that ethanol has ~30% less specific energy per gallon.

>>Kirby6+qy
Figures 8 and 9 show that both volumetric and thermal efficiency can be improved with nonzero ethanol in the right circumstances. Increasing efficiency metrics, while combining figure 7's showing that you get more power with ethanol, means that you're getting more power and doing so with greater efficiency.

You mention fuel efficiency. Figure 6 shows that in some cases, looks like the threshold is between 60 and 80% load, the story flips such that adding ethanol to the mixture reduces sfc. In other words, less fuel is needed for the same amount of power. Meaning, if it takes X hp or kw to sustain whatever test speed you're measuring efficiency at, the amount of fuel you use is lowered. I think this is very important to understand. Figure 6 shows that if your engine is sufficiently loaded, nonzero ethanol can improve the amount of energy you get out of the same mass of fuel relative to pure gasoline, which I believe would be directly proportional to better fuel efficiency.

Now very notably, if less heavily loaded, it seems better to use 0 ethanol. The ability to choose lets it get the best of both worlds.