Straight Vegetable Oil SVO as Diesel replacement fuel

Straight vegetable oil

Straight Vegetable Oil (SVO) is a fuel for diesel engines that can be either pure new vegetable oil or waste vegetable oil that has been cleaned, although this is normally referred to as WVO. Vegetable oil used as fuel in a compression ignition or diesel engine is also referred to as vegidiesel or vegifuel. The most noticeable difference between an engine running on diesel and SVO is that the latter is quieter, but (with rapeseed based SVO) only produces 96% of the equivalent power of diesel.

The original diesel engine was designed to run on peanut oil, so SVO burns well in some diesel engines. However, due to its relatively high viscosity, using SVO with unmodified vehicles can lead to poor atomisation of the fuel, incomplete combustion, coking in the injectors, ring carbonisation, and accumulation of fuel in the lubricating oil.

Application and usability

Most diesel car engines are suitable for the use of SVO with modifications. One common solution is to add an additional fuel tank, one for SVO and a separate tank of diesel (petrodiesel or biodiesel) and an electric valve to switch between them. The viscosity of the SVO is reduced by preheating it using heat from the engine; the engine is started on diesel, switched over to SVO as soon as it is warmed up and switched back to diesel shortly before being switched off to ensure it has no SVO in it when it is started from cold again. In colder climates it is often necessary to heat the SVO’s fuel lines and tank as it can become very viscous. Another common 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 fuel use with one tank.

With unmodified engines the unfavourable effects can be reduced by blending, or “cutting”, the SVO with diesel fuel. 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 to run 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 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.

Properties

The main form of SVO used in the UK is rapeseed oil 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 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.

Examples

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 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 useable in temperatures lower than 17 degrees Celsius.

Taxation of fuel

Taxation on SVO as a road fuel varies from country to country, and it is possible the revenue departments in many countries are even unaware of its use, or feel it sufficiently insignificant to legislate. Germany offers 0% taxation, resulting in their leading on most developments of the fuel use. There seems to be no clear taxation system in the USA, however given the low rate of fuel taxation, it is unlikely to face anything unfavourable, although charges could vary from state to state. In Ireland a pilot scheme is currently running (as of April 2006) whereby eight suppliers have been approved to sell SVO for use as a fuel without the payment of excise duty (VAT at 21% still applies, SVO from any other source still attracts exise duty at 36.8058 cents per litre plus 21% VAT). Despite its use being common in France, it would appear there has been no legislation to cover this.

In the UK, drivers using SVO have been prosecuted for failure to pay duty to Her Majesty's Revenue and Customs. The rate of taxation on SVO was originally set at a reduced rate of 27.1p per litre, but in late 2005, HMRC started to enforce the full diesel excise rate of 47.1p per litre. HMRC argued that SVOs on the market from small producers did not meet the official definition of "biodiesel" in Section 2AA of The Hydrocarbon Oil Duties Act 1979 (HODA), and consequently was merely a "fuel substitute" chargeable at the normal diesel rate. Such a policy seemed to contradict the UK Government's commitments to the Kyoto Protocol and to many EU directives and had many consequences, including an attempt to make the increase retrospective, with one organisation being hit with a £16,000 back tax bill. Such a change in the interpretation of excise duty effectively makes use of SVO in the UK nonviable by all but environmentalists, as the combined price of SVO and taxation for its use considerably exceeds the price of mineral diesel. HMRC's interpretation is being widely challenged by the SVO industry.

Vegetable oil used as fuel

Use of vegetable oil directly as a fuel is one of the most environmentally friendly sources of power, as it is carbon neutral, and unlike biodiesel does not require energy input to perform transesterification or produce glycerine as a waste product. The use of waste vegetable oils is obviously greener, but requires filtering and settling. Some waste may not be suitable.

Sunflower oils are also used for this purpose and with a seven degree Celsius (13 °F) lower freezing point can provide slightly better cold weather starting. It is worth mentioning some indirect injection diesel engines will run happily without alteration, especially in warmer climates or summer. The French XUD series of engine is a good example. One caveat is the injection pump should not use vegetable oil as a lubricant, as due to the higher viscosity when cold it can fail to lubricate and cause premature failure. Inline injection pumps, and mechanical Bosch pumps work well, however CAV/Rotodiesel and similar can fail if run on relatively low vegetable oil to diesel ratios. Petrol may be added as a thinner to the fuel, as most manufacturers recommend this for winter up to ten percent. In practice many people may add slightly more, as the problems of vapour locks are less likely at low temperatures. However, excessive thinning can cause a lack of lubrication in injectors, resulting in their overheating.