Biofuels an overview

Biofuel is a broad term used to describe fuel derived from an organic source. This includes both vegetable oil and biodiesel, which can be burn in an engine as a diesel replacement.

Use as a diesel alternative

In order of safety to the engine and fuel injection equipment, safest first:

Biodiesel

Biodiesel is vegetable oil that has been chemically converted. It is a much thinner liquid than vegetable oil and has a higher Cetane Number, making it suitable for use in most diesel engines.

Twin-tank

The engine is started on diesel, run until hot, and then switched over to vegetable oil. The oil is pre-heated before entering the injection system by the vehicle's hot engine coolant. This thins the oil, improving atomisation and helping to prevent piston ring gumming.

Blending

Vegetable oil is mix with petrol or diesel to thin the resulting liquid. This technique is considered unsuitable for direct injection and common rail engines. Although it cannot be considered as a true biofuel, many people blend waste mineral oils with various viscosity reducing components to produce a usable diesel alternative. Note, the use of mineral oil in road going vehicles is not legal in the UK and may not be legal in other countries.

Direct

Using vegetable oil directly in the tank (aka single tanking) is suitable for only the toughest fuel injection systems and engines (those with indirect injection and Bosch rotary mechanical injection pumps). Long term problems with piston ring gumming and loss of compression may occur.

Engine considerations

Indirect injection engines (those with glow plugs and pre-swirl chambers) are most robust when it comes to running biofuels, as there is more chance of complete combustion due to better mixing with air, higher temperatures in the swirl chamber, and not being sprayed directly into the cylinder - as is the case with direct injection engines.

The primary problem with direct injection is the possibility of unburnt fuel hitting the cylinder walls and being swept up by the piston rings. Eventually this unburnt fuel forms a hard rubbery mass (similar to what can be found on the bottom of a well used frying pan) and prevents the piston rings following the contours of the cylinder, resulting in loss of compression. This build up happens fastest when the engine is cold-started and poorly atomised fuel is injected into a cold cylinder.

If vegetable oil is to be used in a direct injection engine, it must be heated first to reduce its viscosity and aid atomisation (and thus completeness of burn).

Alternatively, biodiesel does not gum up in the same way, and is much thinner to start with.

With all engines, it is advisable to halve the oil service interval when running biofuels. This ensures that the engine oil remains unpolluted by biodiesel or vegetable oil, which, unlike diesel, may polymerise in the oil and block oil-ways.

Fuel system considerations

Rotary pump driven injection

Older vehicles are equipped with mechanical rotary injection pumps, and the robustness of these systems depends on the type of pump. Bosch pumps are very tolerant when it comes to fuel viscosity, and consequently quite hard to break. However, Lucas pumps are much weaker, and can fail quickly when forced to pump cold vegetable oil. Lucas pumps will only last with biodiesel, heated vegetable oil, or suitably thin blends of vegetable oil with pump diesel/petrol.

Electronic fuel injection

Modern fuel systems with ECU controlled electronic injectors are only really suited for running on well made biodiesel. Very modern engines are sensitive to fuel viscosity, so even biodiesel may not be suitable. Do not attempt to run a common rail, electronically injected engine on cold vegetable oil - the result is likely to be expensive to correct!

Diesel Particulate Filters

There is some controversy regarding biofuels and DPFs. The argument against using biofuels in a car equipped with a DPF is the way that some fuel systems clean the DPF in the exhaust, by injecting fuel into the engine post-combustion. This fuel is blown out to the exhaust where it burns out accumulated soot in the DPF. Inevitably such a system will also deposit some of the fuel into the engine oil (sometimes referred to as "bore wash"), which isn't necessarily too much of a problem with diesel, but may be with biofuels, which are more likely to polymerise into gels/solids in the oil.

PLEASE NOTE
Biopowered wiki is aimed at an international audience. To this end all aspects of biofuels are included. As a result some fuels may not be legal for use as road fuel in some countries without payment of duty. The reader should satisfy him or herself as to the legality of use in a specific country. See Rules and Regulations for details pertaining to the UK.