Why can we make only Diesel fuel out of soybeans ect?

I heard a spot on the news today (radio WIBC) concerning governmental, incentive grants for switching vehicles to soybean diesel. Also, in the past I have heard many stories about how we could basically “grow” our own oil, but all of these involve diesel not our “regular” oil (this is a problem since most people are not willing to drive diesel engine vehicles for various reasons).

I don’t think that’s accurate.

I buy regular ‘gasoline’ that is up to 10% ethanol. This is derived from home-grown plants (mostly corn in this area). The state is talking about increasing this to 20%.

This is used in a standard, Chevrolet gasoline engine car.

That’s right. We can produce fuel for gasoline-powered cars from plants. The problem is it’s not very efficient to produce. To make the stuff, you have to put in a lot of energy relative to how much you get out.

It has to do with the carbon chain length.

Our liquid fuels tend to be mixtures of hydrocarbon molecules. The boiling point of hydrocarbons goes up as the molecules get bigger. Methane and ethane have small molecules and so are gases at ambient temperatures, butane is also a gas but condenses into a liquid somewhere around 0 deg C which is why butane lighters can fail in cold weather.

Larger hydrocarbons have increasing boiling points giving liquid fuels such as gasoline, kerosene, diesel etc. Even larger hydrocarbons have solid forms such as paraffin (wax, to Brit dopers.)

Crude oils tend to contain a whole mixture of hydrocarbons from small to large molecules, and we seperate it out into narrower-range mixtures that we can use as fuels.

Vegetable oils tend to consist of larger and more complex hydrocarbons than those found in crude oil, and so are not so versatile as sources of fuel. With a certain amount of processing, vegetable oils can be readily converted into biodiesel which can be fed straight into automobile diesel engines. The processing is fairly simple, you can even do it yourself. The site below describes how you can make biodiesel from vegetable oil combined with methanol, although I don’t know if this the same process used for commercial biodiesel.


I don’t know of any route by which a “bio gasoline” can be made from vegetable oils. Essentially you’d be trying to make a highly volatile mixture of hydrocarbons out of a not-very volatile mixture of hydrocarbons, which can probably be done but may not be economically sensible.

The production of ethanol and methanol from plants is by the fermentation of carbohydrates, and is not directly related to use of vegetable oils as fuels. Gasoline engines can run on ethanol-gasoline blends. With modification, gasoline engines can run on 100% ethanol or methanol.

AFAIK, crude oil is heavy in long chains. I believe that to get enough gasoline out of crude, catlyitic cracking is employed to shorten the chains.

Is there a process for “cracking” fats? Even if there is, gasoline engines seem to be much more picky about octane ratings.

Catalytic “cracking” is done in the petroleum refining business to increase the fraction of smaller molecules. Would that work for soybean, or other vegetable oils?

Well the other question that presents itself from the above discussion is why can’t we make commercially viable engines that will run off of mostly ethanol (as indicated is possible above) or even 21st century diesal engines? What are the commercial trade offs that overcome the benefits of reducing (or possibly elimininating) our dependence upon foreign oil? Wouldn’t this also be a windfall for the American farmer? Would price per gallon costs be comparible?

The best Biodiesel is brewed from Canola oil, due to canola’s antioxidants: this prevents polymerization resulting in gelling and varnishing which would clog fuel filters and injectors, and also preventing injector pump fuel starvation.

IIRC, it requires more barrels of oil to make diesel fuel than it does to make a comparable amount of gasolene. Even with the improved fuel economy diesels have over gas engines, if we all switched to petroleum based diesel cars, our oil consumption would go up. Biodiesel is an option, of course, and some folks claim to have developed a cheap way to synthesize petroleum, but there’s no hard numbers from any reliable sources one way or the other that I’ve seen.

As for switching over to ethanol, well, that’s a whole 'nother kettle of fish. It could be done, but remember everyone in the auto industry has decided that fuel cells are the wave of the future, and that “any day now” they’ll start cranking out fuel cell cars, so why should they invest the necessary billions of dollars it would take to design, build, educate the public and the mechanics on the characteristics of ethanol engines when they’re going to be replaced by fuel cell cars? Depending upon how long it took them to come up with affordable fuel cell cars they might not be able to make back the money they spent on developing the ethanol cars.

We could I suppose but it would take more ethyl alcohol that gasoline. The heat of combustion of gasoline is almost 150% that of alcohol. Given the same efficiency we would be getting more like 20 mi/gal as opposed to 30.

I’m not sure. Vegetable oils are quite different animals from petroleum - they have more complicated molecules (triglycerides, I think?) , free fatty acids, stearin.

Biodiesel is actually very different in composition to petroleum diesel - it just happens to have the correct properties re viscosity, clean burning and autoignition. The diesel cycle is quite fuel-tolerant anyway - diesel engines will run on gasoline, kerosene, straight vegetable oil, just about any liquid that burns and the injectors can squirt as droplets into the cylinders. The trouble with actually doing this is that many of these fuels damage the engines by forming deposits or by other mechanisms.

Getting a liquid fuel with gasoline-like properties from vegetable oils is beyond my very limited chemistry, but with a bit of luck someone else will know about it.

Soybean ethanol isn’t likely to have much effect on foreign oil dependence.

If all U.S. diesel wwere a 2 percent biodiesel blend, it would require the oil from 500 million bushels of soybeans. That amount exceeds the total 2002 soybean production of Iowa, the largest producer.