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As fuel system technology advances, a rise in injection pressures to 2,000bar or more could cause extra problems for those operators wanting to run on biodiesel.

Research by Delphi suggests that biodiesel in blends of more than about 5% poses particular challenges where it has been held in storage tanks long enough for it to start degrading and the higher the percentage bio blend the worse the effects.

Even where biodiesel conforms to the EN14214 standard, increased injection pressures  - leading to higher combustion chamber temperatures - can cause 'lacquering', a build-up of solid deposits, especially around injector nozzle spray holes.

Analysis of the 'crust' has shown it to include lube oil constituents. It is hoped that the introduction of low-SAPS (sulphated ash, phosphorus and sulphur) lube oil, aimed at reducing the accumulation of combustion ash in ceramic particulate filters, will bring a lacquering reduction benefit when engines are fuelled with biodiesel.

Recent moves to smaller injector spray holes (albeit greater in number) have aggravated the lacquering problem, says Delphi. Thus combustion is less complete, so that fuel consumption suffers, with a risk of particulate emissions exceeding legislated limits.

Biodiesel additives are being evaluated by Delphi. Biodiesel tends to reduce particulate and gaseous hydrocarbon emissions. But fuel consumption and engine performance suffer from the fuel's lower calorific value. Oxides  of nitrogen (NOx) emissions are also increased, though not, through the usual cause of higher peak combustion temperatures, but because of the extra oxygen present when biodiesel burns. Should an engine certified as Euro 4 or 5 compliant using ULSD test fuel then be run on a strong, say 30%, biodiesel blend, NOx emissions could, suggests Delphi, rise above the legislated limit.