The economic growth of the industrialised world is due in no small part to the use of fossil fuels, and arguably no fossil fuel has had more impact than oil. During the development of the combustion engine the inventors quickly identified that fuels based on hydrocarbons were the most suitable, and with the growth in popularity of the automobile a rapidly expanding industry was created almost overnight.
The rush was on, not only to locate the necessary reserves of oil, but to refine and improve the quality.
The first iterations of petrol were based on coal tar distillates and distillation of crude oil, the latter being used to power the first four-stroke cycle spark-ignition engine in 1884.
At the time, petrol was considered an undesirable by-product from the manufacture of kerosene, the latter being used extensively for lighting and other commercial and domestic purposes.
As the proliferation of cars grew during the first few decades of the twentieth century, demand for petrol would see its profile as a fuel source rise, quickly surpassing the once mighty kerosene.
Thermal cracking was introduced in 1913 to convert a larger fraction of petroleum into petrol - earlier investigations had shown that the heating of crude oil caused a split-up of molecules, thus increasing the proportion of volatile fractions suitable for petrol manufacture (thermal cracking required elevated pressure for the process). The 1920’s were a time of great innovation in the automotive world, and oil companies needed to invest heavily in refining techniques to ensure their petrol would meet the ever more exacting standards being required to run the latest engines.
During that time it was found that certain silica/alumina-based catalysts accelerated the reaction rate to the extent that high pressure became unnecessary. The advantages of catalytic cracking over thermal cracking were a higher petrol yield and a better quality of product.
In about 1922 Thomas Midgley discovered the effects of tetraethyl lead (TEL), and petrol has never been the same since. TEL had its drawbacks. Lead oxides formed on the combustion chamber and exhaust valves, lead salts (electrically conductive) on spark plug insulators. Ethyl dibromide fluid dealt with the deposits, halo-wax oil served to lubricate the valve stems. Still, it took time for TEL to catch on, as there was no reliable means of testing the anti-knock rating of fuels - until in 1927 another American, Graham Edgar, discovered iso-octane, which he combined with normal heptane to establish the octane number scale that we now all use, know, and to some extent understand.
The Research and Motor Methods
There are actually two octane scales used. One measured knock resistance at very low speeds when the engine was really 'lugging', and was known as the Research Method. The other applied to higher speeds and was the Motor Method, and its figures were somewhat lower. The difference between the two was called the sensitivity number: about 5 or 6 for the best grades of pump petrol, it was becoming more important to reduce the anti-knock properties of premium petrol’s in the hope of reducing emissions of lead and of oxides of nitrogen. The use of the more volatile tetramethyl lead (TML) instead of TEL was a palliative, but there were many that wanted to stop the use of TEL altogther.
During World War 2 enthusiasm ran high for a new super-fuel called triptane, having much of the character of conventional gasolines but so resistant to detonation that an engine designed to exploit it could, in theory, develop three times as much power as one doing its best with 100 RON (Research Octane Number) fuel. Everyone thought triptane would win the war for the Allies until it was calculated that to produce it in sufficient quantities would consume the entire chlorine output of the USA. It is in knock resistance that the secret of power lay. Unfortunately, the octane rating scales did not give a clear measure of this potential, and fuel chemists preferred a scale of performance numbers (PN) which offered a roughly linear rating corresponding to the power output they make possible.
PN and RON scales coincide at 100; but a 72 RON fuel would have a PN of 50, indicating that an engine using it would be limited to half the power output of an engine fully adapted to 100 PN fuel. PN ratings improved over the years, being responsible for a far greater proportion of the improvements in engine power since the pioneering days than anything else - in fact, a greater proportion than all other developments put together. PN ratings rose by 416% in the thirty years ending in 1945 - making it ironic that for some years afterwards, the quality of motor fuels was execrable, only reaching 90 plus RON ratings in 1953.
The Catalytic Cracking Process
Next came a breakthrough in the catalytic cracking process, the initially developed fixed-bed catalytic process was replaced by a fluid-bed process, which allowed for excellent control of temperature and reaction, in the process providing better yields of petrol from the refineries. The introduction of the catalytic cracking process and catalytic reforming in the 1940’s was significant for the manufacture of high-octane petrol components. During the 1950’s automobile manufacturers started to increase the compression ratios in their engines, resulting in higher octane ratings, lead levels, and vapour pressure.
As cars improved, so too did the 'petrol' available at the pump. Over the years, petrol could also contain lead-tetra-ethyl, ethyl dibromide, halowax oil, xylidine, tricresylphosphate, catechol, and various other additives whose presence or absence may depend on the octane rating of the product, the country where it was sold, or the time of year. Even the basic crude oil from which it is derived, will vary in characteristics according to its place of origin: the stuff from the East Indies is very different from Californian, which is again very different from what is found in the eastern states of the USA.
There were other fuels that were used for powering cars, but their cases are little less confusing. Benzol, benzene, or C6H6 (which is another hydro-carbon), comes from coal tar and coal gas. It served as the regular fuel of high-performance engines until the chemists got around to developing high-octane gasoline. Ethyl alcohol, C2HsOH, was produced by fermentation of any carbohydrate such as that from grain or potatoes. Not the best alcohol for motor fuel, it had the saving grace of being potable, but whisky distillers and the like had to be careful not to carry distillation too far lest methanol result, which has until recent times had a bad reputation.
Methanol is methyl alcohol, CH30H, and it can be produced by the destructive distillation of wood, and for that reason it is sometimes called wood alcohol and its properties make it ideal as a fuel for high-compression or (especially) super-charged engines of high specific output. Yet, when methanol is supplied as a racing fuel, it is usually impure: acetone has to be added to cure pre-ignition. Looking back at the formula for these alcohols, it may be noted that as well as carbon and hydrogen, they also contain oxygen. So do all the 'nitro' fuels. Nitrous oxide, for example, employed as a temporary vitaliser in certain selected Spitfire fighters late in World War 2 was, in its liquid form, one-third oxygen by volume. It and the other nitro fuels-nitro-methane, and the like-are chemically related to nitric acid.
Despite being one of the most corrosive agents going, the acid was used raw in some rocket engines of the 1960s and 1970s, combining with kerosene to bum with terrific intensity at temperatures which would melt a piston engine. Whatever the chosen fuel, and however it may burn, it does so by combining with oxygen; and this is why the superfuels carry some oxygen with them, in a more concentrated form than can be found in the ambient nitrogen-diluted atmosphere. Oxygen has the ability to join with practically everything that happens to be in the vicinity, as the basic atom of oxygen is short of a couple of electrons.
Oxygen will react with hydrogen to form water, or with iron to form rust. Special extinguishers are needed for dealing with fuel fires. Most of them rely on blanketing the flames with a gas that will not support combustion, as water wiII only spread the fire and make it worse. Carbon tetrachloride (CT) was once the accepted extinguishant; but when used, it released a gas which was converted to the deadly phosgene if inhaled through a cigarette. Even worse than the petrol fire is the petrol explosion, caused when a certain mixture of air and fuel is ignited in a closed container such as a car, a petrol tank, or a garage.
Petrol too low in octane value will show signs of detonation, bad for performance and bad for the engine itself. 'Octane value' is a common expression which ought to be replaced by anti-knock value, as this is what the octane scale measures: the higher the figure, the higher the resistance to knock or detonation. Octane is simply one of the series of light distillates produced from crude oil, and gets its name from the eight carbon atoms in each molecule. Heptane (seven carbon atoms) is another, and has a pronounced tendency to cause knocking, whereas octane at one time had the highest resistance of all fuels used or known.
But the challenges of making the best fuel possible remained. Petrol must remain liquid in the dead of winter (which is more than benzole would unless treated with additives), so a little toluene or xylene would be added. When carburetors were the norm ice formation inside would stop the flow of fuel to the engine, and in the 1960s and 1970s this was overcome to a large extent by the addition of isopropyl alcohol. To ensure ready starting at low temperatures, high volatility was essential: in winter, the oil companies included particularly 'light' distillates produced in the fractionating columns of refineries where crude oil is broken down into a variety of products including paraffin’s, lubricating oils, diesel oils and tars, et cetera.
Platformate and Activate
A petrol of 87 octane rating was equivalent in it knock resistance to a mixture of 87% octane and 13% heptane. This was a satisfactory measure until the chemists introduced fuels whose octane rating suggested a mixture of more than 100% octane and a negative percentage of heptane. It seemed as if it would be impossible to do, but the demands of military aviation saw drove the development. Petrol with a high aromatic content had a higher anti-knock value than those which had not. Minor improvements continued to be made to petrol formulations to improve yields and octane, including the introduction of so-called performance additives such as “Platformate” and “Activate”.
From the 1970s petrol underwent a slow evolution, most evident to the classic car enthusiast being the phasing out of leaded petrol (unleaded fuels were introduced to protect the exhaust catalysts that were being introduced for environmental reasons).