Straight Vegetable Oil
Straight vegetable oil (SVO) is too thick to get through modern injection pumps and injectors without causing damage, but chemical transformation into biodiesel is not the only way to make it thinner. When heated to 80° Centigrade (180° Fahrenheit), common vegetable oils are no more viscous than cold diesel fuel.
This typically requires an extra tank and engine fuel supply modifications. By pre-heating the SVO with a combination of radiator coolant and electric heat, the SVO runs the diesel engine with no problems. The engine is started on biodiesel (or blend, if below freezing) and run until hot, then an electric valve switches the engine to the SVO tank.
The pollution from SVO is not as well studied as that of biodiesel, but it is thought to be similar to biodiesel -- and much less than petrodiesel pollution. The Danish Center for Plant Oil Technology (http://www.folkecenter.dk/plant-oil/publications/PPO-emissions.htm) has some interesting figures comparing rapeseed (Canola) oil and diesel fuel: 42% lower carbon monoxide, 63% lower unburned hydrocarbons, 19% lower nitrogen oxides, and 42% lower particulates.
Depending on the source of the oil, it has more or less the same energy content as petrodiesel -- more energy than biodiesel. This can be noticeable on under-powered vehicles.
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