Lots of vegetation, especially things like nuts and seeds, contains some kind of oil which has been used for various purposes throughout history, including burning it as a direct source of light and heat. Some natural oil or fat can be obtained from animals too, tallow is a traditional product.
When the internal combustion engine was invented, as a development of the steam engine during the industrial revolution, it ran on "oil" and then began the diversification into several types of internal combustion engines and several types of fuel oil.
The Diesel engine is a type of engine, named after its inventor, in which the fuel is injected into the cylinders when the air in them is highly compressed and so usually hot enough for the fuel to ignite immediately without further help. It is not very fussy about the type of fuel but has to be well built to achieve the high compression necessary. The first ones probably used vegetable oil. Whilst the engine itself is not fussy about the fuel, the tank, pipes, filter and pump have to be suitable and key factors are the viscosity (stickiness/runniness) of the fuel and its tendency to dissolve them.
The more common type of spark-ignition engine is easier to manufacture because it creates less compression. It is more fussy about how well its fuel will mix with air, ignite and burn. The immense demand for this fuel of consistent quality led to the fossil-oil or petr-oleum industry. And here begins the confusion about terminology as well as all sorts of political, commercial and environmental issues. In UK we use the term "petrol" to refer to this fuel and to the spark-ignition engine - though etymologically "petro-" means fossil-based and technically the engine is an Otto engine. By the way LPG (Liquid Petroleum Gas) is an alternative fuel for this kind of engine, not for a Diesel engine.
More to the point, we use the term "Diesel" rather vaguely to refer to the fuel for a Diesel engine, and because it is taxed differently if used, say, in a boat, tractor or generator we use the term DERV (Diesel-engined-road-vehicle) to refer to this fuel for road use. But neither term strictly says anything about whether it's come from a fossil source or from a renewable source such as vegetable or animal fat. When you come across "petro-Diesel" it means fossil fuel for a Diesel engine.
All this is to explain that going back to using vegetable oil as fuel for a Diesel engine is only radical because we have got used to the multi-national fossil-fuel industry mass-producing consistent-quality fuels which are taxed heavily on a complex basis.
Vegetable oil will have more tendency to dissolve materials of vegetable origin such as natural rubber but most of the parts will in practice either be metal or plastic. Viscosity always reduces (becomes more runny) as temperature increases. For this reason, irrespective of the type of fuel, vehicles with Diesel engines may be fitted as standard with heaters for the tanks, pipes or filters, especially useful in winter. These can either be run from the engine cooling system just to improve performance or they can be electrical (which poses safety, and therefore insurance, issues) to be effective when the engine is started from "cold". An alternative is to have two fuel supplies (which, as a modification, affects insurance) switching to a less viscous one for restarting the engine.
The fossil/vegetable issue is important for two reasons. Fossil fuel resources are running out and becoming more costly. Secondly, burning renewable fuel releases carbon (dioxide) that was absorbed from the atmosphere within the past year or so - so is "carbon neutral" in terms of damage to the global atmosphere, whereas burning fossil fuel adds to the atmosphere carbon which would otherwise remain locked away underground.
The taxation issue is complicated because the tax is not specifically used to deal with the effects of pollution. The tax rates can be used to influence people's choice of fuel so setting the tax rates can be done erratically in response to the environment lobby, or to big business! Also part of the tax on fuel will probably be added to the tax on vehicles so that road-users pay for the services they use according, to some extent, to the distance they drive - regardless of pollution issues.
The consistency of quality issue is central for two reasons. First, some modern cars with Diesel engines have become more fussy about the type of fuel. Secondly, when it comes to measuring the pollutants (or indeed other aspects of performance) there's not much point unless you know precisely to which fuel the results apply - not a problem back in the early days.
Finally the industrial issue. The petroleum industry will compete on behalf of its shareholders and the green lobby will takes its usual stance against the multinationals. Here are some of the results ...
Food quality Straight Vegetable Oil (SVO), typically rape seed oil. Easy to buy (supermarket), but you're paying for the "food quality" which you don't need, and you're breaking the law if you use it as road vehicle fuel unless you go to the trouble of paying the correct Road Fuel Tax. Also its viscosity is high which can be a problem under colder conditions though it can be mixed, say 50/50 with "normal" fuel to solve that.
Although essentially agricultural, a more suitable plant species than rape as raw material for vegetable fuel oil could be grown, processed and sold as road vehicle fuel on an industrial scale. This would probably be trans-esterifying to produce a Fatty Acid Methyl Ester (FAME) and the waste by-product glycerol. A lot of rape seed is processed into Rapeseed Methyl Ester (RME) and sold as "Bio-Diesel" although this term is also used generally to describe all sorts of non-fossil fuel. It may be heavily diluted with fossil fuel B5, 5% RME, is quite common, also B10, 10% RME, which are a step in the right direction environmentally and considered acceptable to the general public and car manufacturers' warranties. Or you can find B100 which is all vegetable.
Then there is the idea of using the vegetable oil twice - first in the food industry for cooking and then as fuel. Currently this waste oil goes into animal food - a practice that may not continue much longer, which should mean that there is more reason to use it for fuel, called Modified Waste Vegetable Fat (MWVF). A further economy is to do without hi-tec trans-esterification and burn the glycerol as fuel along with the esters so once the oil has been distributed to chip shops etc the second process of collection of the Waste Vegetable Oil (WVO), processing and redistribution to Diesel vehicles may be possible by small enterprises - the processing can be quite simple and safe and vegetable oil is less flammable than even fossil Diesel so distribution and bulk storage are low hazard. Unfortunately "regulations" all too often swamp promising enterprises such as this and without the quality control that comes with industrial scale it is difficult to generalise about the performance of the product.
As Townace owners we are used to taking the risk of not having all the answers neatly in a manual, and living with the fact that one batch of Aces is not quite the same as the next batch! Also our Diesel engined cars are already reported to give good power, mileage and MOT emission results on types of vegetable oil. My three concerns are ...