[expatpat]

I'm not sure why they need the chemistry. In the UK, they were using used cooking oil directly in their diesel cars. However, the government came down hard on them -- because they were avoiding the fuel tax!

[Jubal Harshaw]

They need the chemistry because transesterification, the reaction they are doing in their soda bottles, is a reaction with a fairly low initial energy cost. In other words, its a reaction that would occur normally in the hot, pressurized environment of the engine. When they do the reaction in a controlled environment (the soda bottles) they get biodiesel and glycerine, and can easily separate the two out. If the reaction was to take place in the engine, they would get (1) esterification of motor oil components and (2) esterification of the glycerine. (1) and (2) add up to varnish, which will destroy the engine.