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Engine ModificationThe aim of this section of Biofuels for Transport is to discuss the enginemodifications that may be required to run biofuels in conventional internalcombustion engines.

The fuels being looked at specifically are biodiesel, used in a compressionignition engine, and bioethanol, used in a spark ignition engine.

Bioethanol Modification(ref21)Octane NumberThe octane number of a petrol fuel is defined as a measure of the

resistance of the fuel to abnormal combustion - known as knocking. Thehigher the fuel octane number, then the less likely it becomes that theengine will be susceptible to knock. The knocking process is caused bythe incomplete combustion of the petrol fuel in the engine cylinder, whichcauses a sudden knock or blow to the piston, which over a period of timewill seriously damage the vehicle engine. By adding an ethanol E10 blendto petrol, we can increase the octane number of the petrol fuel by twopoints. Therefore bio-ethanol is termed as an octane enhancer.

Air Fuel Mix

The air/fuel mixing ratio that is required for 100% petrol fuels in order forcomplete combustion is about 14.6 air : 1 fuel. This means that 14.6 Kg ofair is required for the complete combustion of 1 Kg of non oxygenatedpetrol fuel.

An ethanol E10 blend of fuel will normally have an oxygen content of about3.5% oxygen. The oxygen that is present in the ethanol can affect the air tofuel ratio at which the engine is operating at. Therefore, it is usuallynecessary for certain car engines to have the air : fuel ratio reduced inorder to take into account the oxygen content that is present in the ethanolblend. The air / fuel ratio for a VW Golf running on 22% ethanol is 12.7:1,which is slightly less than the 14.6:1 air / fuel ratio that is used forconventional fuels.

This effect is also applicable to biodiesel as it is also an oxygenated fuel.

The engine management systems that are fitted in most modern motorvehicles will electronically sense and change the air fuel mixing ratio inorder to maintain the stoichiometric ratio when ethanol (oxygenated) fuels

are added to the engine. For some vehicles, the maximum oxygen contentthat can be compensated for is 3.5% oxygen (E10 ethanol fuel blends).

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Older vehicles are usually not fitted with engine management systems,instead they operate with a normal fuel carburettor system. Thus, thecarburettor air fuel mixture must be adjusted manually, in order tocompensate for the increased oxygen content that is present in ethanol

blended fuels.

Fuel FiltersIt maybe necessary to change the vehicles fuel filter more often as ethanolblends can loosen solid deposits that are present in vehicle fuel tanks andfuel lines.

Cold StartingEthanol blends have a higher latent heat of evaporation than 100% petroland thus ethanol blends have a poorer cold start ability in Winter. Therefore

some vehicles have a small petrol tank fitted containing 100% petrol forstarting the vehicle in cold weather.

Engine Modifications for Ethanol blends of 14% to 24%The following engine modifications were carried out by car companies inBrazil, in the 1970s, when vehicles were operating on ethanol blends ofbetween 14 and 24% ethanol:

Changes to cylinder walls, cylinder heads, valves and valve seats Changes to pistons, piston rings, intake manifolds and carburettors Nickel plating of steel fuel lines and fuel tanks to prevent ethanol E20

corrosion Higher fuel flowrate injectors to compensate for oxygenate qualities

of ethanol

Vehicle WarrantiesVehicle owners running their cars on ethanol blends should adhere to therecommendations of the individual car manufacturers. In the UK, nearly allvehicle manufacturers specify that the maximum ethanol blend in petrolshould be no more than 5% ethanol by volume. In the USA, nearly allvehicle manufacturers specify that the maximum ethanol blend in petrolshould be no more than 10% ethanol by volume. Therefore, should avehicle owner choose to use a higher ethanol blend that the manufacturerrecommends, then normally the vehicles warranty would become null andvoid. Most vehicle manufacturers also state that vehicle damage anddriveability problems would occur by using higher ethanol blends that themanufacturers recommendation.

Biodiesel Modification(ref22)

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Almost all modern diesel engines will run biodiesel quite happily providedthat the biodiesel is of high enough quality. Generally speaking biodieselrequires much less engine modification than bioethanol.

Rubber SealsWith some older vehicles rubber seals used in the fuel lines may requirereplacing with non-rubber products such as VITONTM. This is due to theway biodiesel reacts with rubber. If a low blend is used (5% biodiesel forexample) then the concentration of biodiesel isn't high enough to cause thisproblem.

Cold StartingCold starting can sometimes be a problem when using higher blends. Thisis due to biodiesel thickening more during cold weather than fossil diesel.

Arrangements would have to be made for this, either by having a fuelheating system or using biodegradable additives which reduce theviscosity. This effect is only a problem with higher blends.

Oil ChangingIt was noticed that during many field trials that engines running on biodieseltended to require more frequent oil changes. This was generally the casewith blends above 20%. During an ALTENER project where two MercedesBenz buses were run on diesel and biodiesel it was found that the busrunning on biodiesel required an oil change after 12,000 km compaired to

21,000 km for the bus running fossil diesel. It is worth noting however thatthe engine had not been significantly effected in any adverse manner.

Engine TimingFor higher blends engine performance will be improved with a slightchange to engine timing, 2 or 3 degrees for a 100% blend. The use ofadvanced injection timing and increased injection pressure has beenknown to reduce NOx emissions. It is worth noting that catalytic convertersare just as effctive on biodiesel emissions as on fossil diesel.