Question about biodiesel
July 7, 2006 7:20 AM   Subscribe

Why is biofuel diesel, instead of standard gasoline?

I was just reading this aricle about "renewable diesal", made from slaughterhouse waste, sewage, old tires, etc. Then I thought about biodisel, made from plant oils. Why are all these interesting, cleaner-burning and renewable fuels diesel? Would it be possible to make bio-gasoline, usable in today's cars?
posted by yesno to Science & Nature (16 answers total) 1 user marked this as a favorite
The higher fuel economy of diesel engines would be at least one factor.
posted by shepd at 7:26 AM on July 7, 2006

Someone who knows more than me will have a more complete answer, but I believe the difference lies in how diesel and gasoline combust.

In a gasoline engine you use a spark to cause the fuel to ignite. In a diesel engine the heat of the cylinder is used to start combustion. That's why you need a glow plug to start a diesel engine. It heats the engine hot enough so that spontaneous ignition happens when the diesel fuel is introduced into the cylinder.

So, you would have to refine bio-diesel to a point where it could combust at a much lower temp using a spark. I think, if not impossible, it would be expensive and unnecessary since you don't need to refine bio-diesel to use it (you can use the oil from a fryer for instance). More refining would defeat the purpose of using a bio-fuel as it would add additional fuel cost into it's production, and also more emissions in it's production, etc.
posted by qwip at 7:32 AM on July 7, 2006

Diesel fuel is of a lower octane than gasoiline, is less volitile, and much thicker, so there are many differences between gasoiline and diesel engines. Diesel engines ignite the fuel not with a spark, but with high pressure and latent heat in the cylinder. They use incredibly high pressure fuel pumps (cam driven, one on each cylinder on some engines) and are basically running on the verge of detonation ("knocking") all the time, so they require a much more stout engine components.

Consider also that diesel and and gasoline both come from crude, but from different stages of the distillation process: Wikipedia link.

I don't believe biofuel could be distilled in this manner. The neat thing is that because it has some similar properties to diesel it can be run in existing diesel engines.
posted by kableh at 7:33 AM on July 7, 2006

Uncompressed diesel fuel does not burn at a practical temperature. In order for it to combust, it must be brought above a specific pressure and temperature. It's called bio-diesel because it shares those same properties.
posted by 517 at 7:42 AM on July 7, 2006

diesel isn't really a type of fuel, it's a type of engine, based on the diesel cycle. the kind of engine that most gasoline-based cars use is based on the otto cycle. diesel designed the engine to be used with a wide range of fuels - the original fuel was peanut oil.

the fuel used in diesel engines is a type of fuel oil - it is thicker and heavier than gasoline (and dirtier, too - these things are not by any means cleaner-burning) - basically, closer to the tar-and-sludge end of things when you do the fractionation of crude oil in a distillation column. think of diesel like gasoline of lower quality.

i think a really in-depth answer to the question of "why do you get diesel out instead of gasoline?" would depend on knowing the chemistry used to break down the turkeys and shoes etc., and i'm sure that's a closely-guarded trade secret by these guys.

but, a nice hand-wavey argument is to say that they are basically packing a few million years' worth of high-pressure, high-temperature chemistry into a few minutes or hours. it would be a really phenomenal feat to get the same quality out of such a process. i don't think such a thing is beyond reason, but without a very long time to cook, garbage in = garbage out.
posted by sergeant sandwich at 7:44 AM on July 7, 2006

I'm not familiar with all the technical details, but diesel engines achieve extremely high compression, which means they can burn fuels that are not so easy to ignite (such as vegertable oil). In other words they can burn a wide range of fuels. Interestingly, Diesel (the inventor, not the engines) was initially trying to make an engine that would run on peanut oil. Today, a relatively "low grade" refined oil is the one commenly used.

Gasoline, on the other hand, is a highly refined product, with a low ignition point (meaning lower pressure in the cylinder). That's what gasoline engines require, I don't think you could produce this so easily (if at all) form biological sources.
posted by bluefrog at 7:46 AM on July 7, 2006

You can think of gasoline as champagne and diesel fuel as cheap plonk. It's way easier to turn junk into cheap plonk than into champagne. In fact, especially in view of the probable low yields, the energy costs of turning junk into gasoline might well outweigh the energy savings of using it.
posted by unSane at 8:01 AM on July 7, 2006

Boy, I got to be faster... 4 answers came up while I was typing mine...
posted by bluefrog at 8:07 AM on July 7, 2006

Ethanol is the most common bio-fuel replacement for gasoline. Many engines actually run on a high ratio of ethanol to gasoline — up to 60% for flex-fuel engines as I understand. Many regions mandate up to 10% ethanol mixture for retail sale.
posted by clord at 8:32 AM on July 7, 2006

i should add, though, that in order to increase the gasoline yield during normal crude processing, refineries employ a process known as cracking whereby long-chain hydrocarbons like diesel and oils are split into the shorter chain hydrocarbons that make up gasoline.

there is a limit to this process, but presumably this biodiesel could be cracked to yield some amount of gasoline.
posted by sergeant sandwich at 8:35 AM on July 7, 2006

There are a few alternatives to biodiesel, including ethanol as clord mentioned. A current initiative by auto makers and ethanol manufacturers will make sure that E85 (an 85% ethanol blend) will be available throughout the country. Biobutanol is another biofuel that stands to gain more ground due to the increased efficiency of its refinement over ethanol, but it will not work with current automobiles.
posted by mikeh at 9:15 AM on July 7, 2006

We may be slightly muddling things here. Ethanol can indeed be made from biomass, or anything fermentable that will form ethyl alcohol and CO2 as part of the bacterial digestion byproducts of sugars. The ethanol produced can be used in modified gasoline engines. DOE uses an 85% ethanol mixture in some of its fleet cars.

Biodiesel is something else and as discussed variously above, can be used (like many oils) as fuel in diesel engines.

Gasoline is distilled from crude petroleum by heating it, which fractionates the "light" ends (i.e., octanes, etc.) from the heavier ends (read, more tar-like). Bunker fuel, etc., such as used in naval vessels is an example of the heavier fuels produced from the latter. Gasoline comes from the light overheads distilled.

You don't distill gasoline from bio-oil feeds because they don't have the light ends to fractionate like petroleum does.

On preview, thanks Sarge....
posted by Pressed Rat at 9:27 AM on July 7, 2006

Best answer: Qwip and others are correct: biodiesel is used as a diesel fuel mostly because it works best in a diesel engine and does not work in a gasoline engine or as well in a kerosene/AV gas turbine or in a marine fuel oil boiler. That's the whole reason. It has a higher ignition temperature than gas, but ignites in approximately the same conditions as petrodiesel.

Sarge isn't completely correct: diesel is most certainly a fuel specification. All diesel fuels must be in narrowly defined ranges for a whole host of properties, the most important being boiling point range, density, cetane number, sulphur content. The most important specs are published by the API, the US DOD, and ISO. Biofuels also must meet a strict set of standards (about two dozen) to be considered "biodiesel".

There's no technical reason biofuels could not be distilled. It would have to be done under vacuum though which is expensive, but since direct esterification of both animal and plant oils results in something so close to a highly valuable commercial product no one has seriously contemplated it. The chemistry of these fatty acid methyl ester (FAME) compounds is such that they do not make very good gasoline analogues. If you want a biological component or replacement for gasoline, ethanol is a much better choice from a fuel chemistry point of view, as others have stated above (it's a bit of a disaster from an environmental standpoint though).

And finally: You can think of gasoline as champagne and diesel fuel as cheap plonk. That is so not true. Firstly, it's possible for a refinery to convert gasoline to diesel and vice versa, so much so that refiners tend to view these two markets as one from a production point of view. You make more of whichever is the more profitable to make. Secondly, diesel is a better fuel than gasoline, it has a higher energy density meaning you can go farther on less.
posted by bonehead at 10:41 AM on July 7, 2006

"Would it be possible to make bio-gasoline, usable in today's cars?"

I think this is a hell of a question. Like bonehead wrote, the chemistry used to make biodiesel has an end product that is very different from gasoline. It's less volatile, it combusts differently, etc. It's possible to imagine a different chemical reaction that yields relatively short-chain hydrocarbons from the biological feedstocks, producing something much more like gasoline. I don't know of any such chemistry; I'm sure it's possible, but it's probably difficult and expensive (as opposed to the cheap, simple reaction used to make biodiesel).

You can sure as hell bet that someone's out there looking for an easier (read: economically viable) way, though. Maybe with a fancy new catalyst.
posted by mr_roboto at 12:37 PM on July 7, 2006

Er, isn't ethanol bio-gasoline?
posted by toastchee at 4:44 PM on July 7, 2006

Er, isn't ethanol bio-gasoline?

I guess you could think of it that way, but it's a very different thing than biodiesel. Made a different way, from a different group of sources.

To turn the long-chain partially-saturated methyl esters that make up biodiesel into something that would burn in a gasoline engine would, in the end be turning them into an ether or, more likely, a short ester. It would require some very clever chemistry, and right now isn't worth the trouble at all, considering you can get a decent price selling them to truck and marine fleets.
posted by bonehead at 5:00 PM on July 7, 2006

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