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Old 09-18-2013, 10:18 PM   #150 (permalink)
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Originally Posted by Allch Chcar View Post
Wankel Rotaries are especially prone to detonation.
I think you have that backwards. The high surface area to volume ratio and that the combustion chamber is constantly moving into a new, cool, part of the engine means that Wankels are less prone to detonation than piston engines. This is one reason why they have promise - if any ICE does - for running on H2

The consequences of detonation in a Wankel may well be worse than it occurring in a piston engine.

Originally Posted by RedDevil View Post
Isobutanol does indeed have a higher energy density than ethanol, but it is also less volatile. Not sure if it can be used as is.
One factual untruth in the article:
Yeast can produce up to 120 grams of ethanol per liter (15% volume).
Sunflower seeds, linseed etc. contain almost pure biodiesel.
I guess the original quote had more premisses, like "using gentech on previously unproductive organisms".

Imagine the cost of distilling purish isobutanol from a less than 0.2 % mixture.
It will need a lot of time and effort to become practical.
Nevertheless, this is a promising development.
Isobutanol can be used as is in an ICE designed for use with gasoline. (It might be better in an ICE designed for use with it though.)

One problem with isobutanol is that the biological pathway that generates it means less of the energy theoretically available in the feedstock ends up in the fuel than with ethanol.

The gain is in that the isobutanol is easier to separate from the water based fermentation broth than is ethanol.

Those are the things that are considered when figuring out the economics of a biotech research effort to produce fuels (or anything else): What is the maximum theoretical yield and what is the value of the product? If even the theoretical yield is too low, there's no point in trying to figure out how to get (close) to it.

If you are looking at growing/manufacturing/extracting a biofuel you have to consider the yield - potential and realized - at each stage:

Seed crops for biodiesel have good yield (and easy extraction) from the seed but poor yield per acre because the seeds are such a small fraction of the total biomass of the plant.

You also have to consider that to make biodiesel that is compatible with current engines, there is an interesterification step, to swap the glycerol with a small chain alcohol (MeOH usually but it can be EtOH), required.

Anything that can use cellulose (even better with lignin) is potentially good because that is what makes up most of all terrestrial plants.

At the processing/fermentation step, you are looking for the efficiency with which the fermenting organism can convert the feedstock to the product, the concentration at which the fermentation ceases and the speed of fermentation (you might not run it to completion if it slows down with increasing product concentration). The latter two are because of the capital cost of the fermentation vessels, not just the extraction costs.

Yeast, consuming 6 Carbon sugars, are pretty much the best option at the moment, which, regardless of subsidies, is why that is the predominant biofuel.

Algae look good because there's not much "extra" organism that has to be grown, avoiding diversion of resources in the organism away from the desired product(s) and extraction requires little energy.
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