Untreated plant-derived oils have been used to replace diesel in emergency situations and Diesel’s patent included plant-derived oils as one of the fuels. However, long-term use of untreated oil does cause problems with diesel engines. Recently interest has been revived in the use of oils as a renewable and carbon-neutral replacement for diesel.

Oils are produced from plants throughout the world in considerable quantities (Shay, 1993). Plant oils are normally extracted from oil-containing seeds, where the plant uses oil rather than starch as an energy store for the seed. Seed oil can be extracted from a wide range of annual crops such as soybean, sunflower, rapeseed (canola) and the perennial oil palm. A list of high oil-producing plants is given in Table

7.9 where it is clear that perennial crops have a higher yield of oil per hectare. Despite the higher yields from the perennial plants, annual crops like rapeseed and soybean have commanded most interest, probably because there is already a market for their oil and annuals are a more flexible crop which are often grown in rotation.

The advantages of using plant-derived oils are:

• They are liquid.

• Their calorific content is 80% of diesel.

• They are readily available in large quantities disregarding competition with food crops.

• They are renewable/sustainable as derived from crops.

• They are non-toxic and much more biodegradable than diesel.

• They are CO2-neutral, combustion releases CO2 previously fixed by the plant.

• They are contain no sulfur.

However, the following are problems associated with the use of untreated plant oils:

• They have high viscosity.

• They have low volatility and high flash point.

• They contain reactive unsaturated hydrocarbon chains.

• They have carbon deposits.

To function correctly in a diesel engine, the fuel must form a fine mist, which should burn rapidly and evenly. Untreated plant-derived oil contains residual components such as waxes, gums and high molecular weight fatty components, which clog the fuel lines and filters. High oil viscosity cause poor atomization, affecting ignition and combustion, which gives carbon deposits on injectors, combustion chamber walls and pistons. The polymerization of unsaturated fatty acids in the combustion chamber also causes deposits on the wall, and some components mix with the lubricating oils increasing their viscosity (Peterson et al., 1996; Ma and Hanna, 1999). The presence of water in the oils can allow microbial growth that can block the fuel filters. To illustrate the problem of viscosity, a comparison of the properties of diesel and plant oil is shown in Table 7.10.

Different methods have been used to reduce the viscosity of the oil, which includes blending with diesel, microemulsification, pyrolysis and transesterification. Of these methods, only transesterification has been successful, and the mixture of esters formed is called biodiesel.