Diesel engines run on animal fat?

I've heard a few stories recently about people running their diesel vehicles and generators with animal fat that the get from any fast food store essentially free. Does anyone know if this is accurate? Are there a number of modifications to make to the engines? Please advise...I'd really like to go all diesel if that's the case.
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>Thanks!

You can go two ways on this: Animal fats can be converted to biodiesel fairly simply, producing glycerin and a small amount of soap as byproducts which can then be used as if it were regular diesel, or you can install a separate heated fuel tank which you fill with well-filtered grease.

In the latter scenario, you still have to use diesel to start the engine and warm up the tank (the heater is usually cooling water powered) and you have to run on diesel a few minutes before you shut down to make sure the injector pump and injectors have ambient temperature liquid diesel in 'em so you can get started again.

An exceptionally frugal friend of mine has been pouring his bacon grease through a coffee filter straight into the regular fuel tank of his old diesel VW for years (where it is well diluted by petroleum diesel) to get some use out of the grease. . . He does seem to go through fuel filters a lot faster than I do with my diesel pickup, though.

- John.

I've since read a lot about using straight vegetable oil without converting it to biodiesel. The woes are said to be congealing of the oil in the cylinders (coking), causing engine troubles. So, the question: Is the use of the petro-diesel to warm up before and "clean out" after the cure for the congealing woes?