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What Is the Limit in Octane Values ?

28th May 1943, Page 23
28th May 1943
Page 23
Page 23, 28th May 1943 — What Is the Limit in Octane Values ?
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How the Chemist Assists in the Constant Improvementin Motor Fuels

By "Azote"

THE raising of the octane value of motor spirit is, obviously, the trend in fuel development of the future. In the days of distillation, as opposed to the modern system of '` cracking " fuels, iso-octane was-the ideal, so far as anti-knock was concerned, and it was, consequently, given 100 marks. It was allotted that number to indicate that it

was at the top of the bill. • No one imagined that there was, or could be, anything better than 100 octane. Petrol, , at that time, had a rating of between 50 and 60 octane value and this, in a measure, barred the way to outstanding progress in the power output of internal-combustion engines using so-called petroleum spirit. The task, then, of the oil people was to find a fuel that could approach, as nearly as possible, to iso-octane, this substance itself being too scarce to provide an answer to the increasing demand for higher-grade motor fuels.

The first thing was to institute a standard of testing of new fuels, produced with the above target in view. This was done by comparing, by observation, a sample of 'the new fuel on engine test, with a reference fuel, composed of a mixture of iso-octane and heptane (heptane having a reaction to knocking propensities represented by the number 6).

Various proportions of these two compounds were tried, and the nearest mixture to the fuel under test was inspected for its percentage of octane, which settled the octane number of tha new fuel. For instance, if the reference fuel mixture contained 70 per cent, of iso-octane and 30 per cent. of heptane, then 76 was the octane number of the new fuel.

Petrol Not a Simple Fluid Petrol is not a simple fluid. Within the range of its initial and end points of boiling range, of 35 degrees F: to 185 degrees F., or thereabouts, there are some 200 different hydrocarbons. It does not necessarily mean that all samples of petrols contain all these compounds; the source of the crude from which the petrol has been made has an influence upon this point.

Different sources vary considerably in their hydrocarbon content. If there were but one hydrocarbon in petrol, the boiling point would be a point and not a range, but the 'fact that there is a range proves the presence of many compounds in the fuel.

Distillation is the method of separating one liquid from another, a process which is made possible by the fact that various liquids possess different boiling points. Those liquids with the lowest boiling points pass into the gaseous state, leaving those in the higher range as liquids. The boiling points of the hydrocarbons contained in petrol are so close together that they could not be separated by 'the ordinary distillation process. Moreover, the number of liquids possessing boiling points within a suitable range for an aspiration carburetter to vaporize is such a small percentage of the original crudes from which they would be produced that the process would be neither -economic nor practicable on the basis of demand.

The foregoing can be graphically represented, to show the range of compounds suitable for use as motor fuel. Actually, only a small proportion is usable, and the market for what might be termed the by-products was, originally, very small. The range of usable products was considerably added to with the progress of the oil engine, a development which, for obvious reasons, was encouraged by the oil interests.

e Chemistry Lends

a Helping Hand Chemistry was called in to increase the percentage of yield of motor spirit

from the cnrde. The carbon compounds of higher than nine atoms were split up, or cracked, into the compounds of the range C, to C,, and those of the range below were re-formed into that range: the compounds of the paraffin series are as follow:—Pentane, C, H,2; hexane, C. H„; heptane, C, H.; octane, C, 11„; nonane, C, H„.

There are additional compounds in the other series of hydrocarbons which are also contained in petrol fuel, as they have boiling points within the .same temperature range. Incidentally, the process of cracking gave fuels a _somewhat higher octane value than those produced by distillation.

Much research into the intricacies of all the hydrocarbons elicited the general fact that the paraffin series, quoted above, were the least desirable from the point of view of anti-knock value, the aromatics and, to a lesser degree, the naphthenes and olefins being mach , more desirable.

It became necessary, therefore, to transform the paraffins into the more desirable aromatics. The chemists set about taking the paraffin compounds to pieces, and putting them together again, with the chemical composition of the aromatics, changing the physical properties accordingly and obtaining the desired result.

They also found means for dissolving the C., compounds of the various series into the C,-C, liquids, thus obtaining more volatile properties and improved engine starting. Casinghead gas, socalled, was concerned in the making of high octane petrol. This was an extraction from the higher hydrocarbons occurring in natural gas.

Many chemical processes were brought into use in the manufacture of this high-octane petrol, which, when it reached 100, cost £5 per gallon in the laboratory stage. Based on the large quantity now produced, the cost to-day is, of course, far more reasonable.

Some of these chemical processes have extraordinary names, whigh are purely descriptive to the chemist. As examples, we have catalytic hydrogenation, thermal cracking, thermal reforming, catalytic polymerization, sulphuricacid alkylation, and so on. Patent literature to-day is full of new processes, filed in the names of the famo.u.s oil companies.

There does not appear to be a ceiling to the possibilities of octane value, upon which much of the future of the internal-combustion engine will depend. A value of 100, thd dreamed-of pre-war fuel, is now a fact, and 110 and 115 values are spoken of as assured possibilities. Further than this, a value of 150 is no longer a dream. It is spoken of as having reached the laboratory stage, and reference is confidentially made to the semi-commercial stage. Houdry, one of the principal oil interests, has developed a sy'stem for this super-octane fuel, and it is suggested that even more surprises are in store regarding chemically prepared super fuels.

Many of the difficulties of fuelmaking and utilization lie in the multiplicity of different hydrocarbons con

tained in the liquid. The ideal fuel. would be one in which but a single hydrocarbon was concerned, which, as previously mentioned, would have a boiling point instead of a boiling range. It is quite likely that much greater efficiency in me, and economy of operation, would be attached to such a fuel. What is required is some substance which could be carried as a liquid, but which would be burned as a gas, with the efficiency of combustion attaching to a gaseous fuel.

Methane—the Fuel of the Future ?

There is one hydrocarbon that meets all these conclitions; its name is methane and its octane number is 150. It may be that this is the fuel that is darkly hinted at as a future possibility. It is of particular interest, in that its octane number of 150 is natural to it and does not have to be pushed into it by expensive chemical processes. Farther, this fuel can be produced from British coal in Britain. Why not make our own 150-octane fuel?

[The mere availability of fuels having high octane numbers does not necessarily mean that the early post-war era will be one in which weshall be obtaining sensational horse-power figures from comparatively small units. Some authorities are of the opinion that fuels 'having values of 100 and over are only economically practicable with the employment of supercharging, but supercharging is not, of course, essential to the use of such fuels. There is no doubt, however, that fuels with reasonably high octane values will be used, with consequent increased power for a given capacity with the economy resulting directly therefrom.—Ers.]

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