Actually Valis - I need to clarify something.
I mis-understood your statement. Yes, you can burn used vegetable oil directly in the diesel engine, BUT it takes some finnagling to do so.
One needs to add a second fuel tank to the vehicle (which takes up precious space in some of these smaller vehicles).
Then, one needs to route the fuel line from this added fuel tank to a valve - and have the ability to switch between the regular diesel fuel tank - and the vege-oil tank.
The reason is that the car cannot be started on the regular vege-oil - until it is heated up to a really hot temperature. (Not sure what it is, and don't want to do the research to find out the temp right now.)
That's the principle that this works on. The vegetable oil - if heated hot enough - then fed into the diesel engine - will combust just fine.
So - start-up and shut-off of the diesel needs to be on regular diesel - running mode - vegetable oil.
I didn't like that idea - and opted for the bio-diesel route - as it needs NO modifications to the diesel engine. If you run out - filler' up with the good ol' regular petro diesel, and keep going.
Getting the vegetable oil from some of these 'greasy spoons' can be challenging... as it can have all sorts of crud in the oil. Salts, french fry bits, etc.
I always used a good filtering system to clean the oil up before I processed it for my vehicle.
Regards,
Jim TX