You can run a diesel engine on vegetable oil.
Seems like a very green alternative
Yeah, because crop dusters run on fairy farts, and combines and harvesters use unicorns to pull them, and those produce trucks are powered by pixies, and that massive veg-oil manufacturing plant, it's all Minotaurs and goblins, not fossil fuels.
If you want to use vegetable oil, that's fine. But don't delude yourself into believing it's a green alternative, when fossil fuels are used through the entire chain to get that oil to you.
Can't most of that machinery be run on diesel?
Sure, but that is prohibitively expensive.
Typically regular vegetable oil has less energy as well. You can moderate this with blends, but generally you will need a little more veg-oil than real oil, to do the same work.
However, the problem is that the cost per gallon on veg-oil is drastically higher than that of regular oil. Most estimates place a replacement cost at roughly $8/gallon. Suggesting that farmers move their machinery over to veg-oil at $8 a gallon is about as likely to happen, as you voluntarily paying $8/gallon.
Now that isn't to say that if oil actually stops flowing on the Earth, that we can't adjust to it, and move to a vegetable oil system. Yeah, I think we could.
But to suggest it is a green-alternative today, when everything is run on regular oil, and it won't change any time soon... no, it's not a green alternative. Thousands of gallons oil, and tons of coal, and nuclear power was used to make that vegetable oil.
Now even if we imagined a future where oil was declining.... I still don't think that vegetable oil is a solution. I can see that as a solution in a world devastating dystopia.
And the real problem is.... land.
Last estimates are that it would require 440,000 sq miles to produce enough veg-oil to replace current oil usage.
Right now, there are only 650,000 sq miles of farm land in the US. That means 2/3rds of all farm land would have been switched over to producing oil.
Can you image what will happen to food prices in this country, when we go from being a net exporter of food, to a massive net importer? Prices will go crazy.