HIGH CETANE INDEX
The cetane index is a measure of the inflammability of diesel fuel. Cetane is a long-chained hydrocarbon with 16 carbon atoms and ignites particularly well under the influence of high temperatures and high pressure without requiring an igniting flame or spark. It is therefore an ideal fuel for diesel engines. A cetane index of 100 designates a reference fuel. The higher the cetane index of a diesel fuel, the better the ignition and combustion and the more regular and smoother the engine runs. Commonly available diesel fuels have a cetane index of 50 to 52, and values of 53 to 54 are achieved by the addition of ignition accelerators. In this, biodiesel has a natural advantage. Its primary components are similar to cetane and this fuel therefore has a natural cetane index of 56 to 58 and can easily fulfil the requirements of engine manufacturers for high-quality fuels with high inflammability without additives.
PRACTICALLY FREE OF SULPHUR
Whereas the sulphur content of lowsulphur diesel fuel is reduced in the refinery in a high-energy process with additional CO2 emission and a loss of the intrinsic lubricating capability, biodiesel is naturally almost free of sulphur (max. 0.001 percent and thereby at the limit of its detectability). This characteristic of biodiesel also allows the simple and optimum use of an oxidation catalytic converter.
The biodiesel molecule contains around 11 percent of oxygen. This oxygen contingent leads to an improved combustion and thereby to substantially less soot. The residues left inside the engine by the fuel are significantly reduced.
Biodiesel has a very good intrinsic lubrication capability. Trials have shown that biodiesel lies far below the values specified in the standard for mineral oil diesel. The so called HFRR value is a measure of the lubrication capability. In general, the lower the HFRR value, the better the fuel. Highly desulphurised mineral oil diesel fuel has an HFRR value of 500 or higher without additives, but the limit specified by the standard for diesel fuel is 450. Mineral oil diesel fuel therefore requires additives. In contrast, the HFRR value of biodiesel is approx. 200. Biodiesel is therefore suitable as a good lubricating additive to conventional diesel when added in proportions of only 1 percent. In the operation of an engine approved for biodiesel, the engine wear is significantly reduced. This is confirmed by the picture of a diesel engine shown adjacent. After approx. 15,000 service hours, the honing is still perceptible.
One great environmental advantage of biodiesel is its rapid biodegradability. However, this advantage requires particular attention regarding the stability of the fuel. If biodiesel is exposed to a specific "oxidation stress", i.e. high temperatures and frequent contact with (atmospheric) oxygen, or the influences of UV radiation or contact with non-ferrous metals, it ages faster than conventional diesel. In this case, the double bonds in the fatty acids of the biodiesel are broken and they react with oxygen. That is the starting point for the polymerisation of the fuel, i.e. long-chained molecules form which thicken the fuel and lead to blockages in the injection pumps and filters. To completely prevent this effect, which occurs only under extreme conditions, environmentally friendly additives are added during the production of biodiesel, so-called antioxidants.
Biodiesel based on rapeseed oil (RME) has a naturally high resistance to oxidation. This must be preserved for as long as possible. A positive side effect: this also improves the storage stability and thereby the storage capability.
Another factor which can have a destabilising effect on biodiesel is the presence of water. Because biodiesel has hygroscopic properties and actually attracts water, the biodiesel manufa cturers pay particular attention to ensure a very low water content. At the same time, this property means that the buffer capacity or the capability of binding water is significantly higher than that of diesel fuel. The occurrence of free water is therefore substantially reduced. Although it is biologically degradable, biodiesel therefore denies micro-organisms the basis for their development due to this property. Misgivings of bacterial growth in the biodiesel tank or in fuel bearing vehicle components are therefore unjustified.
It is generally applicable that biodiesel must be suitable for use in winter to temperatures of -20 °C (measured as the CFPP value) in the same way as mineral oil diesel. Both fuels receive additives to ensure this. Otherwise, an irreversible flocculation (production of paraffins) occurs in diesel fuel, which blocks the fuel lines, injection pump etc. If this happens, expensive cleaning is necessary. In contrast, biodiesel is only thickened which, in contrast to the paraffin precipitation of mineral oil diesel fuel, is reversible. When the temperature rises, biodiesel returns to a thinner state and it is unnecessary to clean the fuel system. Biodiesel on the basis of rapeseed oil has a CFPP value of approx. -10 to -12 °C without additives as a result of its raw materials.
A series of additives are used in mineral diesel fuel to achieve specific properties of the fuel. There are also properties (winter capability, oxidation stability) of biodiesel which can be improved with additives, but an "additive package" as those mixed with conventional diesel is unnecessary for biodiesel. In this relation, the use of biocides is often discussed because biodiesel is quickly biodegradable. However, these toxic additives are unnecessary because water is necessary for bacterial growth. In contrast to conventional diesel, biodiesel is hygroscopic. Water is absorbed at a molecular level by biodiesel, which prevents all bacterial growth. Practical and analytical results confirm that this quality parameter can be upheld through the entire marketing chain to the end customer. Due to the necessary repetitive alternation between summer and winter qualities, the issue of the water content does not arise in practice.
MIXING WITH MINERAL OIL DIESEL
Although the density of biodiesel is slightly different from that of mineral diesel fuel, both fuels can be mixed in any ratio due to their similar chemical structure. The mixture is stable and cannot be separated by mechanical methods. It is therefore also impossible to extract biodiesel which has escaped to the engine oil with partial flow filters. The subject of mixing will have more importance in the future in view of the more stringent emission requirements of the EURO IV (2005) and EURO V (2008) standards. Due to the different emission effects, it will be necessary for new vehicles to detect whether biodiesel or a diesel/biodiesel mixture is in the tank. With the aid of the biodiesel manufacturers and UFOP and in co-operation with Volkswagen AG, a sensor has been developed which informs the engine management system of the respective ratio of diesel and biodiesel in the fuel tank. This m akes it possible to control the injection point and rate to a value most favourable to optimise reduced emission, an optimum solution to fully exploit the potential emission reduction of biodiesel in the future despite the constantly increasing requirements.
COMBUSTION ODOUR OF BIODIESEL
The typical odour reminiscent of chips occurring during the combustion of biodiesel is produced mainly when the engine is cold or in vehicles without oxidation catalytic converters. The components which cause the odour are unburnt or only partially burnt hydrocarbon compounds. However, the catalytic converters which are standard in new cars break down these compounds efficiently. Particularly the low sulphur content of biodiesel leads to a much improved conversion of toxic substances in the catalytic converter.
HIGHER CONSUMPTION WITH BIODIESEL
Due to the lower specific energy content of biodiesel, a slightly higher consumption must be anticipated. However, in practical operation, this is significantly less than indicated by formal calculations because other favourable parameters of biodiesel allow a more efficient operation of the engine. In fleet trials, consumption rates 0 to 5 percent higher than with the use of diesel fuel were determined. It must also be noted that the engines were optimised for the use of diesel fuel.
SAFETY ASPECT FLASHPOINT
Biodiesel compliant with DIN EN 14124 has a flashpoint of over 110 °C and other properties which indicate a lower potential hazard. For this reason, it is not a hazardous material and its handling is not subject to the operational safety rules. This is a great advantage over mineral oil diesel in storage and handling. Refuelling of utility machines from canisters in road construction, forestry or in leisure sailing is therefore much safer. The same applies to storage and transport in these environmentally sensitive areas.