Many vegetable oils have similar fuel properties to diesel fuel, except for higher viscosity and lower oxidative stability. If these differences can be overcome, vegetable oil may substitute for #2 Diesel fuel, most significantly as engine fuel or home heating oil.
For engines designed to burn #2 diesel fuel, the viscosity of vegetable oil must be lowered to allow for proper atomization of fuel, otherwise incomplete combustion and carbon build up will ultimately damage the engine. Many enthusiasts refer to vegetable oil used as fuel as waste vegetable oil (WVO) if it is oil that was discarded from a restaurant or straight vegetable oil (SVO) to distinguish it from Biodiesel.
The first known use of vegetable oil as fuel for a diesel engine was a demonstration of an engine built by the Otto company and designed to burn mineral oil, which was run off of pure peanut oil at the 1900 World's Fair. When Rudolf Diesel invented the diesel engine, he designed it to run on peanut oil but it was soon discovered that it would operate on cheaper petroleum oil. In a 1912 presentation to the British Institute of Mechanical Engineers, he cited a number of efforts in this area and remarked, "The fact that fat oils from vegetable sources can be used may seem insignificant today, but such oils may perhaps become in course of time of the same importance as some natural mineral oils and the tar products are now."
Periodic petroleum shortages spurred research into vegetable oil as a diesel substitute during the 30s and 40s, and again in the 70s and early 80s when straight vegetable oil enjoyed its highest level of scientific interest. The 1970s also saw the formation of the first commercial enterprise to allow consumers to run straight vegetable oil in their automobiles, Elsbett of Germany. In the 1990s Bougainville conflict, islanders cut off from oil supplies due to a blockade used coconut oil to fuel their vehicles.
Academic research into straight vegetable oil fell off sharply in the 80s with falling petroleum prices and greater interest in biodiesel as an option that did not require extensive vehicle modifications.
Application and usability
While engineers and enthusiasts have been experimenting with using vegetable oil as fuel for a diesel engine since at least 1900, in all the literature, only one peer reviewed study exists that compares long term use of vegetable oil and #2 Diesel as fuels which shows no noticeable difference in rate of deterioration of the engine burning vegetable oil, for one particular model of engine, the German Deutz F3l912W . (#1 Diesel has a cold-weather additive to reduce gelling)
Most diesel car engines are suitable for the use of SVO, also commonly called Pure Plant Oil (PPO), with suitable modifications. Principally, the viscosity of the SVO/PPO must be reduced by preheating it, typically by using heat from the engine or electricity, otherwise poor atomization, incomplete combustion and carbonization may result. One common solution is to add an additional fuel tank for "normal" diesel fuel (petrodiesel or biodiesel) and a three way valve to switch between this additional tank and the main tank of SVO/PPO. The engine is started on diesel, switched over to vegetable oil as soon as it is warmed up and switched back to diesel shortly before being switched off to ensure it has no vegetable oil in the engine or fuel lines when it is started from cold again. In colder climates it is often necessary to heat the vegetable oil fuel lines and tank as it can become very viscous and even solidify. Another solution (the one-tank system) is to add electric pre-heating of the fuel and, if necessary, upgrade the injection pumps and glow-plugs to allow SVO/PPO fuel use with one tank. One tank conversions are most viable in hot climates.
With unmodified engines the unfavourable effects may be reduced by blending, or "cutting", the SVO with diesel fuel; however, opinions vary as to the efficacy of this. Some WVO mechanics have found higher rates of wear and failure in fuel pumps and piston rings due to partially-combusted WVO/SVO droplets carbonizing in those components.
For normal use, without either blending or a second tank and associated modifications in a petrodiesel engine, vegetable oil has to be transesterified to biodiesel.
Many cars powered by indirect injection engines supplied by inline injection pumps, or mechanical Bosch injection pumps are capable of running on pure svo in all but winter temperatures.* Turbo diesels tend to run better due to the increased pressure in the injectors. Pre-CDI Mercedes-Benz vehicles and cars featuring the PSA XUD engine tend to perform well too, especially as the latter is normally equipped with a coolant heated fuel filter.
The main form of SVO used in the UK is rapeseed oil (also known as canola oil, primarily in the United States and Canada) which has a freezing point of -10°C. However the use of sunflower oil, which freezes at -17°C, is currently being investigated as a means of improving cold weather starting. Unfortunately oils with lower gelling points tend to be less saturated (leading to a higher iodine number) and polymerize more easily in the presence of atmospheric oxygen.
Cetane number (combustion quality) is highest with coconut oil, palm stearine, palm kernel, palm oil, palm oleine, lard and tallow. Coconut oil, palm oil, palm stearine, tallow and lard have the lowest iodine numbers.
Some Pacific island nations are using coconut oil as fuel to reduce their expenses and their dependence on imported fuels while helping stabilize the coconut oil market.
Coconut oil is only usable where temperatures do not drop below 17 degrees Celsius (62 degrees Fahrenheit), unless two-tank SVO/PPO kits or other tank-heating accessories, etc. are used. Fortunately, the same techniques developed to use, for example, Canola and other oils in cold climates can be implemented to make coconut oil usable in temperatures lower than 17 degrees Celsius.
With often minimal modification, most residential furnaces and boilers which are designed to burn No. 2 heating oil can be made to burn either biodiesel or filtered, preheated waste vegetable oil. These are generally not as clean-burning as petroleum fuel oil, but if processed at home, by the consumer, can result in considerable savings. Many restaurants will give away their used cooking oil either free or at minimal cost, and processing to biodiesel is fairly simple and inexpensive. Burning filtered WVO directly is somewhat more problematic, since it is much more viscous, but it can be accomplished with suitable preheating. WVO can thus be a very economical heating option for those with the necessary mechanical and experimental aptitude.
Waste vegetable oil
As of 2000, the United States was producing in excess of 11 billion liters of waste vegetable oil annually, mainly from industrial deep fryers in potato processing plants, snack food factories and fast food restaurants. If all those 11 billion liters could be collected and used to replace the energetically equivalent amount of petroleum (a rather utopian case), almost 1% of US oil consumption could be offset. However, usage of waste vegetable oil as a fuel competes with already established usages.
Pure vegetable oil (pure plant oil) or SVO
Pure plant oil (PPO) (or Straight Vegetable Oil (SVO)), in contrast to waste vegetable oil, is not a byproduct of other industries, and thus its prospects for use as fuel are not limited by the capacities of other industries. Production of vegetable oils for use as fuels is theoretically limited only by the agricultural capacity of a given economy.
The UK exported 280 000 tonnes of rapeseed in 2005. If the UK used just its set aside land it could reach its 5% biofuel target without the need for exotic and environmentally damaging oil crop imports.
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