Biodiesel and its Impacts

Biodiesel and its Impacts

This article reviews the long-term storage of fuels containing a % of biodiesel and the impacts biodiesel can have on fuel injection systems. Care should be taken in the long-term storage of fuels and engines served from these storages.

Biodiesel (also known as FAME (fatty acid methyl ester)) %

Current limits for standard automotive diesel in Europe allow 7% Biofuel (B7) as per EN 590. In Australia this percentage is lower, at 5% or B5, however it is expected that Australia will follow Europe in the long run.

All MTU, Mercedes Benz, Detroit and Deutz engines can safely run on B5 and B7 biodiesel blends, it is only when the engine remains unused for a length of time, or the diesel fuel remains in storage for a significant period that problems can arise.

Are all Biodiesels the same?

In Europe mainly rapeseed oil > RME rapeseed acid methyl ester

In Asia mainly palm oil > PME palm acid methyl ester

In Americas mainly soy oil > SME soya acid methyl ester

In Australia > Mostly Tallow (animal fat)

The type of oil used influences the behaviour of the biodiesel in terms of oxidation stability and cold behaviour.

FAME must be blended into fossil diesel fuels (Fuel Standard (Automotive Diesel) Determination 2019)

Dependency of oxidation stability on the type of biodiesel

The higher the amount of unsaturated double bonds in the oil fraction, the poorer the oxidation stability

The higher the amount of saturated double bonds in the oil fraction, the worse the cold stability (waxing)

This results in a biodiesel that has either good oxidation stability but waxes badly in cold weather, or a biodiesel that has good cold weather performance but poor oxidation stability. As Australia is using mostly animal fat-based biodiesel, the biodiesel behaviour sits in the middle of the graph below.



Impacts of Biodiesel on engines

Biodiesel is a natural product and is subject to natural ageing/oxidation. The higher the FAME content in the fuel, the more unstable the overall fuel, i.e. the faster the fuel ages and has negative effects on the overall engine system, e.g. deposit formation, filter clogging etc.

·         Biodiesel can attack elastomer components and lead to leakages

·         Biodiesel can lead to corrosion

·         Biodiesel is hygroscopic, i.e. it attracts water and can dissolve more water than fossil diesel

·         Biodiesel is an ideal food source for microbes (see image below)

·         Biodiesel is a solvent, i.e. it can attack painted surfaces

·         If fuel enters the engine oil, the ageing resistance of the engine oil can also be negatively affected by oxidized biodiesel.



Oxidation stability

Oxidation stability, also called ageing stability, is a measure of the durability of diesel fuel. Oxidation stability depends on number of factors, including the presence of some metals that can be found in fuel systems. Oxidation produces ageing products such as acids (short-chain) and polymers – impacts of this can be seen in the image below. Polymers are no longer soluble in the fuel due to their chain length.

Ageing factors are:

·         Water content in the biodiesel

·         The temperature: the higher the temperature, the faster the ageing

·         The amount of oxygen: the higher the oxygen supply, the faster the ageing.

The presence of catalytic components: Copper, zinc and alloys thereof accelerate ageing – avoid these metals in any part of the fuel supply chain.




Effects of poor fuel oxidation stability on engines

Build-up of deposits on fuel-carrying components, this can be particularly noticeable with filter clogging. If there is fuel dilution in the oil when using biodiesel, this can have a negative influence on the engine oil, this is why OEM’s sometimes mandate reduced oil drain periods when engines use higher percentages of biofuel.

Oxidation of the fuel has several negative influences on chemical fuel-properties:

·         Increase of fuel-acidity

·         Increased risk of corrosion

·         Increase of viscosity




Ageing and thickening of the engine oil sees the formation of oil sludge seen in the filter, cylinder head and oil pan in the images above.


Recommendations for the use of fuel containing biodiesel

Use fuels containing biodiesel with a high initial oxidation stability to delay the ageing of the fuel.

Observe suitable tank maintenance, i.e. low storage temperature, minimise water and oxygen.

Avoid water through suitable water separation – ideally continually polish the bulk fuel storage

Avoid metals that can have a catalytic effect on oxidation, e.g. zinc, copper, copper-containing alloys.

Do not use NBR sealing materials, as these are attacked with increasing biodiesel content.

If necessary, use approved antioxidant additives.

Select suitable engine oil, i.e. at least mtu-category 2 oils and monitor the oil behaviour. If necessary adapt (reduction) oil change interval.

It is recommended that for standby applications where large, long term storage fuel tanks are used, that a suitable testing regime is implemented for the fuel, so its condition, particularly with respect to oxidation and water content are closely monitored. A study done in Germany found that in several standby installations, the fuel quality was so low that is was likely the engine would not start on request.



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