I think in a proper SHTF scenario, a carburetor petrol engine would be the most useful. These can readily be converted to run on syngas (woodgas), which can be generated with basic technology (a crude syngas generator can be made from a few old drums and pipes).
The problem with biodiesel will be obtaining the other reagents. In a SHTF scenario, you won't be able to go to the store and get NaOH or Methanol. These are things that you could make yourself, but require a bit of expertise and effort. As things settled down a bit, perhaps people might start to make biodiesel as a small business/side hustle. Another issue here would be oil availability. Currently, seed oils like canola are relatively cheap, this is only possible due to large scale mechanized farming of them. In a SHTF scenario, where you had to manually plant, harvest and process the seeds to get the oil, the quantity generated would be small, and highly valuable as a lubricant, or for finishing wood products, etc.
For that reason, I'd go with a non-fuel injected petrol engine that is pre-computer control era. Those are getting harder and harder to get hold of though. The .gov is likely to actively ban such engines. They are aware of these scenarios. Need to keep an eye out for a suitable engine and store it somewhere out of sight of prying eyes.
Much to do to prepare... need to stop procrastinating.
For sure
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