Effect of biodiesel on fuel injection systems

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This page reproduces information found in the Joint Fuel Injection Equipment Manufacturers Statement, a study by the following manufacturers into the effect of biodiesel (aka FAME) and associated impurities on diesel fuel injection systems.

  • Delphi Diesel Systems
  • Stanadyne Automotive Corporation
  • Denso Corporation
  • Robert Bosch GmbH

Biodiesel and impurities

Fuel Characteristic Effect Failure Mode
Fatty acid methyl esters (general) Causes some elastomers including nitrile rubbers to soften, swell, or harden and crack Fuel leakage
Free methanol in FAME Corrodes aluminium & zinc
Low flash point
Corrosion of fuel injection equipment
FAME process chemicals Potassium and sodium compounds
Solid particles
Blocked Nozzles
Dissolved water in FAME Reversion of FAME to fatty acid Filter Plugging
Free water in mixtures Corrosion
Sustains bacteria
Increases the electrical conductivity of fuel
Corrosion of fuel injection equipment
Free glycerine Corrodes non ferrous metals
Soaks cellulose filters
Sediments on moving parts
Filter clogging

Injector Coking
Mono- & di-glyceride Similar to glycerine
Free fatty acid Provides an electrolyte and hastens the corrosion of zinc
Salts of organic acids
Organic compounds formed
Corrosion of fuel injection equipment
Filter plugging
Sediments on parts
Higher modulus of elasticity Increases injection pressure Potential of reduced service life
High viscosity at low temperature Generates excessive heat locally in rotary distributor pumps
Higher stressed components
Pump seizures
Early life failures
Poor nozzle spray atomisation
Solid impurities Potential lubricity problems Reduced service life

Ageing products

Fuel Characteristic Effect Failure Mode
Corrosive acids (formic & acetic) Corrodes all metallic parts
may form simple cell
Corrosion of fuel injection equipment
Higher molecular organic acids Similar to fatty acid
Polymerisation products Deposits especially from fuel mixes Filter plugging
Lacquering formation in hot areas

Considerations for homebrew biodiesel

Note that this section is not part of the document above, but our take as biodiesel homebrewers.

Minimising free fatty acid and mono/diglyceride content

Free fatty acids should be removed by the biodiesel process, if the source oil is titrated correctly.

Mono and diglyceride molecules are partly converted vegetable oil (triglyceride) molecules. They are excellent emulsifiers with water, and are often responsible for a white layer in 50/50 soap tests). Complete conversion of vegetable oil into biodiesel will ensure minimum mono and diglyceride content. This can be verified by 3/27 testing the produced fuel.

Removing methanol and glycerol

Water washing will remove methanol and glycerol, as will methanol recovery, settling and dry washing.

Avoiding polymerisation products

Red residue that sometimes drops out when mixing biodiesel with mineral oil - in this case petrol
Same red residue at the bottom of a car's tank baffle

Many biodiesel homebrewers will have seen reddish drop-out in their fuel when mixed with mineral fuels, as highlighted in the polymerisation products row above. If mixing is required, it is advisable to do this outside of the vehicles tank, and settle for at least 24 hours prior to fuelling.

Fuel storage and oxidisation

As most homebrewers use their fuel within a short period of time after production, most of the negative effects of aging are never seen. That said, be aware that contact with copper (processor pipework or cylinders) reduces the shelf life of biodiesel - even small PPM copper content will act as an oxidisation catalyst for the fuel.

Fuel intended for storage should be kept in a airtight container, that is as full as possible to minimise the quantity of air at the top.

Fuel system and engine care

Nylon tubing makes a good substitute for fuel line

Biodiesel can have a detrimental effect on fuel system components such as fuel line and pump seals.

The most common problems are with systems employing mechanical injection pumps.

  • Injector leak-off return pipes rotting - these are relatively easy to replace on most vehicles - nylon air line makes a good biodiesel-proof replacement
  • Injection pump shaft seals leaking
    • for gear driven pumps this can mean leaking of fuel into the crankcase oil (look for increasing engine oil level) and contamination of the fuel with engine oil (look for black deposits when the fuel filter is changed).
    • for belt driven pumps, failure of this seal usually results in fuel leaking out onto the road, but also possible contact with belts - particularly serious if a cam belt is affected, as biodiesel will rapidly weaken it.

Engine rattling due to air ingress is also a sure sign of seal failure somewhere, though if you're lucky the leak won't be at pump but further back in the fuel system, such as the fuel filter or at tank fittings. A section of clear braided hose (commonly 8mm ID) greatly helps with locating the source of air ingress - put between the fuel filter and the pump and backlit with a torch, any air bubbles will be clearly visible.

Peugeot 406 in-tank strainer
Blocked in-tank strainer from a Mitsubishi Delica
Suitable strainer replacement for the engine bay

Other potential issues include a blocked in-tank coarse filter with soaps or polymerisation products. Some biodiesellers like to remove the in-tank filter and fit a coarse filter before the main filter in the engine bay, where it is more easily accessible for cleaning or replacement.

In all case, it is recommended to change the engine oil twice as frequently as the normal service interval. When diesel contaminates engine oil, over time it can "boil off" with engine heat - however, Biodiesel will remain in the oil and potentially polymerise into a gel over time, blocking oilways or the oil filter.