Fuels: A Guide
Don’t shed a tear for me but the new MiTo, 159 and the Brera all have service intervals of 18000 miles and at 18000 miles we change the oil and that is about all we do on the first service, now when I were a lad you had to have your Bertone coupe serviced every 3000 miles, that is quite a change over 30 years and there is one major reason for this and that is the improvement in oil and fuel technologies, fuels in particular have come a long way in that time. Both petrol and Diesel as fuels have approximately the same amount of power stored in them at around 3700kj/m3 and this is virtually the same for all liquid fuels as well as liquefied gasses (I bet you didn’t know that!). For spark ignition (petrol) engines the fuel needed for a particular car has a grade or Octane rating of either 95 or 98 RON, RON stands for research octane number and the higher the number the greater the resistance to pre-ignition or pinking. Regular petrol without additives is very prone to pinking and to increase its resistance various compounds are added at the refinery the most famous of which is tetramethyl lead, Pb(CH2CH3)4 or just lead to you and I.
In 1922, an American called Thomas Midgely (who also invented CFCs and was obviously very fond of the planet and future generations) found that if tetraethyl lead was put into petrol, particles of lead and lead oxide PbO are formed on combustion. This helped the petrol to burn more slowly and smoothly, preventing knocking and giving higher Octane ratings. Using higher-Octane leaded petrol meant that more powerful high-compression car engines could be built. Lead also had the added advantage that it formed a layer between the valve and the valve seat which meant that economy car makers could get away without fitting a separate valve seat inserts into cheap cast iron cylinder heads. The theory behind valve seat regression is that wearing down of the exhaust valve seat is caused by abrasion, corrosion and erosion processes, but mostly by abrasion. Wear is considered excessive when the valve system no longer has the ability to seal the combustion chamber resulting in loss of compression, power and fuel efficiency. Excessive valve seat recession can occur because the normal rotation of the valve causes abrasion between the materials that are caught between the surfaces of the valve face and the valve seat. Researchers have concluded that abrasive particles are formed by a miniscule welding of the valve to the valve seat when the two close due to the force of the valve closing and the temperatures involved, it then pulls the welded particle with it upon opening. The welded base metal from the valve seat then oxidises due to the much higher temperature of the valve. The resulting oxide is abrasive and excessive valve wear is caused by this abrasion. However with an aluminium cylinder head you have to have a steel valve seat insert fitted as the aluminium is too soft on its own to take the required force and this steel seat is hardened to resist wear and therefore as Alfa Romeo’s have Aluminium cylinder heads recession does not occur with Alfa Romeo valve seats. With a lot of Alfa Romeo’s the exhaust valve is also hollow and it is filled with sodium, this melts at the high temperatures the exhaust valve runs at and then transfers the heat to the valve guide this also helps to prevent damage to the valve and seat. It is relatively easy to check that there is not a problem with valve seat wear by regular checks of the exhaust valve clearances.
With the removal of Lead from petrol other Additives have been used to increase the octane rating of the fuels and these include methyl tertiary-butyl ether (MTBE) which is not very toxic but it is not very biodegradable either, and has a strong taste and smell, noticeable at the 15 parts per million level so it is not very pleasant in the environment. Benzene is another additive used in fuel to increase the octane rating but it does have one major disadvantage and this is a quote “The short term breathing of high levels of benzene can result in death” so governments are trying to reduce the amount of benzene in fuel. On most modern fuel injected cars there is a fuel recovery system that uses charcoal filters to absorb the gasses that are released from the fuel which are then burnt in the combustion process when the engine is running. This vapour recovery system helps to stop benzene escaping into the environment which is perceived to be carcinogenic especially in large cities. The last of the commonly used octane boosters is toluene which by the standards of the other chemicals here is comparatively harmless as generally inhalation of toluene fumes is only intoxicating and only in larger doses it is nausea-inducing and it is not believed to have any carcinogenic effects. In the 1980’s a mixture of 86% toluene was used on all of the Formula 1 Turbo cars with a “filler” to bring the octane rating down to the regulation level.
With all the environmental issues caused by burning fossil fuels manufacturers are increasingly turning to Alcohols as a substitute for petrol and the most promising of these is ethanol which is produced from plant material. Ethanol is in fact the same alcohol you find in beers, wines and spirits its chemical formula is C2H5OH. This presents all sorts of problems, one of the major ones is that Ethanol can easily absorb water (water with your whisky Sir) and this means that all of the fuel injection system must be corrosion proof and this is why you cannot readily substitute Ethanol for petrol at the moment. I do however see a time when car engines become multi-fuel and will be able to run efficiently on whatever is in the fuel tank. The use of ethanol as a major source of fuel is also an environmental issue in the fuel verses food debate, to make one gallon of fuel requires 26 lbs (12Kg) of corn and so an acre of land will produce around 330 gallons of fuel which when you use 680 million gallons of petrol per year. This equates to an awful lot of land that is put aside from producing food and this has resulted in the major rises in grain prices over the past year. Interestingly one of the by products of ethanol production is distillers grain which is an extremely good form of protein and can be fed to cows without causing excessive flatulence (I just had to get that in!). This reduces the amount of methane produced by cows and as methane is a greenhouse gas this has an advantage in the climate change debate. Ethanol can also be produced from other forms of plant material which can be more efficient, while corn can produce approximately 35% more energy than it takes to produce, sugar cane can produce up to 800% more than it takes to produce it so it is highly likely that this crop will become more widely used to produce fuel but again this is at the expense of land that is used for food. Ethanol used for fuel is made unfit for Human consumption by the addition of various compounds that are toxic such as methanol or unpleasant such as denatonium benzoate.
Diesel engines work by compression ignition and not by spark ignition, the air in the combustion chamber is compressed by the piston coming up the bore as the gas is compressed it heats up considerably to around 400°C then at the optimum time in the compression cycle diesel is injected directly into the combustion chamber and due to the temperature in the combustion chamber it explodes. Diesel made from crude oil and is a mixture of hydro-carbons that have boiling points in the 180º to 360ºC range. Diesel varies widely in density and this makes it quite difficult to meter the correct amount of fuel at any given rpm and one of the advantages of the common rail (JTD) system is that it can deal more effectively with these differences in density. Additives in Diesel are less widely used than petrol engines, however diesel has a tendency to form crystals of paraffin as the temperature drops towards freezing these crystals then tend to clog the fuel lines and fuel filter. To combat this it used to be common practice to add a very small percentage of petrol to your fuel, this is not recommended in common rail diesel engines instead fuel refiners add flow improvers, these are polymers that impede the growth of the paraffin crystals and so they remain small enough to pass through the fuel lines and filter and do not create problems with starting or running.
Diesel can also be produced from plant matter as a sustainable resource, however it suffers from the same fuel verses food arguments that apply to petrol, increasingly governments are insisting that a percentage of all diesel supplied on the forecourt is bio diesel at the moment there is around 5% bio diesel in the fuel you purchase at the pumps and that percentage should go up to around 10% by 2020. Bio diesel also helps to reduce the amount of sulphur that is emitted from diesel vehicles which reduces the effects of exhaust emissions.
Now if you happen to accidentally put the wrong fuel in your car you will breakdown and here is why, If you put petrol in a diesel the petrol will detonate in the combustion chamber far earlier than diesel will (remember the bit about additives to stop pinking earlier) and this will force the piston the wrong way down the bore and cause the engine to rattle and stop, this way round you may cause damage to your engine if used for a prolonged period of time, however we have never found that it causes any significant damage to lift pumps or injectors if the car is stopped immediately. If you put diesel in your petrol car you will just come to a sad ignominious smoking halt but it is extremely unlikely that you will do any damage to anything other than your pride.