Quote:
Originally Posted by DSilk
Ethanol is a much better fuel power wise? You do realize that ehtanol is far less energy dense than gasoline, which contradicts the statement you are making. What ethanol does accomplish is to help raise the octane level of the fuel (because it is harder to start combustion). But this is only a benefit if you cannot secure normal premium gasoline with an octane rating of 91 to 93 without going to an E85 blend, something that just isn't the case for most of us.
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I see what you are saying, but there is a little more to making power with ethanol blends than just octane.
As you stated, ethanol blends will have less energy density compared to refined gasoline. This means you need more injected volume of an ethanol blend to create an equivalent gasoline burn. That is why there is a very noticeable fuel economy penalty to running an ethanol blend.
Octane is a major part of why there is more power
potential in E85. The equivalent AKI is somewhere around 100-107 depending on winter/summer blend. If we focus just on the M3's S65/MSS60 DME, the optimum AKI is around 93-94 (equivalent to 98RON in Germany), so there is still a bulk of the country who is likely seeing ignition retard even when filling with premium gasoline (ie 91 octane in AZ/CA). If you combine E85 with a more aggressive tune, there is even more power potential above standard premium gasoline, and you can fuel from a pump at a reasonable per-gallon price.
This is all in-line with what you said. Just explained for context.
The other 'power-adder' aspect to ethanol blended fuels is the Latent-Heat reaction when the fuel changes phase from liquid to gaseous (port wetting/evaporation, pre-combustion mixing, etc). True E85 has about 300% more Latent-Heat of Evaporation, which has a significant cooling effect on both the mechanical components in the fuel path and the charge air. The charge air can be measured in principal; with a Delta-T of around 54 degrees F for pure gasoline, and a Delta-T of around 170 degrees F. That reduction in charge-air temperature is good for 5-8% power on its own, without any changes to ignition or combustion cycle.
I would be cautious to confuse the subject of "energy density" with "power"; as you can argue to semantics of the chemistry terms people often use to make a point. But the bottom line is, ethanol blended fuels will make more power all-else-equal (Latent Heat)
and have the potential to make a lot more power (higher AKI), when compared to standard gasoline.