Last reviewed 18 February 2014
A previous article, Engine and fuel types in vehicle selection, described the characteristics of the various types of engines commonly available for road vehicles and identified that any particular engine type is not necessarily limited to a single fuel. In this article, Richard Smith focuses on some of the factors to be considered with particular fuels.
The default choice for commercial vehicles is a compression-ignition engine and, for heavy vehicles, there is generally no alternative since no manufacturer fits spark-ignition engines to such vehicles nowadays. However, such engines may be a viable option for smaller vans and, when the impact of emissions legislation is considered, may be an advantage.
While the predominant fuels are diesel for compression-ignition engines and petrol for spark-ignition ones, both types of engine may be able to be supplied or converted to run on compressed natural gas (CNG) or liquefied petroleum gas (LPG). These fuels have clear cost advantages under the present taxation system and also result in quieter and cleaner running engines. Against this must be set the potential disadvantage of reduced fuel economy.
Most significantly, vehicles, including hybrids, using CNG or LPG are prohibited from using the Channel Tunnel and many tunnels in mainland Europe.
Biofuels are those that mix with the petrol or diesel a proportion of biologically-sourced material. For diesel, the bio element comes from rapeseed or palm oil and for petrol it is in the form of ethanol derived from grain or sugar cane.
Fuel companies are now legally obliged to include 5% bio element in both their standard petrol (E5) and diesel (B5) fuel. These mixes are claimed to present no problems in engines not specifically designed for them, although operators have noted some (see below).
Diesel is also available in a 30% blend (B30) and 100%, but should not be used unless the engine has been certified by the manufacturer as safe for them. E10 petrol (10% ethanol in standard 95 octane gasoline) is sold in France but is not suitable for all engines.
There is pressure for these higher proportion blends to become the legal standard and in some countries bioethanol is available at up to 95%.
Leaving aside the wisdom of devoting large areas of land to the production of biofuel rather than food, there are also some problems experienced by operators when using current blends of biofuels. In the case of both biodiesel and bioethanol, the energy density of the fuel is reduced compared to pure diesel or petrol. This results in reduced power output and increased fuel consumption.
Both types of fuel can also suffer from a tendency to attract water from the atmosphere and this leads to a build up of water in storage tanks. The water separates out and forms a layer on the bottom of the tank, which not only promotes rusting of steel tanks but also means that the fuel pump draws out this water rather than the fuel.
Problems associated with biodiesel
The principal problem noted with biodiesel is that of fuel filter clogging. This arises from a combination of several causes. A particular component of the fuel (sterol glucosides) is prone to formation of a gel (waxing) at lower temperatures and hence blocks the filter. Normal diesel fuel also does this, but during the winter months a special additive is included to prevent it.
Also, unlike normal diesel waxing, the biodiesel gel does not dissolve back into the fuel once the temperature warms up. Since the component is part of the bio additive the problem increases with the proportion of bio element.
The second cause of filter clogging is the growth of microbiological organisms in the fuel. The water which biodiesel absorbs readily from the atmosphere supports the growth of these microbiological organisms in the fuel in storage tanks. If this bacterial growth is allowed to continue it will also contribute to filter clogging.
While gel formation only happens at low temperatures, this process occurs at all temperatures and is likely to be more significant at warmer temperatures.
A third problem with higher-proportion biodiesel blends arises from the solvent effect of these fuels, which can cause break-down of the varnish deposits that build up on the walls of fuel tanks. This results in contamination of the fuel with particulates that will rapidly block filters.
This problem should be solved and not recur when the filter is changed, for the deposits will by then have been cleaned off and trapped in the filter.
It has been claimed that there is no solvent effect at B5 proportions, although this seems to have been contradicted by experience. The effect is stated to be minimal up to B20 but at B30 solvent problems can certainly be expected.
Higher concentrations of bioethanol can give excellent high performance results and many vintage racing cars used pure methanol as fuel. However, the engine has to be specially set up to use such high concentrations and the fuel is also more difficult to vaporise, making it hard to start the engine in cold weather.
Ethanol is corrosive and, over time, can create white deposits in the fuel system, the result of corrosion of the materials the components are made from by the fuel — made worse by the presence of the water in the fuel. Ethanol also dries out rubber components in the fuel system and therefore causes cracked and brittle (hence leaking) fuel lines, seals and diaphragms.
While the attraction of water is a constant problem, the latter issues of corrosion and drying out of rubber components should not affect vehicles built since about 2007, as they will have components specially designed for ethanol. However, it will be wise to confirm the suitability of any given engine for ethanol before using the higher proportion blends.
While the problems associated with the attraction of water by both bioethanol and biodiesel should not be significant for vehicles in regular use, since the fuel tank will be regularly emptied and refilled with fresh fuel, infrequently used vehicles such as recovery vehicles can have problems. Even with regularly used vehicles the fuel tank should be run down nearly to empty from time to time to prevent build up of water in the bottom.
Storage in bulk tanks will be more of a problem, especially when a certain level of fuel is maintained as a buffer stock. This will be exacerbated by the fact that the fuel is not continuously agitated as it would be in a moving vehicle tank. Operators who hold bulk stocks of fuel must be aware of these problems and regularly run down the tank to nearly empty to eliminate a possible build-up of bacteria and water. The solvent effect will also affect the bulk storage tank and, unless steps are taken to prevent it, the fuel drawn from them will continue to contaminate vehicle fuel systems even after a filter change.
There are viable alternatives to diesel-fuelled compression-ignition engines for many types of operation, but also important caveats. The statutory inclusion of a bio element in both petrol and diesel can cause problems for operators, particularly concerning those who have infrequently used vehicles or who hold their own fuel in bulk storage.