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2. Supply and Composition of Aviation Gasolines.

(a) Supply and sources.

The German aviation gasoline volume came very largely from synthetic oil plants that hydrogenated coals and coal tars. A very small volume only came from petroleum while essentially none came from the Fischer-Tropsch plants. Some components in small volume came from various chemical plants.

Parallel to the situation in the United States, great efforts were put forth continually in Germany to increase the supply of aviation gasoline. Much of the new construction was never completed due firstly to Allied bombing and then to termination of the war.

In Table I is given partial breakdown of the sources and volumes of supply of aviation gasolines and their components.

The volume figures given in Table I represent the highest production level in 1963 before bomb damage interfered greatly with production. (The highest production for an entire month was in 1963 and the average daily volume during that month was 52,200 barrels.) At that time when the maximum daily production of total aviation gasoline was about 56,000 barrels, there was under construction or being developed extending to increase that figure to nearly 100,000 barrels. (It is interesting to note that at the time the aviation gasoline production reached the figure to 56,000 barrels per day, the total German motor gasoline production was 55,000 barrels per day.)

(b) Composition and Specifications

There were two (2) grades of aviation gasoline produced in volume in Germany one the B-4 or blue grade and the other the C-3 or green grade. Both grades were loaded with the equivalent of 4.35 cubic centimeters tetraethyl lead per gallon. The B-4 grade was simply a fraction of the gasoline product from coal and coal tar hydrogenation. It contained normally 10 to 15 percent volume aromatics, 45 percent volume naphthenes, and the remainder paraffins. The octane number was 89 by a measurement corresponding to the C.F.R. motor method. The C-3 grade was a mixture of 10 to 15 percent volume of synthetic isoparaffins (alkylates and isooctanes) and 85 percent of an aromatized base stock produced by hydroforming types of operation on coal and coal tar hydrogenation gasolines. The C-3 grade was permitted to contain not more than 45 percent volume aromatics. This aromatic limitation sometimes required that the base stock component include some diluents other than the aromatic fraction, which could then be balanced if necessary by the inclusion of slightly more isoparaffin. (The C-3 grade corresponded roughly to the U. S. grade 130 gasoline, although the octane number of C-3 was specified to be only 95 and its lean mixture performance was somewhat poorer.)

The components of the two grades were therefore simple and few in number. The isoparaffins were produced by standard, well known methods and there was nothing abnormal found in their compositions. The base stocks were fractionated to and points of 300 to 320 degrees Fahrenheit. No normal isopentane separation was carried out and the pentane and butane contents were adjusted simply for vapor pressure control. Small amounts of specially synthesized aromatic compounds were included from time to time but no regular large scale use of such materials was practiced. No aromatic olefines or other special additives were used.

Oxidation inhibitors were not used in the regular blended aviation gasolines. It will be seen that the components were in general of such nature that addition inhibition should not have been necessary. Lead depositions from fuels was an operating problem, however, but no inhibitors were used for its prevention. This lead instability was believed to be related to aromatic content and fear of lead deposits was a reason for the limitation of the aromatic contents of the two grades.

The relative volumes of production of the two grades cannot be accurately given, but in the last war years the major volume, perhaps two-thirds (2/3) of this total has the C-3 grade. Every effort was being made toward the end of the war to increase isoparaffin production so that C-3 volume could be increased for fighter plane use. The isoparaffin usage in that grade had already been cut to a minimum.

In Table II, are given the important RLM (Reichs Luftfahtministerium)specification for aviation gasolines supplied to the Air Ministry. The complete specification sheet is appended. On that RLM sheet are also given specifications for aircraft diesel fuel. (The subject of diesel fuel manufacture in Germany is being covered by a U. S. Naval Technical Mission in Europe Report Entitled, “German Diesel Fuel”.)

Note-The mixture response curve for each gasoline shall at least equal that of a standard reference fuel, supplied by the R.L.M, at all air-fuel ratios between 0.75 and 1.3. The following document transmitted to the Bureau of Ships relates to specifications:

I. Technische Lieferbedingungon für die Fluguotoren-Frontkraftstoffe. (RLM specifications for aviation gasolines).

(c) Engine Testing.

The anti-knock performance of aircraft fuels was evaluated in two (2) different manners: by the octane number, using a test very similar to the C.F.R. Motor Method, and by a mixture response curve. The specifications of B-4 and C-3 fuels include both octane number and the mixture response curves.

Octane number was measured on the one-cylinder “I.G. Prüfmotor”. The technical data for this engine are as follows:

The values obtained with this I. G. test engine agree quite closely with those obtained on the C.F.R. determined on I.G. engines. The test conditions for measurement of aviation fuels were as follows:

The mixture-response curves of aircraft fuels were measured on a B.M.W. (Bayerische Motorenwerke) 132-F single cylinder engine. Liquid injection was employed and the following test conditions were used:

There are attached on the following pages two (2) small photographs which give several comparable mixture response values and plots for different components and fuels.

The first in a plot of air fuel ratio (abscissa) against “useful” pressure in atmospheres. The B-4 and C-3 fuels are shown thereon (MOZ is motor method octane number)

The second is a table with little meaning “Mixture-Response Power Outputs for Aromatic fuels showing relative power outputs of several components at air-fuel ratios of 0.9 and 1.3 and also their motor method and research method octane numbers. The top group is for mixtures of 50 percent values of 73 octane number (unleaded) coal hydrogenation gasoline tested with 4.35 cc tetraethyl lead per gallon and 50 percent volume of each of the components listed, also leaded. The lower group is of well known materials for comparison. (Fliegerbenzol is aircraft fuel; Dehydrier means “from dehydrogenation process”; Aromatisierungs means “produced by a process yielding high aromatic contents.”)

The composition of C-3 with a high aromatic content, resulted in that gasoline having a good rich mixture (less than 1.0) performance. It’s performance of allowable power output at lean mixture was not entirely satisfactory, however. If more isoparaffin had been included, the lean mixture performance would have been improved. This was recognized as the outstanding shortcoming in the German aviation fuel quality position. Had raw materials and equipment been available, more isoparaffins would have been included in the C-3 blend. As isoparaffin content increased the aromatic content could simultaneously have been decreased (by use of base stocks with octane numbers equal to those of the aromatic base stocks) and a gasoline with increased heat content would have resulted. However, because of the relatively greater was of manufacturing aromatics, they were used in large quantity to help gain a satisfactory lean mixture performance, with the result that rich mixture performance was no limiting.

(d) Safety Aviation Fuels

A note should be made regarding the development of safety aviation fuels. The Germans were quite aware of the desirability of safety fuels. Tests had been made with 390 to 660 degrees Fahrenheit fractions of coal and coal tar hydrogenation products but no full scale use of the materials was being made.

Some tests had been made to relate flash point and boiling range of a safety fuel to its resistance to ignition by incendiary bullets. It was concluded from this work that for a safety fuel to be effective, the flash point 20 degrees Fahrenheit and should be in the region of 300 degrees Fahrenheit.

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