Theory of Vegetable Oil Use as a Fuel

1 The Diesel Engine
2 Theory of Vegetable Oil Use as a Fuel
3 Engine suitability
4 Heating the Oil
5 Biodiesel
6 Micro Emulsions and Blends
7 Vegetable Oil Engine Design
8 Vegetable Oil Furnaces and Heaters
9 Oil Types and Filtering
10 Taxation
11 Implications of Vegetable Oil Fuel Use
12 Sources

Put in the simplest terms, the only reason a diesel engine can not run on straight vegetable oil (SVO) is if at ambient temperature the oil is too thick to run through the fuel lines and filters of the engine at the required rate. The design of the engine, especially the fuel system and combustion chamber, and the type of vegetable oil used will dictate at what ambient temperature the engine will start.

Even if the fuel system can pump the thicker oil there is risk of damage to the engine. SVO will not be injected into the cylinders as effectively as diesel fuel, the injector will not provide as fine a fuel mist with this less viscous oil. A mist of larger droplets will not burn as effectively causing difficulty with starting and incomplete combustion which will lead to build ups of carbon and vegetable oil residue. As deposits build in the engine they will cause the engine to run poorly because of low compression due to sticking piston rings or coked valves and poor injection spray pattern from build ups on the injector nozzle. A bad spray pattern will produce different combustion activities with a greater amount of incomplete combustion and possible piston damage due to increased heat on the piston surfaces, this can end in a holed piston .

Engine carbon build ups can be a problem with diesel fuelled engines which have not been run hard/hot enough. It is advisable to work diesel engines hard fairly regularly to burn this carbon from the engine, more so when running on vegetable oil. A standing start to high speed pedal to the metal run or a very long uphill haul has been shown to help. Extended periods at low engine load or tick over are best avoided when possible. Combustion is less complete from when the engine is first started until it reaches operating temperature as the piston rings do not seal as efficiently until the various engine components have expanded with the combustion heat. It is good practice to drive fairly hard upon first starting the vehicle to bring it up to operating temperature as soon as possible.

The greater effort required to pull the vegetable oil through fuel lines and filters can damage the fuel injector pump or the extra force required to move this oil through the pump causes extra stress on the timing belt/chain which could give out prematurely, causing costly engine damage.

Some engines have been found to be able to operate reliably under given conditions without modification when fuelled with certain oils.[10][28][29][30]

Engine Modification

Engines have been designed to overcome potential problems and run on vegetable oil. Heating the fuel system allows the SVO to flow and an improved injector and combustion chamber design give better combustion.

A German company called Elsbett [32] has worked extensively on vegetable oil fuelled engines since the 1970’s and at one stage offered SVO engines, unfortunately low demand due to the high cost of these hand built engines compared to mass produced diesel engines forced their engine out of production. They now offer a service converting existing diesel engines to run on SVO.

Fuel Modification

Another solution is to thin the vegetable oil so that it behaves in a similar way to diesel fuel and can be used in existing diesel engines. There are a number of options

Biodiesel

Biodiesel is a fuel made through transesterification. About 80% vegetable oil is mixed with 20% alcohol and a catalyst. A chemical reaction occurs (transesterification) and you are left with biodiesel and glycerin which separate out when allowed to settle.
Biodiesel in operation is comparable to fossil diesel in most ways.

Micro Emulsions and Fuel Blends

Vegetable oil can be mixed with alcohols, diesel fuel, petrol, surfactants, cetane improvers, water and solvents in varying mixes and proportions to form a micro emulsion or a blend with suitable operating qualities

Heating the Oil

Above 70 degrees Celsius vegetable oils have a viscosity approaching that of diesel fuel. The design of an engines fuel system and combustion chamber will dictate at what temperature vegetable oil can be reliably run as a fuel.

1 The Diesel Engine
2 Theory of Vegetable Oil Use as a Fuel
3 Engine suitability
4 Heating the Oil
5 Biodiesel
6 Micro Emulsions and Blends
7 Vegetable Oil Engine Design
8 Vegetable Oil Furnaces and Heaters
9 Oil Types and Filtering
10 Taxation
11 Implications of Vegetable Oil Fuel Use
12 Sources