The aim of this section of Biofuels for Transport is to discuss the engine modifications that may be required to run biofuels in conventional internal combustion engines.
The fuels being looked at specifically are biodiesel, used in a compression ignition engine, and bioethanol, used in a spark ignition engine.
The octane number of a petrol fuel is defined as a measure of the resistance of the fuel to abnormal combustion - known as “knocking”. The higher the fuel octane number, then the less likely it becomes that the engine will be susceptible to “knock”. The “knocking process” is caused by the incomplete combustion of the petrol fuel in the engine cylinder, which causes a sudden knock or blow to the piston, which over a period of time will seriously damage the vehicle engine. By adding an ethanol E10 blend to petrol, we can increase the octane number of the petrol fuel by two points. 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 for complete combustion is about 14.6 air : 1 fuel. This means that 14.6 Kg of air is required for the complete combustion of 1 Kg of non oxygenated petrol fuel.
An ethanol E10 blend of fuel will normally have an oxygen content of about 3.5% oxygen. The oxygen that is present in the ethanol can affect the air to fuel ratio at which the engine is operating at. Therefore, it is usually necessary for certain car engines to have the air : fuel ratio reduced in order to take into account the oxygen content that is present in the ethanol blend. 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 for conventional 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 motor vehicles will electronically sense and change the air fuel mixing ratio in order to maintain the stoichiometric ratio when ethanol (oxygenated) fuels are added to the engine. For some vehicles, the maximum oxygen content that can be compensated for is 3.5% oxygen (E10 ethanol fuel blends). Older vehicles are usually not fitted with engine management systems, instead they operate with a normal fuel carburettor system. Thus, the carburettor air fuel mixture must be adjusted manually, in order to compensate for the increased oxygen content that is present in ethanol blended fuels.
It maybe necessary to change the vehicles fuel filter more often as ethanol blends can loosen solid deposits that are present in vehicle fuel tanks and fuel lines.
Ethanol blends have a higher latent heat of evaporation than 100% petrol and thus ethanol blends have a poorer cold start ability in Winter. Therefore some vehicles have a small petrol tank fitted containing 100% petrol for starting the vehicle in cold weather.
Engine Modifications for Ethanol blends of 14% to 24%
The following engine modifications were carried out by car companies in Brazil, in the 1970’s, when vehicles were operating on ethanol blends of between 14 and 24% ethanol:
Vehicle owners running their cars on ethanol blends should adhere to the recommendations of the individual car manufacturers. In the UK, nearly all vehicle manufacturers specify that the maximum ethanol blend in petrol should be no more than 5% ethanol by volume. In the USA, nearly all vehicle manufacturers specify that the maximum ethanol blend in petrol should be no more than 10% ethanol by volume. Therefore, should a vehicle owner choose to use a higher ethanol blend that the manufacturer recommends, then normally the vehicles warranty would become null and void. Most vehicle manufacturers also state that vehicle damage and driveability problems would occur by using higher ethanol blends that the manufacturers recommendation.
Almost all modern diesel engines will run biodiesel quite happily provided that the biodiesel is of high enough quality. Generally speaking biodiesel requires much less engine modification than bioethanol.
With some older vehicles rubber seals used in the fuel lines may require replacing with non-rubber products such as VITONTM. This is due to the way biodiesel reacts with rubber. If a low blend is used (5% biodiesel for example) then the concentration of biodiesel isn't high enough to cause this problem.
Cold starting can sometimes be a problem when using higher blends. This is due to biodiesel thickening more during cold weather than fossil diesel. Arrangements would have to be made for this, either by having a fuel heating system or using biodegradable additives which reduce the viscosity. This effect is only a problem with higher blends.
It was noticed that during many field trials that engines running on biodiesel tended to require more frequent oil changes. This was generally the case with blends above 20%. During an ALTENER project where two Mercedes Benz buses were run on diesel and biodiesel it was found that the bus running 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 that the engine had not been significantly effected in any adverse manner.
For higher blends engine performance will be improved with a slight change to engine timing, 2 or 3 degrees for a 100% blend. The use of advanced injection timing and increased injection pressure has been known to reduce NOx emissions. It is worth noting that catalytic converters are just as effctive on biodiesel emissions as on fossil diesel.