How this whole thing works.
I realize that I haven't spent any time explaining why or how I am able to burn vegetable oil in my diesel car - I keep getting questions on how this works, so here's my very non-technical explanation. With pictures.
Here's the deal:
Diesel engines will burn almost anything that is liquid and contains carbon and hydrogen.
The first diesel engine (displayed by Rudolph Diesel at the Paris world fair) ran on Peanut Oil, so vegetable oils have always been a part of the diesel engine's heritage.
Modern diesel engines are designed to run on petroleum fuel (A.K.A.: diesel fuel), but kerosene and other heavy fuel oils will also work well. Compared to vegetable oils, petroleum fuels are not very viscous, so to use vegetable oil in a diesel engine you either have to modify the engine (expensive) or make the oil less viscous (cheap). There are two easy ways to get vegetable oil to be less viscous: additives and heat.
Additives cost money (and are usually petroleum products), but diesel engines produce plenty of extra heat for free!
Vegetable oil gets to around the same viscosity as petroleum fuels when it is heated to about 120F (50C). So, to get vegetable oil to work in a modern diesel engine, all you need to do is heat it before it enters the engine. To do this, there is a kit installed in the car that reroutes the coolant so I can warm up the vegetable oil in my trunk:
My car starts on petroleum diesel, and when the engine reaches it's normal operating temperature (coolant temperature of ~80C), I switch to the vegetable oil tank using a switch by my left knee. The switch stops the diesel fuel and allows vegetable oil to flow into the engine, where it burns just like any other fuel. The only thing I notice is that the engine gets quieter and people outside my car will notice that my exhaust smells of fried foods.
Here's how things look when I first start the car:
And here is how things look when I switch to vegetable oil:
Stopping the engine is a similar process. I don't want to leave vegetable oil in the fuel lines (low temperatures could cause the oil to solidify - this would be bad), so I purge the lines for about 20 seconds with petroleum diesel before shutting off the engine. All this takes is a little forethought. I'm now used to judging about 20 seconds before I get out of the car, so I purge the lines and get the engine ready for starting with clear lines the next time I hop in the driver's seat.
Here's what purge mode looks like before I switch back to diesel mode and turn the engine off: