Vegetable oil can be used as diesel fuel just as it is, without being converted to biodiesel.

The downside is that straight vegetable oil (SVO) is much more viscous (thicker) than conventional diesel fuel or biodiesel, and it doesn't burn the same in the engine -- many studies have found that it can damage engines.

BUT it can be done properly and safely -- IF you get a professional engine conversion. 

There are other approaches, here are the main ones:

1. Just put it in and go.

2. Mix it with diesel fuel or kerosene then just put it in and go.

3. Blend it with an organic solvent additive or with what some companies call "our secret ingredient that we'll tell you about if you pay us" (several versions) or with up to 20% gasoline (petrol), just put it in and go.

4. The only way to use veg-oil is in a properly installed two-tank system where the oil is pre-heated and you start up and shut down on diesel fuel (or biodiesel).

SVO - straight vegetable oil used as diesel fuel (usually new oil, fresh, uncooked)

PPO - pure plant oils, same as SVO: PPO is the term most often used in Europe

WVO - waste vegetable oil (used cooking oil, "grease", fryer oil, probably including animal fats or fish oils from the cooking)

UCO - used cooking oil (what we called it in the first place until everyone started calling it WVO, even if it wasn't necessarily all vegetable)

IDI - Indirect Injection diesel engines: the fuel is injected into a pre-chamber or swirl-chamber before going on to the combustion chamber. Pre-chamber engines are more tolerant of SVO than swirl-chamber engines.

DI - Direct Injection diesel engines: the fuel is injected straight into the combustion chamber. DI diesels are less tolerant of SVO than IDI engines (see The TDI-SVO controversy). Types of DI diesels:

TDI - Turbo Direct Injection

CDI or CRD - Common-rail Direct Injection

PDI or PD - Pump Unit Injection (Direct Injection, each injector has its own pump)

The basic choice for running diesels on biofuels:
-Make biodiesel and just use it, no need to convert the engine, or
-Convert the engine so you can run it on SVO -- no need to process the fuel.

It's not quite that simple. For instance, if you want to use waste vegetable oil, which is often free, you're going to have to process it anyway, though less so than to make biodiesel. And it still might not be very good fuel.

One of the great advantages of biodiesel is that it will run in any diesel engine. The same claim has been made for two-tank SVO fuel systems: "Ready-to-install kit that will allow you to run any diesel on waste vegetable oil." Also in any weather.

In cold weather vegetable oil crystallises, forming solid wax crystals that can quickly block the fuel filters. One solution to the all-weather problem with two-tank kits is to change the filter in winter, using a 30-micron filter instead of the standard 10-micron filter (or less), so the wax crystals just go straight through without blocking the filter and melt in the injection pump, allegedly without causing any stress or damage.

Also going straight through into the injection pump however will be any solid particles of between 10 and 30 microns that the specified standard filter would have stopped.