Pure plant oil (PPO) or straight vegetable oil (SVO) can be used as a fuel. It 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 (176° 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 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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Use in diesel engines
Theoretically it is possible to modify any diesel engine to run on pure plant oils (PPO). The modifications necessary to allow reliable operation vary greatly depending on the design of the engine, the type of oil to be burnt and the ambient temperatures in which the engine will be operated.
For reliable operation with PPO it is highly recommended to perform an engine health check before switching fuels as the use of PPO requires the engine to be in good order.
A widely used method for reliable operation is to convert the engine to dual fuel and fit a PPO two tank system. The engine is started on diesel fuel and switched to PPO as the engine warms. Before the engine is stopped for an extended period the fuel supply is switched back to diesel fuel for enough time to allow the fuel system to be purged of PPO. The engine is then ready to be cold started on diesel fuel.
It is possible however to modify cold start and fuelling parameters to allow engines to be started on PPO - this is known as a PPO single tank system