Do what you want behind the barn, but don't brew your own soydiesel
So, your neighbor has a soybean extruder to make bean meal for his hogs, and the two of you have been thinking about squeezing the oil out of your beans to make soydiesel and save some money on fuel bills next spring. My, you have a plan!
And all you have to prevent is a farmstead-destroying methanol explosion. Or maybe your plan is a bit tamer, and you just want to try some of the soydiesel available from the local co-op, but you're not sure how well it performs. Maybe it is a good time to chew the fat about soydiesel.
Soybean oil-based biodiesel is accelerating in popularity, both for its benefit to the soybean market and for its performance in diesel engines. Purdue agronomist Shawn Conley and Purdue ag engineer Bernie Tao have produced a fact sheet "Biodiesel Qquality: Is all biodiesel created equal?" that will answer some of your initial questions about biodiesel.
Just as ethanol has offset part of our dependence on petroleum, soybean oil can do the same in diesel fuel. After all, Rudolf Diesel didn't have diesel fuel to power his invention, and instead he used vegetable oil (peanut oil if you must know.) So from day-one, diesel engines could run on vegetable oil, but today's high-performance engines were designed for diesel fuel, and accommodations have to be made if part of the fuel is soybean oil, or really soy methyl ester to be exact.
Creation of soydiesel takes a chemical reaction that will be familiar to a fat chemist (not a heavy chemist, but one who studies fats, such as soybean oil.) The soybean oil is combined with methanol to make soy methyl ester, a long chain hydrocarbon similar to diesel fuel, and that is how it can easily blend. The soy methyl ester is also known as B100, or 100% biodiesel, and you'll need to keep track of your percentages.
As an aside, methanol is not only toxic stuff but highly explosive, and you probably don't want to order a tank truck to make your own B100, because of the threat to your neighborhood. There is more liability than your insurance agent will want you to have in playing with methanol.
You may see some references to whether the soy methyl ester is saturated, monounsaturated or polyunsaturated. What you need to know is that those different fatty acid chains in the fuel will perform in different ways in your engine. They will have different cetane points which affects ignition quality, different cloud and stability points, which affects cold weather flowability and release of pollutants.
Conley and Tao compliment soydiesel because it provides a more consistent energy output compared to pure diesel fuel which can vary by 15% due to blending. They say, "In general, however, the average energy output for number-two diesel fuel is approximately eight percent greater than for B100. In a typical B20 blend, this would equate to an approximately one percent loss in fuel economy and an insignificant drop in torque and power." Remember that B20 blend is 20% soy methyl ester and 80% diesel. They say that the soydiesel will have a higher cetane number, which indicates shorter ignition delay.