WVO (Waste Vegetable Oil) or SVO (Straight Vegetable Oil) can be used as fuel for vehicles that use a diesel engine. WVO is that oil that has been recycled and filtered using generally cooked oil from restaurant fryers and SVO is considered pure, uncooked and fresh vegetable oil.  It is also possible to make your own biodiesel from vegetable oil.

Originally, the first diesel engines were designed to run on peanut oil, so it makes sense that a modern diesel engine could use vegetable oil.  In fact, most diesel engines can run without any modifications by pouring vegetable oil straight into the tank.  However, without some modifications there are a host of problems that can occur doing this including poor atomisation of the fuel, incomplete combustion, coking in the injectors, ring carbonisation, and accumulation of fuel in the lubricating oil. These things will lead to poor performance in the best case and possible engine failure in some situations.

With the right modifications to the fuel delivery system, most diesel engines can be used running WVO as a fuel without any of the problems mentioned above.  These modifications include some form of preheating of the fuel and can be done with the original fuel tank or by adding an additional tank.

With two tanks, the vehicle is started on straight diesel and switched over either manually or automatically once the oil is heated to a sufficient level to lower its viscosity.  Just prior to shutdown, the system is again switched back to straight diesel to purge the lines of vegetable oil for the next startup. 

Using one fuel tank only, the SVO or WVO needs to be pre-heated prior to combustion, generally with some form of electric heater, and upgrades are necessary for the injection pumps and glow plugs.

The solution that is right for the vehicle will depend largely on vehicle type and age and, most importantly, the climate.  In cold weather, the viscosity of vegetable oil is so thick that it is absolutely necessary to ensure the oil is preheated sufficiently to prevent damage to the vehicle.  In warmer climates during the summer months it is possible to run without any changes at all, though even this is not recommended.  It is best to make changes to the vehicle that will make sure proper viscosity can be maintained at all times.